Light-Emitting Self-Assembled Materials Based on d8 and d10

Review pubs.acs.org/CR

Light-Emitting Self-Assembled Materials Based on d8 and d10 Transition Metal Complexes Vivian Wing-Wah Yam,* Vonika Ka-Man Au, and Sammual Yu-Lut Leung Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee (Hong Kong)) and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China 3.3.2. Gold(I) Clusters of High Nuclearities 3.3.3. Trinuclear Gold(I) Metallacycles 3.4. Light-Emitting Mercury(II) Self-Assembled Materials 4. Heterometallic Light-Emitting Self-Assembled Materials 5. Conclusion and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments Abbreviations References

CONTENTS 1. Introduction 2. Light-Emitting Self-Assembled Materials of d8 Metal Complexes 2.1. Light-Emitting Rhodium(I) Self-Assembled Materials 2.2. Light-Emitting Iridium(I) Self-Assembled Materials 2.3. Light-Emitting Palladium(II) Self-Assembled Materials 2.4. Light-Emitting Platinum(II) Self-Assembled Materials 2.4.1. Platinum(II) Complexes with Cyanide and/or Isocyanide Ligands and Double Salts 2.4.2. Platinum(II) Complexes with Bidentate Ligands 2.4.3. Platinum(II) Complexes with Tridentate N-Donor Ligands 2.4.4. Platinum(II) Complexes with Tridentate Cyclometalating Ligands 2.4.5. Other Platinum(II) Complexes 2.5. Light-Emitting Gold(III) Self-Assembled Materials 3. Light-Emitting Self-Assembled Materials of d10 Metal Complexes 3.1. Light-Emitting Copper(I) Self-Assembled Materials 3.1.1. Tetranuclear [Cu4X4L4] Complexes 3.1.2. Copper(I) Alkynyl Complexes 3.1.3. Other Copper(I) Complexes 3.2. Light-Emitting Silver(I) Self-Assembled Materials 3.3. Light-Emitting Gold(I) Self-Assembled Materials 3.3.1. Low-Dimensional Gold(I) Complexes © XXXX American Chemical Society

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1. INTRODUCTION The spontaneous self-organization of matters has always been one of the most fascinating wonders in the world. Self-assembly refers to “the autonomous organization of components into patterns or structures without human intervention”, according to a definition by Whitesides.1 The self-assembling properties of organic molecules have resulted in a number of novel structures and properties. The extensive works in supramolecular chemistry, especially after the award of the Nobel Prize to Lehn,2 Cram,3 and Pedersen4 in 1987, have further boosted the development in the studies of self-assembled materials. With the enormously increasing interest in the study of supramolecular chemistry and the need of an in-depth understanding on the structure−property relationship of supramolecular self-assemblies, much attention has been drawn toward the self-assembly of small organic molecules in the solution state.5 Most of such self-assembling processes have been observed to be driven by an interplay of various noncovalent interactions, including hydrogen bonding, π−π stacking, electrostatic, donor−acceptor, and hydrophobic− hydrophobic interactions.6 On the other hand, the selfassemblies of metal complexes have been relatively less explored than the organic systems. The works by Fujita,7 Raymond,8 Stang,9 Nitschke,10 Lehn,2,11 Saalfrank,12 Ward,13 Lindoy,14 Stoddart,15 and many others16−18 have successfully demonstrated how metal complexes could be assembled into different supramolecular structures and architectures. In particular, molecular assemblies based on d8 and d10 transition

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Special Issue: 2015 Supramolecular Chemistry

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Received: February 5, 2015

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DOI: 10.1021/acs.chemrev.5b00074 Chem. Rev. XXXX, XXX, XXX−XXX

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Figure 1. (a) Molecular structures of [Rh(CNR)4]X, R = C6H11 and iPr. (b) Monomeric, dimeric, and trimeric structures of the complex cations in yellow [Rh(CNC6H11)4](BPh4), red [{Rh(CNC6H11)4}Cl]2·0.5C6H6·2H2O, and deep green [{Rh(CNC6H11)4}(SbF6)]3, respectively. Reproduced with permission from refs 31 and 32. Copyright 2006 Royal Society of Chemistry and 2007 American Chemical Society.

2. LIGHT-EMITTING SELF-ASSEMBLED MATERIALS OF d8 METAL COMPLEXES

metal complex systems represent important classes of luminescent self-assemblies.19−22 The versatility in the coordination modes from square planar in d8 metal complex systems to trigonal planar and linear geometries in d10 metal complex systems has enabled the rich structures and topologies of these transition metal complexes. More remarkably, the availability of a range of excited states has enriched these complexes with interesting luminescence properties. In most cases, the self-assembly of molecules involves inter- and/or intramolecular interactions. In addition to the aforementioned noncovalent interactions, the presence of metal−metal interactions in metal complexes has also been observed to be associated with the formation of self-assemblies. By judiciously manipulating the delicate balance between the nature of transition metal centers, ligands, coordination modes, and nuclearities, one will be able to design and synthesize metal complexes with luminescence properties. Together with a careful control of the solvophobicity and solvophilicity of the ligands, as well as subtle changes in the structures of the metal complexes, the resulting interplay of various noncovalent interactions can lead to the generation of interesting supramolecular self-assemblies and/or aggregation phenomenon. This Review will mainly focus on light-emitting self-assembly systems of d8 and d10 metal complexes. The scope of this Review will be confined to homo- and heterometallic d8 and d10 metal-based self-assembled and aggregated materials that involve noncovalent interactions. In many cases, the selection of the ligand will lead to the enforcement of aggregation behaviors of the metal complexes, and thus the assembled materials may not be completely spontaneous in nature. The associated luminescence properties of these d8 and d10 metal complex systems will also be discussed. However, heterometallic complexes with metal centers other than those of d8 and d10 electronic configurations, as well as mixed-valence metal complexes, are excluded from the scope of this Review. Other materials such as cross-linked polymers and nanoclusters will not be discussed here due to the limitation of the scope. Luminescent metal organic frameworks (MOFs) represent an actively developing class of light-emitting assemblies. The extensive works on luminescent MOFs have already led to some comprehensive review articles in recent literature,23−25 and hence this Review will not focus on MOFs. It is envisaged that through the following discussions on the self-assembly and luminescence behaviors of d8 and d10 metal complexes, one could appreciate how the self-organization of metal−ligand complex systems could lead to the construction of molecular functional materials.

2.1. Light-Emitting Rhodium(I) Self-Assembled Materials

Most of the d8 rhodium(I) complexes were found to adopt a square-planar geometry with four coordinating ligands.26−62 Among them, isocyanides are one of the most commonly reported ligands in this class of square-planar rhodium(I) complexes.26−55 This has been attributed to the π-accepting ability of the isocyanide ligands that are capable of stabilizing the electron-rich rhodium(I) metal center, and thus avoiding the occurrence of oxidative addition of the complexes. One typical example is the class of homoleptic tetrakis(isocyanido)rhodium(I) complexes, [Rh(CNR)4]X (R = alkyl or aryl; X = counter-anions). They were known to exist in various crystal forms of different colors, such as red, green, yellow, blue, and violet.26 The variation in the appearance of the crystal forms was associated with the R substituents on the isocyanides, the selection of the noncoordinating counter-anions, and the methods of the recrystallization. McCleverty and co-workers first attributed the intense colors of the crystals to the presence of close Rh(I)···Rh(I) separations in the crystalline states.27 The crystallographic data by Mann and Keller, later on, supported the presence of the short Rh(I)···Rh(I) distances (3.19−3.26 Å) in the dimeric structures for the purple, green, and blue forms of the crystals.28,29 Mann, Gordon, Gray, and co-workers performed a detailed systematic study on the formation of the oligomers of [Rh(CNR)4]X (R = Ph, iPr, t Bu, vinyl, and cyclohexyl; X = Cl−, PF6−, BF4−, ClO4−, and BPh4−) in solution at room temperature.30 On the basis of the UV−vis absorption spectra at various concentrations, the absorption bands at ca. 568 and 727 nm were ascribed to the formation of dimeric and trimeric species at the ground state, respectively, while the absorption band at 411 nm was attributed to the monomeric species.30 Until recently, Balch and co-workers have succeeded in the isolation of the crystals of the trimeric form of [Rh(C NR)4](X) (R = iPr or C6H11; X = Cl, BPh4, or SbF6) (Figure 1).31 [Rh(CNC6H11)4](BPh4) and [Rh(CNiPr)4](BPh4) would exist as yellow crystal forms when they were formed by the slow evaporation of their ethanolic solutions. Their discrete cations showed no close Rh(I)···Rh(I) contacts, with Rh(I)··· Rh(I) distances of 8.6390 and 8.5228 Å, respectively, in the crystal packings.31 Yet, the slow diffusion of hexane into a benzene solution of [Rh(CNC6H11)4]Cl would lead to the formation of red crystals with the dimeric structure, showing a short Rh(I)···Rh(I) distance of 3.287 Å.31 In addition, the deep green crystals could be prepared from the layering of diethyl ether over the benzene solution of [Rh(CNiPr)4]Cl, while those of [Rh(CNC6H11)4](SbF6)] and [Rh(CNC6H11)4](AsF6) could be obtained from the steady evaporation and B


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cooling of the corresponding ethanolic solutions.32 Interestingly, the deep green crystal forms of the complexes would arrange into trimeric structures, showing different Rh(I)···Rh(I) distances for [{Rh(CNiPr)4}Cl]3·0.45H2O (3.1012 and 3.0739 Å) and [{Rh(CNC6H11)4}(AsF6)]3 (3.040 and 3.044 Å).32 On the other hand, the same Rh(I)···Rh(I) distance (3.044 Å) was observed in the trimeric structure for the case of [{Rh(CNC6H11)4}(SbF6)]3.32 It was worthwhile to note that the trimeric structure of the deep green crystal forms would have a significantly shorter distance between the Rh(I) metal centers, as compared to that of the related dimeric structures.31,32 The low-energy absorption bands of the yellow crystals of [Rh(CNC6H11)4](BPh4) (384 nm), red crystals of [{Rh(CNC6H11)4}Cl]2·0.5C6H6·2H2O (464 nm), and deep green crystals of [{Rh(CNiPr)4}Cl]3·0.45H2O (684 nm) (Figure 2) were shown to be consistent with the concentrationdependent electronic absorption measurements.32

[Rh2(bridge)4]2+ in aqueous HCl solution.34 Upon the irradiation of this complex in 12 M HCl solution at 546 nm, a clean conversion to a yellow product with evolution of hydrogen gas was observed. The yellow solution was characterized to comprise [Rh2(bridge)4Cl2]2+.34 In 1978, the same group studied the luminescence property of [Rh(C NC6H4−CH3-p)4](PF6) and a related dinuclear complex, [Rh2(bridge)4](BPh4) (bridge = 1,3-diisocyanopropane). The mononuclear rhodium(I) complex was shown to exhibit an emission band at 697 nm with a luminescence quantum yield of 0.0065 in a concentrated acetonitrile solution of the complex at room temperature.35 The dinuclear complex, [Rh2(bridge)4](BPh4) (bridge = 1,3-diisocyanopropane), has also been found to be highly emissive with a luminescence maximum at 656 nm (ϕ = 0.056).35 On the other hand, [Rh2(TMB4)]2+ (TMB = 2,5-dimethyl-2,5-diisocyanohexane) was another classic dimeric Rh(I) complex with bridging isocyano ligands, as reported by Gray and co-workers in 1979.36 The X-ray crystal showed that the TMB ligands were rigorously eclipsed and the coordination geometry of the Rh atoms was essentially square planar, with a Rh(I)···Rh(I) distance of 3.242(1) Å.36 The absorption and emission spectra of the dinuclear rhodium(I) complexes with the bridging isocyano ligands were examined in acetonitrile solution at room temperature.36 They were found to exhibit an intense low-energy absorption band at ca. 400−600 nm, attributed to the fully allowed 1A1g → 1A2u transition.36 The strong emission originated from an excited state derived from 1 A1g → 1A2u excitation.36 Later, the same group studied the Rh(I)···Rh(I) separation of the dinuclear complex, [Rh2(bridge)4]2+ (bridge = 1,3-diisocyanopropane), in the excited state and found that the Rh(I)···Rh(I) distance would be reduced by ca. 0.3 Å, based on the investigation of the absorption band shape.49 Because the 4dz2 and 5pz orbitals of the two rhodium(I) metal centers would have a strong interaction in the face-to-face conformation, the contraction of the Rh(I)···Rh(I) bond in the excited state was attributed to the excited state originating from the spin-allowed dσ* → pσ transition.49 Recently, Coppens and co-workers utilized DFT calculations to confirm the contraction of the Rh(I)···Rh(I) bond of [Rh2(bridge)4]2+ (bridge = 1,3-diisocyanopropane) to be 0.39 Å.41 Their estimation of the singlet−singlet and singlet−triplet excitation energies, determined from the TD-DFT calculations, was consistent with the experimental results. The same group has also utilized time-resolved crystallographic methods to directly measure the contraction of the Rh(I)···Rh(I) distance in the crystalline state of [Rh2(dimen)4](PF6)2·MeCN (dimen = 1,8-diisocyanomenthane).42 It was found that the lifetime of the triplet excited state of the complex would depend on the temperature. At 23 K, its lifetime would increase to 11.7 μs, suggestive of the presence of a thermally accessible upper state with a shorter lifetime. Apart from the homoleptic dinuclear rhodium(I) complexes, in 1976, Balch first reported the synthesis of heteroleptic dinuclear rhodium(I) isocyanide complexes with diphosphine ligands, [(RNC)2Rh(Ph2PCH2PPh2)2Rh(CNR)2]2+ (R = CH3, nC4H9, C6H11, tC4H9) and isolated them as crystalline PF6− and BPh4− salts.46 The dimeric analogue of the system with a diarsine bridge, [(RNC)2Rh(Ph2AsCH2AsPh2)2Rh(CNR)2][BPh4]2, was also prepared by the reaction of the corresponding isocyanides with the complex precursor, [Rh2(CO)2Cl2(Ph2AsCH2AsPh2)2], followed by the precipitation of the cation with sodium tetraphenylborate.47 Crosby and

Figure 2. (Left) UV−vis absorption spectra for polycrystalline samples of [Rh(CNC6H11)4](BPh4), [{Rh(CNC6H11)4}Cl]2·0.5C6H6· 2H2O, and [{Rh(CNC6H11)4}(SbF6)]3. (Right) Simplified molecular orbital diagram showing the lowest-lying transition through the interactions of the out-of-plane dz2 and pz orbitals for [Rh(CNR)4]n with D4h symmetry. Reproduced with permission from ref 32. Copyright 2007 American Chemical Society.

Apart from the mononuclear rhodium(I) complexes, a number of dinuclear rhodium(I) complexes, [Rh2(bridge)4]2+, consisting of four bridging isocyanido ligands such as 1,3diisocyanopropane, 1,6-diisocyanohexane, 1,5-diisocyano-1,1,5triphenylpentane, 2,5-dimethyl-2,5-diisocyanohexane, 1,8-diisocyanomenthane, etc., were reported by the research groups of Gray,33−38 Mann,39 Miskowski,39 Connick,40 and others.41,42 Together with the examples of the related heteroleptic dinuclear rhodium(I) complexes of [Rh2(bridge)2(dppm)2]2+43−45 and [Rh2(CN−R)4(P∧P)2]2+ (P∧P = bis(diphenylphosphino)methane (dppm) or bis(diphenyarsino)methane (dpam)),46−48 the photophysical and photochemical properties of all of the complexes have been found to be associated with the extent of the Rh(I)···Rh(I) interactions. In 1976, Gray and co-workers first synthesized and characterized a dimeric Rh(I) complex consisting of four 1,3diisocyanopropane (bridge) ligands. 33 This complex, [Rh2(bridge)4]Cl2, was found to undergo oligomerization in a concentrated methanol solution as monitored by the emergence of the low-energy absorption bands at 1140 and 1735 nm.33 One year later, they utilized this complex for the production of hydrogen gas through 546 nm irradiation of C


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reported dinuclear rhodium(I) complexes with bridging diisocyanido ligands.40 In the IR spectroscopic study, the CN bond of [Rh2(dmb)2(dppm)2](BPh4)2 would show a lower stretching frequency at ca. 2164 cm−1, as compared to that of [Rh2(dmb)4](BPh4)2 (2176 cm−1).40 This was attributed to the greater extent of the π-back bonding from the electron-rich rhodium(I) metal center with the less πaccepting dppm ligands to the diisocyanido ligands. The lowenergy absorption bands of [Rh 2 (dmb) 4 ](BPh 4 ) 2 and [Rh2(dmb)2(dppm)2](BPh4) in acetonitrile appeared at 560 and 630 nm, respectively. Because a closer proximity of the two rhodium(I) metal centers was observed, the spin-allowed dσ* → pσ absorption bands were found to be more red-shifted, as compared to the previously reported dinuclear rhodium(I) diisocyanido-bridging complexes.34−42 Both of them were found to display strong luminescence behavior in fluid media at room temperature, that is, [Rh2(dmb)4](BPh4)2 (630 and >840 nm) and [Rh2(dmb)2(dppm)2](BPh4)2 (710 nm). The emission bands at 630 and 710 nm were originated from the singlet 1[dσ* → pσ] excited state, while the emission band beyond 840 nm with lifetime of 11.2 μs was assigned as the phosphorescence of the 3[dσ* → pσ] triplet state.40 Che, Lu, and co-workers reported the aggregation behaviors in aqueous media for a series of tetrakis(aryl-isocyanido)rhodium(I) complexes (Figure 4).50 At an elevated temperature of 80 °C, the wine-red solution would show the absorption band at 530 nm, while upon cooling to room temperature, the solution would turn to blue and exhibit a low-energy absorption band at 625 nm, suggestive of the formation of dimeric and trimeric species or oligomeric aggregates, stabilized by Rh(I)··· Rh(I) interactions. Upon the aging of the solution for more than 12 h, nanowires were observed under electron microscopies with the exhibition of low-energy absorption band (645 nm) and near-infrared (NIR) emission bands (806 nm), which originated from the 4dσ* → 5pσ transition, typical of the presence of Rh(I)···Rh(I) interactions.50 Conductivity studies for the dispersion of the nanowires have also been performed, and it was revealed that the electrical conductivity was comparable to the single crystals of [Rh((CN−R)4]+.50 Grimme and Djukic have utilized density functional theory (DFT-D3) methods and spin-component-scaled second-order Møller−Plesset perturbation theory (SCS-MP2) quantum chemical calculations to give insights into the arrangement of the π−π stacking interactions from the eight aryl-isocyanides, which would overcome the entropy and Coulombic repulsion of the dimeric structure of [Rh(CN−Ph)4]22+ with two positive charges either in the gas phase or in solution (Figure 5).51 The binding energy contributed from the Rh(I)···Rh(I) interactions was found to be smaller than the stabilizing effect

co-workers studied the luminescence property of this class of heteroleptic dinuclear rhodium(I) isocyanide complexes with diphosphine ligands, [Rh2(RNC)4(P∧P)2](X)2 (R = tBu, n Bu, and Ph; P∧P = bis(diphenylphosphino)methane and bis(diphenylarsino)methane; X = PF6− and BPh4−).48 It was found that the luminescence property of all of the complexes, except [Rh2(PhNC)4(dppm)2](PF6)2, would exhibit mostly fluorescence at room temperature, but predominantly phosphorescence at 77 K.48 Particularly, [Rh2(PhNC)4(dppm)2](PF6)2 would exhibit solid-state phosphorescence at 10 500 cm−1 at both room temperature and 77 K, originating from the transition of σ*(4dz2) → σ(5pz).48 Che and co-workers have synthesized the heteroleptic dinuclear rhodium(I) isocyanide complex with diphosphine ligands, [Rh2(bridge)2(dppm)2]2+ (bridge = 2,5-di-isocyano-2,5-dimethylhexane) from the reaction of the homoleptic dinuclear rhodium(I) isocyanide complex, [Rh2(bridge)4]2+, with dppm in acetonitrile solution.45 The low-energy absorption band at 595 nm originated from the 1A1g[(dσ*)2] → 1B1u[(dσ*)1(pσ)1] transition, while the fluorescence (677 nm, τo < 10 ns) at room temperature originated from the 1B1u1(dσ*pσ) excited state in the solution state. The tailing of the emission band (ca. 750 nm) was presumably assigned to the 3B1u state as the lifetime of this specific wavelength was determined to be ca. 1.0 μs.45 Recently, Connick and co-workers employed another bridging ligand 2,2-dimethyl-1,3-diisocyanopropane (dmb), to prepare other dinuclear rhodium(I) complexes, [Rh2(dmb)4](BPh4)2 and [Rh2(dmb)2(dppm)2](BPh4)2 (Figure 3). Their

Figure 3. Molecular structures of the heteroleptic dinuclear rhodium(I) complexes, [Rh2(dmb)4](BPh4)2 and [Rh2(dmb)2(dppm)2](BPh4)2.

X-ray crystal structures as well as their photophysical properties have also been studied.40 The intramolecular Rh(I)···Rh(I) distance of [Rh2(dmb)2(dppm)2](BPh4)2·0.5MeOH·0.2H2O (3.0371 Å) was found to be the shortest among the previously

Figure 4. Molecular structures of the tetra(aryl-isocyano)rhodium(I) complexes. D


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To overcome the electronic repulsion of the rhodium(I) system in the self-assembly process, Jung, Son, and co-workers reported a series of neutral rhodium(I) complexes, cis[Rh(CO)2(H2N−R)Cl], where R substituents are hydrocarbon chains of various lengths.53 The concentration-dependent UV− vis absorption study revealed that [Rh(CO)2(H2N−C4H9)Cl] would exhibit a new broad absorption band at 500−700 nm, typical of the formation of Rh(I)···Rh(I) interactions when the concentration was increased from 0.1 to 50 mM in dichloromethane solution at room temperature (Figure 7). The green crystal was obtained from the evaporation of solvent, and its metallic luster appearance was suggestive of the formation of extended linear chains of Rh(I)···Rh(I) interactions. The X-ray crystal structure of [Rh(CO)2(H2N−CH3)Cl] showed that the complex was arranged in a one-dimensional linear chain with alternative Rh(I)···Rh(I) separations of 3.41 and 3.39 Å.53 In addition, electron microscopic studies revealed the presence of self-assembled aggregates in micrometer thickness and millimeter length. The variation of solid-state colors, ranging from green to violet, was believed to be associated with the different extents of the Rh(I)···Rh(I) interactions. Ko and co-workers reported a series of cis-bis(aryl-isocyano)rhodium(I) complexes bearing imine ligands (Figure 8) that

Figure 5. Molecular interactions in dimeric [Rh(CN−Ph)4]22+. Reproduced with permission from ref 51. Copyright 2011 American Chemical Society.

from the aryl ligands, which provided the π−π stacking interaction for stabilization. Apart from the rhodium(I) isocyanido system that would exhibit extended linear chains of dimeric or oligomeric structures associated with the Rh(I)···Rh(I) interactions, Dunbar and co-workers reported the rhodium(I) system with the mixed acetonitrile and carbonyl ligands, [Rh(CO)2(MeCN)2](BF4), which would form a one-dimensional oligomeric stack in the crystalline state (Figure 6).52 The

Figure 8. Synthetic route to di(isocyano) rhodium(I) diimine complexes.

demonstrate solid-state polymorphism and solution-state thermochromism.54,55 Diffusion of diethyl ether into concentrated solutions of the complexes in acetone, acetonitrile, and other solvents has been shown to form red or dark green crystals. The X-ray structure of the dark green crystals of [Rh(CN−C6H3−(Me)2-2,6)2(4,4′-Me2-bpy)](BF4) showed dimeric structures with alternating Rh(I)···Rh(I) separations of 3.48 and 4.68 Å, while that of the red crystals showed monomeric structures with longer Rh(I)···Rh(I) distances of 4.39 Å.54 Interestingly, when the temperature was decreased to 178 K, a low-energy absorption band would emerge at ca. 626− 705 nm (Figure 9), suggestive of dimer formation in the presence of Rh(I)···Rh(I) interactions. In addition, [Rh(C

Figure 6. Tetramer of [Rh(CO)2(MeCN)2](BF4) cations in the crystal packing. Reproduced with permission from ref 52. Copyright 1999 American Chemical Society.

Rh(I)···Rh(I) distances in the extended chain were found to be 3.1528 and 3.1811 Å, shorter than that of 3.27 Å for the related dimer system. The dark blue solid of the complex would give a yellow solution in acetonitrile, suggestive of the presence of monomeric species. Upon the slow evaporation of the solvent, dark blue crystals would steadily form, indicative of the formation of oligomeric species in the crystalline state.

Figure 7. (a) UV−vis absorption spectra of [Rh(CO)2(H2N−CH3)Cl] at various concentrations. (b) Molecular packing of neutral [Rh(CO)2(H2N−CH3)Cl] complexes. Reproduced with permission from ref 53. Copyright 2009 American Chemical Society. E


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Figure 10. Synthetic route to [Rh(H2bim)(CO)2]+ complex, where H2bim = 2,2′-biimidazole.

also found that 2,2′-biimidazole ligands would form hydrogen bonds with the NO3− and BF4− anions in the crystal state.56 They have also suggested that the HOMO−LUMO energy gap would become narrower upon the shortening of the Rh(I)··· Rh(I) distance, based on TD-DFT calculations.56 The results were consistent with the observation of the red-shifted absorption bands from 430 to 580 nm, leading to the variation of the crystal colors.56 On the other hand, a series of dinuclear rhodium(I) complexes with neutral charge, [{Rh(CO)2Cl}2(R2-bim)] (R = Me or Bn), was reported by the same group (Figure 11).57 Two different color forms of the

Figure 9. UV−vis absorption spectra of (a) [Rh(CN−C6H3−Cl32,4,6)2(4,4′-Cl2-bpy)](BF4) (1.01 mM), (b) [Rh(CN−C6H3−Cl32,4,6)2(bpy)](BF4) (0.95 mM), and (c) [Rh(CN−C6H5)2(bpy)](BF4) (0.98 mM) in acetone at different temperatures. Reproduced with permission from ref 55. Copyright 2011 American Chemical Society.

N−C 6 H 3 −Cl 3 -2,4,6) 2 (bpy)](BF 4 ) and [Rh(CN− C6H5)2(bpy)](BF4) showed red-shifted absorption bands at 834 and 903 nm, respectively, indicative of the presence of the trimeric form at low temperature (Figure 9).55 A further redshifted absorption band in the near-infrared region (990 nm) for [Rh(CN−C6H5)2(bpy)](BF4) was attributed to the tetrameric or oligomeric aggregates with extended Rh(I)··· Rh(I) interactions in solution.55 Notably, this class of compounds showed strong luminescence with emission maxima at ca. 565−588 nm, originated from the 3MLCT [dπ(Rh) → π*(N∧N)] excited state.55 Subsequent work by the same group has led to the isolation of a series of neutral cisbis(aryl-isocyano)rhodium(I) complexes with pyridylindolide ligands.55 The complex coordinated with the 2,4,6-trichlorophenylisocyano ligand was found to exhibit thermochromic behaviors. At low temperature, the solution of this complex in THF was found to change from yellow to green with the emergence of a new absorption band (563−607 nm), resulting from Rh(I)···Rh(I) interactions in the aggregates.55 Haukka and co-workers reported a series of cationic [Rh(H2bim)(CO)2]+ complexes, where H2bim is 2,2′-biimidazole, with Cl− together with the corresponding double salts of [RhCl2(CO)2]−, [FeCl4]−, and [CoCl4]− as the counteranions.56 This class of square-planar rhodium(I) complexes was synthesized by reductive carbonylation of RhCl3, followed by stirring with H2bim on Fe and Co metal surface under the conditions of 125 °C and 50 bar for 3−30 h.56 The X-ray crystal structure showed that the [Rh(H2bim)(CO)2]+ complex would exist as the dimeric form with the formation of π−π stacking interactions from two 2,2′-biimidazole moieties. The Rh(I)···Rh(I) distances were found to range from 3.280−3.770 Å, highly dependent on the steric bulkiness of the counteranions. In the case of [FeCl4]− and [CoCl4]− with tetrahedral geometry, the two rhodium(I) metal centers were far apart from each other, relative to the cases of Cl − and [RhCl2(CO)2]− anions.56 Further explorations by the same group on this class of system have been extended to other counterions such as NO3− and BF4− (Figure 10).56 In the crystalline state, the [Rh(H2bim)(CO)2]+ complex cations were found to stack consecutively to form an infinitely linear chain with Rh(I)···Rh(I) distances of ca. 3.238−3.343 Å, which were highly dependent on the selection of anions, preparation method for recrystallization, as well as the temperature.56 It was

Figure 11. Synthetic route to the dinuclear rhodium(I) complexes with neutral charge, [{Rh(CO)2Cl}2(R2-bim)], where R = Me or Bn.

crystal, reddish-orange and red, were isolated for the complex with the bridging Me2-bim ligand. In the reddish-orange crystal structure, the dinuclear molecules were found to adopt a linear chain with a slightly zigzag fashion, showing a shorter intramolecular Rh(I)···Rh(I) separation of 3.2090 Å, while the intermolecular Rh(I)···Rh(I) separation was found to be longer with a distance of 3.6341 Å.57 For the X-ray crystal structure of the red form, it was found that the complex would adopt a “dimer of dimer” arrangement in which only two [{Rh(CO)2Cl}2(Me2-bim)] units, with intramolecular Rh(I)··· Rh(I) distances of 3.2327 and 3.5197 Å, were joined by another intermolecular Rh(I)···Rh(I) contact of 3.4026 Å.57 However, only yellow crystals could be isolated for the complex with methyl substituents on the 2,2-biimidazole ligand. With the introduction of the sterically bulky benzyl moieties, yellow crystals were obtained, which were ascribed to the relatively long intramolecular Rh(I)···Rh(I) distance in the discrete complex.57 The rhodium(I) terpyridine system has also aroused attention because of the propensity for the formation of Rh(I)···Rh(I) and π−π stacking interactions.58 Hartl and coworkers first reported a series of square-planar [Rh(4′-Rterpy)X] (R = H, Cl, or tert-butyldimethylsilyl-o-carboranyl; X = Cl or Br) complexes (Figure 12).58 The complexes with dark purple or dark blue colors in the solid state were both highly unstable in air because they would be readily oxidized into the rhodium(III) compounds with yellow-to-red appearance. This instability was attributed to the electron-rich property of the rhodium(I) metal center. The X-ray crystal structure of [Rh(terpy)Cl] showed that the complexes would be arranged in sheets with interplanar distances of 3.508 Å, suggestive of the F


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crystallography, showing the bowl-shaped geometry and intramolecular Rh(I)···Rh(I) separations of 3.163−3.144 Å.62 2.2. Light-Emitting Iridium(I) Self-Assembled Materials

As compared to the isoelectronic rhodium(I) system, the tetrakis(isocyanido)iridium(I) complexes, [Ir(CN−R)4]+, received much less attention.63,64 This class of complexes would exhibit a metallic appearance with black-blue color in the solid state. Although there was a lack of X-ray crystal structures, the uniqueness of this intense color was suggestive of the existence of Ir(I)···Ir(I) interactions in one-dimensional infinite chains.63,64 The UV−vis absorption spectra at various concentrations revealed that the complexes would aggregate into oligomeric forms governed by the formation of Ir(I)···Ir(I) interactions in acetonitrile, with the appearance of a low-energy absorption band at 630 nm. Unfortunately, this class of complexes might be subjected to photodecomposition upon excitation because the solution color was changed to orange from blue during the measurement.63,64 On the other hand, the dinuclear iridium(I) complex with the four bridging dimen (1,8-diisocyanomenthane) ligands, [Ir2(dimen)4]2+, was reported by Gray, Mann, and co-workers to possess rich photophysical properties.65 The UV−vis absorption spectrum showed two distinct absorption maxima at ca. 470 and 580 nm at room temperature in fluid solutions. The intense absorption bands were attributed to a 1[dσ* → pσ] transition. It was found that the relative intensity of these two bands was temperaturedependent, with the band at higher energy (ca. 470 nm) losing its intensity to the band at lower energy (ca. 580 nm) as the temperature was decreased.65 Miskowski and co-workers performed resonance Raman spectroscopy to reveal two different ν(Ir···Ir) stretching modes (12 and 48 cm−1), whose intensities were found to be dependent on the excitation wavelength. The lower-frequency stretch was found to show an enhancement upon excitation on the blue edge of the 1[dσ* → pσ] band, while the higher-frequency stretch was found to be enhanced with excitation on the red edge of the 1[dσ* → pσ] system.65,66 It was suggested that [Ir2(dimen)4]2+ would exist as an equilibrium of two isomers with different Ir(I)···Ir(I) contacts in the solution at room temperature.65 Recently, Gray, Mann, Hill, and co-workers reported the M···M bondstretching energy landscapes for [M2(dimen)4]2+ (M = Rh and Ir) and revealed two minima for [Ir2(dimen)4]2+ with two geometric motifs, eclipsed “paddle-wheel” and twisted “propeller” conformations. The former displayed an Ir(I)···Ir(I) distance of ca. 4.1 Å with a twist angle of 0°, while the latter exhibited an Ir(I)···Ir(I) distance of ca. 3.6 Å with a twist angle of ±12°. Unlike [Ir2(dimen)4]2+, the rhodium(I) analogue complex would exist in only one conformation with a Rh(I)··· Rh(I) distance of ca. 4.5 Å with no twisting.66 In 1982, Stobart, Atwood, and co-workers reported the synthesis of bis(cycloocta-l,5-diene) bis(μ-pyrazolyl)diiridium(I), [Ir(COD)(μ-pz)]2, together with the analogues that were incorporated with 3,5-disubstituted μ-pyrazolyl ligands, 3,5bis(trifluoromethyl)pyrazole and 3-methyl-5-(trifluoromethyl)pyrazole.67 These bis(μ-pyrazolyl)diiridium(I) complexes would adopt a boat-shaped conformation with intramolecular Ir···Ir separations of 3.066−3.216 Å.68 Recently, Jones and coworkers reported another related diiridium(I) complex with pyrazolyl-bridging ligands, [Ir(CO)2(3,5-(CF3)2-pz)]2 (3,5(CF3)2-pz = 3,5-bistrifluoromethyl-pyrazolate) (Figure 14).61 The complex was also found to adopt a boat-shaped conformation with an intramolecular Ir(I)···Ir(I) distance of

Figure 12. Synthetic route to the [Rh(4′-R-terpy)X] (R = H, Cl, or tert-butyldimethylsilyl-o-carboranyl; X = Cl or Br) complexes.

formation of π−π stacking interactions.58 However, the shortest Rh(I)···Rh(I) separation was observed to be 4.896 Å due to the lack of Rh(I)···Rh(I) interactions.58 In contrast, the [Rh(4′-tertbutyldimethylsilyl-o-carboranyl-terpy)Cl] complexes would adopt a head-to-tail conformation in the dimeric structure, showing a Rh(I)···Rh(I) separation of 3.1503 Å.58 Ogo and coworkers synthesized a cationic rhodium(I) terpyridine complex with acetonitrile as the auxiliary ligand via reductive elimination of the rhodium(III) precursor, [Rh(terpy)Cl3], with AgNO3, H2, and NaOTf.59 Deep purple crystals of [Rh(terpy)(MeCN)](OTf) were isolated, and it was found that the cationic complex would adopt a zigzag arrangement, in which two dimers of the rhodium(I) terpyridine moieties with Rh(I)···Rh(I) contacts of 3.0693 Å were connected to form an array of tetrameric species with interdimer Rh(I)···Rh(I) distances of 3.1520 Å.59 Another original series of rhodium(I) complexes is the dinuclear [Rh(μ-pz)(L)2]2 system (pz = 1,2-dihaptopyrazolide, L = COD or CO ligand), which was first reported by Trofimenko in 1971.60 The reaction of 3,5-dimethylpyrazolide ions with [(COD)2RhC1]2 or [(OC)2RhC1]2 was found to give a quantitative yield of the corresponding dinuclear [Rh(μpz)(L)2]2 complexes.60 It was not until 2003 that Jones and coworkers prepared the related [Rh{μ-3,5-(CF3)2-pz}(COD)]2 (Figure 13, left) by stirring the reaction mixture of 3,5-

Figure 13. Molecular structures of (left) [Rh{μ-3,5-(CF3)2-pz}(COD)]2 and (right) [Rh{μ-3,5-(C6H4OCnH2n+1)2-pz}(CO)2]2 (n = 4, 6, 8, 10, 12, and 14).

(CF3)2pzLi with [Rh(COD)Cl]2 in diethyl ether solution at 0 °C.61 They have also characterized the complex by X-ray crystallography and found that the dinuclear rhodium(I) complex would adopt a bowl-shaped structure with an intramolecular Rh(I)···Rh(I) distance of 3.164 Å.61 Later, Cano and co-workers reported a series of related dinuclear complexes with 3,5-disubstituted pyrazoles containing long 4-nalkyloxyphenyl chains, [Rh{μ-3,5-(C 6 H 4 OC nH 2n+1 ) 2 -pz}(CO)2]2 (n = 4, 6, 8, 10, 12, and 14) (Figure 13, right).62 Some of the complexes have been characterized by X-ray G


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be axial as found in the symmetrically substituted product, [Ir(Cl)(PPh3)(CO)(μ-pz)]2 (Ir(I)···Ir(I) distance = 2.754 Å), that was formed by the reaction of [Ir(PPh3)(CO)(μ-pz)]2 with CCl4.70 Bis(carbonyl)acetylacetonatoiridium(I), [Ir(CO)2(acac)], is another class of well-known complexes that has been utilized as a precursor in the preparation of metal films and coatings via chemical vapor deposition (CVD).71 Needle-shaped single crystals with light yellow color were obtained by zone sublimation in a furnace under vacuum. The X-ray crystal structure revealed that the Ir(I) complex would adopt a squareplanar geometry with the bidentate acetylacetonato ligand and the two carbonyl groups.71 The coordination of the bidentate acetylacetonato ligand would form a planar six-membered metallacycle with the Ir(I) metal center (Figure 16, left).71 The Ir(I)···Ir(I) distances with the nearest neighbors were found to be 3.242 and 3.260 Å in the head-to-tail conformation, 180° opposite to each other as shown in the crystal packing of the complexes (Figure 16, right).71 Recently, Rohmer, Oro, and co-workers reported the synthesis of a related dinuclear Ir(I) complex, [Ir2(μOPy)2(CO)4] (Opy = 2-pyridonate), prepared by bubbling carbon monoxide into a solution of the complex precursor, [Ir2(μ-OPy)2(COD)2], in toluene.72 The isolated crystals were found to be air-sensitive with metallic-luster appearance.72 Xray crystallography revealed that the Ir(I) metal centers were bridged by two 2-pyridonato ligands with the O,O- and N,Ncoordinations only in the head-to-head configuration (Figure 17, left) with an intramolecular Ir(I)···Ir(I) contact of 2.8693

Figure 14. Synthetic route to the diiridium(I) complex with the pyrazolyl-bridging ligands, [Ir(CO)2(3,5-(CF3)2-pz)]2 (3,5-(CF3)2-pz = 3,5-bistrifluoromethyl-pyrazolate).

3.122 Å. Subsequent work by the same group has led to the isolation of a related diiridium(I) complex, [Ir(P{OR*}Ph2)(CO)(μ-pz)]2, where R* = (1S)-endo-(−)-bornoxy, which would exist as two diastereomers in a 63:37 ratio (Figure 15),

Figure 15. Two diastereomers of diiridium(I) complex, [Ir(P{OR*}Ph2)(CO)(μ-pz)]2, where R* = (1S)-endo-(−)-bornoxy. Reproduced with permission from ref 69. Copyright 1996 American Chemical Society.

as supported by 1H and 31P NMR experiments.69 Further oxidative addition reactions with MeI, as monitored by NMR, resulted in the formation of a diastereomeric pair of diiridium(II) adducts.69 They have also employed dihydrogen in the oxidative addition reactions with the diiridium(I) complex, [Ir(PPh3)(CO)(μ-pz)]2, to form the 1,2-dihydridodiiridium(II) adduct, [IrH(PPh3)(CO)(μ-pz)]2.70 X-ray structure determination showed that the two PPh3 ligands would axially coordinate to the iridium(II) metal centers, while the hydrides would occupy the trans coequatorial positions along the intramolecular Ir(I)···Ir(I) bond, the distance of which was determined to be 2.672 Å.70 Further reaction with CCl4 would replace the hydride to afford the monohydride complex, [Ir2(PPh3)2(CO)2(μ-pz)2H(Cl)], with an intramolecular Ir(I)···Ir(I) separation of 2.683 Å. As evidenced by the X-ray crystal structure, the hydride was equatorial and the triphenylphosphine was in axial position of one of the metal centers.70 However, at another metal center, the chloride would

Figure 17. Crystal structure of the dinuclear Ir(I) complex, [Ir2(μOPy)2(CO)4] (Opy = 2-pyridonate). Reproduced with permission from ref 72. Copyright 2005 American Chemical Society.

Å.72 The intermolecular Ir(I)···Ir(I) separation was determined to be 2.9808 Å in the dimer of dimer arrangement, in which two dinuclear moieties were packed through Ir(I)···Ir(I) interactions in the less hindered O,O-coordination face of the Ir(I) metal centers (Figure 17, right).72 2.3. Light-Emitting Palladium(II) Self-Assembled Materials

The chemistry of organopalladium(II) complexes represents a popular and fascinating research field in constant evolution and has attracted tremendous attention over the past few decades. The complexes are rather easy to prepare and are readily

Figure 16. X-ray crystal structure of (left) Ir(CO)2(acac) and (right) the crystal packing along the c and a directions. Reproduced with permission from ref 71. Copyright 2009 Springer. H


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Figure 18. Molecular structures of a series of dinuclear cyclopalladated [(C∧N)Pd(μ-Cl)]2 complexes with various azobenzene moieties and bridging ligands.

ogens, which exhibited a unique thermal property, achieving higher phase transition temperatures and the reduction of mesophase temperature range, as compared to that of the liquid crystalline ligands alone. Recently, Espinet and co-workers reported a series of imine-derived dimeric palladacycles (Figure 18c).95 Notably, the mesogenic property could be enhanced upon the incorporation of polyether-substituted carboxylate bridging ligands with larger steric bulkiness.95 It was found that this complex would display a large range of smetic A phases below 50 °C.95 When the polyether-substituted carboxylate bridging ligands were replaced by the L-lactate-derived acetate bridge, there would be a positive effect on the mesomorphic property due to the lack of the liquid crystalline behavior of the palladacycles. The mononuclear [(C ∧ N)PdLX], [(C ∧ N)Pd(O ∧ N)], [(C∧N)Pd(O∧O)], and [(C∧N)Pd(N∧N)]+ complexes could be synthesized by the simple stirring of a reaction mixture of the dinuclear species with the monodentate ligands (L) or bidentate ligands (O∧N or O∧O, e.g., 8-hydroxyquinoline or acetylacetone) through the bridge-splitting reactions.85,86 In 1985, the luminescence property of palladacycles was first reported by Kutal, Wakatsuki, and co-workers.96 They reported a series of palladium(II) complexes derived from azobenzenes with unsymmetric substituents, [(C∧N)PdLX] (Figure 19), and the studies of their photophysical and spectroscopic proper-

available among the known transition metal complexes. The synthesis of organopalladium(II) complexes was initiated by the isolation and the characterization of organopalladium(II) complexes with azobenzene derivatives by Cope and coworkers in the mid-1960s. 73,74 Since then, numerous exploration on the utilization of various donor groups apart from azobenzene, such as imines,75,76 pyridines,77,78 thioketones,79 amides,80−82 amidines,80 oxazolines, phosphorus and arsine(III)-containing ligands,83 thioethers,84 and ethers, in the chloro-bridged dimeric palladacycle system has been extensively made. The commonly reported chloro-bridged dimeric palladacycles were derived from tertiary amines and imines because of the formation of five- or six-membered rings.85,86 Tremendous studies on the organopalladium(II) system have been dedicated to the synthesis, structural characterization, methodology in organic synthesis, organometallic catalysis, new molecular materials, photoluminescent palladacycles, supramolecules, and dendrimers.85,86 The dinuclear cyclopalladated [(C∧N)Pd(μ-Cl)]2 complexes with azobenzene moieties were prepared by the reaction of M2[PdCl4] (M = Li or K) with aromatic C−H moiety bearing a planar sp2 nitrogen atom.87−89 Ghedini and co-workers first reported the X-ray crystal structure of di-μ-chloro-bis[(2′,6′dimethylazobenzenato-C 2∧ N 2 )palladium(II)] 90 and di-μchloro-bis[(4-hydroxy-4′-methlyazobenzenato-C 2∧ N 2 )palladium(II)].91 The X-ray crystal structures showed that the two (C∧N)Pd moieties are essentially coplanar with each other, adopting a H-shaped molecular geometry.90,91 The Pd···Pd distance for the complex of 4-hydroxy-4′-methlyazobenzene was determined to be 3.554 Å. In the early 1980s, this class of complexes with hydrocarbon chains at the 4,4′-positions on the azobenzene derivatives (Figure 18a) was first demonstrated to show thermotropic liquid crystal properties.87 The nature of the bridging halides, μ-Cl, Br, and I, would determine the phases and transition temperatures of the complexes (Figure 18b).92−94 Particularly, the chloro-bridged derived complex bearing four aliphatic chains was found to exhibit an unusual mesomorphic phase sequence at elevated temperatures,93,94 from the highly ordered crystalline C phase to the disordered N phase, and then to the ordered SmE mesophase, and finally to isotropic liquid I. Thus, these dinuclear cyclopalladated [(C∧N)Pd(μ-Cl)]2 complexes with azobenzenes bearing long hydrocarbon chains were regarded as nematic palladiomes-

Figure 19. Molecular structure of a series of palladium(II) complexes derived from azobenzenes with unsymmetric substituents, [(C∧N)PdLX]. I


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palladium(II) complexes with 8-hydroxyquinoline, [(C∧N)Pd(O∧N)] (Figure 20b), which exhibited rich luminescence properties arising from triplet ILCT excited states.103 Another class of mononuclear palladacycles, [(C∧N)Pd(O∧O)], represents an interesting self-assembly system that has been utilized as liquid-crystalline organometallic materials.104,105 They could be easily prepared from the bridgesplitting reactions of [(C∧N)Pd(μ-Cl)]2 and monoanionic bidentate O∧O ligands, such as acetylacetone or other derivatives.104,105 The unsymmetric palladacycle cores with long hydrocarbon chains showed a relatively less ordered mesophase at lower temperature, because of the disruption of the H-shaped molecular geometry with weaker intermolecular interactions.104,105 The liquid-crystalline property of chiral mesophases at low temperature was also demonstrated by the palladacycle acetylacetonato complexes (Figure 21a) containing

ties.96 This class of complexes would display an intraligand (IL) transition in the UV−vis absorption spectra. In addition, the luminescence of the complexes with the ortho-metalated 4nitro-4′-(dimethylamino)azobenzene moiety was assigned as an intraligand π → π* origin, while the emission of the 4methoxyazobenzene-derived palladium(II) complexes was attributed to the ligand-centered n → π* fluorescence.96 However, the self-assembly behavior of this class of mononuclear palladacycles [(C∧N)PdLX] has been relatively less explored. In 1987, Watts and co-workers first reported the synthesis of ortho-metalated Pd(II) 2-phenylpyridine (ppy) complexes, [Pd(ppy)(bpy)]Cl, [Pd(ppy)(en)]Cl, and [Pd(ppy)(CO)Cl], where bpy = 2,2′-bipyridine and en = ethylenediamine.97 This class of Pd(II) 2-phenylpyridine complexes was found to exhibit low-energy absorption bands at ca. 360 nm and luminescence at ca. 460 nm at 77 K, attributed to the transitions localized on the ppy− ligands.97 One year later, the spectroscopic and luminescence properties of cis-[Pd(ppy)2]98 and cis-[Pd(bzq)2]99 (bzq = benzo[h]quinoline) were reported by Barigelletti, Balzani, and co-workers, and Gliemann, von Zelewsky, and co-workers, respectively. Both studies suggested that the electronic transitions were mainly localized on the N∧N ancillary ligands. In the early 1990s, Ghedini and co-workers explored the aggregation behaviors upon the introduction of hydrophobic hydrocarbon chains on the [(C∧N)Pd(O∧N)] cores.100,101 They have reported the synthesis of dinuclear cyclopalladated [(C∧N)Pd(μ-Cl)]2 complexes with a series of salicyleneanilines or o-hydroxyazobenzenes to form the first examples of lateral− lateral fused organometallic mesogens.100 They found that the organopalladium(II) complexes with an unsymmetric polar core would exhibit thermotropic properties, either nematogenic or smectogenic phase with lower transition temperatures and higher mesophases stability, as compared to the corresponding halo-bridged dinuclear complex precursor.100 Specifically, they have reported a series of heteroligand complexes of cyclopalladated azo-moiety and a chelating resorcylideneamine bearing chiral terminal tails (Figure 20a).102 It was found that this class of complexes with stereogenic centers in the lateral chains would exhibit a dramatic decrease in the clearing temperatures and promote the presence of smectic and/or nematic chiral phases of the complexes with nonmesomorphic azobenzene precursors.102 Later, the same group also reported a series of unsymmetric cyclopalladated 2-phenylpyridine

Figure 21. Molecular structures of the palladacycle acetylacetonato complexes with (a) the stereogenic centers, S(−)-β-citronellyl and R(−)-2-octyle, in the alkoxy chains of the azobenzene moiety; and (b) polyacrylate.

stereogenic centers, for example, S(−)-β-citronellyl and R(−)2-octyl in the alkoxy chains of the azobenzene moiety.106 In addition, this class of palladacycle acetylacetonato complexes has also been incorporated with polymers to prepare polymeric liquid crystalline organometallic materials (Figure 21b).107,108 Ghedini and co-workers have prepared a film of homopolymers appended with [(C∧N)Pd(O∧O)] complexes.108 By using a laser, permanent gratings could be formed on this film, leading to their potential use as optical storage materials.108 The lack of luminescence for this class of palladacycle acetylacetonato complexes was attributed to the presence of the nπ* character of the azobenzene-centered state. In 1997, the photoluminescence study of this class of palladacycle cores was initiated by Ghedini and co-workers on the heteroligand cyclopalladated complexes with azobenzene and acetylacetone moieties.109,110 Although some of the complexes would exhibit only weak photoluminescence in solution, a series of emissive palladacycles containing the dye, Nile Red, was later reported by the same group (Figure 22).111 This class of complexes would exhibit similar absorption spectra, assigned as the metalperturbed ligand-centered (LC) transitions of the Nile Red ligand.111 The luminescence also showed solvatochromism, with the emission maxima ranging from 585 nm in cyclohexane to 712 nm in methanol spanning across a wide portion of the visible spectrum. In addition, the luminescence quantum yield (ϕ) was found to depend on the solvent polarity. They attributed such luminescence behavior to the presence of a

Figure 20. Molecular structures of (a) the dinuclear cyclopalladated [(C∧N)Pd(μ-Cl)]2 complexes with salicyleneanilines or o-hydroxyazobenzenes and (b) the unsymmetric cyclopalladated 2-phenylpyridine palladium(II) complexes with 8-hydroxyquinoline. J


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Figure 24. Molecular structures of a series of cyclopalladated 5-(1hexyl)-2{[4′-(1-undecyloxy)phenyl]}pyrimidine complexes, [(C∧N)Pd(N∧N)]X.

Figure 22. Molecular structure of a series of emissive palladacycles containing the dye, Nile Red.

result in lamellar mesomorphism, either of SmA or of SmC phase.115 In 2001, Yu and Fujita reported a dinuclear palladium(II) bipyridine complex bridged by two nitrato ligands, cis[Pd2(bpy)2(μ-1,3-NO3)2](NO3)2, that adopt a clip-like geometry with a Pd···Pd contact of 2.848 Å, suggestive of the presence of weak Pd···Pd interactions (Figure 25).116 The two

twisted intramolecular charge transfer (TICT) state on the Nile Red moiety that showed energy comparable to the lowest energy LC state. Tschierske and co-workers investigated the effect of the variation of the substituents on the acetylacetonato ligand on the mesomorphic properties for the [(C∧N)Pd(O∧O)] palladacycle complexes (Figure 23a).112 It was reported that the binary coassembly of palladacycle complexes with trinitrofluorenone (TNF) would induce a McMillan phase, in which the aggregates would orthogonally align themselves with each other. The same group also reported a series of butterfly-like metallomesogen palladacycles that contained para-cyclophanes and 1,3-diketonato moieties (Figure 23b).113 The variation of alkyl chain length on the diketonato ligands would induce changes in the molecular organization as well as the mesophases.113 Ghedini and co-workers reported a class of nematogenic cyclopalladated 5-(1-hexyl)-2{[4′-(1-undecyloxy)phenyl]}pyrimidine complexes, [(C∧N)Pd(N∧N)]X, prepared by the reaction of the corresponding chloro-bridged dinuclear complex, [(C∧N)Pd(μ-Cl)]2, with 2,2′-bipyridine114 and a series of 4,4′-disubstituted-2,2′-bipyridines115 (Figure 24). This class of palladium(II) complexes was the first example of cyclometalated ionic mesogens, which demonstrated an ionic thermotropic liquid crystal property.115 It was found that the nematic mesophase could form only for the complexes with tetrafluoroborate counter-anions, while no mesomorphism could be observed with other counter-anions. On the other hand, the incorporation of disubstituted bipyridines would

Figure 25. Molecular packing of a dinuclear palladium(II) bipyridine complex bridged by two nitrato ligands, cis-[Pd2(bpy)2(μ-1,3-NO3)2](NO3)2. Reproduced with permission from ref 116. Copyright 2001 Royal Society of Chemistry.

[Pd(bpy)] moieties were found to be arranged in a head-tohead stacking with an interplanar angle of ca. 17°, probably due

Figure 23. Molecular structures of palladacycle [(C∧N)Pd(O∧O)], with the variation of the substituents on the (a) acetylacetone, and (b) paracyclophanes and 1,3-diketonato moieties. K


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to the steric bulk of the bridging nitrato ligands. The dimer-ofdimer structure was found to exhibit a zigzag orientation with an intermolecular Pd(II)···Pd(II) separation of 3.117 Å. Yu and co-workers have further utilized this metallo-clips of cis[Pd2(N∧N)2(μ-1,3-NO3)2]2+ moiety to construct metallomacrocycles with 1H-bipyrazole as bridging ligands.117−121 The metallomacrocycles in various geometries were found to selfassemble from the Pd(II)-containing clips with various bridging ligands consisting of multiple pyrazolato units. A wide range of Pd(II)···Pd(II) separations (3.029−3.434 Å) were found in the X-ray crystal structures, which were highly dependent on the nature of the bipyridine and pyrazolato ligands.117−121 In particular, Yam and Yu reported a direct self-assembly methodology in aqueous medium to synthesize a series of positively charged metallomacrocycles where the dimetallic clips of dipalladium(II,II) diimine moieties would function as the corners while the dipyrazolate dianions were the spacers.120 Interestingly, all of these complexes adopted a crown shape with a well-defined cavity, capable of encapsulating solvent molecules and anions. The luminescence of such metallomacrocycles at ca. 620 nm in methanol originated from the triplet excimeric emission of the intraligand excited state resulting from the intermolecular π−π stacking interactions. Further studies on the host−guest interactions between this class of metallomacrocycles and aromatic guests were demonstrated by Yu and co-workers.121 They reported a series of metallomacrocycles with anthracene-, naphthalene-, and benzene-derived dipyrazolate dianions as the spacers.121 Upon the addition of aromatic guests to the metallomacrocycles in DMSO, the vibronic emission band of the metallomacrocycles was found to be significantly enhanced, reflecting a stronger π−π stacking interaction between the receptors and the aromatic guests.121 Parallel to the extensive development on the organopalladium(II) system with bidentate ligands, a family of palladium(II) polypyridine and their related cyclometalated complexes has also attracted enormous attention because of their interesting spectroscopic properties as well as selfassembly behaviors, associated with the π−π and/or Pd(II)··· Pd(II) interactions.122−127 The synthesis and spectroscopic study of the palladium(II) complexes with 6-phenyl-2,2′bipyridines 122−125 and azaindolyl-derived tridentate ligands126,127 have been reported over the past few decades. Most of the complexes were found to be nonemissive in solution at ambient temperature but exhibit yellow-orange luminescence only in the solid state or glass state at 77 K. Specifically, Neve, Campagna, and co-workers reported a series of chloropalladium(II) complexes with carboxy or hydroxy groups at the 4′-position of the C∧N∧N tridentate ligands.124 Xray structure determination showed that the chloropalladium(II) complexes with 4-carboxy-6-phenyl-2,2′-bipyridine would associate into dimeric structures in a head-to-tail fashion with a Pd(II)···Pd(II) distance of 3.27 Å, while the dimers would be separated by interdimer Pd(II)···Pd(II) contacts of 5.41 Å (Figure 26).124 The luminescence of the complexes in the glass state was assigned as triplet metal-perturbed ligand-centered (LC) excited states, with mixing of some triplet MLCT character.124 However, the solid-state luminescence was ascribed to an excimeric emission with the combination of a MMLCT state, as supported by the presence of Pd(II)···Pd(II) interactions in the crystal state.124 Che and co-workers reported a series of mononuclear Pd(II) phosphine complexes and the dinuclear derivatives with bridging phosphine ligands such as bis(diphenylphosphino)methane and 1,5-bis-

Figure 26. Stacking arrangement of Pd2 dimers along the a axis with alternation of short (solid lines) and long (dashed lines) Pd···Pd contacts. Reproduced with permission from ref 124. Copyright 2002 American Chemical Society.

(diphenylphosphino)pentane.128 The X-ray crystal structures of the dinuclear analogues revealed that the intramolecular Pd(II)···Pd(II) contacts were 3.230−3.320 Å, suggestive of minimal metal−metal interactions. Nevertheless, it was evident of the presence of π−π stacking interactions as the interplanar distances between two Pd(C∧N∧N) moieties were about 3.35 Å.128 The low-energy absorption bands at ca. 390 nm were tentatively assigned to MLCT transitions. In addition, these Pd(II) complexes were nonemissive at room temperature but would exhibit strong luminescence at 77 K. The dinuclear Pd(II) analogues bridged with dppm would display dual emissions in MeOH−EtOH glass at 77 K. The high-energy vibronic bands were assigned as a metal-perturbed 3IL transition, while the broad structureless bands were attributed to an excimeric 3IL emission, resulting from π−π stacking interactions.128 Bosnich and co-workers have extensively studied the molecular tweezers of the cofacial bis-terpyridine palladium(II) moiety for molecular recognition as well as self-assembly.129−137 Although the chloropalladium(II) terpyridine system exhibit limited solubility in common organic solvents, the incorporation of 3,5-di-tert-butylphenyl groups onto the linker has helped to solvate the dipalladium(II,II) terpyridine system in acetonitrile solutions.129−137 The two parallel chloropalladium(II) terpyridine moieties were separated by ca. 7.0 Å apart, which offered an opportunity for the potential occurrence of π−π stacking interactions of the planar aromatic guests upon intercalation within the cleft for molecular recognition. One of the single crystals of the bis-Pd(II) complex showed that the two complexes would interpenetrate with each other through the palladium(II) terpyridine moieties.136 The interplanar separation of the terpyridine ligand as well as the Pd(II)···Pd(II) contact were 108 M−1 s−1 even for trialkylamines.307 The result of the transient absorption spectroscopy showed convincing evidence for the formation of the one-electron-reduced Pt(II) complex, even with the presence of quenchers, serving as H atom donors.307 Afterward, the research groups of Ziessel308,309 and Castellano308−312 also carried out transient absorption difference spectroscopic experiments to study this class of alkynylplatinum(II) terpyridine system in the excited state. Later, Sun and co-

in solution has been attributed to the larger splitting of the d orbitals upon the incorporation of the good σ- and π-donating alkynyl ligand.298 The strong σ-donating behavior would raise the energy of the dσ* orbitals, giving rise to an enlargement of the d−d orbital splitting that led to a higher-lying d−d ligand field (LF) excited state. At the same time, the destabilization of the HOMO, which would be a result of the filled−filled pπ(alkynyl)−dπ(Pt) interactions, would lead to the lowering of the energy of the 3MLCT/3LLCT state. Such raising of the nonemissive 3LF excited state and the lowering of the emissive 3 MLCT/3LLCT state would lead to the enhancement of their luminescence properties.298,299 More importantly, upon the introduction of the alkynyl ligand into this class of complexes, the complexes were found to be more soluble in different common organic solvents. The good solubility has provided an excellent prerequisite and criterion for aggregation studies in solution.299 The same group further designed a series of carbon-rich molecular rods with luminescent properties based on the alkynylplatinum(II) terpyridine moiety.300 Interestingly, the extended π-conjugation of alkynyl ligands by an increase in the CC units would lead to unusual blue shifts in the MLCT/LLCT absorption as well as 3MLCT/3LLCT emission of the complexes.300 It was suggested that dπ(Pt) orbital would be stabilized with the more conjugated alkynyl ligands, thus resulting in a blue shift in the electronic absorption and emission spectra.300 The same group also reported a luminescent trinuclear platinum(II) terpyridine complex, [{Pt(tBu3terpy)}3(μ3-η1,η2-CC−)](PF6)4,301 by the reaction of dinuclear [Pt(tBu3terpy)(CC)Pt(tBu3terpy)](OTf)2 with 1 equiv of [Pt( tBu 3terpy)(MeCN)](OTf) 2 in acetone as illustrated in Figure 53.301 The crystal structure revealed that the naked CC2− bridge with two platinum(II) terpyridine moieties at the two termini was connected to the third Pt(II)

Figure 53. Synthetic route of [{Pt(tBu3terpy)}3(μ3-η1,η2-CC−)](PF6)4. Z


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emission behaviors for the alkynylplatinum(II) 4′-p-tolylterpyridine complexes appended with the amine-containing moieties upon the addition of HBF4 and NEt3.329 Yam and coworkers further labeled human serum albumin (HSA) by the reaction of the isothiocyanate and iodoacetamide attached on the alkynylplatinum(II) terpyridine system.323 The reaction resulted in bioconjugates, which were highly colored and exhibited luminescence property upon photoexcitation.323 In addition, the same group demonstrated the fabrication of dyesensitized solar cells (DSSCs) based on the alkynylplatinum(II) terpyridine complexes with an overall power conversion efficiency of 3.6%.325 Besides, the research groups of Eisenberg326−328 and Tung331 utilized this class of complexes to serve as photosensitizers to generate hydrogen gas from aqueous protons. Apart from the above applications derived from the alkynylplatinum(II) terpyridine system, the enhanced solubility also provides an excellent criteria for aggregation studies in solution for the alkynylplatinum(II) terpyridine complexes associated with the Pt(II)···Pt(II) and π−π stacking interactions, which was first reported by Yam and co-workers in the study of an interesting platinum(II) terpyridine complex with butadiynyl ligand, [Pt(terpy)(CC−CCH)]OTf.335 Like other typical platinum(II) polypyridine systems, this complex would exhibit solid-state polymorphism, which showed crystals with two different colors, the dark green and red forms. The dark green crystals would form a linear chain with the shorter intermolecular Pt(II)···Pt(II) distance of 3.388 Å, whereas the red crystals would show a dimeric structure in a zigzag fashion with alternating Pt(II)···Pt(II) distances of 3.394 and 3.648 Å (Figure 55).335 The polymorphism is common in the solid state for the platinum(II) polypyridine systems. However, when either of the two forms of [Pt(terpy)(CC−CCH)]OTf dissolved in acetonitrile or acetone, they would give yellow solutions, characteristic of monomeric species with an identical UV−vis absorption spectrum.335 Of interest, upon increasing the solvent composition of diethyl ether content in solution, the color of the solution changed distinctively from yellow to green then to blue (Figure 56), with a new emerging absorption band at 615 nm concomitant with a drop in absorbance at 416 nm (Figure 57).335 Together with the NIR emission enhancement observed at 785 nm (Figure 57), the drastic color changes were attributed to the formation of Pt(II)···Pt(II) and π−π stacking interactions in the presence of aggregates due to the reduced solvation upon increasing the nonpolar diethyl ether composition.335 The lower-energy absorption band in the UV− vis absorption spectrum at 615 nm was assigned as a metal− metal-to-ligand charge transfer (MMLCT) transition, while the NIR emission band of the 785 nm phosphorescence was derived from 3MMLCT transition.335 This finding establishes the first example of the utilization of the propensity of Pt(II)··· Pt(II) and π−π stacking interactions that induce the selfassembly with luminescence and spectroscopic properties upon a change in the solvent polarity.335 Yam and co-workers further explored the effects of the counter-anions toward the self-assembly properties of this class of complexes.336 It was found that MMLCT bands arising from metal−metal interactions were not observable in the UV−vis absorption and emission spectra of [Pt(terpy)(CC−C CH)](BPh4) in mixtures of acetonitrile and diethyl ether solution, which was in stark contrast to the related analogues of OTf−, PF6−, ClO4−, and BF4− (Figure 58).336 This was rationalized by the larger size as well as the noncoordinating

workers also reported a series of rod-shaped dinuclear chloroplatinum(II) complexes of back-to-back terpyridine ligands with luminescence properties.318 These complexes would exhibit dual emissions, ascribed to the fluorescence and phosphorescence, arising from the singlet and triplet [π−π*]/ILCT/MLCT excited states.318 They further extended the work to the second-order nonlinear optical study of this class of platinum(II) terpyridine complexes.319,320 Particularly, they reported chloroplatinum(II) terpyridine complexes in a dipolar D−π−A fashion with different linkers between the donor and acceptor.319 The two-photon absorption (2PA) cross-section values (σ2) were determined to be up to ca. 600− 2000 GM, as measured by Z-scan experiments.319 It was found that the chloroplatinum(II) complex with the ethynylene linker would demonstrate a stronger two-photon absorption than those with the vinylene linker.319 The rich photophysics and spectroscopic properties of the alkynylplatinum(II) terpyridine system have prompted the groups of Yam,321−325 Eisenberg,326−328 and Tung329−331 to explore their applications, such as in molecular recognition, 321,322,329,332 biomolecular labeling, 323 pH sensing,324,329,330 hydrogen generation,326−328 dye-sensitized solar cells,325 and so on.333,334 The groups of Yam298,324 and Tung329 independently demonstrated molecular recognition and pH sensing properties by utilizing the alkynylplatinum(II) terpyridine and 4′-p-tolyl-terpyridine moieties, respectively. The 3 MLCT emission could be switched on by blocking the photoinduced electron transfer (PET) quenching of the emissive 3MLCT excited state upon the addition of various cations or organic acids to crown ether- or amine-containing moieties.298,324,329 Yam and co-workers have also demonstrated pH sensing by utilizing this class of platinum(II) complexes to exhibit remarkable and reversible color changes upon the consecutive addition of p-toluenesulfonic acid and triethylamine (Figure 54).324 The low-energy absorption band at 546

Figure 54. Molecular structures of alkynylplatinum(II) terpyridine complexes utililized as colorimetric and luminescence pH sensors.

nm was assigned to the amine-containing alkynyl-to-terpyridine LLCT transition, with mixing of a MLCT character at higher energy at 412 nm.324 Upon the addition of p-toluenesulfonic acid, the LLCT absorption band would drop dramatically with the emergence of the MLCT band.324 The presence of a welldefined isosbestic point at 460 nm was suggested to arise from the clean conversion of the complexes between their unprotonated and protonated forms. In addition, the protonated complexes would exhibit a new emission band at 600 nm, assigned to a 3MLCT emissive origin.324 Tung and coworkers have also observed similar reversible absorption and AA


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Figure 55. Molecular packing motifs of cationic [Pt(terpy)(CC−CCH)]+ in dark-green and red crystal forms.

Figure 56. Solutions of [Pt(terpy)(CC−CCH)]OTf (concentration = 1.47 × 10−4 M) in acetonitrile−diethyl ether mixtures demonstrating the remarkable color changes. Reprinted with permission from ref 335. Copyright 2002 American Chemical Society. Figure 58. UV−vis absorption spectra of [Pt(terpy)(CC−C CH)]X, where X = OTf−, PF6−, ClO4−, and BF4−, in an acetonitrile− diethyl ether mixture (concentration = 7 × 10−5 M) at room temperature. Inset: Photograph of the corresponding solutions displaying the remarkable color differences upon solvent-induced aggregation. Reproduced with permission from ref 336. Copyright 2005 Wiley-VCH.

property of BPh4−, which would hinder complex aggregates to form via the Pt(II)···Pt(II) and π−π stacking interactions in the solution. The same groups also described the first report on the induced aggregation of alkynylplatinum(II) terpyridine complexes in the presence of polyelectrolytes through electrostatic interactions.337 It was found that the coassembly between the negatively charged poly(acrylic acid) and the positively charged platinum(II) complexes would form polymer−metal complex aggregates. Because of the formation of complex aggregates via electrostatic assembly, the platinum(II) complex cations were brought into close proximity with each other, facilitating the formation of Pt(II)···Pt(II) and π−π stacking interactions that

could be revealed from the UV−vis absorption and emission spectra (Figure 59). By using the concept of the Pt(II)···Pt(II) and π−π interactions as an “on and off” switch, a label-free method has been utilized in biosensing materials. Yam and co-workers have demonstrated th e aggregat ion beh avior o f

Figure 57. (Left) UV−vis absorption and (right) emission spectral changes of [Pt(terpy)(CC−CCH)]OTf in acetonitrile−diethyl ether mixtures upon increasing the diethyl ether content. Reprinted with permission from ref 335. Copyright 2002 American Chemical Society. AB


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onto the ensembles to switch on the 3MMLCT emission, Yam and co-workers reported a probe to monitor G-quadruplex formation and extended the work as a selective and sensitive K+ ion assay based on G-quadruplex-forming oligonucleotides.339 Addition of K+ ions onto the ensembles of the TGGG oligonucleotide and the cationic platinum(II) complex, [Pt(terpy)(CC−CC−CH2−OH)]OTf, would result in the emergence of a new UV−vis absorption band at 550 nm and a low-energy phosphorescence in the NIR region. The formation of the G-quadruplex driven by the addition of K+ ions through the electrostatic interactions had led to the self-assembly of platinum(II) complexes with the presence of Pt(II)···Pt(II) and/or π−π interactions (Figure 60) to provide label- and

Figure 59. Aggregation of the platinum(II) complex ions in the presence of a base and a poly(carboxylic acid). Reproduced with permission from ref 337. Copyright 2005 Wiley-VCH.

alkynylplatinum(II) terpyridine complexes and the related conformational changes to recognize a range of biomolecules, such as single-strand DNA,338 G-quadruplex,339 heparin,340 glucose,341 lysozyme,342 thrombin,342 and human serum albumin,343 through the monitoring of emission changes in the near-infrared (NIR) region as self-assembly luminescence materials. The initial study of the biological systems involved the interactions of the cationic complexes with single-stranded nucleic acids.338 The cationic platinum(II) complexes were observed to show a strong tendency to assemble onto anionic single-stranded nucleic acids, such as poly(dT)25, poly(dC)25, poly(dG)25, and poly(dA)25, as revealed by the emergence of the low-energy 1MMLCT absorption and 3MMLCT emission bands upon addition of single-stranded nucleic acids to a buffer solution of the platinum(II) complex.338 It was rationalized that electrostatic interactions brought the complex moieties into close proximity and led to the formation of Pt(II)···Pt(II) and/ or π−π interactions. However, the extent of aggregation of the complex cations was highly dependent on the noncovalent interactions with the nucleobases and the secondary structure of the single-stranded nucleic acids.338 The cationic complex particularly exhibited stronger metal−metal and/or π−π interactions in the presence of poly(dT)25, when compared to the addition of poly(dA)25, as revealed by the more obvious MMLCT absorption and emission bands.338 This was rationalized by the stronger hydrophobic interactions between the adenine base and the platinum(II) complex that resulted in the lesser extent of the aggregation of complex molecules, and thus weaker Pt(II)···Pt(II) and/or π−π interactions were observed.338 In addition, the changes in the intensity and the pattern of the CD signals also indicated the assembly of the platinum(II) complexes into helical structures with the formation of metal−metal and π−π stacking interactions.338 To further study the self-assembly properties of the platinum(II) complexes on the anionic single-stranded nucleic acids, organic solvents were added in the binding studies with poly(dA)25 to suppress the hydrophobic interactions.338 Upon the addition of 20% acetonitrile or trifluoroethanol, the hydrophobic π−π stacking interactions between the platinum(II) complexes and the nucleic acid bases were indeed found to be suppressed,338 resulting in the self-assembly of the platinum(II) complexes and the more apparent emergence of the low-energy absorption and emission bands. In addition, the decrease in the CD signals indicated the dissociation of the complexes from the helical single-stranded nucleic acids, thus leading to the loss of helicity under such media.338 The use of positively charged potassium ions to stabilize Gquadruplexes is well-known,344 and Yam and co-workers have employed the combination of DNA with short G-rich sequences and potassium ions to increase the number of nucleotide units for the study. By utilizing the strategy of the self-assembly of alkynylplatinum(II) terpyridine complexes

Figure 60. Schematic representation showing the possible selfassembly of platinum(II) complexes with the formation of Pt···Pt and π−π interactions induced by G-quadruplex formation upon K+ ion binding. Reproduced with permission from ref 339. Copyright 2009 Royal Society of Chemistry.

immobilization-free detection methods for nucleic acids. The conformational changes of the secondary structure for Gquadruplex formation could be monitored by CD spectroscopy.339 On the other hand, the same group demonstrated the disassembly of platinum(II) aggregates of [Pt(terpy)(CC− CC−CH2−OH)]OTf through the enzymatic reaction of nuclease on poly(dT)25.339 Upon introduction of nuclease S1 into the resulting solution of ensembles for the cationic platinum(II) complexes and poly(dT) 25 , the MMLCT absorption and luminescence would diminish after incubation at 37 °C for a few minutes.339 Eventually, both of them would disappear within minutes, suggestive of the cleavage of the oligonucleotides, which would disrupt the Pt(II)···Pt(II) and π−π interactions on the ensembles. After prolonged incubation, the ensembles would completely deaggregate as only short oligonucleotides were present, and thus no low-energy MMLCT spectral bands were detected. This has provided a “proof-of-principle” sensing protocol for the monitoring of nuclease activities. The self-assembly of water-soluble alkynylplatinum(II) terpyridine complexes has also been employed in heparins, which possess the highest negative charge densities among the biological macromolecules.340 Yam and co-workers demonstrated the induced self-assembly of the platinum(II) complexes in the presence of the highly anionic-charged heparin, with the spectral change of the 3MMLCT phosphorescence employed for the quantification of unfractionated heparin (UFH) and the chemically or enzymatically fractionated low-molecular-weight heparin (LMWH) in both the buffer and biological media with a detection range typical of clinical dosage levels.340 This type of sensing protocol has been shown to be capable of differentiating heparin from other structurally similar analogues, such as chondroitin 4-sulfate (ChS) and hyaluronic acid (HA). AC


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thrombin-binding aptamer (TBA) G-quadruplex, as reflected from the exhibition of MMLCT absorption shoulder and NIR emission.342 Because the aptamers have a high affinity toward specific proteins, the addition of selected proteins would displace the discrete platinum(II) complexes from the ensembles, leading to a drop in the NIR 3MMLCT emission (Figure 62). By monitoring the spectral changes due to the dissociation, the two-component aptamer-complex ensemble could lead to the quantification of lysozyme and thrombin as specific probe materials. Catalytic amounts of adenylpyrophosphatase (ATPase) can hydrolyze adenosine triphosphate (ATP), which is an important resource of biological energy, into adenosine diphosphate (ADP) and inorganic phosphate (Pi). To monitor this kind of enzymatic activities in real time, Yam and coworkers employed a water-soluble alkynylplatinum(II) terpyridine complex, [Pt(terpy){CC−C6H4−CH2N(CH3)3}](OTf)2, to differentiate the phosphorylated tyrosine (Tyr)containing phosphopeptide (pP1) and dephosphorylated Tyranalogue P1 on the basis of their different anionic charge densities.345 According to the extent of the spectral changes on the MMLCT absorption and emission bands in the UV−vis absorption and emission studies, the phosphopeptide pP1 would induce the self-assembly of the cationic complexes in aqueous buffer solution to a greater extent, as a result of the two more negative charges on the phosphopeptide pP1 itself. Further utilization of the corresponding enzymes, v-Src kinase and alkaline phosphatase for phosphorylation of P1 and dephosphorylation of pP1, respectively, led to the catalytic conversions of the respective target substrates, which could be monitored by UV−vis absorption and emission spectral changes in real time due to the high sensitivity toward microenvironmental changes brought about by the selfassembly behavior facilitated by Pt(II)···Pt(II) and/or π−π interactions. The same group also reported another two-component ensemble from an anionic synthetic conjugated polymer, PPE− SO3− and [Pt(terpy){CC−C6H4−CH2N(CH3)3}](OTf)2, which is soluble in aqueous media.343 It was found that Förster resonance energy transfer (FRET) would occur from PPE− SO3− to the aggregated platinum(II) complexes on the polymer

They have also presented a selective glucose sensor on the basis of the self-assembly of water-soluble alkynylplatinum(II) terpyridine complexes onto the synthetic boronic acid-derived polymer, poly(3-acrylamidophenylboronic acid) (PAAPBA).341 Glucose was found to convert the boronic acid moieties appended on the polymer first into anionic boronate ester forms. Such formation of polyanionic polymers would attract the cationic platinum(II) complexes via electrostatic interactions, bringing them into close proximity to turn on the metal−metal interactions (Figure 61). Such ensembles of

Figure 61. Schematic representation showing the aggregation of platinum(II) complex molecules onto PAAPBA, leading to the formation of metal−metal interactions. Reproduced with permission from ref 341. Copyright 2011 Royal Society of Chemistry.

platinum(II) complex−polymer, which served as label-free methods for spectroscopic detection, have further been extended for the monitoring of α-glucosidase activity on disaccharides. The multiple negatively charged aptamers are the artificial nucleic acids employed in a diverse field of specific sensing. The same group reported another water-soluble alkynylplatinum(II) terpyridine complex, [Pt(terpy){CC−C6H4−CH2O−C6H3− (CH2OH)2-3,5}]OTf, that would self-assemble with the multiple negatively charged aptamer, either in the randomcoil 42-mer lysozyme aptamer (LA) or in the preformed

Figure 62. Schematic representation showing the disassembly of platinum(II) complex molecules from LA upon lysozyme binding or from TBAhemin upon thrombin binding. Reproduced with permission from ref 342. Copyright 2010 Royal Society of Chemistry. AD


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Figure 63. Schematic representation showing the disassembly of the polymer−metal complex ensemble upon binding of HSA or human telomeric DNA. Reproduced with permission from refs 343 and 346. Copyright 2011 American Chemical Society and 2013 The Royal Society of Chemistry.

Apart from the self-assembly of luminescent Pt(II) complexes for biosensing, the assembled Pt(II) complex system for bioimaging has also aroused considerable interest in recent years.348,349 Upon aggregation, the Pt(II) complexes would exhibit an emission in the NIR region, which is more biologically interesting. This aggregation feature could not be achieved by using octahedral complexes, such as Re(I), Ru(II), and Ir(III) derived complexes for bioimaging applications. Yam and co-workers first demonstrated pH probes derived from alkynylplatinum(II) terpyridine complexes with the turning on and off of NIR luminescence upon self-assembly and disassembly in cell-imaging studies (Figure 64).348 Upon

backbone by tuning the concentration of the complexes so that there is a better spectral overlap between the conjugated polyelectrolyte emission and the absorption of the assembled platinum(II) complexes arising from the Pt(II)···Pt(II) and/or π−π interactions (Figure 63). Further application of human serum albumin (HSA), which would bind to PPE−SO3− under appropriate conditions, resulted in the disassembly of the complexes from the ensemble, leading to ratiometric emission spectral changes in the visible and NIR regions for the sensitive and selective label-free detection of HSA. Polymer−metal complex aggregate showing FRET behavior has also been extended to the sensing of human telomere DNA.343 A series of water-soluble alkynylplatinum(II) terpyridine complexes with various hydrophobicity, steric bulkiness, as well as Förster radius (R0) were reported.346 It was found that [Pt{(CH3 )3 NCH2−C 6H4 −terpy}(CC−C 6H4−CH3 )](OTf) 2 with the extended π-phenyl ring at the 4′-position of the terpyridine ligand gave the highest FRET efficiency, as revealed by the largest Stern−Volmer constant (KSV).346 On the basis of the stronger electrostatic and π−π interactions between the complex cations and the G-quadruplex formed from human telomeric DNA, which are required to destroy the polymer− metal complex aggregates with stronger interactions, a twocomponent ensemble has been demonstrated to exhibit high selectivity and specificity in the detection of human telomere DNA in aqueous buffer solution (Figure 63). Thus, the modulation of the assembly/disassembly processes with emission spectral changes over the visible−NIR region from this polymer−metal complex aggregates has provided the labelfree detection of human telomere DNA with high sensitivity and selectivity.346 Very recently, water-soluble alkynylplatinum(II) terpyridine complexes appended with guanidinium moieties were reported by the same group.347 UV−vis absorption, resonance light scattering, and dynamic light scattering experiments showed that [Pt(terpy)(CC−Ar)][OTf]2 (Ar = C6H4-{NHC( NH2+)(NH2)}-4, C6H4-{CH2NHC(NH2+)(NH2)}-4) would aggregate through the strong and specific electrostatic as well as hydrogen-bonding interactions with citrate.347 The selfassembly of the complexes would lead to the emergence of MMLCT absorption and emission.347 The good selectivity toward citrate has been attributed to the specific interactions between the high charge density of citrate and the guanidinium moiety on the complexes under physiological conditions. Furthermore, their NIR emission property has been utilized for real-time monitoring of the enzymatic activity of citrate lyase.

Figure 64. Molecular structure of the water-soluble alkynylplatinum(II) terpyridine complex, [Pt{terpy(C6H4CH2NMe3-4)-4′}(CC− Ar)](OTf)2 (Ar = C6H3−(OH)2-3,5).

increase in pH value of the media, the phenolic protons of the alkynyl would be deprotonated, making the complexes more hydrophilic. Thus, the complexes would deaggregate in the polar media. Because of the dissociation together with the turning on of the photoinduced electron transfer (PET) process, the NIR emission of the complexes would diminish at pH above 7.6. This kind of water-soluble alkynylplatinum(II) terpyridine complexes has also been demonstrated to serve as an NIR luminescence probe in cell-imaging experiments.348 De Cola and co-workers demonstrated the self-assembly of neutral platinum(II) complexes with the tridentate pyridine bistriazole N∧N∧N ligand as a tool to achieve luminescent nanostructures in cellular compartments (Figure 65).349 Both complexes displayed strong yellow luminescence in their aggregated form arising from 3MMLCT emission. Notably,

Figure 65. Molecular structures of neutral platinum(II) complexes with the tridentate pyridine bis-triazole N∧N∧N ligand. AE


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anions (OTf−, PF6−, Cl−, and BF4−) (Figure 67) that exhibited luminescence and gelation properties, and formed helical supramolecular structures through metal−metal interactions as observed in the TEM and SEM images (Figure 68).354 More

cell internalization and localization experiments showed that the assemblies would target specific parts of the cells, the cytoplasmatic region and the nucleus, as dictated by the tridentate ligand coordinated to the platinum. With the same phosphorescence intensity after cell uptake, the aggregate form was suggested to remain intact inside the cell. Parallel to the development of the self-assembled ensembles and aggregates for recognition and imaging of biological molecules and cells based on luminescent platinum(II) complexes with tridentate N∧N∧N ligands, there have been tremendous interests in the exploration of soft materials derived from this class of Pt(II) complexes. Indeed, the first example of Pt(II)-containing soft materials was reported by Shinkai and coworkers in 2005.350 It was found that the luminescence property could be acquired from the inhibition of oxygen quenching for the triplet excited state in the gel state. A color change from yellow to orange in the sol−gel transition was observed. However, the gelation behavior was attributed solely to the formation of J-aggregates with the formation of π−π stacking interactions. More details will be discussed in section 2.4.4. Pertinent to the participation of Pt(II)···Pt(II) interactions in the gelation, the research groups of Ziessel351 and Yam352−354 independently reported alkynylplatinum(II) terpyridine complexes featuring six long alkoxy chains (Figure 66) and three

Figure 68. SEM images of chiral alkynylplatinum(II) terpyridine complexes in concentrated DMSO gel showing the coil-shaped superstructure in gel network. Reproduced with permission from ref 354. Copyright 2009 Wiley-VCH.

importantly, they have demonstrated the first example of the utilization of counter-anions to alter the chirality of the supramolecular structures in a single gelator molecule. Che and co-workers then reported a series of metallogels of alkynylplatinum(II) terpyridine complexes that do not contain long hydrocarbon chains for hydrophobic−hydrophobic interactions.355,356 Dinuclear alkynylplatinum(II) terpyridine complexes linked by oxadiazole and a mononuclear platinum(II) complex coordinated with biphenylalkynyl were shown to undergo gelation in acetonitrile and acetonitrile−alcohol mixtures, respectively. Because bulky tert-butyl groups were present on the terpyridine ligands, no Pt(II)···Pt(II) interaction was involved in the gelation process. Only π−π stacking interactions were observed and were thought to be associated with the sol−gel transition as well as the luminescence property. Notably, the Cl− salt of the biphenylalkynyl platinum(II) complex with the unsubstituted terpyridine ligand was found to form a brown hydrogel for more than 12 h.356 In sharp contrast to the thermotropic metallogels that typically show luminescence enhancement in the gel form, Yam and co-workers reported an unusual luminescence enhancement property of [Pt(R,R′-bzimpy)(CCR′′)]X (bzimpy = bis(N-alkylbenzimidazol-2′-yl)-pyridine; X = PF6−, OTf−) (Figure 69) during the gel-to-sol phase transition at elevated temperature.357 The unusual luminescence enhancement was rationalized by the fact that a higher degree of freedom for the molecules at the elevated temperature could lead to the formation of molecular assemblies to a greater extent, leading to the “excimeric” emission, which originated from the strengthening of the metal−metal and π−π interactions in the excited state. Subsequent works by Yam, Chow, and co-

Figure 66. Molecular structure of alkynylplatinum(II) terpyridine complexes containing six long alkoxy chains.

long alkoxy chains (Figure 67), respectively, to demonstrate new classes of soft materials via the formation of metal−metal interactions. They found that this kind of metal-containing molecules was capable of forming stable metallogels in dodecane, DMSO, or benzene, and displaying a strong 3 MMLCT emission band in the NIR region, distinctive from the gel derived from their pure organic counterparts. Yam and co-workers also presented the first report on a series of chiral alkynylplatinum(II) terpyridine complexes of various counter-

Figure 67. Molecular structures of alkynylplatinum(II) terpyridine complexes containing three long alkoxy chains. AF


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Figure 69. Molecular structures of [Pt(R,R′-bzimpy)(CCR′′)]X (X = PF6− and OTf−).

Figure 70. Molecular structures of platinum(II) terpyridine complexes with L-valine derived alkynyl ligands.

with the electrostatic interactions of +3 charges on the sterically bulky complex.359 In addition, the propeller structure of the trinuclear complexes with steric bulk hindered the formation of the intermolecular Pt(II)···Pt(II) interactions in such a gel-like supramolecular structure.359 Apart from the numerous examples of emissive gel formation for the platinum(II) N∧N∧N tridentate system, the construction of well-defined soft materials in nanosize with unique morphologies has aroused tremendous interest.350−359 Che reported luminescent soft self-assembled materials derived from a new class of platinum(II) N∧N∧N complexes with hydrogenbonding pyrazolyl and imidazolyl moieties (Figure 71).360 X-

workers have reported a rare example of L-valine-based gels with only one amino acid unit and no long alkyl chains on the platinum(II) terpyridine moiety (Figure 70). The complexes were found to aggregate into gels with the 3MMLCT emission band at 755 nm in acetonitrile.358 Incorporation of an additional L-valine unit into the system would lead to the extensive formation of hydrogen bonds, resulting in precipitation instead. It was concluded that incorporation of only one L-valine unit on the alkynyl ligands could achieve a subtle balance on the platinum(II) terpyridine moiety in the construction of supramolecular architecture. These demonstrated the interplay of multiple noncovalent interactions including hydrogen bonding, π−π stacking interactions, as well as metal−metal interactions for the design of a luminescent gelator in this kind of peptide-containing metallogels. Kato and co-workers reported a series of di- and trinuclear platinum(II) complexes bridged by 2-octylthio-1,3,5-triazine4,6-dithiolate, 2-octadecylthio-1,3,5-triazine-4,6-dithiolate, 2-din-butylamino-1,3,5-triazine-4,6-dithiolate, and 1,3,5-triazine2,4,6-trithiolate.359 For the dinuclear complexes, the two platinum(II) terpyridine moieties adopted a syn-configuration with an intramolecular Pt(II)···Pt(II) distance of 4.3 Å. The low-energy absorption bands (450−500 nm) and the structureless emission band (ca. 622 nm) originated from LLCT transition from triazine thiolates to the terpy ligands. This assignment was further supported by DFT calculations.359 It was found that the dinuclear [{Pt(terpy)}2(L)](PF6)2 (L = 2octadecylthio-1,3,5-triazine-4,6-dithiolate) would form a transparent red gel in acetonitrile, arising from the hydrophobic− hydrophobic interactions among the octadecyl chains. Although Pt(II)···Pt(II) interactions were not found throughout the study, the intermolecular π−π interactions could also play an important role as a driving force for the aggregation. The trinuclear [{Pt(terpy)}3(L)](PF6)3 (L = 1,3,5-triazine-2,4,6trithiolate) with neither a long alkyl chain nor a hydrogenbonding motif would aggregate into red gel-like supramolecular structures.359 The sole π−π stacking interactions would be the dominating driving force for the solvated aggregation, together

Figure 71. Molecular structures of platinum(II) N∧N∧N complexes with hydrogen-bonding pyrazolyl and imidazolyl moieties.

ray crystal structures of the complexes revealed extensive intermolecular Pt(II)···Pt(II) and/or π−π interactions, which were utilized as the driving forces in the construction of nanostructures. One of the complexes gave nanowire structures upon injection of a dimethylformamide solution of the complex into diethyl ether to form precipitates, as revealed from the TEM and SEM images. The selected area electron diffusion (SAED) pattern of an individual nanowire showed sharp diffraction spots with d-spacings of ca. 3.48 Å, suggestive of the involvement of Pt(II)···Pt(II) and π−π interactions in the AG


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construction of the nanowires.360 Another complex yielded micrometer-sized rectangular plates in the solution, and the SAED pattern also supported the presence of two-dimensional Pt(II)···Pt(II) and π−π interactions.360 The charge-transporting property of nanowire was further investigated by using a field-effect transistor (FET) configuration. Because of the formation of Pt(II)···Pt(II) interactions through the overlapping of the dz2 orbitals of the platinum(II) metal centers, a charge-hopping pathway was suggested to favor the high electron mobility of the aggregated complexes.360 Apart from the cationic platinum(II) complexes that could undergo self-assembly into well-defined nanostructures, Yam and co-workers reported an unprecedented class of amphiphilic anionic chloro- and alkynylplatinum(II) 2,6-bis(benzimidazol2′-yl)pyridine (bzimpy) complexes containing hydrophilic sulfonate groups (Figure 72).361 Upon an increase in the

To provide further understanding on the relation between the design of the molecular structures and their molecular packing that led to the different geometries of nanostructures, the same group has presented a series of amphiphilic platinum(II) bzimpy complexes featuring sulfonate groups and different lengths of alkyl chains on the alkynyl ligands (Figure 74).362 It was found that the length of the alkyl chains

Figure 74. Molecular structures of amphiphilic platinate(II) complexes of bzimpy with sulfonate groups and different lengths of alkyl chains.

could govern the molecular packing, associated with the formation of different self-assembled nanostructures. The change in morphology for the self-assembled nanostructures has been rationalized by the use of packing parameters of amphiphilic complexes. Such differences of the morphology would also result in red-shifted emission energy upon an increase in alkyl chain length due to the increase in the hydrophobicity of the complexes. Very recently, the same group reported an amphiphilic alkynylplatinum(II) bzimpy N∧N∧N complex incorporated with triethylene glycol (TEG) units, which would exhibit an unusual thermoresponsive behavior, arising from the transformational morphology with the governing of metal−metal interactions (Figure 75).363 Notably, upon heating the yellow aqueous solution of the complexes, the color would turn to orange-red, as indicated from the UV−vis absorption changes associated with the growth of the absorption tail at 500 nm, assigned as the MMLCT transition. Consistent with the spectral changes in the UV−vis absorption spectra, the 3IL band was found to decrease in intensity, and a structureless 3MMLCT emission band at 710 nm, associated with Pt(II)···Pt(II) and/or π−π stacking interactions, would become prominent on increasing the temperature. In addition, the UV−vis absorption studies showed a large hysteresis via a heating−cooling cycle, indicative of a transformation between two distinctive aggregate states.363 The electron microscopy images revealed that the complexes of the orange-red solution at elevated temperature yield spherical structures with diameters of about 8−10 nm, whereas the complexes of the yellow solution at 0 °C show large bilayered sheet-like structures of over a few millimeters in size. Because the Pt(bzimpy) moiety was positively charged and the TEG chains were not significantly long, the complex was regarded as an ionic amphiphile.363 Therefore, at low temperature, the solubility of the alkyl chains would be lower, and thus the alkyl chains on the complexes would be more rigid and tightly packed, leading to the formation of the bilayered structure. This tight packing would restrict the conformation of the Pt(bzimpy) moiety from coming into close proximity, resulting in

Figure 72. Molecular structures of amphiphilic anionic chloro- and alkynylplatinum(II) 2,6-bis(benzimidazol-2′-yl)pyridine (bzimpy) complexes.

acetone content in water, a programmable transformation of the morphology for the complex aggregates, from bilayer or multilayer vesicles to nanorods, was observed by controlling the polarity of the solvent. Such transformable morphology under external stimulus could also provide a dramatic color change from red solution to yellow and then to blue (Figure 73).361

Figure 73. Solutions of chloroplatinate(II) complex of bzimpy with hydrophilic sulfonate groups in water−acetone mixture. Reprinted with permission from ref 361. Copyright 2011 American Chemical Society.

The corresponding spectroscopic and luminescence changes were further studied by UV−vis absorption and emission spectroscopy, and were observable even with the naked eye. It was rationalized that the spectral and morphological changes by the aggregation−partial deaggregation−aggregation process resulted from the changes in the Pt(II)···Pt(II) and/or π−π interactions.361 This work represents the first report on the selfassembly of anionic platinate(II) with polypyridine-type ligands. AH


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Figure 75. Molecular structures of amphiphilic alkynylplatinum(II) bzimpy complexes incorporated with triethylene glycol (TEG) units.

the absorption and emission of the monomeric Pt(bzimpy) species. In contrast, at elevated temperature, the alkyl chains would achieve higher degrees of freedom, and its hydrophobic nature would direct them to point inward to avoid the unfavorable contact with the formation of micelle cores. The Pt(II) complexes would then be brought into close proximity, with the emergence of the MMLCT absorption and emission bands. Very recently, the same group first incorporated the polyhedral oligomeric silsesquioxanes (POSS) moieties into the alkynylplatinum(II) terpyridine system (Figure 76), which

Figure 77. Molecular structure of neutral Pt(II) complexes with the pincer ligand of 2,6-bis(1H-1,2,4-triazol-5-yl)pyridine N∧N∧N ligands.

pyridine N∧N∧N ligand.367 This complex was observed to show microcrystalline fibers in the length of micrometers in electron microscopic studies. Strong photoluminescence governed by metal−metal interactions was also observed.367 The aggregates displayed a high photoluminescence quantum yield of up to 74% with yellow-orange emission, as a consequence of their highly ordered aggregation, imparted by the noncovalent intermolecular Pt(II)···Pt(II) and π−π interactions. Very recently, they reported the synthesis of a neutral platinum(II) complex with two tetraethylene glycol (TEG) chains.368 Upon addition of cyclodextrin, the platinum(II) complexes would steadily form phosphorescent hydrogels due to the host−guest interactions between cyclodextrin and the tetraethylene glycol tails.368 Bu and co-workers reported the assembly−disassembly process for a series of metal-containing amphiphiles, demonstrating reversible luminescence switching.369 The cationic platinum(II) bzimpy complex appended with two triethylene glycol chains with anionic surfactants would electrostatically self-assemble into nanorods and sheets in aqueous solutions.369 The resulting solutions have been found to show strong luminescence, associated with the presence of metal−metal interactions for the aggregates formed. The aggregates could steadily undergo disassembly upon the addition of α-cyclodextrin, causing the switching off of the 3MMLCT emission band. The same group also reported water-soluble chloroplatinum(II) complexes of 2,6-bis(benzimidazol-2′-yl)-pyridine appended with hexaethylene glycol methyl ether chains with nonemissive property.370 Upon addition of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) salts to the dilute aqueous solutions of the complexes, the complex bearing two hexaethylene glycol methyl ether chains was found to exhibit a remarkable 3 MMLCT luminescence enhancement. It was rationalized by the formation of aggregates, as evidenced by both TEM and DLS experiments in aqueous solutions containing 0.5 wt % BMIMPF6.370 Mononuclear and dinuclear platinum(II) terpyridine-derived metal-containing supramolecular copolymers with amphiphilic properties were prepared by Yam and co-workers (Figure 78).371,372 The diblock copolymer, [(PEO112-terpy)Pt(CC− P2VP)]OTf, was shown to undergo a reversible self-assembly

Figure 76. Molecular structure of alkynylplatinum(II) terpyridine complex with POSS.

would exhibit characteristic nanostructures with distinguishable morphological transformation from rings to rods upon a systematic modulation of the solvent polarity.364 The assembly processes have been further supported by the crosspeaks between the phenyl rings and the terpyridine signals in the 2D H−H NOESY NMR experiments.364 De Cola and co-workers also reported a series of neutral Pt(II) complexes with 2,6-bis(1H-1,2,4-triazol-5-yl)pyridine N ∧ N ∧ N pincer ligands and pyridine as the ancillary ligand.365,366 It was found that this class of complexes was nonemissive in dilute chloroform solution at room temperature. Upon diffusion of n-hexane into the chloroform solution, a highly luminescent yellow gel was formed. The luminescence was characteristic of the presence of metal−metal interactions that resulted from the overlapping of the dz2 orbitals of the central platinum(II) atoms (Figure 77). Electron microscopic studies revealed that the emissive soft material was comprised of a 3D network of fibers. In addition, the complex was found to exhibit a photoluminescence quantum yield (PLQY) of up to 90% in the gel state, while the thin films of the complex in poly(methyl methacrylate) (PMMA) matrixes showed a PLQY of up to 87%. The same group further reported the solvent-induced aggregation of a neutral platinum(II) complex with the tridentate 2,6-bis(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)AI


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Figure 78. Molecular structures of platinum(II) terpyridine-derived supramolecular diblock and triblock copolymers.

ism upon exposure to volatile organic compounds (VOCs).376 It was found that the vapochromic property of the LB films derived from this class of chloroplatinum(II) bzimpy complexes was sensitive to the alkyl chain length and the counteranion effect. Yam and co-workers reported the isolation of a new class of alkynylplatinum(II) 2,6-bis(1-alkylpyrazol-3-yl)pyridine (N5Cn) complexes with amphiphilic properties (Figure 79),

into micelles upon a change in pH and polarity of the solution media. Drastic UV−vis and emission spectral changes could be used to monitor the formation of the micelles because of the modulation of the Pt(II)···Pt(II) and π−π interactions governing the micelle formation.371,372 For the triblock copolymers, [ClPt(terpy)PEO20−PPO70−PEO20(terpy)PtCl](OTf) 2 and [(Ph−CC)Pt(terpy)PEO 2 0 −PPO 7 0 − PEO20(terpy)Pt(CCPh)](OTf)2, spherical micelles were preferentially formed, as observed in TEM and DLS studies at temperatures above the critical micelle temperature.372 The UV−vis and emission spectral changes also indicated the significance of the metal−metal interactions in the micellization. Very recently, the same group reported a series of triblock copolymers, PEO 20 −PPO 70 −PEO 20 , end-capped with alkynylplatinum(II) terpyridine moieties with the functionalization of ammonium as well as pH-responsive amino groups.373 The amino-functionalized metallo-supramolecular triblock copolymers were found to exhibit a dual pH- and temperature-responsive behavior with dramatic spectral changes in the UV−vis absorption and NIR emission upon modulation of temperature and pH values.373 This has been rationalized by the fact that an increase in temperature would cause a collapse of the hydrogen bonding of the PPO block with water molecules.373 The polymeric micelles would shrink and enhance the aggregation of the platinum(II) complex moieties with the growth of the MMLCT absorption and emission bands. When the solution media became more basic, the MMLCT absorption shoulder would exhibit a two-step growth, suggestive of the formation of Pt(II)···Pt(II) and π−π interactions. This is attributed to the fact that the decreased hydrophilicity due to the deprotonation of the positively charged CH2NHMe2+ groups would lead to a greater extent of aggregation of the platinum(II) complex moieties through Pt(II)···Pt(II) and π−π interactions in the basic aqueous medium.373 Langmuir−Blodgett (LB) films with luminescent property represent another interesting type of self-assembled materials. Utilization of Pt(II) complexes to form self-assembled LB films in a controllable manner has been made.374 DeArmond and coworkers reported the LB studies on Pt(II) complexes in 1992.374 The square-planar Pt(II) complex, [Pt(C∧N)2] (C∧N = 2-(2-thienyl)pyridine), was found to horizontally and vertically orient on the film by controlling the surface pressures in a mixture of [Pt(C∧N)2]/stearic acid.374 Later, Haga and coworkers utilized Pt(II)···Pt(II) interactions to control the molecular ordering of square-planar Pt(II) complexes by using Langmuir−Blodgett techniques.375 They reported LB films of a series of chloroplatinum(II) bzimpy complexes,375 demonstrating a strong emission at 650 nm that originated from the metal−metal dσ* to ligand π* charge transfer (MMLCT) transition in the multilayer LB films, as well as the monolayer. Sun and co-workers further utilized this class of chloroplatinum(II) bzimpy complexes in the formation of LB films and showed that the films exhibited selective vapochrom-

Figure 79. Molecular structures of alkynylplatinum(II) 2,6-bis(1alkylpyrazol-3-yl)pyridine (N5Cn) complexes.

which could form reproducible and stable LB films at the air− water interface.377 The 47-layer LB film of alkynylplatinum(II) N5Cn complexes would exhibit a low-energy emission band at 693 nm, indicative of the formation of Pt(II)···Pt(II) and π−π stacking interactions as a consequence of the close molecular packing in the ordered arrangement. This was further supported by LB film characterization through the studies of the π−A isotherm and the XRD pattern. Very recently, the same group presented a new class of platinum(II) complexes with tridentate 2,6-bis(1-alkyl-1,2,3-triazol-4-yl)pyridine and 2,6-bis(1-aryl-1,2,3-triazol-4-yl)pyridine (N7R) ligands to demonstrate the formation of ordered molecular packings in LB films.378 In 2004, Eisenberg and co-workers reported the vapoluminescence property of the chloroplatinum(II) terpyridine complexes appended with the nicotinamide moiety (Figure 80).379 The sorption of methanol, acetonitrile, and pyridine vapor was found to induce a color change of the complex from red to orange, and the emission maximum would be blueshifted from 660 to 630 nm. The photophysical properties of the red form were suggested to arise from a 3MMLCT excited state involving the participation of Pt(II)···Pt(II) interactions,379 whereas the photophysical property of the orange

Figure 80. Molecular structure of chloroplatinum(II) terpyridine complex with nicotinamide moiety. AJ


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form resulted from a 3MLCT excited state originating from the monomeric complex. This work represents the first example of the selective and reversible vapochromic response for the platinum(II) terpyridine system. Later, they further reported a chloroplatinum(II) complex with a pentaphenylbenzene moiety, which was shown to exhibit unusual large band shifts of vapor-induced phosphorescence from the red to the green spectral range of ca. 140 nm upon the sorption of VOCs.380 Similar to the previous case, the metal−metal interactions would be disrupted as a consequence of the sorption of guest vapor. Field and co-workers reported a chloroplatinum(II) terpyridine complex with nitrogen-coordinated thiocyanato ligand, [Pt(terpy)(NCS)]SbF6 (Figure 81, left).381 The crystal packing

served upon exposure to acetonitrile, DMF, or pyridine. The color of the solid was changed from maroon to yellow, probably attributed to the disruption of metal−metal interactions in the solid state.381 Connick and co-workers reported a new class of vapochromic chloroplatinum(II) salts with the tridentate ligand of 2,6-bis(N-methylbenzimidazol-2′-yl)pyridine (bzimpy).382 The complexes with different counter-anions were found to show different sorption abilities toward various guest vapors. Chloride salts were found to change from yellow to red after sorption of methanol, chloroform, ethanol, and acetonitrile vapors. In contrast, the PF6− salt selectively responded to acetonitrile vapor, turning from yellow to violet. Gravimetric studies also revealed that the Cl− analogue would take up two MeOH molecules per formula unit, whereas the PF6− analogue would only take up 1 equiv of MeCN. This strengthening of Pt(II)···Pt(II) interactions in the sorption process was distinctive from the prior examples for the platinum(II) terpyridine system.382 The 3MMLCT emission (600 nm) would be red-shifted to a longer wavelength at ca. 685 nm upon vapor sorption, as expected for stronger Pt(II)···Pt(II) interactions.382 Colón and Connick further reported a zirconium phosphate (ZrP) framework incorporated with platinum(II) terpy and bzimpy complexes that exhibited a rapid and reversible vapochromic/vapoluminescence response upon exposure to VOCs.383 The vapochromic response of [Pt(bzimpy)Cl]+- and [Pt(terpy)Cl]+-exchanged ZrP materials was distinctly different from that of platinum(II) terpy and bzimpy complexes alone. More interestingly, the lower concentration loading of the Pt(II) complexes in ZrP would

Figure 81. (Left) Molecular structure of [Pt(terpy)(NCS)]SbF6. (Right) Perpendicular view showing two complex cations of [Pt(terpy)(NCS)]SbF6·MeCN with the Pt(II)···Pt(II) distance of 3.293 Å. Reproduced with permission from ref 381. Copyright 2010 Royal Society of Chemistry.

revealed a head-to-tail arrangement in the dimeric structure with a Pt(II)···Pt(II) separation of 3.293 Å (Figure 81, right). Reversible vapochromic/vapoluminescence behavior was ob-

Figure 82. Molecular structures of mononuclear and dinuclear alkynylplatinum(II) terpyridine complexes with flexible linker on alkynyl or terpyridine groups. AK


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Figure 83. Molecular structures of a series of dinuclear alkynylplatinum(II) terpyridine complexes bridged by various repeating meta-phenylene ethynylene (mPE) units and binaphthol derivatives.

intermolecular aggregation of the complexes was further supported by the presence of spherical disk-like structures in the TEM images. Yam and co-workers reported the synthesis of a series of dinuclear alkynylplatinum(II) terpyridine complexes with oligomeric bridge constructed from five and six repeating meta-phenylene ethynylene (mPE) units (Figure 83).387 This class of complexes was found to have a strong tendency to fold back onto themselves to form a single-turn helical strand. 2D ROESY NMR experiments have been employed to give insights into the intramolecular self-association when the solvent polarity was increased. The observations of MMLCT absorption and emission have revealed the involvement of Pt(II)···Pt(II) interactions in governing this kind of metalcontaining foldamers. The work was then extended to the studies of chiral foldamers by the incorporation of an inherently chiral binaphthol moiety (Figure 83).388 By using specific enantiomers of the binaphthol moiety, the self-association of the two platinum(II) terpyridine moieties at the terminus would lead to the formation of M- and P-helices individually, as revealed by CD spectroscopy. More recently, a dinuclear alkynylplatinum(II) terpyridine complex linked by an amphiphilic binaphthol bridge featuring two hydrophilic TEG chains (Figure 83) was shown to display cylindrical columnar assemblies in aqueous acetonitrile solutions, mediated by Pt(II)···Pt(II) and π−π stacking interactions.389 The length

result in equally strong or even stronger emission intensity when compared to the neat complexes. A class of dinuclear alkynylplatinum(II) terpyridine complexes, in which the two platinum(II) units were linked through a flexible bridge (Figure 82), was reported by Yam and coworkers to exhibit aggregation and deaggregation processes under the modulation of temperature.384,385 This class of dinuclear alkynylplatinum(II) terpyridine complexes has been demonstrated to serve as synthetic organometallic analogues of DNA hairpin governed by the Pt(II)···Pt(II) and π−π stacking interactions. Their thermochromism together with the drastic solution color changes were studied upon temperature variation. Their aggregation properties were believed to be associated with the propensity of these noncovalent Pt(II)··· Pt(II) and π−π interactions to direct the two terminal platinum(II) terpyridine moieties to come into close proximity. The aggregation of the complexes would mimic a synthetic DNA hairpin. Further applying the two-state van’t Hoff model has allowed the determination of the activation parameters for the deaggregation process.384,385 Che and co-workers have linked the dinuclear platinum(II) terpyridine complexes by the introduction of the ortho-phenylene ethynylene oligomers in the bridging alkynyl ligand.386 The dinuclear complexes were shown to exhibit a red-shifted emission, suggestive of the increase in intermolecular Pt(II)···Pt(II) and π−π interactions upon an increase in water content in acetonitrile solution. The AL


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of the binaphthol bridge was found to be critical for the alignment of the hierarchical helices of helices in the formation of tertiary structures for foldamers. This kind of reciprocal association of multiple helices has resulted in the luminescence enhancement behavior, distinct from that of the pure organic systems. Recently, Wolf and co-workers reported a series of dinuclear alkynylplatinum(II) terpyridine complexes with a flexible terthiophene linker.390 They have isolated X-ray quality crystals for the folded and unfolded states from acetonitrile and chloroform solutions, respectively. The X-ray crystal structure in the folded state is found to be stabilized by the weak intermolecular interactions, including π−π stacking and C−H··· O and C−H···Cl interactions as well as Cl−π interactions.390 Apart from constructing the DNA hairpin mimics and the helical conformation for self-assembled Pt(II) complexes, Bosnich and co-workers have reported the pioneering works on molecular clefts and molecular rectangles by utilizing dinuclear chloroplatinum(II) terpyridine complexes with a rigid bridge to interact with cationic and neutral Pt(II) complexes and other metal complexes, such as those of Pd(II) and Au(III), facilitated by π−π and metal−metal interactions in solution.391−393 They demonstrated a series of molecular receptors consisting of two parallel [M(terpy)Cl]+ moieties, where M is Pd(II) and Pt(II).391 The two cofacially disposed units were found to be separated by 6.4−7.2 Å, which should be capable of accommodating various molecular guests.391 Upon the encapsulation of the neutral [Pt(salap)(NH3)], the host− guest complexes were found to change from light yellow to deep red in color with the emergence of low-energy absorption bands, ascribed to the formation of metal−metal interactions.391 The association constants of these two palladium(II)and platinum(II)-containing tweezers for the host−guest interactions have also been determined, and it was found that the platinum(II)-containing analogue would have a larger value of the association constants, suggestive of a stronger metal− metal interaction in this host−guest complex.391 Interestingly, upon the encapsulation of the anionic nBu4N[Pt(salap)(CN)] or nBu4N[Pt(salap)Cl], there was a slow formation of gels for these solutions, suggesting that extensive association existed.391 The conformation of the host−(neutral)guest complexes was also determined by the 1H−1H NOESY experiment.391 One crystal structure of the platinum host−(neutral)guest complex has also been obtained to confirm the presence of Pt(II)··· Pt(II) interactions (Figure 84).391 They have also utilized temperature-dependent 195Pt NMR studies to quantify the rates of intermolecular exchange between the free and associated guest.391 Later, the same group has demonstrated the encapsulation of Pt(II) and Au(III) complexes by using the Pt(II)-containing tweezers and found that the host−guest stability of adducts formed by the isoelectronic Pt(II) complex with the Pt(II) receptor is more stable.392 The same group then constructed Pt(II)-containing molecular cages by coordinating pyridine-derived linkers with the Pt(II)-containing tweezers.393 X-ray crystallography revealed that the pyridine linkers were perpendicular to the [Pt(terpy)]+ moieties, while the adjacent planes of the [Pt(terpy)]+ moieties in each cleft were essentially parallel to each other, inferring a model of molecular rectangles with an interplanar separation of ca. 6.9 Å (Figure 85).393 They have tried Pt(II) guest complexes of various sizes to be encapsulated by this molecular cage and concluded that the smallest guest could be better accommodated into the binding sites of the receptor, leading to the specific allosterism.393 In 2010, Wenger and co-workers reported a molecular cleft based

Figure 84. Perspective views of the molecular adduct in a “side” view, a “top” view, and a “front” view. Reprinted with permission from ref 391. Copyright 2003 American Chemical Society.

on dinuclear alkynylplatinum(II) bzimpy moieties bridged by a xanthene ligand that exhibited strong luminescence with good solubility in organic solvents (Figure 86).394 Yam and co-workers also revealed the host−guest interactions of versatile molecular tweezers based on the alkynylplatinum(II) terpyridine system, in which cationic, neutral, and anionic Pt(II), Pd(II), Au(II), and Au(III)395 as well as polyaromatic hydrocarbon guests396 could be encapsulated into the binding cleft of the tweezers in a sandwich fashion (Figure 87). This class of bis-alkynylplatinum(II) terpyridine molecular tweezers has served as versatile tweezers to host various guest molecules, including d8 and d10 metal−ligand moieties intercalated into the cavity. More importantly, this class of bis-alkynylplatinum(II) terpyridine molecular tweezers has provided a unique electronic absorption and luminescence behavior for the design of a unique spectroscopic probe in the host−guest interaction. Very recently, the same group reported a series of phosphorescent molecular double-decker tweezers or triple-decker complexes based on the alkynylplatinum(II) terpyridine system.397 The tweezers could accommodate two platinum guest complexes in an unprecedented well-defined and controlled manner. The AM


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Figure 85. X-ray structure of the platinum-containing molecular rectangle. Reproduced with permission from ref 393. Copyright 2005 Wiley-VCH.

self-assembly has also led to drastic color changes together with the turning on of the near-IR emissions as a result of oligomeric Pt(II)···Pt(II) interactions.397 Recently, Wang and co-workers prepared bis-alkynylplatinum(II) terpyridine molecular tweezers with a pyrene motif as the pendant functional group.398 The pendant pyrene was found to be encapsulated by the cavity of an adjacent pair of tweezers to extend the heteroditopic monomers, triggering a head-to-tail supramolecular polymerization process.398 Further addition of anthracene derivatives and bis(2-methoxyethyl) dicyanofumarate to this compartmentalized system would lead to a reversible disassembly and reassembly process.398 Very recently, Wang, Yao, and co-

Figure 86. Molecular structures of the dinuclear alkynylplatinum(II) bzimpy moieties bridged by xanthene ligand. Reproduced with permission from ref 394. Copyright 2010 Elsevier.

Figure 87. Molecular structures of alkynylplatinum(II) molecular tweezers and various metal guests. AN


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series of mononuclear and dinuclear bis-cyclometalated platinum(II) C∧N∧C complexes with 2,6-di(2′-naphthyl)-4-Rpyridine as the tridentate ligand (Figure 88). 407 The

workers reported a heterometallic tweezers/guest complexation of the bis-alkynylplatinum(II) terpyridine tweezers and alkynylgold(III) diphenylpyridine guest appended with benzo21-crown-7 (B21C7) units.399 Further complexation of secondary ammonium salt moieties with the B21C7 crown ether would lead to the formation of supramolecular hyperbranched polymers.399 Maeda and co-workers reported a kind of ion-based assembly between cationic platinum(II) complexes and anionic pyrrolebased π-conjugated moieties.400 XRD analysis and the UV−vis absorption spectroscopy in the solid state have revealed that the ion-based assemblies would exist as Colh mesophase in the dimeric and/or oligomeric stacking structures of platinum(II) complexes.400 Further investigation by microwave-based conductivity measurements showed that this kind of ionbased assembly displayed electrical conductive properties with ambipolar characteristics.400 2.4.4. Platinum(II) Complexes with Tridentate Cyclometalating Ligands. In 1988, von Zelewsky and co-workers first synthesized a class of bis-cyclometalated platinum(II) complexes, [Pt(C∧N∧C)X] (HC∧N∧CH = 2,6-diphenylpyridine; X = Et2S, pyridine, pyrazine).401,402 Until 1999, Rourke and co-workers reported a versatile synthetic route to prepare this class of Pt(C∧N∧C) complexes in high yields.403,404 The research groups of Yam405 and Che406 independently studied their photophysical and spectroscopic properties. Both of the studies did not observe short Pt(II)···Pt(II) contacts in the crystal structures even in the dinuclear complexes with bridging diphosphine ligands.406 Yam and co-workers investigated the photophysical properties and the ion-binding properties of a series of platinum(II) C∧N∧C complexes containing crown ether pendants.405 The complexes would exhibit vibronic emission bands in the glass state, which has been assigned as metal-perturbed 3IL origin. Upon an increase in concentration, broad and structureless emission bands (600−615 nm) were observed. Together with the structureless emission (620−625 nm) in dichloromethane solution at high concentration, they suggested that these low-energy and broad emissions in the glass state and dichloromethane solution at room temperature originated from the self-assembly or the oligomerization of the complexes.405 In addition, they have also performed a comparison study on the cation-binding properties between the C∧N∧C and the terpyridine complexes appended with the same crown ether moiety. It was found that the neutral platinum(II) C∧N∧C complexes would give rise to a larger binding constant than the positively charged terpyridine analogues. Che and co-workers prepared a class of cyclometalated platinum(II) C∧N∧C complexes with 2,6-diphenylpyridine as the tridentate ligand.406 The X-ray crystal structures showed the π−π stacking interactions of the tridentate 2,6-diphenylpyridine ligand in a head-to-tail manner, while there were no significant Pt(II)···Pt(II) interactions.406 The broad and structureless emission bands of the complexes in the solid state were assigned as the 3ππ* excimeric emissions.406 One of the complexes with bis(diphenylphosphino)methane (dppm) bridging ligand was demonstrated to show vapochromism.406 Upon exposure to organic solvents, such as benzene, pentane, dichloromethane, acetone, or methanol guest vapors, the orange crystal would immediately turn to bright yellow.406 The vapochromic property was ascribed to the sorption of organic vapors, which perturbed the π−π stacking interactions in the crystal lattice.406 In 2006, the same group reported a

Figure 88. Molecular structures of mononuclear and dinuclear biscyclometalated platinum(II) complexes with 2,6-di(2′-naphthyl)-4-Rpyridine (R = H, C6H5, C6H4−Br-4, C6H3−F2-3,5).

mononuclear complexes were found to adopt a head-to-tail orientation with intermolecular Pt(II)···Pt(II) interactions of >6 Å.407 Interestingly, the dinuclear system with bridging diphosphine ligands would adopt a syn-conformation with intramolecular Pt(II)···Pt(II) distances of 3.272−3.441 Å.407 This class of dinuclear [{Pt(C∧N∧C)}2(μ-dppm)] system was also shown to exhibit a reversible vapoluminescence response upon exposure to volatile organic compounds.407 Particularly, one of the desolvated complexes would exhibit a fast and reversible vapoluminescence response upon the sorption of halogenated organic solvents, except CCl4 vapors.407 They suggested that this was attributed to the intermolecular C−H··· X and X···X interactions between the platinum(II) complexes and the halogenated organic solvents, resulting in the less condensed solvated structure from the desolvated one, which were further supported by powder X-ray diffraction analysis on both desolvated and solvated crystals of the complex.407 More recently, the same group reported a series of platinum(II) C∧N∧C complexes, [(C∧N∧C)Pt(L)] (L = DMSO or CN− Ar), with the tridentate C∧N∧C ligand derived from carbazole, fluorene, or thiophene moieties.408 It was suggested that the complexes would exhibit extensive C−H···π(CC) as well as π−π stacking interactions with the adjacent complexes.408 However, there was no significant metal−metal interaction as determined from the X-ray crystal structures showing the shortest Pt(II)···Pt(II) distance of 6.967 Å.408 All of the complexes exhibited strong luminescence in solution derived from the 3MLCT/3IL excited state with luminescence quantum yields of ca. 2−26%.408 Given the high luminescence quantum yields, good solubility, and high thermal stability (>300 °C), they have been utilized as phosphorescent dopants for OLED applications, achieving a maximum external efficiency of 12.6%.408 Lin and co-workers reported the synthesis and the luminescence properties of a platinum(II) complex with a pyridine-based NHC pincer ligand.409 This complex was demonstrated to exhibit aquachromic photoluminescence property upon hydration and dehydration. Recrystallization from methanol−diethyl ether has been shown to afford the hydrated form, while recrystallization from CH2Cl2−hexane would yield the anhydrate form.409 The crystal structures of both forms indicated the existence of dimers that showed headto-tail orientations with Pt(II)···Pt(II) contacts of 3.5096(7) and 3.5185(19) Å, respectively.409 The hydrated form would show orange luminescence with a band maximum at 614 nm, whereas the anhydrate form would display green luminescence at 555 nm.409 The two luminescence behaviors could be reversibly switched from one color to the other through the AO


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This class of platinum(II) N∧C∧N complexes was first demonstrated by Kozhevnikov and Bruce in 2008 to show liquid crystal properties.419 They reported the liquid-crystalline (LC) phases for the complexes bearing long alkyl chains (Figure 90). Upon slow cooling of the isotropic liquid to 170

hydration and dehydration processes.409 Later, the same group replaced the chloro ligand with carbonyl ligand and reported vapochromism and vapoluminescence for this class of platinum(II) system upon sorption of different organic vapors.410 Very recently, Yam and co-workers reported the first isolation of a new class of luminescent alkynylplatinum(II) complexes with the tridentate pyridine-based NHC ligand (Figure 89).411 This class of complexes was found to exhibit

Figure 89. Molecular structures of the pincer-type pyridine-based Nheterocyclic carbene alkynylplatinum(II) complexes. Figure 90. Molecular structures of platinum(II) complexes of tridentate N∧C∧N ligand with long alkyl chains.

strong 3MLCT/3LLCT phosphorescence in solution with the emission energy readily tuned by the variation of the electronic effect of the alkynyl ligands. The spectroscopic assignments were further supported by the TDDFT/CPCM calculations. In addition, nanosecond transient absorption (TA) spectroscopy has also been performed to probe the nature of the excited state. Parallel to the extensive studies on the cyclometalated platinum(II) C∧N∧C complexes, Cárdenas, Echavarren, and coworkers first isolated and synthesized the cyclometalated platinum(II) complexes of 1,3-di(2-pyridyl)benzene (N∧C∧N) in 1999.412 The study of luminescence property for this class of system was first reported by Williams and Weinstein in 2003.413 This class of complexes would strongly emit in degassed CH2Cl2 at room temperature, achieving a high luminescence quantum yield of 68%.413 The strong luminescence behavior was associated with the strong ligand field exerted by the N∧C∧N ligand due to the short Pt−C bond.413 Thus, the energy of the nonemissive 3LF state would become higher-lying and the nonradiative deactivation would be less likely to occur. The vibronic-structured emission bands of the complexes were derived from the metal-perturbed 3IL 3[π → π*] excited state.413 Upon an increase in concentration, the complexes would exhibit structureless emissions at ca. 700 nm, assigned as excimeric emissions.413 In 2007, Fattori and Williams have utilized this class of complexes with the tridentate 1,3-di(2pyridyl)benzene N∧C∧N ligand as phosphorescent OLED dopants with high electroluminescence efficiencies.414 The electroluminescence could also be steadily tuned from yellow to greenish-blue by varying the substituents at the 5-position of the central phenyl ring of the tridentate N∧C∧N ligand.414 Later, various groups also studied the spectroscopic and luminescence properties415−417 as well as the second-order nonlinear optical property418 for this class of platinum(II) N∧C∧N complexes. Particularly, Shinozaki and co-workers reported an interesting luminescence property with tunable color from blue to white to orange for the complex, [Pt(Fmdpb)X] (FmdpbH = 4-fluoro-1,3-di(4-methyl-2pyridyl)benzene; X = Cl or CN).417 The dramatic color change was attributed to the monomer−excimer equilibrium in the excited state.417

°C, a columnar phase (Colh) of antiparallel complexes was formed, as supported by structural analysis. The luminescence from the liquid crystalline state was found originate from the monomeric emission. In contrast, the noncrystalline phase formed from the rapid cooling of the isotropic liquid showed an excimeric emission. It was suggested that the noncrystalline phase consisted of a high degree of isotropic grain boundaries such that the complexes were in close proximity to exhibit excimeric emission. In 2009, Che and co-workers reported a series of alkynylplatinum(II) N∧C∧N complexes and studied the optoelectronic properties as well as the formation of nanosized structures.420 One of the complexes showed an infinite linear chain of Pt(II) centers with short intermolecular Pt(II)···Pt(II) separations of 3.383 Å in the X-ray crystal structure.420 Upon an increase in hexane or cyclohexane content in the CH2Cl2 or THF media, the complex solutions would turn to suspensions with intense colors, which were further found to exist as twodimensional nanosheets or nanobelts under TEM and SEM.420 They have also studied the charge-transporting properties of the nanosheets, which were shown to have enhanced forward and backward currents upon photoexcitation, suggestive of the increase in the carrier densities of both the holes and the electrons by the photoirradiation of the nanosheets.420 Recently, the same group has reported a series of luminescent platinum(II) N∧C∧N complexes with 2,6-dimethylphenyl isocyanide by simple ligand substitution reactions in a homogeneous solution (Figure 91).421 One of the complexes was demonstrated to exhibit vapoluminescence response upon exposure to acetonitrile vapor. The blue shift in emission energy with enhanced intensity was rationalized by structural rearrangements exerted by the sorption or desorption of guest molecules, which has been further supported by powder X-ray diffraction. Apart from the tridentate 1,3-di(2-pyridyl)benzene-derived N∧C∧N ligand, Yam and co-workers isolated a new class of cyclometalated chloroplatinum(II) complexes with the tridentate 2,6-bis(N-alkylbenzimidazol-2′-yl)benzene N∧C∧N ligand. The complex was found to exhibit green emission with AP


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SO2 sorption, the coordination geometry was changed from square-planar to pyramidal, leading to the change in the packing index and the density of the materials (Figure 93).427 The utilization of the complexes as a functional SO2 sensor device has been further demonstrated in their study.427

Figure 91. Molecular structures of platinum(II) N∧C∧N complexes with 2,6-dimethylphenyl isocyanide. Figure 93. Crystal structure of the complexes (a) before and (b) after exposure of SO2. Reprinted with permission from ref 427. Copyright 2000 Macmillan Publishers Ltd.

photoluminescence quantum yield (PLQY) of 19% in dichloromethane.422 The vibronic-structured emission bands of the complexes were assigned to the metal-perturbed 3IL [π → π*] excited state.422 The PLQY in thin film was found to be 45%, much higher than that in solution. Further fabrication of multilayer OLEDs by using this class of complexes has led to high current and external quantum efficiencies of 38.9 cd A−1 and 11.5%, respectively.422 Later, the research groups of Yam423,424 and Zhao425 independently reported the incorporation of alkynyl ligands to synthesize a new class of alkynylplatinum(II) complexes bearing 1,3-bis(N-alkylbenzimidazol-2′-yl)benzene as the tridentate ligand and showed that they exhibited strong luminescence properties. Very recently, Yam and co-workers further reported the synthesis and luminescence property for a related series of cyclometalated platinum(II) complexes with the tridentate N∧C∧N ligands of 2,6-bis(benzoxazol-2′-yl)benzene (bzoxb), 2,6-bis(benzothiazol-2′-yl)benzene (bzthb), and 2,6-bis(N-alkylnaphthoimidazol-2′-yl)benzene (naphimb).426 Their vibronic-structured emission bands mainly originated from the 3IL [π → π*(N∧C∧N)] state with mixing of a 3MLCT [dπ(Pt) → π*(N∧C∧N)] excited state.426 It was interesting to note that the emission energies of the complexes could be fine-tuned by the electronic properties of the N∧C∧N ligands, as supported by computational studies.426 In addition, some of these platinum(II) complexes have been utilized as phosphorescent dopants in multilayer organic light-emitting devices, achieving a saturated yellow emission with the Commission International de l’Eclairage (CIE) coordinates of (0.50, 0.49).426 van Koten and co-workers reported a series of platinum(II) complexes with a N∧C∧N pincer ligand, which was demonstrated to show high selectivity toward sulfur dioxide gas.427 This class of complexes would self-assemble in the solid state through intermolecular hydrogen bonding between the chloro ligand and hydroxy group on the pincer ligand (Figure 92). Upon exposure to sulfur dioxide gas for ca. 1 min, the complexes would form the complex-adduct, in which the sulfur dioxide molecule would ligate to the Pt(II) metal center.427 Although the crystallinity of the complexes was retained during

The study of another cyclometalated platinum(II) C∧N∧N complex has also aroused tremendous interest.428−449 In 1990, Constable and co-workers reported the first synthesis of a cyclometalated platinum(II) complex with tridentate 6-phenyl2,2′-bipyridine C∧N∧N ligand, [Pt(C∧N∧N)(MeCN)](PF6).428 X-ray crystal structure showed alternating short and long Pt(II)···Pt(II) distances of 3.28 and 4.59 Å, respectively, indicative of the presence of Pt(II)···Pt(II) interactions with the neighboring complex in the solid crystal lattice.428 Later, the same group reported a related series of cyclometalated platinum(II) complexes with 6-(2-thienyl)-2,2′-bipyridine as the tridentate C∧N∧N ligand, [Pt(thbpy)X], where thbpy = 6(2-thienyl)-2,2′-bipyridine and X = Cl, P(O)(OMe)2, or acetylacetone.429 In 1993, Che and co-workers reported a series of cyclometalated platinum(II) C∧N∧N complexes with the tridentate 2,9-diphenyl-1,10-phenanthroline (dpp) ligand.430 The dimeric structure of [Pt(dpp)(MeCN)](ClO4) showed weak Pt(II)···Pt(II) and π−π interactions in the crystal lattice. However, the UV−vis absorption spectra of [Pt(dpp)(MeCN)](ClO4) at various concentrations showed that there was no ground-state aggregation. Therefore, the low-energy emission band (ca. 700 nm) in acetone solution was assigned as an excimeric emission. The same group has also carried out an extensive study on the synthesis and structural, spectroscopic, and electrochemical properties of a number of mononuclear, dinuclear, and trinuclear cyclometalated platinum(II) C∧N∧N complexes with various kinds of auxiliary ligands such as carbonyl (Figure 94),431 isocyanide (Figure 94),431 phosphine (Figure 95),432−434 alkynyl (Figure 96),435 and others.436,437 Their intriguing photophysical properties were correlated with the propensity of the Pt(II)···Pt(II) and π−π stacking interactions. The cyclometalated isocyanoplatinum(II) C∧N∧N complex, [(C∧N∧N)Pt(CN−C6H3-Me2-2,6)]+, was found to adopt a dimeric structure with alternating short and long intermolecular Pt(II)···Pt(II) separations of 3.383 and 4.603 Å.431 On the other hand, the dinuclear and trinuclear phosphine complexes tethered with bridging phosphine ligands would show intramolecular Pt(II)···Pt(II) distances of 3.194− 3.399 Å, suggestive of the formation of Pt(II)···Pt(II) interactions.432,434 They have also revealed that the triynyl complex, [(C∧N∧N)Pt(CC−CC−CC−TMS)]+, would show an intermolecular Pt(II)···Pt(II) separation of 3.466 Å in the crystal lattice.435 Subsequent to those fundamental structural and photophysical studies, Che and co-workers have also extensively studied the design and synthesis of platinum(II) C∧N∧N

Figure 92. Molecular structures of the platinum(II) N∧C∧N pincer complexes and the complex-adduct with sulfur dioxide. AQ


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Figure 94. Molecular structures of cyclometalated platinum(II) carbonyl and isocyano complexes.

Figure 95. Molecular structures of mononuclear, dinuclear, and trinuclear cyclometalated platinum(II) phosphine complexes.

Figure 96. Molecular structures of cyclometalated platinum(II) alkynyl complexes of C∧N∧N ligands.

Figure 97. Synthesis of adduct of cyclometalated platinum(II) carbonyl and chloro complexes.

Figure 98. Molecular structures of cyclometalated platinum(II) isocyano complexes.

complexes for biological applications,438−442 fabrication of OLEDs,443−446 and self-assembled materials.447−449 They demonstrated the construction of well-defined wire-to-wheel

metamorphism in nanosize from a solution mixture containing the neutral cyclometalated chloroplatinum(II) 6-tolyl-2,2′bipyridine C∧N∧N complex and the cationic cyclometalated AR


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Figure 99. Molecular structures of a series of organoplatinum(II) chloride or sulfate salts with tridentate C∧N∧N, C∧N∧C, and N∧N∧N ligands.

carbonylplatinum(II) 6-tolyl-2,2′-bipyridine complex (Figure 97).447 The two complexes would aggregate into dimeric species with the formation of Pt(II)···Pt(II) interactions, as revealed from the low-energy MMLCT absorption band at 604 nm. Notably, a variety of diverse morphologies for wires, springs, and wheels were observed in field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) images.447−449 However, the luminescent property of the nanostructures was not reported in the study. Later, the same group prepared nanowires with average lengths of 3.8 mm from a series of cyclometalated platinum(II) isocyanide complexes (Figure 98) by precipitation.449 It was found that the platinum(II) isocyanide complexes would selfassemble into nanowires through the directional intermolecular Pt(II)···Pt(II) and π−π stacking interactions. Furthermore, such nanowires were found to possess a variety of interesting properties, such as liquid crystalline, waveguiding, semiconducting, and electroluminescence properties as well as the capability to be fabricated into ambipolar organic light-emitting field-effect transistor (OLEFET) devices with the demonstration of red or NIR luminescence behaviors.449 Che and workers reported a series of organoplatinum(II) chloride or sulfate salts with tridentate C∧N∧N, C∧N∧C, and N∧N∧N ligands (Figure 99).450 The spectroscopic study revealed that upon increasing the water content in methanol, the complexes would show distinct low-energy absorption tails and red phosphorescence with band maxima at 677−655 nm, typical of metal−metal-to-ligand charge-transfer (3MMLCT) excited states. As revealed from POM, these three kinds of complexes with different tridentate ligands would exhibit different liquid crystal phases in the aqueous solution. In addition, the organoplatinum(II) salts with tridentate C∧N∧N and C∧N∧C ligands were found to form supramolecular polyelectrolytes of high viscosity, determined by the Ubbelohde viscometer.450 The liquid crystallinity and viscosity were found to depend on the substituent groups on the cyclometalating ligands. The aqueous solutions of the complexes with methyl substituents were found to preferentially exist as fluids, whereas those of complexes with −CF3 substituents would steadily form gel-like semisolids.450 Further analysis by rheological measurement on the −CF3 analogues indicated that the gel-like semisolid would turn fluidic at 75 °C with high reversibility. Recently, Che and co-workers described Pt(II) C∧N∧N allenylidene complexes that would undergo self-assembly into nanostructures with biological activities (Figure 100).451 The combination of a cationic planar structure and the phosphorescent properties has made these metal allenylidene complexes possible switching probes for DNA molecules in aqueous solutions. They demonstrated that there were 17- and 5-fold luminescence enhancements in the intensity upon the addition of DNA. They have further investigated cell imaging applications for this class of Pt(II) complexes incubated with

Figure 100. Molecular structures of neutral dinuclear cyclometalated platinum(II) complexes containing oligo(oxyethylene) chains as bridges.

HeLa cells. Under the fluorescence microscope, predominant localization in the nuclei was observed, while others would mainly accumulate in the cytosolic region. The same group has also utilized a neutral dinuclear cyclometalated platinum(II) complex containing oligo(oxyethylene) chains as bridges to link up the mononuclear cyclometalated platinum(II) complexes and showed that the complex has a strong tendency to form gel with red photoluminescence. The gel formation was attributed to the coassembly of the dinuclear Pt(II) complexes with the mononuclear complexes, in which the discrete 1D chains were interconnected by the linker to give a giant 3D network of supramolecular structure that resulted in gelation.451 Recently, Hirao and co-workers reported a dinuclear pyridinedicarboxamide−platinum(II) C∧N∧N complex (Figure 101) with tunable luminescence properties based on metal−

Figure 101. Proposed conformational change of dinuclear platinum(II) C∧N∧N complex by methanol studied using 1H NMR spectroscopy. Reproduced with permission from ref 452. Copyright 2013 Royal Society of Chemistry.

metal interactions.452 Conformational changes with the exhibition of 3MLCT and 3MMLCT emission properties were demonstrated. In CH2Cl2, the dinuclear platinum(II) C∧N∧N complex showed a low-energy 3MMLCT emission band. The addition of the first 10% methanol into CH2Cl2 would give rise to an emergence of 3MLCT emission even though the 3 MMLCT emission band was still present. Further addition of methanol led to a drastic increase in the intensity of the higher-energy 3MLCT emission. Structural changes of the AS


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fabricated an efficient white phosphorescent OLED device achieving a maximum EQE of 18.2%. 2.4.5. Other Platinum(II) Complexes. In 1977, the first report of a soluble platinum(II) polyynyl polymer, poly[transbis(tri-n-butylphosphine)platinum 1,4-butadiynediyl], by Hagihara and co-workers464,465 aroused tremendous interest in the development of the organometallic polymeric materials based on the platinum(II) phosphine system. In the early 1990s, the research groups of Lewis466 and Diederich467 reported a versatile synthetic route to prepare linear and cyclic alkynylplatinum(II) complexes by copper(I)-mediated coupling reactions. Particularly, Diederich and co-workers reported the synthesis of linear and cyclic platinum(II) phosphine complexes with tetraethynylethene-derived motifs.468 Sequential deprotections and Hay coupling reactions of the σ-bis(alkynyl) moieties have led to the formation of diplatinated metallacycles, linked by the carbon network derived from tetraethynylethenes.468 Further study on the synthesis and characterization of this class of monodispersed oligomeric tetraethynylethenes bridged by the platinum(II) phosphine moieties was reported by the same group.469 They have utilized oxidative oligomerization to prepare the alkynylplatinum(II) phosphine oligomers under end-capping conditions with the phenylacetylides (Figure 103).469 This class of platinum-containing phosphine

dinuclear platinum(II) complex linked by pyridinedicarboxamide induced by the addition of methanol were also confirmed by 1H NMR experiments. The chemical shift of the amide protons was upfield shifted, suggestive of the weakening of the hydrogen bonding in the interior of the cavity. Because the folded scaffold of pyridinedicarboxamide was stabilized by noncovalent intramolecular hydrogen bonding, the addition of methanol was thought to compete with the existing hydrogen bonding, thus resulting in hydrogen-bonding distortion inside the cavity. In addition, the weakening of the π−π stacking of the platinum(II) C∧N∧N moieties has resulted in the elongation of the Pt(II)···Pt(II) distance and thus the exhibition of the 3MLCT phosphorescence. Sun and co-workers reported a series of luminescent dinuclear cyclometalated platinum(II) 4,6-diphenyl-2,2′-bipyridines with different phosphine ligands.453 In addition, the same group further extended their works to the nonlinear optical study of this class of complexes.454−459 They also introduced fluorenyl-derived moieties on the tridentate C∧N∧N ligand of the cyclometalated platinum(II) complexes.457 It was suggested that the incorporation of the fluorene unit into the platinum complexes would enhance the two-photon absorption (2PA) cross sections of the platinum complexes in the NIR region. The research group of Hollis460,461 and Li462,463 independently reported the blue light-emitting phenyl-based C∧C∧C− NHC chloroplatinum(II) complexes with different synthetic routes in 2012. The emission origin was assigned as originating from a mixture of metal-to-ligand charge-transfer and ligandcentered character.460−463 This class of complexes was found to be thermally stable and was suggested to be suitable for thermodeposition processing.460−463 Hollis and co-workers further demonstrated the synthesis of a Pt2Ag2 cluster by the reaction of this kind of complexes with an excess amount of silver(I) trifluoroacetate.461 An unsymmetric Pt2Ag2 core was found to contain a scalene Pt−Ag−Ag triangle (Ag(1)−Ag(2) = 2.8430(4) Å, Pt(1)−Ag(1) = 3.1413(4) Å, and Pt(1)−Ag(2) = 2.9335(3) Å) and a second Pt−Ag interaction (Pt(2)−Ag(2) = 2.9104(3) Å) linked by trifluoroacetate ligands (Figure 102).461 On the other hand, Li and co-workers focused on the electroluminescence property for this class of complexes and

Figure 103. Molecular structures of alkynylplatinum(II) phosphine oligomers under end-capping conditions with the phenylacetylides.

oligomers was shown to display good solubility in common organic solvents, making the isolation and characterization of these complexes feasible.469 In general, various independent research groups of Stang,470−476Takahashi,477−480 Lewis,481−485 Raithby,486,487 Yam,488−497 Gladysz,498−500 Schanze,501−504 Cooper,505−507 and Wong508−511 and many others512−516 have reported different kinds of alkynylplatinum(II) phosphine complexes, oligomers and metallopolymers featuring unique spacers and chain lengths for the development of supramolecular architectures. They have demonstrated that the alkynylplatinum(II) phosphine moiety could serve as promising and versatile building blocks for the construction of multinuclear complexes with intriguing luminescence properties. Recent works have been extended to the field of nonlinear optical (NLO) properties,492,495−497,501,512−517 photochromic materials,518,519organic light-emitting diodes (OLEDs),508−511 and dye-sensitized solar cells (DSSCs).520−522 The research groups of Stang 470 and Takahashi 477 independently reported the isolation and characterization of metallodendrimers derived from alkynylplatinum(II) phosphine moieties. Both of them utilized the copper-catalyzed coupling reaction of the trinuclear platinum complexes with the triyne moiety for the growth of the dendrimers. However, this divergent synthetic route in the stepwise approach would often lead to the undesirable formation of cross-linked oligomers or

Figure 102. X-ray crystal structure of the Pt2Ag2 cluster (hydrogen atoms, trifuoromethyl groups, and trimethylsilyl groups omitted for clarity). Reproduced with permission from ref 461. Copyright 2012 American Chemical Society. AT


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Figure 104. Synthetic route of platinum(II)-containing molecular squares.

could be regarded as a metal analogue of calix[4]arene (Figure 105).523 Upon the addition of silver(I) salts, the complex would

polymers. To tackle this problem, Takahashi and co-workers utilized the convergent approach to synthesize the giant alkynylplatinum(II) metallodendrimers.478 The third-generation metallodendrimers containing 45 platinum atoms were prepared successfully by employing this strategy.478 Another strategy is regarded as the controllable divergent approach, in which the triethynylbenzene derivatives of different protecting groups would couple with the mononuclear alkynylplatinum(II) complexes.479 This sequential coupling−deprotection− coupling divergent approach would afford alkynylplatinum(II) phosphine metallodendrimers of up to the sixth generation containing 189 Pt atoms with molecular weights as high as 139 750 Da.479 However, a recent study by the same group showed that the higher generations of the metallodendrimers do not have influences on the electronic and luminescence properties.480 Parallel to the development of the dendrimeric platinum(II) phosphine complexes, the research groups of Stang471,472 and Lippert523−525 have also reported the synthesis and characterization of a number of platinum(II)-containing polygons in the early 1990s. Although the luminescence property for the platinum(II)-containing polygons was not determined,471,472,523−525 their pioneering works also inspired a number of subsequent works for the development of platinum(II) phosphine systems.488−516 The classical example of supramolecular coordination platinum(II) complexes was the self-assembly of cis-[Pt(dppp)X 2 ] (dppp = 1,3-bis(diphenylphosphino)propane; X = OTf) with equimolar amounts of 4,4′-bipyridine in dichloromethane to form platinum(II)-containing molecular squares (Figure 104).471 The X-ray structure analysis showed that the complex would adopt a square-planar geometry with an edge-to-edge distance of 11.2 Å and a diagonal distance of 14.3 Å.471 Because of the stacking of the phenyl rings on the phosphine ligands with 4,4′bipyridine, the N−Pt(II)−N angle was found to decrease to 84°, making the entire complex slightly puckered.471 Later, they replaced the 4,4′-bipyridine with other planar π-aromatic linkers, such as 2,7-diazapyrene or 2,9-diazadibenzo[cd,lm]perylene, to obtain the desired tetranuclear platinum(II) squares with deeper cavities.472 Subsequent to these works, the same group has also utilized ferrocene-,526 crown ether-,527 and calixarene-substituted phosphine bridging ligands527 to synthesize a unique class of macrocyclic tetranuclear platinum(II) squares.526,527 Lippert and co-workers also reported the reaction of a uracil-containing platinum(II) complex with equimolar amounts of silver nitrate to yield a cyclic tetranuclear platinum(II) complex, [(en)Pt(uracil)]4(NO3)4 (en = ethylenediamine).523 The X-ray structure showed that the complex

Figure 105. Crystal structure of the tetranuclear complex, [(en)Pt(uracil)]4(NO3)4. Reproduced with permission from ref 523. Copyright 1992 Royal Society of Chemistry.

spontaneously undergo 1,3-alternate/cone conversion to form an octanuclear cone-shaped species, [(en)Pt(uracil)Ag]48+.524 This class of metal analogue of calix[4]arene was demonstrated for the host−guest interaction with Zn2+, Be2+, and La3+ in aqueous solutions arising from the hydrophobic nature of the cone cavity.525 In the middle of the 1990s, Stang and co-workers prepared a series of heterobimetallic molecular squares by utilizing the selfassembly of cis-[Pt(dppp)(p-C6H4CN)2] or cis-[Pt(dppp)(pCCC6H4N)2] with cis-[M(dppp)(OTf)2] (M = Pt, Pd; dppp = 1,3-bis(diphenylphosphino)propane).473,474 These macrocyclic tetranuclear complexes were successfully characterized by fast atom bombardment mass spectrometry and X-ray structural analysis.473,474 Further binding with silver(I) ions through π−π interactions with the acetylene units has also been demonstrated by the same group to prepare a new class of multinuclear macrocyclic complexes.473 These complexes were successfully characterized by multinuclear NMR, IR, and UV spectroscopy as well as FAB mass spectrometry.473 The same group further attempted to bind different organic guest molecules into this coordination cavity.475 The reaction of the multinuclear macrocyclic complexes with an equimolar amount of pyrazine, phenazine, or dipyridyl ketone in CH2Cl2 AU


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at room temperature would yield interesting host−guest complex adducts, which were fully characterized by various physical and spectroscopic techniques.475 Particularly, the multinuclear macrocyclic complexes with phenazine have been structurally characterized by X-ray crystallography (Figure 106).475 The geometry of the macrocyclic complex was nearly

Figure 108. Molecular structures of neutral (left) tetranuclear and (right) octanuclear macrocyclic platinum(II) butadiyne heterocyclynes.

Figure 106. (Left) Molecular and (right) X-ray crystal structure of the host−guest complex with the Ag(I) ions and phenazine. Reproduced with permission from ref 475. Copyright 1998 American Chemical Society.

planar, with the guest phenazine orthogonal to the alkynylplatinum(II) phosphine moieties.475 The silver(I) atoms were located in a pseudo-trans arrangement through πinteractions on the acetylene units.475 Later, the same group incorporated the chiral phosphine ligands, BINAP, at the corners of the macrocyclic complexes to monitor the host− guest interactions by circular dichroism spectroscopy.476 Upon coordination of the silver(I) ions, a significant change in the intensity and the shape of the CD bands was observed. Upon further stoichiometric addition of organic guests, the CD bands were found to decrease. Youngs and co-workers reported the isolation and characterization of a dinuclear platinum(II) phosphine metallacyclyne with double-pocket cyclynes (Figure 107)528 and neutral

Figure 109. X-ray crystal structure of the mixed-metal trigonal bipyramidal cage. Reproduced with permission from ref 530. Copyright 2006 The Royal Society of Chemistry.

complexes was the study of the dinuclear platinum(II) phenylethenylidene complex, [Pt2(μ-CCHPh)(CCPh)(PEt3)3Cl], in 1989.531 It was reported that this complex would exhibit red−orange luminescence in the glass state.531 Demas and co-workers initially assigned the luminescence as originated from the spin-forbidden metal-centered d−p phosphorescence.531 Later, the emissive origin was reassigned to be derived from excited states of spin-forbidden Pt2-toalkenylidene charge transfer character.532 It was suggested that there was no involvement of the alkynyl ligand in the lowestlying excited state, but the electronic effects of the alkynyls might have a subtle influence on the energy of the platinum atoms, thus perturbing the excited-state properties.532 In 1991, DeGraff, Lukehart, Demas, and co-workers reported the siteselective luminescence property of the mononuclear Pt(II) alkynyls.533 The complexes of trans-[Pt(CCR)2(PEt3)2] (R = H, Ph) were shown to exhibit intense structured emissions with a ν(CC) vibrational progression in the glass state. The authors assigned the emissive state to the Pt → π*(CC) MLCT origin.533 Later, Che and co-workers also reported similar vibronically structured emission bands in the glass state for the system of trans-[Pt(CCPh)2(dppm-P)2].534 Further supported by Hückel molecular orbital calculations, the emissive origin was assigned as originating from the platinum-to-alkynyl MLCT character with substantial mixing of the π* orbital of phenylacetylide with the 6pz orbital of Pt

Figure 107. Molecular structure of dinuclear platinum(II) phosphine metallacyclyne with double-pocket cyclynes.

tetranuclear and octanuclear macrocyclic platinum(II) butadiyne heterocyclynes (Figure 108).529 Recently, the same group reported a trigonal bipyramidal cage self-assembled by the condensation reaction between [(Me 3 tacn)Rh(p-C CC6H4N)3] (Me3tacn = N,N′,N″-trimethyl-1,4,7-triazacyclononane) and [(P∧P)Pt(NO3)2] (P∧P = 1,2-bis(dicyclohexylphosphino)ethane).530 X-ray crystal structure analysis revealed that both the N−Pt−N and the C−Rh−C bond angles of the cage were deviated from 90°, showing 84.9° and 87.9°, respectively (Figure 109). The strain of the bonds was thought to arise from the sterically bulky cyclohexyl substituents on the chelating phosphine. Apart from synthetic studies, the first report on the luminescence properties of alkynylplatinum(II) phosphine AV


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Figure 110. Synthetic routes of [Pt2(dppm)2(CCR)4] (R = Ph, Ph−Cl-4, Ph−NO3-4, and SiMe3), [Pt2(dppm)2(CCPh)4{Cu− (MeCN)}2](PF6)2, and [Pt2(dppm)2(CCPh)4{Ag−(MeCN)}2](BF4)2.

and of the 5dyz(Pt) orbital with the π(CCPh) orbitals.534 Analogous to the trans-alkynylplatinum(II) phosphine complexes, cis-[Pt(phen)(CCPh)2] was also reported to have luminescence property in fluid solution at room temperature by the same group.241 The emissive origin was assigned to the 3 MLCT [dπ(Pt) → π*(phen)] excited state.241 Later, Lewis and co-workers further reported the synthesis of a kind of polymeric species, [Pt(PBu3)2CC−R−CC]n (R = pC6H4),481 together with its spectroscopic and luminescence properties.482 Lewis, Marder, Friend, and co-workers then studied the luminescence property for the polymers, [Pt{(C C)m}2(PR3)2]n, which also showed the presence of a vibronically structured emission band, originated from an alkynyllocalized π−π* excited state.483 They have also reported the optical absorption, photoluminescence, and photoinduced absorption of the related polymers, [Pt(PR3)2CC−L−C C]n (R = Et, nBu; L = pyridine, phenylene, or thiophene), together with the comparison of the photophysical property with the corresponding monomers.484 They also synthesized a variety of the polymeric alkynylplatinum(II) phosphine complexes with extended π-conjugation through benzene, anthracene, and thiophene in the backbone.485 Particularly, the polymeric platinum σ-alkynyl complexes with the anthracene spacer units have been shown to exhibit high luminescence quantum yields of up to 0.5.485 In 1978, Stone and co-workers synthesized a diplatinum complex, [(PCy 3 )(CCPh)Pt(μ-CCPh)(μ-SiMe 2 )Pt(PCy3)], containing a μ-phenylethynyl bridging ligand as the first example of this type of complexes.535 Subsequently, Puddephatt and co-workers reported a series of homobimetallic A-frame Pt(II) complexes with the μ-H bridgehead.536−538 Later, Shaw and co-workers reported a class of A-frame dimethyl diplatinum complexes containing bis(diphenylphosphino)methane (dppm) ligands, [MePt(μ-C CR)(μ-dppm)2PtMe]BF4.539 The μ-CCR bridging ligand was found to be symmetrically bonded with the diplatinum core, as revealed from the X-ray crystal analysis.539 Later, Shaw, Pringle, and co-workers also reported other examples of heterobimetallic A-frame Rh(I)−Pt(II) complexes containing μ-CCR bridgeheads.540 In 1987, Pringle and co-workers

reported another homobimetallic Pt(II) A-frame alkynyl complex, [Pt2(μ-dppm)2(μ-CCtBu)(CCtBu)2]+ with the tetraphenylborate salt,541 by the reaction of [PtCl2(dppm)] with LiCCtBu in tetrahydrofuran. The X-ray structure showed that one of the alkynyl ligands would unsymmetrically bridge the diplatinum core. In addition, 31P and 1H NMR experiments revealed the fluxional behavior of this class of complexes in the solution. In 1993, Yam and co-workers utilized a new reaction methodology to prepare this class of complexes.179 The reaction of [Pt(dppm)2]Cl2 with HCCtBu in the presence of Hg(OAc)2 in refluxing ethanol would produce [Pt2(μ-dppm)2(μ-CCtBu)(CCtBu)2](X) (X = Cl− and ClO4−) and [Pt2(μ-dppm)2(μ-CCtBu)(CCtBu)Cl](ClO4).179 Both complexes were found to exhibit solid- and fluid-state photoluminescence with long-lived excited-state lifetimes.179 They further synthesized a series of homobimetallic Pt(II) A-frame alkynyl complexes, [Pt2(μ-dppm)2(μ-C CR)(CCR)2]+ (R = C6H4−OCH3-4, C6H4−CH2CH3-4, C6H4−OCH2CH3-4, C6H4−Ph-4, Ph, tBu), in 1997 and showed that these complexes displayed fluxional behavior.180 The complexes would exhibit vibronic emission bands in the solid state at 77 K with progressional spacings of ca. 2000 cm−1, characteristic of the ν(CC) stretching mode in the ground state.180 In addition, the solid-state emission energies of the complexes at 77 K would follow the order: R = C6H4−Ph-4 < Ph < C 6 H 4 −OCH 3 -4 ≤ C 6 H 4 −CH 2 CH 3 -4 ≤ C 6 H 4 − OCH2CH3-4 < tBu, in line with the increasing π−π* transition energy of the alkynyl ligand.180 All of these suggested the involvement of the π*(CCR) orbital in the transition.180 The emissive origin of this class of complexes was assigned as a pure metal-centered 3[(dσ*)(pσ)] state arising from the Pt(II)··· Pt(II) interactions.180 On the other hand, the long-lived and strong luminescence in the fluid state at room temperature was assigned as derived from an excited state of 3MMLCT [dσ* → pσ/π*] character.180 Yam, Phillips, and co-workers also utilized the resonance Raman spectroscopy to investigate the dinuclear platinum(II) A-frame alkynyl complex, [Pt2(μ-dppm)2(μκCα:η2-CCPh)(CCPh)2]+, together with the mononuclear alkynylplatinum(II) phosphine complex, [Pt(dppm)2(C CPh)2].542 It was found that the dinuclear platinum(II) AAW


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Figure 111. Crystal structure of the complex cation of [Pt2(μ-dppm)2(CCC5H4N)4{Pt(terpy)}4](CF3SO3)8. Reproduced with permission from ref 546. Copyright 2002 American Chemical Society.

Figure 112. Synthesis of [Pt2(μ-dppm)2(CCR)4] by phosphine exchange reaction.

actions.544,545 The same group has further encapsulated copper(I) and silver(I) ions to synthesize tetranuclear mixedmetal platinum(II)−copper(I) and −silver(I) complexes, [Pt2(dppm)2(CCPh)4·{M(MeCN)}2](X)2, where M = Cu+, X = PF6− and M = Ag+, X = BF4− (Figure 110).544,545 The low-energy electronic absorption and emission bands were found to be red-shifted as a result of the higher-lying dσ*(Pt2) orbital (HOMO) arising from the shortening of the Pt(II)··· Pt(II) separation. At the same time, the pσ(Pt2)/σ*(CCR) orbital energy (LUMO) was shown to be lower-lying due to the more electron-deficient alkynyl group with better π-accepting ability upon the coordination of Cu(I) and Ag(I) ions. Subsequent to these works, the same group reported a hexanuclear Pt(II) complex, which was assembled from the face-to-face dinuclear platinum(II) alkynyl complex and 4 mole equiv of [Pt(terpy)(MeCN)]2+ (Figure 111).546 The X-ray structure of this hexanuclear Pt(II) complex showed that the two Pt(II) centers were connected by two dppm ligands to construct an eight-membered ring in a face-to-face arrangement with a Pt(II)···Pt(II) separation of 3.178(1) Å, suggestive of the presence of Pt(II)···Pt(II) interactions.546 It was found that the Pt(II)···Pt(II) separation was contracted by 0.1 Å upon the coordination with four peripheral Pt(II) terpyridine moieties.546 This was attributed to the decrease in the electron density of the dinuclear platinum centers, which favors the formation of metal−metal interactions to compensate for the loss of electron density.546 It was interesting to note that the two adjacent Pt(II) terpyridine moieties were parallel to each other with an interplanar distance of 3.67 Å, suggestive of possible weak π−π interactions.546 However, no Pt(II)···Pt(II) interaction was observed from the Pt(II) terpyridine moieties, as the Pt(II)···Pt(II) distance between these moieties was found to be 5.08 Å.546 In addition, the hexanuclear Pt(II) complex exhibited a low-energy absorption band at 416 nm, assigned as the MLCT transition. The solid state of this complex was found to exhibit strong luminescence in 77 K at ca. 620 nm, which was ascribed to the phosphorescence derived from the 3 MMLCT state. It was found that the emission band was red-shifted relative to that of the complex precursor, which

frame complex would exhibit three strong CC stretching peaks at 2027, 2062, and 2125 cm−1,542 suggestive of three different environments for the alkynyl ligands, while the mononuclear complex would only show a strong CC stretching peak at 2114 cm−1.542 The peak at 2027 cm−1 for the Pt(II) A-frame dinuclear complex was found to be the smallest upon the excitation at the blue edge of the MMLCT band.542 However, when the excitation was shifted to the red edge of the MMLCT envelope, the intensity at 2027 cm−1 was found to increase, relative to the 2062 and 2125 cm−1 peaks.542 The excitation-wavelength dependence was associated with the presence of more than one configuration that contributes to this MMLCT absorption band, corresponding to the photoinduced charge transfer to three different but closely lying π*(CCPh) orbitals, in which the π*(μ-CCPh) was the lowest-lying in energy.542 Thus, the excitation at the red edge of the MMLCT band would excite the MMLCT transition, mainly localized on the π*(μ-CCPh), thus preferentially leading to the resonance enhancement at 2027 cm−1.542 In addition, the progressional spacing of 2000 cm−1 in the 77 K emission spectrum was consistent with the lowest-lying emissive state, which predominantly has MMLCT [dσ* → pσ/π*] character.542 Another related classical example is the face-to-face dinuclear platinum(II) alkynyl complex with bridging phosphine ligands, [Pt2(dppm)2(CCPh)4], which was first synthesized by Pringle and Shaw in 1987.543 Yam and coworkers extended the work and isolated a series of [Pt2(dppm)2(CCR)4] (R = Ph, Ph−Cl-4, Ph-NO3-4, and SiMe3) with various electronic effects of the alkynyl ligands (Figure 110).544,545 It was found that there was a correlation between the Pt(II)···Pt(II) separation and the substituent on the alkynyl ligands, which would alter the photophysical properties of this class of complexes.544,545 In general, the lower energy of the absorption and emission bands was attributed to the stronger π-accepting ability of the alkynyl group as well as the shorter intramolecular Pt(II)···Pt(II) distance.544,545 The luminescence originated from the triplet [dσ*(Pt2) → pσ(Pt2)/ σ*(CCR)] metal−metal-to-ligand charge transfer (MMLCT) excited state arising from Pt(II)···Pt(II) interAX


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emitted at ca. 605 nm.546 This further supported that there was a stronger Pt(II)···Pt(II) interaction in the hexanuclear Pt(II) complex due to the shortening of Pt(II)···Pt(II) separation in the Pt2(dppm)2 core as well as the better π-accepting ability of the ethynylpyridine ligands upon coordination of the Pt(terpy) moieties.546 Furthermore, the hexanuclear Pt(II) complex would exhibit strong luminescence at ca. 520 nm in the glass state, arising from metal-perturbed ligand-centered phosphorescence.546 The same group further modified the synthesis of this class of face-to-face dinuclear alkynylplatinum(II) system by using phosphine exchange reaction (Figure 112).547 Most of the complexes and the mixed-metal tetranuclear platinum(II)− silver(I) and −copper(I) complexes were structurally characterized by X-ray crystallography and were shown to display short Pt(II)···Pt(II) separations of ca. 3.0117−3.3686 Å.547 The nature of both the alkynyl and the bridging diphosphine ligands, the Pt(II)···Pt(II) distance, and the metal encapsulation would influence the lowest-energy absorption and emission bands.547 The lower absorption and emission energies were observed for the cases with stronger π-accepting abilities of the alkynyl ligands, more π-conjugated groups on the bridging diphosphine ligands, and shorter Pt(II)···Pt(II) separations.547 On the basis of the spectroscopic studies together with the TDDFT/CPCM calculations, the origin and the energy of their lowest-energy absorption and emission were assigned as derived from excited states of MLCT origin arising from the Pt d orbitals to the π* orbitals of the alkynyl ligands and the P− Cσ* orbitals of the bridging diphosphine ligands with mixing of intraligand π−π* character of the alkynyl ligands.547 Later, Chen548,549 and co-workers employed the face-to-face dinuclear alkynylplatinum(II) complexes as versatile building blocks for further coordinating with lanthanide(III) moieties to prepare the mixed-metal tetranuclear platinum(II)−lanthanide(III)548 and hexanuclear platinum(II)−lanthanide(III)548,549 (Ln = Eu, Nd, and Yb) supramolecular frameworks. The nature, steric bulkiness, and coordination modes of the additional metal− ligand chromophores would govern the Pt(II)···Pt(II) separations, which perturb the luminescence behavior548 and the energy transfer via sensitization548,549 in this kind of supramolecular self-assemblies. In 2001, the luminescence behavior of dendrimeric platinum(II) phosphine complexes was investigated by Yam and coworkers.489 They synthesized a new series of palladium(II) and platinum(II) phosphine complexes of branched rigid alkynyls.489 These complexes would exhibit high-energy absorption bands, assigned as the mixture of the intraligand [π → π*(C CR)] and MLCT [dπ(M) → π*(CCR)] transitions, with predominantly IL character or, alternatively, a metal-perturbed IL transition. The solid and glass states of these complexes showed intense green to yellow luminescence at 77 K, assigned as originating from the excited state of mixed 3IL 3[π → π*(CCR)]/3MLCT 3[dπ(M) → π*(CCR)] transitions with predominantly IL character.489 The same group further demonstrated the use of multinuclear platinum(II) phosphine complexes containing branched or dendritic phenylene ethynylene,491 truxene,495 or oxadiazole495 as the backbones in the field of NLO studies. Particularly, a dinuclear alkynylplatinum(II) phosphine complex of substituted carbazole with peripheral naphthalene-derived alkynyls was demonstrated to show two-photon absorption and two photoninduced luminescence, achieving a 2PA cross-section (σ2) higher than that of the mononuclear analogue (λex = 740 nm).492 The synthesis of metallodendrimers derived from the

alkynylplatinum(II) phosphine moieties has also been extended to heterobimetallic dendrimers.550 Rodrigues, Wenseleers, and co-workers utilized the 2,4,6-tris(4-ethynyl)phenyl-1,3,5-triazine core to link the alkynylplatinum(II) units with the peripheral ferrocene moieties and showed that they exhibited nonlinear optical property.550 Gladysz and co-workers reported the X-ray crystal structure and the DFT study of the electronic structure for a series of diplatinum(II) polyynediyl phosphine complexes.499,500 They also reported an interesting class of diplatinum(II) polyynediyl phosphine complexes surrounded by sp3 carbon double helices by the synthetic approach of phosphine exchange reaction.498 Rogers, Cooper, and co-workers reported a series of alkynylplatinum(II) phosphine complexes containing paraphenylethynyl oligomers.505 They studied the effects of πconjugation of the phenylethynyl ligands on the electronic structure of platinum(II) phosphine complexes.505 The increase in the π-conjugation would result in the red shift of S0−S1 and T1−Tn transitions, together with the increase in their molar extinctions.505 The spin−orbit coupling effect of the platinum(II) metal center became less important for the ground and excited-state properties, because the S0−S1 transition was more localized on the π-conjugated phenylethynyl oligomers.505 Subsequent to this work, they utilized femtosecond timeresolved fluorescence and transient absorption spectroscopies to study the dynamics of intersystem crossing and the formation of triplet states for the platinum(II) complexes with different extent of π-conjugation of the para-phenylethynyl ligands.507 It was found that intersystem crossing would occur in less than 100 fs for the platinum(II) complexes of the short phenylethynyl ligands.507 In addition, the triplet state relaxation was also determined to be within the picosecond time-scale.507 Schanze and co-workers have conducted a comprehensive study on the fundamental photophysical properties of alkynylplatinum(II) phosphine complexes.502,504,551−555 Particularly, they reported a series of monodispersed alkynylplatinum(II) phosphine oligomers end-capped with naphthalenediimide (NDI)-derived alkynyls.556,557 This kind of organometallic molecular wires was demonstrated to serve as conduits for the transport of electrons and triplet excitons in less than 200 ps on the 3 nm oligomeric chains.556,557 Subsequent to this work, they reported a series of conjugated donor−acceptor dyads bridged by the alkynylplatinum(II) phosphine oligomers.558 In this kind of conjugated donor− acceptor dyads, the (diphenylamino)-2,7-fluorenylene (DPAF) moiety was the electron donor, while the naphthalenediimide (NDI) unit served as the electron acceptor.558 They have performed a detailed photoinduced electron transfer study and found that the rates of charge separation and charge recombination were independent of the length of the spacer, suggesting that the platinum−alkynyl spacer was actively participating in the process of electron transport.558 In 2008, they reported a series of rod-like alkynylplatinum(II) phosphine complexes with hydrophobic alkyl chains at two terminals that would undergo gel formation via H-aggregation (Figure 113).559 The formation of gel in hydrocarbon solvents at very low concentration showed fibrous networks, as observed in TEM. The solution and the gel state were both phosphorescent at ambient temperature. Because two bulky PMe3 ligands were coordinated to the platinum(II) center, no Pt(II)···Pt(II) interactions were observed in the self-assembly process. Notably, the triplet exciton diffusion and trapping processes were first observed in the molecular aggregate system through AY


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Figure 115. Molecular structures of rod-like alkynylplatinum(II) phosphine complexes containing amide groups appended with hydrocarbon tails. Figure 113. Molecular structures of rod-like alkynylplatinum(II) phosphine complexes with hydrophobic alkyl chains at two terminals.

state would have an emission band at 485 nm due to the rigidochromism of the gel state significantly suppressing the nonradiative decay process, thus leading to an enhancement of luminescence property. It is worthwhile to note that the formation of noncovalent helical supramolecular polymers was driven by the cooperative growth mechanism. The very small value of the equilibrium constant of the nucleation step, Ka, was determined to be 3.79 × 10−4, suggestive of a moderate degree of cooperativity in the self-assembly process.560 Yang and co-workers reported the synthesis of alkynylplatinum(II) phosphine derivatives, featuring various geometries, such as linear, triangular, and rectangular shapes. It was revealed that the gelation property was related to the geometry of the complexes, in which the rectangular complexes would exhibit the most efficient gel formation, even at very low concentrations, while the linear and triangular complexes had a limited ability to form metallogels. Their gelation property was mainly attributed to the hydrophobic−hydrophobic interactions and π−π stacking interactions with the luminescence behaviors, originating from an admixture of triplet intraligand π → π* excited states of the alkynyl ligands and dπ(Pt) → π*(CCR) 3MLCT states.561 Using this similar strategy of hydrophobic−hydrophobic and π−π stacking interactions for the gelators, the same group has also prepared alkynylplatinum(II) phosphine complexes with iptycene562 and pyrene moieties563 in the cores and end-capped with 3,4,5-tris(alkoxy)phenyl substituents. It was found that this class of complexes would give rise to the formation of metallogels with luminescence properties. Subsequent to this work, a series of alkynylplatinum(II) phosphine complexes, possessing a por-

the study of phosphorescence spectroscopy. In addition, the mixing of the gel aggregates consisting of 1,4-phenylene as triplet donor (host) with 2,5-thienylene as acceptor (trap) was found to show an efficient triplet energy transfer.559 Very recently, the same group has reported a series of linear and cross-conjugated alkynylplatinum(II) phosphine complexes that contain oligomeric p-phenylene (OPV) π-chromophores with linear and nonlinear optical properties (Figure 114).517 The resultant complexes were found to exhibit very high femtosecond two-photon absorption (2PA) with the crosssection values (σ2) up to 103−104 GM, measured by the nonlinear transmission (NLT) and two-photon excited fluorescence (2PEF) spectroscopies.517 The nanosecond transient absorption study revealed that the chromophores showed strong and broad T−T absorption in both the visible and the NIR region.517 The magnitude enhancement of peak 2PA was attributed to the high heavy atom-assisted, intersystem crossing efficiency along with the good spectral overlap between the broad 2PA spectrum and the T−T absorption wavelength.517 Wang and co-workers reported a series of rod-like alkynylplatinum(II) phosphine complexes containing amide groups appended with hydrocarbon tails (Figure 115).560 The complex could undergo gelation in heptane and hexane at room temperature, attributed to the overlapping of the alkynylplatinum(II) phosphine chromophores to form edgeto-edge J-type aggregation with the formation of π−π stacking. The solution of the complexes was not emissive, while the gel

Figure 114. Molecular structures of alkynylplatinum(II) phosphine complexes that contain oligomeric p-phenylenevinylene (OPV) πchromophores. AZ


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Figure 116. A schematic representation of the enantioselective synthesis of complementary double-helical molecules. Reprinted with permission from ref 566. Copyright 2007 Wiley-VCH.

naphthyl (step A).566 The sequential phosphine exchange reaction with achiral diphosphine ligands (dppm) would generate enantiomerically pure double helices with a desirable helicity (step B) as shown in Figure 116.566 The conformation and helicity of the bridged double helix were further confirmed by X-ray crystal analysis.566 Another example of the utilization of alkynylplatinum(II) phosphine moiety in supramolecular structures was reported by Graf, Hosseini, and co-workers.567 They have coordinated two pyridine-derived alkynyls into the platinum(II) center with the two terminal diphenylphosphine moieties equipped with a pyridine unit (Figure 117). Upon the addition of Ag(I) ions, the open and closed forms of the organometallic turnstile could be achieved. This controllable intramolecular motion in solution was well-characterized by using 1D and 2D NMR experiments. Apart from the extensive studies on the alkynylplatinum(II) phosphine complexes, the research groups of Scandola and Yersin first characterized the electronic states of [Pt(qol)]2 (qol = 8-quinolinolato-O,N) that exhibit very intense red phosphorescence, even in solution at room temperature, originated from

phyrin moiety and hydrocarbon tails in the peripheral positions, have also been prepared, demonstrating J-aggregation in the self-assembly process.564 The emission properties were found to be dependent on the concentration and temperature. Although the complexes could not form stable gels, the polarity of the solvents could induce different kinds of morphological patterns, as observed in the electronic microscopy. The evaporation of hexane solutions would yield microporous films with a honeycomb-pattern, while nanospheres would be formed for the evaporation of methanol solutions. Yashima and co-workers have successfully constructed artificial double helices utilizing the alkynylplatinum(II) phosphine moiety.565 This kind of double helices was stabilized by hydrogen bonding from the amidinium−carboxylate salt bridges, which intertwine the linear alkynylplatinum(II) phosphine moieties to adopt two crescent-shaped strands.565 Recently, they demonstrated the enantioselective synthesis of complementary double-helical molecules by the phosphine exchange reaction.566 The formation of diastereomeric double helices was governed by the employment of the chiral phosphine ligand, 2-(diphenylphosphino)-2′-methoxy-1,1′-biBA


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Chan and co-workers reported a series of conjugated polymers consisting of alternating p-phenylene-ethynylene and [Pt(salphen)] (H2-salphen = N,N′-bis(salicylidene)-1,2phenylenediamine) luminophore units at the para- or metaposition with phosphorescence properties (Figure 118).586 The

Figure 117. Molecular structure of alkynylplatinum(II) complexes with the two terminal diphenylphosphine ligands linked to the pyridine unit upon the addition of silver(I) ion.

the 3ILCT state.568,569 It was not until 2005 that Shinkai and co-workers reported the first example of Pt(II)-containing gels by utilizing 8-quinolinol platinum(II)−chelate moiety bearing 3,4,5-tris(n-dodecyloxy)benzoylamide substituents.350,570 This kind of metallogels would demonstrate a sol−gel transition in most common organic solvents, with the color change from yellow to orange. Under the electron microscope, the presence of well-developed fibrous structures was observed, characteristic of low-molecular weight organogels. In addition, the light orange phosphorescence at 620 nm that originated from the triplet ligand-centered transitions (3π → π* or 3ILCT) was observed in trichloroethene solution.350 Upon formation of gels in p-xylene, the phosphorescence was red-shifted to 650 nm with remarkable luminescence enhancement. The color change of phosphorescence from light orange to red originated from Jaggregation of the chelate moieties with the formation of π−π stacking interactions, while the luminescence enhancement was attributed to the inhibition of the dioxygen quenching for the excited triplet states,350 with the phosphorescence quantum yield of this gel significantly enhanced. Apart from 8-quinolinolato-O,N coordinating ligands, Schiff base ancillary ligands derived from N,N′-bis(salicylidene)-1,2ethylenediamine (H2salen) represent another important class of ligands in coordination chemistry over the past few decades.571−577 Since the early 1980s, the synthesis of the platinum(II) Schiff base complexes had been reported.578−581 Che, Yersin, and co-workers have performed spectroscopic studies on cis-bis(salicylaldiminato)platinum(II) complexes and explored their potential as functional materials for organic lightemitting diodes (OLEDs)582,583 and biosensors.584 Some of the crystal lattices for this class of complexes have been obtained. No metal−metal interactions were observed as the Pt(II)··· Pt(II) distances were found to be ca. 5.922−6.368 Å.582,583 The complexes were shown to adopt a head-to-tail arrangement with the presence of π−π stacking interactions between phenylene and the phenoxide groups.582,583 The strong fluidstate phosphorescence for this class of complex system was assigned as the triplet state of the phenoxide lone pair → π*(imine) with mixing of a 3[Pt(5d) → π*(Schiff base)] MLCT character.582,583 Yang, Zhu, and co-workers reported a class of Pt(II)-based supramolecular hexagons with reversible transformations via alternate irradiation with UV and visible light.585 The transformation behavior of the metallacycles was induced by the photocyclization of the photochromic multibisthienylethene moiety. This was accompanied by the formation of two intense absorption bands at ca. 420 and 628 nm with a dramatic color change from colorless to cyan.585 In addition, the fluorescence for the closed form of the metallacycles was found to be completely quenched upon photocyclization.585

Figure 118. Molecular structures of conjugated polymers consisting of the alternating p-phenylene-ethynylene and [Pt(salphen)] luminophore units.

backbone of the polymers was appended with n-alkoxy groups and acetylated-sugar side-chains. It was found that the “meta”type of the polymers would have a higher propensity to undergo intramolecular aggregation with the formation of π−π stacking interactions. This was further supported by the observation of CD signals, which indicated the handedness of the polymers, arising from the coiling of the backbones.586 The phosphorescence originated from mixed triplet O(p)/Pt(d) → π*(diimine) (3ILCT/3MLCT) excited states.586 In addition, the emission bands from the “meta”-type of polymers would be significantly red-shifted from the corresponding monomers, tentatively ascribed to the coilable polymers with predominant intramolecular π−π stacking interactions.586 Further addition of Cu(II) ions would selectively induce significant quenching effect on the emission, which could be regarded as a kind of sensory materials for detecting Cu(II) ions.586 The same group also developed a class of molecular frameworks derived from dinuclear cis-bis(salicylaldiminato)platinum(II) complexes with various rigid linkers, such as xanthene, dibenzofuran, and biphenylene.587 The X-ray crystal structure showed that the one bridged with xanthene linker would adopt the syn configuration, while the one linked by the biphenylene moiety would prefer the anti configuration for the two [Pt(salphen)] moieties in the crystal state. The luminescence in solution for the complexes was assigned as mixed triplet O(p)/Pt(d) → π*(diimine) excited states. They have also studied the change of the photophysical property toward the addition of Pb2+ ions. They have suggested a plausible mechanism for this kind of Pb2+binding, interplaying with the O-chelation and the perturbation of intramolecular π−π stacking interactions within the [Pt(salphen)] luminophores. Analogous to the strong phosphorescence properties of the cis-coordinated bis(salicylaldiminato)platinum(II) complexes, Naota and co-workers reported a novel class of vaulted transform platinum(II) complexes, which exhibited an intense phosphorescence in the crystalline state.588 However, the trans-coordinated platinum(II) complexes were found to be nonemissive in the solution state due to the high conformational mobility of the solubilizing groups. The strong crystalline-state phosphorescence property for this class of complexes was attributed to the trans-vaulted structure, which serves as a molecular barrier limiting the intermolecular BB


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contacts between d−π conjugation systems, and this would suppress the reduction of energy dispersion.588,589 The same group then demonstrated the gelation into organic liquids by the clothespin-shaped trans-bis(salicylaldiminato)Pt(II) complexes, doubly bridged with polymethylene spacers induced by ultrasound (Figure 119).590 They reported that the Pt(II)

Figure 120. Molecular structures of tetrameric platinum(II) metallacycles. Figure 119. Molecular structures of dinuclear clothespin-shaped transbis(salicylaldiminato)platinum(II) complexes. Reprinted with permission from ref 590. Copyright 2011 American Chemical Society.

relatively limited and under-explored. The lower propensity of luminescence from gold(III) complexes,592,593 as well as the rare observation of Au(III)···Au(III) interactions,594−598 have rendered the design of light-emitting self-assemblies of gold(III) complexes a challenging topic. Even though there has very recently been an increasing interest in the design and synthesis of light-emitting gold(III) complexes, most works have been focused on discrete gold(III) complex molecules.592,593,599−612 Before the 1990s, the investigation of gold(III) complexes was mainly directed toward the synthesis of gold(III) alkyl compounds, such as [AuR2X]2 (R = alkyl group, X = halogen).613 With the exception of a few reports on the photophysics of gold(III) porphyrins,614,615 luminescent gold(III) complexes remained rather unexplored at that time. This was probably due to the electrophilicity of the gold(III) metal center as well as the presence of low-energy d−d ligand field states that would quench the luminescent excited states by thermal equilibration or energy transfer.616 To render the metal center more electron-rich and achieve luminescence of the gold(III) system at ambient conditions, the group of Yam attempted to introduce good σ-donor ligands into the gold(III) metal center and successfully developed the first class of gold(III) complexes with room temperature luminescence, [Au(N∧N)R2]ClO4 (N∧N = bpy, phen, dpphen; R = mes, CH2SiMe3), which were all emissive in the solid state and in acetonitrile solutions at both low and room temperatures.599 The incorporation of good σ-donating ligands was suggested to be functional to raise the energy of the d−d ligand field states and thus improve the chance of obtaining luminescent gold(III) complexes. Later, Che and co-workers prepared the cyclometalated gold(III) chloro complexes, [Au(C∧N∧N-dpp)Cl]+ (HC∧N∧Ndpp = 2,9-diphenyl-1,10-phenanthroline), that were found to be emissive in various media, showing vibronically structured emission bands originating from triplet metal-perturbed intraligand excited states.617 The subsequent report of a related complex, [Au(C∧N∧N)Cl]+ (HC∧N∧N = 4′-(4-methoxyphenyl)-6′-phenyl-2,2′-bipyridine), demonstrated the usefulness of this class of gold(III) compounds for intercalation of calfthymus DNA.618 The same group also utilized the tridentate 2,6-diphenylpyridine (HC∧N∧CH) ligand to prepare a biscyclometalated gold(III) chloro complex, [Au(C∧N∧C)Cl].619 Replacement of the chloro group by various phosphines, 1-

complexes did not exhibit significant emission in common organic solvents. However, they would transform immediately (ca. 10 s) into stable phosphorescent gels with intense yellow 3 MLCT phosphorescence upon brief irradiation of low power ultrasound.590 More interestingly, the relative emission intensity can be precisely controlled by tuning the sonication time. Under electron microscopes, the colloidal aggregates with spherical morphology were observed in the freshly prepared solution.590 However, after ultrasonic irradiation for 3 s, the spherical morphology would transform into long fibers with a uniform width of ca. 50 nm.590 Prolonged sonication yielded higher-order nanostructures consisting of flat bundles, corresponding to the spontaneous gels.590 To elucidate the relevance of the emission properties in the ultrasound-induced gels together with the nanostructures as observed in the electronic microscopy, they suggested that the colloidal spheres were formed by the loose and nonpenetrative intermolecular stacking.590 Upon the irradiation of low power ultrasound, the spheres would turn to the microcrystals consisting of interpenetrating molecular units.590 The microcrystals would steadily aggregate and finally result in the fibrous gel networks under sonication.590 MacLachlan and co-workers reported a class of tetrameric metallacycles, which was constructed by the self-assembly from the coordinating pyridine ligands and the Pt(II) Schiff base moieties (Figure 120).591 Further aggregation of the macrocycles into columnar architectures has been determined by dynamic and static light scattering, NMR spectroscopy, powder X-ray diffraction, as well as transmission electron microscopy.591 In addition, the solvent-free matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) has shown the presence of sequential aggregates from dimers to hexamers due to the transverse columnar aggregation.591 Most interestingly, the columnar architectures of these Pt4 metallacycles appended with the branched 2-hexadecyl substituents would form liquid crystals, showing lyotropic mesophases in nonpolar organic solvents.591 2.5. Light-Emitting Gold(III) Self-Assembled Materials

When compared to the isoelectronic d8 platinum(II) system, works on luminescent gold(III) materials have remained BC


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methylimidazole, and 2-mercaptopyridine resulted in the preparation of a series of related bis-cyclometalated gold(III) complexes that was found to be emissive at 77 K.619,620 A related work by Eisenberg and co-workers in 1998 also demonstrated the low-temperature intraligand emission of various bidentate chlorogold(III) complexes, [Au(N∧N)Cl2]PF6 and [Au(C∧N)Cl2].621 On the other hand, the dithiolate derivatives of these bidentate gold(III) complexes were only found to exhibit solvatochromism with absorption bands from charge-transfer-to-diimine transitions, and no emission was observed.621 In 2001, Laguna and co-workers prepared a class of pentafluorophenyl gold(III) complexes, [{Au(C6F5)3}n(μ-triphos)] (n = 2, 3; μ-triphos = bis(2-diphenylphosphinoethyl)phenylphosphine). Although the dinuclear complex was nonemissive, the trinuclear derivative was found to emit at both ambient and low temperatures, representing a rare example of emissive gold(III) complexes at that time.622 Yam and co-workers utilized alkynyl ligands as a good σdonating group to prepare a class of bis-cyclometalated gold(III) complexes, [Au(C∧N∧C)(CC−R)] (C∧N∧C = 2,6-diphenylpyridine derivatives; R = aryl and alkyl groups). This class of gold(III) complexes was found to be air-stable and thermally stable, and to show intense emission in various media at both low and room temperatures.600−608 This class of complexes was also found to exhibit electrogenerated chemiluminescence623 and electroluminescence.602−608 The rich photo- and electroluminescence of this class of complexes rendered them good candidates for the development of lightemitting materials in organic light-emitting devices (OLEDs).602−608 Extension of the work to gold(III) complexes with dendritic alkynyl ligands further boosted the performance of the gold(III)-based materials for OLED applications.605,606 The same group later demonstrated that N-heterocyclic carbene ligands could also function as a good σ-donor for the preparation of emissive gold(III) complexes,624 which could be used in cytotoxicity studies and photocatalytic reactions.625,626 Subsequent works by Yam and other groups on related complexes with other cyclometalating and ancillary ligands further demonstrated the versatility and rich luminescence properties of the bis-cyclometalated gold(III) system.604−608,627−632 In another work, the encapsulation of some bis-cyclometalated gold(III) complexes into platinum(II)-based tweezers to form supramolecular host−guest assemblies has been demonstrated,395 and more details on these heterometallic systems involving metal−metal interactions have been discussed in section 2.4.3. In 2012, the groups of Che and Yam independently reported a class of cationic cyclometalated gold(III) alkynyl complexes, [Au(R−C∧N∧N)(CC−C6H4−R′)]+, which were emissive in the solid state and in low-temperature glass.633,634 It was interesting to observe that Au(III)···Au(III) interactions existed in the complex, [Au(C∧N∧N)(CC−C6H4−NMe2-4)]+ (Figure 121a), with the shortest separation between two gold(III) centers being 3.495 Å. Together with π−π stacking interactions, the Au(III)···Au(III) interactions have assisted in the selfassembly of the gold(III) complex to form belt-like microstructures with lengths longer than tens of microns and widths in the range of 1−10 μm.633 Another complex, [Au(MeC∧N∧N)(CC−C6H4−C2H5-4)]+ (Figure 121b), was found to exhibit the shortest Au(III)···Au(III) distance of 3.6018 Å, suggestive of some weak aurophilic interactions.634

Figure 121. Structures of (a) [Au(C∧N∧N)(CC−C6H4−NMe2-4)] and (b) [Au(MeC∧N∧N)(CC−C6H4−C2H5-4)].

These works have provided important insights into the future design of gold(III) materials for supramolecular assemblies. Although aurophilic interactions in gold(III) complexes are rare, the self-assembly of gold(III) complexes via other noncovalent interactions has also resulted in the report of gold(III)-based metallogels.635 Recently, Yam and co-workers reported the first examples of luminescent cyclometalated alkynylgold(III) complexes that could self-assemble to form metallogels.635 With long alkoxy substituents in the alkynyl ligand, the gold(III) complexes were found to exhibit gelation properties driven by π−π stacking and hydrophobic−hydrophobic interactions.635 Microscopic studies of the xerogels showed a network of fibrous structures. The gold(III) complexes were found to be emissive at various media at low and ambient temperatures. Variable-temperature UV−vis absorption and emission studies were performed to investigate the gelation properties in detail. When excited at the isosbestic point at the wavelength of 410 nm, the metallogel of the gold(III) complex, [Au(C∧N∧C)(CC−C6H2−(OC18H37)3-

Figure 122. Structure of [Au(C∧N∧C)(CC−C6H2−(OC18H37)33,4,5)].

3,4,5)] (Figure 122), was found to emit at 570−630 nm from a 3 LLCT [π(CC−C6H2−(OC18H37)3-3,4,5) → π*(C∧N∧C)] excited state. Upon phase transition to the sol form, the intensity of the emission band was observed to drop.635 Very recently, metallogels of alkynylgold(III) complexes containing anionic pincer ligands derived from 2,6-bis(benzimidazol-2′yl)pyridine have also been reported.636 The successful preparation of gold(III)-based metallogels has opened new opportunities in the use of transition metal complexes as probes for the detection of microenvironmental changes. In another study, a cyclometalated gold(III) complex functionalized with 2,4-diamino-6-(4-pyridyl)-1,3,5-triazine was found to self-assemble to form supramolecular polymers in acetonitrile by Che and co-workers, but the luminescence properties of the complex have not been studied.637 The same group later reported a luminescent cyclometalated gold(III) allenylidene complex (Figure 123) that was capable to form self-assembled nanostructures.638 The gold(III) complex was BD


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ray crystal structure.649 A study by Holt and co-workers suggested that the emission from this class of copper(I) cubanes was derived from more than one excited states, and the emission at longer wavelengths in the 585 nm region would originate from an excited state involving more than one metal centers, with a Cu(I)···Cu(I) distance of 2.8 Å or less.650 Subsequent works by Vogler and Zink have suggested an emission origin from a 3d10 → 3d9s1 excited state that was strongly modified by Cu(I)···Cu(I) interactions.651,652 In 1985, White and co-workers reported the synthesis and characterization of some [Cu4X4L4] (L = PPh3, X = Cl, Br; L = 2-[bis(trimethylsilyl)methyl]pyridine, X = Cl) clusters.653 Similar to the amine-containing derivatives, the crystal structure of [Cu4I4(PPh3)4] was also found to be monoclinic and to exhibit a cubane structure with almost tetrahedral geometry.653 Eventually in 2010, the group of Ozawa and Toriumi isolated the cubic polymorphs of [Cu4I4(PPh3)4].654 The Cu4I4 core was found to be located on the crystallographic 3-fold axis and trigonally elongated from the regular tetrahedral symmetry.654 The emission properties of the two polymorphs were different. The monoclinic form was found to exhibit greenish-yellow emission at 550 nm, while the cubic form was found to emit at 520 nm in the blue-green emission at 293 K. At 78 K, the emission of the monoclinic form would be red-shifted to 610 nm, while that of the cubic form remained unchanged.654 Together with multi-temperature X-ray crystal structure analyses, it was revealed that the luminescence thermochromism of the monoclinic cubane form was due to the shrinkage of the Cu4 core size at low temperature.654 The group of Ford further investigated the photophysical properties of some tetranuclear copper(I) clusters, [Cu4I4(pyX)4] (X = H, 3-Cl, 4-Ph, 4-PhCH2, 4-tBu), by time-resolved emission spectral studies at 296 K in toluene solutions.655 It was observed that the complexes exhibit dual emission from two uncoupled states. The one in the 473−537 nm region was from a short-lived state, while the other more intense, longlived band at 678−698 nm was from a metal cluster-centered (MCC) state.655 The poor coupling between the two excited states has also been demonstrated by the dynamic quenching studies of the MLCT excited state of [Cu4I4(py)4].656 Time-resolved emission studies have also been carried out on [Cu4I4(py)4] and a series of related tetranuclear copper(I) iodide clusters, [Cu4I4(L)4] (L = 4-tert-butylpyridine, 4benzylpyridine, pyridine-d5, 4-phenylpyridine, 3-chloropyridine, piperidine, PnBu3).657 It was found that the complexes generally exhibited a lower-energy emission band at 678−698 nm and a higher-energy emission at 473−537 nm in toluene solutions at 294 K.657 While the lower-energy band was a dominant feature for all complexes, the higher-energy emission was only observable for copper(I) iodide clusters containing aromatic amine ligands. The sensitivity of the higher-energy band to solvents and the nature of the pyridine ligands was suggestive of its origin as a metal-to-ligand [Cu → π*(L)] or ligand-to-ligand [I− → π*(L)] charge transfer excited state.657 Ford later collaborated with Palke and investigated in detail the origin of the dual luminescence by ab initio calculations at the restricted Hartree−Fock self-consistent field level.658 It was found that the high-energy band originated from an iodide-to-pyridine charge transfer (XLCT) state, while the low-energy emission should be assigned to a transition with both iodide-to-copper ligand-to-metal charge-transfer and metal-centered 3d → 4s states within the Cu4I4 cluster core in a cluster-centered (CC) transition.658

Figure 123. Mesomeric structures of the cyclometalated gold(III) allenylidene complex.

found to exhibit dual emission in dilute dichloromethane solution at 450 and 525 nm, respectively, originating from singlet and triplet ILCT excited states.638 This complex was found to form nanostructures upon deposition from acetonitrile, displaying an entangled fibrillar network in SEM micrographs.638

3. LIGHT-EMITTING SELF-ASSEMBLED MATERIALS OF d10 METAL COMPLEXES Because of the prevalent metallophilic interactions in d10 transition metal systems, there have been extensive studies on the supramolecular assemblies of discrete molecules of Cu(I), Ag(I), Au(I), as well as Hg(II). Very often, the presence of metallophilic interactions would render the coinage metal complexes luminescent, allowing them to be potential candidates for use as functional light-emitting materials. 3.1. Light-Emitting Copper(I) Self-Assembled Materials

There has been a long history of several decades for the development of luminescent copper(I) complexes. Although there have been a number of literature reports in the 1960s for the preparation of phosphine- and arsine-containing complexes of copper(I), very few of them were focused on the spectroscopic properties of the copper(I) complexes.639−641 In 1970, Dori and co-workers investigated the emission properties of various d10 metal phosphines and arsines, and revealed the occurrence of copper(I) luminescence in the visible region.642 Much attention was then drawn toward the photoluminescence properties of the copper(I) system after the late 1970s. 3.1.1. Tetranuclear [Cu4X4L4] Complexes. A classical example of luminescent d10 metal complexes would be the tetranuclear copper(I) clusters, [Cu4X4L4] (X = halogen; L = pyridine, substituted pyridine, saturated amine) (Figure 124). The first example of such tetranuclear complexes, [Cu4I4(py)4] (py = pyridine), was reported by Hardt and co-workers in the 1970s.643−648 The complex was observed to adopt a “cubane” structure with a short Cu(I)···Cu(I) distance of 2.69 Å in the X-

Figure 124. General structure of [Cu4X4L4] cubanes. BE


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The same group also studied the photoluminescence properties of the chloro analogues, [Cu4Cl4L4] (L = pyridine, 4-phenylpyridine, diethylnicotinamide, morpholine, triethylamine), and observed a single emission band in the solid state and in toluene glass at 77 K for complexes with unsaturated amine ligands (L = pyridine, 4-phenylpyridine, diethylnicotinamide).659 The emission maxima were found to be dependent on the nature of the nitrogen donor ligands, and the emissive state was assigned as a triplet MLCT or halide-topyridine charge transfer (XLCT) state rather than a metalcentered excited state.659 This assignment was supported by the observation of substantial chloride character in the HOMO from ab initio studies.659 In 1991, Attar, Nelson, and co-workers reported the preparation of a series of phosphole-containing copper(I) complexes, [LnCuX]m (n = 1, m = 4, L = 1-phenyl-3,4dimethylphosphole (dmpp), 1-phenyldibenzophosphole (dbp); n = 2, m = 2, L = dmpp; n = 3, m = 1, L = dmpp, dbp; X = Cl, Br, I).660 The X-ray structure of [(dmpp)CuI]4 was revealed to be a tetrameric cubane structure in which each of the four copper(I) centers was coordinated to the phosphorus atom of the phosphole ligand and three bridging iodide ligands, with an average Cu(I)···Cu(I) contact of 2.933 Å.660 Using a combination of far-infrared and cross-polarization magicangle-spinning (CP/MAS) 31P NMR spectroscopy, the structures of the other complexes were determined. All of the 1:1 complexes (n = 1) were also found to show a tetrameric cubane structure.660 For the 2:1 complexes, the chloro analogue exhibited an ionic structure, [(dmpp)4Cu]+[CuCl2]−, while the bromo and iodo analogues showed halogen-bridged dimeric structures, [{dmpp}2CuX]2. The 3:1 complexes were found to exhibit mononuclear [L3CuX] structures.660 Lai and Zink later studied the emission properties of one of the clusters, [Cu4I4(dmpp)4].661 In the powder form at 15 K, the complex was found to exhibit a vibronic-structured red emission band at 15070 cm−1. The presence of multiple emissive origins was excluded by time-resolved emission spectroscopic studies.661 Raman spectroscopy and theoretical emission spectrum based on the time-dependent theory calculations were also studied, and it was found that the vibronic nature could be correlated to the distortion of the phosphole moieties. The emission was therefore assigned to originate from a spin-forbidden copper-todmpp MLCT excited state.661 Ford and co-workers later investigated a series of tetranuclear copper(I) clusters, [Cu4X4(dpmp)4] (X = I, Br, Cl; dpmp = 2(diphenylmethyl)pyridine).662 Because of the steric requirement of the bulky dpmp ligand, the isostructural complexes were found to exhibit highly similar Cu(I)···Cu(I) distances of 2.88(1)−2.92(8) Å in the cubane structure.663 In the solid state at 77 K, the complexes were found to emit at 467−505 nm from a XLCT excited state. Interestingly, [Cu4I4(dpmp)4] and [Cu4Br4(dpmp)4] were found to show a weak shoulder at higher temperatures at lower energies, which was assigned to originate from a Cu4X4 core-centered excited state of mixed XMLCT and metal-centered character.662 In 1997, pressure-induced luminescence rigidochromism and its effect on the emission properties of [Cu4I4(py)4] have been studied.664 In benzene solution at ambient pressure, the complex exhibited a cluster-centered emission at 695 nm. A pronounced hypsochromic shift in the emission maximum to 575 nm, which was similar to the solid-state emission (λmax = 580 nm), was observed when the pressure was high enough to cause a phase transition from fluid to solid.664

In 2007, Lee and co-workers utilized calix[4]bis(thiacrown5) (L) with 1,3-alternate conformation as a ligand and synthesized a mixture of three-dimensional copper(I) networks; one was linked by a Cu2I2 rhomboidal unit ([Cu2LI2]n) while the other was linked by a Cu4I4 cubane unit ([Cu4LI4]n) (Figure 125).665 A related silver(I) derivative, [Ag2L](PF6)2,

Figure 125. Crystal structure of [Cu4LI4]n: (left) core Cu4LI4 unit and (right) 3D network linked by Cu4I4 cubane units. Reprinted with permission from ref 665. Copyright 2007 American Chemical Society.

was also synthesized as a discrete endocoordinated complex.665 The copper(I) complex, [Cu2LI2]n, was found to crystallize into colorless crystals. The three-dimensional structure of the complex featured an interconnected layer with a short Cu(I)···Cu(I) distance of 2.654(7) Å.665 However, this complex was nonemissive in nature.665 For [Cu4LI4]n, it was observed to crystallize into pale-yellow polyhedrons. In the three-dimensional polymeric structure, Cu(I)···Cu(I) distances of 2.607−2.995 Å were observed.665 [Cu4LI4]n was found to show an intense emission in the orange-yellow region in the solid state, which originated from the cluster-centered excited state mixed with iodide-to-metal charge transfer (XMCT) character.665 In the next year, the group of Lee utilized the same calix[4]bis(thiacrown-5) ligand and isolated another threedimensional copper(I) network of formula [{(Cu3I3)L(MeCN)}(CH2Cl2)(2H2O)]n (Figure 126) at ambient temperature.666 Upon heating to 175 °C, the loss of acetonitrile would result in the formation of [{(Cu3I3)L}(0.5H2O)]n. Remarkably, this was accompanied by a single-crystal-to-single-crystal transformation from tetrahedral to trigonal coordination geometry that gave rise to a solvato-photoluminescence “off− on” behavior.666 While the acetonitrile-solvated form was nonemissive in nature, [{(Cu3I3)L}(0.5H2O)]n was observed to exhibit an intense yellow emission at 420 nm, assignable to have originated from a cluster-centered excited state with mixed iodide-to-metal charge transfer (XMCT) character.666 Moreover, the synthetic pathway was found to be temperaturedependent. The reaction at −10 °C was found to yield another three-dimensional polymer, [{(Cu2I2)L2(Cu4I4)}(CH2Cl2)(MeCN)]n, which contained both Cu4I4 cubane units and Cu2I2 rhomboidal units that were linked alternately by the ligand L.666 This polymeric complex was observed to show an orange emission at 531 nm in the solid state arising from the Cu4I4 moieties. Lindoy, Lee, and co-workers reported a series of copper(I) halide complexes using various O2S2X-macrocycles (L1, X = S; L2, X = O; L3, X = NH) as a ligand.667 Although the macrocyclic ligands were structurally similar, three different copper(I) three-dimensional networks with different architectures have resulted. The use of L1 as the ligand resulted in the formation of a cofacial dimer, [(Cu2I2)(L1)2], in which the BF


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Figure 126. Synthesis of copper(I) calix[4]bis(thiacrown-5) complexes. Reprinted with permission from ref 666. Copyright 2008 American Chemical Society.

thermochromic tetranuclear copper(I) clusters.669,670 The alkene-containing cluster, [Cu4I4{PPh2(CH2CHCH2)}4], was found to exhibit both mechanochromic and thermochromic luminescence properties.669 At room temperature, the complex was found to exhibit a weak green emission. Upon grinding, the compound would then exhibit an intense yellow emission, and the green emission would be restored at increased temperature. When cooled to 77 K, the complex would exhibit a blue emission, which would turn to purple after grinding.669 Such observations could be attributed to the local distortions in the crystal packing and thus shortening of Cu(I)···Cu(I) distances in the Cu4I4 cluster core, therefore affecting the luminescence properties.669 Using different phosphine ligands, the thermochromic luminescence properties of other tetranuclear copper(I) clusters have been studied.669 With the support of DFT and TDDFT calculations, the emission origin of the copper(I) complexes could generally be assigned to a cluster-centered excited state and a mixed XMCT/XLCT excited state.670 A recent structural investigation by X-ray diffraction and solid-state NMR studies further established the relationship between cuprophilic interactions and the mechanochromic properties of the clusters.671 In 2011, Li and co-workers combined the Cu4I4 cubane moiety with the triangular Cu3(pyrazolate)3 (Cu3Pz3) system to prepare a supramolecular dual emissive and thermochromic supramolecular coordination polymer, [Cu4I4(NH3)Cu3L3]n (L = 3-(4-pyridyl)-5-p-tolyl-pyrazolate).672 The Cu4I4 cluster unit showed a distorted cubane configuration with short Cu(I)··· Cu(I) contacts of 2.6490(5)−2.8043(6) Å.672 With the exception of one Cu(I) site that was coordinated to an NH3 group, the Cu(I) sites in each Cu4I4 unit were bonded to the pyridine in the bridging ligand L, which was in turn coordinated to the triangular Cu3Pz3 moiety to form an extended twodimensional (2-D) layered structure. In the presence of cooperative, interlayer Cu(I)···Cu(I) interactions between the Cu3Pz3 moieties, two adjacent 2-D layers were assembled into a double-layer arrangement.672 Upon excitation at both 370 and 270 nm, the complex was observed to show dual emission at

Cu2I2 rhomboidal core was found to be sandwiched between two L1 ligands.667 The reaction between CuCl and L2 gave rise to a double-stranded one-dimensional polymer, [(Cu2Cl2)(L2)]n, with the ligands linked with Cu2Cl2 rhomboidal units through Cu−S bonds.667 Both [(Cu2I2)(L1)2] and [(Cu2Cl2)(L2)]n were nonemissive. On the other hand, the use of the nitrogen-containing macrocycle L3 resulted in the formation of a luminescent polymeric complex, [(Cu4I4)(L3)2]n, which existed as a two-dimensional polymeric array linked by Cu4I4 units. This complex was shown to exhibit intense green luminescence at 558 nm that originated from a cluster-centered excited state mixed with XLCT character.667 In 2010, Eisenberg and co-workers investigated the structural and photophysical properties of a series of tetranuclear [Cu 4 I 4 (L n ) 2 ] clusters that contained 4,4′-(4,5-di-X-1,2phenylene)bis(1-benzyl-1H-1,2,3-triazole) ligands (X = H, Me, F) (Figure 127).668 As the triazole ligands exhibited both

Figure 127. Structures of the tetranuclear [Cu4I4(Ln)2] clusters.

chelating and bridging coordination modes to the copper(I) centers, the clusters were found to show a distorted “step-type” structure with short intramolecular Cu(I)···Cu(I) distances of 2.6311(5)−2.7465(8) Å, in contrast to the conventional cubane structure of clusters with the Cu4I4 core.668 A related dinuclear complex has also been obtained. All of the clusters were found to be emissive in the solid state at ambient and low temperatures, and the emission band at 495−524 nm was tentatively ascribed to a MLCT excited state.668 In the past decade, Perruchas, Boilot, and co-workers also revisited the cubane [Cu4I4L4] system and prepared a series of BG


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form a distorted octahedron, with two additional iodide groups μ2-bound on each short side of the parallelogram. The P∧N ligands were coordinated to the copper(I) centers on opposing long sides of the parallelogram to give a C2 symmetry (Figure 129).675 For the clusters with tert-butyl and isopropyl

530 and 700 nm, which was assigned to be derived from the cluster-centered excited states of Cu4I4 and excimeric states of [Cu 3Pz3]2 dimer of trimers, respectively.672 Reversible luminescence thermochromism was also observed, and this could be attributed to the consequence of a thermal equilibrium between the two separated, competitive excited states. Very recently, the group of Li extended their work to the preparation of a tetradecanuclear cluster, [Cu6L3(Cu2I2)Cu6L3] (H2L = 3,5bis((3,5-dimethyl-pyrazol-4-yl)methyl)-2,6-dimethylpyridine).673 The cluster was observed to exhibit well-resolved dual emissions, blue phosphorescence from the Cu2I2 unit and red phosphorescence from the Cu6L3 moieties. The good thermal stability and the thermochromic response of the dual emission across a wide temperature range from 120 to 450 K have rendered this Cu6−Cu2−Cu6 cluster a potential luminescent molecular thermometer.673 Pike and co-workers used various N-substituted and N,N′disubstituted piperazines (pip) as bridging ligands to prepare a series of copper(I) halide complexes.674 Depending on the steric effects of the substituents on the pip ligand, the resulting copper(I) halide complexes were observed to exhibit different structures. These included networks of linked Cu4X4 cubanes of [(CuX)4(pip)2], simple [Cu4X4(pip)4] cubanes, as well as chains of linked Cu2X2 rhomboids, [(CuX)2(pip)2].674 The network structure of [(Cu4I4(N,N′-dimethylpiperazine)2)] is depicted in Figure 128 as an illustrative example. In general, the

Figure 129. Crystal structure of [Cu4I4(P∧N)2] (P∧N = 2-[(di-tertbutyl-phosphino)methyl]pyridine. Reprinted with permission from ref 675. Copyright 2012 American Chemical Society.

substituents on the P∧N ligand, the emission spectra showed an emission band at ca. 460 nm at 298 K, assignable to have originated from a triplet halide-to-ligand charge-transfer (XLCT) excited state, while the other clusters showed an additional band at ca. 570 nm that originated from a Cu4I4 cluster-centered (CC) excited state.675 Very recently, the groups of Perruchas and Boilot also utilized cholesteryl-based phosphine ligands to prepare a tetranuclear copper(I) cluster that could form highly emissive gels in cyclohexane (Figure 130).676 Upon excitation at 312

Figure 130. General structure of tetranuclear cholesteryl-based copper(I) clusters.

Figure 128. Crystal structure of [(Cu4I4(N,N′-dimethylpiperazine)2)] showing one layer of 3-D honeycomb network structure. Reprinted with permission from ref 674. Copyright 2012 The Royal Society of Chemistry.

nm, the white gel would show a green cluster-centered emission at 535 nm at room temperature and a blue emission at 435 nm at 77 K involving an iodide-to-phosphine halide-to-ligand charge transfer and a copper-to-phosphine metal-to-ligand charge transfer (XLCT/MLCT) excited-state origin. It was observed that the photoluminescence properties of the copper(I) complex were similar in the xerogel, powder, and gel states.676 SEM and TEM studies showed the formation of an extended three-dimensional globular network upon gelation. Results from FTIR analysis suggested that hydrogen bonding between the spacers and van der Waals’ interactions between the cholesteryl groups were responsible for the gelation process.676 Steffen, Müller, and co-workers prepared a tetranuclear copper(I) phosphinine cluster, [Cu4Br4L4] (L = 2,4-diphenyl-5methyl-6-(2,3-dimethylphenyl)phosphinine) (Figure 131).677 The observed Cu(I)···Cu(I) distances of 3.1304(12)− 3.3087(11) Å showed that there were no appreciable cuprophilic interactions due to the bulky phosphinine

tetrameric complexes were found to show low-energy emission bands assignable to cluster-centered transitions, whereas the dimeric complexes emitted at relatively high energy either from a cluster-centered or from a MLCT origin.674 In particular, the dimeric [Cu2I2(pip)] complexes were found to react irreversibly with gaseous amines to form tetrameric cubanes by replacement of the piperazine ligands. The associated emission color change therefore allowed the dimeric complexes to act as sensor materials for amines.674 Thompson and co-workers prepared a class of [Cu4I4(P∧N)2] clusters that were supported by two P∧N-type ligands, 2-[(di-R-phosphino)methyl]pyridine (R = phenyl, cyclohexyl, tert-butyl, isopropyl, ethyl).675 Instead of the conventional cubane structure, all of the Cu4I4 clusters were shown to adopt an unusual geometry in which the central core consisted of the copper(I) centers arranged in a Cu 4 parallelogram that was μ4-capped by two iodide ligands to BH


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dppm)3(μ3-η1-CCtBu)2]PF6 was also reported.683 However, no photophysical and photochemical studies have been documented at that time. Using a variety of alkynyl ligands, Yam and co-workers performed a systematic and comprehensive study on the photophysical properties of a series of related trinuclear copper(I) complexes with mono- and bicapped alkynyl ligands. Examples of the bicapped copper(I) alkynyl complexes being investigated included [Cu3(μ-dppm)3(μ3-η1-CCtBu)(μ3Cl)]PF6 and [Cu3(μ-P∧P)3(μ3-η1-CCR)2]PF6 (P∧P = dppm, R = Ph, tBu, C6H4NO2-p, C6H4Ph-p, C6H4OMe-p, C6H4NH2-p, C6H4CN-p, nC6H13, nC4H9, C6H4CCPh, C6H4carbazole, naphthalene; P∧P = [(C6H5)2P]2NR, R = nPr, C6H5, C6H4−CH3-p, C6H4−F-p) (Figure 130).684−689 Crystallographic studies showed that the trinuclear copper(I) complex consisted of an isosceles triangular array of copper atoms with a diphosphine ligand bridging each edge to form a roughly planar [Cu3P6]+ core, with short Cu(I)···Cu(I) distances of 2.754(2)− 2.791(2) Å.684 In general, the complexes were found to be emissive in acetonitrile solutions and in the solid state. In the solid state, the complexes exhibited a high-energy emission band at 440−485 nm and a low-energy band at 525−540 nm. Subsequent works by the group also involved the incorporation of two different alkynyl ligands to prepare trinuclear mixcapped copper(I) alkynyl complexes.690 In addition to trinuclear bicapped complexes, the group of Yam also studied a series of trinuclear copper(I) monocapped complexes, including [Cu3(dppm)3(μ3-η1-CCR)]2+ (R = Ph, tBu, C6H4NO2-p, C6H4Ph-p, C6H4OMe-p, C6H4NH2-p, nC6H13) (Figure 133).687,691 Similar to the bicapped counterparts, the

Figure 131. Crystal structure of [Cu4Br4L4]. Reprinted with permission from ref 677. Copyright 2014 American Chemical Society.

ligands.677 At room temperature, the cluster was found to exhibit a strong orange emission at 640 nm in the solid state. With the support of DFT and TDDFT studies, the emission origin has been assigned to a mixture of 3MLCT and 3XLCT states, in which charge transfer was suggested to occur from Cu and Br to the phosphinine ligand.677 The recent collaboration between the groups of Wang, Bian, and Thompson has demonstrated the fabrication of organic light-emitting diodes based on a related dimeric copper(I) complex, [Cu2I2(CPPyC)4] (CPPyC = 3-(carbazole-9-yl)-5((3-carbazol-9-yl)phenyl)pyridine) (Figure 132).678 Upon

Figure 132. Structure of [Cu2I2(CPPyC)4].

codeposition of copper(I) iodide and CPPyC, [Cu2I2(CPPyC)4] would be formed on the thin film. The formation of [Cu2I2(CPPyC)4] was characterized by X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) studies.678 The codeposited film could be employed as the light-emitting layer to give greenemitting OLEDs with an external quantum efficiency (EQE) of up to 15.7% at a luminance of 100 cd m−2.678 The origin of such green emission of the in situ generated copper(I) compound has been assigned to an excited state of 3MLCT and/or 3XLCT character.678 3.1.2. Copper(I) Alkynyl Complexes. In the 1980s, the groups of Naldini and Gimeno have investigated the incorporation of alkynyl ligands into the copper(I) metal center and reported, respectively, the tetranuclear complexes, [Cu(PPh3)(CCPh)]4 and [Cu(μ3-η1-CCPh)(Ph2PpyP)]4 (Ph2PpyP = 2-(diphenylphosphino)pyridine).679,680 Gimeno and co-workers later reported a series of trinuclear copper(I) clusters, including the monocapped complex, [Cu3(μ3-η1-C CPh)2(μ-dppm) 3](BF 4)2,681 and the bicapped complex, [Cu3(μ3-η1-CCPh)2(μ-dppm)3](BF4)2.682 The synthesis of [Cu3(μ-dppm)3(μ3-η1-CCtBu)(μ3-Cl)]PF6 and [Cu3(μ-

Figure 133. Structure of [Cu3(μ-dppm)3(μ3-η1-CCR)2]+.

monocapped copper(I) alkynyls were also found to show intense dual luminescence with long lifetimes in the microsecond range.691 At that time, the higher-energy bands for both the mono- and the bicapped complexes were suggested to be likely derived from a metal-to-ligand charge transfer (MLCT) [Cu → π*(RCC−)] or intraligand [π → π*(RCC−)] excited states, whereas the lower-energy bands were tentatively assigned to originate from an admixture of a cuprophilicitymodified metal-centered 3d94s1 state of Cu(I) and LMCT [RCC− → Cu3] excited states.684,692 In addition, the phosphorescent state of these copper(I) complexes was found to be readily quenched by pyridinium acceptors, and nanosecond transient absorption studies were thus performed to establish the electron-transfer nature of the quenching reactions.692 It was found that these copper(I) alkynyl complexes exhibited intense absorption bands at 790−830 nm in the near-infrared region, which was attributed to the formation of transient CuICuICuII mixed-valence species.692 BI


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complexes.699 It was found that the high-lying occupied molecular orbitals contained substantial character from the antibonding between the Cu(d) orbitals and the ligand orbitals, and these orbitals were also found to be mixed with significant Cu(s,p) character.699 In addition to trinuclear copper(I) alkynyls, Yam and coworkers also reported a tetranuclear copper(I) alkynyl complex, [Cu4(μ-dppm)4(μ4-η1,η2-CC)](BF4)2, which contained an alkynyl ligand in both η1- and η2-bonding modes.700 In the crystal structure, the copper(I) centers were arranged in a distorted rectangular array with the four dppm ligands arranged in a saddle-like fashion (Figure 135). The complex was

The group later incorporated a selection of alkynyl moieties, such as diynyls693 and crown ethers,694 into the copper(I) center to prepare a wide variety of bicapped copper(I) complexes. With the help of density functional theory and extend Hückel calculations in collaboration with the group of Halet, the origin of the lower-energy emission band was assigned to be derived from excited states of substantial LMCT parentage, mixed with a metal-centered nd9(n+1)s1 state, but an involvement of a ligand-centered π → π* excited state of the diynyl moiety would be possible due to the highly structured bands.693 In addition, Yam and co-workers reported a series of luminescent tetranuclear copper(I) phosphine alkynyl complexes, [Cu4(PPh3)4(μ3-η1-CCR)4] (R = C6H4F-p, C6H4Mep, C6H4OMe-p, C6H4Et-p, C6H4Ph-p, C6H4NO2-p), that exhibited a distorted cubane structure similar to the [Cu4X4L4] system (Figure 134).643,655,656,679,680,687,695 Upon

Figure 135. Structure of [Cu4(μ-dppm)4(μ4-η1,η2-CC)]2+.

observed to show intense green and greenish yellow emissions in the solid state and in acetone solution, respectively, and the emission origin was suggested to be associated with a spinforbidden transition from a triplet state of LMCT and metalcentered d−s character.700 The excited-state redox properties were also explored by quenching studies with pyridinium acceptors. Interestingly, 31P and 1H NMR studies revealed that this complex would undergo two independent fluxional processes with similar activation energies in solution, namely the oscillation of the alkynyl moiety inside the copper(I) rectangle and the flipping of the diphosphines.701 The existence of these fluxional processes was confirmed by DFT(B3LYP) calculations. The dinuclear copper(I) alkynyl complex, [Cu2(PPh2Me)4(μ-η1-CCPh)2],702 was also found to exhibit rich photoluminescence in the solid state at room temperature. In the crystal structure, the two copper(I) centers and the bridging carbon atoms were displayed to arrange into a strictly planar rhomboidal array. The complex was found to exhibit a sharp emission maximum at 467 nm and a shoulder to longer wavelength at 509 nm in the solid state at 298 K. This emission band became vibronically structured upon cooling to 77 K, suggestive of the participation of the alkynyl moiety, possibly derived from triplet states involving a LMCT (PhCC− → Cu2) character modified by Cu(I)···Cu(I) interactions.702 The emission properties of two related complexes, [Cu2{2-C(SiMe3)2C5H4N}2] and [{Cu(mes)}5] (mes = mesityl), have also been investigated.702 The complexes were found to show a structureless emission band, respectively, at ca. 520−550 and 650 nm. Given the short Cu(I)···Cu(I) contacts in the structures of the two complexes, an emissive origin derived from the metal-centered d → s state of Cu(I) modified by metallophilic interactions was suggested, although an assignment as LMCT state modified by metallophilic interactions would also be possible given the strong σ-donating properties of the ligands.702 Yam and co-workers also extended their work to the preparation and photophysical studies of a hexanuclear copper(I) alkynyl complex, [Cu3(μ-Ph2PCH2PPh2)3(μ3-η1CCC6H4CC-p)Cu3(μ-Ph2PCH2PPh2)3](BF4)4 (Figure 136).703 In the single-crystal structure, the complex cation was found to arrange into a dumbbell shaped structure that

Figure 134. Structure of [Cu4(PPh3)4(μ3-η1-CCR)4].

photoexcitation, the tetranuclear complexes were generally found to exhibit an intense structureless emission band centered between 516 and 548 nm in the solid state at room temperature. The long emission lifetimes in the microsecond range were suggestive of an origin from spin-forbidden transitions.695 For the emission in dichloromethane solution, the complexes were found to exhibit two bands at ca. 420 and 620 nm, with an additional shoulder at ca. 520 nm on the highenergy side of the band at 620 nm. It was likely that the emissive states originated from triplet excited states of d → s/ LMCT admixture.695 Efforts have also been extended to the preparation of another series of tetranuclear copper(I) clusters, [Cu 4 (PR 3 ) 4 (μ 3 -η 1 ,η 1 ,η 2 -CCR′) 3 ]PF 6 (R = Ph, R′ = C 6 H 4 OMe-p, C 6 H 4 OEt-p, C 6 H 4 O n Bu-p, C 6 H 4 O n Hex-p, C 6 H 4 O n Hept-p, C 6 H 4 Me-p, C 6 H 4 n Bu-p, C 6 H 4 n Hex-p, C6H4nHept-p, C6H4nOct-p, C6H4Ph-p, C6H4Cl-p, C4H3S; R′ = C6H4OMe-p, R = C6H4Me-p, C6H4CF3-p).696,697 X-ray crystallographic studies revealed an open-cube structure of the complex, which consisted of a puckered Cu3C3 sixmembered ring bridged by a copper(I) center, with the C C moiety of the alkynyl ligand in μ3-bridging modes, showing short Cu(I)···Cu(I) distances of 2.446(2)−2.467(2) Å.696 The complex was found to show a long-lived emission at ca. 443− 541 nm in the solid state at 298 and 77 K, some also with an additional weak shoulder at 623−665 nm at room temperature, while an intense band at ca. 675 nm was observed in solutions. The lower-energy band was tentatively assigned as derived from states involving LMCT [CCR → Cu4] character with mixing of a metal-centered d−s state, and the nature of the excited states has been probed by DFT calculations.696,697 Related diynyl derivatives were also prepared and observed to show similar low-energy bands, possibly with the additional mixing of an intraligand π−π*(diynyl) state in the emissive origin.698 Ab initio molecular orbital calculations were also employed to study the electronic structures of the tetranuclear cubane BJ


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Figure 138. Structure of [Cu4(μ-Ph2Ppypz)4(μ4-η1,η2-CC)]2+.

distance of 2.843(1) Å in the complex was indicative of the presence of cuprophilic interactions in the complex.705 Upon photoexcitation, the tetranuclear cluster was found to exhibit a vibronically structured emission, which was tentatively assigned to a [(CC)2− → Cu4] LMCT state mixed with d → s character.705 3.1.3. Other Copper(I) Complexes. In the 1980s, McMillin and co-workers reported the photophysical studies of the dinuclear copper(I) diimine complexes, [Cu(dmp)2]+ (dmp = 2,9-dimethyl-1,10-phenanthroline) and [Cu(dpp)2]+ (dpp = 2,9-diphenyl-1,10-phenanthroline).706−709 These copper(I) complexes were generally found to exhibit structureless emission bands in solutions at room temperature. For example, the emission maximum of [Cu(dmp)2]+ was found to be at 730 nm.706 Such emission bands could be assigned to originate from MLCT states.706−709 The pressure effects on the excited-state dynamics of this class of complexes were later investigated, and it was proposed that an associative mechanism was involved in the nonradiative deactivation of the MLCT excited state of the less bulky [Cu(dmp)2]+.710 Kaim and co-workers reported another luminescent dinuclear copper(I) diimine complex, [(μ-bpym){Cu(PR3)2}2]2+ (bpym = bipyrimidine).711 The complex was found to be strongly emissive in the solid state but would become nonemissive in the solution state or after anion exchange. On the basis of X-ray crystallographic studies, the origin of the solid-state emission was suggested to be derived from the π-stacking of one of the phenyl rings of the phosphine groups with the bpym ligand.711 Blasse, van Koten, and co-workers prepared a series of copper(I) arylthiolate compounds, [CuSC6H4-[(R)-CH(Me)NMe 2 ]-2] 3 (Figure 139) and [CuSC 6 H 4 (CH 2 NMe 2 )-

Figure 136. Structure of [Cu 3 (μ-Ph 2 PCH 2 PPh 2 ) 3 (μ 3 -η 1 -C CC6H4CC-p)Cu3-(μ-Ph2PCH2PPh2)3][BF4]4.

consisted of two triangular arrays of copper(I) centers with a dppm ligand bridging each edge to form two roughly planar Cu3P6 cores bridged by a 1,4-diethynylbenzene unit.703 The hexanuclear copper(I) cluster was found to exhibit intense yellow-orange luminescence at both 298 and 77 K upon photoexcitation at wavelengths longer than 350 nm, which was tentatively assigned to emissive states derived from an alkynylto-metal LMCT transition mixed with d−s character.703 The group of Che then reported a tetranuclear copper(I) cluster, [Cu4(CCPh)4L2] (L = Ph2PCH2(CH2OCH2)2CH2PPh2).704 As shown in Figure 137,

Figure 137. Crystal structure of [Cu4(CCPh)4L2]. Reprinted with permission from ref 704. Copyright 1998 Elsevier B.V.

the crystal structure of the complex displayed a zigzag geometry of the Cu4 unit, in which the central Cu2L2 metallomacrocyclic moiety was encapsulated by two Cu(CCPh)2 fragments.704 The tetranuclear complex was found to emit at 533 nm in the solid state at room temperature, with an additional weak band at 575 nm.704 At 77 K, the emission maximum was observed to blue-shift to 525 nm and the weak band became a weak shoulder. The emission was suggested to have originated from the excited state of the Cu4(CCPh)4 core.704 In 2001, Mak and co-workers reported a luminescent tetranuclear copper(I) complex, [Cu4(μ-Ph2Ppypz)4(μ4-η1,η2CC)](ClO4)2 (Ph2Ppypz = 2-(diphenylphosphino-6-pyrazol1-yl)pyridine) (Figure 138).705 The single-crystal structure of the complex cation featured a butterfly shaped Cu4C2 core, in which the alkynyl moiety bridged the two Cu2 subunits via both η1- and η2-bonding modes.705 The shortest Cu(I)···Cu(I)

Figure 139. Structure of [CuSC6H4-[(R)-CH(Me)NMe2]-2]3.

2]3,712−714 which exhibited a chairlike conformation of the six-membered Cu3S3 ring with three equatorial aryl moieties in the crystal structure. Replacement of an arylthiolate by an alkynyl ligand resulted in the formation of the hexanuclear copper(I) complexes, [Cu 3 {SC 6 H 4 -[(R)-CH(Me)NMe2]}2(CCtBu)]2 and [Cu3{SC6H4(CH2NMe2)}2(C CtBu)]2, which consisted of a six-membered Cu3S2C ring in the boat conformation in the crystal structure.714 In particular, the chiral complexes were found to exhibit triboluminescence, and the emission was assigned to be derived from a LMCT origin.714 In 1992, Ford, Vogler, and co-workers reported the structural and photophysical studies of another hexanuclear copper(I) BK


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cluster, [Cu6(mtc)6] (mtc = di-n-propylmonothiocarbamate) (Figure 140).715 The preparation of this copper(I) cluster was

Figure 142. Structure of [Cu4(μ4-dppm)4(μ4-S)]2+ (phenyl rings omitted for clarity).

yellow-orange to orange emission in both the solid-state and the fluid solutions, which was assigned to an origin containing a large amount of ligand-to-metal charge transfer [LMCT (E2− → Cu4)] character, mixed with metal-cluster-centered transitions.722−725 Ab initio, semiempirical Fenske−Hall and TDDFT calculations have been performed to support this assignment.686,699,726 The strong reducing power of the excited states of these tetranuclear copper(I) complexes has also been demonstrated by oxidative quenching studies with pyridinium acceptors.722−725 Extension of the work on copper(I) chalcogenide complexes to the preparation of a series of dinuclear chalcogenolate complexes, [Cu2(μ-dppm)2(μ-EAr)]BF4 (EAr = SePh, SeC6H4Cl-p, TePh, TeC6H4Me-p), has also been made.727 These dinuclear complexes, with short Cu(I)··· Cu(I) distances of 2.723−2.739 Å, were found to emit in the low-energy region at 600−630 nm, originating from a predominantly metal-centered ds/dp excited state, mixed with some MLCT [Cu → E/dppm] character.727 In another study, Yam and Lo used various bridging diimine ligands to react with [Cu(PPh3)2(MeCN)2]BF4 to prepare a series of luminescent dinuclear copper(I) complexes, [{Cu(PPh3)2}2L]2+ (L = 2,3-bis(2-pyridyl)pyrazine, 6,7-dimethyl2,3-bis(2-pyridyl)quinoxaline, 2,3-bis(2-pyridyl)quinoxaline, 6,7-dichloro-2,3-bis(2-pyridyl)quinoxaline, 2,3-bis(2-pyridyl(benzo[g]quinoxaline).728 These complexes were found to show dual luminescence at 400−500 and 600−750 nm upon excitation at λ > 350 nm. The higher-energy emission band was assigned to be intraligand in nature, while the lower-energy band was suggested to have likely originated from a triplet MLCT state.728 In a related work, the same group reported a crown-ether moiety to prepare a copper(I) phosphine complex, [Cu(PPh 3 ) 2 (L)]BF 4 (L = N-(2-pyridinylmethylene)2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopenta-oxacyclodecin-16-ylamine), which was found to show similar dual emission properties.729 Upon binding of alkali metal and alkaline earth metal cations, the emission intensity would be enhanced. The excited-state and photoluminescence properties of the dinuclear copper(I) complex, [Cu2(C6H5NNNC6H5)2], were investigated by Harvey using extended Hückel molecular orbital (EHMO) calculations and various spectroscopic techniques.730 The crystal structure of the complex displayed a short Cu(I)··· Cu(I) distance of 2.45(2) Å.731 It was observed that the complex exhibited fluorescence at 77 K with a structured emission band at around 600 nm. An emission origin of π−π*/ MLCT parentage was suggested.730 Fackler and co-workers revisited the tetranuclear copper(I) thiolate cluster, [Cu(S2P(OiPr)2)]4,732 and prepared two related clusters with the same ligand, [Cu8{S2P(OiPr)2}6(μ8S)] and [Cu6{S2P(OEt)2}6·2H2O].733 The tetranuclear cluster, [Cu(S2P(OiPr)2)]4, was observed to exhibit luminescence thermochroism. In the solid state at room temperature, the complex was found to emit at 547 nm. At low temperature of

Figure 140. Structure of [Cu6(mtc)6].

previously reported in the literature in 1970.716 In the photophysical studies, a low-energy d → s emission with some mixing of LMCT character modified by Cu(I)···Cu(I) interactions was observed. Similar emission properties were also observed for the silver(I) analogue, [Ag6(mtc)6].715 A class of trinuclear copper(I) halide complexes, [Cu3(μ3dppp)2(MeCN)2(μ-X)2]+ (X = Cl, I), was studied by Che and co-workers.717 The chloride and iodide complexes were found to emit at 530 and 560 nm, respectively, upon photoexcitation. The origin of the emission was assigned to be derived from a metal-centered 3d94s1 exited state modified by cuprophilic interactions and/or mixing with the dppp ligand.717 The same group also synthesized a series of copper(I) complexes, [Cu2(μdppm)2L2]2+, by reacting [Cu2(μ-dppm)2(MeCN)4]2+ (dppm = bis(diphenylphosphino)methane) with substituted pyridines or triphenylphosphine (L).718 The precursor complex was reported in a previous work by Gimeno and co-workers.719 The preparation of [Cu2(μ-dppm)2(MeCN)n]X2 (n = 2 or 4) and [Cu2(μ-dppm)3]X2 (X = BF4 or ClO4, dppm = Ph2PCH2PPh2) was reported, and the crystal structure of [Cu 2 (μdppm)2(MeCN)4](ClO4)2 showed a long Cu(I)···Cu(I) separation of 3.757(3) Å in the complex cation.719 Similarly, the copper(I) complexes reported by the group of Che, [Cu2(μ-dppm)2(py)2]2+ (py = substituted pyridines), were also found to exhibit long Cu(I)···Cu(I) distances of over 3.7 Å.718 These complexes were also found to show long-lived emissive excited states in solutions at ambient temeperature.718 The supramolecular chemistry of copper(I)−sulfur complexes was pioneered by Fackler in the late 1960s.720 A notable example would be the octanuclear copper(I) dithiolate cubic cluster, [Cu8(i-MNT)6]4− (i-MNT = 1,1-dicyanoethylene-2,2dithiol, S2CC(CN)22−) (Figure 141).720,721 Yet, the photo-

Figure 141. Structure of [Cu8(i-MNT)6]4−.

physical properties of these complexes remained rather underexplored. In 1993, a class of luminescent tetranuclear copper(I) complexes with an unsubstituted μ4-chalcogenide ligand, [Cu4(μ4-L)4(μ4-E)]2+ (L = dppm, E = S, Se; L = dtpm, E = S), was reported in the literature by the group of Yam, representing the study on the luminescence of copper(I) chalcogenide clusters.722−725 The structure of [Cu4(μ4dppm)4(μ4-S)]2+ is shown in Figure 142. These copper(I) complexes were generally found to exhibit an intense long-lived BL


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polymer was observed to emit at about 612 nm in the solid state at 20 K, and the emission originated from a MLCT [Cu → π*(pyrazine)] excited state.739 A series of organometallic copper(I) polymers, [{Cu(dmb)2Y}n] (dmb = 1,8-diisocyano-p-menthane; Y = BF4, NO3, ClO4), have also been studied.740 The complex, [{Cu(dmb)2BF4}n], was found to emit at 490 nm in the solid state at 77 K. According to density functional theory calculations, the HOMO was characterized by metal-centered d orbitals, whereas the LUMO was π*(isocyanide) in character. The emission has therefore been assigned to a MLCT [Cu → π*(dmb)] origin.740 Maverick and co-workers reported the tetrameric copper(I) cluster, [CuN(SiMe3)2]4.741 The complex was observed to show an approximately square planar Cu4N4 core with the copper(I) centers at the centers of the sides and the nitrogen atoms at the corners as shown in Figure 145, and the Cu(I)··· Cu(I) distances were in the range of 2.6770(7)−2.6937(7) Å.741 Upon photoexcitation, the tetranuclear copper(I) cluster was shown to give blue-green phosphorescence in the solid state and dichloromethane solution at ambient temperature, and in the solid state at low temperature. Such luminescence was suggested to be associated with a metal-centered excited state.741 In addition, although the copper(I) cluster exhibited low volatility, it could be employed as a precursor for chemical vapor deposition (CVD) of copper metal.741 Fujisawa, Moro-oka, and co-workers reported a pentanuclear aliphatic thiolato copper(I) complex, [NEt4][Cu5(SAd)6] (SAd = adamantine thiolate anion), together with a related mononuclear two-coordinate complex, [NEt4][Cu(SAd)2].742 As shown in Figure 146, the five copper(I) centers in [NEt4][Cu5(SAd)6] were arranged into a trigonal bipyramidal structure. The mono- and pentanuclear complexes were found to emit at 600 and 618 nm, respectively, in the solid state at 140 K, and the emission bands were tentatively suggested to be associated with spin-forbidden transitions.742 Goher and Mak studied a series of copper(I) thiocyanato complexes, [Cu(NCS)L]n (L = methyl nicotinate, ethyl nicotinate) and [HL][Cu(NCS)2] (HL = H-ethyl isonicotinate).743 The crystal structures of the complexes were found to differ from each other, in which [Cu(NCS)(methyl nicotinate)]n was found to form a Cu2S2 cyclic core while [Cu(NCS)(ethyl nicotinate)]n exhibited a CuS2N2 structure (Figure 147), and [HL][Cu(NCS)2] featured a network of ethyl isonicotinate cations and polymeric [Cu(NCS)2]− anions (Figure 148).743 In the solid state at ambient temperature, the [Cu(NCS)L]n complexes were found to emit at 498 and 499 nm, assignable to a MLCT origin, whereas [HL][Cu(NCS)2] exhibited a redshifted emission at 575 nm, which was suggested to have originated from an excited state of substantial XLCT character.743 On the other hand, long-lived low-energy LMCT [E → Cun] emission bands have been observed in acetone solution for a series of trinuclear and hexanuclear copper(I) complexes, such as [Cu3(μ-dppm)3(μ3-SR)2]BF4 (Figure 149) and [Cu6(μP∧P)4(μ3-SePh)4]BF4 (P∧P = dppm, (Ph2P)2NH).744 A related luminescent tetranuclear copper(I) phosphide cluster, [Cu4(μdppm)4(μ4-PPh)](BF4)2, which is isoelectronic to the [Cu4(μdppm)4(μ4-E)]2+ complexes,722 was also reported and crystallographically characterized, in which the four copper(I) centers were arranged in a rectangular array with the bridging dppm ligands in a saddle-like fashion.745 The complex was observed to be emissive in the red region from a [P(phosphide) → Cu4]

77 K, the complex would show dual luminescence with emission maxima at 573 and 647 nm.733 Harvey and co-worker studied the photophysical properties of a trinuclear copper(I) cluster, [Cu3(μ-dppm)3(μ3-OH)]2+,734 the synthesis of which was previously reported in the literature.735 The cluster was observed to emit at 540 and 480 nm in ethanol at room and low temperatures, respectively. Density functional theory calculations were performed to probe the nature of the lowest energy excited states. The Cu−Cu bond length was found to be reduced significantly in the excited state as compared to that in the ground state.734 In addition, the emission band was found to be quenched by acetate and 4aminobenzoate ions.734 The group of Harvey later reported another dinuclear complex, [Cu2(μ-dppm)2(μ-O2CCH3)]+.736 The complex was only emissive at 77 K. According to molecular orbital calculations, the lowest energy excited state was MLCT [copper → phenylphosphine/acetate] in nature.736 A hexanuclear complex, [(CuPPh3)6L2] (H3L = trithiocyanuric acid), was synthesized by Che and co-workers.737 The structure of the complex featured two parallel triazine moieties connected by six Cu−S bridges. The hexanuclear complex was found to emit at 562 nm in the solid state and 580 nm in dichloromethane solution at ambient temperature. The emission was suggested to be derived from an excited state associated with MLCT [Cu → π*(L)] and/or intraligand [π → π*(L)] transitions of the triazine stack.737 A series of tetranuclear copper(I) cluster with tetradentate N2S2 and bidentate N,S Schiff base ligands were reported by Ford and co-workers.738 The crystal structure of one of the complexes, [Cu4(L)2] (L = N,N′-(2,2′-diphenyl)-bis(1,3diphenyl-4-iminomethyl-5-thiopyrazole), was determined, and the Cu(I)···Cu(I) contacts were found to range from 2.77 to 2.96 Å in the structure (Figure 143).738 All of the complexes

Figure 143. Crystal structure of [Cu4(L)2]. Reprinted with permission from ref 738. Copyright 1996 Elsevier B.V.

were found to show a broad emission band upon excitation at 355 nm in the solid state and in solutions at low and room temperatures. The emission origin was assigned to be derived from a cluster-centered mixed state of ds/dp and LMCT triplet characters.738 In 1997, Zink and co-workers prepared a copper(I) polymer, [{(Ph3P)2Cu2(μ-Cl)2(μ-pyrazine)}∞].739 The crystal structure consisted of dimeric copper(I) moieties that were linked together by pyrazine ligands (Figure 144). The Cu(I)···Cu(I) distance of 3.059(1) Å indicated that no significant metallophilic interactions were present in the structure.739 The BM


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Figure 144. Crystal structure of the copper(I) polymer showing three repeating units. Reprinted with permission from ref 739. Copyright 1997 American Chemical Society.

Figure 148. Structure of [Cu(SCN)(ethyl nicotinate)]n. Reprinted with permission from ref 743. Copyright 2000 Elsevier B.V.

Figure 145. Crystal structure of [CuN(SiMe3)2]4 with thermal ellipsoids at the 50% probability level. Dashed line shows crystallographically imposed 2-fold axis. Reprinted with permission from ref 741. Copyright 1998 American Chemical Society.

Figure 149. Structure of [Cu3(μ-dppm)3(μ3-SR)2]+.

[Cu4(μ-dppm)4(μ4-E)]2+ in the order of S > Se > PPh has been ascribed to the lower energy of the filled ligand-based orbitals for sulfide than selenide and phosphide.722,745 In another work, a series of dinuclear copper(I) phosphine complexes, [Cu2(dcpm)2]X2 (dcpm = bis(dicyclohexylphosphinyl)methane, X = ClO4, PF6, BF4, I), [Cu2(dcpm)2(MeCN)2]X2 (X = ClO4, PF6), [Cu2(dmpm)3](ClO4)2 (dmpm = bis(dimethylphosphino)methane), and [(CuI)n(dcpm)2] (n = 2, 4), have been synthesized and found to show photoluminescence properties that were sensitive to the extent of Cu(I)···Cu(I) interactions, which were in turn affected by the interactions between the copper(I) centers with neighboring anions or solvent molecules.746−748 An increase in the interactions between neighboring solvent molecules or anions with the Cu(I) centers in the Cu(I)··· Cu(I) bonded excited state in the crystal lattice would result in lower-energy photoluminescence due to exciplex formation.747 A stronger interaction with the copper(I) centers in the Cu(I)···Cu(I) bonded excited state would result in a lowerenergy photoluminescence, and such photophysical assignment has also been supported by Raman spectroscopic studies.746 For example, the complex [Cu2(dcpm)2](ClO4)2 was observed

Figure 146. Crystal structure of [Cu5(SAd)6]−. Reprinted with permission from ref 742. Copyright 1998 American Chemical Society.

Figure 147. Structure of [Cu(SCN)(methyl nicotinate)]n. Reprinted with permission from ref 743. Copyright 2000 Elsevier B.V.

LMCT excited state mixed with metal-centered character.745 The red shift of the emission energy relative to those of BN


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slow cooling would shift the emission to the yellow region at 615 nm. Both the red and the yellow emissions could be assigned to a triplet metal-centered excited state modified by Cu(I)···Cu(I) interactions.752 Remarkably, the concept of “thermally rewritable phosphorescent paper” was demonstrated by fabricating a polymer-supported coating of the dichroic copper(I) complex onto a piece of polyethylene terephthalate (PET) paper.752 A phosphorescent trinuclear copper(I) pyrazolato complex, [{3,5-(CF3)2pz}3Cu3] (pz = pyrazolate) (Figure 151), was

to show a single low-frequency mode and its overtone for the Cu(I)···Cu(I) stretch. The excited-state Cu(I)···Cu(I) vibration was estimated to occur at 150 cm−1, which was greater than the ground-state value of 104 cm−1, suggestive of stronger Cu(I)··· Cu(I) interactions in the excited state, and the excited-state distortion mainly occurred at the Cu(I)···Cu(I) bond.746 The trinuclear copper(I) complex, [Cu(3,5-Ph2pz)]3 (3,5Ph2pz = 3,5-diphenylpyrazolate), was first reported by Raptis and Fackler.749 The complex was found to exist as a nonplanar metallacycle with a nine-membered Cu3N6 ring with an average Cu(I)···Cu(I) distance of 3.339(1) Å in the structure.749 In the presence of larger substituents on the pyrazolate group, tetrameric species such as [{3,5-(tBu)2pz}4Cu4] and [{3(iPr),5-(tBu)2pz}4Cu4] could also be obtained.750 However, the self-assembly and luminescence properties of this class of complexes were not extensively studied at that time. Through incorporation of dendritic units into the pyrazolate ligands, Aida and co-workers reported a series of trinuclear metallacycles, [M{Cn}Lmpz]3 (M = Cu(I), Ag(I), Au(I); n = 1; m = 2, 3, 4).751 The complexes were found to form columnar stacks with a uniformly sized and highly symmetric structure according to the size-exclusion chromatography (SEC) profiles. When a hot paraffin suspension of the lower-generation complexes (m = 2) was cooled to room temperature, the complexes would self-assemble to form highly luminescent superhelical fibers, while the higher-generation complexes (m = 3, 4) were found to give glassy aggregates.751 In particular, the superhelical fibers of the trinuclear copper(I) complex, [Cu{C1}L2pz]3 (Figure 150), were observed to show an

Figure 151. Structure of [{3,5-(CF3)2pz}3Cu3].

reported by Dias and Omary and co-workers in 2003.753 The crystal structure of the trinuclear complex was found to show weak intramolecular Cu(I)···Cu(I) interactions (3.221−3.242 Å), while no intermolecular Cu(I)···Cu(I) interactions were found between adjacent molecules (3.879−3.893 Å).753 The complex was found to show emission properties that were sensitive to temperature, solvent composition, and concentration, as well as excitation wavelength. In the solid state at ambient temperature, the trinuclear copper(I) complex exhibited orange luminescence. Upon cooling, the emission was shifted to the red region, and an orange band eventually reappeared at 77 K. The orange band at 77 K could be assigned as a mixture of a red emission peak at 665 nm and a yellow shoulder at 590 nm. At higher temperature, the yellow shoulder disappeared and its band broadening at room temperature gave rise to an orange emission.753 On the other hand, the emission of the copper(I) complex in solutions was found to be significantly enhanced at low temperature, associated with Stokes’ shifts of over 18 000 cm−1. It was suggested that the thermochromism was attributed to the contraction of intermolecular Cu(I)···Cu(I) distances in the excited state that would lead to huge distortions, in agreement with the very large Stokes shifts.753 Work has also been extended to the synthesis and photophysical studies of trinuclear copper(I) complexes of other pyrazolates with different substituents.754 Crystallographic studies showed that the derivatives with fluorinated pyrazolates such as [{3-(CF3)pz}3Cu3] and [{3(CF3),5-(Ph)pz}3Cu3] would form trimers that packed in zigzag chains, while the nonfluorinated complexes such as [{3,5-(Me)2pz}3Cu3] and [{3,5-(iPr)2pz}3Cu3] would give dimers of trimers with much shorter intertrimer Cu(I)··· Cu(I) distances.754 Upon photoexcitation, all of the complexes were found to exhibit intense metal-centered phosphorescence.754 The same group also studied a related three-coordinate dinuclear copper(I) complex, [{3,5-(CF3)2Pz}Cu(2,4,6-collidine)]2.755 The complex displayed a planar six-membered Cu(μ-N−N)2Cu ring in its crystal structure (Figure 152) with an average Cu(I)···Cu(I) distance of 3.3940(9) Å.755 Photoexcitation of the complex at room or low temperatures would lead to blue phosphorescenece, which was assigned as mainly derived from a triplet π−π* excited state of the pyrazole ligand.755

Figure 150. Structure of [Cu{C1}L2pz]3.

intense orange emission band at 605 nm upon excitation of the dendritic moiety at 280 nm, involving some energy transfer from the dendritic wedge as an antenna to the trinuclear copper(I) core. Upon dissociation of the fibers in dichloromethane solutions to give discrete metallacycles, the orange emission would disappear with a concomitant growth of the fluorescence from the dendritic moiety at 305 nm.751 Extension of the work to trinuclear copper(I) complexes with long alkyl chains on the dendritic units resulted in the preparation of [Cu{Cn}Lmpz]3 (n = 12, 18; m = 2, 3; Lm = number (m) of dendritic layers; pz = pyrazolate), which could self-assemble to form columnar assemblies via intermolecular Cu(I)···Cu(I) interactions.752 One of the complexes, [Cu{C18}L2pz]3, was found to show dichroic luminescence at room temperature. A hot melt of the complex was found to emit at 650 nm, and such red emission could be preserved upon natural cooling, while BO


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dcpm)2(CN)2] (dcpm = bis(dicyclohexylphosphino)methane), [{Cu(dcpe)} 2 (CN)(μ-dcpe)]PF 6 (dcpe = 1,2-bis(dicyclohexylphosphino)ethane), [{Cu(dppp)} 3 (CN)2 (μdppp)]PF6 (dppp = 1,3-bis(diphenylphosphino)propane), and [Cu(CN)(PCy3)]6.759 The crystal structures of the complexes have been determined, and the structures of [{Cu(dppp)}3(CN)2(μ-dppp)]PF6 and [Cu(CN)(PCy3)]6 are, respectively, shown in Figures 155 and 156. The polymeric

Figure 152. Crystal structure of [{3,5-(CF3)2Pz}Cu(2,4,6-collidine)]2 (hydrogen atoms are omitted for clarity). Reprinted with permission from ref 755. Copyright 2003 American Chemical Society.

Tsubomura and co-workers reported a dinuclear copper(I) complex bridged by a diphosphine ligand, [Cu2(dmp)2(dppe)(MeCN)2]PF6 (dmp = 2,9-dimethyl-1,10-phenanthroline, dppe = bis(diphenylphosphino)ethane) (Figure 153).756 The com-

Figure 155. Crystal structure of [{Cu(dppp)}3(CN)2(μ-dppp)]PF6 with carbon atoms drawn with small circles for clarity. Reprinted with permission from ref 759. Copyright 2005 American Chemical Society.

+

Figure 153. Structure of [Cu2(dmp)2(dppe)(MeCN)2] .

plex was observed to exhibit an emission band at 550 nm in the solid state, which was assigned to a MLCT origin.756 The same group then extended the work to the use of simple diphosphine ligands and prepared a series of mononuclear copper(I) complexes, [Cu(Ph2P(CH2)nPPh2)(dmp)]PF6.757 Remarkably, a dinuclear complex, [Cu2(dppb)2(dmp)2](PF6)2 (dppb = Ph2P(CH2)4PPh2), was obtained when dppb was used as the diphosphine ligand. The dinuclear complex was found to emit intensely at 590 nm, and a triplet MLCT origin was suggested.757 The group of Tsubomura also prepared a dinuclear copper(I) complex with the 2,5-bis(2-pyridyl)pyrazine (2,5bppz) ligand, [{Cu(PPh3) 2}2 (μ-2,5-bppz)](PF6 )2·2CHCl3 (Figure 154).758 The copper(I) centers were shown to adopt

Figure 156. Crystal structure of [Cu(CN)(PCy3)]6. Reprinted with permission from ref 759. Copyright 2005 American Chemical Society.

complex existed as one-dimensional zigzag chains in the crystal structure, while the other polynuclear complexes were arranged to form macrocyclic ring structures. The dinuclear complex, [Cu2(μ-dcpm)2(CN)2], was found to emit at 470 nm in the solid state at room temperature, which was red-shifted from the emission of its polymeric precursor, [{(Cy3P)Cu(CN)}3]∞. In alcoholic glass at 77 K, all of the copper(I) complexes were found to exhibit intense emission at 382−416 nm, assignable to a triplet [3d → (4s,4p)] excited state.759 In 2005, the first structural determination by X-ray diffraction studies of a series of polymeric copper(I) complexes, [CuC CR]n, was reported by Che and co-workers.760 Although the first report of this class of copper(I) polymer could be dated back to the late 1950s,761 the full characterization of such complexes remained scarce, and a rare example of such was the

Figure 154. Structure of [{Cu(PPh3)2}2(μ-2,5-bppz)]2+.

a distorted tetrahedral geometry in the crystal structure. The dinuclear copper(I) complex was found to emit at 700 nm in the solid state, which was ascribed to a MLCT excited state.758 Further reaction of the dinuclear complex with ruthenium(II) species has also led to the preparation of heteronuclear complexes.758 Lai, Che, and co-workers reported the synthesis of a series of copper(I) complexes with phosphine and cyanide ligands.759 These include the polymeric copper(I) complex, [{(Cy3P)Cu(CN)} 3 ] ∞ , and the polynuclear complexes, [Cu 2 (μBP


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report of [(tBuCCCu)24] by Weiss and co-workers.762 In Che’s work, the polymeric copper(I) complexes were found to exhibit different packing patterns upon variation of the substituents on the alkynyl moiety. In particular, [(CuC CtBu)n]·C6H6 (Figure 157) was shown to give an unusual Cu20

excited state from the [Cu2(μ-X)2] core to 1,8-nap with the support of molecular orbital calculations.764 Since 2005, various highly emissive dinuclear copper(I) complexes have been reported by Peters and co-workers. In contrast to most previously reported copper(I) complexes, the amide-bridged dinuclear complex, [Cu(PNP)]2 (PNP = bis(2(diisobutylphosphino)phenyl)amide), was found to exhibit a high photoluminescence quantum yield of over 0.65 and an emission lifetime of about 10 μs.765 The complex was observed to display a diamond structure with a Cu2N2 core, with a short intramolecular Cu(I)···Cu(I) distance of 2.6245(8) Å.765 The rich luminescence properties, together with the exhibition of reversible redox couples of the complex, suggested them to be good candidates as photoreductants or photosensitizers. Subsequent works by the same group demonstrated the potentials of this class of self-assembled dinuclear copper(I) complexes as emitters in organic light-emitting diodes (OLEDs). In particular, the tert-butyl analogue, [Cu(PNP-tBu)]2, was found to exhibit an E-type delayed fluorescence mechanism, giving rise to emission from a triplet and a singlet excited state.766 The complex was employed in the fabrication of vapor-deposited OLED devices as the emissive layer, and a maximum external quantum efficiency of 16.1% has been obtained, demonstrating an important approach to harvest triplet excitons for highly efficient OLEDs.766 The group of Kim reported the synthesis of the copper(I) dithioether-containing coordination polymer, [Cu3I3L]n (L = 1,4-bis((cyclohexylthio)acetyl)piperazine).767 The three-dimensional coordination polymer was found to exhibit a staircase polymer chain structure that was made up of “threerunged ladder” Cu3I3 structural motifs (Figure 159). A short intramolecular Cu(I)···Cu(I) contact of 2.6175(9) Å was observed, while the intermolecular Cu(I)···Cu(I) distance of 3.1686(17) Å was suggestive of little metallophilic interactions.767 The complex was found to show a broad emission band at 546 nm, which was assigned to a mixture of iodide-to-

Figure 157. Crystal structure of the Cu20 molecular cluster, [(CuC CtBu)n]·C6H6. All hydrogen atoms are omitted for clarity. Reprinted with permission from ref 760. Copyright 2005 Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim.

catenane cluster structure with various types of CCtBu → Cu coordination modes. The Cu(I)···Cu(I) distances in the structure were found to vary from 2.498(1) to 3.482(1) Å.760 On the other hand, an extended sheet structure with discrete zigzag Cu4 subunits connected through bridging alkynyl groups was observed for [(CuCCnPr)n], whereas [(CuCCPh)n] was found to give an infinite chain structure with extended Cu(I)···Cu(I) ladder. The polymeric copper(I) complexes were found to show different emission properties in the solid state due to their different packing arrangements. The emission origins were, however, all suggested to have originated from the triplet π → π* excited states of the alkynyl unit.760 Tsuge and co-workers prepared a series of copper(I) halides with N-heteroaromatic ligands, [Cu2(μ-X)2(PPh3)(L)n] (X = Br, I; L = various N-aromatic ligands; n = 1, 2).763 These complexes were found to emit from red (450 nm) to blue (707 nm) in the solid state at room and low temperatures.763 In the crystal structure, the complexes consisted of planar [Cu2(μX)2] units with rather long Cu(I)···Cu(I) distances of 2.872− 3.303 Å.763 The group later incorporated 1,8-naphthyridine (1,8-nap) ligand into the copper(I) center to prepare two derivatives, [Cu2(μ-X)2(1,8-nap)(PPh3)2] (X = I, Br) (Figure 158).764 In contrast to the other analogous compounds, the 1,8nap derivatives were shown to adopt a butterfly like shape with short Cu(I)···Cu(I) contacts of 2.613(5)−2.6271(4) Å.764 Red phosphorescence was observed in their solid-state emission spectra, and it has been assigned to a triplet charge-transfer

Figure 159. Crystal structure of a [Cu3I3L3] unit: (a) arbitrary view with selected atomic labeling; and (b) side view of (a) showing the “three-runged ladder” Cu3I3 in nearly horizontal lines. Reprinted with permission from ref 767. Copyright 2007 Elsevier B.V.

Figure 158. Structure of [Cu2(μ-X)2(1,8-nap)(PPh3)2]. BQ


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Figure 160. (a) Crystal structure of [(CuCN)20(pip)7] (hydrogen atoms have been omitted for clarity); and (b) projection down crystallographic a axis. Atom identities for all ball and stick projections: black spheres = Cu atoms, gray spheres = cyano C/N atoms; wire ffame = pip ligand. Reprinted with permission from ref 776. Copyright 2008 American Chemical Society.

and 545 nm, and the lowest energy excited state was assigned to an intraligand π−π* state of the binap moiety.773 The dinuclear complex, [Cu(binap)I]2, was found to exhibit a single emission band at 582 nm, which was assigned to originate from an iodide-to-binap LLCT state.773 The group later reported the photophysical studies of the copper(I) quinap (1-(2-diphenylphosphino-1-naphthyl) derivatives, [Cu(quinap)2]PF6 and [Cu(quinap)I]2.775 In this case, the complexes were observed to be phosphorescent only in the solid state, and the emission origin was suggested to be MLCT in character.775 Using CuCN and amine ligands as building blocks, Pike, Patterson, and co-workers synthesized a series of copper(I) complexes that could self-assemble into network structures in aqueous solutions.776 The crystal structure of [(CuCN)20(pip)7] (pip = piperazine) is shown in Figure 160 as a representative example. Most of the copper(I) cyanide complexes were emissive at room temperature, exhibiting a blue emission band in the 450 nm region, which originated from a MLCT origin.776 Subsequent work by the group involved the preparation of luminescent metal−organic networks from the reactions between CuSCN with substituted pyridines, and these complexes were found to exhibit MLCT emissions in the yellow to green regions.777 Using N,N′-bis(5,7-dimethyl-1,8-naphthyridine-2-yl)amine and phosphine ligands, Che and co-workers reported two tetranuclear copper(I) complexes that could precipitate as crystalline quasi-2D sheet-like nanostructures.778 Short intramolecular Cu(I)···Cu(I) contacts of 2.6107 and 2.8223 Å were observed in the presence of the less bulky dppm ligand.778 The amorphous particles of one of the complexes were found to show an orange emission at 600 nm. Upon transition to crystalline nanosheets, the emission maximum was blue-shifted to 560 nm in the greenish yellow region with a substantial enhancement in the intensity.778 Such a change in the photophysical properties was rationalized by DFT calculations. Recent work by the group of Fackler resulted in the preparation of a hexanuclear cyclic guanidinate copper(I) cluster, [Cu6(TEhpp)4](PF6)2 (H(TEhpp) = (3,3,9,9-tetraethyl-1,5,7-triazabicyclo[4.4.0]dec-4-ene).779 As depicted in Figure 161, the six copper(I) centers were arranged in a distorted octahedral geometry in the crystal structure, with Cu(I)···Cu(I) distances ranging from 2.5404(10) to 2.9212(13) Å. Each of

copper charge transfer (XMCT) and d−s transitions by Cu(I)···Cu(I) interactions within the Cu3I3 clusters.767 After their reports of the structures and photoluminescence of copper(I) cyanide networks with various amine ligands,768−770 Pike, Patterson, and co-workers explored the ligand-dependent luminescence properties upon the reversible reaction between CuCN and a number of amines.771 While CuCN only emitted weakly at 392 nm, CuCN samples exposed to liquid or vapor amines were found to exhibit emission of different colors across the visible region. Authentic samples of [(CuCN)Ln] crystals formed by the neat reactions between CuCN and amines were determined by X-ray crystallography, and it was observed that the CuCN molecules were arranged into a chain with one or two amine ligands per copper(I) site.771 Exposure of CuCN to structurally very similar amines were found to induce similar color changes in the emission color, offering the system a potential advantage for the independent monitoring of multiple chemically related analytes.771 Further studies by Patterson’s group using copper(I) thiocyanate led to the production of CuSCN networks with different conformations.772 In general, compounds with a CuSCN·L ratio of 1:2 were arranged into zigzag or helical chains. For compounds with a CuSCN:L ratio of 1:1, ladder or sheet structures resulted from the presence of S,S,Nthiocyanate bridging.772 The [(CuSCN)L2] (L = substituted pyridine) complexes were found to show yellow-green phosphorescence at room temperature, while the complexes with aliphatic amines were nonemissive. The emissive origin was therefore suggested as derived from MLCT states associated with pyridine π* orbitals.772 In 2008, Vogler and co-workers reported the luminescence studies of the copper(I) complexes, [Cu(binap)2]PF6 and [Cu(binap)I]2 (binap = 2,2′-bis(diphenylphosphino)-1,1′binaphthyl).773 Both complexes were emissive in the solid and solution states at ambient temperature. These complexes were found to show emission behaviors that were rather different from those of related mononuclear copper(I) complexes, [Cu(binap)(1,2-diimine)], which exhibited MLCT [Cu → 1,2-diimine] emission bands without the direct involvement of the binap moieties.774 In dichloromethane solutions, [Cu(binap)2]PF6 was found to show an intense emission band at 412 nm and weaker emission bands at 510 BR


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Figure 163. General structure of [(P∧N)3Cu2I2].

showed that this class of complexes generally featured a butterfly shaped Cu2I2 core that was surrounded by three P∧N ligands. The Cu(I)···Cu(I) distances of the complexes were 2.69−2.83 Å, indicative of no significant metallophilic interactions.781 In the presence of different P∧N ligands, the powders of this series of complexes were found to exhibit photoluminescence from blue to red (λmax = 481−713 nm) across the visible spectrum upon photoexcitation. High photoluminescence quantum yields of up to 96% have been achieved.781 The emission properties of [(2(diphenylphosphino)pyridine)3Cu2I2] were studied in detail as a representative example, and it was found that the emission at room temperature could be assigned to TADF, rendering the complexes potential candidates for singlet harvesting in OLEDs.781 Kato and co-workers recently reported the halide-bridged dinuclear copper(I) complexes, [Cu2(μ-X)2(DMSO)2(PPh3)2] (X = I, Br).782 Interestingly, the iodo analogue was found to show both photochromic and vapochromic luminescence properties.782 The O-coordinated linkage isomer, Cu2I2[O,O], was observed to emit strongly at 435 nm from an excited state of mixed 3XLCT and 3MLCT characters.782 This blue emission would diminish upon irradiation of UV light, with a concomitant growth of a new green emission band at 500 nm, and the emission color would eventually turn yellowish green (λem = 540 nm) upon prolonged UV irradiation.782 Exposure to saturated DMSO vapor at 90 °C would recover the blue emission. Such observation could be attributed to the linkage isomerization of the DMSO ligand to the S-coordinated conformation, Cu2I2-[O,S] and Cu2I2-[S,S], and eventually expulsion of the DMSO ligands upon UV exposure (Figure 164).782 With the support of DFT calculations, it was suggested that the removal of the DMSO ligand would lead to a contraction of the Cu2I2 core and thus an increase in the extent of Cu(I)···Cu(I) interactions, shifting the emission origin from the 3XLCT/MLCT mixed state to the 3CC state and giving rise to the yellowish green emission.782 The groups of Fenske783 and Coucouvanis784−788 have reported a number of copper(I) clusters of high nuclearities in the literature, with the major focus mainly concentrating on the structural properties of the clusters. Recently, Fuhr, Fenske, and co-workers also investigated the photophysical properties of some copper(I) clusters and reported a series of di-, tetra-, and heptanuclear copper(I) phenylthiolate clusters.789 The crystal structure of one of the heptanuclear complexes, [Cu7(p-S− C6H4−NMe2)7(PPh3)4], is shown in Figure 165. It was observed that there were no Cu(I)···Cu(I) interactions in the dinuclear complexes. The tetranuclear complexes showed Cu(I)···Cu(I) separations of 2.727−3.048 Å, indicative of very weak to no cuprophilic interactions, while the Cu(I)··· Cu(I) interactions were stronger in the heptanuclear complexes (2.652−2.939 Å).789 In the solid state at room temperature, the

Figure 161. Crystal structure of [Cu6(TEhpp)4](PF6)2 (hydrogen atoms and anions have been omitted for clarity). Reprinted with permission from ref 779. Copyright 2012 Springer.

the copper(I) centers was connected to two TEhpp ligands in an almost linear geometry with an average N−Cu−N angle of 164.8°.779 In the solid state, the hexanuclear cluster was observed to emit at 537.5 nm. The emission origin was assigned to be of MLCT parentage, modified with some Cu(I)···Cu(I) character.779 In recent years, several classes of copper(I) complexes have been developed for use as singlet harvesting materials for the fabrication of efficient organic light-emitting diodes (OLEDs). For instance, in 2013, Wesemann, Yersin, and co-workers reported a series of halide-bridged copper(I) aminophosphine complexes, [Cu(μ-X)(PNMe2)]2 (X = Cl, Br, I; PNMe2 = Ph2P-(o-C6H4)N(CH3)2) and [Cu(μ-I)(PNpy)]2 (PNpy = Ph2P-(o-C6H4)-NC4H8) (Figure 162).780 The intramolecular

Figure 162. Structures of [Cu(μ-X)(PNMe2)]2 and [Cu(μ-I)(PNpy)]2.

Cu(I)···Cu(I) distances in the complexes were found to range from 2.559 to 2.983 Å.780 The complexes [Cu(μ-Cl)(PNMe2)]2 and [Cu(μ-Br)(PNMe2)]2 were shown to exhibit intense green emission, whereas [Cu(μ-I)(PNMe2)]2 and [Cu(μ-I)(PNpy)]2 showed intense blue luminescence in the solid state. The emission origin was suggested to be a mixture of triplet MLCT and XLCT states according to DFT calculations.780 The complexes were found to show a significant decrease in the emission decay lifetime by almost 2 orders of magnitude when the temperature was increased from about 60 to 300 K. Such decrease in the decay lifetime was suggested to be associated with the thermal population of a short-lived singlet state, and the emission mechanism was proposed to be a thermally activated delayed fluorescence (TADF) process.780 The groups of Baumann, Monkowius, Yersin, and Bräse collaborated to report a series of dinuclear copper(I) halide complexes that contained different diphenylphosphinopyridinetype P∧N ligands (Figure 163).781 X-ray diffraction studies BS


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Figure 164. Photochromic and vapochromic luminescence properties of [Cu2(μ-I)2(DMSO)2(PPh3)2]. Reprinted with permission from ref 782. Copyright 2013 American Chemical Society.

Figure 166. Crystal structures of (a) [Cu12S6(dpppt)4] and (b) [Cu12S6(dppo)4]. Reprinted with permission from ref 791. Copyright 2014 The Royal Society of Chemistry.

3.2. Light-Emitting Silver(I) Self-Assembled Materials

As compared to the related copper(I) and gold(I) congeners, the luminescence of silver(I) compounds is relatively less extensively reported in the literature.22 This is mainly because of the thermal instability and light sensitivity and thus ease of photodecomposition of a number of silver(I) containing complexes under ambient conditions. Despite the welldocumented luminescence properties of free silver(I) ions doped in a matrix of alkali metal halides,792−794 the photophysical studies of silver(I) complexes and assemblies were rather rare in the old days. In 1975, Teo and Calabrese reported the complete structural study of a series of tetrameric silver(I) complexes, [(Ph3P)4Ag4X4] (X = Cl, Br, I),795 although the synthesis of such complexes could be dated back to the 1930s.796 The tetranuclear silver(I) complexes could exist in a cubane or a chairlike conformation, with short Ag(I)···Ag(I) distances of 3.115(2)−3.769(3) Å.795 The luminescence properties of this class of complexes were revealed when Vogler and co-workers studied the photophysical properties of [Ag4Cl4L4] (L = P(OCH3)3, P(C6H5)3).797,798 In toluene solutions at 77 K, the tetranuclear silver(I) complexes were found to emit at 480−483 nm, which was assigned to a mixed d−s/LMCT excited-state origin.798 The group of Zink then reported the chair and cubane isomers of the complex, [Ag4I4(PPh3)4].799 Both forms of the complex were found to be photoluminescent, with the emission origin assigned to a metal-centered excited state modified by Ag(I)···Ag(I) interactions. Because of the more extensive delocalization over the d and s orbitals of the silver(I) centers, the cubane form was shown to emit at a longer wavelength.799 In 1992, Ford and Vogler reported the photophysical studies of the hexanuclear silver(I) complexes, [Ag6(mtc)] and [Ag6(dtc)6] (mtc = di-n-propylmonothiocarbamate, dtc = din-propyldithiocarbamate).715 The two complexes were only emissive at low temperature. The complex, [Ag6(mtc)], was found to exhibit a structureless orange emission band at 644 nm in the solid state and at 607 nm in toluene solution, assignable to have originated from the monothiocarbamate

Figure 165. Crystal structure of [Cu7(p-S−C6H4−NMe2)7(PPh3)4]. Reprinted with permission from ref 789. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim.

copper(I) clusters were found to emit at 480−560 nm. The emission bands originated from LMCT excited states, with modification by Cu(I)···Cu(I) interactions for the tetranuclear and heptanuclear complexes.789 In a related study, the same group prepared a series of copper(I) chalcogenido complexes, [Cu2(ER)2(dpppt)2] (dpppt = 1,5-bis(diphenylphosphino)pentane; E = S, R = C4H3S, C6H4OMe-p; E = Se, R = C6H5, C6H4SMe-p, C6H4NMe2-p).790 It was found that only the complexes with electron-rich thiolate ligands, [Cu 2 (SC 6 H 4 OMe-p) 2 (dpppt) 2 ] and [Cu 2 (SeC 6 H 4 NMe 2 p)2(dpppt)2], were emissive at room temperature. The complexes were found to emit at 465 and 480 nm, respectively, in the solid state at 298 K.790 Recently, the same group with Eichhöfer also reported the decanuclear copper(I) clusters, [Cu12S6(dpppt)4] (dpppt = Ph2P(CH2)5PPh2) and [Cu12S6(dppo)4] (dppo = Ph2P(CH2)8PPh2).791 As shown in the crystal structures in Figure 166, the structure of the complex mainly consisted of an octahedron of nonbonding sulfur atoms with copper(I) centers bridging the 12 edges. The phosphine ligands were found to bridge the copper(I) centers differently in the two complexes.791 Both of the complexes were shown to emit in the red region at 648−665 nm in the solid state at room temperature.791 BT


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cluster.715 On the other hand, [Ag6(dtc)6] was found to show a green emission from the dithiocarbamate cluster at 545 and 550 nm, respectively, in the solid state and in toluene. The emissive states of both complexes were tentatively assigned as a triplet cluster-centered (CC) excited state of mixed d → s and LMCT character.715 A series of trinuclear silver(I) phosphines, [Ag3(dppp)2(MeCN)2(ClO4)2]+ and [Ag3{HC(PPh2)3}2]3+, were prepared by Che and co-workers and reported to be luminescent.800,801 In particular, the crystal structure of [Ag3(dppp)2(MeCN)2(ClO4)2]+ showed short Ag(I)···Ag(I) distances of 2.943(2) and 3.014(2) Å. This complex was found to emit at 467 nm in the solid state, attributed to an excited state derived from a metal-centered dσ* → pσ transition.800 Recently, related work was done by the same group to synthesize [Ag 3 (dcmp) 2 ](ClO 4 ) 3 (Figure 167) and

crystal structure.807,808 Together with theoretical calculations by extended Hückel and ab initio calculations, it was suggested that the excited-state Ag(I)···Ag(I) interactions were significant in the formation of luminescent exciplexes in Tl[Ag(CN)2].807,808 It was subsequently observed that upon UV irradiation at low temperature, the emission intensity of Tl[Ag(CN)2] would decrease and a nonemitting species would be produced.809 The process was reversible upon heating the sample to ambient temperature to regenerate the original emitting species. A light-induced electron transfer mechanism was proposed for the photochemical reaction. The original emitting species, the [Ag(CN)2]3− trimer, was reduced to the nonemitting [Ag(CN)2]34− trimer by Tl(I).809 The reversibility of this reaction has illustrated an example of applications in optical memory. In addition to the copper(I) analogues, the group of Yam based on the synthetic procedure for copper(I) alkynyl complexes by Naldini and co-workers679 prepared and reported a series of trinuclear mono- and bicapped silver(I) alkynyl complexes, including [Ag3(P∧P)3(μ3-η1-CCR)]2+ (P∧P = dppm, R = Ph, C 6 H 4 −OCH 3 -4, C 6 H 4 −NO 2 -4; L = [(C6H5)2P]2NnPr(nPrPNP), R = Ph) (Figure 168) and

Figure 167. Structure of [Ag3(dcmp)2]3+.

[Ag3(dcmp)2Cl2]ClO4.802 Short Ag(I)···Ag(I) contacts of 2.9280(7)−3.0737(8) Å were observed in the trinuclear complexes.802 In particular, [Ag3(dcmp)2](ClO4)3 was found to emit at 375 nm from a metal-centered excited state with the complex molecules arranging into infinite silver(I) linear chains.802 Another trinuclear silver(I) phosphine complex, [Ag3(μ3-dppnt)2]3+ (dppnt = 2,7-bis(diphenylphosphino)-1,8napthyridine), was also reported.803 This complex was found to emit at 550 nm in acetonitrile at 298 K, and the emission was assigned to a ligand-centered excited-state origin.803 In 1994, Harvey and co-workers reported a series of dimeric silver(I) isocyanide complexes, [Ag2(μ-dmb)2(μ-X)2] (dmb = 1,8-diisocyano-p-menthane; X = Cl, Br, I).804 The complexes were found to be emissive at 440−480 nm both in ethanol glass and in the solid state at 77 K. The emission maxima and lifetimes were found to be sensitive to the nature of halides and the Ag(I)···Ag(I) distance in the complex. These, together with results from extended Hückel molecular orbital calculations, were suggestive of an assignment of the emission origin to a triplet [Ag2(4d)/X(p) → π*(dmb)] MLCT/XLCT excited state.804 Patterson and co-workers studied the photophysical properties of the layered complexes, Eu[Ag(CN)2]3 and Dy[Ag(CN)2]3, and then investigated the excited-state properties of [Ag(CN)2]−.805,806 Excitation of the host ion, [Ag(CN)2]−, would result in the emission of the Eu3+ or Dy3+ ions, while the emission of the anion itself was completely quenched, suggestive that a host-to-guest energy transfer process has taken place.805,806 The same group later reported the complex, Tl[Ag(CN)2], and observed the presence of Ag(I)···Ag(I) interactions associated with Ag(I)···Ag(I) distances as short as 3.11 Å.807,808 The group later studied the emission spectra of the compound as a function of temperature.807,808 The emission spectrum of single crystals of the compound at 10 K was found to show a broad emission band centered at ca. 420 nm with two excitation maxima, each of which had a different temperature dependence behavior for its corresponding emission spectra. This was consistent with the existence of two different sites with close Ag(I)···Ag(I) distances in the

Figure 168. Structure of [Ag3(P∧P)3(μ3-η1-CCR)]2+.

[Ag3(P∧P)3(μ3-η1-CC−C6H4−NO2-p)]+.810 An independent work on [Ag3L3(μ3-η1-CC−Ph)]+ was also reported in the literature.811 The crystal structures of the complexes mainly featured a triangular array of silver(I) centers bridged by the phosphino ligands, with Ag(I)···Ag(I) distances ranging from 2.8946(8) to 3.4030(6) Å, indicative of very weak to no Ag(I)···Ag(I) interactions.810 All of these silver(I) clusters were found to exhibit a vibronic-structured emission band at 428− 627 nm at ambient and low temperatures in the solid state, which could be assigned to a 3LMCT origin with some mixing of metal-centered d−s/d−p states, or, alternatively, a 3LMMCT assignment has been made.810 In addition, the group also reported a series of related bicapped silver(I) diynyl derivatives, [Ag3(μ-dppm)3(μ3-η1-CCCCR)2]PF6 (R = Ph, C6H4− OCH3-p).693 As compared to the copper(I) analogues, the complexes were found to emit at higher energies. For instance, [Ag3(μ-dppm)3(μ3-η1-CCCCPh)2]PF6 was found to emit at 479 nm, while [Cu3(μ-dppm)3(μ3-η1-CCCCPh)2]PF6 emitted at 533 nm in acetone solution.693 The emissive state was suggested to involve substantial ligand-to-metal charge transfer [RCCCC− → Cu3] LMCT character, with an involvement of a ligand-centered π → π*(diynyl) state and possibly some metal-centered state.693 A hexanuclear silver(I) alkynyl cluster, [Ag 3 (μPh2PCH2PPh2)3(μ3-η1-CCC6H4-CC-p)Ag3(μPh2PCH2PPh2)3][BF4]4, has also been reported.703 The complex was found to exhibit intense green-yellow emission at both ambient and low temperatures. A possible assignment for the origin of the emission involved emissive states derived BU


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(diphenylphosphinyl)-2,2′-bipyridine).815 As shown in Figure 170, the presence of additional iodide or bromide ions would result in the formation of a new pale yellow species with the Ag4 faces capped by the halide ions, while the addition of halide ions in the presence of acetonitrile or DMSO would yield another colorless species.815 All of the tetranuclear silver(I) clusters were found to be emissive in solution and in the solid state. The dichloromethane solution of [Ag4(P2-bpy)2](BF4)4 was shown to emit at about 475 nm at 298 K, assignable to a metal-centered excited state.815 In addition, the gas sensing properties of the halide-free complex cation, [Ag4(P2-bpy)2]4+, were demonstrated. The presence of carbon monoxide would quench the emission and the emission intensity could be completely recovered upon purging of nitrogen, whereas exposure to nitrogen monoxide would lead to irreversible quenching of the emission.815 The groups of Che and Phillips reported the spectroscopic studies of a series of dinuclear silver(I) complexes, [Ag(PCy3)(O2CCF3)]2, [Ag2(μ-dcpm)2]X2 (X = CF3SO3, PF6; dcpm = bis(dicyclohexylphosphino)methane), and [Ag2(μ-dcpm)(μO2CCF3)2].816 Crystallographic studies showed the presence of short Ag(I)···Ag(I) distances of 2.8892(9)−3.095(1) Å in the structures of all of the complexes.816 The resonance Raman spectrum of [Ag2(μ-dcpm)2](CF3SO3)2 showed that all of the Raman intensity appeared in the Ag−Ag fundamental stretching of 80 cm−1, indicative of the presence of Ag(I)··· Ag(I) interactions in the complex.816 The [Ag2(μ-dcpm)2]X2 complexes were found to be emissive at 417−420 nm in the solid state at room temperature, and such emission bands were suggested to have originated from the formation of exciplexes, associated with the weak metal−anion interactions in the complexes.816 The collaboration between Bu, Yam, and Shionoya led to the report of a series of dinuclear or polymeric silver(I) complexes with polypyridine bridging ligands, [Ag(L1)(MeCN)]2[BF4]2· 2CHCl 3 , [Ag(L 2 )(MeCN)] 2 (ClO4 ) 2 , {[Ag 2 (L 3 )(NO 3 ) 2 ]· MeCN}∞, and [Ag2(L3)(NO3)2]∞ (L1 = 2,3-di-2-pyridylqui-

from a triplet alkynyl-to-metal LMCT parentage mixed with a metal-centered state with large intraligand π−π* character.703 Yam and co-workers also reported the first series of silver(I) chalcogenide clusters of the structure, [Ag4(μ-dppm)4(μ4-E)]2+ (E = S, Se, Te) (Figure 169).812 The silver(I) centers in the

Figure 169. Structure of [Ag4(μ-dppm)4(μ4-E)]2+ (E = S, Se, Te) (phenyl rings on P have been omitted for clarity).

tetranuclear cluster were quadruply bridged by a chalcogenide atom with short Ag(I)···Ag(I) distances of 3.038(2)−3.357(1) Å.812 The silver(I) chalcogenide clusters were found to show long-lived green to orange emission in acetonitrile solution and in the solid state. Given the σ-donating nature of the chalcogenide ligands and with the support of theoretical studies, the emission was assigned to have originated predominantly from a LMCT [(E2−) → Ag4] excited state, probably with mixing of a metal-centered (d−s/d−p) Ag(I) state.812−814 A class of polymeric silver(I) isocyanide complexes, [{Ag(dmb)2Y}n] (Y = BF4, PF6, NO3, CH3CO2, ClO4), was reported.740 The complexes were observed to emit at 467−492 nm in the solid state and at 435−502 nm in ethanol glass at low temperature. A polyexponential emission decay lifetime was also observed. With the support of molecular orbital calculations, the emission was assigned to be derived from a MLCT [Ag → π*(dmb)] excited state.740 In 1999, Catalano and co-workers reported a tetranuclear complex, [Ag 4 (P 2 -bpy) 2 ](BF 4 ) 4 (P 2 -bpy = 6,6-bis-

Figure 170. Structure of [Ag4(P2-bpy)2](BF4)4 and its reactions with halides and coordinating solvents. BV


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Figure 171. (Top) Crystal structure of a dinuclear box segment of {[Ag2(L3)(NO3)2]·MeCN}∞ and (bottom) its one-dimensional zigzag chain structure. Reprinted with permission from ref 817. Copyright 2001 American Chemical Society.

noxaline, L2 = 2,3-di-2-pyridyl-5,8-dimethoxyquinoxaline, L3 = 2,3,7,8-tetrakis(2-pyridyl)-pyrazino[2,3-g]quinoxaline).817 The dinuclear complexes exhibit similar structures with the two Ag(I) centers of each complex being spanned by two ligands to form a box-like cyclic dimer, whereas the polymeric complexes consisted of a one-dimensional zigzag chain formed by the replication of cyclic box-like dinuclear units (Figure 171).817 The complexes were found to be emissive in glass and in the solid state at low temperature, showing emission bands originated from intraligand excited states. Only [Ag(L2)(MeCN)]2(ClO4)2 was emissive at room temperature because of the electron-donating effect of the methoxy substituent in the ligand.817 In 2002, a class of luminescent hexanuclear silver(I) chalcogenolate complexes, [Ag6(μ-dppm)4(μ3-EAr)4](PF6)2 (EAr = SPh, SC 6 H 4 Me-p, SePh, SeC 6 H 4 Cl-p), were reported.818 In the crystal structures of [Ag6(μ-dppm)4(μ3SC6H4Me-p)4](PF6)2 and [Ag6(μ-dppm)4(μ3-SeC6H4Cl-p)4](PF6)2, the Ag4E4 (E = S or Se) core unit was arranged in a distorted cubane structure, and the remaining two silver(I) centers were capped onto the opposite faces of the cubane, each of them being linked to two bridging dppm ligands.818 Weak argentophilic interactions associated with short Ag(I)··· Ag(I) distances of 3.3289−3.5184 Å were observed. Although the complexes were nonemissive at room temperature, photoexcitation at 77 K resulted in an intense orange-red emission at 666−746 nm. The emission was tentatively assigned to be derived from excited states with an admixture of MMLCT and MC (ds/dp) character.818 The photoluminescence properties of a series of tetranuclear silver(I) alkynyl complexes, [Ag 4 {μ-(CC) x C 6 H 4 Rp}4(PCy3)y] (x = 1, y = 2, R = H, CH3, OCH3, CCPh; x = 2, y = 4, R = H), were reported by Che and co-workers.819 The crystal structures of [Ag4{μ-CCC6H4R-p}4(PCy3)2] generally exhibited a planar Ag4 core arranged in a parallelogram, while the structure of [Ag4{μ-(CC)2C6H5-p}4(PCy3)4] (Figure 172) consisted of two interpenetrating tetrahedrons that are made up of four silver(I) centers and four alkynyl moieties to form a twisted cubane structure (Figure 173).819 In the solid state at ambient and low temperatures, the complexes

Figure 172. Crystal structure of [Ag4{μ-CCC6H5}4(PCy3)2]. Reprinted with permission from ref 819. Copyright 2002 American Chemical Society.

Figure 173. Crystal structure of [Ag4{μ-(CC)2C6H5-p}4(PCy3)4]. Reprinted with permission from ref 819. Copyright 2002 American Chemical Society.

BW


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tetrafluoroborate and hexafluorophosphate were used as the anion. The OTf and DOS derivatives were found to exhibit a structureless emission band at 460 and 486 nm, respectively, in the mesophase upon excitation at 335 nm. The large Stokes shifts and short lifetimes of the emission band suggested an origin from a distorted state, probably an excimeric excited state arising from Ag(I)···Ag(I) interactions in the columnar structure.822 Zheng, Coppens, and co-workers observed that the otherwise unstable dimer, [{Ag(NH3)2}2]2+, could be isolated in the presence of hydrogen bonding in supramolecular frameworks to give rise to stable compounds of the formulas [{Ag(NH3)2}2][(H2thpe)2]·4.25H2O and [{Ag(NH3)2}-H2O-{Ag(NH3)2}]-[(H2-thpe)2]·benzene (H3thpe = tris(hydroxyphenyl)ethane).823 The crystals of the complexes were found to emit at 530 and 560 nm, respectively, at 90 K, tentatively assigned to emissive states from molecular aggregation.823 In another study, the synthesis of a series of silver(I) bispyrazolato complexes, ranging from a mononuclear complex to hexanuclear clusters and a one-dimensional polymer, has been reported.824 The structures of the polynuclear complexes are shown in Figure 176. Both ligand-supported (2.874(1)− 3.333(2) Å) and unsupported (3.040(1) Å) Ag(I)···Ag(I) interactions were observed in the silver(I) complexes.824 The oligomeric and polymeric species were found to be emissive in the solid state, with the emission origins assignable to intraligand (IL) and MLCT (d−π) transitions.824 Zhang, Harvey, and co-workers reacted [Ag2(PhPPy2)2(NCCH3)2](ClO4)2 (PhPPy2 = bis(2-pyridyl)phenylphosphine) with ammonium chloride to obtain a onedimensional polymer, [{Ag2(PhPPy2)2Cl}(ClO4)]n, which was of the type [MMX]n.825 From X-ray crystallography, it was found that the polymer was composed of a linear array of dinuclear Ag2(PhPPy2)2 units linked by bridging Cl anions (Figure 177). Short Ag(I)···Ag(I) distances of 3.0942(11) Å were observed in the polymeric structure.825 The Ag(I) polymer was found to exhibit a broad structureless emission band with a maximum at 520 nm in the solid state at room temperature. With the support of DFT and TDDFT calculations, the emission origin was assigned to a mixture of MLCT and XLCT excited states.825 Quite recently in 2011, Omary and co-workers prepared the silver(I) ion pair, [(3,5-(CF 3 ) 2 Pz)(AgL) 2 ] + [Ag 5 (3,5(CF3)2Pz)6(MeCN)]− (L = 2-(N,N-diethylanilino-4-yl)-4,6bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine), by reacting the cyclic trinuclear complex, [(3,5-(CF3)2Pz)Ag]3, with the triazine ligand.826 The head-to-tail stacking of the cation resulted in the formation of a one-dimensional columnar structure that was held together by π−π stacking, Ag(I)···Ag(I), and Ag(I)···π interactions. The crystals of the complex showed dual emission, in which a higher-energy green emission band at 525 nm dominated at lower temperatures and a lower-energy orange emission band at 600 nm dominated at higher temperatures. It was suggested that the green emission was intraligand in nature, while the orange phosphorescence was associated with the aggregated stacks of the coordinated ligand in the extended structure.826 The collaborative work between the groups of Zang and Gao resulted in the preparation of a high-nuclearity silver(I) thiolate cluster with nest-like structure, [Ag33S3(StBu)16(CF3COO)9(NO3)(MeCN)2](NO3), while the use of NO3− bridging ligands would lead to the formation

were generally found to exhibit emission that was characteristic of the triplet intraligand π−π* excited state of the alkynyl moieties.819 Catalano and Malwitz reported the preparation of a trinuclear N-heterocyclic carbene-containing silver(I) complex, [(μ-NHC)3Ag3](BF4)3.820 In the X-ray crystal structure (Figure 174), the Ag(I) centers were observed to be arranged in an

Figure 174. Crystal structure of [(μ-NHC)3Ag3](BF4)3. Reprinted with permission from ref 820. Copyright 2003 American Chemical Society.

almost equilateral fashion with short Ag(I)···Ag(I) distances of 2.7249(10)−2.7718(9) Å.820 The acetonitrile solution of the trinuclear silver(I) complex was found to emit at 435 nm. As the NHC ligand was found to show a similar emission band at 450 nm, the emission of the trinuclear species was tentatively assigned to an intraligand origin, although the possibility of a metal-centered or MLCT state could not be ruled out.820 Liquid crystal properties of silver(I) complexes have also been reported in the literature,821 but the corresponding luminescence studies mainly commenced in the past decade. In 2006, a series of silver(I) bipyridine complexes containing the chiral S-(−)-β-citronellyl group (Figure 175) were reported to

Figure 175. Structure of the chiral S-(−)-β-citronellyl-containing silver(I) complexes.

show liquid crystal behaviors dependent on the nature of counter-anions.822 The liquid crystalline silver(I) complexes were investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), and temperaturedependent powder X-ray diffraction (XRD). It was found that in the presence of triflate (OTf) or dodecylsulfate (DOS) anions, the silver(I) complexes would form a columnar hexagonal mesophase at room temperature, which was associated with a columnar helical supramolecular structure.822 On the other hand, no liquid crystals would form when BX


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Figure 176. Structures of the polynuclear silver(I) bispyrazolato complexes.

and 617 nm in chloroform solution and in the solid state, respectively. The origin of the emission could tentatively be assigned to a LMCT [S2− → Ag33] excited state with mixing of the metal-centered excited state.827 The complex was found to show reversible thermochromic luminescence properties, in which the solid-state emission of the complex shifted from yellow to orange upon cooling to 77 K, accompanied by an increase in the emission intensity.827 A similar thermochromic phenomenon was also observed for the Ag31 cluster. The thermochromic luminescence properties could be explained by the change in the coordination mode of the NO3− ligands in the cluster at reduced temperature, which in turn resulted in shorter Ag(I)···Ag(I) and Ag−S distances.827 Such a structure− property relationship was found to be in accordance with the results from variable-temperature single-crystal X-ray diffraction (VT-SCXRD) studies.

Figure 177. Crystal structure of [{Ag2(PhPPy2)2Cl}(ClO4)]n. Reprinted with permission from ref 825. Copyright 2010 American Chemical Society.

3.3. Light-Emitting Gold(I) Self-Assembled Materials

of another nest-like cluster, [Ag31S3(StBu)16(NO3)9]n (Figure 178).827 The crystal structure of the Ag33 cluster was observed to be stabilized by Ag(I)···Ag(I) interactions (2.869(6)− 3.357(2) Å).827 The Ag33 cluster was found be strongly emissive at room temperature, with emission maxima at 583

Because of the more pronounced metallophilic interactions in gold(I) complexes, which was named “aurophilicity” by Schmidbaur,828,829 extensive works have been directed toward the self-assemblies of the gold(I) system.22 When two gold metal centers were arranged in close proximity with a separation of ca. 2.70−3.30 Å, a distance shorter than the sum of two van der Waals’ radii (ca. 3.40 Å), closed-shell Au(I)···Au(I) interactions with a strength comparable to that of hydrogen bonding (5−15 kcal mol−1) would occur.828,829 Aurophilic interactions have resulted in a variety of assemblies of discrete gold compounds, and, in particular, gold clusters with high nuclearities. The presence of aurophilic interactions has also been supported by theoretical studies. For instance, in 1991, Pyykkö and Zhao reported the ab initio calculations on the model dimer, [(ClAuPH3)2].830 The results suggested that the Au(I)···Au(I) interactions were caused by electroncorrelation effects rather than hybridization, and were also strengthened by relativistic effects.830 Omitting the 6p or the 5d functions of the gold(I) centers from the basis set would, respectively, result in the disappearance of a large portion or the entire Au(I)···Au(I) interaction, suggestive of the involvement of the metal orbitals in the aurophilic interactions.831 In most

Figure 178. Side view of the discrete nest-like silver thiolate cluster of [Ag33S3(StBu)16(CF3COO)9(NO3)(MeCN)2](NO3). Reprinted with permission from ref 827. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim. BY


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cases, the presence of aurophilic interactions was found to be significantly influential on the photophysical properties of the gold(I) complexes. This would result in striking luminescence properties that rendered the gold(I) complexes potential candidates for supramolecular and optoelectronic applications. After the first observation of luminescence in gold(I) complexes by Dori and co-workers in 1970,642 extensive research interest has been attracted toward the studies of the photophysical properties of gold compounds. 3.3.1. Low-Dimensional Gold(I) Complexes. Gold(I) chalcogenides represented one important class of luminescent gold(I) clusters. In the crystal structure of the gold(I) thiolato complex reported by Zanazzi and co-workers, [Au(dta)4] (dta = dithioacetate) (Figure 179), the four gold(I) centers were

Figure 180. Structure of [Au2(PPh3)2(i-MNT)].

the complex was found to emit at 525 nm with biexponenial decay.842 The related complex, [(nBu4N)2{Au(i-MNT)}2] (Figure 181), was also reported and characterized by X-ray

Figure 181. Structure of [{Au(i-MNT)}2]2−.

crystallography.842 In the crystal structure, the sulfur atoms were coordinated to the gold(I) centers in a linear manner, and a very short Au(I)···Au(I) distance of 2.796(1) Å was observed in the structure. This complex was similarly observed to show a broad emission at 510 nm with biexponential decay under the same conditions.842 The group of Zink later revisited the emission properties of [Au2(PPh3)2(i-MNT)].843 At 20 K, the crystals of the complex were observed to show a vibronicstructured band with progressional spacings in both a 1410 cm−1 ν(CC) stretching mode of the dithiolate ligand and a 480 cm−1 mode that involved gold−dithiolate stretching. With the help of Raman spectroscopy and theoretical studies using the time-dependent theory of electronic spectroscopy, the charge transfer nature of the emission bands was established. The analogous complex, [(AuAsPh3)2(i-MNT)], was found to show a red shift in the emission spectrum, which allowed the assignment of the emission origin as derived from a dithiolateto-gold LMCT state.843 A similar assignment of the emission origin to the LMCT state has also been made for a related trinuclear complex, [Au3(i-MNT)2(PEt3)2]−.844 Rogers, Mason, and co-workers reported the synthesis and spectroscopic studies of the dinuclear gold(I) complex cation, [Au2(dmpm)2]2+ (dmpm = Me2PCH2PMe2).845 In the crystal structure of the perchlorate and bromide salts, short aurophilic separations of 3.028(2) and 3.023(1) Å were, respectively, observed. The Au(I)···Au(I) distances were observed to be even shorter when dmpe (Me2PCH2CH2PMe2) was used as the bridging phosphine ligand.845 Electronic absorption and magnetic circular dichroism studies showed the presence of Au2-localized dσ* → pσ transitions and several higher-energy AuP2-localized transitions.845 Meanwhile, the independent works by Fackler846 and Che847,848 demonstrated the photophysical properties of the cationic gold(I) phosphine compound, [Au2(dppm)2]2+ (Figure 182), and found that the long-lived yellow emission of the complex was likely to come from a Au(I)···Au(I) bonded emissive excited state. Such observation was in line with the short Au(I)···Au(I) contact of 2.931(1) Å in the crystal structure of [Au2(μ-dppm)2](BF4)2, suggestive of the presence of some aurophilic interactions.849 A

Figure 179. Structure of [Au(dta)4].

arranged to form a rhombus in which the dta ligands were arranged alternately above and below the Au4 plane, with an average Au(I)···Au(I) distance of 3.013 Å.832,833 Vogler and Kunkely later studied the photophysical properties of [Au(dta)4] and another tetrameric gold(I) complex, [Au(pip)Cl]4 (pip = piperidine).834 Both complexes exhibited a weak to moderately intense emission band at 700−743 nm in ethanol glass at 77 K, and the emission origin could be assigned to a metal-centered excited state. The group of Patterson investigated the luminescence properties of various dicyanoaurate(I) salts, including Cs[Au(CN)2]835 and K[Au(CN)2].836 The crystal structures of these metal salts, M[Au(CN)2], were found to exhibit a layered structure in which layers of [Au(CN)2]− anions were found to alternate with layers of M+ cations.837 The short Au(I)···Au(I) distances between adjacent M[Au(CN)2] layers would give rise to emission bands attributable to aurophilic interactions. The microcrystalline samples of Cs[Au(CN)2] and K[Au(CN)2] were found to show an aurophilicity-modified emission band at 458 and 390 nm, respectively.835,836 Nagle and co-workers further investigated the effects of a magnetic field on the emission properties of Cs[Au(CN)2] at low temperatures from 2 to 20 K.838 They observed that the 458 nm emission band showed a reduced decay lifetime at 4 K. At a magnetic field strength of 1.0 T, a blue shift was observed, and this emission band at 416 nm (τ < 10 ns) was assigned to be fluorescent in nature.838 Later, Yersin collaborated with Patterson and others to further explore the photophysical properties of related compounds, including Eu[Au(CN)2]3·3H2O, Tl[Au(CN)2], and Tb[Au(CN)2]3·3H2O.839−841 It was observed that for Eu[Au(CN)2]3·3H2O and Tl[Au(CN)2], a relatively small reduction in the aurophilic distance under increased pressure would cause a large reduction in the band gap energy and hence a significant red shift of the lowest-energy electronic transitions.839,840 In 1988, Fackler and co-workers reported the luminescent dimeric gold(I) thiolate complex, [Au2(PPh3)2(i-MNT)] (iMNT = S2C2(CN)22−) (Figure 180).842 In acetonitrile at 77 K,

Figure 182. Structure of [Au2(dppm)2]2+. BZ


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related complex, [Au 2 (dmpm) 3 ] 2 + (dmpm = bis(dimethylphosphino)methane), was found to exhibit roomtemperature emission at a similar energy but with a small quantum yield, tentatively assigned to one of the spin−orbit states of the dδ*pσ triplet.850,851 Yam and co-workers then reported the cationic complexes, the two-coordinate [Au3(dmmp)2]3+ (Figure 183) and the three-coordinate

Figure 183. Structure of [Au3(dmmp)2]3+.

[Au3(dmmp)3]3+, and observed that the extent of Au(I)··· Au(I) interactions would affect the photoluminescence properties of the complexes.852,853 The two-coordinate species, [Au3(dmmp)2]3+, was found to exhibit dual emission in acetonitrile solution, in which the higher energy emission originated from an IL excited state, and the lower-energy emission was assigned to a 3[(dδ*)1(pσ)1] excited state.852 Systematic studies extending from Au2 to Au3 in both [Au2(P∧P)2]2+ and [Au2(P∧P)3]2+ to [Au3(P∧P∧P)2]3+ and [Au3(P∧P∧P)3]3+ have been performed.853,854 The long-lived triplet excited state of [Au3(dmmp)2]3+ was also demonstrated to be effective in the photolytic cleavage of plasmid pBR322 DNA.854 With an additional dmmp ligand, the three-coordinate species, [Au3(dmmp)3]3+, was found to show emission at higher energies than its two-coordinate counterpart, due to the larger repulsion and thus weaker aurophilic interactions between the Au(I) centers.853 Che and co-workers reported the preparation of the dinuclear gold(I) isocyanide complexes, [Au2(dmb)(CN)2] (dmb = 1,8-diisocyano-p-menthane) and [Au2(tmb)(CN)2] (tmb = 2,5-diisocyano-2,5-dimethylhexane).855,856 Crystallographic studies revealed the presence of a short intramolecular Au(I)···Au(I) distance of 3.536 and 3.21 Å in the complexes, respectively.855 Upon photoexcitation, [Au2(dmb)(CN)2] was found to emit at about 468 nm in degassed dichloromethane.855 Later, the same group reported the dinuclear phenylethynylgold(I) complexes with different alkyl and aryl isocyanide ligands, [{Au(CCPh)}2(L)] (L = 2,5-diisocyano2,5-dimethylhexane, 1,8-diisocyano-p-menthane).857 The intermolecular Au(I)···Au(I) distances were, respectively, found to be 3.565(2) and 3.485(3) Å, suggestive of negligible aurophilic interactions in the gold(I) isocyanide complexes.857 The complexes were found to exhibit long-lived emission in fluid solutions at room temperature, which has been assigned to emissive states of 3π−π* origin.857 The group also studied the emission properties of a series of [Au2(dcpm)2]Y2 (dcpm = bis(dicyclohexylphosphinyl)methane, Y = ClO4, PF6, OTf, [Au(CN)2], Cl, I).858−860 Remarkably, it was found that the emission of the 3dσ*pσ excited state was observed in the nearUV region, while the luminescence of the complexes in the visible region at around 500 nm was believed to originate from exciplexes.858,859 The self-assembly of a dinuclear gold(I) complex, [Au2(tmb)Cl2] (tmb = 2,5-dimethyl-2,5-diisocyanohexane), was reported by Michel, Harvey, and co-workers.861 As shown in Figure 184, the molecular structure of the complex

Figure 184. (Top) Crystal structure of [Au2(tmb)Cl2]. Hydrogen atoms have been omitted for clarity. (Bottom) Stereoview of the crystal packing. Reprinted with permission from ref 861. Copyright 1991 American Chemical Society.

showed an anti configuration without intramolecular Au(I)··· Au(I) interactions. Interestingly, the molecular packing revealed a multi-one-dimensional network with short intermolecular Au(I)···Au(I) contacts of 3.3063(3) Å.861 This complex was found to exhibit a phosphorescence band at 417 nm in the solid state upon photoexcitation.861 Gray and co-workers investigated the photophysical properties of [Au2(μ-dcpe)2]2+ and [(dcpe)2Au2(μ-dcpe)]2+ (dcpe = 1,2-bis(dicyclohexylphosphino)ethane).862,863 The X-ray crystal structure of [Au2(μ-dcpe)2]2+ showed a short Au(I)···Au(I) distance of 2.936 Å, which indicated the presence of some aurophilic interactions. For [(dcpe)2Au2(μ-dcpe)]2+, the two gold(I) centers were separated by 7.0501 Å as they were bridged by an extended dcpe ligand to give two isolated AuP3 distorted trigonal planar units.862 The complex [Au2(μdcpe)2]2+ was found to emit at 489 nm in the solid state at 77 K, while [(dcpe)2Au2(μ-dcpe)]2+ exhibited an emission at 501 nm in the solid state and at 508 nm in acetonitrile solution at 298 K. The latter complex was observed to show a large Stokes shift of ca. 7342 cm−1, which was suggestive of a large excited-state distortion. This was mainly due to the strengthening of the Au−P bond and thus the contraction of the AuP3 core in the excited state upon depopulation of the dσ* orbitals.862 Che and co-workers also reported a trinuclear gold(I) phosphine complex, [Au3{HC(PPh2)3}2Cl]2+.801 In the crystal structure, the three gold(I) centers were arranged in a nearly equilateral triangle with M−M−M angles close to 60° and short Au(I)···Au(I) distances of 2.9220(8)−3.0889(8) Å.801 The degassed acetonitrile solution of the complex was found to give an emission band centered at ca. 537 nm with a lifetime of 11 μs. The observation of such a relatively long-lived excited state suggested that the emission probably arose from a triplet state that originated from the transition between the 6p and 5d orbitals.801 CA


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The groups of Che and Mak later reported the preparation of the tetranuclear complex, [Au4(dpmp)2(SCN)2]2+, and the trinuclear complex, [Au3(dpmp)2]3+.864 The crystal structures showed that the complexes were arranged into nearly linear Au4 and Au3 chains in the presence of weak intramolecular Au(I)··· Au(I) interactions.864 Both complexes were found to be emissive at room temperature and a red shift in the dσ* → p σ transition energy from [Au 2 (dppm) 2 ] 2 + 8 4 7 to [Au3(dpmp)2]3+ to [Au4(dpmp)2(SCN)2]2+ was observed, which could be rationalized by the bonding interactions between adjacent AuP2 moieties.864 In another work, a dinuclear three coordinate gold(I) complex without Au(I)···Au(I) interactions, [Au 2 (μdpppy)3]2+ (dpppy = 2,6-bis(diphenylphosphine)pyridine), was studied.865 A long Au(I)···Au(I) contact of 4.866 Å was displayed in the crystal structure of the complex. The complex was found to exhibit an intense emission band at 520 nm in solution at 298 K, tentatively assigned to an intraligand origin.865 Gold(I) complexes with linear isocyanide and alkynyl ligands have been reported to form luminescent complexes stabilized by intra- and/or intermolecular aurophilic interactions.866−870 Earlier examples include the gold(I) complexes by Che and coworkers, [Au2(dmb)(CN)2] (dmb = 1,8-diisocyano-p-menthane), 866 [N(PPh 3 ) 2 ][Au(CCPh) 2 ], [Au(PPh 3 )(C CPh)],867 and [{Au(CCPh)}2(μ-dppe)],866 which were found to be emissive at 420−468 nm in dichloromethane solution. A trinuclear complex, [Au3(μ2-dppm)2(CCPh)2][Au(CCPh)2], was also reported to be luminescent in the solid state and in solution.868 In addition, the crystal structure of the complex cation revealed that the gold(I) centers were arranged in an isosceles triangle with intramolecular Au(I)··· Au(I) distances of 3.083(2) and 3.167(2) Å.868 The collaborative work between the groups of Mingos and Yam resulted in a series of dinuclear gold(I) complexes containing an alkynyl bridge, [{(Ph) n (Np) 3−n P}Au(CC)Au{P(Ph)n(Np)3−n}] (n = 0−3).869 With more naphthyl substituents, a red shift in the emission band was observed. A related compound, [(Me3P)AuCC-2,5-C6H2R2-CCAu(PMe)3] (R = H, CH3) (Figure 185), was reported by

Figure 186. Structures of [Au2(P∧P)(SR)2].

same group via extended X-ray absorption fine structure (EXAFS) experiments on related gold(I) complexes suggested that the presence of Au(I)···Au(I) interactions did not necessarily cause a large perturbation in the luminescence properties of the complexes.872 A pentanuclear gold(I) thiolate complex, [Au5(μ-L)3(μdppm)2]2+ (L = quinoline-2-thiolate), was reported.873 The Au(I)···Au(I) distances in the crystal structure of the complex were observed to be 2.936(3)−3.351(3) Å.873 In acetonitrile solution, the pentanuclear complex was found to exhibit dual emission properties with emission bands at 500 and 605 nm. The higher-energy emission originated from either a ligandcentered or a MLCT [Au → dppm/π*(L)] triplet state. The lower-energy emission was ascribed to a metal-centered Au(ds/ dp) triplet state, probably with some mixing of LMCT and ligand-centered character.873 In 1996, Yam and co-workers reported the synthesis and characterization of the tetranuclear complexes, [Au4(tppb)(CCR)4], and the related dinuclear complexes, [Au2(dppb)(CCR)2] (R = C6H13, Ph, 4-MeO-Ph); tppb = 1,2,4,5tetrakis(diphenylphosphino)benzene; dppb = 1,4-bis(diphenylphosphino)benzene) (Figure 187).874 In particular,

Figure 187. Structures of [Au4(tppb)(CCR)4] and [Au2(dppb)(CCR)2].

the crystal structures of [Au 2 (dppb)(CCPh) 2 ] and [Au4(tppb)(CCPh)4] were determined. Although no short Au(I)···Au(I) contact was observed in the dinuclear complex, the crystal structure of the tetranuclear complex was found to exhibit extensive aurophilic interactions associated with a short Au(I)···Au(I) distance of 3.1541 (4) Å between adjacent Au(I) units.874 Remarkably, the aurophilic interactions would bring the two adjacent Au(CCPh) moieties into close contact, leading to a crossed geometry with a P−Au−Au−P torsion angle of 75.31(6)° that rendered the molecule to be in a distorted anthracene-like structure consisting of a central benzene ring with two fused Au2PC2P six-membered rings at the two opposite sides.874 The dinuclear and tetranuclear complexes were found to be emissive in both the solid state and dichloromethane solution, and the origin of the luminescence was consistent with a σ → π*(Phbridge) excited state that was not perturbed by Au(I)···Au(I) interactions.874 The synthesis and photophysical properties of a class of luminescent dinuclear gold(I) thiolates, [Au2{Ph2PN(R)PPh2}-

Figure 185. Structure of [(Me3P)AuCC-2,5-C6H2R2−CCAu(PMe)3].

Puddephatt and co-workers after a few years.870 In the crystal structure, the molecules were packed into polymeric zigzag chains with short intermolecular Au(I)···Au(I) distances. The dichloromethane solution of the complexes would emit at ca. 415 nm from a [π → π* (alkynyl)/]/[σ(Au−P) → π*(alkynyl)] excited state, while in the solid state, the complexes would emit at ca. 540 nm from a metal-centered excited state.870 Elder, Bruce, and Bruce investigated a series of dinuclear gold(I) thiolato phosphine complexes, [Au2(P∧P)(SR)2], in 1993 (selected complexes shown in Figure 186).871 It was found that intramolecular Au(I)···Au(I) interactions were present in the cyclic derivatives or in the presence of the shortest bridging dppm ligand, associated with LMCT transitions of the lowest energies.871 Further studies by the CB


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Figure 188. Crystal structures of (a) [(8-qns)2Au(AuPPh3)2]BF4 and (b) [{(8-qns)2Au(AuPPh3)2}]2(BF4)2. Reprinted with permission from ref 881. Copyright 1991 American Chemical Society.

bis[(triphenylphosphine)gold(I)]oxonium tetrafluoroborate,879 Che and co-workers prepared the related gold(I) thiolate complexes, [Au(PPh3)(8-qnS)] (8-qnS = quinoline-8-thiolate) and [{Au(PPh3)}2(8-qnS)]BF4.880 The crystal structure of the dinuclear complex showed two independent units with short intramolecular Au(I)···Au(I) distances of 2.991(2)−3.081(2) Å and a Au−S−Au angle of 80.6(1)°.880 In dichloromethane solution, both complexes exhibited a weak emission at 460− 477 nm, while the dinuclear complex showed an additional lowenergy emission at 640 nm, assignable to a metal-centered 5d(dσ*) → 6p(pσ) excited state, with mixing of some S → Au charge transfer character.880 In polar solvents such as acetonitrile, quenching of the low-energy emission band was observed, which has been rationalized by the elimination of aurophilic interactions from an isomerization process that occurred at high solvent polarity.880 In 2003, Tzeng and coworkers further explored the structure and photophysical properties of the trinuclear complex, [(8-qns)2Au(AuPPh3)2]BF4 (Figure 188a).881 This trinuclear complex was observed to show intramolecular Au(I)···Au(I) contacts of 3.0526(3) and 3.0952(4) Å. Further dimerization of the complex would result in the formation of a hexanuclear complex, [{(8-qns)2Au(AuPPh3)2}]2(BF4)2 (Figure 188b), with a short intermolecular aurophilic distance of 3.1135(3) Å and intramolecular aurophilic contacts of 3.0526(3)−3.1135(3) Å.881 The complex [(8-qns)2Au(AuPPh3)2]BF4 was found to emit at 587 nm in the solid state, which was assigned to originate from a S → Au charge-transfer excited state modified by metal-centered 5d(dσ*) → 6p(pσ) character.881 A series of polynuclear halogold(I) complexes, [Au2(μdppm)X2] (X = Cl, I), [Au3(μ3-tppm)X3] (tppm = tris(diphenylphosphino)methane; X = Cl, I), and [Au3(μ3dppp)X 2 ] + (dppp = bis(diphenylphosphinomethyl)phenylphosphine; X = Cl, I), were reported by Che and coworkers.882 The crystal structures of [Au3(μ3-dppp)X2]+ displayed short intramolecular Au(I)···Au(I) distances of 2.946(3)−3.136(1) Å.882 The complexes were observed to show dual luminescence behavior at low temperature. The high-energy emission of the complexes at 480−530 nm in the solid state was assigned to a mixed state involving both intraligand and MLCT [Au → π*(phosphine)] character. For the low-energy emission at 570−700 nm, it was suggested to be originated from a 3MMLCT [Au → π*(phosphine)] state, with increasing 3LLCT [halide → π*(phosphine)] character when

(SR′)2] (R = C6H11, R′ = C6H4F-p, C6H4Cl-p, C6H4Me-p; R = Ph, R′ = C6H4Me-p; R = iPr, R′ = C6H4Me-p), were reported by the group of Yam.875 In the X-ray crystal structure of [Au2{Ph2PN(C6H11)PPh2}(SC6H4F-p)2], a highly unsymmetrical geometry with negligible to no aurophilic interactions was observed, possibly due to the steric requirements of the ligands. The complexes were found to emit in the blue-green region in the solid state and in fluid solution at ambient temperature, assignable to a metal-perturbed ligand-centered emission.875 Upon cooling to 77 K, the complexes were found to exhibit a higher-energy band in the blue-green region as well as a lowerenergy band in the orange region, suggestive of two closely lying excited states. The lower-energy emission band was tentatively assigned to emissive states derived from thiolate-togold LMCT transitions.875 The tetranuclear gold(I) complexes, [Au4X4(dptact)] (dptact = 1,4,8,11-tetra(diphenylphosphinomethyl)-1,4,8,11-tetraazacyclotetradecane; X = Cl, Br, I), were also reported.876 The crystal structure of [Au4Cl4(dptact)] revealed that the tetranuclear moieties were held together by aurophilic interactions to form a two-dimensional polymer with the shortest intermolecular Au(I)···Au(I) distance being 3.104(1) Å.876 The complexes were found to be dual emissive at 77 K. The high-energy emission at 470−530 nm originated from an intraligand/ MLCT [Au → π*(phosphine)] state, whereas the low-energy emission at 600−700 nm was ascribed to a 3LMCT [X → Au] state mixed with a metal-centered 3MC(ds/dp) state.876 Balch, Tinti, and co-workers studied the structural and photophysical properties of the gold(I) complexes, [{(Me2PhP)AuX}n] (X = Cl, Br, I; n = 2, 3).877,878 The presence of short Au(I)···Au(I) distances of 3.091(2)− 3.230(2) Å was indicative of a certain degree of aurophilic interactions in the complexes.877 The complexes were found to exhibit two emission bands in the solid state at low temperature, including a higher-energy structured emission band at about 360 nm and a lower-energy structureless band at 630−730 nm.878 The higher-energy emission band was assigned to a 3π−π* state localized on the phenyl moiety, whereas the lower-energy emission band was suggested to be attributed to a metal-centered excited state with increasing halide-to-gold LMCT character upon going from the chloro to the iodo complexes.878 Following the report of the ligand scrambling phenomenon upon addition of [Au(PPh3)]+ to a solution of (8-quinolinyl)CC


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going from chloride to iodide. The emission assignment was also supported by EHMO calculations.882 A class of annular dinuclear gold(I) diphosphine dithiolate complexes, [Au2(P∧P)(S∧S)] (P∧P = dmpm, dmpe, dppm, dppe; S∧S = dtc (S2CNEt2−), i-mnt (S2C2(CN)22−)), were reported by Lin, Wang, and co-workers.883,884 It was observed from the crystal structure that the two gold(I) centers were bridged by a diphosphine ligand on one side and a dithiolate on the other. Short intramolecular aurophilic interactions were observed in the crystal structures. However, the complexes were found to be different in their molecular packing, some of which were found to give an extended polymeric structure, while some were found to be monomeric in nature.883,884 The complexes were found to be luminescent. In general, the complexes were found to exhibit 1MC emissions in acetonitrile solution at room temperature, while in low-temperature glass, thiolate-to-gold 3LMCT emissions were observed.883 Earlier works on gold(I) complexes have provided important guiding principles for the design of self-assembly materials from a structural and spectroscopic perspective. Subsequent works in the literature over the past few decades have illustrated how one could judiciously design transition metal complex molecules to give self-assemblies with desirable properties.5,612 Mills and co-workers reported the preparation of a dinuclear gold(I) complex, [Au2{μ-(Ph2P)2N−nC18H37}2I2], and its use in the fabrication of thin film oxygen sensors.885 The complex exhibited an eight-membered ring structure as shown in Figure 189. The dinuclear gold(I) complex was found to exhibit an

Figure 191. Structure of [Au(S2CN(C5H11)2)]2.

appeared as bright orange microcrystals and exhibited intense luminescence at 631 nm upon excitation at 366 nm. However, the dried form of the complex appeared colorless and became nonemissive. It would become orange and emissive again upon exposure to polar aprotic solvent vapors, such as dichloromethane and acetone.886 The single-crystal structures of both forms of the complex were determined. The orange DMSOsolvated form of the complex was found to self-assemble into an infinite chain of dimeric molecules along the c axis, with short intermolecular Au(I)···Au(I) distances of 2.9617(7) Å and intramolecular Au(I)···Au(I) distances of 2.7690(7) Å.886 On the other hand, the dimers in the colorless form were found to have the shortest intermolecular Au(I)···Au(I) separation of 8.135 Å, despite a short intramolecular Au(I)···Au(I) distance of 2.7653(3) Å within the dimers.886 In 1998, the group of Yam reported the crown ethercontaining gold(I) phosphine complexes, [Au2(P∧P)(S− B15C5)2] and [Au2(P∧P)(S−B18C6)2] (S−B15C5 = 4mercaptobenzo-15-crown-5, S−B18C6 = 4-mercaptobenzo18-crown-6; P∧P = dcpm, dppm) (Figure 192), which were

Figure 189. Structure of [Au2{μ-(Ph2P)2N−nC18H37}2I2].

Figure 192. Structures of [Au2(P∧P)(S−B15C5)2] and [Au2(P∧P)(S− B18C6)2].

emission maximum at 530 nm with a lifetime of 9.8 μs when incorporated in a polystyrene film. This emission band was readily quenched by oxygen with a Stern−Volmer constant of 5.35 × 10−3 Torr−1. The reversible change in the emission intensity of the film in an alternating atmosphere of nitrogen and oxygen is illustrated in Figure 190. In another example, Eisenberg and co-workers reported a vapochromic gold(I) dithiocarbamate complex, [Au(S2CN(C5H11)2)]2 (Figure 191).886 The solvated form of the complex

found to emit intensely in the solid state and in fluid solutions at ambient and low temperatures.887−889 The crown ether moieties in the complexes were found to bind metal cations of certain appropriate sizes. The S−B15C5 ligand in the bis-crown unit was designed on purpose to bind the large-size potassium ion in a 1:1 binding ratio.887,889 Such complexation mode should allow the potassium ion to be sandwiched between the two crown ether moieties of [Au2(P∧P)(S−B15C5)2] upon binding, such that the two gold(I) centers would be brought into close proximity to switch on the Au(I)···Au(I) interactions between them. This was supported by the appearance of a lowenergy emission at 720 nm, tentatively assigned to orginate from a ligand-to-metal−metal charge transfer (LMMCT) [RS− → Au2] excited state.887,889 This has represented the first report of chemosensing by the switching on and off of metallophilic interactions in gold(I) complexes. Similar findings have also been observed with the binding of Rb+ or Cs+ ion to the S− B18C6 bis-crown unit of [Au2(P∧P)(S−B18C6)2].888,889 The same group later synthesized a dinuclear and a tetranuclear gold(I) complex with crown ether-functionalized alkynyl and phosphine ligands.890,891 These complexes were also found to show changes in their absorption and emission spectra upon binding of alkali metal cations. In the same year, Che and Mak reported a two-coordinated looplike gold(I) complex, [Au(dpdo)]ClO4 (dpdo = 1,8-

Figure 190. Emission intensity of a [Au2{μ-(Ph2P)2N−nC18H37}2I2] film exposed to an alternating atmosphere of 100% oxygen and 100% nitrogen over a period of 1 h. Reprinted with permission from ref 885. Copyright 1997 American Chemical Society. CD


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bis(diphenylphosphino)-3,6-dioxaoctane), that could readily bind a phosphine unit to give intense luminescence.892 Addition of PPh3 to a nonemissive degassed acetonitrile solution of [Au(dpdo)]ClO4 would readily switch on an intense yellow emission band at 510 nm, which came from a three-coordinated gold(I) phosphine species.892 The binding of dppm ligands would also give a similar observation, but with a red-shifted emission band at 610 nm. This has been ascribed to the presence of bonding interactions between neighboring AuP3 moieties as shown in Figure 193.892

Figure 194. Structure of [Au(4,6-Me2pym-2-S)]2.

higher-energy emission was assigned to an intraligand or a goldto-pyrimidine charge transfer state, while the lower-energy emission was tentatively assigned to a Au2 excited state.895 A tetranuclear gold(I) metallacycle with bridging thiolate ligands, [{Au2(μ-SPh)(PPh2O)(PPh2OH)}2] (Figure 195), was

Figure 195. Structure of [{Au2(μ-SPh)(PPh2O)(PPh2OH)}2].

reported by Puddephatt and co-workers in 2000.896 The crystal structure of the complex showed that the four gold(I) centers were arranged in a distorted square with very weak to negligible intramolecular Au(I)···Au(I) interactions (3.538(1)−3.645(1) Å) and some intermolecular Au(I)···Au(I) interactions (3.0733(5)−3.0756(5) Å). The tetranuclear complex was found to emit at 420 nm in the solid state in KBr matrix at room temperature, and this emission was assigned to a LMCT [S → Au] excited state.896 Wong and co-workers reported a series of bis(alkynyl)gold(I) complexes with various bridging ligands, [LAuCCRC CAuL] (L = tertiary phosphines; R = fluorene-2,7-diyl, dihexylfluorene-2,7-diyl, 9-((ferrocenylphenylene)methylene)fluorene-2,7-diyl, fluoren-9-one-2,7-diyl, 9-(dicyanomethylene)fluorene-2,7-diyl).897 The presence of bulky phosphine ligands prevented the close approach of the two gold(I) centers, and thus there were no appreciable Au(I)···Au(I) interactions in the complexes.897 In general, the solid-state emission spectra of the complexes at 290 K featured emission bands at 567−736 nm, tentatively assigned to originate from triplet π−π* and/or σ−π* excited states.897 Harvey and co-workers prepared a series of gold(I) complexes by functionalizing the upper-rim of calix[4]arene ligands with chloro(isocyanide)gold(I) moieties.898 The resulting mononuclear and tetranuclear gold(I) complexes (Figure 196) were found to emit at about 450 nm in ethanol solutions and in the solid state at room temperature, and the

Figure 193. Crystal structure of [Au(dpdo)]+. Reprinted with permission from ref 892. Copyright 1998 The Royal Society of Chemistry.

In 1999, the groups of Ma and Che together reported the use of the dinuclear gold(I) complex, [Au2(dppm)Cl2], as the emitting layer for the fabrication of single-layer organic lightemitting diodes.893 It was observed that the emission color of the device, which originated from the triplet 3MC emission of the complex, was dependent on the deposition rate. This has been explained by the different extents of aurophilic interactions and thus different aggregation modes of the complex during the deposition processes.893 The turn-on voltage of the devices was found to be below 1 V, indicative of a low carrier injection barrier. It was suggested that the presence of inter- and intramolecular aurophilic interactions might provide new channels for the transport of carriers and therefore reduced the resistance and improved the charge mobility.893 The same group later extended their work to the preparation of electroluminescent devices based on another dinuclear gold(I) complex, [Au2(dppm)2(SO3CF3)2], as well as a tetranuclear copper(I) complex, [Cu 4 (CCPh) 4 L 2 ] (L = 1,8-bis(diphenylphosphino)-3,6-dioxaoctane).894 In 1999, Eisenberg and co-workers reported a dimeric gold(I) complex, [Au(4,6-Me2pym-2-S)]2 (4,6-Me2pym-2-S = 4,6-dimethylpyrimidinethiolate) (Figure 194).895 Intramolecular aurophilic interactions (2.7365 Å) were observed to exist in the dimeric complex. Instead of packing into an extended chain via intermolecular aurophilic interactions, the dimeric molecules were found to stack in a manner that the pyrimidine ligands in adjacent molecules overlapped with one another in the presence of π−π stacking interactions.895 The dimeric complex was found to emit at 516 nm in the solid state, and at 416 and 532 nm in solution and low-temperature glass. The

Figure 196. Structures of the calix[4]arene-containing chloro(isocyanide)gold(I) complexes. CE


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emission was assigned to π−π* phosphorescence.898 Comparison of the solution and solid-state spectra showed the absence of shifts in the phosphorescence, suggestive of the lack of appreciable aurophilic interactions in the tetranuclear gold(I) complex.898 In 2002, Fackler and co-workers reported another example of luminescent one-dimensional chains of gold(I) complexes, [(TPA)2Au][Au(CN)2] and [(TPA)AuCl] (TPA = 1,3,5triaza-7-phosphaadamantane) (Figure 197), which were stabi-

colorless polymorph was found to emit at 424 nm, while the yellow polymorph was emissive in the yellow-green region at 480 nm, and both emissions have been assigned to a metalcentered emission modified by aurophilic interactions.903 Subsequent work by the group of Balch resulted in the preparation of a series of organogold(I) isocyanides, [(RNC)AuCN] (R = Cy, nBu, iPr, Me, tBu).904 All of the complexes were found to emit at 371−430 nm in the solid state.904 These complexes could undergo self-assembly through Au(I)···Au(I) interactions to give different structures. The simplest structures were the one-dimensional infinite chains formed by [(CyNC)AuCN] and [(tBuNC)AuCN], in which the individual molecules packed about their centers of symmetry to form a zigzag chain with an Au−Au−Au angle of roughly 130°.904 In [(nBuNC)AuCN], two linear chains of the gold(I) complexes further aligned with each other via intermolecular Au(I)···Au(I) interactions to give side-by-side chains (Figure 198). For

Figure 197. Structure of [(TPA)AuCl].

lized by weak Au(I)···Au(I) interactions.899 The former complex, [(TPA)2Au][Au(CN)2], was found to exist as an extended linear chain along the c-axis with an intrachain Au(I)···Au(I) distance of 3.457(1) Å. Although the single crystals were nonemissive in nature, the powder of the complex was observed to show intense green luminescence at ambient temperature. This has been attributed to the formation of localized surface defects that comprised dimeric or oligomeric units, which would result in shorter Au(I)···Au(I) distances and accounted for the emission.899 For the latter complex, [(TPA)AuCl], it was observed to form a helical structure with an intrachain Au(I)···Au(I) distance of 3.396(2) Å and a pitch of 3.271 Å. The crystals of this complex were found to weakly emit at 580 nm at low temperature.899 The group of Eisenberg also reported a series of dinuclear gold(I) complexes, [Au2{S2P(OR)2}2] (R = Me, Et, nPr, nBu), that were observed to exhibit rich photoluminescence.900 Similar to an earlier structural report in the literature,901 the complexes were found to arrange into an extended chain structure with both intra- and intermolecular aurophilic interactions.900 The polycrystalline sample of [Au2{S2P(OMe)2}2] was found to exhibit white emission at 77 K and showed an emission maximum at 422 nm with a long emission tail extending throughout the visible region at 298 K.900 Timeresolved emission decay studies showed that such a white emission band, as well as the multiple emission of the other complexes, were contributed by a mixture of higher-energy components assignable to 1MC and 3MC states, as well as a lower-energy component from a 3LMCT state.900 Gold(I) isocyanides represented another important class of gold(I) complexes that showed rich self-assembly properties.902,903 Balch and co-workers revisited the crystal structures and the photoluminescence properties of the gold(I) complex, [(C6H11NC)2Au](PF6), that was reported by them in the 1970s.902,903 The complex was found to exist in two polymorphs, a colorless form and a yellow form. In the colorless form, aurophilic interactions were present to arrange the gold(I) centers in an extended chain with Au(I)···Au(I) distances of 3.1822(3) Å. On the other hand, every four independent molecules of the complex cation would arrange themselves to form a kinked, slightly helical chain with very short Au(I)···Au(I) distances of 2.9643(6)−2.9803(6) Å.903 When dissolved in organic solvents, the electronic absorption spectra of both polymorphs looked essentially the same and both forms were nonemissive, probably due to the disappearance of Au(I)···Au(I) interactions. In the solid state, the

Figure 198. Structure of [(nBuNC)AuCN]. Reprinted with permission from ref 904. Copyright 2003 American Chemical Society.

[(MeNC)AuCN] and [(iPrNC)AuCN], the molecules were found to arrange themselves in a multilayered sheet-like structure with Au(I) centers held together in each layer by Au(I)···Au(I) interactions.904 In the related gold(I) isocyanide halides, [(CyNC)AuX] (X = Cl, Br, I), the presence of the halo ligands would cause steric hindrance in the overall packing of the complexes and reduce the extent of Au(I)···Au(I) interactions.905 The complexes were found to phosphoresce in the orange region with a large Stokes’ shift of ca. 21 000 cm−1, suggestive of a large distortion in the structures of the excited states. Omary and co-workers observed a different kind of packing in another isocyanido gold(I) complex, [(p-tosylCH2NC)AuCl].906 The complex molecules were found to form crossed dimers with a short Au(I)···Au(I) separation of 3.0634(4) Å. The complex exhibited a high-energy emission at ca. 478 nm, which was tentatively assigned as an excimeric emission.906 On the other hand, Yam and co-workers prepared a class of luminescent gold(I) phosphine mono- and diynyl complexes with nuclearity ranging from 1 to 3.907 The structure of [(dppm)2Au3{CCC(CH2)Me}2][Au{CCC(CH2)Me}2] was determined by X-ray crystallography. The three gold(I) centers in the complex cation were arranged in a triangular array with intramolecular aurophilic interactions associated with short Au(I)···Au(I) distances of 3.1152(9)− CF


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3.1538(9) Å.907 This class of gold(I) alkynyl complexes was found to emit in the solid state and in solutions with long lifetimes at room temperature. In general, the emission energies of the complexes were found to be dependent on the nature of the alkynyl ligands, as well as the nuclearities of the complexes. In the solid-state emission spectra, the dinuclear and trinuclear complexes were found to exhibit emission bands of lower energies as compared to the mononuclear counterparts, and this has been attributed to the presence of weak aurophilic interactions in the complexes in the di- and trinuclear complexes.907 In dichloromethane solutions, it was observed that the low-energy emission disappeared in the dinuclear complexes but persisted in the trinuclear complexes. It was suggested that the intermolecular Au(I)···Au(I) interactions in the dinuclear complexes were destroyed in the solution state, rendering their emission similar to the mononuclear counterparts.907 On the other hand, the intramolecular Au(I)···Au(I) interactions in the trinuclear complexes remained intact in the solution state, thus resulting in the persistence of the lowenergy luminescence.907 Using various 2-(diphenylphosphino)-1-methylimidazole (dpim) as the ligand, Catalano and co-workers prepared the homo- and heteronuclear complexes, [Ag2(dpim)2(MeCN)2]2+, [Au2(dpim)2]2+ (Figure 199), and [AuAg(dpim)3]2+.908

interactions (2.9235(4) Å), which in turn switched on the LMMCT emission of the gold(I) complex.909 Similar luminescence tribochromism behaviors have also been observed in a class of gold(I) benzimidazolethiolate complexes.910 Nakamoto, Yanagida, and co-workers reported a series of bis(benzenethiolato)aurate(I) complexes, n Bu 4 N[Au(SC6H4R)2], that exhibited solid-state luminescence ranging from 438 to 529 nm at room temperature.911,912 As an extension of this work, the group then reported a dinuclear gold(I) complex, ( n Bu 4 N) 2 [Au 2 (S 2 -1,3-C 6 H 4 ) 2 ] (Figure 200).913 The complex has a twisted 12-membered ring

Figure 200. Structure of [Au2(S2-1,3-C6H4)2]2−.

structure with negligible intra- and intermolecular interactions.913 The complex was found to show an intense blue emission at 437 nm in acetonitrile solution, which originated from a triplet ligand-centered state of the benzenedithiolate moiety.913 In 2004, Yip and co-workers reported the self-assembly of a luminescent gold(I) molecular rectangle, [Au4(μ-PAnP)2(μbipy) 2 ](OTf) 4 (PAnP = 9,10-bis(diphenylphosphino)anthracene, bipy = 4,4′-bipyridine).914 This complex was synthesized by connecting two [Au2(μ-PAnP)(OTf)2] units with two 4,4′-bipyridine ligands. The size of the rectangular cavity was determined to be 7.921(3) × 16.76(3) Å, which was suitable for binding various aromatic molecules.914 The complex cations were found to pack in an intriguing twodimensional mosaic structure in the solid state, in which each ion was surrounded by four adjacent ions at each of its corners with edge-to-face π−π interactions. The aerated acetonitrile solution of the gold(I) rectangle was found to emit at 480 nm with a small Stokes shift, assignable to a 1π−π* origin.914 Upon binding of aromatic guests such as naphthalene and phenanthroline, the fluorescence of the complex would be quenched. The quenching mechanism was suggested to be due to either structural changes of the molecular rectangle or fast energy transfer to a nonemitting charge transfer excited state.914 On the other hand, the trinuclear gold(I) complex, [(μ3S){Au(CNC7H13)}3](SbF6), was observed to show polymorphism at different temperatures (Figure 201).915 At ambient conditions, the gold(I) centers were arranged to form an isosceles triangular array that was stabilized by aurophilic interactions. Reversible changes from an orthorhombic space group to a monoclinic one were found to occur

Figure 199. Structure of [Au2(dpim)2]2+.

Crystallographic studies revealed the presence of metal··· metal interactions in these complexes.908 The complexes were all found to be emissive in the solid state and in solution. In particular, the Au(I)−Au(I) dimer was observed to show intense mechanochromic luminescence, in which the solid-state emission would shift from blue (λmax = 483 nm) to orange (λmax = 548 nm) upon grinding.908 Such unusual luminescence of the Au(I) dimer has tentatively been ascribed to the formation of exciplexes.908 Eisenberg and co-workers reported the gold(I) complexes, [Au2(μ-TU)(μ-dppm)]Y and [Au2(μ-Me-TU)(μ-dppm)]Y (TU = 2-thiouracil; Me-TU = 6-methyl-2-thiouracil; Y = CF3COO, NO3, ClO4, Au(CN)2), which were found to exhibit luminescence tribochromism, in which the luminescence of the complex would be switched on upon grinding of a solid sample.909 The crystal structures of [Au2(μ-TU)(μ-dppm)]+ showed that the two gold(I) centers were bridged by a thiouracilate ligand on one side and a dppm ligand on the other, and the intramolecular aurophilic separation was 2.8797(4) Å. Neighboring molecules were aligned in a headto-tail conformation to give an extended arrangement of the dinuclear gold(I) complex in a helical structure with weak intermolecular aurophilic interactions (3.3321(5) Å).909 The helical form of the gold(I) complex was weakly emissive. However, grinding of a polycrystalline sample of [Au2(μTU)(μ-dppm)]Y would lead to intense bright blue or cyan luminescence at room temperature, due to the formation of an emissive dimeric form with stronger intermolecular aurophilic

Figure 201. Phase change behavior of [(μ3-S){Au(CNC7H13)}3](SbF6). CG


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Figure 202. Photoisomerization of [Au4(dppm)2(CC−L−CC)2]. Reprinted with permission from ref 923. Copyright 2007 American Chemical Society.

schlieren texture of the nematic (N) phase.918 The complexes were found to emit in the yellow-green region, with the emission origin assigned to intraligand excited states. The mesophases of the complexes were also found to be emissive in nature. Upon an increase in temperature, the complexes would transform to the smectic-C mesophase, accompanied by a decrease in emission intensity, as increasing temperature would increase the molecular mobility in the mesophase and would render quenching processes more efficient.918 The same group later incorporated crown ether moieties into the gold(I) isocyanides to prepare a series of related complexes, [Au(C6F4OCH2C6H4OCnH2n+1-p)(CNR)] (n = 4, 8, 10, 12) and [Au(C6F4OCH2C6H4-3,4,5-(OCnH2n+1)3)(CNR)] (n = 4, 8, 12).919 These fluoroaryl-containing gold(I) complexes were found to exhibit liquid crystal properties.919 All of the complexes were luminescent in the solid state and in solution at room and ambient temperatures. In addition, they were also found to emit in the mesophase and in the isotropic liquid at moderate temperatures.919 The formation of linear chains by gold(I) complexes could also be stabilized by ligand-unsupported Au(I)···Au(I) interactions. In [Au(NH3)2]X (X = ClO4, NO3), the gold(I)based cations were arranged in a staggered chain conformation with short Au(I)···Au(I) contacts of 2.990(1)−3.091(1) Å.920 The perchlorate salt, [Au(NH3)2]ClO4, was found to emit at 470−600 nm at ambient temperature, attributed to the presence of Au(I)···Au(I) interactions. The silver(I) analogues of these complexes have also been reported and observed to form similar one-dimensional chains.920 The group of Yam extended their work to the preparation and photophysical studies of a series of alkynylgold(I) bis(diphenylphosphino)alkyl- and aryl-amine complexes, [{Ph2PN(R)PPh 2}Au2(CCR′) 2] (R = nPr, R′ = Ph, C 6 H 4 OMe-p, C 6 H 4 Me-p, C 6 H 4 Me-p, C 6 H 4 Cl-p; R = C6H4OMe-p, R′ = Ph).921 The crystal structures of [{Ph2PN(nPr)PPh2}Au2(CCR′)2] (R′ = Ph, C6H4OMe-p) revealed the presence of intramolecular aurophilic interactions in the complexes (Au(I)···Au(I) = 2.8408(8)−3.0708(7) Å).921 The complexes were emissive in low temperature glass and in the

upon cooling, and this would cause a perturbation of the Au(I)···Au(I) distances between adjacent complex molecules and thus a change in the emission maxima at different temperatures. The gold(I) complex was found to emit at 667 nm at room temperature. Upon cooling to cryogenic temperature, it was found to exhibit dual emission at 490 and 680 nm.915 It was notable that such polymorphism was influenced by the nature of the counteranions. No phase change behaviors occurred when PF6− was used as the counteranion despite its similarity in structure. Patterson and co-workers also extended their work to heterometallic systems and studied the excited-state properties of mixed-metal systems with different metal ratios, such as La[Ag 0 . 78 Au 0 . 22 (CN) 2 ] 3 , La[Ag 0 .5 5 Au 0 . 45 (CN) 2 ] 3 , La[Ag0.33Au0.67(CN)2]3, and La[Ag0.19Au0.81(CN)2]3.916 It was found that all of the mixed-metal complexes were emissive at room temperature and exhibited emission bands at about 370− 420 nm, with energies between the emission spectra of the pure Ag(I) and Au(I) analogues.916 The observation of a single band maximum in the emission profile suggested that the excited state was delocalized over the Au(I) and Ag(I) centers in the mixed-metal complexes.916 With rational molecular design, the self-assembly of gold(I) complexes has been demonstrated to exhibit liquid crystalline properties.917 One of the first examples of luminescent liquid crystals based on d10 metal systems was the gold(I) isocyanide complexes prepared by Espinet and co-workers.918 In 2005, the group of Espinet reported a series of rod-like gold(I) isocyanide complexes, [Au(C6F4OCmH2m+1)(CN− C6H4C6H4OCnH2n+1)] (m = 2, n = 4, 10; m = 6, n = 10; m = 10, n = 6, 10).918 The crystal structure of [Au(C6F4OC2H5)(CNC6H4C6H4OC4H9)] revealed that aurophilic interactions were absent in the rod-like structure of this class of gold(I) complexes. Instead, intermolecular Fortho···Fmeta interactions with exceptionally short F···F distances of 2.66 Å existed to stabilize the molecular assemblies in the absence of other significant intermolecular interactions.918 All of the complexes were found to be mesomorphic. Polarized optical microscopy showed a fan-shape texture of the smectic A (SmA) phase and a CH


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Figure 203. Structure of [Au{C(OMe)NHMe}2]X.

extended linear columns with significant Au(I)···Au(I) interactions, as well as hydrogen bonding between the cations and anions. However, the chlorobenzene-solvated form was found to arrange in kinked columns, and it was likely that such disorder would cause a shift in the emission color to green.924 A related series of gold(I) carbene complexes, [Au{C(OMe)NHMe}2]X (X = CF3SO3, PF6, CF3CO2, ClO4, I) (Figure 203), was found to show similar polymorphism, in which the complexes were arranged into linear stacks with significant aurophilic interactions, accompanied by a variation of their emission colors from blue to yellow.925 The group of Balch also reported a dimeric complex, [Au2(μ-dppe)2Br2], that exhibited similar polymorphism and vapoluminescence properties, in which the dimer crystallized into orange- and green-emitting forms in the presence of different solvents.926 Ito, Sawamura, and co-workers reported the mechanochromic luminescence properties of the complex, [(C6F5Au)2(μ-1,4diisocyanobenzene)] (Figure 204).927 The gold(I) complex,

solid state. The glass luminescence was assigned to be derived from triplet states of a [π(ArCC) → π*(PNP)] LLCT character, with mixing of a [π(ArCC) → 6s/6p(Au)] LMCT character. The solid-state emission was similarly assigned, but with modifications by aurophilic interactions.921 DFT calculations were also performed to support the assignment. Luminescent phosphine gold(I) thiolate complexes, [(R3P)Au{SC(OMe)NC6H4NO2-4}] (R = Et, Cy, Ph) and [(Ph2PR-PPh2){AuSC(OMe)NC6H4NO2-4}2] (R = CH2, (CH2)2, (CH2)3, (CH2)4, Fc), were reported with full structural characterization by X-ray crystallography.922 It was observed that the gold(I) centers in the complexes were linearly coordinated by phosphorus and thiolate-sulfur with weak intramolecular Au···O interactions. Photoexcitation of the complexes at wavelengths longer than 350 nm would result in green and blue emission in the solid state and in solution, respectively. The emission was tentatively assigned to originate from a 3[n(S) → π*(C6H4NO2)] intraligand donor−acceptor charge transfer excited state, with mixing of some 3[S(3p) → Au(6s/6p)] LMCT character.922 Variations in the nature of the ancillary phosphines, as well as the presence or absence of aurophilic interactions, were found to cause negligible changes in the photophysical properties of the complexes. In addition, the ferrocene-containing derivative was found to be nonemissive under ambient conditions due to the intramolecular quenching effects of ferrocene.922 The group of Yam also reported the dinuclear and tetranuclear azobenzene-containing gold(I) alkynyl phosphine complexes, [{Au(PPh3)}2(CC−L−CC)] and [Au4(dppm)2(CC−L−CC)2] (HCC−L−CCH = 4,4′-diethynylazobenzene).923 The crystal structure of the tetranuclear complexes has been determined, and it was confirmed that the two azobenzene units in the complex were arranged in the trans conformation. Intramolecular aurophilic interactions (3.131(2)−3.221(18) Å) were also observed.923 Although the complexes were nonemissive in nature, they were observed to exhibit photoisomerizaation upon excitation into the IL π−π* transition at 360 nm, while irradiation with visible light at 486 nm would result in the reverse isomerization process.923 The introduction of Ag(I) ions into the tetranuclear complex would inhibit the trans−cis photoisomerization by locking up the complex in a sandwich binding fashion to give [{Au4(dppm)2(CC−L−CC)2}Ag2]2+. The addition of nBu4NCl would restore the capability of the complex to undergo photoisomerization.923 The photoisomerization process is summarized in Figure 202. In 2007, Balch and co-workers reported that the colorless crystals of the complex, [Au{C(NHMe)2}2](EF6) (E = P, As, Sb), exhibited different emission depending on the conditions of recrystallization.924 The crystals of the complexes were blueemitting in the absence of solvates. Recrystallization of the AsF6− salt with benzene or acetone gave blue-emitting crystals, while recrystallization with chlorobenzene resulted in greenemitting crystals.924 In general, the crystals were packed to form

Figure 204. Structure of [(C6F5Au)2(μ-1,4-diisocyanobenzene)].

which appeared as a white powder, was found to exhibit intense blue emission upon irradiation by UV light at 365 nm.927 Upon grinding, a drastic change of the emission color to yellow occurred. Exposure to organic solvents such as dichloromethane, ethyl acetate, and diethyl ether would regenerate the blue luminescence (Figure 205).927 The crystal structure of the complex revealed that the shortest Au(I)···Au(I) distance was 5.19 Å, which was too long for any significant aurophilic

Figure 205. Photographs showing [(C6F5Au)2(μ-1,4-diisocyanobenzene)] ([Au]) on an agate mortar under UV irradiation with black light (365 nm), unless otherwise noted: (a) [Au] powder after grinding the right-half with a pestle; (b) the same sample under ambient light; (c) entirely ground powder of [Au]; (d) partial reversion to the blue luminescence by dropwise treatment using dichloromethane onto the center of the ground powder; (e) powder after treatment with dichloromethane; and (f) repetition of the yellow emission by scratching the powder with a pestle. Reprinted with permission from ref 927. Copyright 2008 American Chemical Society. CI


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interactions.927 Because the unground solid, single crystals and solvated form of the complex were all found to exhibit blue luminescence, and given the vibronic-structured emission profile, the blue emission was suggested to be originated from an intraligand π−π* origin.927 According to X-ray diffraction (XRD) studies, the grinding process would convert the sample from crystals to a metastable amorphous phase, in which Au(I)···Au(I) interactions were suggested to be present and would lead to the broad lower-energy luminescence.927 Treatment of organic solvents would reverse the amorphous phase to crystalline nature, thus restoring the blue luminescence. Espinet and co-workers reported a series of gold(I) isocyanide complexes, [AuX(CNR)] (X = Cl, C6F5; R = C6H4N=NC6H4OCnH2n+1, n = 4, 8, 12), which contained an azo group on the isocyanide ligand.928 In addition to liquid crystal properties, the complexes were all found to be photosensitive in solution. Upon UV irradiation, photoisomerization of the azo moiety would occur in solution as well as in the mesophase.928 Similar photoisomerization properties were also observed in a series of azo-containing gold(I) and silver(I) bipyridine complexes.929 However, none of these complexes were observed to be photoluminescent.928,929 Thermochromic behaviors have been observed in the metallopolymers, [Au(C6F5)(PVP)] and [Au(C6Cl5)(PVP)] (PVP = poly(4-vinylpyridine)).930 Both metallopolymers were found to emit in the solid state, with the chlorophenyl analogue showing a yellow emission at room temperature and a blue emission at low temperature, while the fluorophenyl analogue could exhibit a structureless emission band that could be finetuned upon variation of the temperature.930 Because both interand intrachain Au(I)···Au(I) interactions would be possible in the polymeric structure, the emission bands have been assigned to phosphorescence from Au(I)···Au(I) bonded excimeric triplet states of different aggregated species.930 Mohr and co-workers prepared the dinuclear gold(I) complex, [Au2(μ-C10H6){μ-Ph2P(CH2)2PPh2}], which was found to show a vibronic-structured green emission typical of the intraligand transition of naphthalene-based systems.931 The oxidative addition reactions of the dinuclear complex with halogens in dichloromethane at ca. −70 °C resulted in the formation of the dihalodigold(II) species, [Au2X2(μ-C10H6){μPh2P(CH2)2PPh2}] (X = Cl, Br, I), which were stable to air and moisture. Raman spectroscopic studies showed Au−Au stretching vibrations at 174 and 135 cm−1, for the chloro and bromo derivatives, respectively, suggestive of the presence of a Au−Au bond in the dihalodigold(II) species. In 2009, the group of Yam utilized 1,1,1-tris(diphenylphosphinomethyl)ethane as a tridentate linker to prepare a trinuclear gold(I) complex with three oligoethersubstituted alkynyl moieties (Figure 206).932 This complex displayed selective binding properties toward Mg2+ ions, in

which the encapsulation of a Mg2+ ion by the three oligoether pendants would bring the three gold(I) centers into close proximity to switch on aurophilic interactions, leading to the appearance of a low-energy emission band at 675 nm in the solution state.932 Earlier work of Yam and co-workers demonstrated the cation-binding studies of a series of gold(I) calix[4]crown alkynyl complexes in association with spectral changes.933,934 As an extension of this work, a series of calixarene-containing gold(I) isocyanide complexes, [{calix[4]arene(OCH2CONHC6H4CC)2}{Au(CNR)}2] (R = benzo[15]crown-5, benzo[18]crown-6) (Figure 207), was synthesized.935

Figure 207. Structures of [{calix[4]arene-(OCH2CONHC6H4C C)2}{Au(CNR)}2].

Upon metal ion-binding in a sandwich fashion, the resulting host−guest assembly was found to show a shorter intramolecular Au(I)···Au(I) separation, and a new emission band in the low-energy region was observed.935 Work has been extended to the preparation of a bis-alkynyl calixarene gold(I) isocyanide complex that showed selective binding toward Al3+ ions.936 Upon cation binding, reorganization of the lower rim of the calixarene moiety would occur to bring the two Au(I) metal centers into close proximity, with a short Au(I)···Au(I) distance of 3.062 Å. This would switch on the aurophilic interactions and result in a visual change in the emission color from green to orange-red.936 DFT calculations have also been performed to probe the ion-binding mechanism and the nature of the excited states.935,936 The same group further explored the cation- and anionbinding properties of a ditopic receptor, [(B15C5Ph2P)Au(SC6H4NHCONHC6H5)].937 The complex featured both a benzo-15-crown-5 moiety in the auxiliary phosphine ligand for cation-binding, as well as a urea-functionalized thiolate ligand for anion-binding. The dichloromethane solution of the complex was found to show a high-energy emission at ca. 451 nm and a low-energy emission at ca. 530 nm. The lowerenergy emission was assigned to originate from a thiolate-togold LMCT excited state, while the high-energy emission was assigned to states arising from the metal-perturbed intraligand transition.937 Using 1H NMR and ESI−MS techniques, the anion- and cation-binding properties, salt extraction ability, and cooperativity of the complex have been studied. Extension of this work has been made to the synthesis, photophysical studies, and anion-binding properties of a series of bis(dicyclohexylphosphino)methane-containing gold(I) thiolate complexes with urea receptors.938

Figure 206. Structures of the trinuclear gold(I) complexes. CJ


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In 2010, Yersin and co-workers investigated the temperaturedependent emission decay dynamics of the luminescent gold(I) complexes, [Au(dipnc)(PPh 3 )] (dipnc = 7,8-bis(diisopropylphosphino)-nido-carborane) and [Au(dppnc)(PPh3)] (dppnc = 7,8-bis(diphenylphosphino)-nido-carborane) at temperatures of 1.5 ≤ T ≤ 300 K.939 The complexes were observed to exhibit intense green luminescence at room temperature. The nature of the lowest-energy triplet states was probed by the analysis of the emission decay dynamics. The magnitudes of zero-field splitting of the emitting states of [Au(dipnc)(PPh3)] and [Au(dppnc)(PPh3)] were, respectively, found to be 47 and 29 cm−1, which were relatively small as compared to triplet states assigned to be of significant metal character.939 It was thus suggested that there involved significant contributions of phosphine ligand orbitals in the lowest triplet states of the gold(I) phosphine complexes. In addition, the high emission quantum yields of up to 75% suggested the potential of these complexes for material applications.939 By incorporating various carbon-donor ligands such as Nheterocyclic carbenes, alkynyls, and isocyanides into the gold(I) center, Che and co-workers reported a series of gold(I) complexes, [Au(NHC)(CCAr)] and [Au(CNAr)2]+, that were found to emit at room temperature.940 The complexes of the formula [Au(NHC)(CCAr)] were observed to show vibronically structured emission bands that were assigned to either 1π−π* excited states localized on the NHC moiety or 3 π−π* excited states of the alkynyl moiety, while in general the solid-state emission bands of [Au(CNAr)2]+ were attributed to intermolecular aurophilic and π−π stacking interactions.940 Some of the complexes were observed to form nanostructures upon reprecipitation. In particular, the gold(I) isocyanide complexes were observed to form nanowires and nanobelts in THF/water mixtures, which was attributed to the presence of intermolecular hydrogen-bonding interactions between the C− H bond of the alkyl substituents on the isocyanide and the F atom on the triflate anion.940 Wong and co-workers recently reported a class of hybrid Langmuir−Blodgett (LB) films consisting of an alkynylgold(I) complex, [PPh 3 PAuCC−(N-butylcarbazole)−CC− AuPPh3], and a polyoxometalate (POM).941 The gold(I) complex was found to form stable and well-defined LB films in pure water and POM subphases. The presence of POM would lead to a certain degree of quenching of the luminescence of the alkynylgold(I) complex, and the [Au]/ POM hybrid films were observed to show a strong photovoltage effect in the UV−visible region.941 By changing the bridging carbazole bis-alkynyl ligand into a 9,9-bis(4ethynylphenyl)fluorene ligand, luminescent LB films with near-white emission have been obtained because of the dual emissive nature of the hybrid material.942 On the other hand, the gold(I) complex, [Au2(dppp)(BIT)2]·(CF3CO2)2 (dppp = bis(diphenylphosphino)propane; BIT = 2-benzimidazolethiol), was found to readily interconvert between two forms (Figure 208).943 The discrete dinuclear complex was found to exhibit intramolecular aurophilic interactions with a Au(I)···Au(I) distance of 3.1312(5) Å.943 In the presence of a base, the acetate moiety would be eliminated to give [Au2(dppp)(BIT)2], in which intermolecular aurophilic interactions would become prevalent to assemble the molecules into a linear chain, thus switching on the luminescence of the complex at 493 nm.943

Figure 208. Luminescence switching of [Au 2 (dppp)(BIT)2 ]· (CF3CO2)2.

Using the dansyl (5-dimethylamino-naphthalene-1-sulfonyl) group as the ligand, a series of mono-, di-, and trinuclear gold(I) complexes (Figure 209) have been synthesized by Chao

Figure 209. Structures of the mono-, di-, and trinuclear gold(I) complexes with the dansyl (5-dimethylamino-naphthalene-1-sulfonyl) ligand.

and co-workers.944 According to the crystal structures, the intramolecular Au(I)···Au(I) distances of the trinuclear complex (3.0529(7)−3.1408(8) Å) were found to be shorter than those of the dinuclear complex (3.3902(7) Å).944 All of the complexes were found to be emissive and showed a highenergy emission band in the solid state at room temperature, assignable to a charge-transfer excited state from a lone pair of the NMe2 moiety to the π* orbital of the naphthalene ring of the dansyl-containing unit.944 The dinuclear complex was found to show an additional low-energy emission at 590 nm from an excimeric excited state.944 With the use of extensive transient spectroscopic techniques, Che and co-workers recently extended their earlier work on mono- and dinuclear gold(I) complexes with oligo(p-phenyleneethynylene) (PE) ligands945 and explored the spectroscopic properties of a series of gold(I) derivatives containing oligo(oor m-PE) units.946 The structures of the dinuclear complexes are shown in Figure 210. With the exception of the dinuclear gold(I) complexes containing only one PE unit, the complexes were observed to show dual luminescence properties. The nature and dynamics of the excited states were probed with femtosecond (fs) transient absorption, fs time-resolved fluorescence, and nanosecond time-resolved emission spectroscopy.946 The origin of the dual emission has been assigned to the singlet and triplet π−π* ligand-centered excited states. In addition, the room-temperature fluorescence of the complexes was found to be a combination of prompt fluorescence and delayed fluorescence, in which the delayed fluorescence was shown to arise from triplet−triplet annihilation (TTA) from the T1 state, and the TTA process would be facilitated upon an extension of the length of the PE ligand.946 CK


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Figure 212. Structures of the amphiphilic gold(I) phospholes.

plexes were shown to self-assemble in DMSO−water mixture to form sheet-like nanostructures, which were suggested to be comprised of bilayer aggregates. The self-assembly process was suggested to proceed via an isodesmic mechanism and had been driven by hydrophobic−hydrophobic interactions. As revealed by the nucleation-elongation model, an increase in the extent of the π-conjugated surface at the alkynyl moiety would result in a higher tendency for the aggregation to proceed and represent an example of tunable self-assemblies. Very recently, Tanase and co-workers used tetraphosphine ligands to synthesize a series of luminescent tetranuclear gold(I) phosphine complexes, syn-[Au4(meso-dpmppm)2]X4, anti-[Au4(meso-dpmppm)2]X4 (Figure 213), and syn-[Au4(rac-

Figure 210. Structures of the gold(I) oligo-PE complexes.

The group of Chao reported a series of alkynylgold(I) urea complexes, [R′3PAuCCC6H4-4NHC(O)NHC6H4-R-4] (R′ = cyclohexyl, R = NO2, CF3, Cl, H, CH3, tBu, OCH3; R′ = phenyl, R = NO2, OCH3; R′ = 4-methoxyphenyl, R = H, OCH3) (Figure 211).947 The complexes were generally found

Figure 211. Structures of the alkynylgold(I) urea complexes.

to show blue-green emission at 443−476 nm, which originated from the triplet ππ* excited state of the alkynyl moiety.947 Some of the complexes were found to bind fluoride ions to give a visible color change.947 The same group later extended their work to other gold(I) complexes,948,949 and prepared a trinuclear complex that showed selective binding toward Ag+ ions.950 Kuroiwa and co-workers used amphiphilic diblock copolypeptides with the structural formula, poly-L-lysine-block-Lleucine, to direct the self-assembly of [Au(CN)2]− moieties.951 It was observed that the addition of the block copolypeptide into solutions of [Au(CN)2]− would lead to the observation of a new emission band that originated from metallophilic interactions in the gold(I) self-assemblies. The formation of nanostructures was also observed in transmission electron microscopy (TEM).951 In recent years, increasing interest has been drawn toward the photophysical properties of gold(I) phosphole complexes. For instance, in 2003, Wu, Réau, and co-workers prepared a conjugated phosphole-containing gold(I) complex that exhibit both photo- and electroluminescence, representing a new type of materials for OLEDs.952 The rich luminescence properties of other phosphole derivatives and their gold(I) complexes were reported by the groups of Réau953 and Baumgartner.954,955 Despite the increasing attention toward gold(I) phosphole luminescence, corresponding studies on the self-assemblies of this class of complexes remained scarce. Very recently, the group of Yam developed a novel class of amphiphilic alkynylgold(I) phospholes (Figure 212).956 The complexes were all found to be luminescent in methanol at room temperature. The majority of the complexes were observed to exhibit dual emission properties attributed to singlet and triplet π−π* ligand-centered excited states. As revealed by transmission electron microscopy, the amphiphilic gold(I) com-

Figure 213. Structure of anti-[Au4(meso-dpmppm)2]X4.

dpmppm)2]X4 (X = PF6, BF4, OTf) (dpmppm = bis[(diphenylphosphinomethyl)phenylphosphino]methane) (Figure 214).957 The crystal structures of the complexes were

Figure 214. Structure of syn-[Au4(rac-dpmppm)2]X4.

studied by X-ray diffraction analysis. It was observed that the four gold(I) centers in syn-[Au4(meso-dpmppm)2]X4 complexes were arranged into bent Au4 chains that were bridged by the meso-dpmppm ligands. Short Au(I)···Au(I) distances of 2.942− 2.968 Å were indicative of the presence of intramolecular aurophilic interactions.957 In anti-[Au4(meso-dpmppm)2]X4, the gold(I) centers exhibited a relatively linear alignment with stronger aurophilic interactions (2.8658(2)−2.8981(3) Å).957 For syn-[Au4(rac-dpmppm)2]X4, the gold(I) centers were arranged in a zigzag manner with intramolecular Au(I)··· Au(I) distances of 2.9545(6)−3.0065(7) Å.957 These tetranuclear gold(I) complexes were found to be phosphorescent in the solid state at 475−515 nm with quantum yields of 0.67− 0.85. With the support of computational studies, the emission CL


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was assigned to originate from the triplet excited states of the cluster centers.957 In addition, it was found that the emission would be readily quenched by the presence of chloride ions by both dynamic and static quenching processes, rendering this class of complexes potential materials for anion sensing.957 3.3.2. Gold(I) Clusters of High Nuclearities. Besides discrete complex molecules, the presence of extensive aurophilic interactions has been found to support the selfassembled structures in a number of higher-ordered gold(I) polynuclear clusters.828,829 Over the past decade, chalcogenide ligands have been employed to prepare a variety of luminescent gold(I) clusters.958 An earlier example would be the luminescent dodecanuclear gold(I) cluster, [Au 12 (μdppm)6(μ3-S)4](PF6)4 (Figure 215), reported by the group of

Figure 217. Structure of [Au6{μ-Ph2PN(p-CH3C6H4)PPh2}3(μ3S)2]2+.

complex was found to emit in the orange region in the solid state at ambient and low temperatures, and the emission origin was similarly assigned to a mixture of LMCT and MC states. The large Stokes’ shift of 15 052 cm−1 suggested a highly distorted structure of the complex in the excited state. By employing selenide ligands in place of sulfide ligands, the same group recently reported a series of decanuclear and hexanuclear gold(I) derivatives with short aurophilic contacts.962 These complexes were found to show intense green and/or orange emission upon photoexcitation in the solid state and in solution at room and low temperature, and such emission properties were highly sensitive to the nuclearities and the nature of the chalcogenide ligand. Tentative assignments of the emission origin to excited states derived from phosphine-centered intraligand (IL) transition or metal-centered (ds/dp) mixed with LMMCT (E → Au) transition have been suggested.962 Fenske and co-workers prepared the gold(I) selenide complex, [Au18Se8(dppe)6]Br2 (dppe = bis(diphenylphosphino)ethane).963 It was observed that multiple Au(I)···Au(I) interactions existed within the cluster and gave rise to a long-lived red emission of the complex in the solid state. The decanuclear analogue, [Au10Se4(dpppe)4]Br2 (dpppe = bis(diphenylphosphino)pentane) (Figure 218), was found to emit in the near-infrared region at 880 nm in dichloromethane solution.963

Figure 215. Structure of [Au12(μ-dppm)6(μ3-S)4]4+.

Yam in 1999.959 The gold(I) cluster was found to exhibit dual emission, showing an orange-red emission in the solid state and a green emission in the solution state at room temperature, which has been assigned to originate from triplet LMCT states mixed with metal-centered states modified by Au(I)···Au(I) interactions. Subsequent work by the same group resulted in the report of another luminescent decanuclear gold(I) cluster, [Au10(μPNP)4(μ3-S)4](PF6)2 (PNP = Ph2PN(nPr)PPh2) (Figure 216).960 The crystal structure of the complex featured a

Figure 216. Structure of [Au10(μ-PNP)4(μ3-S)4]2+.

Figure 218. Structure of [Au18Se8(dppe)6]2+.

propeller-shaped structure of S4 symmetry formed by four PNP-Au2 units, with the principal symmetry axis passing through the two interstitial gold(I) centers in the core of the cluster.960 A significant amount of aurophilic interactions was found to exist in the decanuclear gold(I) cluster. The complex also showed dual emission, in which the higher-energy green emission band was assigned to a metal-perturbed intraligand transition while the lower energy orange-red emission originated from an LMCT excited state mixed with metalcentered (MC) states modified by Au(I)···Au(I) interactions.960 A related hexanuclear complex, [Au6{μ-Ph2PN(pCH3C6H4)PPh2}3(μ3-S)2](ClO4)2 (Figure 217), has also been prepared.961 This complex exhibited a heterocubane structure with two S atoms occupying two of the apical positions. The

Using various alkynylcalix[4]crown-6 ligands, the group of Yam reported a class of tetranuclear gold(I) clusters in 2004.964 The perspective drawing of one of the complexes is shown in Figure 219. The complex was found to feature a double-cage structure in which the four gold(I) metal centers were arranged in a rhomboidal manner and capped by the two alkynylcalix[4]crown-6 ligands on the two ends. Short intramolecular Au(I)··· Au(I) distances of 3.1344(8) and 3.2048(8) Å were found to be present in the complex.964 This class of tetranuclear gold(I) complexes was found to exhibit solid-state luminescence at 590−620 nm at both ambient and low temperatures, which was assigned to be derived from metal-cluster-centered (ds/dp) excited states that were modified by aurophilic interactions and CM


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Figure 219. Crystal structure of the tetranuclear gold(I) alkynylcalix[4]crown-6 complex. Reprinted with permission from ref 964. Copyright 2004 Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim.

mixed with metal-perturbed intraligand π−π* (CC) states.964 The presence of Au(I)···Au(I) interactions has been reported to direct the self-assembly of gold(I) centers to form polynuclear supramolecular structures such as gold rings. In 2005, Yu, Yam, and co-workers reported the assembly of a chiral hexadecanuclear gold(I) ring, [(dppm)2Au4(pipzdtc)]4(PF6)8 (dppm = bis(diphenylphosphino)methane; pipzdtc = piperazine-1,4-dicarbodithiolate) (Figure 220).965 It was remarkable that this chiral Au16 complex was made up of four achiral tetrameric units linked by Au(I)···Au(I) interactions, with an average distance of 2.90 Å between the gold(I) metal centers.965 The Au16 complex was found to show an intense green emission in the solid state from S → Au LMCT excited states modified by Au(I)···Au(I) interactions.965 In another work, a shuttle-like Au12 complex, [Au12{R(NCS2)2}6], was reported (Figure 221).966 The assembly of the gold(I) centers was directed by Au(I)···Au(I) interactions (2.902−3.143 Å) to form a chiral cyclic framework with D2 symmetry. At low temperature, the complex exhibited a longlived red phosphorescence, tentatively assigned to a metalcentered excited-state origin.966 In 2008, the self-assembly of a crown-like Au36 ring complex (Figure 222) was reported.967 It was suggested that the selfassembly was a spontaneous process directed by strong Au(I)··· Au(I) interactions during crystallization. In the crystal structure, six Au6 monomeric units were held together by intramolecular aurophilic interactions (average Au(I)···Au(I) distance = 2.890 Å).967 The Au6 monomer was found to exhibit a low-energy absorption band, assigned to the ligand-to-metal−metal charge transfer LMMCT (thiocarbamate-to-gold−gold) transition, while the cluster was found to exhibit a low-energy absorption tail.967 However, the solid samples and solutions of the complex were nonemissive, attributed to the presence of the ferrocenyl groups that quenched the LMMCT excited states.967 Similar aurophilicity-directed spontaneous assembly also resulted in a Au18 thiacrown complex, [Au18(μ-dpepp)6(μ3S)6](PF6)6 (Figure 223).968 The self-assembly of six [Au3(μdpepp)] units through ligand-supported Au(I)···Au(I) interactions in the presence of six bridging sulfide ligands has resulted in the formation of a Au18 ring that highly resembled the thiacrown ether, [18]aneS6, in appearance.968 The Au18 ring could function like a thiacrown ether moiety to bind soft silver(I) ions, giving rise to pronounced spectroscopic changes. Reversible binding could be achieved by the addition of acetonitrile without disruption of the host macrocycle. In 2011, it was demonstrated by Konishi and co-workers that with the use of a diphosphine ligand, a cluster-to-cluster transformation through a growth/etching process could be performed to generate the cluster cations, [Au8(dppp)4Cl2]2+

Figure 220. (a) Structure and (b) crystal structure of [{(dppm)2Au4(pipzdtc)}4]8+. Reprinted with permission from ref 965. Copyright 2005 American Chemical Society.

and [Au8(dppp)4]2+ (dppp = 1,3-bis(diphenylphosphino)propane), from the precursor cluster, [Au6(dppp)4](NO3)2.969 The octanuclear clusters were observed to contain edge-fused gold tetrahedral motifs that were isomeric to each other, in which the dppp ligands functioned to interlock the octanuclear core to stabilize the cluster skeleton.969 The complex containing [Au8(dppp)4Cl2]2+ was found to exhibit an emission band at 600 nm, while the one with [Au8(dppp)4]2+ showed negligible emission.969 This highlighted that the luminescence CN


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Also in 2011, Koshevoy reported a series of octanuclear gold(I) clusters, [Au 8 (CC t Bu) 6 (P ∧ P) 2 ] 2+ (P ∧ P = PPh2C6H4PPh2, PPyr2C6H4PPyr2, PPh2CCPPh2).970 Extensive intramolecular aurophilic interactions were observed in the structure of the complexes. All of the complexes showed luminescence in both solution and solid states. For P∧P = Ph2PC6H4PPh2, Pyr2PC6H4PPyr2, the clusters were shown to exhibit thermochromism, in which a red shift was observed at increased temperatures. For P∧P = Ph2PCCPPh2, the crystal form was observed to emit at 730 nm, while grinding would result in the replacement of this emission band by a new intense band at 560 nm.970 Results from EXAFS experiments suggested that both the thermochromic and the mechanochromic behaviors could be attributed to the change in Au(I)··· Au(I) distances and thus crystal lattice arrangements in the clusters.970 Very recently, the group of Tsutsumi prepared a series of liquid crystalline gold(I) complexes, [C5H11NC−Au−C CC6H4(OCnH2n+1)-p] (n = 5−8).971 The complexes were found to emit in the blue region in the solid state and in the liquid crystal phases. Crystallographic studies showed that the gold(I) isocyanide complexes were arranged in dimers with very weak or no aurophilic interactions between two neighboring molecules, and it was suggested that such dimers were also present in the liquid crystal phase to act as the mesogen unit.971 For [C5H11NC−Au−C CC6H4(OC8H17)-p], the dimer had the longest alkoxy chains and thus the largest aspect ratio, so only this complex was observed to show a Sm phase while the others showed a less ordered N phase.971 An addition reaction has also been employed to induce the self-assembly of luminescent gold(I) clusters through aurophilicity-directed cluster-to-cluster transformation.972 The reaction between the chlorogold(I) precursor, [vdpp(AuCl)2] (vdpp = vinylidenebis(diphenylphosphine)), and H2S was found to yield a series of polynuclear gold(I) μ3-sulfide clusters with different nuclearities and structures.972 The identities of these clusters and the mechanism of the transformation process were characterized spectroscopically. Single-crystal structures revealed the existence of substantial Au(I)···Au(I) interactions in the complexes, imparting the complexes with rich luminescence properties in the solid state and in solutions.972 The aurophilicity-directed self-assembly of a new class of gold(I)-containing metallosupramolecular cages and cage-built two-dimensional arrays, [{Au8L2}n] (n = 1 or ∞, L = tetrakisdithiocarbamato-calix[4]arene), was recently reported by Yu

Figure 221. (a) Structure and (b) crystal structure of [Au12{R(NCS2)2}6]. Reprinted with permission from ref 966. Copyright 2008 The Royal Society of Chemistry.

Figure 222. Structure of the Au36 cluster.

properties of the complexes were highly dependent on the geometry.

Figure 223. (Left) Structure and (right) crystal structure of [Au18(μ-dpepp)6(μ3-S)6]6+. Reprinted with permission from ref 968. Copyright 2010 American Chemical Society. CO


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and Yam.973 According to synchrotron radiation X-ray diffraction structural analyses, the complexes were demonstrated to exhibit a quadruple-stranded helicate dimeric cage structure with different packing modes, depending on the extent of inter- and intramolecular Au(I)···Au(I) interactions. The complexes were found to show a green phosphorescence in dichloromethane solution, tentatively assigned to originate from triplet metal-centered ds/dp and LMCT excited states modified by Au(I)···Au(I) interactions.973 Upon binding silver(I) ions, the formation of Au(I)···Ag(I) metallophilic interactions would result in a new Au(I)···Au(I)···Ag(I) 3 LMMCT emission in the red region (Figure 224).973 Such luminescence switching properties have illustrated that the gold(I) supramolecular cages could serve as a highly selective sensor for the detection of silver(I) ions.

nanoclusters were observed to exhibit rich photoluminescence properties.978−982 3.3.3. Trinuclear Gold(I) Metallacycles. Apart from polynuclear clusters, extensive works have also been conducted on trinuclear gold(I) metallacycles.592 The relatively planar triangular structure of the complexes, together with the propensity of inter- and/or intramolecular Au(I)···Au(I) interactions, have given rise to various gold(I) molecular triangles with rich luminescence properties. In 1997, Balch and co-workers reported a trinuclear gold(I) complex, [Au3(CH3NCOCH3)3], that exhibited solventstimulated luminescence.983 After the irradiation of UV light at 366 nm, the subsequent contact of the trinuclear gold(I) complex with a drop of chloroform would lead to intense yellow luminescence. Emission studies in the solid state showed that the complex exhibited dual emission at room temperature, with a short-lived emission at 446 nm and a long-lived component at 552 nm, in which the latter band correlated well to the solvent-triggered emission.983 Crystallographic studies showed that the trinuclear gold(I) complexes were arranged into two types of stacks. Complex molecules in the more ordered stacks were arranged into a prominent trigonal prismatic array with intermolecular Au(I)···Au(I) distances of 3.346(1) Å.983 It was believed that the extended supramolecular assembly of the complex in the stacks was involved in the solvent-triggered luminescence, in which charge or electron separation might have occurred, and recombination might then take place in the molecular stacks upon solvent exposure. The highly ordered stacks would allow migration of charge through the stacks for efficient energy transfer from the bulk to the surface, whereas the disordered stacks would function as traps for energy storage.983 In a related work, electron-accepting TCNQ and C6F6 molecules were sandwiched between various trinuclear gold(I) complexes to form acid−base adducts, resulting in the perturbation of aurophilic interactions and hence the luminescence properties of the complexes.984 The group of Balch also reported the gold(I) pyridine trimer, [Au3(NC5H4)3], which could self-assemble via Au(I)···Au(I) interactions to give two distinct structural motifs, discrete dimers and extended chains, both of which were supported by aurophilic interactions.985 The complex was found to emit at 425 and 490 nm, respectively, in pyridine solution and in the solid state. It was remarkable that prolonged standing in the atmosphere or immersion in 4 M hydrochloric acid would render the crystal an hourglass shape due to the deposition of metallic gold during a chemical reaction occurring within the crystals.985 The long alkyl chain-containing trinuclear gold(I) pyrazole complex (Figure 226) reported by the group of Aida and coworkers in 2005 represented one of the most remarkable examples of luminescent triangular d10 metallacycles.986 In hexane, the complex could self-assemble in the presence of Au(I)···Au(I) interactions to form a metallogel that gave a red emission at 640 nm. When doped with silver(I) ions, the emission color of the gel would be switched to blue at 458 nm. The red emission color could then be recovered upon dedoping.986 Upon gel-to-sol phase transition at increased temperature, the intensity of the red emission would diminish due to the gradually weaker aurophilic interactions. Doping of silver(I) in the sol form would lead to a green emission at 501 nm, thereby demonstrating an interesting red-green-blue (RGB) switching phenomenon of the trinuclear gold(I) complex.986

Figure 224. Schematic diagram showing the reversible change in structure and solution color upon binding of Ag+ by [Au8L]. Reprinted with permission from ref 973. Copyright 2014 American Chemical Society.

Koshevoy et al. recently prepared a series of hexanuclear clusters, [Au6(CCR)4(PR′2−X−PR′2)2]2+ (Figure 225) by treating the homoleptic decanuclear precursors, (AuCCR)10, with [Au2(PR′2−X−PR′2)2]2+.974 The crystal structures of some of the complexes have been determined by X-ray diffraction studies, and it was observed that the crystal structure mainly consisted of two linear gold(I) bis-alkynyl anionic fragments that were connected by a Au(I)···Au(I) interaction but twisted with respect to each other.974 The alkynyl ligands were coordinated to gold(I) metal centers through both σ- and π-coordination modes, and the complexes were found to exhibit intense emission in the solid state at ambient temperature. Computational studies by DFT methods showed that the emission origin was dominated by contributions from Au(I) and π-alkynyl orbitals. The very high luminescence quantum yields of 0.61 to unity of these gold(I) clusters further highlighted the potential of these complexes for optical applications.974 In addition to the aforementioned gold(I) clusters, recently there has been a number of reports on gold nanoclusters, especially gold thiolate clusters, in the literature.975−977 The details of these nanoclusters will not be discussed here as the oxidation states of the gold centers are not well-defined. However, it is important to note that some of these gold CP


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Figure 225. Synthesis of [Au6(C2R)4(PR′2−X−PR′2)2]2+.

gle-crystal structure (Figure 227).988 In the structural framework, the three gold(I) centers were bridged by the imidoyl

Figure 226. Structure of the trinuclear gold(I) pyrazole complex.

In the next year, Coppens and co-workers investigated the complex, [Au(3,5-iPr2Tz)]3.987 The crystal structures of the complex at 95 and 298 K have been determined. At both temperatures, the trinuclear molecule was found to dimerize to give a fully overlapping dimer-of-trimer structure with intermolecular Au(I)···Au(I) interactions. When the temperature was decreased from room temperature to 95 K, crystal contraction occurred with a change in the intermolecular Au(I)···Au(I) distances. A change in effective symmetry from D3 to C2 was also observed at 95 K.987 The crystals of the complex were found to be emissive, showing low-energy structureless bands at ca. 650 and 750 nm, assignable to the excimeric emission from the dimer-of-trimer unit. Such lowenergy bands could also be observed in dilute dichloromethane solutions of the complex, which suggested the presence of aurophilic interactions even in dilute solutions.987 The complex was found to show molecular recognition properties, in which the addition of acids and aromatic molecules such as benzene has been found to result in quenching of the excimeric emission, where the intertrimer interactions were removed by protonation and π-intercalation, respectively.987 Espinet and co-workers reported a trinuclear gold(I) complex, [Au(2-PyNCNHMe)]3, and determined its sin-

Figure 227. Crystal tructure of [Au(2-PyNCNHMe)]3. Reprinted with permission from ref 988. Copyright 2006 American Chemical Society.

groups. The average Au(I)···Au(I) distance of 3.2532(9) Å was suggestive of the presence of weak intramolecular aurophilic interactions.988 The trinuclear species would dimerize with an adjacent molecule, and the intermolecular Au(I)···Au(I) separation within each pair was found to be 3.2465(12) Å, but the steric hindrance of the pyridine substituents would prevent the molecules from further self-assembling into a columnar array.988 The complex was found to emit at 523 nm in the solid state and at 536 nm in dichloromethane solution at 298 K, which was attributed to the presence of intramolecular Au(I)···Au(I) interactions.988 On the other hand, replacement of the N−H unit on the ligand by an oxygen atom would result in the formation of a tetranuclear complex, [Au(2-PyN COMe)4].988 The tetranuclear gold(I) complex was observed CQ


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Figure 228. Trinuclear imidazolato-bridged gold(I) metallacycle forming a three-dimensional supramolecular trigonal prismatic array. Reprinted with permission from ref 989. Copyright 2010 American Chemical Society.

phosphorescence at 693 nm, attributable to aurophilic interactions, upon excitation by UV light.990 Dipping of the film into a solution of AgOTf in THF would lead to the appearance of a new emission band at 486 nm. Such observations have been explained by the permeation of Ag(I) ions into the nanochannels of [Au3Pz3]/silicahex, which would switch on the heterometallic Au(I)···Ag(I) interactions to give the green emission.990 The mechanism of the ion permeation is illustrated in Figure 229. Very recently, Li and co-workers incorporated thiophene moieties into the trinuclear gold(I) pyrazolate skeleton to yield a dual emissive complex, in which white light emission could be achieved via the combination of the higher-energy monomeric blue-green emission and the lower-energy excimeric orange-red emission.991 At a concentration of 8 × 10−4 M in dichloromethane, the dual emission band of the complex was found to extend across 400−700 nm, showing a quantum yield of 0.413 with CIE coordinates of (0.31, 0.33).991 In the solid state, the complex was found to crystallize into two polymorphs, one showing orange-red emission and the other showing white emission, associated with different crystal packing and Au(I)···

to emit at 436 nm in the solid state at room temperature. The lower emission energy of the tetranuclear complex in comparison to the trinuclear counterpart was also in line with the observation of a shorter intramolecular Au(I)···Au(I) distance of 3.08 Å in the tetranuclear complex.988 In 2010, Fujita and co-workers reported a trinuclear imidazolato-bridged gold(I) metallacycle that could form a three-dimensional supramolecular trigonal prismatic array within organic-pillared cages (Figure 228).989 Substantial Au(I)···Au(I) interactions were observed within the assembly, associated with Au(I)···Au(I) distances of 3.206−3.230 Å.989 The host−guest interactions between the organic-pillared cages and the gold(I) metallacycles were found to result in quenching of the emission of the gold(I) metallacycle, suggestive of the potential in using this class of gold(I) complexes for molecular recognition and sensing. In 2012, Aida and co-workers confined the trinuclear pyrazolate complex, [Au3Pz3], in the nanoscopic channels of mesoporous silica to prepare a nanocomposite with a onedimensional cylindrical structure ([Au3Pz3]/silicahex).990 The spin-coated nanocomposite was found to exhibit red CR


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one-2,7-diyl, 9-(di-cyanomethylene)fluorene-2,7-diyl; R′ = Me, Ph).897 The complexes were found to emit at 444−597 nm in the solid state at 290 K, which was assigned to a triplet π−π* and/or σ−π* origin.897 The same group also reported a class of mercury(II) polyyne polymers containing 9,9dialkylfluorene groups with triplet π−π* emissions.994 Crystal structures of the dinuclear mercury(II) analogue, [MeHg−C C−R−CC−HgMe] (R = 9,9-dioctylfluorene), revealed the presence of weak intermolecular Hg(II)···Hg(II) interactions (3.738 and 4.183 Å). Such metallophilic interactions allowed the molecules to be linked together to form a loose polymeric three-dimensional structure.994 Subsequent works by the same group have been extended to the preparation of other luminescent mercury(II) alkynyl complexes and polymers.995−1000 Another example of light-emitting mercury(II) self-assembly would be the luminescent trinuclear mercury(II) complex, [(oC 6 F 4 Hg) 3 ], reported by Gabbai ̈ and co-workers in 2002.1001,1002 The complex was observed to dimerize in a cofacial manner with an intermolecular Hg(II)···Hg(II) distance of 3.512 Å, and it was found to emit at 450−520 nm at 298 and 77 K in the solid state.1001,1002 The mercury(II) complex would form 1:1 adducts with aromatic compounds such as biphenyl, naphthalene, and pyrene to form supramolecular stacks that exhibited intense emissions derived from the monomeric phosphorescence of the aromatics.1003,1004 A few years later, the group of Gabbai ̈ observed that, in addition to [(o-C6F4Hg)3], pentafluorophenylmercury chloride (C 6 F 5 HgCl) would also react similarly with aromatic compounds.1005 Both [(o-C6F4Hg)3] and C6F5HgCl were allowed to interact with phenanthrene to obtain supramolecular adducts. Single-crystal X-ray analysis showed that the adducts would form extended binary stacks with Hg(II)−π interactions.1005 Upon photoexcitation, the phosphorescence of phenanthrene was observed at room temperature for the [(oC 6F4Hg)3]·phenanthrene adduct and at 77 K for the C6F5HgCl·phenanthrene adduct, suggestive of the stronger spin−orbit coupling in [(o-C6F4Hg)3].1005

Figure 229. Phosphorescence color change of [Au3Pz3]/silicahex resulted from the permeation of Ag+ ions into the silica nanochannels. Reprinted with permission from ref 990. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim.

Au(I) distances in the structures. This work has highlighted the potential of this class of trinuclear gold(I) metallacycles for applications in organic light-emitting devices (OLEDs). Wolf and co-workers recently prepared a series of trinuclear gold(I) thienyl pyrazolate complexes and investigated their luminescence properties.992 In the crystal structure of a representative complex, [Au3(PzT)3] (Figure 230), the intra-

Figure 230. Structure of [Au3(PzT)3].

molecular aurophilic separations fell within 3.30−3.38 Å, while the intermolecular aurophilic distances were found to be 3.2170(7), 3.5841(7), and 3.8049(7) Å.992 The presence of intra- and intermolecular Au(I)···Au(I) interactions was observed to stabilize the complex molecules in a staggered extended one-dimensional structure. The complex [Au3(PzT)3] was found to emit at 445 nm in the solid state at room temperature, and the emission was assigned to the thienyl pyrazolate ligand-centered fluorescence. Cooling to 77 K would lead to the appearance of a vibronic-structured emission band at 540 nm, which could be assigned to the ligand-centered phosphorescence.992

4. HETEROMETALLIC LIGHT-EMITTING SELF-ASSEMBLED MATERIALS Apart from the extensive works on homometallic complexes, the tendency of d8 and d10 metals to self-assemble via metal··· metal interactions has led to the construction of a number of heterometallic self-assembly materials. In particular, heterometallic clusters have contributed to the majority of the heterometallic self-assemblies that are emissive in nature. In the following, literature examples of light-emitting heterometallic complexes of only d8 and d10 metal centers will be highlighted. Heterometallic complexes that contain metal centers other than those of d8 and d10 electronic configurations, as well as mixedvalence metal complexes, are excluded from the scope of this Review. However, it is worth noting that the reaction of d8 and d10 metal complexes with other transition metal centers1006−1014 and lanthanides1015−1022 has also led to the preparation of a variety of other luminescent materials. Earlier in 1988, Abu-Salah and co-workers synthesized and characterized a series of heterometallic clusters, including the pentanuclear complexes, [Au 3 Cu 2 (CCPh) 6 ] − and [Au3Ag2(CCPh)6]−.1023−1025 In the crystal structure, the central Cu(I) unit was linked to three tetranuclear [Ag2Cu2(CCPh)4] subclusters by Cu(I)···Ag(I) interactions, and such heterometallic interactions were also present in each

3.4. Light-Emitting Mercury(II) Self-Assembled Materials

As compared to the coinage metal triads, copper(I), silver(I), and gold(I) systems, the self-assemblies of mercury(II) complexes, and, in particular, luminescent mercury(II) selfassemblies, remained rather scarce and under-explored in the literature. One reason might be due to the much weaker Hg(II)···Hg(II) interactions as compared to other metallophilic interactions.993 In 2001, Wong and co-workers reported a series of mercury(II) alkynyl complexes, [R′HgCCRCCHgR′] (R = fluorene-2,7-diyl, dihexylfluorene-2,7-diyl, 9((ferrocenylphenylene)methylene)fluorene-2,7-diyl, fluoren-9CS


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→ dσ*z2 phosphorescence, and the short-lived emission at higher energy was assigned to the corresponding fluorescence.1034 The emission of [AuPt(CN)2(μ-dppm)2]PF6, which was reported earlier by Che and co-workers to be an IL emission of dppm,1035 was reassigned to a pσz → dσ*z2 phosphorescence on the basis of additional data from transient decay and polarization measurements.1034 Another platinum(II)−silver(I) cluster, [Pt2(dppy)4(μ3-S)Ag3(μ3-S)2Pt2(dppy)4]3+, was reported by the group of Yam in 1996.1036 Crystallographic studies showed that the silver(I) centers were arranged in an isosceles triangle sandwiched between two [Pt2(μ-S)2(dppy)4] moieties. Both homometallic Ag(I)···Ag(I) and heterometallic Pt(II)···Ag(I) interactions were observed. The cluster was found to emit at ca. 560 nm in the solid state and in ethanol−methanol glass, which has been assigned to originate from the Ag3/Ag−S or Pt−Ag metalcentered cluster core.1036 The gold(I) complex, [Au(dpnapy)3]+ (dpnapy = 7diphenylphosphino-2,4-dimethyl-1,8-naphthyridine), was reported by Che and co-workers to exhibit a strong affinity toward Cu(I) and Cd(II) ions.1037 The reaction with Cu(I) would result in the formation of the heterometallic complex, [AuCu(dpnapy)3](ClO4)2. The crystal structure of the Au(I)− Cu(I) complex is shown in Figure 231.1037 The Au(I) and

of the subclusters.1023−1025 The structure and bonding properties of the complexes were later investigated by Schmidbaur and co-workers.1026 A related series of d10 heterometallic clusters, [nBu4N][Au3M2(CCC6H4R-p)6] (M = Cu, R = OMe, OnBu, OnHex, Me, Et; M = Ag, R = Et, OnHex), was synthesized by Yam and co-workers.1027 The clusters were found to be luminescent in solution and in the solid state at ambient and low temperatures. Detailed computational studies were performed, and the involvement of alkynyl moieties in the excited state has been suggested.1027 The incorporation of receptor units, including benzo-15-crown5, oligoether, and urea moieties, yielded a related series of luminescent Au(I)−Cu(I) clusters, [{Au3Cu2(CCPh)6}Au3{PPh2−C6H4−PPh2}3]2+.1028 These complexes were found to exhibit intense red emission at 619−630 nm in dichloromethane solution at 298 K. With different receptor units, the clusters were demonstrated to be efficient chemosensors for various cations or anions.1028 Since 1990, Forniés and co-workers reported a series of heteronuclear alkynyl complexes, [Pt2M4(CCR)8] (M = Cu, Ag, Au).1029,1030 The single-crystal structure of the silver(I) analogue showed that the metal atoms were arranged in a distorted octahedral fashion, such that the two platinum(II) metal centers were trans to each other. Several years later, the groups of Yam and Forniés independently reported the luminescence studies of various related heteronuclear clusters.1031,1032 Extension of this work to diyne-containing complexes has also been made by the group of Yam.1031 The crystal structure of [Pt2Cu4(CCR)8] was observed to feature a dimeric structure with ligand-unsupported Pt(II)···Pt(II) interactions (Pt(II)···Pt(II) distance = 3.116(2) Å).1031 Short Pt(II)···Cu(I) distances of 2.931(2)−3.021(2) Å were also observed, suggestive of the presence of weak heterometallic metal···metal interactions.1031 In the absence of bulky alkynyl ligands, the heterometallic complexes were found to show aggregation in the solution state, accompanied by an increase in intensity of the color of the complexes in solutions, as well as polymorphism in the solid state. Concentration-dependent UV−vis absorption studies supported the formation of dimeric structures in concentrated solutions.1031 The complexes were also found to be emissive in the solid state as well as in lowtemperature glass, in which a red shift in the luminescence energies was observed from the monomeric to dimeric to trimeric forms.1031 In 1994, Che and co-workers synthesized a heterometallic Au(I)−Cu(I) complex, [{Au(PPh 3 )(μ-C 7 H 5 N 2 )Cu(μC7H5N2)}2] (C7H5N2 = 7-azaindolate).1033 Despite the observation of short Au(I)···Cu(I) and Cu(I)···Cu(I) distances of 3.0104(6) and 2.941(1) Å, respectively, in the tetranuclear complex, theoretical studies by extended-Hückel molecularorbital calculations demonstrated that there only existed weak heterometallic Au(I)···Cu(I) interactions while cuprophilic interactions were absent.1033 The tetranuclear complex and its monomeric precursor, [Au(PPh3)(C7H5N2)], were both found to be emissive in acetonitrile solution. Both complexes were found to emit at about 510 nm, assignable to an intraligand excited state.1033 In the next year, the group of Crosby performed photophysical studies on a variety of heterometallic complexes, [Pt(CN)2Rh(tBuNC)2(μ-dppm)2]PF6, [Pt2(CN)4(μ-dppm)2], [AuRh(tBuNC)2(μ-dppm)2](PF6)2, [AuIrCl(CO)(μ-dppm)2]PF6, and [AuPt(CN)2(μ-dppm)2]PF6.1034 The long-lived emission of the former four complexes was assigned to a pσz

Figure 231. Crystal structure of [AuCu(dpnapy)3](ClO4)2. Reprinted with permission from ref 1037. Copyright 1998 The Royal Society of Chemistry.

Cu(I) metal centers both exhibited a trigonal planar geometry and formed a face-to-face structure. The Au(I)···Cu(I) distance of 4.469 Å indicated the absence of metallophilic interactions in the complex.1037 The heterometallic complex was observed to exhibit an emission band at 530 nm, which was attributed to an admixture of d(Au) → π*(dpnapy) and d(Cu) → π*(dpnapy) MLCT excited states.1037 Laguna, Pyykkö, and co-workers studied the heterometallic complex, [Au2Ag2(C6F5)4(OCMe2)2]n (Figure 232), and investigated its photophysical properties.1038 The Au2Ag2 core structure of the complex was found to form extended polymeric chains via intermolecular Au(I)···Au(I) interactions (3.1674(11) Å). Intramolecular Ag(I)···Ag(I) (3.1810(13) Å) and Au(I)···Ag(I) interactions (2.7829(9)−2.7903(9) Å) were also observed within the core unit. The Au(I)−Ag(I) complex was found to emit at 546 nm at 298 K.1038 The emission was assigned to be derived from metal-centered excited states. As both the gold(I) and the silver(I) precursor complexes were CT


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Figure 233. Structure of [Pt 2 Ag 4 (CCCCC 6 H 4 −CH 3 4)8(THF)4].

(MeCN)4]}(PF6)5 (dfpma = bis(difluorophosphine)-methylamine).1041 The complex was found to be arranged into an extended chain structure in the crystal packing. As shown in Figure 234, two [Rh2(dfpma)2(MeCN)4] moieties were

Figure 232. Structure of part of the polymeric chain of [Au2Ag2(C6F5)4(OCMe2)2]n. Reprinted with permission from ref 1038. Copyright 2000 American Chemical Society.

nonemissive in nature, it was suggested that the emission resulted from the heterometallic metal···metal interactions in the complex, and such an assignment was in accordance with TD-DFT studies.1038 The reaction between the platinum(II) alkynyl complex, [Pt2(dppm)2(CCPh)4], and [Cu(MeCN)4]PF6 resulted in the preparation of the tetranuclear heterometallic complex, [Pt2(dppm)2(CCPh)4{Cu(MeCN)}2](PF6)2.1039 The tetranuclear complex cation was found to adopt a face-to-face arrangement similar to the precursor complex, in which each platinum(II) center was coordinated to two trans alkynyl ligands and two trans dppm to form an eight-membered ring. A shorter Pt(II)···Pt(II) distance (3.0124(9) Å) was observed in the heterometallic complex when compared to the precursor compound (3.437(1) Å), which could be ascribed to the encapsulation of copper(I) centers by two adjacent alkynyl ligands that pulled the platinum(II) centers into close proximity due to the reduced donor strength of the alkynyl moieties upon coordination into copper(I).1039 The heterometallic complex was emissive in the solid state and in solutions at both room and low temperatures. In the solid state and glass state at 77 K, the complex was observed to exhibit dual emission properties. A similar emission profile could also been seen for the glass emission of the platinum(II) precursor complex. The higher energy bands at 460−495 nm were tentatively assigned as the intraligand phosphorescence of the bridging diphosphine ligands, while the lower energy bands at 540−600 nm were suggested to be derived from the 3MMLCT state.1039 The heteronuclear complex was found to emit at lower energies than the precursor complex, which could be attributed to the larger extent of Pt(II)···Pt(II) interactions in the former. The same group later extended their efforts to the preparation of a mixed-metal Pt(II)−Ag(I) complex, [Pt 2 Ag 4 (CCC CC6H4-CH3-4)8(THF)4] (Figure 233).1040 The structure of the heteronuclear complex exhibited a distorted octahedral array of Pt2Ag4 with two platinum(II) centers in a mutually trans disposition and the four silver(I) centers in the equatorial plane π-bonded by the alkynyl moieties. The heteronuclear complex was nonemissive in the solid state at 298 K, but exhibited a low-energy emission at 542−588 nm in solution at 298 K as well as in the solid state and glass at 77 K, which was tentatively assigned as derived from a LMMCT origin.1040 In 2002, Nocera and co-workers reported a heterometallic Rh(I)−Ag(I) complex, {[Rh 2 (dfpma) 2 (MeCN) 4 ] 2 [Ag-

Figure 234. (a) Monomer and (b) chain structures of {[Rh2(dfpma)2(MeCN)4]2[Ag(MeCN)4]}5+. Reprinted with permission from ref 1041. Copyright 2002 American Chemical Society.

sandwiched between every two [Ag(MeCN)4]+ units in the structure. The intra- and intermolecular Rh(I)···Rh(I) distances were, respectively, found to be 3.0498(10) and 3.902(2) Å.1041 The heterometallic complex was observed to emit weakly at 641 and 815 nm at room temperature, which was assigned to a dz2σ* → pzσ excited state derived from the Rh2 moiety.1041 A series of mixed-metal Au(I)−Cu(I) and Au(I)−Ag(I) η2alkynyl complexes, [{η2-(R3P)Au{CCC(CH2)Me}}2Cu]PF6 (R = Ph, p-Tol) and [(μ-dppf)Au2{η2-CCC(CH2)Me}2M]X (M = Cu, X = PF6; M = Ag, X = OTf), were prepared from the reaction of the gold(I) alkynyl precursor complexes, [(R3P)Au{CCC(CH2)Me}] (R = Ph, p-Tol) and [(μ-dppf)Au2{CCC(CH2)Me}2], with copper(I) or silver(I) ions.1042 It was observed that the coordination to other metal ions would cause a significant perturbation in the luminescence properties of the complexes. Photoexcitation of [(R3P)Au{CCC(CH2)Me}] at λ = 350 nm led to an orange-red emission band at ca. 606−664 nm, assignable to a metal-perturbed 3[π → π*(CC)] IL or a 3[σ(Au−P) → π*(CC)] MLCT origin, although the possible assignment of the emission as derived from states associated with the copper(I)-π-alkynyl core could not be excluded.1042 The emission was of lower energy when compared to the precursor compound, [(R3P)Au{CCC(CH2)Me}], because of the CU


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reduction in the π*(CC) orbital energy upon π-coordination to copper(I). On the other hand, [(μ-dppf)Au2{CCC( CH2)Me}2] was nonemissive in nature because of the quenching effect of the ferrocene unit. However, the encapsulation of copper(I) or silver(I) has been observed to switch on the luminescence of the complex, in which the resulting mixed-metal complexes, [(μ-dppf)Au2{η2-CCC( CH2)Me}2Cu]PF6 and [(μ-dppf)Au2{η2-CCC(CH2)Me}2Ag]OTf, were found to emit at ca. 565−583 nm in the solid state at room temperature. The Au(I)−Cu(I) derivative was found to emit at lower energy than the Au(I)−Ag(I) derivative, and such an emission was tentatively assigned to be derived from states of a LMCT parentage mixed with a metalcentered nd9(n+1)s1 state, with some π → π*(CC) IL character.1042 In 2004, a class of gold(I)−silver(I) clusters, [Au3(μ3E)Ag(PPh2py)3](BF4)2 (E = O, S, Se; PPh2py = diphenylphosphino-2-pyridine) and [Au3(μ3-S)Ag(PPh24-Mepy)3](BF4)2 (Figure 235), was reported by Laguna and co-

centered excited state modified by metallophilic interactions.1045 With the utilization of various N-heterocyclic carbene (NHC) ligands, the group of Catalano prepared a series of heterometallic NHC complexes. In 2004, they reported the synthesis of a chiral heterometallic coordination polymer, {[AuAg[(py)2im]2(NCCH3)](BF4)2}n.1046 The helical polymeric species was found to exhibit an extended chain structure with short Ag(I)···Au(I) distances of 2.8359(4)−2.9042(4) Å (Figure 236).1046 The heterometallic complex was found to

Figure 235. Structure of [Au3(μ3-E)Ag(PPh2py)3]2+ and [Au3(μ3S)Ag(PPh24-Mepy)3]2+.

workers.1043 The structure of the cluster featured a Au3Ag core with a chalcogenide ligand capping the gold(I) centers. Intramolecular Au(I)···Au(I) and Ag(I)···Au(I) interactions were present to stabilize the structure. In addition, intermolecular aurophilic interactions associated with a short Au(I)···Au(I) distance of 3.059(3) Å were present to connect two adjacent Au3Ag moieties.1043 The complexes were found to be highly emissive, showing an emission band from ca. 466 to 670 nm in the solid state at low temperature when the chalcogenide ligand was changed from O to S to Se.1043 By reacting the polymeric precursor complex, (MC CC6H4R-4)n (M = Cu, Ag; R = H, CH3, OCH3, NO2, COCH3), with [M′2(μ-Ph2PXPPh2)2(MeCN)2](ClO4)2 (M′ = Ag, Cu; X = NH, CH2), Chen and co-workers prepared a series of hexa-, octa-, and hexadecanuclear Ag(I)−Cu(I) alkynyl clusters.1044 The complexes were found to exhibit phosphorescence in the solid states and in fluid solutions at 298 and 77 K. Clusters with electron-deficient 4-acetylphenylalkynyl and 4nitrophenylalkynyl ligands were found to show an intraligand π−π* emission. For the other clusters, the emissive origin was tentatively derived from a 3LMCT(CCC6H4R-4 → Ag4Cu2 or Ag6Cu2) excited state, mixed with a cluster-centered d → s character.1044 Through the cyclotrimerization of arylacetylenes, Chen and co-workers also reported a series of heteronuclear clusters, [Au5Ag8(μ-dppm)4{1,2,3-C6(C6H4R-4)3}-(CCC6H4R-4)7](SbF6)3 (R = H, CH3, tBu).1045 The cluster featured a Au5Ag8 core connected by the ligands in the presence of intramolecular Au(I)···Au(I), Ag(I)···Ag(I), and Au(I)···Ag(I) interactions. The clusters were found to exhibit red luminescence from 630 to 680 nm, assignable to be originated from an admixture of MLCT (Au5Ag8 → CCC6H4R-4) state and a metal cluster-

Figure 236. Crystal structure drawing showing the extended structure of one hand of {[AuAg[(py)2im]2(NCCH3)](BF4)2}n and the extended, helical metal core with alternating Au(I)···Ag(I) separations of 2.8359(4) and 2.9042(4) Å. Reprinted with permission from ref 1046. Copyright 2004 American Chemical Society.

emit at 515 nm in the solid state, possibly from a ligandcentered excited state or a state modified by metallophilic interactions.1046 Subsequently, works by the same group resulted in the preparation of a class of related luminescent heterometallic complexes, most of which were found to exhibit short metal···metal distances in the extended chain structures.1047−1049 The reaction between [M2(dppm)2]2+ (M = Au, Ag, Cu) with various transition metal precursor complexes led to the formation of a series of heterometallic d8−d10 complexes, such as [PtCu2(edt)(μ-SH)(dppm)3]+ (edt = 1,2-ethanedithiolate), [PtCu 2 (diimine) 2 (edt)(μ-dppm) 2 ] 2 + , [Au I I I Cu I 8 (μdppm)3(tdt)5]+ (tdt = toluene-3,4-dithiolate), [AuIII3AgI8(μdppm)4(tdt)8]+, [AuIIIAuI4(μ-dppm)4(tdt)2]3+, and [PtM(dppm)2(CCR)2]+.1050−1052 Metal···metal interactions between d8 and d10 metal centers were observed in these complexes. The Pt(II)−M complexes were found to exhibit long-lived emission. For example, the PtCu2 complexes showed phosphorescence in the solid state at room temperature and in acetonitrile glass at 77 K, and the emission was assigned as CV


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derived from a triplet ditholate-to-Pt LMCT excited state.1050 For the series of Au(III)−M complexes, only the AuIIIAuI4 derivative was emissive in the solid state at 298 K and in low temperature glass. The crystal structure of this complex displayed short Au(I)···Au(I) distances of 3.1253(13)− 3.1668(9) Å, while the Au(III)···Au(I) distances were much longer (3.431−4.177 Å).1051 Laguna and co-workers reported the heterometallic cluster, (NBu4)2[Au(3,5-C6F3Cl2)2Ag4(CF3COO)5], which displayed a square pyramidal AuAg4 structure stabilized by Ag(I)···Ag(I), Au(I)···Ag(I) interactions and Au−C−Ag (3c, 2e) bridges.1053 In contrast to other Au(I)−Ag(I) complexes that showed rich photoluminescence, the lack of Au(I)···Au(I) interactions probably rendered the AuAg4 cluster nonemissive in the solid state and weakly emissive in chloroform solution with π−π* emission from the perhalophenyl moieties.1053 The group later used perhalophenyl ligands to prepare a luminescent Au(I)− Cu(I) isocyanide cluster, [AuCu(C6F5)2(NC−Ph)2].1054 The complex was found to dimerize to form a tetranuclear Au2Cu2 unit with intramolecular Au(I)···Cu(I) and intermolecular Au(I)···Au(I) interactions. Intense luminescence at ca. 442 nm was observed in the solid state at 298 and 77 K, which was suggested to arise from the cluster core.1054 However, variation of the isocyanide ligand led to the preparation of nonemissive analogues, [AuCu(C6F5)2(NC−CH3)2] and [AuCu(C6F5)2(NC−CHCH−Ph)2], which did not dimerize via Au(I)···Au(I) interactions in the crystal structure. Such observations were again suggestive of the importance of the presence of aurophilic interactions to the photoluminescence of the heteronuclear complexes.1054 In 2006, the groups of Schmidbaur and Rösch collaborated to report the pentanuclear heterometallic self-assembled species, [(Me3P)2Ag]+[Ag2(PhCCAuCPh)3]−.1055 Crystal structure analysis showed that the anion consisted of three rodshaped [PhCCAuCCPh]− anions linked by two silver(I) ions to give a Ag2Au3 core unit in the presence of Ag(I)···Au(I) metallophilic interactions. Computational studies by density functional theory calculations suggested that the anionic species was largely ionic in nature. The heteronuclear aggregate was highly emissive in the solid state and in solution at room temperature, showing yellow-green emission at 534−556 nm.1055 A series of heterometallic chalcogenido clusters, [E(AuPPh2py)3M]2+ and [E(AuPPh2CH2CH2py)3M]2+ (E = O, S, Se; M = Ag, Cu), were synthesized by Gimeno and coworkers.1056 The Au(I)−Ag(I) clusters were found to selfassemble into Au3Ag units, while the Au(I)−Cu(I) clusters were shown to isomerize into two forms.1056 The clusters were all luminescent, showing emission bands, which could be assigned to orginate from an [E2− → Au3M] LMCT excited state. The emission energy was found to span across the visible region depending on the nature of the chalcogenido ligand.1056 The groups of Amouri and Yam reported the self-assembly of a series of mixed-metal coordination polymers with quinonoid backbones with coinage metal as nodes using [Cp*M(η4benzoquinone)] (M = Rh, LRh; M = Ir, LIr) as the starting material.1057 These include the one-dimensional (1D) coordination polymers, [{Ag3(LM)2(MeCN)2(OTf)}{LM}(OTf)2]n and [{Cu2(LIr)2(MeCN)2(OTf)2}{LIr}(OTf)]n, and the twodimensional (2D) network [{Ag2(MeCN)2}{LIr}2(OTf)2]n. In particular, the 2D coordination polymer was found to involve C−H/π interactions between 1D polymers of enchained macrocycles featuring Ag(I)···Ag(I) interactions. The lumines-

cence properties of the 1D polymers, [{Ag3(LM)2(MeCN)2(OTf)}{LM}(OTf)2]n, which exhibited a shorter Ag(I)···Ag(I) contact of 3.37 Å than the 2D polymer, have been studied. The complexes were found to show weak emissions in acetonitrile solutions at 298 K at 406−421 nm, which was assigned to the metal-perturbed intraligand emission of benzoquinone, suggestive of some dissociation of the polymers into oligomers that were in equilibrium in the solution state.1057 The vapochromic properties of a series of heterometallic clusters, [{Ag2L2[Au(C6F5)2]2}n] (L = Et2O, Me2CO, THF, MeCN), were studied by López-de-Luzuriaga, Facker, and coworkers.1058 In the crystal structure of [{Ag2(THF)2[Au(C6F5)2]2}n], homometallic Au(I)···Au(I) (3.1959(3) Å), Ag(I)···Ag(I) (3.2291(6) Å), and heterometallic Au(I)···Ag(I) interactions (2.7267(5)−2.7903(9) Å) were observed.1058 Exposure of a solid sample of the heterometallic complexes to VOC vapors would lead to substitution of the solvent moieties by the new VOC, and the substitution would occur in the order MeCN > Me2CO > THF > Et2O, with diethyl ether being the easiest to be substituted.1058 The Au(I)−Ag(I) complexes were found to be emissive in the solid state at ambient and low temperatures, and the emission was suggested to have originated from the tetranuclear cluster cores modified by Au(I)···Au(I) or Au(I)···Ag(I) interactions and by molecular aggregation.1058 Since 2008, Koshevoy and co-workers have devoted their studies to heteronuclear coinage metal clusters.1059−1061 Earlier work of the group has resulted in the report of the Au6Cu6 cluster, [Au 3 (PPh 2 (C 6 H 4 ) 2 PPh 2 ) 3 {Au 6 Cu 6 (CCPh) 12 }](PF6)3, which was self-assembled from simple gold(I) and copper(I) precursors.1059 The Au9Cu6 cluster was found to consist of a central [Au6Cu6(CCPh)12] fragment that was surrounded by an outer [Au(PPh2(C6H4)2PPh2)]3 fragment in the presence of Au(I)···Au(I) interactions (Figure 237). The

Figure 237. Structure of [Au3(PPh2(C6H4)2PPh2)3{Au6Cu6(C CPh)12}]3+ (alkynyl moieties have been omitted for clarity).

central fragment was in turn made up of six linear [PhC CAuCCPh] rod-shaped moieties held together by Au(I)··· Au(I) and Cu(I)···Au(I) interactions, as well as π-CC−Cu bonding.1059 The cluster was found to exhibit an intense triplet emission band at ca. 590 nm in solution and in the solid state.1059 With the support of subsequent theoretical studies as well as photophysical studies of a Au(I)−Ag(I) analogue, the origin of the emission band could be assigned to metal-centered excited states associated with the cluster core.1060,1061 Extension of the work to heterometallic clusters of different nuclearities with different metal centers has been made by the group of Koshevoy.1062,1063 In 2009, a series of Au(I)−Cu(I) clusters, [{Au3Cu2(CCC6H4X)6}Au3(PR2C6H4PR2)3][PF6]2 (X = NO2, H, OMe, NMe2; R = C6H5, NC4H4), were prepared.1062 The complexes similarly consisted of a central CW


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luminescence upon coordination to the Cu2X2 moieties, but the nature of the excited states remained elusive.1068 In 2009, the group of Wang reported a strongly luminescent heterometallic cluster with a hypercoordinated carbon center, [Au6Ag2(C)(dppy)6](BF4)4.1070 The gold(I) and silver(I) centers were found to be arranged into a bicapped octahedron in the crystal structure, in which the gold atoms formed an octahedron with two silver atoms each capping one of the two Au3 triangles. The Au(I)···Au(I) and Au(I)···Ag(I) distances of 2.9134(8)−3.0164(6) Å were illustrative of the presence of intramolecular metallophilic interactions.1070 In contrast to the homometallic precursor complex, [Au6(C)(dppy)6](BF4)2, which was almost nonemissive in solutions, the heterometallic cluster was found to exhibit an intense red emission at 625 nm in dichloromethane solution at 298 K, which could be assigned to a cluster-based metal-centered excited state. In particular, the complex was found to exhibit good photostability. This has been attributed to the enhanced rigidity of the structure as the additional silver(I) centers could reduce the fluxionality of the dppy ligands, and the dppy ligands themselves could also protect the cluster from quenchers.1070 Another type of Au(I)−Ag(I) cluster, [Au14Ag4(C CPh)12(PPh2C6H4PPh2)6](PF6)4, was prepared in 2010.1071 In the crystal structure, the molecule consisted of two [Au7Ag2(CCPh)6(PPh2C6H4PPh2)3]2+ units connected by unsupported Au(I)···Au(I) interactions of the [Au4]2+ core. Each of the Au7Ag2 cores was in turn made up of an alkynyl [Au3Ag2(CCPh)6]− moiety surrounded by a [Au4(PPh2C6H4PPh2)3]3+ unit.1071 In dichloromethane solution at room temperature, the complex was found to exhibit an intense red emission at 649 nm irrespective of the excitation wavelength (400−520 nm), which originated from a triplet metal-centered {[Au4]2+ → [Au3Ag2(CCPh)6]−} transition.1071 Upon encapsulation by silica nanoparticles, the heteronuclear cluster was demonstrated to be a dye suitable for one-photon and two-photon imaging in human mesenchymal stem cells. In 2010, Laguna and co-workers reported a series of Au(I)− Ag(I) clusters with mechanochromic and vapochromic behaviors (Figure 239). 1072 The complex, [Au2 Ag 2 (4-

heterometallic alkynyl cluster wrapped by an outer Au(I) phosphine “belt”. The complexes all exhibited intense emission that originated from a triplet metal-centered excited state with a high photoluminescence quantum yield of up to 0.96, and showed negligible quenching of luminescence under aerated conditions. It was suggested that the bulky ancillary and bridging ligands in the outer shell would offer protection of the emissive cluster core, resulting in the intense luminescence that was insensitive to oxygen.1062 The two-photon absorption (TPA) and two-photon-induced luminescence properties of these Au(I)−Cu(I) clusters were also investigated, and the variation of the nature of substituents on the ligands provided a large TPA cross-section in biological windows from 760 to 840 nm, making the clusters potential candidates for applications in time-resolved luminescence imaging.1063 Using the same concept, the group subsequently developed a number of self-assembled heteronuclear coinage metal clusters with highly intense luminescence.1064−1067 Examples of the deviatives with high nuclearities included [{Au8Ag10(C CPh)16}{(PhCCAu)2PPh2(C6H4)3PPh2}2]2+,1066 [Au12Ag12(CCPh)18X3(P3P)3]3+ (X = Cl, Br, I; P3P = 4,4″PPh2(C6H4)3PPh2), and [Au21Ag30(CCPh)36Cl9(P5P)3]6+ (P5P = 4,4′′′′-PPh2(C6H4)5PPh2).1067 X-ray diffraction study revealed the topology of the molecular cation, in which the heterometallic linear-like cluster core was wrapped by gold(I) phosphine moieties in the presence of homo- and heterometallic metal···metal interactions.1066,1067 Catalano and co-workers investigated the addition reaction of copper(I) halides with some gold(I) N-heterocyclic carbene complexes, and prepared a series of tri- and dinuclear Au(I)− Cu(I) heteronuclear complexes.1068,1069 The structures of the complexes are shown in Figure 238. X-ray crystallography

Figure 238. Structures of the heteronuclear Au(I)−Cu(I) complexes.

demonstrated that the trinuclear complexes exhibited a “butterfly-shaped” Cu2X2 core coordinated to two imine ligands, with the gold(I) metal center in close proximity to the center of the Cu(I)···Cu(I) linkage.1068 The gold(I) and two copper(I) centers in the trinuclear complexes were arranged in a triangular fashion. The Au(I)···Cu(I) distances in the complexes were found to range from about 2.8 to 3.0 Å, while the Cu(I)···Cu(I) distances varied from about 2.5 Å in the iodo complexes to about 2.9 Å in the chloro complexes.1068 The complexes were all found to be emissive in the solid state. The results showed a large perturbation of the gold(I)

Figure 239. Mechano- and vapochromic behaviors of [Au2Ag2(4C6F4I)4]n.

C6F4I)4]n, was shown to be mechanochromic in nature, in which the yellow emission of the heterometallic complex would turn to orange upon grinding.1072 In the presence of organic solvents, the complex would form adducts with the formula, [Au2Ag2(4-C6F4I)4L2]n·xL, where L = Me2CO (x = 2, 3), THF (x = 0, 4), and MeCN (x = 0, 5).1072 The emission color could be rapidly switched by exposure to different solvents, ranging CX


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revealed the presence of S-supported metallophilic interactions in the structure.1076 The heteronuclear complex was found to show a ligand-centered emission at 374 nm in low-temperature glass, probably from the phenyl or carboxylato moieties. More interestingly, the complex was found to show cytotoxicity against HeLa, A2780, and A2780cis cell lines in the in vitro studies.1076 The group of Gimeno recently reported a series of heterometallic complexes, [AuAg(OTf)(μ-SC 6F 5 )(P ∧ N)] (P∧N = PPh2py, PPh2CH2CH2py, PPhpy2), [AuCu(μ-SC6F5)(P ∧ N)(NCMe)]PF 6 (P ∧ N = PPh 2 py, PPh 2 CH 2 CH 2 py, PPhpy2), [Au2M(μ-SC6F5)(μ-PPh2py)2]X (M = Ag, X = OTf; M = Cu, X = PF6), and [Au4M(μ-SC6F5)3(μ-PPh2py)4]X3 (M = Ag, X = OTf; M = Cu, X = PF6).1077 The complexes were found to crystallize into different structures that were supported by metallophilic interactions.1077 The heterometallic complexes were observed to exhibit dual emissive properties. In general, at 77 K, the complexes were observed to show a higher-energy IL(monophosphine) emission band and a lower-energy 3 LMMCT (S → Au−M) emission band that was modulated by the heterometal.1077 Wang and co-workers reported the formation of a gold(I)− copper(I) cluster with a hypercoordinated carbon, [CAu6Cu2(dppy)6](BF4)4.1078 As shown in Figure 241, the

from vivid red to bright yellow. XRD studies revealed that most of the adducts were arranged into infinite polymeric chains in the presence of Au(I)···Au(I) interactions.1072 The group later extended the work to the synthesis of a related heteronuclear complex, [Au2Ag2(2-C6F4I)4(tfa)2]2− (tfa = trifluoroacetate).1073 The addition of a few drops of acetonitrile to the powder of the complex was found to trigger a dramatic change in the emission color from green to yellow to orange-red, associated with the formation of a polymeric compound, [Au2Ag2(2-C6F4I)4(MeCN)2]n.1073 Time-resolved emission studies demonstrated the formation of a metastable yellowemitting intermediate, revealing a two-stage scenario for the aurophilicity-driven self-assembly process. The vapochromism of a heteronuclear N-heterocyclic carbene (NHC)-containing complex, [Au{im(CH2py)2}2{Cu(MeCN)2}2](PF6)3, was reported by Strasser and Catalano in 2010.1074 Upon exposure to methanol, the emission color of the complex was found to reversibly change from blue to green, and a new species, [Au{im(CH2py)2}2{Cu(MeOH)}2](PF6)3, was formed.1074 It was observed that in the latter complex, the Au(I) NHC moieties were coordinated to Cu(I) and resulted in heterometallic Au(I)···Cu(I) interactions associated with a short distance of 2.7195(7) Å between the Au(I) and Cu(I) centers.1074 The partial and complete loss of methanol from the complex would further shift the emission to the yellow and yellow-orange regions, respectively. This was suggested to be associated with the ligand exchange reactions of the heterometallic complex that resulted in an on−off switching of the metallophilic interactions.1074 Similar vapochromism was also observed in the presence of acetone and water, but with emission maxima at different energies. Using gold(I) bis-alkynyl complexes as the starting material, Eisenberg and co-workers synthesized a series of luminescent gold(I)−copper(I) alkynyl clusters, [Au4Cu4(ethisterone)8] (Figure 240), [PPN][Au 3 Cu 2 (ethisterone) 6 ], [PPN]-

Figure 241. Crystal structure of [CAu6Cu2(dppy)6](BF4)4. Reprinted with permission from ref 1078. Copyright 2011 The Royal Society of Chemistry.

Figure 240. Structure of [Au4Cu4(ethisterone)8].

single-crystal structure of the complex featured a CAu6Cu2 core in which the six gold(I) metal centers were arranged into a trigonal prism, and each of the two opposite Au3 faces was capped by a copper(I) metal center.1078 Both homo- and heterometallic metal···metal interactions were present, as revealed by the short Au(I)···Au(I) and Au(I)···Cu(I) distances of 2.7529(6)−2.8672(7) and 2.855(1)−2.907(2) Å, respectively.1078 The Au(I)−Cu(I) complex was observed to exhibit red photoluminescence in dichloromethane solution at 619 nm at room temperature, and it was also intensely emissive in the solid state.1078 Meyer and co-workers used thioether-substituted pyrazole ligands to prepare a series of heterometallic complexes with an inner gold ring framed with an outer silver ring, [AuAg(μL)(BF4)]4 (Figure 242).1079 In the crystal structures, each of the four gold(I) metal centers was coordinated to two pyrazolate nitrogen atoms in a linear fashion. The gold(I) centers were aligned into a square with intramolecular Au(I)··· Au(I) distances of 3.03−3.21 Å. The distances between the outer Ag(I) centers and the inner Au(I) centers were found to

[Au3Cu2(1-ethynylcyclopentanol)6], and [PPN][Au3Cu2(1ethynylcyclohexanol)6] (PPN = bis(triphenylphosphine)iminium).1075 In particular, [PPN][Au3Cu2(1-ethynylcyclopentanol)6] and [PPN][Au3Cu2(1-ethynylcyclohexanol)6] were respectively observed to crystallize into two and three polymorphs with different solid-state emission colors ranging from blue to yellow.1075 Crystallographic studies showed that the intramolecular Au(I)···Au(I) and Cu(I)···Au(I) distances would vary from one polymorph to another.1075 All of these Au(I)−Cu(I) complexes were emissive in the solid state at ambient and low temperatures. DFT and TD-DFT calculations suggested that the emissive origin of [Au4Cu4(ethisterone)8] was mainly a 3CC state, while for the other Au3Cu2 clusters, their emission originated from a ligand-to-cluster charge transfer state.1075 The collaborative work between the groups of Laguna and Sordo led to the report of a dinuclear complex, [AgAu(PPh3)2(cpa)] (Hcpa = 2-cyclopentylidene-2-sulfanylacetic acid).1076 The short Ag(I)···Au(I) distance of 3.0463(10) Å CY


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Blanco and co-workers reported the heterometallic Au(I)− Ag(I) clusters, [Ag 12 Au 10 (CCPh) 17 (OTf) 5 (PPh 3 ) 3 ], [Ag26Au20(CCPh)34(OTf)12(PPh3)6(tht)2], and [Ag14Au10(CCPh)17(OTf)6(PPh3)3(py)4]OTf.1081 The heterometallic complexes were all found to be strongly emissive in solution and in the solid state at 298 and 77 K. The complexes were found to emit at 570 nm, which was assigned to be derived from a mixture of metal-to-alkynyl MLCT and metalcluster-centered excited states.1081 In low-temperature glass, the complexes were found to show an additional high-energy emission band at about 450 nm from IL alkynyl and/or phosphine excited states.1081 Recent work by Chen and co-workers led to the report of a Ag16Cu9 heterometallic alkynyl cluster.1082 As shown in Figure 244, the core structure was formed by Ag(I)···Ag(I) and

Figure 242. Structure of [AuAg(μ-L)(BF4)]4.

be 2.84−2.90 Å, indicative of some hetermometallic interactions.1079 These complexes were observed to emit in 2methyltetrahydrofuran glass at 77 K. Vibronically structured emission was observed at 400−450 nm region, the origin of which was tentatively assigned to ligand-centered excited states.1079 The group of Wang also reported a series of gold(I)− silver(I) clusters, [SAu3Ag(L)3](BF4)2 (L = 2-diphenylphosphino-4-methylpyridine, 2-diphenylphosphinopyridine, 2-diphenylphosphinopyrimidine).1080 The complexes were found to exist in two polymorphs with different emission colors. For [SAu3Ag(2-diphenylphosphino-4-methylpyridine)3](BF4)2, recrystallization from diffusion of diethyl ether vapor into a concentrated solution of the complex in acetonitrile would lead to the isolation of the blue-emitting form with intramolecular Au(I)···Au(I) distances of 3.0993(5)−3.2053(5) Å in the two Au3Ag subunits and a short intermolecular Au(I)···Au(I) distance of 2.9584(6) Å between the subunits.1080 On the other hand, the recrystallization from acetonitrile would give rise to a yellow-green-emitting polymorph with intramolecular Au(I)···Au(I) distances of 3.1150(3)−3.4616(3) Å and two intermolecular Au(I)···Au(I) distances of 3.4616(3) Å.1080 The interconversion between the two polymorphs could also be reversibly achieved by removing diethyl ether from the blueemitting polymorph and exposing the other polymorph to diethyl ether vapor, as illustrated in Figure 243. The emissive states of these heterometallic clusters have been assigned to a LMMCT state mixed with metal-centered (ds/dp) character.1080

Figure 244. Structure of the Ag16Cu9 cluster. Reprinted with permission from ref 1082. Copyright 2012 American Chemical Society.

Ag(I)···Cu(I) linkages stabilized by three triphosphine ligands, and it was special that the alkynyl ligands exhibited five types of asymmetric bonding modes in the cluster.1082 Upon excitation at λ > 300 nm, the cluster was found to exhibit an emission band in the near-infrared region in dichloromethane solution and in the solid state at room and low temperatures, which was suggested to have originated from a triplet cluster-centered excited state modified by extensive metallophilic interactions.1082 The group of Chen also reported a series of intensely emissive heteronuclear PtAg2 alkynyl complexes with bis(diphenylphosphinomethyl)phenylphosphine (dpmp) ligands (Figure 245).1083 The structures of the complexes were found to be stabilized by substantial Pt(II)···Ag(I) interactions, and the PtAg2 centers were observed to be doubly linked by the dpmp ligands. The complexes showed rich luminescence in the solid state and in dichloromethane solutions at room temperature. Upon modulation of the substituents as well as the conjugation of the alkynyl moieties, the emission maxima of

Figure 243. Reversible interconversion between the two polymorphs. Luminescence spectra were taken under the irradiation of 365 nm. Reprinted with permission from ref 1080. Copyright 2012 The Royal Society of Chemistry.

Figure 245. Structure of the PtAg2 alkynyl complexes. CZ


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the complexes could be readily tuned from 463 to 661 nm across the visible region.1083 The emissive origin was assigned to 3LLCT [π(CCR) → π*(dpmp)] and 3IL [π → π*(C CR)] excited states with significant PtAg2 cluster centered 3[d → p] character. In addition, the solid-state phosphorescence of the complexes was found to show reversible stimuli-responsive behavior toward the exposure of organic solvent vapors, which has been ascribed to the perturbation of Pt(II)···Ag(I) metal− metal interactions in the excited states.1083 The groups of Czerwieniec, López-de-Luzuriaga, and Yersin together reported the gold(I) complexes, [(Au{4-C6F4(4C6BrF4)})2(μ-L∧L)] (L∧L = dppm, dppb), which would further react with silver trifluoroacetate to form the luminescent heterometallic complexes, [Ag2Au2{4-C6F4(4C6BrF4)}2(CF3CO2)2(μ-L∧L)]n.1084 As shown in the crystal structure of [Ag 2 Au 2 {4-C 6 F 4 (4-C 6 BrF 4 )} 2 (CF 3 CO 2 ) 2 (μdppm)]n in Figure 246, the heterometallic complexes were

Figure 247. Structures of [Ag 4 (PPPP) 2 ] 4+ , [Au 4 (PPPP) 2 ] 4+ , [AuAg3(PPPP)2]4+, and [Au2Cu2(PPPP)2(NCMe)4]4+.

more extensive metallophilic interactions inside the cluster core.1085 The heteronulear complexes, [Pt 2 Ag(μ-dpppy) 2 (C CC 6H4 R-4)4 ](ClO4 ) (R = H, CH3; dpppy = 2,6-bis(diphenylphosphino)pyridine) (Figure 248), were reported by

F i g u r e 2 4 6 . C r y s t a l s tr u c t u r e o f [ A g 2 A u 2 { 4 - C 6 F 4 ( 4 C6BrF4)}2(CF3CO2)2(μ-dppm)]n. Reprinted with permission from ref 1084. Copyright 2013 The Royal Society of Chemistry.

arranged into extensive polymeric chains with alternating Au2 and Ag2 units with Au(I)···Ag(I) interactions (2.7984(8)− 2.9106(10) Å) along the chain.1084 The crystals of the Au(I)− Ag(I) complexes were found to exhibit blue luminescence at 436−439 nm. With the support of DFT and TDDFT calculations, the emission was assigned to originate from a ligand-to-ligand charge transfer (LLCT) state from the perhalophenyl groups to the diphosphine ligand.1084 Very recently, a class of tri- and tetranuclear coinage metal clusters with polydentate phosphine ligands, [AuM2(PPP)2]3+ (M = Au, Cu, Ag), [AuAg3(PPPP)2]4+, and [Au2Cu2(PPPP)2(NCMe)4]4+, as well as their homonuclear analogues, [Ag4(PPPP)2]4+ and [Au4(PPPP)2]4+, were prepared by Koshevoy and co-workers.1085 The structures of the tetranuclear complexes are shown in Figure 247. In general, the trinuclear clusters featured a linear metal core, while the tetranuclear clusters were arranged in a planar star-shape. The incorporation of isocyanide moieties into the tetranuclear cluster core would result in a structural motif with a Au2 unit bridged by two phosphines that were linked to two spatially separated Cu(I) centers. With the exception of the tetrasilver(I) cluster, all complexes were emissive in the solid state, exhibiting phosphorescence at 450−563 nm that originated from triplet metal cluster-centered excited states. In particular for the tetranuclear clusters, an increase in the number of gold(I) centers would lead to an increase in the luminescence quantum yield in the solid state, possibly associated with the

Figure 248. Crystal structure of [Pt2Ag(μ-dpppy)2(CCC6H4CH34)4]+. Reprinted with permission from ref 1086. Copyright 2014 The Royal Society of Chemistry.

Wei and co-workers.1086 The complexes were synthesized by the self-assembling reaction between anionic [Pt(CCC6H4R4)4]2− and cationic [Ag2(μ-dpppy)3]2+ precursors. Both complexes were strongly emissive in the solid state but weakly emissive in solutions at ambient temperature. The incorporation of the complexes into monodispersed silica nanoparticles was demonstrated to enhance the luminescence in the solution state. By measuring the luminescence quenching by cyanide anions in buffer solution, the hybrid material was shown to be an effective sensor for anion recognition.1086 Ng, Li, and co-workers recently reported a heterometallic cluster in which a silver(I) center was sandwiched between two trinuclear gold(I) pyrazolate moieties, [Ag{(AuL)3}2]PF6 (HL = 3-(2-thienyl)-5-phenyl-1H-pyrazole).1087 In the crystal structure in Figure 249, the naked Ag(I) center was observed to be bound solely by six short Au(I)···Ag(I) contacts of 2.772(1)−2.831(1) Å.1087 The formation of this heterometallic species was found to be associated with reversible morphological changes, in which an increase in the concentration of Ag+ would give rise to the formation of yellow-emitting nanodots of increasing sizes.1087 The switching-on of the yellow emission could also be achieved by grinding a solid sample of DA


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fundamental guiding principles for the design of light-emitting transition metal-based materials. In combination with conventional chemical modifications, the manipulation of noncovalent inter- and intramolecular interactions would offer an important strategy for the fine-tuning of the structural, morphological, and spectroscopic properties of the transition metal complexes and their self-assemblies. The exhibition of rich emission properties in the solid state as well as in solutions has enabled a wide diversity of applications. In particular, many of the metal-containing luminophores have been observed to exhibit long-lived emissive excited states with lifetimes in the microsecond regime. This renders the metal complexes attractive candidates in chemosensing and biomedical applications, including molecular recognition, biolabeling, and molecular imaging,149,1089,1090 as well as in materials and energy applications such as in the fabrication of phosphorescent OLEDs for full-color display and solid-state lighting.1091−1093 A number of d8 and d10 metal complexes, with cis-platin and auranofin being the classical examples, have been shown to be cytotoxic in nature.1094−1098 These materials have been employed as molecular medicines for the suppression of tumor growth in cancer treatment and some as photosensitizers for photodynamic therapy.1099 The construction of metallosupramolecules and the study of their supramolecular interactions with biological molecules may open new opportunities for the exploration of artificial biomimetic molecules, enabling the understanding and mimicking of the three-dimensional structures of DNA, proteins, and other important biomolecules. In addition, this may provide insights into the understanding of the thermodynamics of protein-like folding and unfolding processes. The modulation of aggregation and supramolecular assembly behavior of luminescent transition metal complexes has given rise to obvious spectroscopic changes, which may be exploited for the detection of biomolecules and enzymatic activities, representing novel strategies for early diagnosis of diseases such as cancer, cystic fibrosis, Alzheimer’s disease, Parkinson’s disease, and others. Apart from their potential applications in biomedical sciences, the ability to align and orient molecules in a desirable fashion may also lead to efficient artificial photosynthesis. Inspired by photosynthesis in nature, the ability to manipulate supramolecular control over the synthesis of metal-containing chromophoric materials for the harvesting of sunlight as well as novel materials for water-splitting would also represent research areas of topical interest.1100−1102 In addition, alignment and supramolecular control and hierarchical assembly of functional metal complexes also play an important role in determining the charge transport and optical properties of materials for efficient optoelectronics and electronics, such as organic light emitting d i o d e s ( O L E D s ) , 1 0 9 1 − 1 0 9 3 o r g a n i c p h o t o v o l t ai c s (OPVs),1103,1104 organic field effect transistors (OFETs),1105 organic memory devices,1106,1107 and others. All of these applications are highly related to our daily life, and hence one should not underestimate the importance of supramolecular metal-containing materials. The uniqueness of d8 and d10 metal−ligand chromophores and luminophores lies in their low-dimensionality and their ability to form metal−metal interactions that could not be reproduced by their related octahedral counterparts. Their uniqueness has led to the opening of a whole new area and dimension of supramolecular self-assembled materials research with unlimited opportunities and horizons. To date, luminescent d8 and d10 metal-based self-assembled materials

Figure 249. Crystal structure of [Ag{(AuL)3}2]PF6. Reprinted with permission from ref 1087. Copyright 2014 American Chemical Society.

[(AuL)3] with AgPF6. The processes could be reversed by respectively diluting the solution or adding coordinating solvents such as methanol or acetonitrile to the solid sample.1087 It was suggested that the yellow emission originated from the triplet metal-centered (3MM) excited state of the heterometallic Au(I)−Ag(I) adduct.1087 On the basis of the self-assembly of simple starting materials, [Au(NHC)2]Cl (NHC = N-heterocyclic carbene) and K[M(CN)2] (M = Au or Ag), Che and co-workers reported the preparation of the polymeric double salts, [Au(NHC)2][M(CN)2] (Figure 250).1088 Interesting nanostructures of the

Figure 250. Structure of [Au(NHC)2][M(CN)2].

double salts, ranging from nanoplates to x-shaped superstructures, were obtained by various precipitation methods using water as a benign solvent. X-ray diffraction analysis of the double salts revealed the presence of extensive metallophilic and π−π stacking interactions in the structures.1088 At room temperature, most of the NHC-containing double salts were found to exhibit blue emission in the solid state. It was suggested that the extended metal···metal interactions in the complexes were responsible for the blue phosphorescence, as well as the direction of the anisotropic growth of the quasi-2D molecular packing in the nanostructures.1088 The electroluminescence properties of the double salts as light-emitting materials in polymer light-emitting devices (PLEDs) were also investigated.

5. CONCLUSION AND OUTLOOK The study of supramolecular materials based on d8 and d10 metal complexes has continuously been an immense field of research due to their intriguing photophysical and self-assembly properties. The extensive research on such transition metal complexes over the past few decades has established DB


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have been demonstrated to show different functionalities, with applications in diverse fields ranging from biomedical chemistry to materials science. An in-depth understanding of the structure−property relationship of these luminescent metalbased materials will contribute to the improvement and optimization of their properties and functions. With further exploration of the structures and properties of d8 and d10 transition metal complexes, it is anticipated that new molecules with unique properties will be created. Novel applications and functions will become possible, thereby demonstrating the beauty and relevance of supramolecular chemistry and transition metal chemistry in opening new dimensions and opportunities in multidisciplinary research.

Postdoctoral Fellow in the Department of Chemistry at The University of Hong Kong. Her research interests include the design and synthesis of luminescent transition metal complexes, in particular gold(III) complexes, for supramolecular assemblies and functions.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

Sammual Yu-Lut Leung received his B.Sc. (Hons) in 2009 and his Ph.D. in 2014 under the supervision of Prof. V. W.-W. Yam at The University of Hong Kong. After the completion of his Ph.D., he is currently a Postdoctoral Fellow in the Department of Chemistry at The University of Hong Kong. His research interests include the design and synthesis of organometallic transition metal complexes with luminescence property as building blocks for the construction of functional molecules for materials science and supramolecular assembly.

Biographies

ACKNOWLEDGMENTS V.W.-W.Y. acknowledges support from The University of Hong Kong and the University Research Committee Strategic Research Theme on New Materials. This work was supported by the University Grants Committee Areas of Excellence Scheme (AoE/P-03/08), the French National Research Agency (ANR)/Research Grants Council (RGC) Joint Research Scheme (A-HKU704/12), and General Research Fund (GRF) from the Research Grants Council of Hong Kong Special Administrative Region, China (HKU 7060/12P, HKU 7051/13P and HKU 17305614). V.K.-M.A. and S.Y.-L.L. acknowledge the receipt of University Postdoctoral Fellowships from The University of Hong Kong.

Vivian Wing-Wah Yam obtained both B.Sc. (Hons) and Ph.D. degrees from The University of Hong Kong and is currently the Philip Wong Wilson Wong Professor in Chemistry and Energy and Chair Professor of Chemistry there. Her research interests include photophysics and photochemistry of transition metal complexes, supramolecular chemistry, and metal-based molecular functional materials for luminescence sensing, optoelectronics, optical memory, and solar energy conversion.

ABBREVIATIONS bathophen 4,7-diphenyl-1,10-phenanthroline bzq benzo[h]quinoline bpy 2,2′-bipyridine BSEP 1,1′-bis(2-sulfoethyl)-4,4′-bipyridinium inner salt CC cluster-centered CD circular dichroism COD 1,5-cyclooctadiene Colh columnar hexagonal mesophase CVD chemical vapor deposition DFT density functional theory dimen 1,8-diisocyanomenthane DLS dynamic light scattering dmb 2,2-dimethyl-1,3-diisocyanopropane DMF dimethylformamide DMSO dimethyl sulfoxide dpam bis(diphenylarsino)methane dppm bis(diphenylphosphino)methane

Vonika Ka-Man Au received her B.Sc. (Hons) (Major in Chemistry and Minor in Philosophy) in 2007 from The University of Hong Kong. She then obtained her Ph.D. in 2012 from the same university under the supervision of Prof. V. W.-W. Yam. She is currently a DC


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dppp DSSCs en EQE FET FE-SEM

1,3-bis(diphenylphosphino)propane dye-sensitized solar cells ethylenediamine external quantum efficiency field-effect transistor field-emission scanning electron microscopy FRET Förster resonance energy transfer GPC gel permeation chromatography H2bim 2,2′-biimidazole HOMO highest occupied molecular orbital IL intraligand ILCT intraligand charge transfer IR infrared KSV Stern−Volmer constant LB Langmuir−Blodgett LC ligand-centered LED light-emitting diode LLCT ligand-to-ligand charge transfer LMCT ligand-to-metal charge transfer LUMO lowest unoccupied molecular orbital MALDI-TOF MS matrix assisted laser desorption ionization time-of-flight mass spectrometry MCH methylcyclohexane MLCT metal-to-ligand charge transfer MMLCT metal−metal-to-ligand charge transfer MOF metal organic framework NHC N-heterocyclic carbene OLED organic light-emitting device OLEFET organic light-emitting field-effect transistor PET photoinduced electron transfer phen 1,10-phenanthroline phq 2-phenylquinoline PLQY photoluminescence quantum yield POM polarized optical microscopy POSS silsesquioxanes ppy 2-phenylpyridine pz 1,2-dihaptopyrazolide SAED selected area electron diffusion SBLCT sigma-bond-to-ligand charge transfer SEM scanning electron microscopy SmA smectic A SmC smectic C TD-DFT time-dependent density functional theory TEG triethylene glycol TEM transmission electron microscopy terpy terpyridine THF tetrahydrofuran TNF trinitrofluorenone UV−vis ultraviolet−visible VOCs volatile organic compounds WAXS wide-angle X-ray scattering XLCT halide-to-ligand charge transfer XMCT halide-to-metal charge transfer XRD X-ray diffraction

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