Photophysical Properties of Organoplatinum(II) Compounds and

Article pubs.acs.org/accounts

Photophysical Properties of Organoplatinum(II) Compounds and Derived Self-Assembled Metallacycles and Metallacages: Fluorescence and its Applications Manik Lal Saha,*,† Xuzhou Yan,*,†,§ and Peter J. Stang*,† †

Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112, United States Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States

§

CONSPECTUS: Over the past couple of decades, coordinationdriven self-assembly has evolved as a broad multidisciplinary domain that not only covers the syntheses of aesthetically pleasing supramolecular architectures but also emerges as a method to form new optical materials, chemical sensors, theranostic agents, and compounds with light-harvesting and emissive properties. The majority of these applications depend upon investigations that reveal the photophysical nature and electronic structure of supramolecular coordination complexes (SCCs), including twodimensional (2D) metallacycles and three-dimensional (3D) metallacages. As such, well-defined absorption and emission spectra are important for a given SCC to be used for sensing, bioimaging, and other applications with molecular fluorescence being an important component. In this Account, we summarize the photophysical properties of some bis(phosphine)organoplatinum(II) compounds and their discrete SCCs. The platinum(II) based organometallic precursors typically display spectral red-shifts and have low fluorescence quantum yields and short fluorescence lifetimes compared to their organic counterparts because the introduction of metal centers enhances both intersystem crossing (ISC) and intramolecular charge transfer (ICT) processes, which can compete with the fluorescence emissions. Likewise ligands with conjugation can also increase the ICT process; hence the corresponding organoplatinum(II) compounds undergo a further decrease in fluorescence lifetimes. The use of endohedral amine functionalized 120°-bispyridyl ligands can dramatically enhance the emission properties of the resultant organoplatinum(II) based SCCs. As such these SCCs display emissions in the visible region (ca. 400−500 nm) and are significantly red-shifted (ca. 80−100 nm) compared to the ligands. This key feature makes them suitable as supramolecular theranostic agents wherein these unique emission properties provide diagnostic spectroscopic handles and the organoplatinum(II) centers act as potential anticancer agents. Using steady state and time-resolved-spectroscopic techniques and quantum computations in concert, we have determined that the emissive properties stem from the ligand-centered transitions involving πtype molecular orbitals with modest contributions from the metal-based orbitals. The self-assembly and the photophysics of organoplatinum(II) ← 3-substituted pyridyl based SCCs are highly diverse. Subtle changes in the ligands’ structures can form molecular congener systems with distinct conformational and photophysical properties. Furthermore, the heterometallic SCCs described herein possess rich photophysical properties and can be used for sensing based applications. Tetraphenylethylene (TPE) based SCCs display emissions in the aggregated state as well as in dilute solutions. This is a unique phenomenon that bridges the aggregation caused quenching (ACQ) and aggregation induced emission (AIE) effects. Moreover, a TPE based metallacage exhibits solvatoluminescence, including white light emission in THF solvent, and can act as a fluorescence-sensor for structurally similar ester compounds.

■

INTRODUCTION

processes until the thermodynamically controlled superstructure(s) are formed. Various approaches have been developed by Fujita,2 Stang,7 Raymond,8 Mirkin,3 Newkome,9 Schmittel,10 Nitschke,11 Mukherjee,1 and others to prepare SCCs.12 A plethora of bis(phosphine)organoplatinum(II) ← pyridyl based supramolecular structures, including 2D metallacycles and 3D metallacages and other nanoscopic multicomponent materials

Coordination-driven self-assembly via the spontaneous formation of metal−ligand bonds has become a widespread strategy for preparing discrete supramolecular coordination complexes (SCCs).1−6 This technique has benefited from the highly directional and predictable nature of the metal−ligand coordination spheres that aid the rational design of desired architectures. At the same time, the bond energies of third row metal−ligand bonds are moderate (ca. 15−25 kcal/mol), allowing the system to undergo successive error-correction © 2016 American Chemical Society

Received: August 15, 2016 Published: October 13, 2016 2527

DOI: 10.1021/acs.accounts.6b00416 Acc. Chem. Res. 2016, 49, 2527−2539

Article

Accounts of Chemical Research

have been well-studied, suggesting that these materials are potentially useful for liquid crystallinity, chemosensing, photovoltaic effect, etc.16−18 These results and the intrinsic photophysics of other platinum(II) based organometallic systems have motivated us to systematically investigate the photophysical properties of bis(phosphine)organoplatinum(II) precursors and their SCCs, as summarized herein.

with well-defined sizes, shapes, and geometries, have been constructed using the directional bonding approach wherein the geometric information on the building units (i.e., bond angles, bond lengths, number of coordination sites, etc.) dictate the outcome of the self-assembly reactions (Figure 1).7

■

ORGANOPLATINUM(II) ACCEPTORS In order to better understand the photophysical properties of SCCs, we also investigated their precursor platinum(II) building units. In collaboration with professor Han, we used quantum computations, that is, density functional theory (DFT) and time-dependent density functional theory (TDDFT), as well as steady-state and time-resolved spectroscopic techniques to examine the photophysical properties of a number of organoplatinum(II) compounds.19−21 The 120° acceptor 1 is particularly interesting because its absorbance and emission properties are highly solvent dependent, providing a means to tune its absorbance and emission wavelengths (Figure 2). Specifically the absorption and fluorescence maxima of 1 display spectral red-shifts in alcoholic solvents compared to those observed in hexane.19 To understand this phenomenon, rigorous DFT/TDDFT computations were performed. The results suggest that (1) the intermolecular hydrogen bonding interactions between 1 and

Figure 1. Design strategy for metallosupramolecular rhomboid, hexagons, and tetragonal prism via the directional-bonding approach.

Recent efforts include functionalized supramolecular ensembles such as crown ethers, carboranes, cavitands, dendrimers, saccharides, fullerenes, ferrocenyl units, etc. built into the metallosupramolecular scaffolds by either pre- or post-selfassembly functionalization techniques.1,12 The resultant SCCs oftentimes display better selectivity as compared to the parent scaffolds in host−guest chemistry, cavity-induced catalysis, sensing, etc. However, in many instances the introduction of a functional unit has resulted in a substantial decomposition of the SCC, indicating that the orthogonality13 between the functional unit and the SCC is crucial to keep the metallosupramolecular core intact. With this in mind, functional supramolecular polymers have also been prepared by hierarchical self-assembly techniques that combine metal−ligand interactions and hydrogen bonding or host−guest interactions in a single process.12,13 Studies focusing on the photophysical properties of SCCs are rare, though the understanding of the absorption and emission properties of a given SCC is important for applications using molecular fluorescence. Some emissive SCCs have been prepared by attaching well-known fluorophores, such as anthracene, pyrene, boron-dipyrromethene (BODIPY), naphthalene diimides, etc. to the donor building units or by choosing phototophysically active metal ions as the acceptor units.14 However, little has been reported regarding the emission of a given SCC due to perturbations, like geometrical modifications, solvent and counterions effects, etc.15 In contrast the tunable luminescent and electronic properties of linear organometallic acetylides, in particular platinum(II)-acetylides,

Figure 2. (A) Chemical structure of organoplatinum(II) compound 1, (B) its steady-state absorption (solid lines) and fluorescence (dashed lines) spectra, and (C) schematic representation of the different mechanisms of fluorescence emission in different solvents. Panels B and C adapted with permission from ref 19. Copyright 2010 American Chemical Society. 2528


Article


Figure 3. (A) Chemical structures of organoplatinum(II) of compounds 2−5 and (B) their HOMO and LUMO molecular orbitals. Panel B adapted with permission from ref 20. Copyright 2011 American Chemical Society.

a 1,3-bis[trans-Pt(PEt3)2(I)(ethynyl)]benzene unit as the structural backbone (Figure 3A), yet compound 2 displays a fluorescence maximum at 432 nm while the fluorescence maxima of 3 and 4 are red-shifted with respect to that of 2 by 34 and 36 nm, respectively. The fluorescence lifetime values of these compounds (τ) follow the order: 3 (7.87 ns) > 2 (1.83 ns) ≫ 4 (0.01 ns), suggesting that τ decreases with an increase in conjugation of the substituents. Conversely, the value of τ for compound 5 is much higher than that of 4 (2.75 ns vs 0.01 ns), although the former has a large conjugation effect. This indicates that the presence of platinum(II) centers in compounds 2−4 definitely plays a role in the emissions. To better understand these effects, we computed the frontier molecular orbitals and the corresponding orbital energies for compounds 2−5. The computed energy gap between the HOMO and LUMO of these compounds correlates well with the observed fluorescence maxima and is in the order of 2 (4.26 eV) > 5 (4.15 eV) > 3 (3.99 eV) > 4 (3.20 eV). The HOMO and LUMO orbitals of compounds 2−4 are further indicative of the strong intramolecular charge transfer (ICT) feature from the bimetallic platinum donor to different substituents (acceptors). As shown in Figure 3B, in each case the electron densities of the HOMO and LUMO orbitals are separately populated on the 1,3-bis[trans-Pt(PEt 3 ) 2 (I)(ethynyl)]benzene unit and on the corresponding substituent, respectively. Despite this common nature, the electron density of the LUMO for complex 4 is largely populated on the substituent moiety; while it is only partially populated on the substituents for complexes 2 and 3. This implies that the ICT character of complex 4 is much stronger than that of complexes 2 and 3, likely due to the larger conjugation effect of the anthracene unit compared with the ferrocene and crown ether

the alcoholic solvents reduce the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the alcohol complexes of 1 by ca. 0.2 eV as compared to that of 1. (2) The fluorescence states of 1 and alcohol complexes of 1 correspond to the HOMO → LUMO transitions and are described as the metal-to-ligand charge transfer (MLCT) states. This is further supported by the fact that compound 1 displays larger Stoke shifts in alcoholic solvents (ca. 33 nm in hexanol and 47 nm in ethanol) than that in hexane (ca. 24 nm, see Figure 2B). (3) The emissions of 1 in the alcoholic solvents follow a different mechanism compared to that in hexane. The fluorescent state of 1 in hexane is located above a nonfluorescent (dark) state. Thus, upon excitation compound 1 can either relax from the fluorescent state to the ground state via fluorescence emission or first decay to the dark state using a nonradiative pathway and then further relax to the ground state through additional nonradiative transitions (Figure 2C). In contrast, this nonradiative channel is absent in alcoholic solvents, because the fluorescent state is lower in energy than the nonfluorescent state due to intermolecular hydrogen bonding. This further implies that 1 should exhibit higher fluorescence lifetime values in the alcoholic solvents compared with that in hexane, as experimentally observed. Specifically, the fluorescence lifetimes of 1 in hexanol, ethanol, and hexane are 12.54, 12.85, and 4.26 ns, respectively. Substituent Effect

The covalent attachment of functional units to an organoplatinum(II) fluorophore can significantly modify its emission properties by the molecular conjugation effect.20 For instance, compounds 2−4 are structurally analogous and share 2529


Article


compounds 6−8 with an increase in the emission wavelength, while they remained almost constant for ligands 9−11. This difference suggests that either multiple emitting species or emissive states are likely to be present for the compounds 6−8 compared with the ligands 9−11 and the relatively low-lying fluorescence states of these complexes possess a shorter lifetime. The nature of the optical transitions for these organometallic compounds was determined using DFT computations, which indicate that they are ligand-centered π−π* transitions. Furthermore, the low-lying excited-state transition calculations indicate that compounds 7 and 8 have different mechanisms of fluorescence emission than that of compound 6 (Figure 4B). Accordingly the bright states of compounds 7 and 8 are located above several dark states that have ICT features and cause a nonradiative internal conversion (IC) process, leading to a significant fluorescence quenching. This IC process is absent for compound 6, wherein the dark states are located above the fluorescence state. Consequently, compound 6 can be directly photoexcited to the bright state and then decays to the ground state from the first excited state via an irradiative process.21 The introduction of the platinum(II) centers further shifts the fluorescence states of compounds 6−8 closer to the lowlying triplet excited state, thereby increasing the possibility of an intersystem crossing (ISC) effect. Based on these observations and on literature reports,16,22 we conclude that the introduction of platinum(II) centers can significantly increase the rate of ISC or other nonradiative processes (e.g., ICT) that compete with the emissive process and result in a low fluorescence quantum yield, spectral red-shifts, and short excited-state lifetime in comparison with the free ligands.

moieties. Thus, the ICT characters of these complexes follow the exact opposite order (i.e., 4 ≫ 3 > 2) to that of their fluorescence lifetime values, demonstrating that ICT processes can significantly reduce excited state lifetimes. At the same time, the electron density of the LUMO for compound 5 is delocalized on both the donor and acceptor units, suggesting that the ICT process in compound 5 is much weaker compared to those observed for the organoplatinum(II) compounds 2−4. Evidently, the presence of platinum(II) centers can improve the ICT character in organoplatinum(II) compounds and hence results in shorter fluorescence lifetimes compared to the free ligands. The Role of Platinum(II)

To evaluate the role of the platinum(II) ions in detail, we compared the photophysics of three organoplatinum(II) compounds, 6−8 and their corresponding organic precursors 9−11 (Figure 4A). The UV−Vis and fluorescence data reveal

■

BIS(PHOSPHINE)ORGANOPLATINUM(II) ← PYRIDYL BASED SUPRAMOLECULAR COORDINATION COMPLEXES The above studies have provided a better understanding of fluorescent phenomena in organoplatinum(II) acceptors and paved the way for the investigation of more complex SCCs. Although at times the metallacycles and metallacages preserve the absorption and emission characteristics of their precursors, the interplay between metal centers and organic ligands may cause additional novel effects (e.g., significant spectral redshifts), which may not be predicted by considering the precursors in isolation. In the ensuing section, we will describe some of our SCC-systems that have displayed interesting photophysics.

■

TWO-DIMENSIONAL METALLACYCLES

Organoplatinum(II) ← 3-Substitued Pyridyl Based Metallacycles

Figure 4. (A) Chemical structure of compounds 6−11 and (B) schematic representation of the different mechanisms of fluorescence emission for compounds 6−8. Panel B adapted with permission from ref 21. Copyright 2012 American Chemical Society.

Coordination-driven self-assembly of various pyridyl ligands (e.g., mono- to hexapyridines) and their complementary platinum acceptors have been well explored for constructing discrete SCCs. Consequently, the 4-substituted pyridine linkers have emerged as the standard building blocks in coordinationdriven self-assembly reactions, while the use of 3-substituted pyridyl ligands is still limited. This is because the former always maintain the 180° bonding directionality of their nitrogen atoms irrespective of any rotation of the rings, whereas the latter can adopt any dihedral angle between 0 and 180° (Figure 5), thereby increasing the possibility of having a mixture of

that in each case: (1) the absorption and emission spectra of the organometallic compounds become wider and are significantly red-shifted compared with the spectra for the ligands, and (2) the organometallic compound has a lower fluorescence quantum yield (ΦF) and displays a shorter fluorescence lifetime relative to the organic counterpart. The time-resolved fluorescence decay data further indicate that the average fluorescence lifetime values decreased for 2530


Article


the 1H and 31P NMR spectra of 16, while the boat form of 15 was not observed in the NMR spectra. DFT computations on both rhomboids allow the modeling of these conformers’ structures; in both cases the chair forms are more stable than the boat counterparts and the energy gap between the two conformers of 16 is less than that of 15. Likely the close proximity of the platinum-phosphine groups makes the boat form of 15 destabilized relative to its chair counterpart, and hence the chair-15 ⇌ boat-15 equilibrium exclusively shifts to the left (Scheme 1B). The absorption and fluorescence spectra of these rhomboids are shown in Figure 6. The absorption bands for rhomboid 16 are red-shifted and become wider compared with rhomboid 15. This indicats that the additional ethyne spacer of ligand 13 increases the molecular conjugation effect, causing the redshifting and broadening of the corresponding rhomboid. The fluorescence spectra of these assemblies are equally broadened and show two high-energy emission peaks between 350 and 400 nm, which can be tuned by different excitation wavelengths. Furthermore, a low-energy shoulder at ca. 416 nm was observed in the fluorescence spectrum of 16. Based on the TDDFT computational data, we conclude that the shoulder and the high-energy emission bands of 16 can be attributed to its boat and chair conformers, respectively.24 The nature of the low-lying excited states for both rhomboids as well as both conformers of rhomboid 16 are different, despite the fact that they are structurally similar. For rhomboid

oligomers rather than a discrete product, even when reacted with directionally fixed metal acceptors.

Figure 5. Schematic representation of the directionalities of the two nitrogen atoms in 4- and 3-substituted bis-pyridyl ligands.

Nevertheless a suite of discrete metallacycles and metallacages were prepared from flexible 3-substituted bis-pyridines and organoplatinum(II) reagents, in nearly quantitative yields.23 For instance, rhomboids 15 and 16 (Scheme 1A) self-assembled upon mixing the 90° acceptor 14 and 1,2-bis(3pyridyl)ethyne (12) or 1,4-bis(3-pyridyl)-1,3-butadiyne (13) in a 1:1 ratio. Notably the introduction of the additional ethyne chain in the ligand core of 13 results in significantly different conformational properties of the rhomboids. Both the chair (as major) and the boat (as minor) conformers were detected in

Scheme 1. (A) Self-Assembly and (B) Two Different Possible Conformations of Rhomboids 15 and 16

2531


Article


emissive. Moreover the ΦF value of the endohedral amine functionalized rhomboid 21 is 7-fold higher than that of its exocounterpart 22 (28% vs 4%). Both rhomboids display appreciable red-shifted lower-energy absorption bands relative to the free ligands, while the degree of red-shifting of their emission bands is highly dependent on the position of the amine-group on the ligand core. The exo-functionalized rhomboid 22 exhibits a negligible (ca. 1 nm) red-shift in the emission spectra compared to the donor 18. Conversely, the emission band of the endo-functionalized assembly is redshifted by over 100 nm, far into the visible region. The latter feature has also been observed in a series of rhomboids, hexagons, and smaller model fragments that were prepared using endohedral aniline ligands and complementary organoplatinum(II) acceptors. For example, the rhomboid 24 (Scheme 2) and the hexagon 26 (Scheme 3) display a single broad emission band at 493 nm (ΦF = 3.7%) and 505 nm (ΦF = 15%), which are, respectively, 76 and 71 nm red-shifted compared to the ligands. At the same time, these SCCs have much lower ΦF values and exhibit blue-shifted emissions relative to that of rhomboid 21 (λem = 522 nm and ΦF = 28%). These results indicate that this effect is not limited to a specific metallacycle, ligand, or organoplatinum(II) acceptor; rather, the use of the endohedral aniline-based ligands in these self-assembly processes ensures the innate red-shifted emissive characters of the resultant assemblies. Whereas the choice of organoplatinum(II) acceptors, conjugation length of donors, solvents, and shape and size of the final constructs have only a little-to-moderate influences on the emission of SCCs of this type.26 To understand the nature of the optical transitions for these assemblies, DFT/TDDFT calculations were carried out for each assembly. The results indicate that the emissive properties are caused by ligand-centered transitions involving π-type symmetric molecular orbitals with modest contributions f rom metal-based atomic orbitals.26 In all cases, the quantum yields of the assemblies are low compared to that of the ligands, suggesting that the presence of the platinum(II)-centers can diminish the quantum yields of the SCCs in a similar fashion to what had been previously observed for their organoplatinum(II) acceptors (vide supra). Despite the “heavy atom effect” of the platinum centers,26 quantum yield values as high as 28% (e.g., 21) were still observed for endohedral amine-functionalized bis(phosphine)organoplatinum(II) rhomboids.

Figure 6. Steady-state absorption and fluorescence spectra at different excitation wavelengths for 15 and 16 in dichloromethane (CH2Cl2). Adapted with permission from ref 24. Copyright 2010 American Chemical Society.

15, the low-lying excited states are metal-centered (MC), intraligand (IL), intraligand charge transfer (ILCT), and MLCT states. While they have IL, ILCT, and MC characters for boat-16 and IL, ILCT, and metal-to-metal charge transfer (MMCT) features for chair-16. This example demonstrates how a subtle difference in the (conjugation) length of the donors can significantly influence the conformational properties, spectral shifts, and the nature of the excited states of congener metallosupramolecular systems. The influence of the different angularities of the donor atoms on the photophysical properties of isomeric organoplatinum(II) ← bispyridine → organoplatinum(II) SCCs was also probed. The results suggest that the 4-substituted pyridine based triads display a significantly red-shifted absorption and emission spectra and have higher quantum yields relative to their 3substituted pyridine counterparts. The TDDFT data and the excited-state lifetime measurements further indicate that the nature of the Pt(II) ← N bond in the HOMO, the radiative and the nonradiative rate constants, and the extent of ISC effect are significantly different in the two systems and are responsible for the observed differences.25 Aniline Based Metallacycles

Self-assembled SCC species that have aniline-based 120° donor ligands exhibit much more intense emission than their parent phenyl based counterparts. For example, the amine-functionalized D2h-rhomboids 21 and 22 (Scheme 2) are highly emissive, whereas the parent rhomboid 23, itself, is nonScheme 2. Self-Assembly of Rhomboids 21−24

2532


Article

Accounts of Chemical Research Scheme 3. Self-Assembly of Hexagon 26

Figure 7. (A) Rhomboids 27−31 and their (B) photophysical data and (C) emission profiles and photographs (inset). Panels B and C adapted with permission from ref 27. Copyright 2013 American Chemical Society.

Figure 8. (A) Rhomboid 32 and (B) its low temperature (93 K) excitation and fluorescence spectra. Panel B adapted with permission from ref 29. Copyright 2010 American Chemical Society.

the pendent functional groups provides a linear relationship for these rhomboids. The emission wavelength of a given rhomboid can be predetermined using the Hammett constant of its pendent functionality; however, the relationship between the Hammett constants and the quantum yields is not fully understood.27 Light-emitting SCCs have also been constructed by tethering a known fluorophore into the precursor units. For example, Pistolis et al. prepared a series of green-light emitting multiBODIPY SCCs,28 including rhomboid 32 (Figure 8), which largely preserves the photophysical properties such as high quantum yield, fluorescence lifetime, and anisotropy of its luminescent precursor.29 Although this technique has seen extensive use in recent years, the emissions of the resultant

The photophysical properties of these rhomboids can be further tuned by introducing an additional functionality in the para-position of the central aniline group, as shown in Figure 7. Rhomboids 27−31 are characterized by a high-energy absorption band between 305 and 318 nm and a less intense band at longer wavelengths that range from 420 to 480 nm. At the same time, their emission profiles span the entire visible region (476−581 nm), and the quantum yield increases from 27 to 29 and then decreases from 29 to 31. The low-energy absorption and emission bands become significantly red-shifted with an increase in the electron-donating ability of the pendant functionality, while the high-energy absorption bands remain almost unaffected. Plotting of the wavenumbers (cm−1) of the λmax of the emissions against the Hammett σpara constants for 2533


Article

Accounts of Chemical Research Scheme 4. Self-Assembly of Heterometallic Rectangles 36 and 37

Figure 9. (A) Chemical structure of squares 38 and 39 and (B) emission quenching of complex 38 by picric acid and Stern−Volmer plot (inset). Panel B adapted with permission from ref 33. Copyright 2011 Royal Society of Chemistry.

Heterometallic Metallacycles

assemblies oftentimes do not differ from that of the fluorophore-appended precursors, decreasing their bioimaging applications.30 This is because in such cases it is impossible to differentiate the emission signals of the assembly from the decomposition product(s) that could form during the cell studies. In contrast, the photophysics of the endohedral aminefunctionalized organoplatinum(II) rhomboids and their precursor ligands are significantly different, making them suitable as bioimaging agents. Rhomboids 29 and 30 were thus used to monitor the uptake and localization of these complexes in tumor cells with laser-scanning confocal microscopy.31 The results also confirmed that they further exhibited a substantial efficacy in reducing the tumor growth and tumor burden in mouse xenograft models, suggesting that organoplatinum(II)based SCCs of this type can be considered as a novel class of supramolecular theranostic agents, which combine cancer diagnostic and therapeutic activity in a single entity.

Using ligand 33 and dicarboxylate-bridged arene-Ru precursor 34 or 35 we, in collaboration with professor Chi, prepared heterometallic Ru−Pt supramolecular rectangles 36 and 37 and studied their photophysical properties (Scheme 4). The UV− Vis spectra of these assemblies are similar, exhibiting MLCT and intramolecular charge transfer absorption bands tailing up to 700 nm. However, different emission spectra were observed. Specifically, rectangle 37 displays significantly red-shifted and much more intense emission bands as compared to those observed for rectangle 36 and ligand 33, indicating that the presence of the tetracene moiety of the acceptor unit strongly influences its emissive properties, while only the ligand based emissions are prominent in the fluorescence spectrum of 36.32 We have also demonstrated that heterometallic squares 38 and 39 (Figure 9) can serve as “turn-off” sensors for nitroaromatic explosives over various other electron rich and poor aromatic analytes, including benzene, xylene, 2,4,6-TMP, 2534


Article


Figure 10. (A) Self-assembly of SCC 41, (B) its emission quenching by C60, and the corresponding (C) Stern−Volmer plot. Panels A−C Adapted with permission from ref 39. Copyright 2012 American Chemical Society.

benzoic acid, and phloroglucinol.33 As such both assemblies and the ligand display similar emission spectra, indicating that the platinum(II)-ethynyl unit of the ligand core is responsible for the emissive properties of these self-assemblies. The emission intensity of the squares gradually decreased upon the incremental addition of electron-deficient nitroaromatics, such as nitrobenzene, 1,3,5-trinitrobenzene, and 2,4,6-trinitrophenol (i.e., picric acid, PA), with PA displaying the highest quenching effect. The calculated Stern−Volmer quenching constants from the PA titration profile are 6.72 × 105 M−1 for 38 and 7.00× 105 M−1 for 39. The exact mechanisms of quenching are yet to be understood; however the formation of nonemissive donor (SCC)−acceptor (nitroaromatics) complexes were also observed in analogous cases,34 suggesting that it is likely to be a static quenching mechanism.

aromatic molecules, such as anthracene, phenanthrene and pyrene derivatives, within a platinum(II)-based M2L4 metallacage (L = 3,3′-((4,5,6-tris(methoxymethoxy)-1,3-phenylene)bis(anthracene-10,9-diyl))dipyridine).37 Nitschke and co-workers also combined the photophysical and guest-binding properties of a BODIPY- and pyrene-containing FeII4L6 tetrahedral cage to tune the emission colors of the corresponding host−guest complexes.38 Notably, when excess perylene (3 equiv with respect to the cage) was used as a guest, a white light emitting ensemble was formed with a quantum yield of 0.11. Guest-Induced Quenching

Guest molecules can also act as luminescence quenchers, resulting in the formation of dark host−guest complexes from the emissive hosts. For instance, the bowl-shaped carbazole containing heterometallic SCC 41 (Figure 10) displays absorption bands at λmax = 309, 345, and 384 nm corresponding to the intra- and intermolecular π−π*, MLCT, and weak d−d transitions, respectively, and a broad emission band centered at λemiss = 372 nm. This emission band quickly depleted upon the gradual addition of fullerene C60, because the resultant 1:1 inclusion complex suffers from charge transfer interactions.39 Using the symmetry interaction approach,1 Duan, He, and coworkers prepared an emissive M4L4 molecular tetrahedra wherein four cerium(IV) ions act as the vertices and four tridentate N′,N″,N‴-nitrilo-tris-4,4′,4″-(2-hydroxybenzylidene)-benzohydrazid ligands define the faces. The encapsulation of a molecule of 2-phenyl-4,4,5,5-tetra-methylimidazolineyloxyl-3-oxide (PTIO) completely quenches its emission. Remarkably, the addition of 0.45 mM NO to the host−guest system containing 15 μM of the cage and 0.3 mM PTIO again turns on the fluorescence emission with a 12-fold increase of intensity compared to the free cage. This is because the oxidation of NO to NO2 via PTIO eliminates the fluorescence quencher and results in an ON−OFF−ON fluorescence signaling.40

■

THREE-DIMENSIONAL METALLACAGES Although the preparations of metallacycles and metallacages are similar, the well-defined cavities of the latter make them suitable for encapsulating a variety of species, including fluorophores. Oftentimes the host frameworks quench the guest emissions, either due to the heavy-atom effects of the transition metals or by the formation of nonemissive chargetransfer complexes. However, methodologies have been developed to circumvent this issue. For example Fujita, Yoshizawa, and co-workers observed that the encapsulation of an electron-poor guest within a platinum(II)-based M6L12L23 (L1= panels, e.g., 2,4,6-tri(pyridin-4-yl)-1,3,5-triazine, and L2 = pillars, e.g., pyrazine) trigonal coordination cage can significantly minimize the charge-transfer interactions and results in a highly emissive host−guest complex.35 Likewise we reported that the encapsulation of a coronene molecule by a platinum(II)-based M6L12L23 metallacage, with a different pillar (L2= sodium fumarate), can dramatically enhance its photophysics because of the encapsulation induced core-to-cage charge transfer (CCCT) process, representing a novel strategy for optimizing the efficacy of photosensitizers.36 Yoshizawa et al. demonstrated that the emission color of a BODIPY dye can be readily tuned by its pair wise encapsulation with planar 2535


Article


Figure 11. (A) Multicomponent self-assembly of the metallacage 44 and (B) photographs of its emission colors in CH2Cl2/hexane mixtures. The fractions of hexane are shown at the top. (C) The CIE diagram of 44. (D) The fluorescence spectra and photograph (inset) of 44 in different ester solvents. Panels A−D adapted with permission from ref 42. Copyright 2015 Nature Publishing Group.

Aggregation-Induced Emission (AIE) and Platinum(II) ← Pyridyl Coordination

TPPE donors within the prismatic core restricts their intramolecular motions, thereby imparting the emissive behavior of the metallacage in dilute solution. Upon molecular aggregation in CH2Cl2/hexane solutions, 44 showed markedly enhanced fluorescence as reflected by the change of ΦF values from 10.8% to 51.3%, in a 9:1 hexane/CH2Cl2 mixture. Hence, this example suggests that a careful combination of the AIE effect and coordination driven self-assembly can preserve the emissive behavior of a SCC at most concentration regimes.42 This chemistry has also been explored by others, leading to the formation of SCCs based fluorescence sensors for heparin,43 nitroaromatics,44 etc. A control experiment with the parent nonfunctionalized analog of 44 demonstrated that the PEG chains influenced the emission behavior of 44, though the exact role of the PEG chain on the emission is not well-understood. The aforesaid enhancement of ΦF was also accompanied by a change in emission colors from yellow to yellowish green to pale blue when the hexane content of a CH2Cl2 solution of 44 was increased from 0% to 30% to 90%, thus demonstrating how molecular aggregation can lead to tunable colors of a SCC (Figure 11B). Moreover different emission colors were observed in different solvents, as shown in the Commission Internationale de L′Eclairage (CIE) chromaticity diagram (Figure 11C). The gradual aggregation of 44 in THF led to a white light emission, from an initial yellow to final pale-blue

Traditional chromophores oftentimes fluoresce in dilute solutions but suffer from aggregation-caused quenching (ACQ) in the condensed state due to the formation of excimers and exciplexes. Tang and co-workers described an opposite effect known as AIE in which fluorophores are nearly nonemissive as discrete molecules but display strong fluorescence in concentrated solution or in the solid state.41 Tetraphenylethylene (TPE) is one of the prominent examples of this class in which the ethylenic CC bond twisting and the low-frequency phenyl ring rotations nonradiatively dissipate the exciton energy in dilute solutions, while the suppression of molecular rotations through tight intermolecular packing can turn on its emission. Despite these advances, how to preserve the light-emitting properties of a given chromophore at both low- and high-concentration ranges remains a challenge. To address this issue, a poly(ethylene glycol) (PEG) decorated tetragonal prismatic platinum(II) metallacage 44 was prepared by combining tetra-(4-pyridylphenyl)ethylene (TPPE, 42), dicarboxylate ligand 43, and cis-(PEt3)2Pt(OTf)2 14 in a 1:2:4 ratio (Figure 11A). The CH2Cl2 solution of metallacage 44 (c = 10.0 μM) displayed bright-yellow fluorescence with a ΦF value of 10.8%. In sharp contrast, the TPPE ligand remains nonemissive under the identical conditions, suggesting that the immobilization of the two 2536


Article


Figure 12. (A) Self-assembly of metallacages 49−51 and (B) their quantum yields in CH2Cl2/hexane mixtures. The fractions of hexane are shown at the inset. Panels A and B adapted with permission from ref 15. Copyright 2016 American Chemical Society.

spectroscopic techniques were used in concert to examine the photophysics of these systems. The presence of the platinum(II) centers generally introduces a significant number of low-lying dark states that result in a low fluorescence quantum yield and short excitedstate lifetime in comparison with the free ligands. However, methodologies have been introduced and developed to prepare highly emissive platinum(II) based SCCs that exhibit lowenergy absorption and emission bands, long-lived and lowenergy excited states, high-quantum yields, and tunable emission properties, features that are useful in bioimaging, sensing, and photocatalysis. Specifically, the aniline-containing and the AIE-active SCCs have proven useful for sensing and imaging applications. We hope that the specifics described herein will also help the design of future assemblies when emissive properties are desired.

emission. The coordinates for the white-light emission in the CIE diagram (0.313, 0.344) are close to those of pure white light emission (0.333, 0.333). Evidently partial aggregationinduced white-light emission from a single SCC in a pure solvent at room temperature was achieved, which complements the other known white light emission mechanisms, including ratiometric emission from organic or supramolecular fluorophores,38 excited state intramolecular proton transfer, etc.45 Methyl formate, ethyl formate, ethyl acetate, and butyl acetate are structurally similar ester compounds, yet the metallacage 44 emits pale-yellow, indigo, blue, and cyan lights, respectively, upon dissolution into them, due to the solubility differences of 44 in these four solvents. The twisted ligand conformations within the rigid skeleton, MLCT process, and solvent-dependent aggregation processes may contribute to this multicolor luminescence. The normalized emission spectra of 44 in the ester solvents display emission bands that are reasonably separated from each other (Figure 11D). Hence, 44 may be used as a fluorescent sensor for structurally similar ester compounds.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].

Anion Effect

We have also investigated the influence of counterions on the photophysics of AIE-active discrete based metallacages by examining the organoplatinum(II) ← pyridyl based cages 49− 51 (Figure 12A).15 They are structurally similar and only differ in the associated anions, yet in dilute solutions as well as in the aggregated states the molar absorption coefficients, the fluorescence emission intensities, and the quantum yields of these cages follow the order 49 > 50 > 51, indicating that hexafluorophosphates as counteranions enhance the photophysics of the corresponding cage over triflate or nitrate anions (Figure 12B). The origin of these differences is not wellunderstood; however we suppose that the counteranions may influence the solubility of these cages, thereby modulating their aggregation behaviors.

Notes

The authors declare no competing financial interest. Biographies Manik Lal Saha received his B.Sc. (Honours), M.Sc., and Ph.D. in chemistry from the University of Calcutta (India), IIT Kanpur (India), and the University of Siegen (Germany), respectively. He is currently investigating the photophysical and biological properties of organoplatinum(II) based supramolecular systems as a postdoctoral fellow in the group of Peter Stang at the University of Utah. Xuzhou Yan received his Ph.D. in chemistry from Zhejiang University in 2014. Then he joined Stang’s group as a postdoctoral fellow at the University of Utah. In August 2016, he moved to Stanford University to continue his postdoctoral research under the supervision of Professor Zhenan Bao. His current research interests are the design and application of light-emitting metal−organic materials in molecular electronics.

■

CONCLUSIONS We describe here a number of recent studies that discuss the origins and characteristics of fluorescence in either organoplatinum(II) compounds or organoplatinum(II) ← pyridyl based metallacycles and metallacages. Since the emission properties of a given system are complex and a complete understanding of those requires both experimental and theoretical investigations, a combination of DFT/TDDFT quantum computations and steady-state and time-resolved

Peter J. Stang is the David P. Gardner distinguished professor of Chemistry. He is a member of the US National Academy of Sciences and a foreign member of the Chinese Academy of Sciences, the recipient of the Chinese Government “International Cooperation 2537


Article


Charge Transfer and Fluorescence of Bimetallic Platinum Complexes. J. Phys. Chem. A 2011, 115, 6390−6393. (21) Chen, J.-S.; Zhao, G.-J.; Cook, T. R.; Sun, X.-F.; Yang, S.-Q.; Zhang, M.-X.; Han, K.-L.; Stang, P. J. Experimental and Theoretical Study on the Photophysical Properties of 90° and 60° Bimetallic Platinum Complexes. J. Phys. Chem. A 2012, 116, 9911−9918. (22) Flynn, D. C.; Ramakrishna, G.; Yang, H. B.; Northrop, B. H.; Stang, P. J.; Goodson, T. Ultrafast Optical Excitations In Supramolecular Metallacycles with Charge Transfer Properties. J. Am. Chem. Soc. 2010, 132, 1348−1358. (23) Chi, K.-W.; Addicott, C.; Arif, A. M.; Das, N.; Stang, P. J. Bidentate Ligands Capable of Variable Bond Angles in the SelfAssembly of Discrete Supramolecules. J. Org. Chem. 2003, 68, 9798− 9801. (24) Zhao, G.-J.; Northrop, B. H.; Stang, P. J.; Han, K.-L. Photophysical Properties of Coordination-Driven Self-Assembled Metallosupramolecular Rhomboids: Experimental and Theoretical Investigations. J. Phys. Chem. A 2010, 114, 3418−3422. (25) Pollock, J. B.; Cook, T. R.; Schneider, G. L.; Lutterman, D. A.; Davies, A. S.; Stang, P. J. Photophysical Properties of Endohedral Amine-Functionalized Bis(phosphine) Pt(II) Complexes as Models for Emissive Metallacycles. Inorg. Chem. 2013, 52, 9254−9265. (26) Pollock, J. B.; Cook, T. R.; Stang, P. J. Photophysical and Computational Investigations of Bis(phosphine) Organoplatinum(II) Metallacycles. J. Am. Chem. Soc. 2012, 134, 10607−10620. (27) Pollock, J. B.; Schneider, G. L.; Cook, T. R.; Davies, A. S.; Stang, P. J. Tunable Visible Light Emission of Self-Assembled Rhomboidal Metallacycles. J. Am. Chem. Soc. 2013, 135, 13676−13679. (28) Karakostas, N.; Mavridis, I. M.; Seintis, K.; Fakis, M.; Koini, E. N.; Petsalakis, I. D.; Pistolis, G. Highly Efficient And Unidirectional Energy Transfer Within A Tightly Self-Assembled Host−Guest Multichromophoric Array. Chem. Commun. 2014, 50, 1362−1365. (29) Kaloudi-Chantzea, A.; Karakostas, N.; Raptopoulou, C. P.; Psycharis, V.; Saridakis, E.; Griebel, J.; Hermann, R.; Pistolis, G. Coordination-Driven Self Assembly of a Brilliantly Fluorescent Rhomboid Cavitand Composed of Bodipy-Dye Subunit. J. Am. Chem. Soc. 2010, 132, 16327−16329. (30) Cook, T. R.; Vajpayee, V.; Lee, M. H.; Stang, P. J.; Chi, K.-W. Biomedical and Biochemical Applications of Self-Assembled Metallacycles and Metallacages. Acc. Chem. Res. 2013, 46, 2464−2474. (31) Grishagin, I. V.; Pollock, J. B.; Kushal, S.; Cook, T. R.; Stang, P. J.; Olenyuk, B. Z. In vivo anticancer activity of rhomboidal Pt(II) metallacycles. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 18448−18453. (32) Vajpayee, V.; Song, Y. H.; Yang, Y. J.; Kang, S. C.; Cook, T. R.; Kim, D. W.; Lah, M. S.; Kim, I. S.; Wang, M.; Stang, P. J.; Chi, K.-W. Self-Assembly of Cationic, Hetero- or HomonuclearRuthenium(II) Macrocyclic Rectangles and Their Photophysical, Electrochemical, and Biological Studies. Organometallics 2011, 30, 6482−6489. (33) Vajpayee, V.; Kim, H.; Mishra, A.; Mukherjee, P. S.; Stang, P. J.; Lee, M. H.; Kim, H. K.; Chi, K.-W. Self-assembled Molecular Squares Containing Metal-Based Donor: Synthesis and Application In the Sensing Of Nitro-Aromatics. Dalton Trans. 2011, 40, 3112−3115. (34) Yan, X.; Wang, H.; Hauke, C. E.; Cook, T. R.; Wang, M.; Saha, M. L.; Zhou, Z.; Zhang, M.; Li, X.; Huang, F.; Stang, P. J. A Suite of Tetraphenylethylene-Based Discrete Organoplatinum(II) Metallacycles: Controllable Structure and Stoichiometry, AggregationInduced Emission, and Nitroaromatics Sensing. J. Am. Chem. Soc. 2015, 137, 15276−15286. (35) Ono, K.; Klosterman, J. K.; Yoshizawa, M.; Sekiguchi, K.; Tahara, T.; Fujita, M. ON/OFF Red Emission from Azaporphine in a Coordination Cage in Water. J. Am. Chem. Soc. 2009, 131, 12526− 12527. (36) Yang, Y.; Chen, J.-S.; Liu, J.-Y.; Zhao, G.-J.; Liu, L.; Han, K.-L.; Cook, T. R.; Stang, P. J. Photophysical Properties of a Post-SelfAssembly Host/Guest Coordination Cage: Visible Light Driven Coreto-Cage Charge Transfer. J. Phys. Chem. Lett. 2015, 6, 1942−1947. (37) Yamashina, M.; Sartin, M. M.; Sei, Y.; Akita, M.; Takeuchi, S.; Tahara, T.; Yoshizawa, M. Preparation of Highly Fluorescent Host−

Award in Science and Technology” (2016), the ACS Priestley Medal (2013), and the US National Medal of Science (2011).

■ ■

ACKNOWLEDGMENTS We thank the NSF (Grant 1212799) for financial support. REFERENCES

(1) Chakrabarty, R.; Mukherjee, P. S.; Stang, P. J. Supramolecular Coordination: Self-Assembly of Finite Two- and Three-Dimensional Ensembles. Chem. Rev. 2011, 111, 6810−6918. (2) Harris, K.; Fujita, D.; Fujita, M. Giant Hollow MnL2n Spherical Complexes: Structure, Functionalisation and Applications. Chem. Commun. 2013, 49, 6703−6712. (3) Lifschitz, A. M.; Rosen, M. S.; McGuirk, C. M.; Mirkin, C. A. Allosteric Supramolecular Coordination Constructs. J. Am. Chem. Soc. 2015, 137, 7252−7261. (4) Oliveri, C. G.; Ulmann, P. A.; Wiester, M. J.; Mirkin, C. A. Heteroligated Supramolecular Coordination Complexes Formed via the Halide-Induced Ligand Rearrangement Reaction. Acc. Chem. Res. 2008, 41, 1618−1629. (5) Brown, C. J.; Toste, F. D.; Bergman, R. G.; Raymond, K. N. Supramolecular Catalysis in Metal−Ligand Cluster Hosts. Chem. Rev. 2015, 115, 3012−3035. (6) McConnell, A. J.; Wood, C. S.; Neelakandan, P. P.; Nitschke, J. R. Stimuli-Responsive Metal-Ligand Assemblies. Chem. Rev. 2015, 115, 7729−7793. (7) Seidel, S. R.; Stang, P. J. High-Symmetry Coordination Cages via Self-Assembly. Acc. Chem. Res. 2002, 35, 972−983. (8) Caulder, D. L.; Raymond, K. N. Supermolecules by Design. Acc. Chem. Res. 1999, 32, 975−982. (9) Eryazici, I.; Moorefield, C. N.; Newkome, G. R. Square-Planar Pd(II), Pt(II), and Au(III) Terpyridine Complexes: Their Syntheses, Physical Properties, Supramolecular Constructs, and Biomedical Activities. Chem. Rev. 2008, 108, 1834−1895. (10) Saha, M. L.; Neogi, S.; Schmittel, M. Dynamic heteroleptic metal-phenanthroline complexes: from structure to function. Dalton Trans. 2014, 43, 3815−3834. (11) Nitschke, J. R. Construction, Substitution, and Sorting of Metallo-organic Structures via Subcomponent Self-Assembly. Acc. Chem. Res. 2007, 40, 103−112. (12) Cook, T. R.; Stang, P. J. Recent Developments in the Preparation and Chemistry of Metallacycles and Metallacages via Coordination. Chem. Rev. 2015, 115, 7001−7045. (13) Saha, M. L.; De, S.; Pramanik, S.; Schmittel, M. Orthogonality in Discrete Self-Assembly − Survey of Current Concepts. Chem. Soc. Rev. 2013, 42, 6860−6909. (14) Xu, L.; Wang, Y.-X.; Yang, H.-B. Recent Advances in the Construction of Fluorescent Metallocycles and Metallacages via Coordination-Driven Self-Assembly. Dalton Trans. 2015, 44, 867−890. (15) Yan, X.; Wang, M.; Cook, T. R.; Zhang, M.; Saha, M. L.; Zhou, Z.; Li, X.; Huang, F.; Stang, P. J. Light-Emitting Superstructures with Anion Effect: Coordination-Driven Self-Assembly of Pure TetraphenylethyleneMetallacycles and Metallacages. J. Am. Chem. Soc. 2016, 138, 4580−4588. (16) Zhou, G.-J.; Wong, W.-Y. Organometallic Acetylides of Pt(II), Au(I) and Hg(II) As New Generation Optical Power Limiting Materials. Chem. Soc. Rev. 2011, 40, 2541−2566. (17) Wong, K. M.-C.; Yam, V. W.-W. Self-Assembly of Luminescent Alkynylplatinum(II) Terpyridyl Complexes: Modulation of Photophysical Properties through Aggregation Behavior. Acc. Chem. Res. 2011, 44, 424−434. (18) Wang, W.; Yang, H. B. Linear Neutral Platinum−Acetylide Moiety: Beyond the Links. Chem. Commun. 2014, 50, 5171−5186. (19) Zhao, G.-J.; Northrop, B. H.; Han, K.-L.; Stang, P. J. The Effect of Intermolecular Hydrogen Bonding on the Fluorescence of a Bimetallic Platinum Complex. J. Phys. Chem. A 2010, 114, 9007−9013. (20) Zhao, G.-J.; Yu, F.; Zhang, M.-X.; Northrop, B. H.; Yang, H. B.; Han, K.-L.; Stang, P. J. Substituent Effects on the Intramolecular 2538


Article

Accounts of Chemical Research Guest Complexes with Tunable Color upon Encapsulation. J. Am. Chem. Soc. 2015, 137, 9266−9269. (38) Neelakandan, P. P.; Jiménez, A.; Nitschke, J. R. Fluorophore Incorporation Allows Nanomolar Guest Sensing and White-Light Emission in M4L6 Cage Complexes. Chem. Sci. 2014, 5, 908−915. (39) Shanmugaraju, S.; Vajpayee, V.; Lee, S.; Chi, K.-W.; Stang, P. J.; Mukherjee, P. S. Coordination-Driven Self-Assembly of 2D-Metallamacrocycles Using a New Carbazole-Based Dipyridyl Donor: Synthesis, Characterization, and C60 Binding Study. Inorg. Chem. 2012, 51, 4817−4823. (40) Wang, J.; He, C.; Wu, P.; Wang, J.; Duan, C. An AmideContaining Metal Organic Tetrahedron Responding to a SpinTrapping Reaction in a Fluorescent Enhancement Manner for Biological Imaging of NO in Living Cells. J. Am. Chem. Soc. 2011, 133, 12402−12405. (41) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Aggregation-Induced Emission. Chem. Soc. Rev. 2011, 40, 5361−5388. (42) Yan, X.; Cook, T. R.; Wang, P.; Huang, F.; Stang, P. J. Highly Emissive Platinum(II) Metallacages. Nat. Chem. 2015, 7, 342−348. (43) Chen, L.-J.; Ren, Y.-Y.; Wu, N.-W.; Sun, B.; Ma, J.-Q.; Zhang, L.; Tan, H.; Liu, M.; Li, X.; Yang, H.-B. Hierarchical Self-Assembly of Discrete Organoplatinum(II) Metallacycles with Polysaccharide via Electrostatic Interactions and Their Application for Heparin Detection. J. Am. Chem. Soc. 2015, 137, 11725−11735. (44) Chowdhury, A.; Howlader, P.; Mukherjee, P. S. AggregationInduced Emission of Platinum(II) Metallacycles and Their Ability to Detect Nitroaromatics. Chem. - Eur. J. 2016, 22, 7468−7478. (45) Wu, J.; Zhang, H.; Du, S. Tunable Luminescence And White Light Emission of Mixed Lanthanide−Organic Frameworks Based on Polycarboxylate Ligands. J. Mater. Chem. C 2016, 4, 3364−3374.

2539


Photophysical Properties of Organoplatinum(II) Compounds and

Recommend Documents