Dual Emission of a Cyclic Hexanuclear Gold(I) Complex. Interplay

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C: Physical Processes in Nanomaterials and Nanostructures

Dual Emission of a Cyclic Hexanuclear Gold(I) Complex. Interplay between Au and Au Ligand-Supported Luminophores 3

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Pengfei Ai, Matteo Mauro, Andreas A. Danopoulos, Alvaro Muñoz-Castro, and Pierre Braunstein J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10190 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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The Journal of Physical Chemistry

Dual Emission of a Cyclic Hexanuclear Gold(I) Complex. Interplay between Au3 and Au 2 LigandSupported Luminophores Pengfei Ai,a Matteo Mauro,b Andreas A. Danopoulos,a,c Alvaro Muñoz-Castro,d and Pierre Braunsteina a

Université de Strasbourg, CNRS, CHIMIE UMR 7177, Laboratoire de Chimie de

Coordination, 4 rue Blaise Pascal, 67081 Strasbourg Cedex, France b

Université de Strasbourg, CNRS, Institut de Physique et Chimie des Matériaux de

Strasbourg, UMR 7504, F-67000 Strasbourg, France c

Inorganic Chemistry Laboratory, Chemistry Department, National and Kapodistrian

University of Athens, Panepistimiopolis, Zografou 157 71, Greece d

Laboratorio de Química Inorgánica y Materiales Moleculares, Facultad de

Ingeniería, Universidad Autonoma de Chile, El Llano Subercaseaux 2801, Santiago, Chile.

Abstract Finding diverse and tunable molecular structures is relevant towards the design of functional nanostructures. The photoluminescence of complex 1, featuring ligandsupported hexanuclear Au(I) framework compromising a triangular Au 3 core of which each apex is connected to an external Au(I) center has revealed a remarkable dual emission at room temperature. The emission bands display maxima centered at λ em = 512 and 694 nm with Stokes shift of 19530 and 14410 cm-1 and are attributed to the radiative relaxation of two excited-states centered onto the central Au 6 skeleton arising from 5dσ∗→6pσ excitation. As suggested by the strikingly different dioxygen dependency of the relative intensity of the two emission bands, the observed dual emission can be tentatively attributed to the incomplete equilibration between two close-lying emissive excited states with singlet and triplet character, most likely due to slow intersystem crossing (ISC) process, yielding green fluorescence and red phosphorescence, respectively; this phenomenon is rarely observed in heavy element compounds. Based on theoretical calculations, these excited-states originated from two different substructures-luminophores of the molecular skeleton, mainly i) the Au 3 core, --

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and ii) one specific Au 2 unit. Thus, the dual luminescence of 1 originates from the noteworthy inclusion of two luminophores within the overall molecular structure. Both solution and solid-state emission spectra show similar characteristics owing to the intramolecular nature of the suggested luminescence mechanism. Such luminophores can be envisaged as novel metalloligands to be incorporated in larger gold nanoclusters towards the development of intense luminescent molecular devices.

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Introduction Polynuclear coinage-metal complexes with d10 electronic configuration often exhibit interesting chemical and physical properties. Their unique features arises from the interactions between closed-shell centers, which promote the formation of building blocks leading to functional molecular and supramolecular materials with tailored and fascinating properties.1–10 In particular, numerous gold(I) complexes possess remarkable luminescence properties with long-lived emission and sizable Stokes shifts which turn them into promising luminescent materials for various practical applications.8,11,12 Their inherent photophysical properties are strongly related to attractive metal-metal interactions between their closed-shell metal centers,13 known as aurophilic interactions,14–17 of which the strength of which comparable to that of hydrogen bonds. Such metallophilic interactions are reinforced by relativistic effects, leading to Au(I)···Au(I) separations shorter than the sum of the van der Waals radii (3.6 Å) and providing a rich structural diversity.15,18–20 The formation of trinuclear gold(I) complexes with a Au 3 unit dates back to the 70’s,21–24 where the luminescent properties of [Au 3 {MeN=C(O)Me} 3 ] were characterized by a long-lived yellow emission in the solid-state owing to the columnar intermolecular stacks,25–27 and a short-lived emission in solution owing to the supported Au 3 core. The recent synthesis of the ligand-supported hexanuclear gold(I) complex [Au 2 Cl(µ-P-C-κP,κC,κN)] 3 (1) displaying a central Au 3 core connected to three peripheral Au(I) centers forming a three-fold symmetric scaffold of Au 2 units, resulted from the use of a bulky N-phosphanyl-functionalized NHC moiety acting as tridentate ligand (Scheme 1),28 represented an interesting extension to the family of well characterized ligand-supported Au 3 complexes.21–24,29

Scheme 1. Schematic representation of 1, denoting the central Au 3 core in blue and three peripheral Au(I) in red.

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In 1, the diverse Au···Au interactions appeared promising for a closer inspection of the nature and origin of the associated photophysical properties with a detailed theoretical analysis, starting from the previously reported structure of 1 should serve the goal of understanding of the photophysical behavior. Indeed, upon excitation at selected wavelengths, exploration of low-lying excited states by means of theoretical calculations suggests the formation of states featuring shorter Au⋅⋅⋅Au contacts, which in turn might yield interesting luminescence features. In particular, more covalent Au⋅⋅⋅Au bonds, lead to distortions and d10-d10 separations shorter than those found in the experimental ground state structure of 1.28 Herein, we now unravel the interesting luminescent properties of 1, showcasing a noteworthy dual emission of intramolecular origin with a sizable Stokes shift, of intramolecular origin persisting in both solution and the solid-state. Similar behavior is limited only to a few dinuclear complexes.30–32 Futhermore, the possible excited states involved were evaluated via relativistic DFT and TD-DFT calculations, in order to rationalize the radiative pathways and related structural rearrangements. In this paper, we report a dual luminescent behavior centered on ligand-supported Au 3 and Au 2 efficient emissive subunits of the Au 6 cluster 1, which opens new perspectives for the design of multiple luminescent gold clusters

Experimental Details Chemical characterization. The chemical characterization of the complexes has been previously reported.28 Photophysical characterization. Absorption spectra were measured on a double-beam Shimadzu UV-3600 UV-Vis-NIR spectrophotometer and baseline corrected. Solution sample of complex 1 were prepared in CH 2 Cl 2 at concentration 5.0×10-5 M. Steadystate excitation and emission spectra at room temperature in CH 2 Cl 2 were recorded on a HORIBA Jobin-Yvon IBH FL-322 Fluorolog 3 spectrometer equipped with a 450 W xenon arc lamp as the excitation source, double-grating excitation and emission monochromators (2.1 nm mm−1 of dispersion; 1200 grooves mm−1) and a TBX-04 single-photon-counting as the detector. Emission and excitation spectra were corrected for source intensity (lamp and grating) and emission spectral response (detector and

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grating) by standard correction curves. All solvents used were spectrophotometric grade. Deaerated samples were prepared by freeze-pump-thaw technique. Computational investigation. Relativistic density functional theory calculations33 were carried out by using the ADF code,34 incorporating scalar corrections via the ZORA Hamiltonian.35 We employed the triple-ξ Slater basis set, plus two polarization functions (STO-TZ2P) for valence electrons, within the generalized gradient approximation (GGA) according to the Perdew-Burke-Ernzerhof (PBE) exchangecorrelation functional36,37 because of its improved performance on long-range interactions and relatively low computational cost when employed for ligand protected gold clusters.38–42 The frozen core approximation was applied to the [1s2-4f14] shells for Au, [1s2-2s2] for P, and [1s2] for C and N, leaving the remaining electrons to be treated variationally. Geometry optimizations of both ground- and excited-states were performed without any symmetry restrain, via the analytical energy gradient method implemented by Versluis and Ziegler.43 An energy convergence criterion of 10‐4 hartree,

a gradient convergence criteria of 10‐3 hartree Å-1 and a radial convergence criteria of

0.01 Å were employed for the evaluation of the relaxed structures. Excitation energies were calculated via TD-DFT considering the van Leeuwen−Baerends (LB94) xc-

functional, which offers more accurate values because of its correct asymptotic behavior.44 It has been employed for other gold cluster structures.42,45–49 The characterized excited states were further optimized to lead to two specific structures, namely XS1 and XS2, see text.

Results and Discussion The reaction between N,N′-diphosphanylimidazol-2-ylidene (P−C−P), a phosphanylfunctionalized NHC ligand, and [AuCl(tht)] (tht = tetrahydrothiophene) resulted in the cleavage of one tBu 2 P-N imid bond of P−C−P and the formation of a cyclic, hexanuclear complex [Au 2 Cl(µ-P-C-κP,κC,κN)] 3 (1) (Figure 1), in which a central Au 3 core is surrounded by three peripheral Au(I) centers, establishing three ligand-stabilized Au 2 units,28 and involving four different bond types, Au-C, Au-P, Au-Cl and Au···Au, denoting the NHC-Au, phosphanyl-Au, terminal Cl-Au, and aurophilic d10-d10 type interactions,15 respectively. In 1, multiple independent aurophilic interactions generate

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six Au···Au separations, ranging from 3.444 to 3.543 Å (av. 3.490 Å) in the central Au 3 unit, and from 3.043 to 3.130 Å (av. 3.080 Å) in the peripheral Au 2 units.

Figure 1. Structural representation of 1 in the electronic ground-state (GS). C: Black; H: White; N: Blue; P: Organge; Cl: Green; Au: Yellow. Photophysical characterization. The spectra recorded for compound 1 are displayed in Figure 2. In dilute CH 2 Cl 2 solution, complex 1 displays an intense and narrow absorption band centered at 283 nm with ε = 3.0×104 M-1 cm-1 that can be ascribed to the singlet-manifold character,

also

1

[5dσ*→6pσ] transition with mainly metal-centered (MC)

comparable

to

related

d8-d8

and

d10-d10

systems

reported

elsewhere.30,31,50,51 The process involving the 283 nm excitation can be formally seen as corresponding to the promotion of electron density from the antibonding d z 2 to the bonding 6s/6p orbital that results in increased metal···metal interaction. Upon irradiation with λ exc = 300 nm, CH 2 Cl 2 samples of 1 show broadband photoluminescence with two featureless bands centered in the green and red portions of the spectrum, with λ em 512 and 694 nm and with Stokes shift of 19530 and 14410 cm-1, respectively. Interestingly, under identical experimental conditions, the relative intensity of these two bands strongly depends on the presence of quenching dioxygen molecules in solution. In particular, the green emission band appears to be insensitive to the presence of dioxygen, the red emission band displays a sizeable increase upon carefully degassing the sample by freeze-pump-thaw cycles and their relative intensity, namely I 512 /I 694 , where I 512 and I 694 are the emission intensities at λ em = 512 and 694 nm,

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respectively, decreases from 3.90 to 0.65. This finding demonstrates the different origin of the two bands, the green emission being attributed to a fluorescence process arising from a singlet-manifold excited-state whereas the red emission might be phosphorescence in nature. Nonetheless, they are both attributed to an excited state with [pσ→dσ*] character. This dual emission might arise from the incomplete equilibration of these two excited states at room temperature, most likely due to slow ISC processes, which in turn make fluorescence a competitive deactivation channel over ISC. Noteworthy, both emission bands display virtually identical excitation spectra that show a maximum at 303 nm, supporting the idea that they share a common excitation process. Nonetheless, a small difference between the absorption and excitation spectra can be noticed that may indicate that the observed absorption band results from the overlap of different electronic transitions with different character and a lower-lying and less intense band at ca. 300 nm may be present underneath the intense one, the latter being at the origin of the two emitting excited states. As a consequence, 1 shows efficient dual emission centered at the Au 6 core with a metal-centered (MC) character. This remarkable behavior is scarce in multinuclear heavy-element complexes, having been observed in some diplatinum d8-d8 complexes,30,31 and Tb-Pt structures,32 and being more common in mononuclear complexes of 2-phenylpyridine and 2,2-bipyridines, where metal-to-ligand charge transfer (MLCT) transitions,52–56 are involved, constituting examples for intramolecular singlet-triplet and triplet-triplet dual luminescent behavior. As shown in Figure 2, excitation of a solid state sample of 1 at λ exc = 284 nm gives rise to dual luminescence as well, although the two maxima were hypsochromically shifted compared to the CH 2 Cl 2 solution by 3840 and 4311 cm-1 for the green and red band (λ em = 430 and 662 nm, respectively),. This finding supports the intramolecular nature of the dual emission and rules out the presence of impurity and intermolecular energy transfer processes. Although d10 metal complexes are well know to establish (extended) intermolecular interactions with metal···metal character in condensed phases, the lack of such interactions in 1 may be attributed to the bulky nature of the ligand substituents, i.e. tert-butyl groups, while the hypsochromic shift may be the result of the more rigid environment experienced by the molecules of 1 in the solid state that disfavor large geometrical distortions.

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Figure 2. Left: Experimental photophysical properties of 1 in dilute CH 2 Cl 2 at room temperature. Dashed traces: absorption (black) and excitation spectra recorded at λ em = 510 (green) and 690 nm (red). Solid traces: emission spectra recorded for airequilibrated (gray) and degassed (black) sample upon excitation at λ exc = 300 nm under identical experimental conditions. Arrows designate the y-axis scale (left or right) related to the spectra in the circle. Right: comparison between emission in airequilibrated (gray) and degassed (black) CH 2 Cl 2 solution upon excitation at λ exc = 300 and solid-state sample (red) upon excitation at λ exc = 284 nm.

In order to establish the nature of each absorption band, 1 was investigated by relativistic DFT and TD-DFT methods. The calculated structure for 1 exhibits average Au···Au distances of 3.518 Å in the central Au 3 unit, and of 3.141 Å at the peripheral Au 2 units, in good agreement to the experimental findings (see above). The calculated absorption spectrum exhibits two relevant bands at 307 and 282 nm, in good agreement with the experimental values, where the calculated peak at 307 appears as a weak band overlapping with the main 283 nm band (Figure 2), since the calculated oscillator strength for the former is lower compared to the latter, (f = 0.19 and 0.30, respectively). The electronic structure of 1 is given in Figure 3 and illustrates the character of frontier orbitals involved in the optical transitions. As a result, the first peak calculated at 307 nm involves a transition from the HOMO, dominated by a 5d-6s-Au hybridization, to a bonding orbital of main 6p-Au character within both Au 3 core and Au 2 units (LUMO+2), leading to a cluster-centered transition. The peak at 282 nm involves a transition from the HOMO to a ligand-6p-Au based orbital (LUMO+5), which is ascribed as a metal-to-metal ligand charge transfer (M-MLCT).

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Figure 3. Schematic representation of the electronic structure of 1, in the ground-state (GS), and the two respective emissive excited-states, XS1 and XS2 respectively. Isosurfaces of relevant molecular orbitals are highlighted in green, the bonding orbital accounting for the enhanced Au-Au interaction, which is further stabilized in both excited-states.

Further inspection of the relevant excited states indicates that the transition accounting for the peak calculated at 307 nm, can be assigned to a S 0 →S 3 transition from the ground state singlet (S 0 ) to the third singlet state (S 3 ) leading to relaxed excited states (metastable structures) with different geometries than the ground state (S 0 ) (Figure 3). This feature was associated with the dual emission and the large Stokes shift described for diplatinum d8-d8 complexes family,30,31 where the Pt···Pt bond shortens in the excited state. In 1 the picture is more complicated since the central skeleton involves six aurophilic Au···Au separations. From the excited S 3 state, two possible structures were obtained, namely XS 1 and XS 2 , owing to the population of the bonding orbital within the Au 3 core and Au 2 units, respectively. Such possibilities account for a main Au 3 bond enhancement, leading to a distortion of the Au 3 triangle, and a selective Au···Au shortening within one Au 2 unit. This geometry relaxation from ground state (GS) to both excited states (XS1 and XS2) structure after photoexcitation at λ exc = 300 nm (calc. 307 nm) results in a remarkable effect on the relevant frontier orbital gap. For the electronic third singlet state, S 3 , at the GS structure, the LUMO+2 orbital is further stabilized, leading to the LUMO for XS1

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and XS2, with a smaller HOMO-LUMO gap going from 3.439 eV at GS, to 1.942 and 1.734 eV, respectively, which is the emissive state. In addition, the HOMO is destabilized in comparison to the ground state, leading to a decrease in the frontier orbital gap. Thus, the geometrical relaxation of the excited state is the main origin of the large observed Stokes shift observed for 1. The electronic S 1 state in XS1 is favored by 29.2 kcal∙mol-1 in comparison to the S 3 state at the ground state geometry (GS). Furthermore, the S 1 state in XS2 is favored by 39.6 kcal∙mol-1 in comparison to the S 3 state at the ground state geometry, indicating that XS1 can decay non-radiatively via an intersystem crossing to XS2, or decay radiatively to the electronic ground state (S 0 ). The latter stabilization of XS2 allows a further intersystem crossing to a triplet T 1 state favored by 4.1 kcal∙mol-1 in comparison to the respective S 1 state in the XS2 structure. This allows us to calculate the emission energies from 1 upon excitation at 300 nm via the energy gap between the S 0 and S 1 states at the calculated excited state structures (∆SCF), and via TDDFT at the excited state geometries. The calculated S 1 →S 0 emission in the XS1 structure amounts to 1.659 eV (747 nm), and the T 1 →S 0 emission from XS2 to 1.499 eV (826 nm), via the ∆SCF method (Table 1). In contrast, the TDDFT method provides values of 2.318 eV (535 nm) calculated for the S 1 →S 0 emission from XS1, and for the T 1 →S 0 emission from XS2 to 1.619 eV (766 nm), which approximate better the observed emission bands at 2.422 eV (512 nm) and 1.787 eV (694 nm), showing a deviation of about 0.1 eV, in the range of the TDDFT method error. Hence, hereafter we will discuss values from the TDDFT method. Thus, the emission peak observed at 512 nm is assigned to green fluorescence, whereas the 694 nm emission to red phosphorescence, in agreement to experimental observations (see above). The discussed photoluminescent mechanism of 1 is summarized in Figure 4, which exhibits an interesting case for the violation of Kasha's rule where the emissive state is initially located above the lowest excited state, which then is further stabilized resulting in the discussed luminescent behavior. Hence, the Au 6 skeleton in 1 is found to provide two novel luminophores given by, a) the geometrical modifications centered at the Au 3 core; and b) the contraction of a single specific Au 2 unit, which are accounted by XS1 and XS2 excited state geometry, respectively.

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Figure 4. Schematic Jablonski diagram summarizing the photoluminescence mechanism of 1 upon photo-excitation.

Table 1. Calculated and experimental emission bands (solution 5.0×10-5 M of 1 in DCM), for each respective excited-state geometry. ∆(SCF)

∆(TDDFT)

exp. λ em

eV

1.660

2.318

2.422

nm

747

535

512

eV

1.500

1.619

1.787

nm

826

766

694

XS1 S 1 →S 0 XS2 T 1 →S 0

Figure 5. Structural representation of 1 at the excited-states XS1 and XS2. Distances for XS2 given in Å. The calculated excited states for 1 feature interesting variation of the Au···Au separations (Figure 5). In XS1, the initial C 3 axis of symmetry is lost owing to the shortening of two Au···Au edges from ~3.5 to ~2.7 Å (Table 2), while a third edge is

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elongated to 3.7 Å. When considering the peripheral Au 2 units, two of them experience a decrease of their Au···Au distance from ~3.1 to ~2.9 Å, whereas the third is elongated to about ~3.8 Å. Thus, in the excited state, some aurophilic Au···Au contacts are shortened, which strengthens the corresponding Au-Au bonding and further stabilizes the metastable situation of the XS1 geometry enough to exhibit photoluminescence properties. For XS2, a related effect is found, where a specific Au···Au contact is shortened from ~3.1 to ~2.7 Å, the remaining d10-d10 separations deviating slightly from the ground state. In addition, the corresponding Au-Cl is changing. Both structures optimized at S 1 and T 1 states shows similar structural parameters.

Table 2. Selected distance at ground-state and the respective emissive excited-states. Values in Å. GS

GS(exp)a

1XS1

3XS2

3.522

3.543

2.702

3.504

3.516

3.486

2.713

3.541

3.516

3.444

3.710

3.544

3.142

3.066

2.854

2.717

3.149

3.130

2.935

3.113

3.132

3.043

3.811

3.131

2.301

2.300

2.315

2.438

2.300

2.294

2.332

2.304

2.300

2.285

2.288

2.303

Au 3

Au 2

Au-Cl

a

Values taken from ref. 25. In order to gain further insight into the shortening of the aurophilic Au···Au

contacts upon photo-excitation and the associated enhancement of the covalent character of gold(I)-gold(I) interaction, we applied the Boys-Foster localization scheme57 of molecular orbitals to obtain localized molecular orbitals (LMO’s), as is

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implemented in the ADF code.58 Such LMO's provide an informative picture for the analysis of the bonding situation in organometallic systems.59–61 In the ground state, three main bond types were encountered (Figure 6), accounting for the Au-C, Au-P, Au-Cl bonds supporting the Au 6 skeleton. Interestingly, in the excited state, the LMOs define a fourth bond type accounting for the enhanced Au-Au bonding interaction, as suggested by the short separation between such d10-d10 centers in the range of ~2.7 Å. This supports the idea that the driving force of photoluminescence in compounds featuring aurophilicity is given by the strengthening Au-Au bonding in the excited state, by switching from interaction of mainly dispersion/correlation origin, to a more covalent character, in a long to short-contact shift upon selected photoexcitation. The population of each LMOs is consistent with this picture, showing 1.55 ē in the main Au 3 core bonding at XS1, and 1.48 ē in one specific Au 2 unit.

Figure 6. Localized molecular orbitals (LMOs) of 1 at GS, XS1 and XS2 states, corresponding to Au-C, Au-P and Au-Cl bonds with occupation of ~2.00 ē, and photoinduced Au-Au bond at XS1 and XS2 (occupation is given).

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Lastly, the dual emission of compound 1 in the solid state is displayed in Figure 2. As mentioned above, dual emission was observed for 1 in the solid state as well. Interestingly, the crystal packing induces a hypsochromic shift of the observed emission bands compared with the spectrum recorded in fluid solution. The peak attributed to XS1 is shifted to shorter wavelength by about 80 nm, whereas the emission from XS2 is shifted to a lesser extent. This is understood in terms of the larger structural modification required for XS1 avoiding a fully geometrical relaxation and the subsequent radiative decay. The XS2 state offers a smaller geometrical distortion, and hence it is suggested that this emissive state is preferred for further deactivation after excitation of 1 in solid-state.

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Conclusions By investigating the photoluminescence characteristics of 1, we unraveled a remarkable behavior due to a dual luminescence upon excitation consisting of a green fluorescence and red phosphorescence: upon excitation at 300 nm, two emission bands are observed at 512 and 694 nm. This phenomenon has scarcely been observed for heavy-element complexes. A theoretical evaluation of the excited states indicates that the luminescent properties are inherent to the Au 6 skeleton of 1, where two novel substructure-luminophores could be identified involving mainly the central Au 3 core, and one specific Au 2 unit, accounting for the green and red emission, respectively. Interestingly, in the computed structures of each excited state notorious aurophilic Au···Au contractions are observed, leading to photoinduced increased covalent character within the Au 6 skeleton, a characteristic of luminescent aurophilic structures. Complex 1 therefore represents a novel addition to heavy-element dual emissive complexes. Thus, the use of isolated or combined Au 3 and Au 2 ligand-supported luminophores, may develop to a useful strategy to generate tunable novel multiple emission structures involving larger gold nanoclusters, towards obtaining intense luminescent molecular devices.

Acknowledgments This work was supported by FONDECYT 1140359 and 1180683. We thank the CNRS and MESR (Paris) for funding and we are grateful to the China Scholarship Council for a Ph.D. grant to P.A.

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Dual Emission of a Cyclic Hexanuclear Gold(I) Complex. Interplay

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