Remarkable Aurophilicity and Photoluminescence Thermochromism


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Remarkable Aurophilicity and Photoluminescence Thermochromism in a Homoleptic Cyclic Trinuclear Gold(I) Imidazolate Complex Mukunda M. Ghimire, Vladimir N. Nesterov, and Mohammad A. Omary* Department of Chemistry, University of North Texas, Denton, Texas 76203, United States S Supporting Information *

have demonstrated PLT in sandwich adducts of gold(I) carbeniate or imidazolate CTCs with Ag+ and Tl+.9 Recently, Raithby and co-workers have reported PLT in gold(I) pyrazolate CTCs that are believed to be related to changes in intermolecular aurophilic interactions.10 PLT has also been reported in gold(I)−copper(I) phosphine−pyridine complexes by the Catalano group, which also exhibited luminescence rigidochromism.11 Herein, we report the novelty of the CTC tris[(μ2-1-ethylimidazolato-N3,C2)gold(I)] ([Au3(Etim)]3, 1) toward solid-state intermolecular aurophilic interactions as well as distinct PLT behavior by describing its synthesis and temperature-dependent single-crystal X-ray structural and photophysical properties. The synthesis of 1 entailed the reaction of Ph3PAuCl with 1ethyl-2-lithiumimidazole to obtain colorless crystals of X-ray quality (see the Supporting Information for synthetic details). A single-crystal X-ray diffraction investigation reveals CTC rings each exhibiting a gold(I) trimetallic core incorporated by three imidazolate ligands in the same plane of the nine-membered Au3N3C3 ring, as shown in Figures 1 and S1−S3. Complex 1 crystallizes in the space group P21/c with two independent molecules in the asymmetric unit (Tables S1 and S2). In the crystal structure at 100(2) K, discrete dimers are linked by pairwise Au···Au contacts whereby the trimer molecules are stacked in two different pairs, giving two distinct intermolecular Au···Au distances of 3.0662(3) Å (Figure 1a) and 3.1407(3) Å (Figure 1b). To the best of our knowledge, an intermolecular aurophilic interaction of 3.0662(3) Å is the shortest ever pairwise intermolecular aurophilic distance among gold(I) CTCs; a Cambridge Structural Database (CSD)12 analysis performed herein reveals an intermolecular aurophilic distance of 3.077(2) Å in [Au3(NC5H4)3], reported by the Balch group,6d as the closest to that in 1 among gold(I) CTCs in the CSD to date. The crystal structure of 1 at 296(2) K (Figures 1c,d) also shows the same space group but amounts to a discernible yet expected positive thermal expansion versus the 100(2) K structure. This is reinforced by the changes in the crystal density (3.001 vs 3.073 g cm−3) and cell volume (3879.2 vs 3788.1 Å3) at 296(2) versus 100(2) K, respectively. Similarly, the intermolecular Au···Au distances are significantly elongated to 3.1099(4) Å (Figure 1c) and 3.1868(4) Å (Figure 1d) at 296(2) K as opposed to 3.0662(3) Å (Figure 1a) and 3.1407(3) Å (Figure 1b) at 100(2) K by maintaining similar crystal packing motifs. The intramolecular Au···Au distances have also slightly changed, ranging

ABSTRACT: A new aurophilically-bonded cyclic trinuclear gold(I) complex, tris[μ2-(1-ethylimidazolato-N3,C2)gold(I)] ([Au3(EtIm)3], 1), has been synthesized and characterized by temperature-dependent crystallographic and photophysical investigations. The crystal packing of 1 reveals two independent molecules in the unit cell, signifying two distinct pairs of dimer-of-trimer units convened by pairwise intermolecular Au···Au interactions of 3.0662(3) and 3.1407(3) Å at 100 K, representing the shortest pairwise intermolecular aurophilic interactions among all cyclic trimetallic gold(I) complexes to date. Remarkably, crystals of 1 exhibit gigantic photoluminescence thermochromism of 10164 cm−1from violet to red!attributed to internal conversion between a higherenergy (T2 → S0; λmax ∼409 nm) and lower-energy (T1 → S0; λmax ∼700 nm) phosphorescent band below and above 200 K, respectively, likely representing an excited-state phase change.

C

yclic trinuclear complexes (henceforth referred to as CTCs) of d10 monovalent coinage metals are coordination compounds well-known for their remarkable solid-state photoluminescence (PL), ground-state metallophilic (M···M) interactions, and excited-state covalent M−-M bonding.1−3 Gold(I) CTCs that are commonly associated with intermolecular aurophilic (Au···Au) interactions have received the most attention among them.4 Comprehensive synthetic, structural, theoretical, and photophysical studies have been performed on a variety of gold(I) CTCs with carbeniate, pyrazolate, imidazolate, and triazolate ligands.5,6 Intermolecular metallophilic interactions of monovalent coinage metal CTCs are known to be highly sensitive toward external stimuli such as temperature, pressure, solvent liquid or vapor, etc. This behavior has given rise to discoveries of novel phenomena such as solvoluminescence, luminescence rigidochromism, luminescence vapochromism, and concentration luminochromism for such CTCs.6 The temperature-dependent luminescence color changea phenomenon originally described by Hardt and Pierre7 for the copper(I) “cubane” cluster Cu4py4I4 (where py = pyridine) complexes as “luminescence thermochromism” and later extensively studied by Vitale and Ford8is often described as a consequence of changes in the phase or molecular structure. In collaboration with the Coppens5a and Dias5b groups, respectively, we have investigated gold(I) triazolate and pyrazolate CTCs that display significant photoluminescence thermochromism (PLT).5 Burini and co-workers © 2017 American Chemical Society

Received: July 6, 2017 Published: September 28, 2017 12086

DOI: 10.1021/acs.inorgchem.7b01679 Inorg. Chem. 2017, 56, 12086−12089

Communication

Inorganic Chemistry

Figure 1. ORTEP plots of dimer-of-trimer units of complex 1 at 100(2) K (a and b) and 296(2) K (c and d) and the molecular structure of 1 (e).

from 3.438−3.488 Å (Figure S2) at 100(2) K to 3.432−3.475 Å (Figure S3) at 296(2) K. As expected, crystals of 1 are luminescent at room temperature. More surprisingly, when the sample was cooled to 77 K and irradiated with the short wavelength of a hand-held UV lamp, it showed a hypsochromic shift of its emission color (Figure 2, inset). The PL color change is reversible, as shown in the PL spectra for crystals of 1 vs temperature in Figures 2 and S4.

The emission spectra consist of a high-energy (HE) band in the violet/blue region with a maximum near 400 nm at T < 200 K and a low-energy (LE) emission in the red/near-IR region with a maximum near 700 nm at T ≥ 200 K. Both the HE and LE bands exhibit a broad, unstructured emission profile suggestive of excimeric emissions from dimer-of-trimer units. The excitation profiles are temperature-independent, unlike the emission spectra for the HE and LE bands with an excitation peak maximum near 310 nm (Figures 2 and S4). The UV−vis electronic absorption spectrum of a dichloromethane solution of complex 1 (Figure S5) shows blue-shifted absorption bands at λ < 300 nm. The emission bands of solid 1 decay with a singleexponential time constant for both the HE and LE bands (Figure 2), each gradually increasing within the microsecond domain upon cooling, suggesting a reduction in the nonradiative decay rate, as expected. A dichloromethane solution of 1, however, does not exhibit any detectable PL at either room or cryogenic temperatures, which is also valid for the PL of 1 doped in a polymer matrix (polyacrylonitrile). These observations negate molecular excited-state assignments such as metal-to-ligand charge transfer or intraligand transitions and underscore our supramolecular assignment. Although luminescence from solids of two-coordinate gold(I) CTCs is well-known, it is quite uncommon that the metalcentered emission shifts profoundly toward the near-UV region from the near-IR region, as depicted in Figures 2, S4, and S6, when cooled toward cryogenic temperatures. A comparison of the HE (409 nm at 120 K) versus LE (700 nm at 240 K) emission maximum amounts to a PLT shift of 10164 cm−1 (Figure S6), spanning the entire visible region, from violet to red! On the basis of the structural and photophysical properties of 1, we ruled out a ground-state structural phase transition as the reason for this humongous PLT shift because the excitation profiles are similar for both the HE and LE emission bands,

Figure 2. PL excitation (left) and emission (middle and right) spectra of solid 1 vs temperature. The inset shows photographs of the luminescence of 1 taken while the solid is being irradiated with a hand-held UV lamp (short wavelength) at 77 K (left) and 298 K (right). PLT reversibility holds for all spectra/pictures. 12087

DOI: 10.1021/acs.inorgchem.7b01679 Inorg. Chem. 2017, 56, 12086−12089

Communication

Inorganic Chemistry demonstrating that the ground-state origin of the excitation route for both emission bands is the same (given that the Franck− Condon vertical excitation occurs at the ground-state geometry). Following excitation, relaxations to two different excimeric states with Au−Au covalent bonds are responsible for the HE and LE phosphorescence bands. The sudden change in the HE versus LE emission intensity at ca. 200 K (Figures 2 and S4), however, indicates an excited-state phase change in the supramolecular packing of molecules, which is undetectable by “dark” temperature-dependent crystallographic investigations and requires time-resolved or steady-state “photocrystallography” experiments.13 Multiple excimeric states are quite common for CTCs of gold(I) and other coinage metals.5b,14 This situation is, therefore, in contrast to the dual emission seen in Cu4L4X4 (L = pyridine and X = iodide) cluster systems because of halide-to-metal charge-transfer (3XMCT) and cluster-centered (3CC) phosphorescent transitions.8 Another possibility that we rule out is fluorescent HE versus phosphorescent LE emission, as was rarely observed previously [e.g., by the Eisenberg group15 for gold(I) O,O′-dialkyldithiophosphate and our group16 for an ion-paired gold(I) cluster complex] because both bands exhibit microsecond lifetimes. This assignment is also consistent with the crystallographic data, which do not entail any space group changes upon temperature variation while contractions in the intermolecular distances and unit cell volume occur (Tables S1 and S2). We also rule out the possibility that temperaturedependent intermetal contractions are responsible for this drastic PLT because such effects usually show gradual red shifting in the emission bands due to corresponding contractions in molecular HOMO−LUMO gaps or semiconductor valence−conduction band gaps, as those seen in the tetracyanoplatinates(II).17 One would also be eager to invoke that the two different dimer-oftrimer units are responsible for these two emission bands such that the unit with the shorter intertrimer distance is responsible for the LE emission while the unit with the longer intertrimer distance is responsible for the HE emissionsomewhat akin to the energy transfer/migration argument suggested by Buchner et al. on platinum(II) extended-chain systems.18 However, we believe that the emission bands are due to the combination of both dimer-of-trimer units with a small difference in the rather broad emission envelope that is not resolvable under the pertinent experimental conditions. The spectral overlap between a potential donor emission and acceptor absorption in the two sites is vanishingly small (>1 eV separation between the HE emission and LE excitation maxima!), whereas energy transfer to a trap state is more common in extended-chain states rather than molecular dimer/excimer states. We propose that an internal conversion process represents the reason for the colossal PLT of 1. The illustration depicted in Figure 3 shows a possible qualitative model of the pertinent photophysics. At high enough temperature (T ≥ 200 K), relaxation to the lowest emissive state takes place, as explained by Kasha’s rule,19 to give rise to the red LE emission band (T1 → S0; centered at ∼700 nm). Such a relaxation is hindered at lower temperatures (T < 200 K), hence giving rise to a higher-lying Au−Au bound excimeric state with near-UV emission (T2 → S0; centered at ∼400 nm), likely representing an excited-state structural phase change in supramolecular packing at ∼200 K. Such an internal conversion process has led to a phosphorescence color change from near-UV to near-IR upon heating from 4 to 298 K. Even though this process is in contradiction with Kasha’s rule, there are numerous exceptions to such a mere “rule

Figure 3. Qualitative description of PLT in solid 1, showing internal conversion processes within 3MM emitting states.

of thumb” (e.g., multiple PL bands seen in pyrene,20 mercury clusters,21 and mixed-metal (Au/Cu) complexes)3 in addition to the aforementioned precedents in homometallic CTCs. In conclusion, 1 manifests a new class of gold(I) CTCs with the shortest intermolecular Au···Au distances known to date. Solid 1 exhibits remarkable PLT spanning the entire visible region. Further characterization of 1 is underway both to theoretically investigate the possible existence of any covalency in the unusually short intertrimer distances, given our team’s recent finding for mixed-metal CTCs with rather short d10−d10 distancesalbeit between copper(I) and gold(I)that suggested ground-state bond formation,3 and to assess the potential application of 1 and congeners thereof toward solid-state lighting/display devices (OLEDs/LEDs), solar cells, and photocatalytic hydrogen production.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01679. Further experimental details (PDF) Accession Codes

CCDC 1561852 and 1561854 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] ORCID

Mohammad A. Omary: 0000-0002-3247-3449 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work is supported by Robert A. Welch Foundation Grant B1542 and NSF Grant CHE-1413641. 12088

DOI: 10.1021/acs.inorgchem.7b01679 Inorg. Chem. 2017, 56, 12086−12089

Communication

Inorganic Chemistry



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DOI: 10.1021/acs.inorgchem.7b01679 Inorg. Chem. 2017, 56, 12086−12089