Remarkable Aurophilicity and Photoluminescence Thermochromism in a Homoleptic Cyclic Trinuclear Gold(I) Imidazolate Complex

Abstract

A new aurophilically-bonded cyclic trinuclear gold(I) complex, tris[μ-(1-ethylimidazolato-N,C)gold(I)] ([Au(EtIm)], 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-from violet to red!-attributed to internal conversion between a higher-energy (T → S; λ ∼409 nm) and lower-energy (T → S; λ ∼700 nm) phosphorescent band below and above 200 K, respectively, likely representing an excited-state phase change.

Luminescence Thermochromism of Gold(I) Phosphane–Iodide Complexes: A Rule or an Exception?

“…[13] In addition, complex 7 does not show de-tectableL El uminescence in DMSO, in both liquid and frozen forms, which confirms its crucial dependence on the presence of weak intra-(aurophilic) andi ntermolecularb ondingt hat is disrupted in liquid medium and in the ground amorphous state. Indeed, grinding the crystalline samples completely removes the thermochromic properties of compounds 6 and 7 and makes them virtually nonemissive at room temperature; similar crystal-only behavior has been described for imidazolate Au 3 phosphor.…”

“…In the case of 6 and 7 , the exact nature of the barrier between T 2 and T 1 , however, remains an open question. Examples of pronounced luminescence thermochromism, which can be treated in terms of two distinct excited states, has been described for a very limited number of gold(I) complexes, including a few dinuclear compounds with the XantPhos ligand . For the last case, the large hypsochromic shift of the emission has been tentatively explained by short Au⋅⋅⋅Au contacts, which cannot contract further upon cooling …”

“…Nevertheless, ligand Xh as an oninnocent influence on luminescencee fficiency because the iodide induces aboutathreefold increase in the radiativer ate constant (k r = 1.5 10 5 s À1 for 10 vs. 5.8 10 4 s À1 for [(AuCN)(tpdp)]), and consequently,e nhances the quantumy ield of 10 (F em = 0.35) by af actor of four,w ith respectt oi ts cyanide congener (F em = 0.08). [13,33] The crucial importance of the ordered solid matrix is demonstrated by bimetallic luminophores 6 and 7,w hich reveal the presence of two coupled triplet excited states T 2 and T 1 of IL 3 pp*a nd 3 (X + M)C (mixedw ith 3 XLCT) origin, respectively.D ue to the energy barrier between T 2 and T 1 ,c rystalline 6 and 7 exhibit HE and LE emissionsw ith thermally variable intensity ratios. Eur.J.…”

“…[10] Gold(I) ions have ah igher oxidation potential than that of coppera nd silver metals, and therefore, are less prone to participate in MLCT transitions. [13] Essentially, this appealing photophysical behavior could not be correlated with changes of intra-or intermolecularm etal-metal distances or phase changes.T he intramolecular switching of luminescence origin upon changing the temperatureh as only been described forafew di-and triphosphane halide [14] and cationic [15] gold(I) species, for which blueshifted IL phosphorescence prevails at low temperature, whereas metal-dominated LE emission with poor quantum yields (F em % 1-2 %) is observed at 298 K. [14a] Al imited understanding of the factors that guide the thermochromic properties of Au I -based materials preventst he rational designo ft hese complexes on the molecular and macroscopic levels. [11] Ar emarkable luminescence thermochromism has been reported for [Au 3 (pyrazolate) 3 ], the emission of which is of 3 CC type (both at 77 Ka nd 298 K), and was proposed to be strongly affected by intermoleculars pacing.…”

“…This process generates two different types of intermolecular aggregates at 77 K through modulation of aurophilic contacts, and consequently causes two phosphorescence signals with maxima at λ=490 and 680 nm . A very large emission thermochromism of 10 164 cm −1 , ascribed to internal conversion between T 2 and T 1 excited states, has been described for a trinuclear imidazolate Au I cluster . Essentially, this appealing photophysical behavior could not be correlated with changes of intra‐ or intermolecular metal–metal distances or phase changes.…”