Photophysics of Bis(thiocyanato)gold(I) Complexes:  Intriguing Structure−Luminescence Relationships

The electronic structure of bis(thiocyanato)gold(I) complexes is studied both experimentally and theoretically. Temperature-dependent photoluminescence studies for K[Au(SCN)2] reveal two unstructured luminescence bands: a strong green phosphorescence band (τ77K = 45.4 μs) and a weak blue fluorescence band (τ77K = 24.4 ns) that becomes well-resolved by cooling toward 4 K or by time-resolved measurements, representing a rare case for Au(I) compounds whereby both fluorescence and phosphorescence are observed simultaneously. Quantum mechanical calculations for dimeric models indicate Au−Au covalent bond formation in the T1 lowest triplet excited state (2.62 Å; υAu - Au = 180 cm-1), compared to corresponding values of 2.95 Å and 84 cm-1, respectively, for the aurophilically bound S0 ground state. Intriguing structure−luminescence relations exist for bis(thiocyanato)gold(I) complexes with different cations such as K+, Rb+, n-Bu4N+, and Cs+ in which the salts with shorter Au···Au nearest-neighbor separations show blue shifts in the phosphorescence emission energies as well as smaller Stokes' shifts, contrary to the expected trends. We have also observed significantly red-shifted phosphorescence energies and larger Stokes shifts in frozen solutions of K[Au(SCN)]2 compared to those for the crystals. The computational data suggest that the emission energy is sensitive to the counterion, in support of the experimental photoluminescence data. Full optimizations of the T1 states for isolated dimeric models in vacuum predict a drastic rearrangement in the T1 states in contrast to the S0 ground state and provide a physical basis for understanding the experimental photophysical results for this class of compounds.