Synthesis, Characterization, and Properties of Tetraphenylethylene-Based Tetrakis-NHC Ligands and Their Metal Complexes.

The development of highly emissive dinuclear AgI or AuI complexes [M2 L](PF6 )2 (L=2 a, 2 b; M=Ag, Au) derived from tetraphenylethylene (TPE)-based tetrabenzimidazolin-2-ylidene ligands is reported. The new complexes exhibit a remarkable fluorescence enhancement compared to their parent benzimidazolium salts. The quantum yield (ΦF ) value for salt H4 -2 a(PF6 )4 in dilute solution (c=10-5  m) was found to be less than 1 %, whereas its metal complexes show ΦF values up to 55 % at similar solution concentration. This observation can be attributed to the rigidification of the TPE skeleton upon metalation resulting in a restriction of the intramolecular rotation of the phenyl groups. Functionalization of the complexes [M2 2 b](PF6 )2 (M=Ag, Au) with terminal coumarin groups and subsequent photoirradiation yielded the complexes [M2 3 b](PF6 )2 (M=Ag, Au) bearing a new type of ligand with an unaffected TPE moiety.