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

The development of highly emissive dinuclear Ag-I or Au-I complexes [M2L](PF6)(2) (L=2a, 2b; 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 (Phi(F)) value for salt H-4-2a(PF6)(4) in dilute solution (c=10(-5)m) was found to be less than 1%, whereas its metal complexes show Phi(F) 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 [M(2)2b](PF6)(2) (M=Ag, Au) with terminal coumarin groups and subsequent photoirradiation yielded the complexes [M(2)3b](PF6)(2) (M=Ag, Au) bearing a new type of ligand with an unaffected TPE moiety.