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 [M 2 L](PF 6 ) 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 ( Φ F ) value for salt H 4 ‐ 2 a (PF 6 ) 4 in dilute solution ( c= 10 −5   m ) was found to be less than 1 %, whereas its metal complexes show Φ 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 2 b ](PF 6 ) 2 (M=Ag, Au) with terminal coumarin groups and subsequent photoirradiation yielded the complexes [M 2 3 b ](PF 6 ) 2 (M=Ag, Au) bearing a new type of ligand with an unaffected TPE moiety.