Photoluminescence of Tetra(benzimidazole)phenylethene\nDerivatives and Their Carbene Metallacycles in Aggregates and Langmuir–Blodgett\nFilms

Molecular structure, external stimuli,\nand environmental factors\nall have a strong effect on the internal molecular rotation and vibration\nof aggregate-induced emission (AIE) luminogens. Here, we report the\nAIE effect for several newly synthesized amphiphilic tetra(benzimidazole)phenylethene\n(TBiPE) derivatives and their binuclear N-heterocyclic carbene silver\n(NHC-Ag) metallacycles in solutions, aggregates, and Langmuir–Blodgett\n(LB) films. Monolayer behaviors and microscopic images indicate that\nintroducing alkyl chains and binuclear NHC-Ag metallacycles can facilitate\nthe formation of a well-defined insoluble monomolecular layer at the\nair–water interface. Absorption and luminescence spectral features\nsuggest that both the TBiPEs’ cyclization structure and binuclear\nNHC-Ag metallacycles can provide additional driving forces to restrict\nthe internal molecular motion of the tetraphenylethene (TPE) unit,\nthus enhancing the AIE effects. Further, external stimuli and environmental\nfactors such as poor solvent addition and LB film deposition also\nplay important roles in the photoluminescent intensity, maximum wavelength,\nand lifetime. These internal and external factors can result in around\n30–40 nm blue-shift for the maximum luminescence wavelength\nand 2–3 times shorter for the luminescent lifetime. These phenomena\ncan be attributed to the reason that the nonradiative energy transfer\nefficiency is weakened because of the enhanced hydrophobic interaction\nand metal–carbene coordination as well as the closely packed\narrangement of AIEgens in the LB films; consequently, the radiation\nenergy transfer efficiency increased.