Precise Excited-State Engineering in Benzimidazole Derivatives via Donor–Acceptor Architecture for Organic Temperature-Responsive Phosphorescence

Herein, we report precise excited-state engineering in benzimidazole (NBd) derivatives via a donor–acceptor (D–A) architecture for organic temperature-responsive phosphorescence (RTP). Four NBd derivatives (1-NBdCN, 1-NBdBrCN, 2-NBdCN, and 2-NBdBrCN) with an electron acceptor at position 1 and position 2 were designed and synthesized. The position of the electron acceptor enables 1-NBdCN and 1-NBdBrCN with temperature-responsive phosphorescence and endows 2-NBdCN and 2-NBdBrCN with red-shifted phosphorescence, showing a remarkable site effect. First, phosphorescence properties of the four NBd derivatives were studied in solution at 77 K. 1-NBdCN and 1-NBdBrCN give out yellow afterglow in toluene solution but bluish-green afterglow in ethanol solution, displaying solvent-dependent double phosphorescence components. 2-NBdCN and 2-NBdBrCN emit orange afterglow both in toluene solution and in ethanol solution, suggesting position 2 leads to a red shift of phosphorescence. Second, copolymerized methyl methacrylate/acrylic acid copolymer (MA/AA) films of the four NBd derivatives all demonstrate RTP. 2-NBdCN and 2-NBdBrCN kept the orange afterglow whether at room temperature or at 77 K. However, 1-NBdCN and 1-NBdBrCN exhibit distinct temperature-responsive phosphorescence color change from bluish green to yellow with increasing temperature, indicating temperature-dependent expression of double phosphorescence components. TD-DFT and SOC results reveal that 1-NBdCN and 1-NBdBrCN have two phosphorescence pathways for triplet excitons from T₁ and T₂ due to their sensitive molecular conformations. Therefore, the double phosphorescence components of 1-NBdCN and 1-NBdBrCN arise from T₁ and T₂, which are greatly affected by solvent polarity and temperature. This study gives a deep insight into the site effects of electron acceptors on RTP and paves a way for intelligent organic phosphorescence materials.