Substrate-Responsive Pillar[5]arene-Based Organic Room-Temperature Phosphorescence

Room-temperature phosphorescence (RTP) is a photophysical phenomenon typically associated with a long-lived emission that can be detected by the unaided eye. Several natural proteins display RTP, as do certain artificial polymers. In both cases, the RTP is ascribed to effective intramolecular through-space electronic communication. However, small molecules with internal electronic communication that enable RTP are relatively rare. Herein, we describe an alkyl halide-responsive RTP system consisting of a meta-formylphenyl-bearing pillar[5]arene derivative that supports effective through-space charge transfer (TSCT) within the pillararene cavity. Treatment with bromoethane, a heavy atom-containing guest for the pillar[5]arene host, serves to enhance the emission. An isomeric para-formylphenyl-bearing pillar[5]arene system proved ineffective in producing an RTP effect. Quantum chemical calculations based on single-crystal X-ray diffraction analyses provided insights into the structural determinants governing TSCT between the 1,4-dimethoxybenzene donor units and the formylphenyl groups of the pillar[5]arene, as well as the associated energy gaps and intersystem crossing channels. We believe that the present system and the associated mechanistic analysis provide the foundation for design of new small molecule with tunable RTP features.