Macrocyclization-induced phosphorescence enhancement of pyridinium-based macrocycles

Abstract

Organic room temperature phosphorescence (RTP) has exhibited various applications in optoelectronics and photobiology. Reported here is an effective phosphorescence enhancement strategy through the macrocyclization of phosphorescent pyridine/pyridinium units by methylene linkers. A pyridine macrocycle was synthesized by the condensation of the 3,5-bis(2,4-dimethoxyphenyl)pyridine monomer with paraformaldehyde under the catalysis of a Lewis acid. The methylation reaction of the pyridine macrocycle with methyl iodide and subsequent ion exchange with ammonium hexafluorophosphate and tetrabutyl ammonium halide gave pyridinium macrocycles. Compared with pyridinium monomers, the macrocycles exhibited enhanced phosphorescence with up to 59-fold prolonging of lifetime and 3.7-fold increase of quantum yields. A mechanism study revealed that macrocyclization restrained the rotation/vibration of luminescent units and therefore suppressed nonradiative decay. Moreover, the macrocyclization slowed down the nonradiative and radiative decay processes of the triplet state. Such macrocyclization-induced phosphorescence enhancement (MIPE) would provide a novel and general strategy for enhancing organic room temperature phosphorescence (RTP) and find wide applications beyond phosphorescence.