Photo-activated time-dependent color-changeable ultralong organic room temperature phosphorescence by co-doping strategy

Abstract

In this work, we for the first time report photo-activated time-dependent color-changeable ultralong organic room temperature phosphorescence (UORTP) by co-doping derivatives (CN2BdBr and CN2BCzBr) of two phosphorescence units, H-benzo[f]indole (Bd) and 7H-benzo[c]carbazole (BCz). BCz is a newly reported phosphorescence unit. Two donor–acceptor (D–A) type derivatives (CNCzBCzBr and CN2BCzBr) were synthesized and their ultralong phosphorescence properties were studied in solution, the pure powder state and the PMMA film. In solution, both compounds are not ultralong phosphorescence active at room temperature but show an intense green afterglow at 77 K. Their pure powders cannot emit an afterglow at room temperature due to their high non-radiation rate but give out a yellow afterglow at 77 K because enhanced intermolecular interactions in the aggregated state reduce the energy level of triplet excitons and lead to a red shift. Amazingly, both compounds demonstrate photo-activated green UORTP in the PMMA film and their afterglow lifetimes are longer than that of CN2BdBr. ESR spectra verify that ultralong phosphorescence is activated and enhanced due to the consumption of oxygen in the PMMA film, which is similar to Bd and its derivatives. Finally, CN2BCzBr and CN2BdBr are co-doped into the PMMA film and photo-activated time-dependent color changeable UORTP is successfully achieved because the afterglow lifetime difference is large enough and two kinds of phosphorescence units can function independently. To our best knowledge, we for the first time propose a co-doping strategy and realize unique photo-activated time-dependent color-changeable UORTP.