Modulating room-temperature phosphorescence of triazine–carbazole systems with stimuli-responsive behaviors through conjugation expansion

Abstract

Carbazole and its derivatives constitute some of the most intensively investigated building blocks in organic optoelectronics for constructing diverse materials especially in organic room temperature phosphorescence (RTP) molecules. Benzocarbazoles with expanded conjugation generally exhibit superior RTP performance in either host–guest systems or small molecules, although indole impurities were found to be key in stimulating the RTP behavior of carbazole containing materials. Here, by mimicking the structure of a star RTP molecule of 4,6-diphenyl-2-carbazolyl-1,3,5-triazine (DPhCzT), two RTP molecules were designed and constructed by replacing the carbazole with benzocarbazoles. Indeed, better photophysical properties were observed, showing not only significantly red-shifted emission bands but also significantly improved RTP performance with a lifetime of up to 1.02 s for afterglow and the extraordinary stimulus-responsive behaviors that can be activated upon photoirradiation and deactivated under ambient conditions when doped in polymethyl methacrylate (PMMA). Compared to the weak, short-lived and static emission of DPhCzT in PMMA, significantly photoactivated ultralong RTP was realized due to the systematic extension of the π-conjugation in modulating the optical bandgap and promoting the intersystem crossing along with the rigidified matrix microenvironment confinements to suppress non-radiative decays. With the long-lived, photoactivatable, and color-switchable afterglow, these polymeric composite films demonstrate promising applications in erasable optical printing and dynamic information encryption. This work demonstrates a versatile strategy for designing diversified RTP materials with fast stimulus-responsive behavior through the expansion of π-conjugation and also provides fundamental insights into the dynamic stimuli-responsive process for the development of high-performance RTP materials and devices.