Two multifunctional stimuli-responsive materials with room-temperature phosphorescence and their application in multiple dynamic encryption

Abstract

Multifunctional stimuli-responsive materials with room-temperature phosphorescence (RTP) has been demonstrated to be one of the most important members of the stimuli-responsive material family and their development remains a great challenge at this stage. Herein, we successfully synthesized two coordination polymers (CPs), formulated as {[M(cbbpy)(HBTC)(H₂O)]·2H₂O}_n (M = Zn (1) and Cd (2)), using a viologen ligand 1-(3-carboxybenzyl)-4,4′-bipyridinium chloride (Hcbbpy) and trimesic acid (H₃BTC) by tuning different metal ions. Complexes 1 and 2 show excellent chromic behavior in response to X-ray and UV light. In particular, complex 1 exhibits thermochromism. Moreover, complexes 1 and 2 exhibit dual fluorescence and RTP emissions in the solid state, emitting yellow afterglow of 1.5 s and 1 s at room temperature (297 K) and cyan afterglow of 6.5 s and 5 s at a low temperature (77 K), respectively, when turning off the UV lamp. A hypothetical mechanism is proposed via theoretical calculations that the fluorescence corresponds to the transfer of electrons from the viologen-based moiety to O1 and O2 of complexes. And the phosphorescence corresponds to the intersystem crossing (ISC) process with electrons being transferred from the viologen moiety through the metal atom to the carboxyl group of trimesic acid in an excited triplet state and then transition back to the ground state. In addition, multiple encryption and anti-counterfeiting were successfully designed through photochromism, fluorescence and RTP of these complexes. This work not only provides a new strategy for the design and synthesis of multifunctional materials, but also develops a new dynamic photo-controlled afterglow switching.