Long-Lived Luminescence Emitted from Imide Compounds Dispersed in Polymer Matrices after Continuous Ultraviolet Irradiation and its Relation to Oxygen Quenching

Long-lived room-temperature delayed phosphorescence, called prolonged irradiation-induced delayed luminescence (PIDL), was observed in air for three imide compounds (ICs) dispersed in polymethyl methacrylate under UV irradiation after an induction time of 1.60-6.78 min, whereas the ICs alone showed only fluorescence after a short period of UV irradiation. The PIDL emission continued as an afterglow for several seconds (0.36-1.30 s) after the irradiation was stopped. In the initial stage of irradiation, the ICs in the excited triplet state (T-1) generated via intersystem crossing were immediately quenched by ground state triplet molecular oxygen (O-3(2)), and the excited state energy of ICs was transferred by photosensitization to O-3(2), generating singlet oxygen (O-1(2)), causing a gradual depletion of the concentration of O-3(2) in the host matrix. When most of the O-3(2) adjacent to the ICs had been consumed, the ICs started to emit a PIDL. The induction time until the appearance of PIDL was linearly proportional to the O-3(2) concentration. Optical measurements and numerical simulations based on the Jablonski diagram support the PIDL generation mechanism and reveal the relationships between the rate constants and oxygen permeabilities of the host matrices.