Prolonged irradiation-induced delayed luminescence of PMMA-dispersed imide compounds containing ether- and thioether-bridged cores

Abstract

Time variations in the photoluminescence (PL) properties of four types of imide compounds (ICs) with ether (OD-IC), thioether (SD-IC), dibenzofuran (BO-IC), and dibenzothiophene (BS-IC) cores dispersed in polymer films were investigated to elucidate the effects of the introduction of sulfur atoms and the structural rigidity of the ICs on their PL properties under continuous UV irradiation. These ICs exhibit photoactivated delayed phosphorescence (PH) from the excited triplet state (T₁), which is called prolonged irradiation-induced delayed luminescence (PIDL), after a few minutes of induction time, during which only fluorescence is observed. This result is because oxygen quenching efficiency gradually weakens as the ground-state oxygen decreases via energy transfer from the ICs in the T₁ state. As compared with the ICs containing ether linkages (OD-IC, BO-IC), the PHs of the ICs containing thioether linkages (SD-IC and BS-IC) were significantly enhanced by the heavy atom effect, and these showed bathochromic (red) shifts of the PL peaks owing to the extended π-conjugation. Furthermore, the ICs containing five-membered cores (BO-IC and BS-IC) demonstrated longer PIDL lifetimes and shorter induction times than those of the ICs containing flexible linkages (OD-IC and SD-IC), owing to their structural rigidity. In particular, BS-IC exhibited an excellent PIDL properties such as the largest intensity (I = 2480), a long lifetime (τ_PIDL = 410 ms), and a short induction time (t_ID = 1.55 min) owing to the rigid core containing a sulfur atom. Based on the results, this study provides a valuable strategy for developing novel PIDL materials.