Color-tunable persistent luminescence in molecular polymorphs of ionic co-crystals

Abstract

Room temperature phosphorescence (RTP) with long afterglow from molecular crystals has been the subject of growing interest for researchers over the last decade due to its potential applications in human lives. Highly efficient RTP materials in small low-molecular-weight systems are still limited owing to the singlet state (S₀) to triplet state (T₁) spin forbidden process and weak spin–orbital coupling. The crystal engineering strategy has been proven useful for the development of long persistent luminescent (LPL) materials. Here we developed two hydrated polymorphic co-crystals that were “engineered” with tautomeric 2-amino-6-methyl-1,4-dihydropyrimidin-4-one (AHMP) and saccharin. Both the polymorphs exhibit bright ultralong phosphorescence with AS-I showing longer RTP lifetime. The different cyan and yellow afterglow colors of AS-I and AS-II indicate that their photoluminescence properties can be highly tuned through changes in both their chemical structure and solid state packing. This is interpreted in terms of different singlet–triplet intersystem crossing (ISC) processes between the S₁ and the low-lying triplet states in the two forms. Hole–electron analysis of these two forms suggests that the spin–vibronic coupling between the local and charge transfer excited states contributes vitally to their persistent luminescence. The significance for the photoluminescence of AS-I and AS-II is the strong spin-orbital coupling (SOC) between the singlet and triplet states of AHMP and saccharin molecules, in which AS-I is more favorable by up to an order of magnitude than AS-II. Therefore, this research leads to appreciation that stacking state regulation in polymorphic co-crystals is promising to achieve ultralong phosphorescence for small molecules.