Molecular Switches Guided by a Reversible Access to Room-Temperature Phosphorescence and ESIPT Fluorescence

This work contributes to luminescent molecular switches featuring several emission pathways, which can be activated by external stimuli. We designed Zn complexes with phenylbenzothiazole-based α-aminomethylphosphine oxide (L) and isolated them as crystalline phases, α-[ZnL₂Cl₂], [ZnL(EtOAc)Cl₂], [ZnL₂Cl₂]·1.5CH₂Cl₂, and [ZnL₂Cl₂]·1.5CHCl₃. They feature an intramolecular hydrogen bond of medium strength, capable of excited-state intramolecular proton transfer (ESIPT), as well as able for intersystem crossing between singlet and triplet states. Since neither of these processes is predominant, one or the other can occur depending on a slight change in a molecular geometry. The crystalline phases reveal red-colored ESIPT fluorescence, while a metastable amorphous phase β-[ZnL₂Cl₂] with a similar structure of the coordination center reveals yellow-colored room-temperature phosphorescence. Combined experimental and quantum-chemical TD-DFT study clarified the dual emission behavior for the polymorphs α-[ZnL₂Cl₂] and β-[ZnL₂Cl₂], which is attributed to the high dependence of the probability of the excited-state processes on the geometry of the phenylbenzothiazole moiety. The reversible phase transition, accompanied by the change in the emission mechanism (ESIPT fluorescence vs phosphorescence), can be manipulated by fuming with CHCl₃ and Et₂O, respectively. We have demonstrated good adhesive properties of the polymer-free β-[ZnL₂Cl₂] film toward glass and plastic, naked-eye color response to fuming with Et₂O, and easy recovery with CHCl₃.