Solid State ESIPT Emission Switching Containing Hydroquinone and p-Benzoquinone Units and its Charge-Transfer Complexes

Abstract

To achieve coexistence of ESIPT (Excited State Intramolecular Proton Transfer) emission and charge transfer (CT) interactions, we synthesized the hydroquinone (H2Q) derivative 1, which introduces a hydroxyl group into 2-(2hydroxyphenyl)benzothiazole (HBT) π-core, and its oxidized form of p-benzoquinone (p-BQ) derivative 2. Molecules 1 and 2 are an electron donor and acceptor, respectively. In solution, it exhibited fluorescence behavior dependent on solvent polarity. ESIPT fluorescence was observed in non-protic solvents, while non-ESIPT fluorescence was confirmed in polar protic-solvents. In the solid state, 1 showed excellent ESIPT luminescence, attributed to the fixation of molecular conformation within the crystal. In contrast, 2 exhibited no fluorescence behavior in the solid state. Furthermore, the CT complexes were formed using electron donor molecule 1 and electron acceptors of p-BQ and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ), and their crystal structures and physical properties were investigated. The electrochemical properties, electronic states, optical properties, and crystal structures of molecules 1, 2, and the CT complexes were elucidated. These results contribute to the development of novel multifunctional emission materials that simultaneously utilize ESIPT fluorescence and CT interactions.