2-(2-hydroxyphenyl)-benzothiazole (HBT)-rhodamine dyad: acid-switchable absorption and fluorescence of excited-state intramolecular proton transfer (ESIPT).

Dyad was prepared by link rhodamine and excited state intramolecular proton transfer (ESIPT) chromophore 2-(2-hydroxyphenyl)-benzothiazole (HBT) using Click reaction, with the goal to switch the absorption/emission property of ESIPT chromophore. The photophysical properties of the dyad were studied with steady state and time-resolved absorption and emission spectroscopy. In the absence of acid, that is, with rhodamine is in spirolactam structure, ESIPT was observed, the enol form emission of HBT unit was observed at 404 nm in protic solvents. In aprotic solvents, emission of the keto form of HBT was observed at 543 nm. With addition of acid such as trifluoroacetic acid, the rhodamine unit transforms to the opened amide structure, intense absorption band at 554 nm developed, as well as a strong fluorescence band at 579 nm; in EtOH, the enol emission of HBT at 406 nm was not quenched by the resonance energy transfer (RET), thus, dual fluorescence was observed. In dichloromethane, however, the fluorescence of the keto form of HBT unit was completely quenched. Thus, the absorption and emission of the ESIPT chromophore were switched by a acid/base-activatable rhodamine chromophore. Such studies will add additional modulability to the ESIPT chromophores.