Al3+ regulated competition between TICT and ESIPT of a chemosensor

Abstract

Twisted intramolecular charge transfer (TICT) and excited state intramolecular proton transfer (ESIPT) are of fundamental importance during the excited state dynamics of dyes and fluorescent sensors. Co-existence of the two processes usually complicates the whole photophysical process of dyes and leave the fluorescence detection mechanism of sensors uncovered. In this contribution, competition between the two processes are observed during the excited state relaxation of a fluorescence sensor 1-[(2-Amino-phenylimino)-methyl]-naphthalen-2-ol (short for L). Al3+ is proved to be capable of regulating this competition during the excited state deactivation process. With the addition of Al3+, the original barrier for the ESIPT process is removed and ESIPT fluorescence peak of the sensor is significantly enhanced. TICT process, which is originally barrier-less, is prohibited by the strong coordination between Al3+ and the sensor. The regulation of TICT and ESIPT process to achieve Al3+ detection is interesting and intrinsically different from the previously proposed sensing mechanism. The newly proposed mechanism is quite rare in the field of metal ion detection and may inspire scientists to design sensors from a different perspective.