Photophysics of a coumarin-based fluorescence probe for Al³⁺: Relations between multiple ESIPT processes and TICT...

Excited state intramolecular proton transfer (ESIPT) and twisted intramolecular charge transfer (TICT) are fundamental processes of the photophysical dynamics. However, relations between them are usually not clear as no or oversimplified theoretical studies are presented. Herein, this paper performs a comprehensive investigation on the photo-dynamics of a coumarin-based fluorescence sensor (PCEH, Pyridine-2-carboxylic Acid [1-(4-hydroxy-2-oxo-2H-chromen-3-yl)-ethylidene]-hydrazide) for Al³⁺ by performing density functional theory (DFT) and time dependent density functional theory (TDDFT) studies. Multiple ESIPT and TICT processes are observed with both competing and cooperating relations. We find out that the low fluorescence of the sensor originates from three easily accessible TICT states. One of them can be reached by the rotation of the N5-C6 single bond. The other two are initiated by an early stage ESIPT process followed by the rotation of C2-C3 or C3-N4 bonds. Interestingly, none of them is initiated by the direct CN isomerization, which has been generally accepted to induce fluorescence quenching. Afterwards, the sensing mechanism for Al³⁺ as well as its selectivity is clarified.