Ab initio insights into the mechanism of excited-state intramolecular proton transfer triggered by the second excited singlet state of a fluorescent dye: an anti-Kasha behavior

An excited-state intramolecular proton transfer (ESIPT) reaction triggered by the second excited singlet S2 state is found by sophisticated ab initio calculations to be an ultrafast process for a novel fluorescent dye of 2-((5-(3-hydroxy-4-oxo-4H-chromen-2-yl) thiophen-2-yl) methylene) malononitrile, 3-HTC-DiCN, which can thoroughly compete with internal conversion from the S2 state to the lowest S1 state, thus leading to the remarkable enhancement of fluorescence quantum yield arising from the proton-transfer tautomer emission as well as the corresponding decrease of the normal emission quantum yield. This is in apparent discrepancy to Kasha's rule.