Exceptional case of Kasha's rule: Emission from higher-lying singlet electron excited states into ground states in coumarin-based biothiol sensing

Abstract A coumarin-based fluorescent sensor (7-Acryloxy-4-methylcoumarin, AMC ) for biothiols has been studied by a combination of quantum chemical calculation and experimental investigation. The AMC selectively reacts with mercapto biomolecules via Michael addition, which can be served as a ratiometric fluorescent sensor for thiol-containing biomolecules. The AMC sensor shows a selectivity towards biothiols with sensitivity in the order of cysteine > glutathione > homocysteine. Moreover, the AMC sensor can detect the cysteine concentration within the intracellular level. The results of the theoretical investigations on the electron excited states show that the lack of overlapping between MOs in transitions of the lower-lying singlet electron excited states and the small energy gap between electron excited states are the cause of fluorescence from the higher-lying singlet electron excited states, as an exception from Kasha's rule. The fluorescence from the higher-lying singlet electron excited states leads to marked fluorescence enhancement at long wavelength in the products of addition reaction of biothiols to AMC sensor.