Can Coumarins Break Kasha’s Rule?

Abstract

Coumarin C-2 was reported ( Signore et al., J. Am. Chem. Soc. , 2010 , 132 , 1276 and Brancato et al., J. Phys. Chem. B , 2015 , 119 , 6144 ) to break Kasha's rule. However, the two lowest excited singlet states ofC-2 are separated by less than 0.5 eV. To slow down the S2 → S1 internal conversion and thus to enable the Kasha's rule-breaking S2 fluorescence, a much larger energy separation seems to be necessary. Thus, the photophysical behavior reported for C-2 raised very basic questions concerning mechanisms of nonradiative transitions in organic molecules. Herein we reinvestigated luminescence of C-2 and found that thoroughly purified C-2 does not show any dual fluorescence in steady-state experiments, contrary to the previous findings. The higher-energy emission, previously erroneously assigned as S2 → S0 fluorescence of C-2, stems from persistent impurity of the synthetic precursor (C-1).