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 of C-2 are separated by less than 0.5 eV. To slow down the S-2 -> S-1 internal conversion and thus to enable the Kasha's rule-breaking S-2 fluorescence, a much larger energy separation seems to ...
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 of C-2 are separated by less than 0.5 eV. To slow down the S-2 -> S-1 internal conversion and thus to enable the Kasha's rule-breaking S-2 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 S-2 -> S-0 fluorescence of C-2, stems from persistent impurity of the synthetic precursor (C-1).
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