Can Coumarins Break Kasha’s Rule?

• Published in: The journal of physical chemistry letters Vol. 10; no. 21; pp. 6468 - 6471
• Main Authors: Shafikov, Marsel Z, Brandl, Fabian, Dick, Bernhard, Czerwieniec, Rafał
• Format: Journal Article
• Language: English
• Published: American Chemical Society 07.11.2019
• ISSN: 1948-7185; 1948-7185
• DOI: 10.1021/acs.jpclett.9b02292

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 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).