Issue 8, 2012

Remarkable fluorescence enhancement of benzo[g]chromen-2-ones induced by hydrogen-bonding interactions with protic solvents

Abstract

A coumarin analogue, 8-methoxy-4-methyl-2H-benzo[g]chromen-2-one (MMBC), is almost non-fluorescent in non-polar media, whereas it exhibits dramatically enhanced fluorescence in polar protic solvents. This study investigates the mechanistic features of the significant solvent effects on the fluorescence properties of MMBC and a related compound, 4-methyl-2H-benzo[g]chromen-2-one (MBC), by time-resolved fluorescence and photoacoustic measurements and by theoretical calculations. Time-resolved photoacoustic measurements reveal that the extremely fast non-radiative processes of MBC and MMBC in non-polar solvents can be attributed predominantly to internal conversion. The internal conversion rates of MBC and MMBC are remarkably reduced in a rigid matrix of 3-methylpentane at 77 K, suggesting that internal conversion is a thermally activated process. The photophysical properties of MBC and MMBC examined in selected solvents with different polarities and hydrogen-bond donating abilities show that the internal conversion rate is greatly reduced by hydrogen-bonding interactions with protic solvents. Furthermore, remarkable fluorescence enhancement is observed by adding a small amount of trifluoroethanol to n-hexane solutions of MBC and MMBC, indicating that internal conversion is suppressed by formation of hydrogen-bonded complexes with protic solvents. In light of theoretical considerations based on time-dependent density functional theory (TD-DFT) and INDO/S-CI calculations, the occurrence of fast internal conversion in MBC and MMBC can be explained in terms of the proximity effect, i.e., pseudo Jahn–Teller coupling between energetically close S1 and S2 states.

Graphical abstract: Remarkable fluorescence enhancement of benzo[g]chromen-2-ones induced by hydrogen-bonding interactions with protic solvents

Supplementary files

Article information

Article type
Paper
Submitted
05 Mar 2012
Accepted
06 May 2012
First published
14 Jun 2012

Photochem. Photobiol. Sci., 2012,11, 1368-1376

Remarkable fluorescence enhancement of benzo[g]chromen-2-ones induced by hydrogen-bonding interactions with protic solvents

A. Kobayashi, K. Takehira, T. Yoshihara, S. Uchiyama and S. Tobita, Photochem. Photobiol. Sci., 2012, 11, 1368 DOI: 10.1039/C2PP25055A

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