Intramolecular fluorescence quenching in aminocoumarines. Identification of an excited state with full charge separation

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Abstract
The fluorescence of 6-aminocoumarin (A6C) is very weak and anomalously redshifted as compared to 7-substituted analogs 4-methyl-7-aminocoumarin (A7C) and 4-methyl-7-diethylaminocoumarin (DEA7C). The solvatochromic redshift in homologous alcohols is much stronger for A6C, pointing to an increased dipole moment of the emitting state. A twisted intramolecular charge transfer (TICT) state with full charge separation is made responsible for this emission. This is rationalized by molecules-in-molecules calculations. The strong Stokes shift in n-hexane and in ethanol glass at 77.degree. K suggests that A6C emits from a TICT state even under these conditions.

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