Abstract

Non radiative relaxation of various N-substituted ethyl-ester-Rhodamine dyes (scheme A) was explained by assuming an intramolecular rotational motion towards a low lying non emissive TICT-like state with charge localization [1]. Stationary and time-resolved fluorescence experiments evidenced that fluorescence quenching in those Rhodamines is linked with the rotation of the amino group. On the other hand, if the o-carboxyphenyl group is exchanged for a p-dimethylamino electron donor group (scheme B), the fluorescence quantum yield is very low in neutral ethanol but increases by more than two orders of magnitude in acidic ethanol solutions, where the electron donating property of the anilino group can be blocked by protonation [1, 2].

© 1992 The Author(s)