ON THE MECHANISM OF RADIATIONLESS DEACTIVATION OF RHODAMINES

Absorption and emission (fluorescence spectra and radiative decay curves) spectroscopies are used to determine the photophysical properties of rhodamines with non-alkylated amino groups (rhodamine 110 and 123) in water and in ethanol. The results are compared to those previously reported for rhodamines with alkylamino groups. Different mechanisms for internal conversion such as the amino group rotation hindered by solvent viscosity, amino N-H vibrations, the formation of a twisted intramolecular charge transfer state (TICT) and the open-close umbrella-like motion model (ULM) are considered. Both the TICT and the ULM models can explain the experimental results if an intramolecular xanthene-COOR interaction and specific rhodamine-solvent interactions are taken into account.