PHOTOPHYSICS OF THE LACTONE FORM OF RHODAMINE-101

The photophysics of the lactone form of the rhodamine 101 has been investigated in nonpolar and polar aprotic solvents. A single broad luminescence band in nonpolar or weakly polar solvents is observed from a charge-transfer (CT) state (with the dipole moment of almost-equal-to 26 D) produced in the electron-transfer reaction in the excited state. In more polar solvents a second fluorescence band appears, revealing the dissociation of the C-O lactone bond. The excited singlet state of the zwitterion (Z) is formed. Temperature-dependent measurements show that the zwitterion form is produced in all solvents; it is, however, quenched in less polar media. It has been found that the quantum efficiency of population of the Z form in the excited singlet state does not depend on the solvent and equals 0.23 +/- 0.03, which indicates purely intramolecular control of the branching into CT and zwitterion excited states. The excited state reaction in frozen solvents led to intense phosphorescence from a low lying (pi,pi*) triplet state of the Z form. The ratio of fluorescence to phosphorescence intensities of the zwitterion (1:3) in rigid glass implies a supposition that the Z form is created in singlet and triplet states according to their spin statistical factors. The results of this work force us to verify the existing views on the spectroscopy of rhodamines and the role of the solvent in photophysics of these molecules. The observed intramolecular quenching of zwitterions has been ascribed to deactivation to a higher triplet, most probably of (n,pi*) nature. We suspect this mechanism to be responsible for the thermally activated nonradiative process in rhodamines. The lack of phosphorescence of rhodamine 101 in protic solvents is explained by an increase of energy of the 3(n,pi*) state, presumably due to the hydrogen bonding. The data do not support the transition to the TICT state as a mechanism of thermally activated quenching of rhodamines.