Theoretical investigation of the effect of the solvent, hydrogen bond and amino group on the isomerization of Rhodamines

The isomerization between two forms of Rhodamine dyes, namely the lactone and zwitterion, have been investigated theoretically in solution phase at the B3LYP/6-31 + G(d,p) level of theory with the polarizable continuum model (PCM). The isomerization energy barrier between the lactone and zwitterion of Rhodamine 110 in solvents of different polarity is calculated. The solvent polarity has little effect on the isomerization energy barrier of Rhodamine 110, though a linear correlation between the energy barrier and solvent polarity is found. The effect of the hydrogen bonds at five sites of Rhodamine 110 on the energy barrier of isomerization is investigated by the inclusion of an explicit methanol molecule in addition to the PCM approach. The energy barrier of Rhodamine dyes with different amino groups is calculated at the PCM-B3LYP/6-31 + G(d,p) level of theory to clarify the role of the amino groups in the isomerization of Rhodamines. The change of the energy barrier is explained by the electrostatic repulsion between the xanthene ring and the carboxyl group as evidenced by the linear relation between the energy barrier of the isomerization and the lactonic C–O distance under different conditions. The relative energies of lactone and zwitterion, which are affected by solvent, hydrogen bonds and the amino groups, are also discussed.