DFT/TD-DFT study of the structural and spectral properties of two forms of Rhodamine B

The absorption wavelengths of the two forms of Rhodamine B, cation and zwitterion, were investigated by Time-Dependent Density Functional Theory (TD-DFT) in combination with polarizable continuum model. The redshift in absorption spectrum of cation relative to zwitterion is attributed to strong inductive effect of carboxyphenyl group and weak electrostatic repulsion between xanthene ring and carboxyphenyl group. The absorption wavelengths of cation and zwitterion decrease linearly with increase of solvent polarity in normal alcohols since in high polar solvents electrostatic repulsion between xanthene ring and carboxyphenyl group increases and affects xanthene π conjugation system. The absorption wavelengths in water and formamide show a deviation from linear relationship because large dielectric constant hinders electrostatic repulsion between carboxyphenyl group and xanthene π system. The hydrogen bonds affect absorption wavelengths because hydrogen bonds could affect conjugation between amino N atoms and xanthene π system or electrostatic repulsion between carboxyphenyl group and xanthene ring. These results indicate electrostatic repulsion between carboxyphenyl group and xanthene ring plays a big role in determining absorption spectrum of Rhodamine B.