Study of solvent effects on the molecular structure and the reorganization energies of $4-nitro-4^\prime -dimethyl-aminoazobenzene$ using resonance Raman intensities

Resonance Raman spectroscopy was used to probe the effect of solvent polarity on the molecular structure and isomerization dynamics of $4-nitro-4^\prime -dimethylaminoazobenzene$ (DA). In addition, the influence of the polarity and the relaxation behavior of the solvent on the mode-specific vibrational and solvent reorganization energies was investigated. Raman spectra were recorded in solvents with different polarity parameters, n-hexane and benzene, using 11 excitation wavelengths in the range 450-550 nm, scanning every 10 nm using the tunable laser pulse output of an optical parametric oscillator. It was observed that the solvent polarity plays a major role in influencing the excited-state potential energy surfaces of DA. The plot of resonance Raman intensities versus excitation wavelength (Raman excitation profiles) of the Franck-Condon-active fundamentals of DA shows that in n-hexane, the locally excited state is more favorable than that in benzene where the charge-transfer state is more stable. The distortions along the N-N and C-N stretching vibrations of DA in n-hexane and benzene infer isomerization via rotation in n-hexane and inversion in benzene. The increase in solvent and vibrational reorganization energies with increasing solvent polarity parameter suggests greater distortion in the Franck-Condon excited state following photoexcitation