Study of solvent effects on the molecular structure and the reorganization energies of 4‐nitro‐4′‐dimethylaminoazobenzene using resonance Raman intensities

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AbstractResonance Raman spectroscopy was used to probe the effect of solvent polarity on the molecular structure and isomerization dynamics of 4‐nitro‐4′‐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. Copyright © 2001 John Wiley & Sons, Ltd.