Solvatochromism of 4-(diethylamino)-4’-nitroazobenzene: explanation based on CNDO/S calculation results

Abstract

Solvatochromism of 4-(diethylamino)-4’-nitroazobenzene was observed by visible absorption spectroscopy, and mechanism of the shift of absorption maxima was explained on the basis of semi-empirical molecular orbital calculation results. The wavelength of the absorption maximum measured for methanol solution was longer than that measured for cyclohexane solution; solvatochromic color change was observed. Dipole moments of the ground state and excited state of the azo compound calculated by the CNDO/S method indicated that the excited state was more polar than the ground state. This polarization at the excited state corresponds to so-called charge transfer excitation, and the polarization results in lowering of excitation energy in polar solvent. Similar tendency was also confirmed by the density functional theory calculations. The experiment and molecular orbital calculation carried out in this work were suitable for promoting the understanding of the solvatochromic shift mechanism of absorption wavelength.