Assignment of the π → π* and n → π* Transitions to the Spectral Bands of Azobenzene and Dimethylaminoazobenzene

Abstract

A critical analysis of the UV–Vis absorption spectra of azobenzene (AB) and dimethylaminoazobenzene (DAB) used as an optical probe was performed based on the key empirical regularities of the n → π* transitions of nitrogen- and carbonyl-containing compounds. The spectrum of DAB has an n → π* band located in the same range ν = 28 000–33 000 cm^–1 (λ = 350–300 nm) as the n → π* band of protonated azobenzene AB⁺H, but differs in the presence of a vibronic structure. It was also shown that the low-frequency n → π* band belonging to AB lies in the range where the π → π* band of AB is located with a maximum at a wavelength λ_max ≈ 320–340 nm (ν_max = 30 300–29 400 cm^–1), in contrast to the conventional assignment of this band to the Vis band at 440 nm. This n → π * band is not found in the spectra of AB solutions in polar media, but appears in the nonpolar medium due to the presence of a vibronic structure.