Studies on the UV-vis absorption spectra of azo dyes: Part 25. analysis of the fine structure of the π1 →π 1* band of 4'-donor-sub

Abstract

The electronic absorption spectra of 4'-donor-substituted 4- N,N-diethyl-aminoazobenzenes in n-hexane at room temperature have been measured. The π 1 → π 1 * band exhibits a vibrational fine structure. Systematic variation of the substituents allows assignments to be made for these sub-bands. The first sub-band is due to a vibronic transition from the ground state to the planar excited state. The intense second sub-band represents the NN stretching vibronic transition between the ground state and a planar vibrational excited state. The third sub-band is attributed to torsional vibrations around the Ph—N() bonds in the excited state. Only this vibronic transition enables coupling to occur between the n s, π 1 * singlet state and the π 1, π 1 * singlet state, resulting in the 'borrowing of intensity by the n s→π 1 * band from the π 1→π 1 * band.