STUDIES ON THE UV-VIS ABSORPTION-SPECTRA OF AZO DYES .25. ANALYSIS OF THE FINE-STRUCTURE OF THE PI-1-]PI-1-STAR BAND OF 4'-DONOR-SUBSTITUTED 4-N,N-DIETHYLAMINOAZOBENZENES

The electronic absorption spectra of 4'-donor-substituted 4-N,N-diethylaminoazobenzenes in n-hexane at room temperature have been measured. The pi-1 --> pi-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), pi-1* singlet state and the pi-1, pi-1* singlet state, resulting in the 'borrowing' of intensity by the n(s) --> pi-1* band from the pi-1 --> pi-1* band.