EXCITED-STATES OF AZO-COMPOUNDS .2. VIBRATIONAL STRUCTURE OF THE ELECTRONIC ABSORPTION-SPECTRA OF 4,4'-DI-SUBSTITUTED AZOBENZENE DERIVATIVES

The electronic absorption spectra of azobenzene and its fourteen symme trical and non-symmetrical derivatives substituted in both para positi ons by electron-donating substituents were studied in 2-methyl-tetrahy drofuran solvent at room (293 K) and low (103 K) temperatures. The mai n effect destroying the vibronic structure of the absorption spectra o f the azobenzene derivatives was found - the ground state non-planarit y caused by torsions around the N-Ph bond. The central azo bond stretc hing was interpreted as the dominant vibrational mode in absorption sp ectrum. It has been shown that increasing electron-donating ability of azobenzene substituents in para positions increases the planarity of the molecules in the ground state and decreases the difference between equilibrium azo bond lengths in the ground and first excited singlet pipi states.