Spectroscopic and electrochemical properties of para-methoxyazobenzene derivatives

Abstract

Ultraviolet photoelectron spectroscopy, gas-phase absorption spectroscopy, polarographic half-wave reduction potentials and INDO/S-CI molecular-orbital calculations have been employed to investigate the electronic structure of some p-methoxyazobenzene derivatives. The photoelectron bands in the low-energy region of the spectrum have been assigned. The a_g(n_–) and the a_u(π₁) ionizations display almost the same sensibility towards the substituent. The observed trends are qualitatively predicted by calculation. The electronic transitions are interpreted with the aid of INDO/S-CI calculations. Use is also made of the approach of Haselbach and Schmelzer approach. Reversible half-wave reduction potentials are used as a measure of the solution electron affinity of the molecule. Good linear relations are found between E_1/2 and E_LUMO and the Hammett σ constant. Correlations between electrochemical and spectroscopic data provide evidence that variations in the experimental transition energies along the series, with the exception of the nitro-derivatives, originate from changes in the (–J+ 2K) and ΔG_solv terms.