Correlation of the photoelectron and electronic spectra of thiochromones and thiochromanones with their electrochemical data

Abstract

The gas phase ionization potentials, electrochemical redox potentials and spectroscopic properties of a series of thiochroman-4-one and thiochromone derivatives have been studied. A dramatic shift in the energies of the lowest vacant and highest occupied molecular orbitals of the parent thiochromanone as a function of the addition of a double bond and/or oxidation of the sulphur atom was observed. This shift in energy of the molecular orbitals was reflected in their spectroscopic characteristics. The lowest singlet (and triplet) state of compounds 1–3 in solution is π,π* in nature, while that of compounds 4–6 is n,π*. These results are best explained in terms of substituent effects on the energetics of the acetophenone moiety. The change in the nature of the lowest excited state from π,π* (1–3) to n,π* (4–6) should result in quite different types of photochemistry for the two series.A linear free-energy relationship between the singlet and triplet energies and the absolute difference between the oxidation and reduction potentials of the two series was found. These correlations have been utilized to estimate the half-wave oxidation potentials of compounds 3–6. A correlation was found to exist between the gas phase ionization potential and the solution electrochemical oxidation potential.