Protonation effect on the excited state behaviour of EE-1-(n-pyridyl)-4-phenylbutadienes (n = 2, 3 and 4)

Abstract

The acid–base equilibria of three aza-derivatives of EE-1,4-diphenylbutadiene and the excited state properties of their neutral and protonated forms have been studied in aqueous solutions. The prevalent relaxation of the neutral molecules is photoisomerization by a diabatic singlet mechanism accompanied by substantial internal conversion while the quantum yield of the radiative pathway is very small, particularly for the ortho- and para-substituted pyridines. Protonation affects only slightly the isomerization yield of these two compounds while drastically decreasing that of the meta-substituted analogue. Molecular orbital calculations and comparison with the corresponding azastilbenes indicate that the first excited singlet state of the protonated meta compound has a net charge transfer character, which leads to an increase of the torsional barrier in S₁. Application of the Förster cycle to the three compounds showed that they become much stronger bases in S₁. The 3-pyridyl derivative proved to be peculiar also in this respect since its equilibration with the proton occurs during the S₁ lifetime thus allowing the acid–base equilibration in the excited state to be directly monitored by fluorimetric titration.