Rotamerism and electronic spectra of aza-derivatives of stilbene and diphenylbutadiene. A combined experimental and theoretical study

The experimental results on the rotameric equilibrium and electronic spectra of aza-derivatives of trans-stilbene and 1,4-diphenylbutadiene, have been rationalized by a theoretical study which combines simple ab initio calculations of molecular energies for the ground state with a theoretical analysis of the splitting of the conjugation band developed at CS INDO CI level. All results indicate that the stable conformer of each ortho aza-derivative is that corresponding to A species. As suggested by the 1H-NMR experiments, the ab initio geometry of ZE-2-pyridylphenylbutadiene is consistent with the presence of the N.H intramolecular hydrogen bond. As regards the Franck-Condon excited states of aza-derivatives, our theoretical results show that the first singlet excited state has (piH, piL*) character in all compounds except for E-4,4'-dipyridylethene, where S1 has (n, pi*) character in non-polar solvent. In this last compound, the theoretical study of solvatochromism indicates a crossing between the 1(n, piL*) and 1(piH, piL*) states which occurs in solvents of high polarity. The inclusion of the most important doubly- and triply-excited configurations in the CI calculations shows that the 1A(g)- excited state is above the spectral region analyzed.