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 Cl level. All results indicate that the stable conformer of each ortho aza-derivative is that corresponding to A species. As suggested by the H-1-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 (pi(H), pi(L)(*)) character in all compounds except for E-4,4'-dipyridylethene, where S-1 has (n, pi*) character in non-polar solvent. In this last compound, the theoretical study of solvatochromism indicates a crossing between the (1)(n, pi(L)(*)) and (1)(pi(H), pi(L)(*)) 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 (1)A(g)(-) excited state is above the spectral region analyzed. (C) 2002 Elsevier Science B.V. All rights reserved.