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

• Published in: Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy Vol. 59; no. 1; pp. 75 - 86
• Main Authors: Baraldi, Ivan, Spalletti, Anna, Vanossi, Davide
• Format: Journal Article
• Language: English
• Published: England Elsevier B.V 2003
• Subjects: 1H-NMR; Ab initio and CS INDO CI calculations; aza-Derivatives of stilbene and 1,4-diphenylbutadiene; Butadienes - chemistry; Chemistry - methods; Electronic spectrum; Hydrogen Bonding; Magnetic Resonance Spectroscopy; Models, Chemical; Rotamerism; Spectrophotometry - methods; Stilbenes - chemistry; Ab initio and CS INDO CI calculations; Electronic spectrum; 1H-NMR; aza-Derivatives of stilbene and 1,4-diphenylbutadiene; Rotamerism
• ISSN: 1386-1425
• DOI: 10.1016/S1386-1425(02)00117-8

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 (π
H, π
L*) character in all compounds except for
E-4,4′-dipyridylethene, where S
1 has (n, π*) character in non-polar solvent. In this last compound, the theoretical study of solvatochromism indicates a crossing between the
1(n, π
L*) and
1(π
H, π
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
1A
g
− excited state is above the spectral region analyzed.