Theoretical study of trans-cis photoisomerism in polymethine cyanines

• Published in: Chemical physics Vol. 123; no. 1; pp. 103 - 112
• Main Authors: Momicchioli, Fabio, Baraldi, Ivan, Berthier, Gaston
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 15.06.1988; Elsevier
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Photochemistry; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); Potential energy curve; Frontier orbital; Theoretical study; Configuration interaction; Cis trans isomerization; Photochemical reaction; Energy level; Energy barrier; Cyanine dye; Intramolecular charge transfer; Model compound; Model potential; Quantum theory; Organic compounds; INDO method
• ISSN: 0301-0104
• DOI: 10.1016/0301-0104(88)87036-8

The trans-cis photoisomerism of polymethine dyes has been interpreted so far using different and rather inconsistent models of the potential energy surfaces. In order to search for a unified electronic model, we tackled the problem again from an intramolecular point of view. Our study consisted in qualitative MO considerations followed by explicit (CS INDO) calculations of the S 0, T 1 and S 1 potential energy curves for a proper model system: pentamethine cyanine isomerizing around the 2–3 and 3–4 bonds. Torsional energy levels for the calculated potential curves were also obtained. The main conclusions are: (i) the photoreaction proceeds through a “non-spectroscopic” (perp) S 1 minimum which exists also in the isolated molecule, (ii) this twisted excited species has charge transfer character (TICT), as opposed to the biradicaloid character of the “non-spectroscopic” S 1 minimum involved in the trans→cis photoisomerization of olefines (e.g., stilbene). The possible consequences on the dynamics of the excited state relaxation in non-polar solvents are envisaged.