Quantum-chemical study of the photoisomerization and photocyclization reactions of styrylquinolines: Potential energy surfaces

Abstract

The cross sections of potential energy surfaces (PES) for the S0 and S1 states were calculated by the semiempirical PM3 and PM3-CI (8 × 8) methods, respectively, along the reaction coordinate of the isomerization and cyclization of 2- and 4-styrylquinolines (SQ). The PES of the S0 state exhibits three minima separated by the transition-state barriers of isomerization and cyclization corresponding to three isomeric SQ forms, the E- and Z-isomers and the dihydrogenated cyclic product. On the PES of the S1 state, the “perpendicular minimum” at dihedral angle values of ∼ 90° corresponds to the transition state of the isomerization reaction and the pericyclic minimum with a distance of 1.7–2.0 A between the atoms involved in cyclization corresponds to the transition state of the cyclization reaction. With simultaneous scanning of the PES of the S1 state along the isomerization and cyclization reaction coordinates, the minimal-energy path was found for 4SQ, which makes it possible to explain the formation of the photocyclization product in the single-photon process upon irradiation of the E-isomer. It was found that the PM3 method overestimates the stability of the structures in which the aromatic ring is oriented perpendicular to the plane of the molecule, resulting in virtual minima on the PES of the S1 states.