Does one-photon photocyclization of trans-diarylethylenes involve adiabatic trans-to-cis photoisomerization? potential energy surface calculations for 1-styrylnaphthalene

Abstract

Potential energy surface (PES) for 1-styrylnaphthalene was calculated by PM3 method for the S 0 state and PM3-CI(2x2) method with configuration interaction for the S 1 state. Scanning PES along both isomerization and cyclization reaction coordinates enabled to reveal the minimum energy path (MEP) with low barriers on the S 1 PES from E-isomer to dihydrocyclophotoproduct (DHP). This is consistent with formation of the photocyclization product in one-photon process during irradiation of E-isomer. Additionally, the MEP was found to bypass the coordinate region of Z-isomer, i.e. one-photon E-isomer-to-DHP photocyclization does not demand participation of the excited Z-isomer. Therefore, adiabatic trans-to-cis isomerization is likely not an intermediate stage on the E-isomer photocyclization pathway, and experimentally observed one-photon formation of the DHP from the E-isomer is likely not an evidence for adiabatic trans-to-cis photoisomerization, as it is usually assumed. According to the results obtained, two photochemical reactions of E-isomer, photoisomerization to Z-isomer and photocyclization to DHP, are not consecutive but parallel reactions with branching at perpendicular conformer on the S 1 PES. Copyright © 2012 Wiley Periodicals, Inc.