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 S0 state and PM3-CI(2x2) method with configuration interaction for the S1 state. Scanning PES along both isomerization and cyclization reaction coordinates enabled to reveal the minimum energy path (MEP) with low barriers on the S1 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 S1 PES. © 2012 Wiley Periodicals, Inc.