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

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. © 2012 Wiley Periodicals, Inc.