Exploration of acyclic 1,3-diene excited-state energy surfaces

Photochemistry and photophysics of four asymmetrically substituted 1,3-dienes were investigated in detail. They are trans, trans-1-fluoro-2,4-hexadiene (EE-FHD), trans, trans1,1,1-trifluoro-2,4-hexadiene (EE-TFH), trans, trans-2,4-hexadien-1-ol (EE-HDO), and trans, trans-6,6-dimethyl-2,4-heptadiene (EE-DMH). Direct photoisomerization of these 1,3-dienes showed an electrostatic control on the regioselectivity, which may imply that excited zwitterionic state is involved in the reaction pathway. Solvent, wavelength, and temperature dependence of the regioselectivity has indicated a barrier on the lowest excited state, which may be caused by 11Bu-21Ag internal conversion. Whereas cis-E isomerization was the major photochemical event of FHD, TFH, HDO, and DMH, ring closure to cyclobutene was the dominant photoreaction for 2,3-dimethyl-1,3-butatdiene (DMB). Isotope and temperature effects on the photocyclization of DUB implied that an energy barrier might exist on the excited state pathway toward cyclobutene formation.; On the basis of the photochemistry of these acyclic 1,3-dienes, we postulated a new excited state reaction surface which involves both ionic-like 1 1Bu state and covalent-like 21Ag state, and the interplay between these two.