Electrostatic control of the regioselectivity in the photoisomerization of trans,trans-1-fluoro-2,4-hexadiene: evidence for competing conical intersections

trans,trans-1-Fluoro-2,4-hexadiene (EE-FHD) was used to explore the excited potential energy surfaces of acyclic 1,3-dienes. Our investigations show that the regioselectivity of double-bond photoisomerization is primarily controlled by the charge-stabilizing characteristic of substituents on the double bonds of the diene. Furthermore, changes in the photoregioselectivity with solvent polarity suggest that the excited unsymmetrical diene returns to the ground state by competing pathways which traverse polarized 1Bu states and drop into two different conical intersections