Adiabatic S1 9,10-Dichloroanthracene Formation from Electronically Excited 9,10-Dichloroanthracene/1,3-Cyclohexadiene Photoadducts

Article abstract of DOI:10.1021/ja00347a045

The photochemistry and fluorescence of the two major photoadducts of 9,10-dichloroanthracene (DCA) and 1,3-cyclohexadiene (CHD) are described.The 1,2-adduct, 1,2-A (<2_?s + 2_?s> to the 1,2-positions of DCA),gives DCA and CHD as main photoproducts in several solvents, φ_-1,2-A ca. φ_DCA ca. φ_CHD ca. 0.3.It exhibits fluorescence characteristic of the naphthalene moiety (350-435 nm) and, in addition, gives DCA fluorescence (390-560 nm).The pronounced oxygen quenching effect of the latter shows that cleavage of singlet excited 1,2-A, ¹1,2-A^*, is in part adiabatic, φ_1DCA* ca. 0.007-0.015.The 9,10-adduct, 9,10-A (<4_?s + 4_?s> to the 9,10-position of DCA), is efficiently destroyed by light, φ_-9,10-A ca. 1.0, but gives several products in addition to DCA and CHD, φ_DCA ca. φ_CHD ca. 0.11.Its fluorescence is identical with DCA fluorescence, corresponding to φ_1DCA* ca 0.0041-0.0068, depending on the solvent.The observations are discussed by using the current theoretical model for electrocyclic photochemical reactions.They suggest that if pericyclic minima are involved in adduct photocleavage and formation, they are isolated in the potential energy surface and do not readily interconvert.