Meta-ortho effect in organic photochemistry: Mechanistic and exploratory organic photochemistry

In early research, the meta effect was based on one-electron, Huckel computations and experimental observation, which revealed a selective transmission of electron density to the meta and ortho positions on an aromatic ring in the first excited singlet. However, attention was focused on the meta site. More recent results have confirmed electron density transmission to the ortho site as well. Typical examples involve benzylic cations. Not only an the S-1 cations selectively stabilized by meta-methoxy groups compared with para-methoxy substituents but also the corresponding meta-substituted radicals prove of higher energy than the para-substituted counterparts. Additionally, the S-0 - S-1 energy gap is dramatically smaller for the meta-substituted cations and ion pairs compared with the gap for the para-substituted counterparts. Also, the radicals and radical pairs exhibit much larger ground-state - excited-state energy separations. With a closer approach of surfaces, the excited-state ion pairs have an avenue for radiationless decay to ground state not available to the radical pairs. An ab initio computational search for conical intersections was carried out for the 3,5-dimethoxybenzyl cation and radical. This revealed the presence of a degeneracy in the cation at a geometry only slightly perturbed from that of the S-1 minimum. A parallel computation on the 3,5-dimethoxybenzyl radical led to the nearest approach of excited- and ground-state surfaces that was large in comparison and at a Fiery high energy Feint on the excited-state hypersurface. The geometries of the minimized excited-state species were obtained and the reaction hypersurface found to provide an available route for facile decay of the meta ion pairs to ground state.