Meta−Ortho Effect in Organic Photochemistry: Mechanistic and Exploratory Organic Photochemistry

Abstract

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 are the S1 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 S0 − S1 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 avenu...