Photochemically Generated Ion and Radical Pairs. Self-Destructive Charge-Transfer Complexes

Irradiation of CT complexes between 4-methoxy-4’-X-bicumenes, la—e (X = H, a; OMe, b; Me, c, CF₃, d, CN, e) and tetranitromethane (2) results in efficient generation of radical and ion pairs (Φ≈ 0.20). The identity of the final products depends on the dynamics of these pairs, which in turn is dramatically influenced by substitution and solvent. In all cases, 2*^-dissociates rapidly (<3 ps) into (NO₂)₃C^- (4^-) and N0₂*. In CH₂C1₂, la*⁺reacts with 4- (aromatic trinitromethylation) and undergoes C—C bond fragmentation before diffusional separation of fragments can occur. Thus, a tetrad of reactive intermediates (cumyl radical, NO₂*, p-methoxycumyl cation and 4-) are produced in a single solvent cage. The cumyl radical is oxidized by 2, and both cumyl cations undergo α-trinitromethylation. The cumyl cations undergo trinitromethylation in the original solvent cage which is kinetically equivalent to a contact ion pair (CIP). In CH₃CN the fragmentation is accompanied by aromatic nitration (radical collapse). In this solvent, the p-methoxycumyl cations produced by cleavage reaction undergo trinitromethylation at the CIP stage, but cumyl cations produced by thermal oxidation of the cumyl radicals are trapped by 4^- at the solvent-separated ion pair stage. In CH₂C1₂, 1b-c^*+ undergo exclusively fragmentation, completely within the solvent cage. The cleavage of 1d-e*⁺ is much slower, and the radical cations undergo instead aromatic trinitromethylation. The observed substituent effect on the rate of cleavage (p⁺ = -2.2) indicates significant charge transfer across the scissile bond in the transition state for this process. The products of these reactions are predominantly derived from ion annihilation. The radical coupling processes are limited to radical cation/radical collapse that lead to nitrated products.