Spectral and Kinetic Properties of Radical Cations Derived from Oxoisoaporphines: Relevance to Electron-Transfer Processes Involving Phytoalexins

The thermally induced intermolecular electron transfer reaction in acetonitrile between the tetracyanoethylene (TCNE), a π-electron acceptor with a large electron affinity, and six oxoisoaporphines (2,3-dihydro-7<i>H</i>-dibenzo­[<i>de</i>,<i>h</i>]­quinolin-7-one, 5-methoxy-2,3-dihydro-7<i>H</i>-dibenzo­[<i>de</i>,<i>h</i>]­quinolin-7-one, 1-azabenzo­[<i>de</i>]­anthracen-7-one, 5-methoxy-1-azabenzo­[<i>de</i>]­anthracen-7-one, 7<i>H</i>-benzo­[<i>e</i>]­perimidin-7-one, and 2-methyl-7<i>h</i>-benzo­[<i>e</i>]­perimidin-7-one) is reported. Spectral and kinetic characteristics are presented for radical cations derived from these six oxoisoaporphines either generated by a thermal reaction or generated radiolytically in argon-saturated 1,2-dichloroethane, oxygen-saturated acetone, and acetonitrile. The radical cations of oxoisoaporphines are insensitive to oxygen and are mostly characterized by absorption maxima of their most intense bands located at λ_max = 400–410 nm, except of the radical cations derived from 2,3-dihydrooxoisoaporphines. For the latter compounds, the absorption maxima of the most intense absorption bands are located at λ_max = 290–295 nm. Their locations are independent of the presence of functional groups and the solvents used. They are formed in bimolecular processes with pseudo-first-order rate constants ranging from 2.1 × 10⁵ to 1.5 × 10⁶ s^–1 (in solutions containing 10^–4 M of the substrate), depending on the derivative and the solvent used. They are stable either when formed via the electron-transfer reaction with TCNE or when generated in isolation in pulse radiolysis of Ar-saturated 1,2-dichloroethane. In acetone and acetonitrile they decay predominantly by first-order kinetics with the first-order rate constants ranging from 2.3 × 10⁴ to 5.1 × 10⁴ s^–1. Formation of dimeric radical cations for all of the oxoisoaporphines studied was observed in acetonitrile solutions, and for azaoxoisoaporphines also in acetone solutions. The experimental spectra show a reasonably good agreement with the ZINDO/S semiempirical quantum mechanical calculations of radical cation absorptions