Time-resolved spectroscopy and charge-transfer photochemistry of aromatic EDA complexes with X-pyridinium cations

Abstract

Direct photoexcitation of 1 : 1 aromatic EDA complexes with various N-substituted X-pyridinium cations (X = nitro, fluoro, methoxy and acetoxy) is achieved by the specific irradiation of their charge-transfer (CT) absorption bands. Time-resolved picosecond spectroscopy refers to charge-transfer activation by the identification of the aromatic cation radical as the initial transient (T₁) formed in a photoinduced electron-transfer together with the X-pyridinyl radical. The homolytic fragmentation of the latter varies with the X-substituent in the order X = NO₂ > F > AcO > CH₃O, and the addition of X˙ to the aromatic donors leads to a series of cyclohexadienyl adducts that are identified as longer-lived transients (T₂) by time-resolved (nanosecond/microsecond) spectroscopy. The phototransients T₁ and T₂ together account for the different types of aromatic product (resulting from ring substitution, side-chain substitution and dimerization) that are generated by steady-state CT photochemistry of the aromatic EDA complexes with X-pyridinium cations.