Photoinduced electron transfer dynamics in porphyrin donor dyads

Intramolecular photoinduced electron transfer (PET) processes occurring in dyads with a free base porphyrin-tetraazaanthracene donor (P) and either a tetracyanonaphthoquinidodimethane (TCQ) or benzoquinone (BQ) acceptor linked by a rigid six σ-bond polynorbornane bridge (6]) have been investigated. For P6]BQ, PET in the polar solvent benzonitrile (_s = 25.9) occurs with a rate constant (k_PET) of 1.6 × 10⁸ s^?1 but is not evident in solvents less polar than tetrahydrofuran (_s = 7.52). For P6]TCQ, highly efficient forward PET occurs in both polar and non-polar solvents (k_PET > 2 × 10¹⁰ s^?1). For P6]TCQ the lifetime of the resulting charge-separated state decreases markedly with increasing solvent polarity. The results are discussed in the context of the likely mechanisms for electronic coupling and current theories for PET processes in such linked molecular systems.