Synthesis and photophysical studies of a porphyrin-viologen dyad covalently linked by a flexible seven-atom chain

A dyad with terminal porphyrin (P) and viologen (MV²⁺) chromophores covalently linked by a flexible seven-atom hexanoate chain has been synthesized. Ab initio and semi-empirical solvent continuum calculations suggest that the preference for 'extended' or 'folded' conformations of the dyad depend strongly on solvent. Fluorescence lifetime and transient absorption studies in acetonitrile indicate that approximately 80% of the dyad population are in a suitable conformation to undergo photoinduced electron transfer (PET) to produce P⁺-MV⁺ with a rate of 5×10⁸ s^-1. A long-lived charge-separated state lifetime of 170 ns is observed. It is proposed that electrostatic repulsion between the positively charged end groups resulting from PET increases the average donor-acceptor separation thus retarding the charge recombination process.