Ultrafast electron transfer in a porphyrin-amino naphthalene diimide dyad

Photo-induced electron transfer in a covalently linked zinc(II) tetraphenylporphyrin-amino naphthalene diimide dyad (ZnTPP-ANDI) is reported. The fluorescence of ZnTPP-ANDI is strongly quenched in both toluene and benzonitrile solvents compared to emission from ZnTPP. Ultrafast pump-probe spectroscopy has identified transient absorptions attributable to the ZnTPP•+ radical cation and the ANDI•− radical anion. It is shown that electron transfer (ET) can occur directly upon excitation to the S2 state of the porphyrin followed by rapid charge recombination to form S1 that subsequently undergoes a further slower ET to ANDI. The kinetics of charge separation (CS) and charge recombination (CR) for the latter ET process are strongly solvent dependent with a dramatically accelerated charge recombination rate (kCR) in the more polar solvent benzonitrile (kCR = 1.59 × 1011 s−1) compared to toluene (kCR = 8 × 109 s−1), in which inter-system crossing (ISC) from the CS state to form the lowest porphyrin triplet state is the dominating decay pathway (kCR/ISC = 3.46 × 1010 s−1). The implications of the results for designing molecular systems to potentially utilise ET following S2 excitation are discussed.