Ultrafast electron transfer in a porphyrin-amino naphthalene diimide dyad

Photo-induced electron transfer in a covalently linked zinc(11) 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 center dot+ radical cation and the ANDI(center dot-) radical anion. It is shown that electron transfer (ET) can occur directly upon excitation to the S-2 state of the porphyrin followed by rapid charge recombination to form S-1 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 (k(CR)) in the more polar solvent benzonitrile (k(CR) = 1.59 x 10(11) s(-1)) compared to toluene (k(CR) = 8 x 10(9) s(-1)), in which inter-system crossing (ISC) from the CS state to form the lowest porphyrin triplet state is the dominating decay pathway (k(CR/ISC) = 3.46 x 10(10) s(-1)). The implications of the results for designing molecular systems to potentially utilise ET following S-2 excitation are discussed. (C) 2012 Elsevier B.V. All rights reserved.