Linkage dependent charge separation and charge recombination in porphyrin-pyromellitimide-fullerene triads

A homolocous series of zincporphyrin (ZnP)-pyromellitimide (Im)-C-60 linked triads where the pyromellitimide (Im) moiety is incorporated as an intermediate acceptor between the above two chromophores with a linkage of different spacers, ZnP-Im-CH2-C-60, ZnP-IM-C-60, and ZnP-CH2-Im-C-60 as well as the reference dyads (ZnP-Im-CH2-ref, ZnP-Im-ref, and ZnP-CH2-Im-ref) have been prepared to investigate linkage dependence of photoinduced electron transfer (ET) and back ET to the ground state in the triads. Time-resolved transient absorption spectra of the triads measured by picosecond laser photolysis as well as the fluorescence lifetimes in THF reveal the occurrence of photoinduced ET from the singlet excited state of the ZnP to the Im moiety to give the initial charge-separated state, i.e., the zincporphyrin radical cation (ZnP.+)-imide radical anion (Im(.-)) pair, followed by a charge shift (CSH) to produce the final charge-separated state, the ZnP.+-C-60(.-) pair. The rate constants of photoinduced ETs in ZnP-Im-C-60 (1.8 x 10(10) s(-1)) and ZnP-Im-CH2-C-60 (1.3 x 10(10) s(-1)) in THF are much larger than those in ZnP-CH2-IM-C-60 (2.9 x 10(9) s(-1)) and ZnP-CH2-Im-ref (1.9 x 10(9)) s-1). The larger charge separation (CS) rates in the former case are ascribed to the relatively strong electronic coupling because of the absence of a methylene linkage between the ZnP and the Im moieties in ZnP-Im-C-60 and ZnP-Im-CH2-C60 as compared to the triad and dyad with the methylene linkage. The transient absorption spectra of the final charge-separated state, the ZnP.+-C-60(.-) pair, have been also measured by nanosecond laser photolysis. It has been found that the rate constants of charge recombination (CR) of ZnP.+-Im-C-60(.-) are temperature independent, but that the CR rate constants of ZnP.+-Im-CH2-C-60(.-) exhibit an Arrhenius-like temperature dependence with an activation energy of 0.13 eV which corresponds to the energy difference between ZnP.+-Im(.-)CH(2)-C-60 and ZnP.+-Im-CH2-C-60(.-). This indicates that the relatively strong electronic coupling without methylene linkage in ZnP-IM-C60 results in the preference of the tunneling superexchange ET over the sequential ET in the CR process which requires the thermal activation to reach the higher energy state (i.e., ZnP.+-Im(.-)-C-60), whereas the sequential ET predominates in the triads with the methylene linkage.