Photoinduced multistep electron transfer in an oligoaniline-oligo(p-phenylene vinylene) - perylene diimide molecular - array

Photoinduced multistep electron transfer has been studied in two sym. oligoaniline-oligo(p-phenylene vinylene)-perylene diimide-oligo(p-phenylene vinylene)-oligoaniline (OAn-OPV-PERY-OPV-OAn) multichromophore arrays with fluorescence and transient absorption spectroscopy in the femtosecond and nanosecond time domains. The arrays consist of a sym. donor(2)-donor(1)-acceptor-donor(1)-donor(2) arrangement, with two OAn-OPV segments coupled to a central PERY unit via a direct linkage (1) or a satd. spacer (2). Photoexcitation gives the OAn-OPV+.bul.-PERY-.bul.-OPV-OAn as the primary charge-sepd. state. For 1 the transfer is extremely fast (kCS > 1000 ns-1) in polar and apolar solvents, while the rate consts. for recombination differ and are significantly higher in polar solvents than in apolar solvents because recombination occurs in the Marcus inverted region. Charge sepn. and charge recombination are slower in 2, because the satd. spacer reduces the electronic coupling between OPV donor and PERY acceptor. The primary OAn-OPV+.bul.-PERY-.bul.-OPV-OAn charge-sepd. state can rearrange in a charge-shift reaction to the OAn+.bul.-OPV-PERY-.bul.-OPV-OAn state. This charge shift is exergonic and competes with fast charge recombination. In polar solvents the efficiency of the charge shift is about 0.22 and 0.28 for 1 and 2, resp., and only weakly dependent on the polarity. In toluene the two charge-sepd. states are nearly isoenergetic for 1, and hence, no shift is obsd. in toluene. The OAn+.bul.-OPV-PERY-.bul.-OPV-OAn charge-sepd. state has a long lifetime as a result of the negligible interaction between the distant OAn+.bul. and PERY-.bul. redox sites and can be obsd. up to several microseconds.