Journal of Physical Chemistry A, Vol.108, No.40, 8201-8211, 2004
Photoinduced multistep electron transfer in an oligoaniline-oligo(p-phenylene vinylene)-perylene diimide molecular array
Photoinduced multistep electron transfer has been studied in two symmetrical oligoaniline-oligo(p-phenylene vinylene)-perylene diimide-oligo(p-phenylene vinylene)-oligoaniline (CAn-OPV-PERY-OPV-OAn) multi chromophore at-rays with fluorescence and transient absorption spectroscopy in the femtosecond and nanosecond time domains. The arrays consist of a symmetrical 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 saturated spacer (2). Photoexcitation gives the OAn-OPV+.-PERY-.OPV-OAn as the primary charge-separated state. For 1 the transfer is extremely fast (kCS > 1000 ns(-1)) in polar and apolar solvents, while the rate constants for recombination differ and are significantly higher in polar solvents than in apolar solvents because recombination occurs in the Marcus inverted region. Charge separation and charge recombination are slower in 2, because the saturated spacer reduces the electronic coupling between OPV donor and PERY acceptor. The primary OAn-OPV+.-PERY-.-OPV-OAn charge-separated state can rearrange in a charge-shift reaction to the OAn(+.)-OPV-PERY-.-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, respectively, and only weakly dependent on the polarity. In toluene the two charge-separated states are nearly isoenergetic for 1, and hence, no shift is observed in toluene. The OAn(+.)-OPV-PERY-.-OPV-OAn charge-separated state has a long lifetime as a result of the negligible interaction between the distant OAn(+.) and PERY-. redox sites and can be observed up to several microseconds.