Distance-Dependent Electron Transfer Kinetics in Axially Connected Silicon Phthalocyanine-Fullerene Conjugates

The effect of donor-acceptor distance in controlling the rate of electron transfer in axially linked silicon phthalocyanine-C₆₀ dyads has been investigated. For this, two C₆₀-SiPc-C₆₀ dyads, 1 and 2, varying in their donor-acceptor distance, have been newly synthesized and characterized. In the case of C₆₀-SiPc-C₆₀ 1 where the SiPc and C₆₀ are separated by a phenyl spacer, faster electron transfer was observed with k_cs equal to 2.7×10⁹ s^-1 in benzonitrile. However, in the case of C₆₀-SiPc-C₆₀ 2, where SiPc and C₆₀ are separated by a biphenyl spacer, a slower electron transfer rate constant, k_cs=9.1×10⁸ s^-1, was recorded. The addition of an extra phenyl spacer in 2 increased the donor-acceptor distance by ∼4.3 Å, and consequently, slowed down the electron transfer rate constant by a factor of ∼3.7. The charge separated state lasted over 3 ns, monitoring time window of our femtosecond transient spectrometer. Complimentary nanosecond transient absorption studies revealed formation of ³SiPc* as the end product and suggested the final lifetime of the charge separated state to be in the 3-20 ns range. Energy level diagrams established to comprehend these mechanistic details indicated that the comparatively high-energy SiPc^.+-C₆₀^.- charge separated states (1.57 eV) populated the low-lying ³SiPc* (1.26 eV) prior returning to the ground state.