Adiabatic photoinduced electron transfer and back electron transfer in a series of axially substituted silicon phthalocyanine triads

A series of new electron acceptor bearing silicon phthalocyanine (SiPc) triads have been synthesized, using the six-coordinated nature of the central silicon atom, by attachment of two electron-acceptor units, fullerene SiPc-(C₆₀)₂, trinitrofluorenone SiPc-(TNF)₂, and trinitrodicyanomethylenefluorene SiPc-(TNDCF)₂. The redox and photophysical properties of SiPc triads in benzonitrile are determined to evaluate the energy of the charge-separated (CS) states and driving force of photoinduced electron transfer in SiPc triads. Photoexcitation of SiPc triads in benzonitrile results in efficient formation of the CS states, which were detected by femtosecond laser flash photolysis measurements. The fate of the CS state whether it decays to the triplet excited-state of the SiPc unit or to the ground-state was examined by nanosecond laser flash photolysis measurements for the longer time scale. The driving force dependence of the rate constants of photoinduced electron transfer and back electron transfer in SiPc triads is analyzed in light of the Marcus theory of adiabatic intramolecular electron transfer.