Axially assembled photosynthetic reaction center mimics composed of tetrathiafulvalene, aluminum(iii) porphyrin and fullerene entities

The distance dependence of sequential electron transfer has been studied in six, vertical, linear supramolecular triads, (TTF-Ph_n-py → AlPor-Ph_m-C₆₀, n = 0, 1 and m = 1, 2, 3), constructed using tetrathiafulvalene (TTF), aluminum(iii) porphyrin (AlPor) and fullerene (C₆₀) entities. The C₆₀ and TTF units are bound to the Al center on opposite faces of the porphyrin; the C₆₀ through a covalent axial bond using a benzoate spacer, and the TTF through a coordination bond via an appended pyridine. Time-resolved optical and EPR spectroscopic methods and computational studies are used to demonstrate that excitation of the porphyrin leads to step-wise, sequential electron transfer (ET) between TTF and C₆₀, and to study the electron transfer rates and exchange coupling between the components of the triads as a function of the bridge lengths. Femtosecond transient absorption studies show that the rates of charge separation, k_CS are in the range of 10⁹-10¹¹ s^-1, depending on the length of the bridges. The lifetimes of the charge-separated state TTF^•+-C^•-60 obtained from transient absorbance experiments and the singlet lifetimes of the radical pairs obtained by time-resolved EPR are in good agreement with each other and range from 60-130 ns in the triads. The time-resolved EPR data also show that population of the triplet sublevels of the charge-separated state in the presence of a magnetic field leads to much longer lifetimes of >1 μs. The data show that a modest stabilization of the charge separation lifetime occurs in the triads. The attenuation factor β = 0.36 Å^-1 obtained from the exchange coupling values between TTF^•+ and C^•-60 is consistent with values reported in the literature for oligophenylene bridged TTF-C₆₀ conjugates. The singlet charge recombination lifetime shows a much weaker dependence on the distance between the donor and acceptor, suggesting that a simple superexchange model is not sufficient to describe the back reaction.