title: Axially assembled photosynthetic reaction center mimics composed of tetrathiafulvalene, aluminum(III) porphyrin and fullerene entities.
abstract: 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 10(9)-10(11) s(-1), depending on the length of the bridges. The lifetimes of the charge-separated state TTF˙(+)-C₆₀˙⁻ 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 C₆₀˙⁻ 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.
authors.affiliation: Department of Chemistry, Brock University, 500 Glenridge Ave., St. Catharines, Ontario, Canada L2S 3A1. ppoddutoori@brocku.ca avde@brocku.ca.
authors.forename: Prashanth K
authors.initials: PK
authors.lastname: Poddutoori
authors.forename: Gary N
authors.initials: GN
authors.lastname: Lim
authors.forename: Atula S D
authors.initials: AS
authors.lastname: Sandanayaka
authors.forename: Paul A
authors.initials: PA
authors.lastname: Karr
authors.forename: Osamu
authors.initials: O
authors.lastname: Ito
authors.forename: Francis
authors.initials: F
authors.lastname: D'Souza
authors.forename: Melanie
authors.initials: M
authors.lastname: Pilkington
authors.forename: Art
authors.initials: A
authors.lastname: van der Est