Influence of Bridge Structures on the Electron Transfer in Donor-Acceptor Systems

Doctoral thesis, 2007

In this Thesis, porphyrin-based donor-acceptor systems have been used to study photoinduced electron transfer processes. Principally, two different sets of systems have been investigated. The first is a series in which a ferrocene-fullerene donor-acceptor couple is bridged by conjugated zinc porphyrin oligomers (Fc-Pn-C60, n = 1, 2, 4), and the second is a system containing a zinc porphyrin donor connected by a secondary electron acceptor to a gold(III) porphyrin primary acceptor. Strong inter-porphyrin conjugation in the oligomer series was achieved by using a butadiyne-link. This link allows for a broad distribution of dihedral conformations, which has important effects on the photophysical properties of the oligomers. The ground-state absorption spectrum is in effect an average of all dihedral conformations. The photophysics of the dimer was studied extensively, and it was found that the perpendicular and planar conformations can be addressed selectively. It was furthermore found that the ground-state barrier for rotation is approximately 0.7 kcal/mol (BLYP/6-31G(d)), whereas there is a strong bias towards the planar conformation in the excited state. Conformation is critical for the electronic coupling between the porphyrin macro-cycles and has significant effect on the electron transfer properties of these systems. By using the dimer as electron donor, considerable control over the electron transfer rate can be exercised by selective excitation of planar or perpendicular conformations. The main objective for investigating the series of conjugated porphyrin oligomers was to establish how suitable they are for use in future molecular electronic applications. This was done by photo excitation of the Pn-bridge, which results in the charge separated state Fc+-Pn-C60– by a sequence of electron transfer steps. The subsequent charge recombination was then monitored. It was found that the oligomers mediate charge very efficiently at distances up to 65 Å for n = 4 with remarkably weak distance dependence. The conformational flexibility of these systems has important implications for the mediation of electronic coupling, as this is expected to be a strong function of porphyrin-porphyrin dihedral angle. It was found that the charge recombination rate increased by an order of magnitude when Fc-P2-C60 was constrained to a planar conformation by the addition of a bi dentate ligand. Further, as a part of this study, the triplet energies of the oligomers were determined. In the second set of donor-acceptor systems, two electron transfer processes are viable: a direct, long-range electron transfer, and a sequential electron transfer. High solvent polarity stabilises an intermediary state, where the electron is localised on the anthracene-based bridge unit, which thus serves as a secondary electron acceptor. It is shown in this work that the two mechanisms operate in parallel, and moreover, that the direct electron transfer is faster than the sequential mechanism by approximately a factor of two, at room temperature. As the temperature dependence of the sequential mechanism is stronger than that of the direct mechanism, temperature served as an important means to control the competition. In butyronitrile, the sequential mechanism was essentially switched off relative to the direct electron transfer at low temperatures.