In the pressing search for alternative energy sources, photosynthetic reaction centers, i.e. supramolecular assemblies in which electron donating and accepting molecules are covalently linked forming so called polyads (dyads, triads...), belong to a presently active research area. Particular attention is drawn by covalently linked porphyrin-fullerene dyads for which several complexes differing in the relative spatial arrangement and distance of the donor (porphyrin) and acceptor (fullerene) have been proposed.
A doubly linked porphyrin-fullerene dyad is studied by molecular dynamics (MD) simulations and quantum mechanical calculations. The exploration of the conformational space of the dyad in polar and non-polar solvent, as well as in vacuum reveals unexpected conformational flexibility of a molecule in which the linkers are presumably restricting the conformational freedom. It is further revealed by density functional theory (DFT) calculations on two representative MD conformations that differences in the relative spatial arrangement and distance of porphyrin and fullerene can induce important changes in some of the dyad properties. The difference in the total energy of the selected conformers is about ten times larger than the thermal energy (kT) at 300 K, which is significantly favoring the population of the lower energy conformer (LEC) at thermal equilibrium. In addition, a higher amount of ground state charge transfer interaction between porphyrin and fullerene is found for LEC as compared to the high energy conformer. Also changes in the electronic structure of the low and high energy conformations are revealed by the DFT calculations.
The excited states of both LEC and HEC are investigated by employing the time dependent DFT with several exchange-correlation functionals. A clear dependence of the energy of the first excited state on the two conformations of the dyad is observed. This is mainly assigned to the differences in the relative positions of porphyrin and fullerene in LEC as compared to HEC. At the same time, the excitation that corresponds to the formation of a locally excited singlet porphyrin state shows little dependence on the structural characteristics of LEC and HEC and on the exchange-correlation functional.
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