| Abstract |
Τhe research on the utilization of solar radiation is necessary for switching to green energy sources, with a large variety of solar technologies being available today. Scientists are inspired from nature and the process of photosynthesis, designing artificial photosynthetic systems which mimic the electron/energy transfer reactions occurring in a photosynthetic center. We can think of the photosynthetic center as a kind of a nanoscale photovoltaic device that converts light into electrochemical energy. It consists of a highly organized series of electron donors (chlorophylls) and electron-acceptors (benzoquinones) that interact supramolecular within the protein matrix, through a series of sequential energy and electron transfers CO2 and H2O converted to glucose and energy (ATP). In this context, the design and formation of highly order molecules with self-assembling properties, which mimic biological processes, finds rapid growth.
In this study the new hybrid materials that were synthesized and studied consist of metalloporphyrins or metallophthalocyanines as electron donors and functionalized fullerenes (C60) as electron acceptors. The fullerenes are covalently linked with oligo phenylene-vinylenes (oPPVs) that carrying pyridyl moieties. The electron donors communicate electronically with fullerenes through oPPVs and the formation of the nanohybrids achieved through self-assembly and supramolecular interaction. Specifically, rigid linear π-conjugated oligomers of various lengths, or dendrimer-type branched ones were synthesized both carrying pyridyl groups at one end and fullerenes at the other. The nano - hybrids that resulted were studied for their photophysical properties.
The π-conjugated oligomers (oPPVs) were used as coupling agents with the metalloporphyrins or metallophthalocyanines through the pyridyl groups they're bearing; they also serve the role of energy/charge transfer with no losses, acting as molecular wires.
A systematic study of various parameters which affect the photophysical behavior and therefore the resulting activity of the hybrids have been achieved. Factors such us the nature of the photo- and redox- units, electron donor - acceptor distance, solvent, molecular topology, and the nature of the bridge, were studied in the present work.
The effect of the length of conductive "molecular wires" i.e. the distance between the electron donor-acceptor, on the electron transfer rate has been studied. For this purpose functionalized fullerenes with linear π-conjugated oligomers of different length, were used.
Initially substituted fullerenes with linear oPPVs of different length, which carry a pyridyl group, were coordinated with zinc porphyrin to form the corresponding hybrids. From the photophysical studies found that the electron transfer takes place from the porphyrin to the fullerene in the hybrids. The lifetime of the radical ion pair is dependent upon the length of the bridge linking the donor and the acceptor. Subsequently, the photophysical properties of formed hybrids between a zinc phthalocyanine and the substituted fullerenes were examined. These derivatives showed similar behavior on the effect of the length of the bridge, to the corresponding porphyrin hybrids.
In order to examine the binding constant (kass) of electron donor - acceptor a ruthenium porphyrin chromophore was used. Ruthenium is coordinating to the nitrogen of the pyridine quite strong and the formed hybrids exhibit high stability. Furthermore increasing the binding sites between donor and acceptor, increasing of the binding constant is observed at the same time. Therefore, appropriately substituted fullerenes were synthesized, bearing two pyridyl groups which interact with the metal centers of the chromophore.
In conclusion, efficient electron transfer - from the chromophores to the fullerenes - was found in all nanohybrids. The lifetime of the separated radical ion pairs is ns scale. Finally, substituted porphyrins with oPPV derivatives were synthesized and studied as photosensitizers in solar cells.
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