Design, synthesis and applications of donor-acceptor systems for artificial photosynthesis and solar cells

摘要

Artificial photosynthesis (AP) is the process of mimicking natural photosynthesis for effective conversion of light energy into a more accessible form of energy. These APs consist of three main components; a photoantenna, the reaction centre and an energy storage system. On the basis of a literature search of AP, we have studied the excited-state dynamics of two multichromophoric arrays composed of a NDI centre, attached to which are four zinc or free-base porphyrins connected to the core via aniline bridges. These pentads have been investigated by using a combination of stationary and ultrafast spectroscopies to gain an understanding of their photophysical properties. Spectroscopic results confirm that these pentads can act as efficient photoantennae, absorbing over the complete visible region. They absorb at a wavelength of around 700 nm, which indicates an excited state transition to the S1 state that is delocalised over the whole pentad. Furthermore higher energy absorption bands are shown by transitions centred on the porphyrins of the multichromophoric arrays. The synthesised pentads were studied in polar and non-polar solvents. These multichromophoric arrays show a charge-separated state under the S1 state in polar solvents. Thermally-activated hole transfer from the S1 state results in populated excited states within a few picoseconds (ps), and the vibrationally hot porphyrin excited states exhibit sub-ps non-equilibrium electron transfer. Further contribution to AP is made by the design of simple donor-acceptor (D-A) dyads. These synthesised dyads demonstrate good results by being able to produce a charge gradient across a membrane, which clearly indicate that they can act as efficient AP antennae. In the dyad MKSB4.1, the process of conversion of light energy into a proton potential is achieved by transporting electrons across a lipid bilayer. This active dyad is composed of a zinc-porphyrin as a donor and a napthalene diimide (NDI) as an acceptor, which are linked by a dithiophene spacer for effective electron transfer. Initially, excitation of the zinc-porphyrin after incorporation into a lipid bilayer results in charge separation to produce an oxidation potential near the outer surface and a reduction potential near the inner surface of the lipid bilayer. This process of creating potentials leads to transmembrane electron transfer. On the other hand, dye-sensitised solar cells (DSSCs) are a growing research area due to their potential to meet our need for a clean source of energy, in this case solar. To contribute to the development of DSSCs, herein, I have designed and studied a donor-acceptor model in which there is a direct linkage of oligothiophenes to the nitrogen atom of a donor amine. AG3 consists of diphenylamine as an electron donor and cyanoacrylic acid as an electron acceptor, linked through an extended oligothiophene π-spacer unit. AG3 is highly soluble in common organic solvents and showed an intense spectral response, compared to a fewer-thiophene analogue. When tested in a conventional solvent-based solar cell (100 mW.cm-2, AM1.5G), AG3 afforded a power conversion efficiency (PCE) of 5.9%. AG3 also achieved a PCE of 4.3% in ionic liquid and 5.2% in cobalt-mediated electrolytes.