The storage of solar energy in chemical fuel is currently under intense investigation, but many of the challenges associated with this goal remain un- surmounted. The investigation of supramolecular assemblies, in which one unit harvests visible light and transfers energy or electrons to another catalytic unit capable of energetically uphill chemical transformations, is of importance. This thesis describes the synthesis, electrochemistry, photochemistry and visible light CO2 photoreduction ability of several rhenium tricarbonyl appended zinc porphyrins. The basic structure comprises a zinc tetraphenyl porphyrin in which one phenyl possesses a NHCO group bound to bipyridine. The bipyridine is complexed to a Re(CO)3X unit.
Introduction of a methoxybenzamide spacer produced [Dyad 2 Pic][OTf] and its bromide analogue. Synthetic methodology was developed for the introduction of a methylene spacer to produce [Dyad 3 Pic][OTf] and its bromide analogue. Further synthetic efforts produced routes to novel porphyrin and rhenium complexes possessing phosphonic acid groups for anchoring to metal oxide surfaces. In the ground states of the two dyads there is little interaction between the metalloporphyrin and rhenium units. However, emission quenching of the porphyrin by the rhenium was demonstrated. Remote site photosubstitution was observed in [Dyad 2 Pic][OTf]. Electrochemical and steady state emission studies indicate that electron transfer is energetically favourable in all systems except Dyad 3 Br. All the catalysts developed produced catalytic quantities of CO under visible light in CO2 saturated DMF/TEOA 5/1. Activity increases in the order Dyad 1 < Dyad 2 < particle system < two components < Dyad 3, reaching a turnover number of 360 for [Dyad 3 Pic][OTf]. TRIR experiments demonstrated that the lifetime of charge separation in the dyads increases in the order [Dyad 1 Pic][OTf] < [Dyad 2 Pic][OTf] < [Dyad 3 Pic][OTf] and is
335 ps for [Dyad 3 Pic][OTf], consistent with catalytic activity.
Reaction mixture analysis by UV-Vis and NMR spectroscopy and ESI mass spectrometry indicated hydrogenation of the porphyrin, the product of which is also active for CO2 photoreduction. ESI-MS studies showed that substitution of the picoline ligand by TEOA and DMF occurred.
The Re and porphyrin units were successfully immobilised on TiO2 nanoparticles. The Re catalyst anchored on TiO2 is active for CO2 photoreduction under visible light. Addition of the porphyrin, and its selective excitation, produced catalytic quantities of CO (TON = 32). This confirmed sensitisation of the Re complex around the TiO2 particle. Electrochemical and steady state emission studies indicate a porphyrin to TiO2 then TiO2 to Re electron transfer mechanism is favourable.
