Solar Fuels by Means of Photo-induced Electron Transfer in Supramolecular Metal Complexes

Light absorption does not just lead to bond breaking and dissociation. It can also result in light emission and electron transfer; these processes offer many opportunities, especially if linked to the response of a supramolecular system. We have used metalloporphyrins as selective long-wavelength chromophores in supramolecular photochemical experiments, using light absorption to:

• observe photo-induced charge separation on picosecond¹ and nanosecond timescales.
• cause steady state ligand substitution at a site remote from the position of light absorption (Fig. 1).^1,2
• investigate photocatalytic properties related to sustainable fuels (Fig. 2).

The intimate mechanism of such processes requires spectroscopy on a time-scale as short as picoseconds. We have used IR spectroscopy to define the time sequence of the processes that occur in the metalloporphyrins (Fig. 1) on both picosecond¹ and nanosecond timescales. Remarkably, we can detect that photo-induced electron transfer from the metalloporphyrin to the rhenium bipyridine unit has already taken place only 1 ps after the laser shot, forming a charge separated or radical state that in some cases last for several microseconds.

We are working in collaboration with four other research groups in the UK to develop a Solar Nanocell, an artificial device that uses visible-light toward the tandem conversion of CO₂ and alkanes to CO and alcohols, respectively (Fig. 2) SolarCAP Consortium for Artificial Photosynthesis. The supramolecular metalloporphyrin–rhenium tricarbonyl bipyridine complexes are currently being studied to photochemically reduce CO₂ to CO by selectively light-excitation of the metalloporphyrin core.

Selected Publications

1. Comparison of rhenium-porphyrin dyads for CO₂ photoreduction: photocatalytic studies and charge separation dynamics studied by time-resolved IR spectroscopy, Chem. Sci. 2015, 5, 6847-6854.
2. CO₂ photoreduction with long-wavelength light: dyads and monomers of zinc porphyrin and rhenium bipyridine, Chem. Commun. 2012, 48, 8189-8191