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Controlling light-induced properties by molecular reorganisation

Final Activity Report Summary - THREE-ELECTRON-BOND (Controlling light-induced properties by molecular reorganisation)

The Sun is the only source of sustainable energy and can satisfy all worlds' energy demands if harnessed efficiently. The main objective of this Fellowship was to develop a novel and original idea to efficient harnessing of solar energy.

A basic system for solar-to-chemical energy conversion should: (1) absorb light by a chromophore to separate positive and negative charges in a local so-called charge-separated state; (2) restrict recombination of the charges and engage in a cascade of 'dark' electron transfer steps away from the local charge separated state to generate an independently reactive 'electron/hole' pair. The latter should have sufficient energy to drive exothermic chemical changes.

Whilst many compounds have been developed over the past decades which effectively absorb visible light, the energy wasting back electron transfer process often leads to low efficiency. The key challenges remain stabilisation of the local charge-separated state, and creation of efficient electron transfer cascades which lead to distant separated charges capable of performing catalytic redox processes.

Our work addressed the challenges above in the following way.
1. Developing a conceptually new approach to the stabilisation of the local charge separated state in transition metal chromophores via light-induced structural reorganisation - the formation of a transient 3-electron sulfur-sulfur bond (S?S) on a metal template, which acts as a reservoir for an absorbed light quanta.
2. Electron transfer cascades are created by modifying the chromophoric core by electron Acceptors and Donors. The D and A moieties possess distinct visible and IR spectra in various redox states, acting as essential spectroscopic probes to monitor electron transfer in real time.

The excited state dynamics initiated by an absorption of visible light and the accompanying structural changes were followed by a combination of transient absorption, time-resolved infra-red and time-resolved resonance Raman spectroscopies on femto- to millisecond time scales. The dynamics of electron transfer are modelled within the framework of Marcus theory to evaluate reorganisation energies and corroborate the structural changes. The combination of structural reorganisations with electron transfer cascades allows for tuning of the lifetime of charge separation by several orders of magnitude.

The knowledge obtained allowed us to design molecular systems for photocatalysis in conjunction with semiconductor electron reservoirs.