Charge transfer in a donor–bridge–acceptor ensemble is shown to be affected by exciting the vibrational modes of the hydrogen-bonded bridging groups.
The transfer of electrons through molecules is crucial to various biological and artificial processes, and is complicated by a molecule's many thermally accessible conformations. Now, Igor Rubtsov from Tulane University and colleagues from the USA and Cyprus have shown1 how such molecular 'flexibility' can be used advantageously to gain control over electron-transfer kinetics.
Using a 'three-pulse experiment', Rubtsov and co-workers investigated how electron transfer in a hydrogen-bonded donor–bridge–acceptor ensemble is affected when the vibrational modes of the bridging moieties are excited. They used an ultraviolet pulse to photoinduce a charge-separation reaction between an anthracene-derived acceptor and a dimethylaniline-containing donor. This was followed by an infrared pulse aimed at vibrationally exciting the C=O and NH2 groups of the bridge. The final pulse probed the absorption of the ensemble in the visible region, enabling the progress of the reaction to be observed.
On comparing electron transfer in the ensemble with and without excitation, the team found that the yield of the charge-separated ('reacted') state — and thus the transfer rate — was lower with vibrational excitation. Rubstov and colleagues suggests that vibrationally induced disruption of the hydrogen bonding — and subsequent reduction in donor–acceptor coupling — could be one possible mechanism that is responsible for the behaviour.
Lin, Z. et al. Modulating unimolecular charge transfer by exciting bridge vibrations. J. Am. Chem. Soc. 10.1021/ja907041t (2009).
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Armstrong, G. Good vibrations. Nature Chem (2009). https://doi.org/10.1038/nchem.516