Electron transfer is common in biochemistry, electrochemistry and redox reactions, but isn’t fully understood. New research now shows that the rate at which an electron leaves its parent atom may be at the mercy of the solvent.
Majed Chergui of the Ecole Polytechnique Fédérale de Lausanne in Switzerland and co-workers used ultrafast spectroscopy to follow the loss of an electron from iodide ions in water after the ions have been excited with ultraviolet light. They found that, like a car waiting at a junction for a gap in the traffic, the electron can only depart from the iodine atom when a void opens up among the jostling solvent molecules. This means that electron ejection has a broad distribution of lifetimes, between about 100 and 400 femtoseconds (10-15 s).
The researchers chose to look at electron transfer from iodide because, being a single atom, it has no internal motions to complicate the process, so that the loss of the electron is entirely dictated by motions of the solvent – making it ideal for studying how the solvent affects the process.
After excitation of the iodide with a UV laser, an electron jumps off and is solvated by water molecules, leaving behind a neutral iodine atom. Previous studies have shown that the electron is detached in about 0.2 picoseconds (10-12 s), although it takes up to 1ps for the solvent to rearrange to fully solvate the electron.
The electron loss happens in two stages. First, light absorption promotes the formation of an excited state called the charge-transfer-to-solvent (CTTS) state, in which the iodide’s electron cloud expands rapidly. ‘The solvent is left in a highly non-equilibrium situation when a whole charge, which was moments before strongly solvated, expands so fast,’ explains Stephen Bradforth, a laser spectroscopist at the University of Southern California in Los Angeles, who has examined this process using quantum-chemical simulations in collaboration with Pavel Jungwirth of the Academy of Sciences of the Czech Republic in Prague.
By looking at the decay of fluorescence from the photoexcited ion, Chergui and colleagues deduce that the first stage of the reaction – when the solvent moves outward to accommodate the expanded CTTS electron cloud – takes about 60fs. Then the electron ‘buds off’ from the diffuse cloud – but it needs to find a solvent void to move into. ‘In some cases the electrons have to wait for a void, in others, it’s already there upon excitation because of the random distribution of solvent cage configurations,’ Chergui explains.
The results agree with the picture that emerged from Bradforth and Jungwirth’s simulations, showing that the irregular, random arrangements of solvent molecules not only influence the shape of the diffuse CTTS state but also dictate the rate at which it can evolve to eject an electron.
‘It’s is a fine piece of experimental work,’ says Jungwirth, ‘and a nice example of the solvent being more than just a “neutral” medium, becoming directly part of the reaction process.’