Directing the path of light-induced electron transfer at a molecular fork using vibrational excitation

Ultrafast electron transfer in condensed-phase molecular systems is often strongly coupled to intramolecular vibrations that can promote, suppress and direct electronic processes. Recent experiments exploring this phenomenon proved that light-induced electron transfer can be strongly modulated by vibrational excitation, suggesting a new avenue for active control over molecular function. Here, we achieve the first example of such explicit vibrational control through judicious design of a Pt( II )-acetylide charge-transfer donor–bridge–acceptor–bridge–donor ‘fork’ system: asymmetric 13 C isotopic labelling of one of the two –C≡C– bridges makes the two parallel and otherwise identical donor→acceptor electron-transfer pathways structurally distinct, enabling independent vibrational perturbation of either. Applying an ultrafast UV pump (excitation)–IR pump (perturbation)–IR probe (monitoring) pulse sequence, we show that the pathway that is vibrationally perturbed during UV-induced electron transfer is dramatically slowed down compared to its unperturbed counterpart. One can thus choose the dominant electron transfer pathway. The findings deliver a new opportunity for precise perturbative control of electronic energy propagation in molecular devices. With recent and improved understanding of how nuclear and electronic degrees of freedom can interact with each other comes the opportunity to directly control electronic processes. Now it has been shown that ultrafast vibrational excitation can direct light-induced intramolecular electron transfer along a specific path.