Looking for chiral recognition in photoinduced bimolecular electron transfer using ultrafast spectroscopy

• Published in: Physical chemistry chemical physics : PCCP Vol. 25; no. 16; pp. 11111 - 1112
• Main Authors: Verma, Pragya, Nançoz, Christoph, Bosson, Johann, Labrador, Géraldine M, Lacour, Jérôme, Vauthey, Eric
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 26.04.2023; The Royal Society of Chemistry
• Subjects: Chemistry; Chirality; Diffusion; Electron transfer; Molecular dynamics; Quenching; Recognition; Stereoselectivity; Tryptophan
• ISSN: 1463-9076; 1463-9084; 1463-9084
• DOI: 10.1039/d3cp00760j

Occurrence of chiral recognition in bimolecular photoinduced electron transfer (ET) is difficult to identify because of the predominant role of diffusion. To circumvent this problem, we apply a combination of ultrafast time-resolved fluorescence and transient electronic absorption to look for stereoselectivity in the initial, static stage of ET quenching, where diffusion is not relevant. The fluorophore and electron acceptor is a cationic hexahelicene, whereas the quencher has either stereocentered (tryptophan) or axial (binaphthol) chirality. We found that, in all cases, the quenching dynamics are the same, within the limit of error, for different diastereomeric pairs in polar and medium-polar solvents. The same absence of chiral effect is observed for the recombination of the radical pair, which results from the quenching. Molecular dynamics simulations suggest that the distribution of inter-reactant distance is independent of the chirality of the acceptor and the donor. Close contact resulting in large electronic coupling is predicted to be possible with all diastereomeric pairs. In this case, ET is an adiabatic process, whose dynamics do no longer depend on the coupling, but are rather controlled by high-frequency intramolecular modes. No significant stereoselectivity could be identified in photoinduced electron transfer in the static regime. Sufficiently high electronic coupling can be achieved with all diastereomeric pairs for this process to occur in the adiabatic regime.