Nature’s fastest photoreceptors rhodopsin converts light into motion by C=C isomerisation through a conical intersection (CI), reaching ~0.65 quantum yield (QY) on a sub-picosecond timescale [1]. Synthetic photoswitches built on the same coordinate rarely approach this, and the shortfall matters: in photopharmacology and optogenetics the required light dose scales inversely with extinction coefficient and QY. Theory attributes rhodopsin’s efficiency to vibrational promoter modes synchronising with the reaction coordinate at the CI [2], [3]; coherent nuclear motion surviving the crossing is seen in rhodopsin and the biomimetic NAIP switches[4], but rarely tested elsewhere.
Our oxindole switch is a minimal analogue of the protein’s electrostatic control: the donating strength of a single hydroxyl is set by protonation state. This one handle switches the mechanism itself [5], [6]. OxI-OH decays biexponentially (0.25, 1.3 ps) through a twisted dark state to a pyramidalised CI; OxI-O⁻ bypasses it, decaying in ~200 fs by axial rotation to a geometrically distinct CI. Yet deprotonation raises Z→E QY (20→25%) while lowering E→Z (33→19%) — mechanism and efficiency decouple.
We apply sub-15 fs broadband (300–700 nm) transient absorption to both isomers in both protonation states, resolving excited-state coherences by linear-prediction singular-value decomposition. If synchronisation governs efficiency, coherence amplitude along each CI’s branching coordinate should track the direction-specific QY; its absence would sever the two. This pair isolates CI topography from molecular identity, giving the first test of vibrational synchronisation in a synthetic switch where the mechanism changes but the chromophore does not. A positive correlation would make electron-donating strength a rational handle on isomerisation efficiency, not only on absorption.
[1] R. W. Schoenlein, L. A. Peteanu, R. A. Mathies, and C. V. Shank, ‘The first step in vision: femtosecond isomerization of rhodopsin’, Science, vol. 254, no. 5030, pp. 412–415, 1991.
[2] M. Filatov and M. Olivucci, ‘Designing conical intersections for light-driven single molecule rotary motors: From precessional to axial motion’, J. Org. Chem., vol. 79, no. 8, pp. 3587–3600, 2014.
[3] A. Blanco-Gonzalez, M. Manathunga, X. Yang, and M. Olivucci, ‘Comparative quantum-classical dynamics of natural and synthetic molecular rotors show how vibrational synchronization modulates the photoisomerization quantum efficiency’, Nat. Commun., vol. 15, no. 1, p. 3499, Apr. 2024, doi: 10.1038/s41467-024-47477-0.
[4] M. Gueye et al., ‘Engineering the vibrational coherence of vision into a synthetic molecular device’, Nat. Commun., vol. 9, no. 1, p. 313, 2018.
[5] M. Mgbukwu et al., ‘Tuning the Photoisomerization Mechanism of Oxindole Switches with Electron-Donating Substituents’, J. Phys. Chem. B, vol. 129, no. 15, pp. 3839–3850, 2025.
[6] M. Mgbukwu, C. Granados, A. Khodko, S. Haacke, O. Kornilov, and J. Léonard, ‘Ultrafast photoreaction dynamics of oxindole-based molecular switches’, in Advances in Ultrafast Condensed Phase Physics IV, SPIE, 2024, p. PC129920F.
