Fig. 5: Temperature dependence of trans(A)–cis(B) and cis(B)–trans(A) photoisomerization kinetics.

a, Potentials are shown as adiabatic states that contain the A1 (PDB 7QLM and 7QLJ) and A2 (PDB 7QLN and 7QLO) off-state and B1 (PDB 7QLK) and B2 (PDB 7QLL and 7QLI) on-state structures as shown. The protonated anionic BH state is a putative intermediate that connects the photocycle (Fig. 1). Cryo-trapping of the unrelaxed on state resolved the double-well structural features of the B1 and B2 on states (Supplementary Sections 9 and 11 provide crystallographic details and full thermodynamic modelling, respectively). b, Arrhenius plots for the on→off (black) and off→on (red, with low-temperature on→off pre-conversion; blue, with on→off pre-conversion at 296 K) kinetics under continuous illumination at 473 nm and 405 nm, respectively, showing convex behaviour in both directions. The off→on kinetics that included high-temperature annealing before conversion (blue) showed significantly reduced kinetics and shifting of the transition temperature to higher values. Both high- and low-temperature regions involved photoisomerization in both on→off and off→on directions, as shown from X-ray-crystal structural analysis (Supplementary Section 9). The error bars in b are the standard error.