The chromophore of bacteriorhodopsin is all-trans retinal with a protonated Schiff base (PSBR). Upon absorption of light, PSBR undergoes ultrafast sub-ps isomerization inside the protein cavity, whereas the isolated chromophore exhibits a longer excited-state decay on a ps timescale. Here, we explore the impact of backbone modifications of PSBR on the photoinduced excited-state dynamics of the isolated chromophore and show that the 11-locked dimethylated analogue of PSBR exhibits not only the accelerated photoisomerization dynamics, but it also becomes unidirectional – like that observed in the protein environment. By using the invariant multistate multireference perturbation theory, XMCQDPT2, we perform relaxed potential energy scans along the isomerization coordinates in the excited state on the way to conical intersections with the ground state. The modified transition state theory is applied to estimate excited-state lifetimes based on energy barriers located along the reaction pathways. The reaction coordinate is treated explicitly, and the corresponding partition functions are calculated based on energy levels found as numerical solutions to the one-dimensional Schrödinger equation for intramolecular rotation, using the matrix Numerov method. A proper account of the pre-exponential factor, which depends on an internal rotational constant, is found to be a prerequisite for calculating the accurate isomerization rate constants. Strikingly, locking the Schiff-base chromophore backbone at the C11=C12 bond in the all-trans configuration, which inhibits the dominant isomerization path in PSBR, decreases the isomerization lifetime to a sub-ps timescale. The increased rate of isomerization is traced to changes in the excited-state energy barriers due to steric hindrance exerted by the chemical modification, where isomerization of the locked chromophore proceeds along C9=C10 rather than the preferred C11=C12 isomerization path. Moreover, the isomerization potential becomes highly asymmetric, which results in unidirectional rotation in the excited state. Our results show that the chemical modifications of the retinal backbone can be used to mimic the protein environment, and these findings can be helpful for the rational design of molecular photoswitches and molecular motors.

Steered ultrafast photoisomerization dynamics in the locked Schiff-base retinal: a detailed XMCQDPT2 study

ACS Spring Meeting 2021

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