Light Driven Ultrafast Bioinspired Molecular Motors: Steering and Accelerating Photoisomerization Dynamics of Retinal

Photoisomerization of retinal protonated
Schiff base in microbial
and animal rhodopsins are strikingly ultrafast and highly specific.
Both protein environments provide conditions for fine-tuning the photochemistry
of their chromophores. Here, by combining time-resolved action absorption
spectroscopy and high-level electronic structure theory, we show that
similar control can be gained in a synthetically engineered retinal
chromophore. By locking the dimethylated retinal Schiff base at the
C11C12 double bond in its trans configuration (L-RSB), the excited-state decay is rendered from
a slow picosecond to an ultrafast subpicosecond regime in the gas
phase. Steric hindrance and pretwisting of L-RSB are found to be important
for a significant reduction in the excited-state energy barriers,
where isomerization of the locked chromophore proceeds along C9C10 rather than the preferred C11C12 isomerization path. Remarkably, the accelerated
excited-state dynamics also becomes steered. We show that L-RSB is
capable of unidirectional 360° rotation from all-trans to 9-cis and from 9-cis to all-trans in only two distinct steps induced by consecutive
absorption of two 600 nm photons. This opens a way for the rational
design of red-light-driven ultrafast molecular rotary motors based
on locked retinal chromophores.

微生物与动物视紫红质（rhodopsins）中的视黄质子化希夫碱（retinal protonated Schiff base）光异构反应，均展现出显著的超快特性与高度特异性。两类蛋白质环境均可为其生色团的光化学过程提供精细调控的条件。本研究结合时间分辨动作吸收光谱法与高精度电子结构理论，证明人工工程化设计的视黄质生色团亦可实现同类调控效果。通过将二甲基化视黄质希夫碱的C11=C12双键锁定为反式构型（L-RSB），气相环境下其激发态衰变可从缓慢的皮秒量级被调控至超快的亚皮秒区间。研究表明，L-RSB的空间位阻与预扭转效应可显著降低激发态能垒，使得该锁定生色团的异构化沿C9=C10路径进行，而非其优先选择的C11=C12异构化路径。尤为值得注意的是，加速后的激发态动力学同时实现了定向调控。本研究证实，L-RSB仅需通过连续吸收两束600 nm光子诱导的两个独立步骤，即可完成从全反式到9-顺式、再从9-顺式返回全反式的单向360°旋转。这一发现为基于锁定视黄质生色团的红光驱动超快分子旋转马达的合理设计开辟了全新途径。