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.微生物视紫红质与动物视紫红质（microbial and animal rhodopsins）中质子化视黄醛席夫碱（retinal protonated Schiff base）的光异构反应兼具极致超快性与高度特异性。两类蛋白环境均可为其生色团的光化学过程提供精细调控的条件。本文结合时间分辨动作吸收光谱（time-resolved action absorption spectroscopy）与高精度电子结构理论（high-level electronic structure theory），证实人工工程化视黄醛生色团（synthetically engineered retinal chromophore）亦可实现同类调控效应。研究将二甲基化视黄醛席夫碱（dimethylated retinal Schiff base）的C11=C12双键锁定为反式构型（简称L-RSB），使气相中的激发态弛豫从缓慢的皮秒级过程转化为超快亚皮秒级过程。研究发现，L-RSB的空间位阻与预扭转可显著降低激发态能垒，令该锁定生色团的异构化沿C9=C10双键路径进行，而非常规偏好的C11=C12异构化路径。尤为关键的是，加速后的激发态动力学还具备定向调控特性。本文证实L-RSB可通过两次连续吸收600 nm光子诱导的两个独立步骤，实现单向360°旋转：从全反式（all-trans）转化为9-顺式（9-cis），再从9-顺式还原为全反式构型。该研究为基于锁定视黄醛生色团的红光驱动超快分子旋转马达（molecular rotary motors）的理性设计开辟了可行路径。