手性烯烃光驱动分子马达设计的理论研究

In recent years, considerable progress has been made in the study of artificial light-driven rotary molecular motors based on chiral overcrowded alkenes, which has attracted particular attention in the field of Molecular Devices and Machines. Through consecutive cis–trans photoisomerizations about the C–C double bond with specific UV lights and thermal helix inversion steps on the ground state, where the latter dictate the overall mechanical property (rotational frequency), these light-driven rotary molecular motors achieve a full 360° unidirectional rotation. The aim of the project is to study in detail the features of both excited state and ground state potential energy surfaces (PESs) of chiral alkene light-driven rotary molecular motors by means of time-dependent density functional theory (TD-DFT) and density functional theory (DFT) calculations, respectively. By exploring the working mechanisms of the rotary cycle of light-driven molecular motors, we are able to put forward suggestions for theoretically redesigned molecular motors with depressed energy barriers in the ground state helix inversion processes, using methods such as sterically active groups with different size, dipole alkane chain, and alkanes with different aromatic or heterocyclic rings on either side. The mechanical properties of light-driven rotary molecular motors in ambient conditions (solvent effect or absorbed on solid surfaces) will be estimated by using the hybrid Quantum Mechanics and Molecular Mechanics (QM/MM) approach. Being able to complement experimental endeavors to design functional light-driven molecular machines and to promote the development of molecular engineering and organic molecular materials are the objectives of the research.

近年来，人工合成手性烯烃类光驱动旋转分子马达的研究取得了长足的进展，成为分子器件与机器领域的研究热点。围绕烯烃碳—碳双键的光致异构化和分子在基态的热力学螺旋转变共同驱动马达产生360°持续定向旋转，其中基态螺旋转变的快慢决定了马达的主要机械性能（旋转频率）。本项目以具有手性特征的烯烃类光驱动旋转分子马达为研究对象，利用含时密度泛函理论（TD-DFT）和密度泛函理论（DFT），分别研究分子激发态和基态势能面性质。通过揭示马达的工作原理，对分子进行理论设计，采用改变立体中心活性基团尺寸、使用偶极特性烯烃、烯烃两侧使用不同芳香环或杂环等手段，探讨降低基态螺旋转变过程中势垒高度的方法，利用量子力学/分子力学（QM/MM）杂化方法预测环境因素（如溶液环境或吸附在固体表面）对马达机械性能的影响。目的在于为功能型光驱动分子机器的设计与合成提供理论指导，促进分子马达机械性能与有机分子材料研究的进一步发展。

2016年诺贝尔化学奖授予了在分子机器与合成领域做出突出贡献的三位科学家，其中B. L. Feringa正是由于首次实现了人工合成手性烯烃类光驱动旋转分子马达而获此殊荣。本项目利用含时密度泛函理论（TD-DFT）和密度泛函理论（DFT）对手性烯烃类光驱动旋转分子马达的激发态和基态转变过程展开研究，理论揭示了Feringa光驱动旋转分子马达的工作机理，确定了马达分子在基态热力学转变过程中的决速步骤。进一步以具体实验合成手段为参考，理论上提出具有不同峡湾区结构的马达分子设计方案，实现了一种具有MHz旋转频率的分子马达改进设计方案。光驱动旋转分子马达在转动过程中分子结构具有周期性变化，每一个360°转动过程中分子构型会在四个基态稳定结构中逐步转变。通过构建具有三明治结构的分子器件，结合密度泛函理论和非平衡格林函数方法，对器件的电荷输运特性进行理论计算，探讨了将光驱动旋转分子马达置于金属电极之间实现一种分子光开关的可能。光驱动旋转分子马达光能向机械能转变过程是通过分子在激发态上的弛豫运动实现的，对该过程的深入理解需开展光驱动分子马达激发态动力学行为研究，为此我们发展了一套自旋翻转含时密度泛函理论方法，通过改变密度泛函中Hartree-Fock交换作用的比例，确定分子自旋态能量的最优计算方案，为下一步开展激发态动力学性质计算提供了基础。