Institute of Chemistry for Life and Health Sciences (i-CLeHS), Chimie ParisTech, PSL Research University, CNRS, F-75005 Paris, France.
CEA/DAM/DIF, 91297 Arpajon Cedex, France.
J Chem Theory Comput. 2020 Nov 10;16(11):7017-7032. doi: 10.1021/acs.jctc.0c00762. Epub 2020 Oct 5.
Photonastic materials present a directional and repeatable deformation of the material shape due to transduction from light energy to mechanical energy. Among these materials, light-responsive polymers, composed of photochromic molecules embedded in a polymer matrix, are of high interest. The present work aims at laying the foundation stone of the modeling of the photomechanical behavior of such systems by proposing a computational strategy that is able to investigate (i) the impact of the polymer matrix on the photochromic properties of a dithienylethene (DTE) switch and (ii) the impact of the photochromic reaction on the polymer environment. Contrary to previous approaches, the present model is able to propose a realistic arrangement of the photochrome embedded in the polymer film, thanks to the adaptation of the so-called "controlled-like polymerization algorithm" [Lemarchand, C. A.; 2019, 50, 224902]. Our strategy relies on molecular dynamics (MD) simulations and time-dependent density functional theory (DFT) calculations. Careful analysis of MD trajectories and comparison with simulations in solution have shown the rigidification of the DTE molecule due to the presence of the polymer chains, which hindered the interconversion between the DTE open-form isomers and can probably modify the photocyclization quantum yield. Besides, the UV-vis absorption properties of the DTE open-form isomers are more impacted by the polymer embedding than its closed-form counterpart. Concerning the impact of the photoreaction on the polymer matrix, the time evolution of the pressure tensor and of the atomic displacements in the matrix have shown that (i) the cyclization reaction has a negligible impact; (ii) the cycloreversion reaction induces a locally large and anisotropic pressure increase and leads to a collective displacement of the polymer matrix away from the reactive center; and (iii) the characteristic time scale associated with these coupled processes is below 1 ps. Therefore, the two processes involved in photonastic motions, namely, the photoreaction and the relaxation of the polymer matrix after the photoswitch structural change, cannot be decoupled.
光致动材料由于光能到机械能的转换而呈现出材料形状的定向和可重复变形。在这些材料中，由光致变色分子嵌入聚合物基质组成的光响应聚合物引起了人们的极大兴趣。本工作旨在通过提出一种能够研究（i）聚合物基质对二噻吩乙烯（DTE）开关光致变色性质的影响和（ii）光致变色反应对聚合物环境的影响的计算策略，为这类系统的光机械行为建模奠定基础。与以前的方法相比，由于适应了所谓的“类似控制的聚合算法”[Lemarchand，CA；2019 年，50 卷，224902]，本模型能够提出光致变色分子在聚合物膜中的实际排列。我们的策略依赖于分子动力学（MD）模拟和时变密度泛函理论（DFT）计算。对 MD 轨迹的仔细分析和与溶液模拟的比较表明，由于聚合物链的存在，DTE 分子的刚性化，这阻碍了 DTE 开环异构体之间的互变异构，并且可能改变光环化量子产率。此外，DTE 开环异构体的紫外可见吸收性质比其闭环对应物受聚合物嵌入的影响更大。关于光反应对聚合物基质的影响，压力张量和基质中原子位移的时间演化表明：（i）环化反应的影响可以忽略不计；（ii）环还原反应导致局部大且各向异性的压力增加，并导致聚合物基质从反应中心集体位移；（iii）与这些耦合过程相关的特征时间尺度小于 1 ps。因此，光致动运动涉及的两个过程，即光致反应和光开关结构变化后聚合物基质的弛豫，不能解耦。
