光开关π共轭聚合物的多副本偏置采样
Multi-replica biased sampling for photoswitchable pi-conjugated polymers
摘要：
近年来，π共轭聚合物由于其在π共轭主链周围的光依赖性扭转重组而引起了人们的极大兴趣，这决定了其独特的发光性能。出于对设计具有可调光开关路径的共轭聚合物的兴趣，我们设计了一个计算框架来增强扭转构象空间的采样，并同时估计基态与激发态自由能的差异。该方案是基于一个组合的哈密顿量交换（REM），平行偏置元自洽，和自由能微扰理论。在我们的方案中，每个REM副本样本的中间非物理状态之间的基态和前两个激发态，其特征在于在B3 LYP/6- 31 G * 水平的理论的TD-DFT模拟。我们将该方法应用于9，9-二辛基芴的5-mer，并发现在照射后，该系统可以经历从155到-155度的二面角反转，越过从基态（S 0）的0.1 eV降低到第一（S1）和第二（S2）激发态的0.05 eV和0.04 eV的势垒。此外，S1和更多的S2被预测为稳定共面二面角，局部自由能最小值位于+-44度。S1态和S2态的自由能垒分别为0.08 eV和0.12 eV时，可以将这种构象捕获在远离155度处的全局自由能最小值的盆地中。模拟结果与实验发射光谱进行了比较，显示与我们的框架提供的预测定量协议。
英文摘要：
In recent years, pi-conjugated polymers are attracting considerable interest in view of their light-dependent torsional reorganization around the pi-conjugated backbone, which determines peculiar light-emitting properties. Motivated by the interest in designing conjugated polymers with tunable photoswitchable pathways, we devised a computational framework to enhance the sampling of the torsional conformational space and at the same time estimate ground to excited-state free-energy differences. This scheme is based on a combination of Hamiltonian Replica Exchange (REM), Parallel Bias metadynamics, and free-energy perturbation theory. In our scheme, each REM replica samples an intermediate unphysical state between the ground and the first two excited states, which are characterized by TD-DFT simulations at the B3LYP/6-31G* level of theory. We applied the method to a 5-mer of 9,9-dioctylfluorene and found that upon irradiation this system can undergo a dihedral inversion from 155 to -155 degrees crossing a barrier that decreases from 0.1 eV in the ground state (S0) to 0.05 eV and 0.04 eV in the first (S1) and second (S2) excited states. Furthermore, S1 and even more S2 were predicted to stabilize coplanar dihedrals, with a local free-energy minimum located at +-44 degrees. The presence of a free-energy barrier of 0.08 eV for the S1 and 0.12 eV for the S2 state can trap this conformation in a basin far from the global free-energy minimum located at 155 degrees. The simulation results were compared with the experimental emission spectrum, showing a quantitative agreement with the predictions provided by our framework.
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