Alguire Ethan C, Subotnik Joseph E, Damrauer Niels H
Department of Chemistry and Biochemistry, University of Colorado at Boulder , Boulder, Colorado 80309, United States.
J Phys Chem A. 2015 Jan 15;119(2):299-311. doi: 10.1021/jp510777c. Epub 2014 Dec 31.
Singlet fission (SF) offers opportunities for wavelength-selective processing of solar photons with an end goal of achieving higher efficiency inexpensive photovoltaic or solar-fuels-producing devices. In order to evaluate new molecular design strategies and for theoretical exploration of dynamics, it is important to put in place tools for efficient calculation of the electronic coupling between single-exciton reactant and multiexciton product states. For maximum utility, the couplings should be calculated at multiple nuclear geometries (rather than assumed constant everywhere, i.e., the Condon approximation) and we must be able to evaluate couplings for covalently linked multichromophore systems. With these requirements in mind, here we discuss the simplest methodology possible for rapid calculation of diabatic one-electron coupling matrix elements-based on Boys localization and rediagonalization of molecular orbitals. We focus on a covalent species called BT1 that juxtaposes two tetracene units in a partially cofacial geometry via a norbornyl bridge. In BT1, at the equilibrium C2v structure, the "nonhorizontal" couplings between HOMOs and LUMOs (t(HL) and t(LH)) vanish by symmetry. We then explore the impact of molecular vibrations through the calculation of t(AB) coupling gradients along 183 normal modes of motion. Rules are established for the types of motions (irreducible representations in the C2v point group) that turn on tHL and tLH values as well as for the patterns that emerge in constructive versus destructive interference of pathways to the SF product. For the best modes, calculated electronic coupling magnitudes for SF (at root-mean-squared deviation in position at 298 K), are within a factor of 2 of that seen for noncovalent tetracene dimers relevant to the molecular crystal. An overall "effective" electronic coupling is also given, based on the Stuchebrukhov formalism for non-Condon electron transfer rates.
单线态裂变（SF）为太阳能光子的波长选择性处理提供了机会，最终目标是实现更高效率的廉价光伏或太阳能燃料生产设备。为了评估新的分子设计策略并进行动力学的理论探索，建立用于有效计算单激子反应物和多激子产物态之间电子耦合的工具非常重要。为了实现最大效用，耦合应在多个核几何构型下进行计算（而不是假设在任何地方都是恒定的，即康登近似），并且我们必须能够评估共价连接的多发色团系统的耦合。考虑到这些要求，我们在此讨论基于分子轨道的博伊斯定域化和重新对角化快速计算非绝热单电子耦合矩阵元的最简单方法。我们重点研究一种称为BT1的共价物种，它通过降冰片基桥在部分共面几何构型中并列两个并四苯单元。在BT1中，在平衡C2v结构下，HOMO和LUMO之间的“非水平”耦合（t(HL)和t(LH)）因对称性而消失。然后，我们通过计算沿183个正常运动模式的t(AB)耦合梯度来探索分子振动的影响。我们建立了开启tHL和tLH值的运动类型（C2v点群中的不可约表示）的规则，以及在通往SF产物的途径的相长干涉与相消干涉中出现的模式的规则。对于最佳模式，计算得到的SF电子耦合幅度（在298 K时位置的均方根偏差处）与与分子晶体相关的非共价并四苯二聚体的耦合幅度相差不到两倍。基于非康登电子转移速率的斯图切布鲁霍夫形式，还给出了一个整体的“有效”电子耦合。
