Aix Marseille University, CNRS, ICR, Marseille 13013, France.
Instituto de Química, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Rio de Janeiro 21941-909, Brazil.
J Am Chem Soc. 2022 Dec 28;144(51):23492-23504. doi: 10.1021/jacs.2c10129. Epub 2022 Dec 19.
Singlet fission in covalently bound acene dimers in solution is driven by the interplay of excitonic and singlet correlated triplet (TT) states with intermediate charge-transfer states, a process which depends sensitively on the solvent environment. We use high-level electronic structure methods to explore this singlet fission process in a linked tetracene dimer, with emphasis on the symmetry-breaking mechanism for the charge-transfer (CT) states induced by low-frequency antisymmetric vibrations and polar/polarizable solvents. A combination of the second-order algebraic diagrammatic construction (ADC(2)) and density functional theory/multireference configuration interaction (DFT/MRCI) methods are employed, along with a state-specific conductor-like screening model (COSMO) solvation model in the former case. This work quantifies, for the first time, an earlier mechanistic proposal [Alvertis et al., 17558] according to which solvent-induced symmetry breaking leads to a high-energy CT state which interacts with the correlated triplet state, resulting in singlet fission. An approximate assessment of the nonadiabatic interactions between the different electronic states underscores that the CT states are essential in facilitating the transition from the bright excitonic state to the (TT) state leading to singlet fission. We show that several types of symmetry-breaking inter- and intra-fragment vibrations play a crucial role in a concerted mechanism with the solvent environment and with the symmetric inter-fragment torsion, which tunes the admixture of excitonic and CT states. This offers a new perspective on how solvent-induced symmetry-breaking CT can be understood and how it cooperates with intramolecular mechanisms in singlet fission.
在溶液中的共价键联薁二聚体中的单重态裂变是由激子和单重态相关三重态（TT）态与中间电荷转移态的相互作用驱动的，这一过程对溶剂环境非常敏感。我们使用高水平的电子结构方法来研究连接的四并苯二聚体中的这一单重态裂变过程，重点是由低频非对称振动和极性/极化溶剂引起的电荷转移（CT）态的对称破缺机制。我们采用了二阶代数图论构造（ADC(2)）和密度泛函理论/多参考组态相互作用（DFT/MRCI）方法的组合，以及前者中的特定状态的导体相似屏蔽模型（COSMO）溶剂化模型。这项工作首次定量地证明了一个早期的机制性提议[Alvertis 等人，17558]，根据该提议，溶剂诱导的对称破缺导致高能 CT 态与相关三重态相互作用，从而导致单重态裂变。对不同电子态之间非绝热相互作用的近似评估强调，CT 态在促进从明亮激子态到（TT）态的转变中起着至关重要的作用，从而导致单重态裂变。我们表明，几种类型的对称破缺的片段间和片段内振动与溶剂环境以及对称的片段间扭转一起协同作用，从而调节激子和 CT 态的混合。这为理解溶剂诱导的对称破缺 CT 如何以及它如何与单重态裂变中的分子内机制合作提供了新的视角。
