Zeng Weixuan, El Bakouri Ouissam, Szczepanik Dariusz W, Bronstein Hugo, Ottosson Henrik
Department of Chemistry, University of Cambridge Cambridge CB2 1EW UK
Department of Chemistry - Ångström Laboratory, Uppsala University 751 20 Uppsala Sweden
Chem Sci. 2021 Mar 25;12(17):6159-6171. doi: 10.1039/d1sc00382h.
The exact energies of the lowest singlet and triplet excited states in organic chromophores are crucial to their performance in optoelectronic devices. The possibility of utilizing singlet fission to enhance the performance of photovoltaic devices has resulted in a wide demand for tuneable, stable organic chromophores with wide S-T energy gaps (>1 eV). Cibalackrot-type compounds were recently considered to have favorably positioned excited state energies for singlet fission, and they were found to have a degree of aromaticity in the lowest triplet excited state (T). This work reports on a revised and deepened theoretical analysis taking into account the excited state Hückel-aromatic (instead of Baird-aromatic) as well as diradical characters, with the aim to design new organic chromophores based on this scaffold in a rational way starting from qualitative theory. We demonstrate that the substituent strategy can effectively adjust the spin distribution on the chromophore and thereby manipulate the excited state energy levels. Additionally, the improved understanding of the aromatic characters enables us to demonstrate a feasible design strategy to vary the excited state energy levels by tuning the number and nature of Hückel-aromatic units in the excited state. Finally, our study elucidates the complications and pitfalls of the excited state aromaticity and antiaromaticity concepts, highlighting that quantitative results from quantum chemical calculations of various aromaticity indices must be linked with qualitative theoretical analysis of the character of the excited states.
有机发色团中最低单重态和三重态激发态的确切能量对其在光电器件中的性能至关重要。利用单重态裂变来提高光伏器件性能的可能性导致对具有宽S-T能隙（>1 eV）的可调谐、稳定有机发色团有广泛需求。最近，西巴拉克罗特型化合物被认为具有有利于单重态裂变的激发态能量，并且发现它们在最低三重态激发态（T）中具有一定程度的芳香性。这项工作报告了一种经过修订和深化的理论分析，该分析考虑了激发态休克尔芳香性（而不是贝尔德芳香性）以及双自由基特征，旨在从定性理论出发，以合理的方式基于该骨架设计新的有机发色团。我们证明，取代基策略可以有效地调节发色团上的自旋分布，从而操纵激发态能级。此外，对芳香特征的深入理解使我们能够展示一种可行的设计策略，通过调整激发态中休克尔芳香单元的数量和性质来改变激发态能级。最后，我们的研究阐明了激发态芳香性和反芳香性概念的复杂性和陷阱，强调各种芳香性指数的量子化学计算的定量结果必须与激发态特征的定性理论分析联系起来。
