Bedogni Matteo, Giavazzi Davide, Di Maiolo Francesco, Painelli Anna
Department of Chemistry, Life Science and Environmental Sustainability, Università di Parma, 43124 Parma, Italy.
J Chem Theory Comput. 2024 Jan 23;20(2):902-913. doi: 10.1021/acs.jctc.3c01112. Epub 2023 Nov 22.
The inversion of the lowest singlet and triplet excited states, observed in several triangle-shaped organic molecules containing conjugated carbon and nitrogen atoms, is an astonishing result that implies the breakdown of Hund's rule. The phenomenon attracted interest for its potential toward triplet harvesting in organic LEDs. On a more fundamental vein, the singlet-triplet (ST) inversion sheds new light on the role of electron correlations in the excited-state landscape of π-conjugated molecules. Relying on the celebrated Pariser-Parr-Pople model, the simplest model for correlated electrons in π-conjugated systems, we demonstrate that the ST inversion does not require triangle-shaped molecules nor any specific molecular symmetry. Indeed, the ST inversion does not require strictly non-overlapping HOMO and LUMO orbitals but rather a small gap and a small exchange integral between the frontier orbitals.
在几个含有共轭碳和氮原子的三角形有机分子中观察到的最低单重态和三重态激发态的反转，是一个惊人的结果，这意味着洪德规则的失效。该现象因其在有机发光二极管中进行三重态俘获的潜力而受到关注。从更基本的层面来看，单重态 - 三重态（ST）反转揭示了电子关联在π共轭分子激发态图景中的作用。依靠著名的巴黎 - 帕尔 - 波普尔模型（π共轭体系中相关电子的最简单模型），我们证明ST反转不需要三角形分子，也不需要任何特定的分子对称性。实际上，ST反转并不严格要求最高占据分子轨道（HOMO）和最低未占据分子轨道（LUMO）严格不重叠，而是要求前沿轨道之间有小的能隙和小的交换积分。
