Department of Chemistry, Loughborough University, Loughborough LE11 3TU, United Kingdom.
J Chem Theory Comput. 2023 Apr 25;19(8):2340-2352. doi: 10.1021/acs.jctc.3c00125. Epub 2023 Apr 6.
The ability to tune excited-state energies is crucial to many areas of molecular design. In many cases, this is done based on the energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). However, this viewpoint is incomplete neglecting the many-body nature of the underlying excited-state wave functions. Within this work, we highlight the importance of two crucial terms, other than orbital energies, that contribute to the excitation energies and show how to quantify them from quantum chemistry computations: a Coulomb attraction and a repulsive exchange interaction. Using this framework, we explain under which circumstances the lowest excited state of a molecule, of either singlet or triplet multiplicity, is not accessed via the HOMO/LUMO transition and show two paradigmatic examples. In the case of the push-pull molecule , we highlight how the lowest triplet excited state is a locally excited state lying below the HOMO/LUMO charge transfer state due to enhanced Coulomb binding. In the case of the naphthalene molecule, we highlight how the HOMO/LUMO transition (the state) becomes the second excited singlet state due to its enhanced exchange repulsion term. More generally, we explain why excitation energies do not always behave like orbital energy gaps, providing insight into photophysical processes as well as methodogical challenges in describing them.
调控激发态能量的能力对于分子设计的许多领域至关重要。在许多情况下，这是基于最高占据分子轨道（HOMO）和最低未占据分子轨道（LUMO）的能量来实现的。然而，这种观点忽略了基础激发态波函数的多体性质，是不完整的。在这项工作中，我们强调了除轨道能量之外的两个关键术语的重要性，它们对激发能有贡献，并展示了如何从量子化学计算中量化它们：库仑吸引和排斥交换相互作用。使用这个框架，我们解释了在什么情况下，分子的最低激发态（无论是单线态还是三线态）不会通过 HOMO/LUMO 跃迁来实现，并展示了两个典型的例子。在推挽分子的情况下，我们强调了由于增强的库仑结合，最低三重激发态是一个局域激发态，位于 HOMO/LUMO 电荷转移态之下。在萘分子的情况下，我们强调了 HOMO/LUMO 跃迁（1 态）如何由于其增强的交换排斥项而成为第二激发单线态。更一般地，我们解释了为什么激发能并不总是像轨道能隙那样表现，为理解光物理过程以及描述它们的方法学挑战提供了深入的见解。
