Sobolewski Andrzej L, Domcke Wolfgang
Institute of Physics, Polish Academy of Sciences, PL-02-668 Warsaw, Poland.
Department of Chemistry, Technical University of Munich, D-75747 Garching, Germany.
Phys Chem Chem Phys. 2023 Aug 23;25(33):21875-21882. doi: 10.1039/d3cp01666h.
The inversion of the energies of the lowest singlet (S) and lowest triplet (T) excited states in violation of Hund's multiplicity rule is a rare phenomenon in stable organic molecules. S-T inversion has significant consequences for the photophysics and photochemistry of organic chromophores. In this work, wave-function based computational methods were employed to explore the possibility of S-T inversion in hexagonal polycyclic aromatic and heteroaromatic compounds. In these molecules, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are two-fold degenerate. The HOMO-LUMO transition gives rise to three singlet and three triplet excited states. While the singlet-triplet energy gap , defined as the energy difference between the S state and the T state, is clearly positive for benzene, it is predicted to be close to zero for borazine, the boron nitride analogue of benzene. Although decreases with increasing size of hexagonal polycyclic aromatics, it remains positive up to circumcoronene (19 rings). However, symmetry-preserving substitution of C-C pairs by B-N groups in the interior, keeping the conjugation of the outer rim intact, results in compounds with robustly negative . These findings establish the existence of a new family of boron carbon nitrides with inverted singlet-triplet gaps.
最低单重态（S）和最低三重态（T）激发态的能量反转违反了洪德多重性规则，这在稳定的有机分子中是一种罕见的现象。S-T反转对有机发色团的光物理和光化学有重大影响。在这项工作中，采用基于波函数的计算方法来探索六角形多环芳烃和杂芳烃化合物中S-T反转的可能性。在这些分子中，最高占据分子轨道（HOMO）和最低未占据分子轨道（LUMO）是双重简并的。HOMO-LUMO跃迁产生三个单重态和三个三重态激发态。虽然单重态-三重态能隙（定义为S态和T态之间的能量差）对于苯来说显然是正的，但对于苯的氮化硼类似物硼嗪，预计其接近零。尽管随着六角形多环芳烃尺寸的增加而减小，但直到外周环烯（19个环）它仍为正。然而，在内侧用B-N基团对称地取代C-C对，同时保持外缘的共轭完整，会导致具有强烈负能隙的化合物。这些发现证实了存在一类新的具有反转单重态-三重态能隙的硼碳氮化物。
