Department of Chemistry , Technical University of Munich , D-85747 Garching , Germany.
Department of Chemistry , University of Washington , Seattle , Washington 98195 , United States.
J Phys Chem A. 2019 Sep 26;123(38):8099-8108. doi: 10.1021/acs.jpca.9b06215. Epub 2019 Sep 12.
According to Hund's rule, the lowest triplet state (T) is lower in energy than the lowest excited singlet state (S) in closed-shell molecules. The exchange integral lowers the energy of the triplet state and raises the energy of the singlet state of the same orbital character, leading to a positive singlet-triplet energy gap (Δ). Exceptions are known for biradicals and charge-transfer excited states of large molecules in which the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are spatially separated, resulting in a small exchange integral. In the present work, we discovered with ADC(2), CC2, EOM-CCSD, and CASPT2 calculations that heptazine (1,3,4,6,7,9,9b-heptaazaphenalene or tri--triazine) exhibits an inverted S/T energy gap (Δ ≈ -0.25 eV). This appears to be the first example of a stable closed-shell organic molecule exhibiting S/T inversion at its equilibrium geometry. The origins of this phenomenon are the nearly pure HOMO-LUMO excitation character of the S and T states and the lack of spatial overlap of HOMO and LUMO due to a unique structure of these orbitals of heptazine. The S/T inversion is found to be extremely robust, being affected neither by substitution of heptazine nor by oligomerization of heptazine units. Using time-resolved photoluminescence and transient absorption spectroscopy, we investigated the excited-state dynamics of 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a chemically stable heptazine derivative, in the presence of external heavy atom sources as well as triplet-quenching oxygen. These spectroscopic data are consistent with TAHz singlet excited state decay in the absence of a low-energy triplet loss channel. The absence of intersystem crossing and an exceptionally low radiative rate result in unusually long S lifetimes (of the order of hundreds of nanoseconds in nonaqueous solvents). These features of the heptazine chromophore have profound implications for organic optoelectronics as well as for water-splitting photocatalysis with heptazine-based polymers (e.g., graphitic carbon nitride) which have yet to be systematically explored and exploited.
根据 Hund 规则，在闭壳分子中，最低三重态（T）的能量低于最低激发单线态（S）。交换积分降低了三重态的能量，提高了相同轨道特征的单线态的能量，导致正的单线态-三重态能隙（Δ）。已知双自由基和大分子的电荷转移激发态是例外，其中最高占据分子轨道（HOMO）和最低未占据分子轨道（LUMO）在空间上分离，导致交换积分较小。在本工作中，我们通过 ADC(2)、CC2、EOM-CCSD 和 CASPT2 计算发现，庚嗪（1,3,4,6,7,9,9b-庚嗪或三嗪）表现出倒置的 S/T 能隙（Δ≈-0.25 eV）。这似乎是第一个在其平衡几何形状下表现出 S/T 反转的稳定闭壳有机分子的例子。这种现象的起源是 S 和 T 态的 HOMO-LUMO 激发特征几乎是纯的，以及由于庚嗪这些轨道的独特结构，HOMO 和 LUMO 之间没有空间重叠。发现 S/T 反转非常稳健，既不受庚嗪取代的影响，也不受庚嗪单元聚合的影响。使用时间分辨光致发光和瞬态吸收光谱，我们研究了 2,5,8-三（4-甲氧基苯基）-1,3,4,6,7,9,9b-庚嗪（TAHz）的激发态动力学，TAHz 是一种化学稳定的庚嗪衍生物，存在外加重原子源以及三重态猝灭氧。这些光谱数据与 TAHz 单线态激发态衰减一致，不存在低能三重态损失通道。不存在系间窜跃和异常低的辐射速率导致 S 寿命异常长（在非水溶剂中约为数百纳秒）。庚嗪发色团的这些特征对有机光电学以及基于庚嗪的聚合物（例如石墨碳氮化物）的水分解光催化具有深远的意义，这些聚合物尚未得到系统的探索和利用。
