Dinkelbach Fabian, Bracker Mario, Kleinschmidt Martin, Marian Christel M
Institute of Theoretical and Computational Chemistry, Heinrich-Heine-University Düsseldorf, 40225 Düsseldorf, Germany.
J Phys Chem A. 2021 Nov 25;125(46):10044-10051. doi: 10.1021/acs.jpca.1c09150. Epub 2021 Nov 10.
Heptazine derivatives are promising dopants for electroluminescent devices. Recent studies raised the question whether heptazines exhibit a small regular or an inverted singlet-triplet (IST) gap. It was argued that the S ← T reverse intersystem crossing (RISC) is a downhill process in IST emitters and therefore does not require thermal activation, thus enabling efficient harvesting of triplet excitons. Rate constants were not determined in these studies. Modeling the excited-state properties of heptazine proves challenging because fluorescence and intersystem crossing (ISC) are symmetry-forbidden in first order. In this work, we present a comprehensive theoretical study of the photophysics of heptazine and its derivative HAP-3MF. The calculations of electronic excitation energies and vibronic coupling matrix elements have been conducted at the density functional theory/multireference configuration interaction (DFT/MRCI) level of theory. We have employed a finite difference approach to determine nonadiabatic couplings and derivatives of spin-orbit coupling and electric dipole transition matrix elements with respect to normal coordinate displacements. Kinetic constants for fluorescence, phosphorescence, internal conversion (IC), ISC, and RISC have been computed in the framework of a static approach. Radiative S ↔ S transitions borrow intensity mainly from optically bright E' π → π* states, while S ↔ T (R)ISC is mediated by E″ states of n → π* character. Test calculations show that IST gaps as large as those reported in the literature are counterproductive and slow down the S ← T RISC process. Using the adiabatic DFT/MRCI singlet-triplet splitting of -0.02 eV, we find vibronically enhanced ISC and RISC to be fast in the heptazine core compound. Nevertheless, its photo- and electroluminescence quantum yields are predicted to be very low because S → S IC efficiently quenches the luminescence. In contrast, fluorescence, IC, ISC, and RISC proceed at similar time scales in HAP-3MF.
七嗪衍生物是有前途的电致发光器件掺杂剂。最近的研究提出了七嗪是否呈现小的正则或反转单重态 - 三重态（IST）能隙的问题。有人认为，在IST发射体中，S←T反向系间窜越（RISC）是一个向下的过程，因此不需要热激活，从而能够有效地捕获三重态激子。这些研究中未确定速率常数。对七嗪的激发态性质进行建模具有挑战性，因为荧光和系间窜越（ISC）在一级近似下是对称禁阻的。在这项工作中，我们对七嗪及其衍生物HAP - 3MF的光物理进行了全面的理论研究。电子激发能和振子耦合矩阵元的计算是在密度泛函理论/多参考组态相互作用（DFT/MRCI）理论水平上进行的。我们采用有限差分法来确定非绝热耦合以及自旋 - 轨道耦合和电偶极跃迁矩阵元相对于正则坐标位移的导数。荧光、磷光、内转换（IC）[1]、ISC和RISC的动力学常数是在静态方法的框架内计算的。辐射性S↔S跃迁主要从光学明亮的E'π→π态借用强度，而S↔T（R）ISC由具有n→π特征的E″态介导。测试计算表明，与文献报道一样大的IST能隙会适得其反，并减缓S←T RISC过程。使用绝热DFT/MRCI单重态 - 三重态分裂为 - 0.02 eV，我们发现七嗪核心化合物中振子增强的ISC和RISC很快。然而，预计其光致发光和电致发光量子产率非常低，因为S→S IC有效地淬灭了发光。相比之下，HAP - 3MF中的荧光、IC、ISC和RISC在相似的时间尺度上进行。 [1]注：原文中IC未全拼，翻译时补充完整为“Internal Conversion”。
