Tada Kohei, Kitagawa Yasutaka
Research Institute of Electrochemical Energy, Department of Energy and Environment (RIECEN), National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan.
Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan.
Phys Chem Chem Phys. 2023 Nov 29;25(46):32110-32122. doi: 10.1039/d3cp04187e.
The diradical state is an important electronic state for understanding molecular functions and should be elucidated for the design of functional molecules and their application to molecular devices. The density functional theory calculation with plane-wave basis and correction of the on-site Coulomb parameter (DFT+/plane-wave calculation) is a good candidate of high-throughput calculations of diradical-band interactions. However, it has not been investigated in detail to what extent the DFT+/plane-wave calculation can be used to calculate organic diradicals with a high degree of accuracy. In the present study, using typical organic diradical molecules (bisphenalenyl molecules) as model systems, the discrepancy in the optimum values between the two electronic states (open-shell singlet and triplet) that compose the diradical state is detected. The calculated results show that the reason for this value discrepancy is the difference in electronic delocalisation due to π-conjugation between the open-shell singlet and triplet states, and that the effect of discrepancy becomes large as diradical character decreases. This indicates that it is necessary to investigate the value discrepancy with reference to the calculated results by more accurate methods or to experimental values when calculating organic diradicals with low diradical character. For this investigation, the local magnetic moments, unpaired beta electron numbers, and effective magnetic exchange integral values can be used as reference values. For the effective magnetic exchange integral values, the effects of discrepancy are partially cancelled out. However, because the effects may not be completely offset, care should be taken when using the effective magnetic exchange integral value as a reference. Furthermore, a comparison of DFT+ and hybrid-DFT calculations shows that the DFT+ underestimates the HOMO-LUMO gap of bisphenalenyls, although a qualitative discussion of the gap is possible.
双自由基态是理解分子功能的重要电子态，对于功能分子的设计及其在分子器件中的应用而言，该电子态应予以阐明。采用平面波基组并校正在位库仑参数的密度泛函理论计算（DFT+/平面波计算）是双自由基带相互作用高通量计算的一个良好候选方法。然而，关于DFT+/平面波计算在多大程度上可用于高精度计算有机双自由基，尚未进行详细研究。在本研究中，以典型的有机双自由基分子（双苯并菲烯基分子）作为模型体系，检测了构成双自由基态的两个电子态（开壳单重态和三重态）之间最优值的差异。计算结果表明，该值差异的原因是开壳单重态和三重态之间由于π共轭导致的电子离域差异，并且随着双自由基特征的降低，差异效应变大。这表明在计算低双自由基特征的有机双自由基时，有必要参照更精确方法的计算结果或实验值来研究该值差异。对于此研究，局部磁矩、未配对β电子数和有效磁交换积分值可作为参考值。对于有效磁交换积分值，差异效应会部分抵消。然而，由于这些效应可能不会完全抵消，在将有效磁交换积分值用作参考时应谨慎。此外，DFT+和杂化DFT计算的比较表明，尽管对能隙进行定性讨论是可能的，但DFT+低估了双苯并菲烯基的HOMO-LUMO能隙。
