Pios Sebastian, Huang Xiang, Sobolewski Andrzej L, Domcke Wolfgang
Department of Chemistry, Technical University of Munich, 85747 Garching, Germany.
Institute of Physics, Polish Academy of Sciences, PL-02-668 Warsaw, Poland.
Phys Chem Chem Phys. 2021 Jun 16;23(23):12968-12975. doi: 10.1039/d1cp02026a.
It has recently been shown that cycl[3.3.3]azine and heptazine (1,3,4,6,7,9,9b-heptaazaphenalene) as well as related azaphenalenes exhibit inverted singlet and triplet states, that is, the energy of the lowest singlet excited state (S1) is below the energy of the lowest triplet excited state (T1). This feature is unique among all known aromatic chromophores and is of outstanding relevance for applications in photocatalysis and organic optoelectronics. Heptazine is the building block of the polymeric material graphitic carbon nitride which is an extensively explored photocatalyst in hydrogen evolution photocatalysis. Derivatives of heptazine have also been identified as efficient emitters in organic light emitting diodes (OLEDs). In both areas, the inverted singlet-triplet gap of heptazine is a highly beneficial feature. In photocatalysis, the absence of a long-lived triplet state eliminates the activation of atmospheric oxygen, which is favourable for long-term operational stability. In optoelectronics, singlet-triplet inversion implies the possibility of 100% fluorescence efficiency of electron-hole recombination. However, the absorption and luminescence wavelengths of heptazine and the S1-S0 transition dipole moment are difficult to tune for optimal functionality. In this work, we employed high-level ab initio electronic structure theory to devise and characterize a large family of novel heteroaromatic chromophores, the triangular boron carbon nitrides. These novel heterocycles inherit essential spectroscopic features from heptazine, in particular the inverted singlet-triplet gap, while their absorption and luminescence spectra and transition dipole moments are widely tuneable. For applications in photocatalysis, the wavelength of the absorption maximum can be tuned to improve the overlap with the solar spectrum at the surface of earth. For applications in OLEDs, the colour of emission can be adjusted and the fluorescence yield can be enhanced.
最近的研究表明，环[3.3.3]嗪和七嗪（1,3,4,6,7,9,9b-七氮杂菲）以及相关的氮杂菲呈现出反转的单重态和三重态，即最低单重激发态（S1）的能量低于最低三重激发态（T1）的能量。这一特性在所有已知的芳香族发色团中是独一无二的，对于光催化和有机光电子学的应用具有重要意义。七嗪是聚合材料石墨相氮化碳的结构单元，石墨相氮化碳是析氢光催化中广泛研究的光催化剂。七嗪的衍生物也被确定为有机发光二极管（OLED）中的高效发光体。在这两个领域中，七嗪的反转单重态-三重态能隙都是一个非常有利的特性。在光催化中，不存在长寿命的三重态消除了大气氧的活化，这有利于长期运行稳定性。在光电子学中，单重态-三重态反转意味着电子-空穴复合的荧光效率有可能达到100%。然而，七嗪的吸收和发光波长以及S1-S0跃迁偶极矩难以调节以实现最佳功能。在这项工作中，我们采用高水平的从头算电子结构理论来设计和表征一大类新型杂芳香族发色团，即三角硼碳氮化物。这些新型杂环继承了七嗪的基本光谱特征，特别是反转的单重态-三重态能隙，同时它们的吸收和发光光谱以及跃迁偶极矩具有广泛的可调性。对于光催化应用，可以调节最大吸收波长以改善与地球表面太阳光谱的重叠。对于OLED应用，可以调节发射颜色并提高荧光产率。
