Local-Excitation versus Charge-Transfer Characters in the Triplet State: Theoretical Insight into the Singlet–Triplet Energy Differences of Carbazolyl-Phthalonitrile-Based Thermally Activated Delayed Fluorescence Materials

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• 作者：Kyungeon Lee ; Dongwook Kim
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：December 15, 2016
• 年：2016
• 卷：120
• 期：49
• 页码：28330-28336
• 全文大小：387K
• ISSN：1932-7455

文摘

The singlet–triplet energy differences, ΔE_ST, of a series of carbazolyl-phthalonitrile (CzPN) derivatives were calculated at the levels of density functional theory (DFT) and time-dependent (TD) DFT using the gap-tuned, range-separated ωB97X functional. The studied CzPN derivatives include 4-(9H-carbazol-9-yl)phthalonitrile (CzPN), 4,5-di(9H-carbazol-9-yl)phthalonitrile (2CzPN), 3,4,5-tris(9H-carbazol-9-yl)phthalonitrile (3CzPN), and 3,4,5,6-tetra(9H-carbazol-9-yl)phthalonitrile (4CzPN). As additional Cz substituents are introduced, both the HOMO–LUMO energy gap, ΔE_H–L, and ΔE_ST continuously decrease. Both natural transition orbital analysis and a quantitative assessment of the local-excitation (LE) and charge-transfer (CT) contributions to the excited states consistently demonstrate that the S₁ states of all of the CzPN derivatives have a predominantly CT nature. In contrast, in the T₁ state, the LE feature is dominant, but the CT character increases with the number of Cz groups. The decomposition of excitation energy reveals that, in addition to the spatial separation of HOMO and LUMO, a significant CT nature in the T₁ state is essential for a further reduction in ΔE_ST. Moreover, the relative proportions of LE and CT characters in the T₁ states of the CzPN derivatives can be modulated with ΔE_H–L.