多共振热激活延迟荧光过程的理论研究

doi:10.6023/A22110472

摘要/Abstract

本工作借助第一性原理和动力学演化, 系统地研究了四个叔丁基-咔唑及吩噻嗪取代的硼-氮化合物(BCz-BN、2PTZ-BN、Cz-PTZ-BN和2Cz-PTZ-BN)的多共振热激活延迟荧光的高效发光机制. 结果表明上述分子T₁与T₂间的内转换速率远大于其它辐射与非辐射速率, 同时T₂到S₁的反向系间窜越速率也高于T₁到S₁的反向系间窜越速率, 因此其多共振热激活延迟荧光过程应遵循T₁→T₂→S₁→S₀的路径. 进一步动力学演化表明, T₁与T₂之间的内转换主要发生在演化初期, 随着时间的推移, 能量逐渐由T₂向S₁转移, 并最终在S₁完成荧光发射. 上述研究揭示了多共振延迟荧光的微观本质, 为未来设计及合成新的多共振热激活延迟荧光分子提供了理论依据.

关键词: 光致发光, 多共振热激活延迟荧光, 密度泛函理论, 激发态, 旋轨耦合, 反向系间窜越

In recent years, the multiple resonance thermally activated delayed fluorescence (MR-TADF) has become a hot topic due to the high fluorescence quantum yield and small Stokes shift. In traditional TADF molecules, the lowest triplet excited state (T₁) can convert into the lowest singlet excited state (S₁) through the reverse intersystem crossing (RISC), and then return to the ground state (S₀) by the radiative fluorescence emission. However, recent studies found that in MR-TADF molecules, the higher triplet excited state (T₂) might play an important role. Thus, the detailed mechanism for the luminescence in MR-TADF molecules, especially for the effect of higher triplet excited state, has still been an indefinite problem. In this work, the luminescence mechanism of four newly reported MR-TADF molecules (BCz-BN, 2PTZ-BN, CZ-PTZ-BN and 2Cz-PTZ-BN) which are substituted by tert-butyl-carbazole and phenothiazine groups has been systematically investigated by the first-principle calculation and kinetic time evolution. The results showed that the phenothiazine group can significantly increase the spin-orbit coupling (SOC) and increase the fluorescence quantum yield. More importantly, by computing the rate constants for the TADF process, we found that the internal conversion (IC) rate between two triplet excited states (T₁ and T₂) is much faster than the radiative and non-radiative rates, and meanwhile the rate of the reverse intersystem crossing of T₂→S₁ is also faster than the one of T₁→S₁. Therefore, the TADF process of the above molecules should follow the T₁→T₂→S₁→S₀ process. The above findings were also confirmed by the further kinetic time evolution. In the early stage, the system was mainly dominated by the internal conversion between T₁ and T₂. With the time evolution, the energy gradually transferred from T₂ to S₁, and finally emitted fluorescence on S₁. However, if T₂ is not involved in the time evolution, the TADF process will be significantly slowed down, which could further confirm the importance of T₂ in the TADF process. This work reveals the nature of the TADF process which could be of great importance for designing and synthesizing new TADF molecules.

Key words: photoluminescence, multiple resonance thermally activated delayed fluorescence, density functional theory, excited state, spin-orbit coupling, reverse intersystem crossing

引用此文

张少秦, 李美清, 周中军, 曲泽星. 多共振热激活延迟荧光过程的理论研究[J]. 化学学报, 2023, 81(2): 124-130.

Shaoqin Zhang, Meiqing Li, Zhongjun Zhou, Zexing Qu. Theoretical Study on the Multiple Resonance Thermally Activated Delayed Fluorescence Process[J]. Acta Chimica Sinica, 2023, 81(2): 124-130.

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图/表 7

图1. BCz-BN、2PTZ-BN、Cz-PTZ-BN和2Cz-PTZ-BN的化学结构

Figure 1. The chemical structures of BCz-BN, 2PTZ-BN, Cz-PTZ-BN and 2Cz-PTZ-BN

图2. B3LYP/6-31G(d)水平下对S1、T1和T2进行自然跃迁轨道(NTO)分析. ∠C1C2N3C4和∠C5C6N7C8为硼-氮π平面左右两侧的二面角

Figure 2. Natural transition orbital (NTO) analysis for S1, T1 and T2 at the B3LYP/6-31G(d) level. ∠C1C2N3C4 and∠C5C6N7C8 are dihedral angles between the left and right sides of the boron-nitrogen π plane

	Method	BCz-BN	2PTZ-BN	Cz-PTZ-BN	2Cz-PTZ-BN	MUE
UV-Vis (S₀→S₁)	B3LYP	2.81	2.65	2.77	2.83	0.12
	CAM-B3LYP	3.29	3.19	3.30	3.36	0.64
	wB97XD	3.35	3.25	3.35	3.41	0.69
	M062X	3.25	3.14	3.25	3.30	0.59
	CC2	2.77	2.54	2.70	2.85	0.13
	Exp.	2.70	2.67	2.58	2.64	—
FL (S₁→S₀)	B3LYP	2.70	2.37	2.46	2.60	0.09
	CAM-B3LYP	3.20	2.92	3.06	3.16	0.63
	wB97XD	3.27	2.98	3.13	3.21	0.70
	M062X	3.15	2.86	2.99	3.09	0.57
	CC2	2.60	2.23	2.40	2.64	0.11
	Exp.	2.53	2.39	2.43	2.46	—
PH (T₁→S₀)	B3LYP	2.34	2.04	2.12	2.12	0.17
	CAM-B3LYP	2.57	2.44	2.50	2.54	0.19
	wB97XD	2.68	2.50	2.58	2.61	0.27
	M062X	2.72	2.45	2.53	2.55	0.24
	CC2	2.52	2.11	2.25	2.42	0.09
	Exp.	2.43	2.21	2.32	2.34	—

表1. 不同泛函计算所得的垂直激发能(UV-Vis)、荧光发射能(FL)和磷光发射能(PH), 单位为电子伏特(eV)

Table 1. The computed vertical excitation (UV-Vis) energy, fluorescence emission (FL) energy and phosphorescence emission (PH) energy with the different functionals (in eV)

	BCz-BN		2PTZ-BN		Cz-PTZ-BN		2Cz-PTZ-BN
	π→π*	n→π*	π→π*	n→π*	π→π*	n→π*	π→π*	n→π*
S₁	98%	0%	85%	15%	87%	13%	84%	16%
T₁	94%	0%	85%	15%	88%	12%	85%	15%
T₂	84%	0%	86%	14%	88%	12%	86%	14%

表2. BCz-BN、2PTZ-BN、Cz-PTZ-BN与2Cz-PTZ-BN四个分子不同激发态下π→π*与n→π*占比

Table 2. The proportion of π→π* and n→π* in different excited states for BCz-BN, 2PTZ-BN, Cz-PTZ-BN and 2Cz-PTZ-BN

图3. (a) BCz-BN, (b) 2PTZ-BN, (c) Cz-PTZ-BN和(d) 2Cz-PTZ-BN的分子速率常数以及能级示意图. k1RISC、k2RISC分别为T1→S1、T2→S1之间的反向系间窜越速率, k1ISC、k2ISC分别为S1→T1、S1→T2之间的系间窜越速率, kIC为T2→T1之间的内转换速率, kRIC为T1→T2之间的反向内转换速率. 括号内为激发态能量. <Sn|HSOC|Tn>为单三态之间的耦合常数

Figure 3. The rate constants and energy levels for (a) BCz-BN, (b) 2PTZ-BN, (c) Cz-PTZ-BN and (d) 2Cz-PTZ-BN. k1RISC and k2RISC are the reverse intersystem crossing rates for T1→S1 and T2→S1, respectively; k1ISC and k2ISC are the intersystem crossing rates for S1→T1 and S1→T2, respectively; kIC is the internal conversion rate for T2→T1 and kRIC is the reverse internal conversion rate for T1→T2. Excited state energy levels are shown in parentheses. <Sn|HSOC|Tn> is the spin-orbit coupling constant between singlet and triplet states

图4. BCz-BN(黑)、2PTZ-BN(红)、Cz-PTZ-BN(蓝)和2Cz-PTZ-BN(粉)电子态演化曲线: (a) 包含T1、T2、S1和S0; (b) 包含T1、S1和S0

Figure 4. The time evolution of population of electronic states (a) with T1, T2, S1 and S0, (b) with T1, S1 and S0 for BCz-BN (black), 2PTZ-BN (red), Cz-PTZ-BN (blue) and 2Cz-PTZ-BN (pink)

图5. (a) BCz-BN, (b) 2PTZ-BN, (c) Cz-PTZ-BN和(d) 2Cz-PTZ-BN激发态的演化. 其中黑色虚线(PL)表示发光过程, 红色实线表示T2, 蓝色实线表示T1, 粉色实线表示S1

Figure 5. The evolution of the population for the excited states of (a) BCz-BN, (b) 2PTZ-BN, (c) Cz-PTZ-BN and (d) 2Cz-PTZ-BN. The black dotted line represents the photoluminescence (PL), T2, T1 and S1 are labeled with solid red line, solid blue line and solid pink line, respectively

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