Theoretical Study on the Multiple Resonance Thermally Activated Delayed Fluorescence Process

doi:10.6023/A22110472

Abstract

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

Cite this article

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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Fig. & Tab. 7

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

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	—

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%

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

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

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)

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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多共振热激活延迟荧光过程的理论研究