Theoretical Study on the Performance of Through-Space Charge-Transfer Thermally Activated Delayed Fluorescence

doi:10.6023/A26040128

Abstract

Through-space charge-transfer thermally activated delayed fluorescence (TSCT-TADF) materials have attracted considerable interest due to their considerable potential for high-performance, particularly non-doped organic light-emitting diodes (OLEDs). Introducing neutral bulky shielding groups into TSCT frameworks has been demonstrated as an effective strategy for developing efficient non-doped emitters; however, the precise molecular-level regulatory mechanisms remain to be fully elucidated. In this study, based on density functional theory (DFT) and the polarizable continuum model (PCM), starting from the experimentally reported high-efficiency molecule 3Ph-SPT, a series of derivative molecules including Ph2Ad-SPT, Ph2Ad-SDT, and Ph2Ad-SST were theoretically designed to elucidate the synergistic regulatory effects of "neutral group engineering" and "donor-atom engineering" on the performance of TSCT-TADF materials. A systematic analysis was conducted on their geometric structures, electronic properties, absorption/emission behaviors, intersystem crossing/reverse intersystem crossing (ISC/RISC) processes, as well as radiative and non-radiative decay dynamics. The results indicate that neutral group engineering—replacing the terphenyl unit with a diphenyladamantyl group—effectively tunes the excited-state energy levels and geometries, reducing the singlet-triplet energy gap to －0.007 eV and suppressing excessive S₁→S₀ geometric relaxation. Further donor-atom engineering exhibits distinct effects: O→2(-CH₃) substitution degrades performance owing to an increased ΔE_ST and weakened spin-orbit coupling (SOC), whereas O→S substitution synergistically enhances SOC (to 0.132 cm⁻¹) and markedly reduces the reorganization energy, thereby elevating the RISC rate to 2.10×10⁷ s⁻¹ and demonstrating superior performance over the prototype molecule, furthermore, preliminary investigations in the thin-film state also expect excellent TADF performance. This work theoretically clarifies the microscopic mechanism of synergistically regulating the performance of TSCT-TADF materials through neutral-group engineering and donor-atom engineering, providing clear theoretical guidance for the rational design of non-doped TADF materials that exhibit both high exciton utilization efficiency and high luminescent efficiency.

Key words: through-space charge transfer, thermally activated delayed fluorescence, neutral-group engineering, donor-atom engineering, density functional theory, excited-state dynamics

Cite this article

Haobo Wan, Wenbin Xie, Lei Yang, Xueqin Ran, Quanyou Feng, Linghai Xie. Theoretical Study on the Performance of Through-Space Charge-Transfer Thermally Activated Delayed Fluorescence[J]. Acta Chimica Sinica, 2026, 84(9): 1475-1486.

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Fig. & Tab. 12

Figure 1. Chemical structure of the investigated molecules

	S₀/S₁
	3Ph-SPT	Ph2Ad-SPT	Ph2Ad-SDT	Ph2Ad-SST
ϕ₁	－60.3/－60.2	－60.6/－66.3	－61.6/－69.5	－61.1/－68.8
ϕ₂	－99.7/－103.0	－101.2/－105.5	－105.3/－122.4	－109.9/－114.3
\|ϕ₁－ϕ₂\|	39.4/42.8	40.6/39.2	43.7/52.9	48.8/45.5
MPP_D	0.035/0.033	0.035/0.034	0.067/0.061	0.052/0.045
MPP_A	0.016/0.016	0.016/0.017	0.016/0.027	0.020/0.022
l	0.530/0.525	0.532/0.527	0.510/0.515	0.544/0.523

Table 1. Selected geometric parameters (ϕ1, ϕ2, l, MPPD, MPPA) of the investigated molecules in toluene solution for the S0 and S1 states (ϕ1 and ϕ2 in (°); l, MPPD, and MPPA in nm)

Figure 2. Ground-state geometric structures of the 3Ph-SPT (a), Ph2Ad-SPT (b), Ph2Ad-SDT (c), and Ph2Ad-SST (d) molecules in toluene solution. (Saturated hydrogen atoms are omitted for clarity)

Figure 3. Geometric changes between the S0, S1, and T1 states for 3Ph-SPT (a), Ph2Ad-SPT (b), Ph2Ad-SDT (c), and Ph2Ad-SST (d)

Figure 4. Frontier molecular orbital distribution of the investigated molecules

	S₀-S₁
	λ_abs/nm	ΔE/eV	Main transition	λ_em/nm	ΔE/eV	Main transition
3Ph-SPT	338	3.67	HOMO→L＋2 (74%)	523	2.37	HOMO→LUMO (98%)
Ph2Ad-SPT	335	3.69	HOMO→L＋3 (91%)	530	2.34	HOMO→LUMO (98%)
Ph2Ad-SDT	284	4.35	H－3→L＋1 (54%)	488	2.54	HOMO→LUMO (97%)
Ph2Ad-SST	283	4.38	HOMO→LUMO (29%)	512	2.42	HOMO→LUMO (98%)

Table 2. Maximum absorption and emission wavelengths, vertical excitation energies, and main orbital contributions of the investigated molecules

Figure 5. Excited-state energy levels of the investigated molecules

Figure 6. Natural transition orbital (NTO) analysis for the S1 and T1 states (based on S0 structures) of 3Ph-SPT (a), Ph2Ad-SPT (b), Ph2Ad-SDT (c), and Ph2Ad-SST (d) in toluene solution

Figure 7. Distribution of electron (pink) and hole (white) wavefunctions for the molecules 3Ph-SPT (a), Ph2Ad-SPT (b), Ph2Ad-SDT (c), and Ph2Ad-SST (d)

	λ_ISC	λ_RISC	λ_S0-S1	E(S₁)*	E(T₁)*	ΔE_ST*	f	SOC
3Ph-SPT	0.009	0.004	34.803	2.731	2.711	0.020	0.0055	0.082
Ph2Ad-SPT	0.035	0.060	33.388	2.705	2.712	－0.007	0.0056	0.086
Ph2Ad-SDT	0.018	0.007	32.953	2.880	2.817	0.063	0.0051	0.054
Ph2Ad-SST	0.015	0.007	41.489	2.849	2.830	0.019	0.0051	0.132

Table 3. Key parameters of excitation dynamics for the investigated molecules (λS1-S0 in kJ/mol, SOC in cm⁻1, and other parameters in eV, * denotes adiabatic energies based on the optimized S1/T1 structures)

	k_ISC/s⁻¹	k_RISC/s⁻¹	k_r/s⁻¹
3Ph-SPT	7.39×10⁶	6.79×10⁶	1.35×10⁶
Ph2Ad-SPT	6.27×10⁶	4.95×10⁶	1.33×10⁶
Ph2Ad-SDT	1.63×10⁵	1.00×10⁴	1.43×10⁶
Ph2Ad-SST	1.74×10⁷	2.10×10⁷	1.30×10⁶

Table 4. Excited-state dynamics rates of the investigated molecules

Figure 8. Photophysical properties of Ph2Ad-SST molecules in thin film. (a) Spatial distribution of HOMO and LUMO and HOMO-LUMO energy gap. (b) Ultraviolet absorption spectrum and fluorescence emission spectrum

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