Key Takeaways & Executive Findings
- •• • D-A dihedral angles below 25° and interfragment distances within 4 Å are established as geometric criteria for high-efficiency TSCT-TADF emitters, validated by both theoretical predictions and experimental evidence. • • The newly designed molecular libraries, using DPXZ donor and QAO acceptor with benzene or carbazole bridges, achieve predicted delayed fluorescence efficiencies up to 96% and an average of 70% in thin film systems. • • The study integrates first-principles calculations, energy decomposition analysis, and statistical modeling across 24 experimentally reported and 54 newly designed TADF molecules, providing a robust theoretical framework for molecular design. • • The rational design strategy based on sub-25° D-A dihedral angles directly addresses the bottleneck of balancing small ΔE_ST and high oscillator strength, enabling efficient triplet harvesting and high PLQY in OLEDs.
Abstract
Intramolecular through-space charge-transfer (TSCT)-enabled thermally activated delayed fluorescence (TADF) emitters have shown exceptional potential for advancing organic light-emitting diode (OLED) technologies, owing to their efficient utilization of triplet excitons and optimized photophysical properties. To date, the intrinsic correlation among molecular geometries, intramolecular non-covalent interactions, and photophysical properties in TSCT-TADF emitters remains unconfirmed, and this study theoretically clarifies this critical correlation. Specifically, through integrating molecular engineering, screening strategies, first-principles calculations, energy decomposition analysis, and statistical modeling, we systematically investigated 24 experimentally reported TADF molecules, and 54 newly designed structures in both solution and thin-film environments. We establish a clear geometric criterion for high-efficiency TSCT-TADF emitters: donor-acceptor (D-A) dihedral angles below 25° and interfragment distances within 4 Å—conditions validated by both theoretical predictions and experimental evidence. Based on this insight, we designed two novel molecular libraries with benzene- or carbazole-derivative bridges, using O-bridged triphenylamine (DPXZ) as the donor and quinolino[3,2,1-de]acridine-5,9-dione (QAO) as the acceptor. Our calculations confirm that sub-25° D-A dihedral angles correlate with exceptional delayed fluorescence efficiency, with predictions reaching up to 96% and an average of 70% for the new thin film systems. This study provides a rational design strategy for high-performance TSCT-TADF emitters, significantly advancing the molecular-level understanding of through-space interactions and accelerating the discovery of tailored, efficient OLED materials.
1. Introduction
Organic light-emitting diodes (OLEDs) incorporating thermally activated delayed fluorescence (TADF) emitters have been extensively investigated in recent years, driven by their ability to harvest triplet excitons via reverse intersystem crossing (RISC). This mechanism enables near-unity internal quantum efficiency (IQE), with state-of-the-art TADF-based OLEDs achieving external quantum efficiencies (EQEs) exceeding 40%. However, efficient triplet-to-singlet conversion through RISC necessitates a small energy gap (ΔE_ST) between the lowest triplet (T1) and singlet (S1) excited states, which typically requires spatial separation of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Yet, such separation often reduces fluorescence oscillator strength (f), leading to low photoluminescence quantum yields (PLQY). This trade-off has hindered the development of high-efficiency TADF emitters.
Recent advances in intramolecular through-space charge transfer (TSCT) TADF emitters offer a novel design paradigm to minimize ΔE_ST while enhancing f by strategically linking donor and acceptor moieties within rigid frameworks. However, the intrinsic correlation among molecular geometries, intramolecular non-covalent interactions, and photophysical properties remains unconfirmed. This study addresses this gap by systematically investigating a large set of experimentally reported and newly designed molecules, establishing clear geometric criteria for high-efficiency TSCT-TADF emitters. The findings provide a rational design strategy that directly tackles the bottleneck of balancing ΔE_ST and f, potentially accelerating the discovery of tailored, efficient OLED materials.
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Sai Guo, Xin Lv, Xue Li, Kang Zhou, Qinwei Chen, Qing Li, Peifeng Su, Lingyi Meng, Can-Zhong Lu (2026). Strategic Dihedral Angle Engineering for High-Efficiency Through-Space Charge Transfer TADF Emitters. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3770-9
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Frequently Asked Questions
What are the specific geometric parameters that ensure high-efficiency TSCT-TADF emitters, and how were they validated?
The study establishes that D-A dihedral angles below 25° and interfragment distances within 4 Å are critical for high-efficiency TSCT-TADF. These criteria were validated by both theoretical predictions and experimental evidence from 24 reported molecules and 54 new designs.
How do the newly designed molecular libraries achieve high delayed fluorescence efficiencies, and what are the predicted values?
The libraries use DPXZ as donor and QAO as acceptor with benzene or carbazole bridges. Calculations show that sub-25° D-A dihedral angles correlate with exceptional delayed fluorescence efficiency, with predictions up to 96% and an average of 70% in thin film systems.
What computational methods were employed to derive the structure-property relationships?
The study integrates first-principles calculations, energy decomposition analysis, and statistical modeling. Specifically, density functional theory (DFT) and time-dependent DFT were used, along with dispersion corrections and solvent models, to accurately predict photophysical properties.
How does this design strategy overcome the traditional trade-off between small ΔE_ST and high oscillator strength?
By controlling the D-A dihedral angle and interfragment distance, the TSCT-TADF emitters achieve spatial separation of HOMO and LUMO to minimize ΔE_ST while maintaining sufficient orbital overlap to preserve oscillator strength, thus enabling high PLQY and efficient RISC.
What is the industrial relevance of these findings for OLED manufacturing?
The rational design strategy provides clear geometric guidelines for synthesizing high-efficiency TADF emitters, potentially reducing trial-and-error in materials development. Achieving predicted efficiencies up to 96% could lead to OLEDs with higher EQE and lower efficiency roll-off, which is critical for commercial display and lighting applications.
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