Key Takeaways & Executive Findings
- •• • The multiple-excitation configuration, involving HOMO→LUMO and HOMO→LUMO+1 singlet configurations, lowers the S1 energy, reducing ΔE_ST compared to the single-excitation picture where ΔE_ST is governed by 2K_HL. • • The empirical expression ΔE_ST ≈ f(2K_HL, ΔE_LUMO–LUMO+1) was validated against a test set of MR-type emitters, indicating that the energy gap between LUMO and LUMO+1 is a key parameter for tuning ΔE_ST. • • For IV-DABNA, STEOM-DLPNO-CCSD calculations quantified the contributions of excitation configurations to S1 and T1 states, providing a concrete example of how multiple-excitation effects manifest in a real MR-TADF emitter. • • This design principle offers a new strategy to achieve small ΔE_ST in MR-TADF emitters, which is essential for efficient reverse intersystem crossing (RISC) and high OLED efficiency, potentially overcoming the limitations of conventional single-excitation approaches.
Abstract
The pursuit of efficient organic light-emitting diodes (OLEDs) has been significantly advanced by thermally activated delayed fluorescence (TADF) materials, particularly those employing multiple-resonance (MR) effects. However, achieving small singlet–triplet energy gaps (ΔE_ST) in MR-TADF emitters remains a critical challenge. This work introduces a fundamental design principle based on multiple-excitation configurations to reduce ΔE_ST. In the single-excitation case, both S1 and T1 states are described by a simple HOMO→LUMO excitation, leading to a large exchange energy (2K_HL). In contrast, the multiple-excitation case incorporates electronic interaction between singlet configurations (1Φ_H→L and 1Φ_H→L+1), which lowers the S1 energy and reduces ΔE_ST. The authors propose an empirical expression ΔE_ST ≈ f(2K_HL, ΔE_LUMO–LUMO+1) and validate it using a test set of MR-type emitters. For the representative emitter IV-DABNA, excited-state energies and difference densities calculated at the STEOM-DLPNO-CCSD level reveal the contributions of excitation configurations to S1 and T1 states. This work provides a new avenue for molecular engineering of MR-TADF emitters, potentially stimulating renewed interest in excited-state design principles for future OLED technologies.
1. Introduction
Conventional TADF emitters rely on charge-transfer states to minimize exchange energy, but often suffer from broad emission and poor color purity. Multiple-resonance (MR) TADF materials, such as DABNA derivatives, achieve narrowband emission through alternating boron and nitrogen atoms, yet they typically exhibit larger ΔE_ST than desired, limiting their efficiency in deep-blue OLEDs. The fundamental bottleneck lies in the single-excitation description, where S1 and T1 are both dominated by HOMO→LUMO transitions, resulting in a large exchange energy (2K_HL) that hinders efficient upconversion.
This work addresses this bottleneck by introducing a multiple-excitation configuration that couples the HOMO→LUMO singlet with the HOMO→LUMO+1 singlet, effectively lowering S1 and reducing ΔE_ST. The authors propose an empirical relationship linking ΔE_ST to the exchange energy and the LUMO–LUMO+1 gap, and validate it across a series of MR emitters. This approach provides a concrete design rule for achieving small ΔE_ST without sacrificing color purity, offering a pathway to more efficient and stable OLEDs.
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Walia R, Xiong X, Fan XC, et al. (2026). Multiple-Excitation Configurations Reduce Singlet–Triplet Energy Gaps in Multiple-Resonance Thermally Activated Delayed Fluorescence Emitters. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3651-1
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Frequently Asked Questions
How does the multiple-excitation configuration quantitatively reduce ΔE_ST compared to the single-excitation case?
In the single-excitation case, ΔE_ST is approximately 2K_HL, the exchange energy between HOMO and LUMO. In the multiple-excitation case, the interaction between the HOMO→LUMO and HOMO→LUMO+1 singlet configurations introduces an additional stabilization term (ΔE_corr) that lowers S1, thereby reducing ΔE_ST. The empirical expression ΔE_ST ≈ f(2K_HL, ΔE_LUMO–LUMO+1) suggests that a smaller LUMO–LUMO+1 gap enhances this stabilization, leading to smaller ΔE_ST.
What is the role of the LUMO+1 orbital in achieving small ΔE_ST in MR-TADF emitters?
The LUMO+1 orbital participates in the multiple-excitation configuration, enabling electronic interaction that lowers S1. The energy gap between LUMO and LUMO+1 (ΔE_LUMO–LUMO+1) is a critical parameter: a smaller gap facilitates stronger interaction and greater reduction in ΔE_ST. This provides a design handle for molecular engineering.
How was the empirical expression validated, and what is its predictive power?
The expression was validated using a test set of MR-type emitters, comparing predicted ΔE_ST values with experimental measurements. The agreement indicates that the expression captures the essential physics, allowing for rapid screening of candidate molecules without expensive calculations.
What are the implications for the design of deep-blue OLED emitters?
Deep-blue OLEDs require both high color purity and high efficiency. MR-TADF emitters offer narrowband emission, but their ΔE_ST often limits RISC efficiency. By applying the multiple-excitation principle, one can reduce ΔE_ST while maintaining the narrow emission, potentially achieving high-efficiency deep-blue OLEDs with improved stability.
Are there any limitations or trade-offs with this approach?
The approach relies on the availability of suitable LUMO+1 states and may require careful tuning of molecular orbitals. Additionally, the empirical expression is validated on a limited test set; further studies are needed to confirm its generality across diverse MR-TADF architectures.
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