Enhancing Multi-Resonance Thermally Activated Delayed Fluorescence Emission via Through-Space Heavy-Atom Effect

Organic Polymeric Materials Organic Chemistry Physical Chemistry Chemistry Materials Science Materials Chemistry
Multi-Resonance Thermally Activated Delayed Fluorescence Heavy-Atom Effect Organic Light-Emitting Diodes Narrow-Band Emission Efficiency Roll-Off

Academic Background

Organic light-emitting diode (OLED) technology has made remarkable progress in recent years, especially in the field of thermally activated delayed fluorescence (TADF) materials. TADF materials achieve high-efficiency emission by converting triplet excitons into singlet excitons through the reverse intersystem crossing (RISC) process. However, traditional TADF materials often face issues of efficiency roll-off and spectral broadening, particularly in multi-resonance (MR) TADF materials. MR-TADF materials achieve narrowband emission by introducing electron-rich nitrogen atoms and electron-deficient boron atoms, which reduce structural relaxation. Nevertheless, such materials typically have a low RISC rate (kRISC), resulting in efficiency roll-off problems.

To address this issue, researchers have proposed enhancing the spin-orbit coupling (SOC) effect by introducing heavy atoms (such as bromine, iodine, sulfur, selenium, etc.), thereby accelerating the RISC process. Traditional approaches for introducing heavy atoms usually involve direct conjugation to the MR chromophore via conjugated pathways, which often leads to spectral broadening and redshift. Therefore, researchers are exploring a new strategy of introducing heavy atoms through short-range spatial interactions, referred to as the “intramolecular external heavy-atom effect,” to avoid the negative impacts associated with conjugation pathways.

Source of the Paper

This paper was jointly authored by Qi Zheng, Yang-Kun Qu, Peng Zuo, and others from the Institute of Functional Nano & Soft Materials (FUNSOM) at Soochow University. The paper was published in the journal Chem on April 10, 2025, with the title “Enhancing Multi-Resonance Thermally Activated Delayed Fluorescence Emission via Through-Space Heavy-Atom Effect.”

Research Process and Results

1. Molecular Design and Synthesis

The researchers designed a series of MR-TADF molecules based on spiro skeletons, incorporating heavy atoms via short-range spatial interactions. Specifically, they attached a MR chromophore (BNCZ) at the C1 site of the spiro skeleton and anchored heavy-atom groups (such as sulfur, selenium, etc.) at the center of the spiro core. To validate this design, five molecules were synthesized: CH2-SFBN, O-SFBN, S-SFBN, Se-SFBN, and CO-SFBN, each corresponding to different heavy or light atom modifications.

The synthesis process involved lithium-halogen exchange, nucleophilic reactions, and Friedel-Crafts ring closure reactions. The structures of all compounds were characterized by nuclear magnetic resonance (NMR) and mass spectrometry (MALDI-TOF), and exhibited good thermal stability.

2. Single-Crystal Structure and IGMH Analysis

To investigate the short-range spatial interactions between the heavy atoms and MR chromophores, the researchers performed single-crystal X-ray diffraction analysis on these compounds. Results showed that the vertical distances between the heavy atoms and MR chromophores in all these molecules were less than 3 Å, indicating a significant steric hindrance effect. In addition, independent gradient model based on Hirshfeld partition (IGMH) analysis further confirmed the spatial interactions between heavy atoms and MR chromophores.

3. Theoretical Calculations

The researchers used density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to study the effects of heavy-atom modification on molecular geometry and optoelectronic properties. The results indicated that the introduction of heavy atoms significantly enhanced the SOC effect, particularly for Se-SFBN, whose SOC matrix element () was one order of magnitude higher than that of other molecules.

4. Photophysical Properties

The absorption, fluorescence, and phosphorescence spectra of these molecules in dilute toluene solutions were measured. All molecules showed narrowband emission, with full width at half maximum (FWHM) in the range of 23-25 nm and emission peaks between 488-492 nm. In addition, the delayed lifetimes (τD) of these molecules were significantly shortened, especially for Se-SFBN, whose τD was only 8.98 μs, and the kRISC reached 1.05 × 10^5 s^-1, two orders of magnitude higher than those of light-atom-modified molecules.

5. Electrochemical Properties

Through cyclic voltammetry (CV), the researchers estimated the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of these molecules. The results demonstrated that heavy-atom modification had little impact on these molecules’ energy levels, which was consistent with the photophysical properties of the MR chromophore.

6. OLED Device Performance

The researchers fabricated OLED devices using these compounds as the emitting layer and measured their electroluminescent (EL) performances. Results showed that S-SFBN and Se-SFBN achieved maximum external quantum efficiencies (EQEmax) of 36.6% and 35.6%, respectively, with significantly reduced efficiency roll-off. Especially, Se-SFBN maintained an EQE of 22.1% at a brightness of 1000 cd/m^2, demonstrating excellent device performance.

Conclusions and Significance

This study successfully achieved efficient narrowband emission in MR-TADF materials and significantly reduced efficiency roll-off by introducing heavy atoms via short-range spatial interactions. The researchers’ proposal of an “intramolecular external heavy-atom effect” provides a new avenue for designing efficient OLED materials. Additionally, this work demonstrates the potential of spatial interactions in tuning photophysical properties, laying a foundation for the future development of more efficient luminescent materials.

Research Highlights

  1. Novel Molecular Design: Introduction of heavy atoms via short-range spatial interactions avoids spectral broadening and redshift problems caused by conventional conjugated pathways.
  2. Efficient RISC Process: Heavy-atom modification significantly enhanced the SOC effect and accelerated the RISC process, especially with Se-SFBN achieving a kRISC of 1.05 × 10^5 s^-1.
  3. Outstanding OLED Performance: S-SFBN and Se-SFBN achieved maximum external quantum efficiencies of 36.6% and 35.6%, respectively, with significant reductions in efficiency roll-off.
  4. Theoretical Validation: Through DFT and TD-DFT calculations, the researchers systematically analyzed the impact of heavy-atom modification on the optoelectronic properties, providing theoretical support for the experimental results.

Other Valuable Information

The researchers also conducted detailed studies on the spatial interactions between heavy atoms and MR chromophores using single-crystal X-ray diffraction and IGMH analysis, providing a structural basis for understanding the intramolecular external heavy-atom effect. Furthermore, this study highlighted the important role of the spiro skeleton in regulating molecular geometry and optoelectronic properties, offering reference for the future design of novel luminescent materials.