To address the long-standing challenge of balancing narrowband emission and efficient reverse intersystem crossing (RISC) in multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters, Duan and co-workers have developed a novel phosphorus-carbon-bridged cyclization strategy. This work, recently reported, synergistically enhances molecular rigidity and spin-orbital coupling (SOC) via phosphorus/sulfur heavy-atom effects, yielding high-performance blue emitters with exceptional color purity and electroluminescence efficiency. Below is a concise overview of the study’s background, theoretical calculations, experimental spectroscopy, and device performance.
A critical dichotomy in MR-TADF research
MR-TADF emitters, featuring alternating electron-rich nitrogen and electron-deficient boron atoms, enable narrowband emission and small singlet-triplet energy gaps (Δ E ST ) via short-range charge transfer (SRCT). However, their low intrinsic RISC rates (10 3 –10 4 s −1 ) cause severe efficiency roll-off in organic light-emitting diodes (OLEDs). While intramolecular cyclization has emerged as a key strategy to rigidify molecular skeletons and narrow emission spectra, existing designs face a trade-off: cyclized derivatives either retain low RISC rates or suffer from drastically broadened emission (e.g., sulfur-bridged emitters with FWHM > 50 nm). This dichotomy drives the quest for novel cyclization motifs that simultaneously optimize spectral precision and triplet harvesting.
Theoretical calculations: synergistic optimization and upconversion acceleration via phosphorus bridging
Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have been employed to validate the design rationale. First, frontier molecular orbital (FMO) analysis confirmed that the two new emitters—BCzBN-PO with a phenylphosphine oxide bridge and BCzBN-PS with a phenylphosphine sulfide bridge—retained the characteristic alternating localization of highest occupied molecular orbitals (HOMOs) on nitrogen atoms and lowest unoccupied molecular orbitals (LUMOs) on boron atoms, preserving the MR-TADF core feature. Second, phosphorus bridging effectively rigidified the molecular skeleton, suppressing high-frequency C–C scissoring and rocking vibrations (around 1450, 1510, and 1650 cm −1 ), which reduced reorganization energy ( λ ≈ 0.11 eV compared to 0.12 eV for the parent BCzBN) and laid the foundation for narrowband emission. Natural bond orbital (NBO) analysis further revealed π-σ* and n*-π* hyperconjugation between the phosphorus bridge and the MR core, weakening the electron-donating ability of carbazole substituents and inducing blue-shifted emission. More importantly, phosphorus (P) and sulfur (S) atoms introduced significant heavy-atom effects, with the SOC matrix element between S 1 and T 1 states (⟨S 1 |H SOC |T 1 ⟩) increasing from 0.058 cm −1 for the parent BCzBN to 0.071 cm −1 for BCzBN-PO and drastically to 2.86 cm −1 for BCzBN-PS, promising accelerated RISC processes. Theoretical predictions also indicated blue-shifted S 1 energy levels for BCzBN-PO (2.85 eV) and BCzBN-PS (2.82 eV) relative to BCzBN (2.78 eV), consistent with the electron-withdrawing nature of P(V) fragments.
Experimental spectroscopy: narrowband emission and high RISC rates
Spectroscopic measurements in dilute toluene (1×10 −5 mol L −1 ) validated the theoretical insights. The two emitters exhibited sharp blue fluorescence peaks, with BCzBN-PO peaking at 467 nm and BCzBN-PS at 474 nm, and both achieved ultra-narrow FWHMs of 19 nm and 18 nm respectively—slightly narrower than the parent BCzBN. Small Stokes shifts (11–12 nm) and weak solvatochromism (7–8 nm redshift when switching from low-polarity cyclohexane to high-polarity dichloromethane) confirmed the SRCT character of the emission. Under nitrogen atmosphere, both emitters reached near-unity photoluminescence quantum yields (PLQYs), with 99% for BCzBN-PO and 98% for BCzBN-PS. Under air, the PLQYs dropped to 26% and 8%, indicating high delayed emission ratios of 73% and 90% respectively—a signature of efficient TADF behavior. Transient photophysical measurements further revealed fast RISC rates ( k RISC ) of 1.1×10 5 s −1 for BCzBN-PO and 8.5×10 5 s −1 for BCzBN-PS, nearly two orders of magnitude higher than conventional MR-TADF emitters. This significant enhancement, particularly pronounced for BCzBN-PS, was attributed to the synergistic heavy-atom effect of phosphorus and sulfur.
Device performance: ultra-high efficiency and narrowband electroluminescence
OLEDs with both non-sensitized and TADF-sensitized emitting layers (EMLs) have been fabricated to evaluate the practical application potential of the emitters. Non-sensitized devices with binary EMLs (2 wt% dopant in PPF host) exhibited narrowband emission with FWHM < 30 nm and maximum external quantum efficiencies (EQEs) exceeding 20%, where BCzBN-PS-based devices showed lower efficiency roll-off due to their faster RISC rate. To optimize exciton harvesting, the team further constructed TADF-sensitized devices by incorporating a TADF sensitizer (4tCzBN), which achieved cutting-edge performance: BCzBN-PO-based devices delivered a maximum EQE of 41.2% with a FWHM of 25 nm and CIE coordinates (0.13, 0.15), while BCzBN-PS-based devices set a higher benchmark with a maximum EQE of 43.0%, a narrower FWHM of 23 nm, CIE coordinates (0.12, 0.16), and only 30.1% efficiency roll-off at 1000 cd m −2 (compared to 25.9% for BCzBN-PO).
The relevant results were published in Science Bulletin .
The doctoral candidate Tianjiao Fan from the Department of Chemistry and the doctoral candidate Qiwei Liu from Laboratory of Flexible Electronics Technology, Tsinghua University, are the first authors of the paper, who contributed equally to the work. Professor Lian Duan from the Department of Chemistry/Laboratory of Flexible Electronics Technology, Tsinghua University, and Associate Researcher Dongdong Zhang from the the Department of Chemistry, Tsinghua University are the co-corresponding authors.
Science Bulletin
Experimental study