en.Wedoany.com Reported - Chinese researcher Ying Fu and their team have developed a structurally regulated pyrene-imidazole molecular system that enhances the electroluminescent performance of deep-blue organic light-emitting diodes (OLEDs) through precise excited-state engineering. Without introducing entirely new molecular frameworks, the study employed a linkage position regulation strategy within a donor–π–acceptor framework, synthesizing and directly comparing the para-linked isomer p-PyI-PBO and the meta-linked isomer m-PyI-PBO, which share identical donor and acceptor units. This established a clear structure–photophysics–device relationship, elucidating the impact of linkage position on molecular conformation, local excited/charge transfer (LE/CT) balance, HLCT characteristics, and exciton utilization efficiency.
Theoretical calculations revealed that p-PyI-PBO exhibits mixed LE/CT characteristics in the T₃ and T₄ states, with S₁–T₃ and S₁–T₄ energy gaps of 0.12 eV and 0.16 eV, respectively. In contrast, m-PyI-PBO shows HLCT characteristics in the T₂ state, with a smaller S₁–T₂ energy gap of 0.09 eV, indicating a more favorable reverse intersystem crossing (RISC) channel. Spin-orbit coupling (SOC) analysis further confirmed these findings: p-PyI-PBO exhibited ⟨T₃|ĤSOC|S₁⟩ and ⟨T₄|ĤSOC|S₁⟩ values of 0.1204 cm⁻¹ and 0.0860 cm⁻¹, respectively, while m-PyI-PBO showed a ⟨T₂|ĤSOC|S₁⟩ value of 0.1308 cm⁻¹, indicating stronger singlet–triplet coupling. Meanwhile, the large S₁–T₁ energy gaps (approximately 0.64–0.91 eV) for both compounds effectively ruled out the conventional thermally activated delayed fluorescence (TADF) mechanism.
Femtosecond transient absorption (fs-TA) spectroscopy measurements (with an excitation wavelength of 320 nm) revealed the kinetic processes of m-PyI-PBO in solution: ground-state bleaching signals appeared in the 320–370 nm range, stimulated emission from the S₁ state was observed at 400–460 nm, and excited-state absorption signals were located above 460 nm, confirming that the dynamics are dominated by the S₁ state. Kinetic analysis at 544 nm yielded two decay components: τ₁ of 8.97 ps, attributed to ultrafast excited-state relaxation, and τ₂ of 4.09 ns, corresponding to fluorescence decay. Transient features in the 560–600 nm range indicated the presence of higher excited states, potentially involving rapid S₁→Tₙ intersystem crossing or singlet–triplet equilibrium, confirming that emission primarily originates from directly populated singlet states with HLCT characteristics.
In device testing, a non-doped m-PyI-PBO OLED achieved a maximum external quantum efficiency (EQE) of 9.52%, with deep-blue emission and minimal efficiency roll-off at high luminance. The performance of doped devices was further enhanced, achieving a maximum EQE of 13.22%, deep-blue CIE coordinates of (0.16, 0.06), an exciton utilization efficiency of approximately 79.7%, and exhibiting preferential horizontal dipole orientation.
The researchers noted that this achievement represents a significant advancement in pyrene-imidazole (PyI)-based luminescent materials without extending π-conjugation or introducing heavy-atom effects. This study highlights the effectiveness of precise linkage position regulation in activating previously underutilized excited-state pathways. By enabling hot-exciton-assisted triplet-to-singlet conversion while maintaining deep-blue purity and device stability, this strategy provides a generalizable molecular design approach for efficient deep-blue fluorescent OLEDs.
