Towards efficient near-infrared fluorescent organic light-emitting diodes

Near-infrared (NIR, 700–1000 nm) emitters are sought for applications including photodynamic therapy, security/defense, and Li-Fi optical networking due to their invisibility to the human eye and the advantages of organic materials such as mechanical flexibility, conformability, and biocompatibility. However, organic NIR emission efficiency is limited by (i) aggregation quenching arising from planar conformations and formation of poorly emissive H-aggregates in extended-conjugation systems, and (ii) the energy-gap law (E_G-law), which predicts an exponential increase in non-radiative decay as the energy gap decreases, typically reducing photoluminescence yield with decreasing E_G. While dilution or molecular design (e.g., polyrotaxanes) can mitigate aggregation, and hybrid perovskites or quantum dots can achieve high EQE, heavy metal content raises toxicity concerns for many applications. Heavy-metal-free strategies leveraging triplet harvesting (e.g., TADF/RISC) have yielded high efficiencies but predominantly at wavelengths <800 nm. Fluorescent porphyrin oligomers with meso-ethyne or meso-butadiyne links offer tunable conjugation enabling emission from visible to >850 nm while retaining solution PL efficiencies around 30%. This work targets the simultaneous mitigation of E_G-law-driven non-radiative losses and aggregation quenching to realize efficient heavy-metal-free NIR OLEDs beyond 800 nm.

- Prior NIR OLEDs have employed rare-earth and transition metal complexes, small molecules, conjugated polymers, and hybrids, but heavy-metal content (perovskites, quantum dots, phosphorescent complexes) limits biocompatible/wearable uses. - Aggregation quenching in planar, long-conjugation emitters has been mitigated via dilution in solid solutions and molecular design, including polyrotaxanes. - The energy-gap law for radiationless transitions predicts exponentially increasing non-radiative rates with decreasing energy gap, often correlating with reduced PLQY; experimental disentanglement from aggregation effects is challenging. - Heavy-metal-free NIR emission leveraging triplet upconversion (TADF/RISC) has achieved high efficiency but typically at <800 nm. - Fluorescent porphyrin oligomers with meso-ethyne or meso-butadiyne linkages provide strong intramolecular coupling, tunable emission up to >850 nm, and decent solution PLQYs (~30%).

- Materials and molecular design: Synthesized a series of linear meso-butadiyne-linked zinc porphyrin oligomers I-PN(THS) (N = 1–6 units). Added trihexylsilyl (THS) side chains on aryl groups to provide steric hindrance and suppress π–π aggregation. The closed-shell Zn(II) centers do not strongly affect photophysics; analogous Mg(II) or free-base oligomers expected to be similar. - Optical characterization in solution: Measured absorption and photoluminescence (PL) spectra of I-PN(THS) in toluene at ~1×10^-6 M. Excitation wavelengths: 450 nm for steady-state PL spectra; 405 nm for time-resolved PL. PL lifetimes obtained from mono-exponential decays. Derived radiative (k_r) and non-radiative (k_nr) rates from PLQY and lifetime (τ) via k_r = PLQY/τ and k_nr = (1−PLQY)/τ. Analyzed dependence of ln(k_r) and ln(k_nr) on energy gap (E_g) from spectral peak positions. - Solid-state host selection and blends: Chose polymer hosts F8BT and TFB based on charge-transport properties and spectral overlap; selected F8BT as optimal. Prepared F8BT:I-P6(THS) blend films with various hexamer concentrations (e.g., 1.0%, 2.5%, 5.0%). Recorded absorption and PL spectra; quantified PLQY over full spectrum and fraction of NIR emission (>700 nm). Assessed spectral shifts and narrowing indicating increased planarity/reduced torsional disorder in solid state. - Device fabrication: OLED stack: ITO substrate / PEDOT:PSS hole-transport layer / TFB electron/exciton blocking layer / F8BT:I-P6(THS) NIR-emitting layer / Ca/Al cathode. Energy level alignment summarized from literature. Evaluated electroluminescence (EL) spectra, peak wavelength, and external quantum efficiency (EQE). Reported average and best-device EQEs. - Modeling: Developed a quantitative model for internal quantum efficiency (IQE) in guest–host blends where the host supports triplet-to-singlet conversion via triplet–triplet annihilation (TTA) and/or thermally activated delayed fluorescence (TADF). Compared model predictions with time-resolved EL to infer contributions from TADF in addition to TTA.

- Suppressed energy-gap-law behavior: The increase of non-radiative rate (k_nr) with decreasing E_g is dramatically reduced compared to prior fluorophores/phosphors; the logarithmic rate of variation in k_nr vs E_g is suppressed by about an order of magnitude relative to previous reports (~10 eV^-1). From data of ln(k) vs E_g, the absolute slope of ln(k_r) is −0.86 ± 0.16 eV^-1, highlighting differing trends for k_r and k_nr with E_g. - Enhanced radiative rate with oligomer length: Oscillator strength of the Q_x transition increases with N, boosting k_r as E_g decreases via stronger intramolecular electronic coupling and transition dipole alignment. - High PLQY despite smaller gaps: In toluene solutions, PLQY saturates at ~28 ± 3% for N > 2, over three times the monomer PLQY (8%), despite red-shifted emission (630 to 800 nm). PL lifetimes are singlet-like and decrease with N (~1.9 ns for N=1 down to ~1.0 ns for N>3) due to increases in both k_r and k_nr. - NIR emission fraction: Fraction of emitted photons >700 nm rises from 13% (monomer) to >90% (dimer), reaching 100% for the hexamer. - Solid-state spectral changes: In F8BT blends, the I-P6(THS) Q_x absorption band red-shifts by ~75 nm relative to solution and narrows by ~50 nm, with redistributed oscillator strength, consistent with increased planarity and reduced torsional heterogeneity in the solid state. - Blend PL performance: Representative PLQYs and NIR fractions for F8BT:I-P6(THS) films: F8BT neat QY = 0.54 (NIR ~3%); 1.0% I-P6 QY = 0.17, NIR = 52%; 2.5% QY = 0.13, NIR = 62%; 5.0% QY = 0.06, NIR = 83%. - Device performance: OLEDs with F8BT:I-P6(THS) emitters exhibit EL peaking at ~850 nm. Average maximum EQE ~1.1%, with best devices reaching up to 3.8%, representing state-of-the-art for heavy-metal-free fluorescent emitters beyond 800 nm. - Triplet-to-singlet conversion in host: A quantitative IQE model and time-resolved EL support the presence of TADF in addition to TTA in the host, enhancing singlet formation beyond spin-statistical limits.

The study addresses two critical barriers for NIR organic emitters: aggregation quenching and energy-gap-law-driven non-radiative losses. By extending conjugation in meso-butadiyne-linked porphyrin oligomers and adding bulky THS side chains, the design suppresses intersystem crossing (via mismatch in spatial extent of singlet vs triplet excitons and reduced exciton–vibration coupling) and aggregation. The result is an atypical trend where PLQY does not diminish with decreasing E_g, contrary to the common corollary of the E_G-law, due to simultaneous enhancement of k_r and suppression of increases in k_nr. Embedding the optimized hexamer I-P6(THS) into F8BT provides efficient energy transfer and benefits from host-mediated triplet-to-singlet conversion (TTA and TADF), increasing singlet yields during electrical excitation. The observed EL at 850 nm with high EQE benchmarks demonstrates that heavy-metal-free fluorescent emitters can perform efficiently deep in the NIR when both molecular engineering and host selection are optimized. The solid-state-induced planarity improves spectral purity and NIR fraction, while side-chain engineering mitigates aggregation losses. The modeling framework further clarifies the role of TADF alongside TTA in enhancing device IQE, in agreement with time-resolved EL signatures.

This work presents a generalizable strategy for high-luminance, heavy-metal-free NIR fluorescent emitters by concurrently mitigating energy-gap-law limitations and aggregation quenching. Linear meso-butadiyne-linked porphyrin oligomers with THS side chains exhibit suppressed growth of non-radiative rates with decreasing energy gap and increased radiative rates via enhanced oscillator strength. In F8BT host matrices, I-P6(THS)-based OLEDs deliver EL at ~850 nm with average EQE ~1.1% and peak EQE up to 3.8%, the highest reported for heavy-metal-free fluorescent emitters beyond 800 nm. A quantitative IQE model incorporating TTA and TADF, supported by time-resolved EL, elucidates triplet-to-singlet upconversion contributions in the host. Future work could optimize host–guest energy alignment and charge balance, explore alternative side-chain motifs to further suppress aggregation at higher loadings, extend oligomer design to tailor E_g and oscillator strength, and investigate long-term operational stability and bio-compatibility for application-specific deployment.

- While the increase of non-radiative rates with decreasing energy gap is strongly suppressed, the solution PLQY plateaus at ~28%, leaving room for further improvement. - In solid-state blends, PLQY decreases with increasing dopant concentration (e.g., 0.17 at 1.0% to 0.06 at 5.0%), indicating residual aggregation or non-radiative pathways at higher loadings. - Devices based on TFB hosts underperform compared to F8BT, highlighting sensitivity to host selection and suggesting incomplete optimization of charge balance and energy transfer pathways. - The IQE model supports contributions from both TTA and TADF, but their relative weights are not fully disentangled experimentally. - Long-term operational stability and environmental robustness were not detailed in the provided text.