Near-infrared (NIR) emitters (700 nm < λ < 1000 nm) are crucial for applications ranging from photodynamic therapy to Li-Fi networking. Organic NIR emitters offer advantages over inorganic counterparts due to their flexibility, conformability, and biocompatibility. However, their efficiency is limited by the energy gap law (*E_G*-law), which dictates an exponential increase in non-radiative transition rates as the energy gap decreases, and aggregation quenching, which reduces emission due to intermolecular interactions. Current research often focuses on rare-earth and transition metal complexes, but these often contain toxic heavy metals. Heavy-metal-free approaches, such as using triplets or thermally activated delayed fluorescence (TADF), have achieved NIR emission but mainly at wavelengths below 800 nm. This study explores fluorescent porphyrin oligomers, whose conjugation length can be tuned to achieve NIR emission while maintaining high photoluminescence efficiencies in solution.

Existing research on NIR organic light-emitting diodes (OLEDs) primarily uses rare-earth and transition metal complexes, small molecules, conjugated polymers, and their combinations. However, the efficiency of these materials is hindered by the energy gap law and aggregation quenching. Strategies to mitigate aggregation include diluting chromophores in solid solutions or employing molecular design techniques such as threading into cyclodextrin rings. While hybrid organic/inorganic materials like perovskites and quantum dots offer high efficiency, their heavy metal content limits their applicability. Heavy-metal-free approaches leveraging triplets or TADF have shown some success, but high efficiencies have been limited to wavelengths below 800 nm. Fluorescent porphyrin oligomers with tunable conjugation lengths represent a promising alternative.

This research investigated a series of linear meso-butadiyne-linked zinc porphyrin oligomers (I-PN(THS)) with varying lengths (1–6 repeat units). The meso-butadiyne bridges enable intramolecular electronic coupling, delocalizing the singlet exciton and suppressing intersystem crossing (ISC). The trihexylsilyl (THS) side chains prevent aggregation quenching. The optical properties of the oligomers were studied in diluted toluene solutions and in solid-state thin films blended with poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) or poly(9,9-dioctylfluorene-alt-N-(4-sec-butyl-phenyl)-diphenylamine) (TFB) polymer hosts. Absorption and photoluminescence (PL) spectra were measured. PL quantum yield (PLQY) and lifetimes were determined. A quantitative model was developed to describe the quantum efficiency of devices incorporating a guest-host emitter blend, considering triplet–triplet annihilation (TTA) and TADF.

The study found that increasing the length of the porphyrin oligomers significantly increased the oscillator strength of the Qx transition, leading to a redshift of the emission wavelength and increased radiative rate. Importantly, the increase in the non-radiative rate with decreasing energy gap was significantly slower than expected based on the energy gap law, suggesting that the suppression of ISC plays a crucial role. The THS side chains effectively prevented aggregation quenching. OLEDs incorporating the hexamer (I-P6(THS)) in F8BT exhibited an average maximum external quantum efficiency (EQE) of 1.1%, reaching up to 3.8% in the best devices. This represents the highest EQE reported for a heavy-metal-free NIR fluorescent emitter above 800 nm. The developed quantitative model supports the presence of TADF in addition to TTA as a mechanism for generating singlets via triplet conversion, further explaining the high efficiency.

The results demonstrate a successful strategy for designing efficient heavy-metal-free NIR OLEDs. The combination of increasing the porphyrin oligomer length to enhance radiative rate and suppress non-radiative decay, along with the addition of THS side chains to prevent aggregation, effectively counters the limitations imposed by the energy gap law. The high EQEs achieved highlight the potential of this approach for various applications. The developed model provides insights into the device physics, confirming the contribution of TADF in addition to TTA, which contributes to improved efficiency. Further optimization of the device architecture and material selection could lead to even higher efficiencies.

This study presents a significant advancement in the development of efficient heavy-metal-free NIR OLEDs. By synergistically addressing the energy gap law and aggregation quenching, the researchers achieved unprecedented EQEs for a fluorescent NIR emitter. The results showcase a promising design strategy for high-performance NIR emitters, opening avenues for applications in various fields. Future research could focus on exploring different polymer hosts and optimizing device structures to further enhance efficiency and stability.

The study primarily focuses on a specific type of porphyrin oligomer and polymer host. The generalizability of the findings to other materials needs further investigation. The developed model, while providing valuable insights, is a simplification of the complex processes within the OLED device. More sophisticated models incorporating additional factors could provide a more comprehensive understanding.