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The development of efficient and biocompatible organic near-infrared emitters is attractive for many applications, spanning from photodynamic therapy [1] to light fidelity (Li-Fi) all-optical networking systems. [2][3][4] In particular, the range 700-1000 nm is interesting for medical applications, given the semitransparency of biological tissue in this spectral interval, [5] and we will specifically refer to this range as near-infrared (NIR) in the following text. Compared to conventional inorganic materials, organic NIR emitters are interesting also for their mechanical conformability, which makes them appealing for the integration in flexible and stretchable devices. [6] Furthermore, the metal-free organic light-emitting materials can be a cheap and biocompatible alternative to inorganic ones for application in wearable, implantable, or in vivo medical applications, such as for sensing of body temperature, heart and respiration rates, blood pressure, glucose level, and oxygenation. [7] In the search for ever-higher efficiencies, several classes of materials have been investigated, such as perovskite-structured methylammonium lead halides, [8][9][10] quantum dots, [11] and organometallic phosphorescent complexes. [12][13][14][15][16][17][18][19] However, although such hybrid materials afford substantial electroluminescence (EL) external quantum efficiency (EQE) in the NIR, in some cases exceeding 10% [8,10] or even 20% or so, [13] their use of heavy, toxic, and/or costly metals is not ideal for manufacturing, sustainability, environmental impact, and, in perspective, biocompatibility. Furthermore, in such hybrid systems, and in general in materials that leverage triplet excitons to boost the EQE, [20,21] exciton recombination dynamics typically fall in the hundreds of nanoseconds or even in the microsecond (or longer) range, which intrinsically limits the bandwidth when integrated in devices for telecommunications. For Li-Fi applications, [2][3][4] fluorescent molecular and polymeric materials are preferred, given that the typical fluorescence lifetime of these materials is of the order of few nanoseconds or less, thereby ideally allowing data transmission rates up to the Gb s −1 regime.In the last decade, scientists have attempted different strategies to develop heavy-metal-free NIR fluorescent organic light-emitting diodes (OLEDs), with chemical design essentially revolving around the careful combination of donor and acceptor groups to both tune the spectral range (up to 1000 nm) and maximize the EQE. [22][23][24][25][26][27][28][29][30][31][32][33] Very recently, we have, for Due to the so-called energy-gap law and aggregation quenching, the efficiency of organic light-emitting diodes (OLEDs) emitting above 800 nm is significantly lower than that of visible ones. Successful exploitation of triplet emission in phosphorescent materials containing heavy metals has been reported, with OLEDs achieving remarkable external quantum efficiencies (EQEs) up to 3.8% (peak wavelength > 800 nm). For OLEDs incorporating f...

The energy gap law (EG-law) and aggregation quenching are the main limitations to overcome in the design of near-infrared (NIR) organic emitters. Here, we achieve unprecedented results by synergistically addressing both of these limitations. First, we propose porphyrin oligomers with increasing length to attenuate the effects of the EG -law by suppressing the non-radiative rate growth, and to increase the radiative rate via enhancement of the oscillator strength. Second, we design side chains to suppress aggregation quenching. We find that the logarithmic rate of variation in the non-radiative rate vs. EG is suppressed by an order of magnitude with respect to previous studies, and we complement this breakthrough by demonstrating organic light-emitting diodes with an average external quantum efficiency of ~1.1%, which is very promising for a heavy-metal-free 850 nm emitter. We also present a novel quantitative model of the internal quantum efficiency for active layers supporting triplet-to-singlet conversion. These results provide a general strategy for designing high-luminance NIR emitters.