Progresses and Perspectives of Near-Infrared Emission Materials with “Heavy Metal-Free” Organic Compounds for Electroluminescence

Organic/polymer light-emitting diodes (OLEDs/PLEDs) have attracted a rising number of investigations due to their promising applications for high-resolution fullcolor displays and energy-saving solid-state lightings. Near-infrared (NIR) emitting dyes have gained increasing attention for their potential applications in electroluminescence and optical imaging in optical tele-communication platforms, sensing and medical diagnosis in recent decades. And a growing number of people focus on the “heavy metal-free” NIR electroluminescent materials to gain more design freedom with cost advantage. This review presents recent progresses in conjugated polymers and organic molecules for OLEDs/PLEDs according to their different luminous mechanism and constructing systems. The relationships between the organic fluorophores structures and electroluminescence properties are the main focus of this review. Finally, the approaches to enhance the performance of NIR OLEDs/PLEDs are described briefly. We hope that this review could provide a new perspective for NIR materials and inspire breakthroughs in fundamental research and applications.

To date, the materials with NIR-emitting are mainly grouped into two categories: inorganic luminescent materials including rare earth metals [29][30][31] and alkaline earth metal luminescent materials [32], and organic luminescent materials covering transition metal complexes [33][34][35][36], small molecules [37][38][39] and polymers [40][41][42]. According to the mechanism of luminescence, these emitters are divided into two types, fluorescent [43] and phosphorescent materials [44][45][46]. The luminescence is generally defined as the radiation Considering that the price and rarity of the heavy metals elevate the cost and limit of their mass processing and limit the future application, a growing number of people focus on the pure organic NIR electroluminescent materials to gain more design freedom with a cost advantage. The mechanical adaptability of organic NIR light-emitting materials also makes them have broad application prospects in flexible and stretchable devices. In addition, metal-free organic light-emitting materials could be used as biocompatible substitutes for inorganic materials, and could be used in implantable, wearable or medical applications in vivo. As shown in Figure 1, in addition to traditional fluorescent materials that just could utilize the singlet excitons, there are several new types of materials that can greatly improve quantum efficiency by taking advantage of singlet and triplet excitons at the same time. (1) The materials with triplet-triplet annihilation (TTA) [47,48] can provide 62.5% energy utilization through two triplet excitons annihilating to form a higher energy triplet exciton. (2) For doublet [49,50], only one electron occupies the highest singly occupied molecular orbital (SOMO). When this electron is excited to the lowest singly unoccupied molecular orbital (SUMO), the SOMO is empty, and transition of the excited electron back to the SOMO is totally spin-allowed. (3) The hybridized local and chargetransfer state (HLCT) material [51][52][53] with the "hot excitons" which could undergo a reverse intersystem crossing process (RISC) through the high-lying channel, and then the excitons could go through a radiative transition to the low-lying locally excited (LE) state to produce a radiative exciton ratio that break through the limit of 25% of spin statistics. (4) The thermally activated delayed fluorescence (TADF) [54][55][56] materials with small singlet-triplet energy gap (ΔEST) make use of efficient reverse intersystem crossing (RISC) Considering that the price and rarity of the heavy metals elevate the cost and limit of their mass processing and limit the future application, a growing number of people focus on the pure organic NIR electroluminescent materials to gain more design freedom with a cost advantage. The mechanical adaptability of organic NIR light-emitting materials also makes them have broad application prospects in flexible and stretchable devices. In addition, metal-free organic light-emitting materials could be used as biocompatible substitutes for inorganic materials, and could be used in implantable, wearable or medical applications in vivo. As shown in Figure 1, in addition to traditional fluorescent materials that just could utilize the singlet excitons, there are several new types of materials that can greatly improve quantum efficiency by taking advantage of singlet and triplet excitons at the same time.
(2) For doublet [49,50], only one electron occupies the highest singly occupied molecular orbital (SOMO). When this electron is excited to the lowest singly unoccupied molecular orbital (SUMO), the SOMO is empty, and transition of the excited electron back to the SOMO is totally spin-allowed. (3) The hybridized local and charge-transfer state (HLCT) material [51][52][53] with the "hot excitons" which could undergo a reverse intersystem crossing process (RISC) through the high-lying channel, and then the excitons could go through a radiative transition to the low-lying locally excited (LE) state to produce a radiative exciton ratio that break through the limit of 25% of spin statistics. (4) The thermally activated delayed fluorescence (TADF) [54][55][56] materials with small singlet-triplet energy gap (∆E ST ) make use of efficient reverse intersystem crossing (RISC) from the lowest triplet state (T 1 ) to the lowest singlet state (S 1 ) so that the theoretical internal quantum efficiency can reach 100%.

Figure 2.
Absorption spectra of the entire tetraphenoxy-substituted rylenediimide series in CHCl3 (adapted from ref. [65]).  As previously mentioned, the effects on tuning the energy gap are mainly related to the individual molecules without consideration of intermolecular interaction. The intermolecular interaction, for instance, the molecular π-π stacking, hydrogen bonding and charge transfer could also have the influence on altering the band gap of the molecules in the solid states [71][72][73][74][75]. According to the above approaches, researchers have designed and organized NIR luminescent materials.

NIR Fluorescent Materials Based on Polymers
Conjugated polymers with fluorescence units have attracted a multitude of attention due to the academic and commercial value when used as the active materials in PLEDs [76]. The turn-on voltage, color purity, and stability of the devices should be optimized to accommodate PLEDs. Some of the principal advantages of conjugated polymers are easy manufacture, solution processability, low-cost, flexibility and suitability to form large area surfaces [77]. The synthetic organic flexibility is the most obvious feature of the conjugated polymers. Through the manipulation of the structures of the monomer and polymer, the    As previously mentioned, the effects on tuning the energy gap are mainly related to the individual molecules without consideration of intermolecular interaction. The intermolecular interaction, for instance, the molecular π-π stacking, hydrogen bonding and charge transfer could also have the influence on altering the band gap of the molecules in the solid states [71][72][73][74][75]. According to the above approaches, researchers have designed and organized NIR luminescent materials.

NIR Fluorescent Materials Based on Polymers
Conjugated polymers with fluorescence units have attracted a multitude of attention due to the academic and commercial value when used as the active materials in PLEDs [76]. The turn-on voltage, color purity, and stability of the devices should be optimized to accommodate PLEDs. Some of the principal advantages of conjugated polymers are easy manufacture, solution processability, low-cost, flexibility and suitability to form large area surfaces [77]. The synthetic organic flexibility is the most obvious feature of the conjugated polymers. Through the manipulation of the structures of the monomer and polymer, the As previously mentioned, the effects on tuning the energy gap are mainly related to the individual molecules without consideration of intermolecular interaction. The intermolecular interaction, for instance, the molecular π-π stacking, hydrogen bonding and charge transfer could also have the influence on altering the band gap of the molecules in the solid states [71][72][73][74][75]. According to the above approaches, researchers have designed and organized NIR luminescent materials.

NIR Fluorescent Materials Based on Polymers
Conjugated polymers with fluorescence units have attracted a multitude of attention due to the academic and commercial value when used as the active materials in PLEDs [76]. The turn-on voltage, color purity, and stability of the devices should be optimized to accommodate PLEDs. Some of the principal advantages of conjugated polymers are easy manufacture, solution processability, low-cost, flexibility and suitability to form large area surfaces [77]. The synthetic organic flexibility is the most obvious feature of the conjugated polymers. Through the manipulation of the structures of the monomer and polymer, the physical, thermal, optical, and electrochemical properties could be adjusted for specific applications.

NIR Fluorescent Materials Based on Small Molecules
Due to the parity-forbidden radiative 4f-4f transitions of the rare earth ions, the corresponding LEDs usually have a nonmeasurable or very low EQE and low power output. In contrast, the luminescence of organic molecules originates from their allowed S 1 -S 0 transitions and thus free from the luminescence efficiency limitation. By using phosphorescent heavy metal complexes that can effectively harvest both the singlet and triplet excitons. Unfortunately, the EL quantum efficiency drops rapidly at high current densities. Therefore, in order to develop NIR-OLEDs/PLEDs with a high EQE, the research on efficient and stable fluorescent NIR-emitting materials is continuing.
Initially, attempts were made to construct NIR luminescent materials using molecules with a large area of conjugated systems. In 2006, Kageyama et al. [95] investigated that OLED ( Figure 8) using tris(8-quinolinolato)aluminum (Alq 3 ) highly doped with N,N -bis(neopentyl)-3,4:9,10-perylenebis(dicarboximide) (M1) as an emitting layer exhibit nearinfrared EL with a peak at 805 nm originating from M1 aggregates ( Figure 9). Phthalocyanines are known to be organic semiconductors and have attracted much attention because of their high chemical stability, various synthetic modifications, epitaxial growth of thin films by organic molecular beam epitaxy and unique absorption bands extending from the ultraviolet region to infrared region [96,97]. Cheng et al. [98] reported the OLED device used purple phthalocyanine (M2) single crystal as an active light-emitting layer with the emission of 936 nm ( Figure 9). And Du et al. fabricated NIR OLEDs based on Tetra (2-Isopropyl-5-methylphenoxyl) substituted phthalocyanine (M3) [99] and tetra-(methoxyphenoxy) substituted phthalocyanine (M4) (Figure 9) [100]. The EL intensity at about 910 nm in the devices based on M3 was increased by about 14 times compared with the intensity at about 930 nm in the devices based on M2 in the same device structures. The improvement in the EL intensity was attributed to the large steric hindrance of non-peripheral phenoxyl substituent of M3. The emission of the NIR-OLEDs based on M4 was observed near 891 nm. Sharbati et al. [101] demonstrated an efficient NIR electroluminescence from OLED based on imine oligomer (E)-N-((E)-3-((E)-(4-iodophenyl-imino)methyl)benzyldine)-4-iodobenzenamine (M5) (Figure 9). Electroluminescence with peak emission wavelengths of 800 nm and maximum EQE of 1.9% were observed. Mateo-Alonso et al. [102] presented an extended and distorted member 7,8,15,16,23,24-hexaazatrianthrylene (M6) of the Ncontaining starphene family due to their excellent electron-transporting ability ( Figure 9). The electroluminescence of the OLEDs based on M6 appeared at substantially higher wavelengths (centred at 848 nm) than the previously reported heterojunctions with hexaazatrinaphtylene (HATANT) derivatives [103,104], which illustrated that electron-deficient N-containing starphenes could be considered a general platform to prepare and fine-tune the properties of NIR-OLEDs. member 7,8,15,16,23,24-hexaazatrianthrylene (M6) of the N-containing starphene family due to their excellent electron-transporting ability ( Figure 9). The electroluminescence of the OLEDs based on M6 appeared at substantially higher wavelengths (centred at 848 nm) than the previously reported heterojunctions with hexaazatrinaphtylene (HATANT) derivatives [103,104], which illustrated that electron-deficient N-containing starphenes could be considered a general platform to prepare and fine-tune the properties of NIR-OLEDs.  Among many low band gap organic compounds, the D-A type of chromophores are particularly of interest to researchers as potential NIR chromophores because their band gap levels and other properties can be readily tuned through a variety of donors and acceptors [105]. In 2008, Wang et al. [106] synthesized the NIR fluorescent compounds M7 and M8 with a combination of triphenylamine (TPA), thiophene, and benzo[1,2-c:4,5- Figure 8. Current density-voltage-luminance characteristics of the M1-based OLED device. The upper inset shows a photo of near-infrared EL obtained for the device at a drive voltage of 9.4 V, taken with an infrared scope (adapted from ref. [95]). member 7,8,15,16,23,24-hexaazatrianthrylene (M6) of the N-containing starphene family due to their excellent electron-transporting ability ( Figure 9). The electroluminescence of the OLEDs based on M6 appeared at substantially higher wavelengths (centred at 848 nm) than the previously reported heterojunctions with hexaazatrinaphtylene (HATANT) derivatives [103,104], which illustrated that electron-deficient N-containing starphenes could be considered a general platform to prepare and fine-tune the properties of NIR-OLEDs.  Among many low band gap organic compounds, the D-A type of chromophores are particularly of interest to researchers as potential NIR chromophores because their band gap levels and other properties can be readily tuned through a variety of donors and acceptors [105]. In 2008, Wang et al. [106] synthesized the NIR fluorescent compounds M7 and M8 with a combination of triphenylamine (TPA), thiophene, and benzo[1,2-c:4,5- Among many low band gap organic compounds, the D-A type of chromophores are particularly of interest to researchers as potential NIR chromophores because their band gap levels and other properties can be readily tuned through a variety of donors and acceptors [105]. In 2008, Wang et al. [106] synthesized the NIR fluorescent compounds M7 and M8 with a combination of triphenylamine (TPA), thiophene, and benzo[1,2c:4,5-c ]bis([1,2,5]thiadiazole) (BBTD) in the D-A-D system that could lead to the longest emission wavelength and the narrowest band gap ( Figure 10). The TPA unit was used as donor with prominent hole-transporting ability and the BBTD-type unit was used as acceptor, which is known to possess substantial quinoidal characters within a conjugated backbone, allowing for greater electron delocalization and thus lowering of the band gap. The OLED device (M7) with exclusive NIR emission at 1050 nm and an EQE of 0.05% has been realized. By doping M8, the emission wavelength can be ex-  Figure 10) [107]. The non-planar TPA unit is available to improve carrier-transporting prop-erties and suppress aggregations. The emission-peak maxima of NIR-OLEDs based on these compounds are all above 1000 nm, and the longest EL is at 1220 nm for M12. Nondoped OLED (M10) achieved NIR emission exclusively at 1080 nm with EQE of 0.73%. Then Wang et al. [108] want to utilize the guest-host system with several requirements be considered and met. At first, the host materials should have high film-forming ability and carriertransport ability. Secondly, the guest materials should have high emission efficiency. Thirdly, the emission of the host should overlap well with the absorption of guest, which is in favor of the energy transfer. Alq 3 was choosed as the host due to its widespread application as a host for organic NIR fluorescent chromophores. A series of a D-A-D type of NIR fluorescent chromophores (4,9- phenazine (M15)) were designed as the guest (Figure 10), which based on [1,2,5]thiadiazolo [3,4-g]quinoxaline (TQ) as an acceptor and TPA as a donor due to the D-A charge transfer absorption bands should be more suitable for the EL band of Alq 3 . The doped OLEDs emit NIR light from 748 to 870 nm with high efficiency and radiance. Particularly, the device containing M13 as a dopant exhibits an exclusive NIR EL at 752 nm with an EQE of 1.12% and the largest radiance of 2880 mW Sr −1 m −2 . And Xue et al. [109] reported NIR-OLEDs based on two D-A-D conjugated oligomers, 4,8bis ( Figure 10), which had the same donor components. A maximum EQE of 1.6% and a maximum power efficiency of 7.0 mW/W were achieved in devices based on M16, whose emission peaks appeared at 692 nm. With a stronger acceptor and thus a reduced energy gap, longer wavelength NIR emissions peaked at 815 nm was achieved in M17 based devices, although the maximum EQE was reduced to 0.51% due to the lower fluorescent quantum yield of the NIR emitter. Using the sensitized fluorescent device structure, the efficiencies were further increased by two to three times, leading to a maximum EQE of 3.1% for M16 and 1.6% for M17 based devices. In 2011, Reynolds et al. [110] showed a family of D-A-D oligomer M18 (Figure 10), which used the 3,4-ethylendioxythiophene as the donor and BT as the acceptors. Introducing a functional end group tetrahydropyran (THP) onto these oligomers provided an opportunity for incorporating the π-conjugated system covalently into a more complex system, where the charge-transporting conjugated units could be used to fabricate solution-processable electrochromic devices. PLEDs based on M18 showed the NIR emission peaked at 730 nm. Energy transfer from the matrix to the emitting oligomer can be achieved, resulting in PLEDs with pure oligomer emission. In 2012, Wang et al. [43] obtained a family of D-A-D type NIR fluorophores (4,9- [1,2,5]thiadiazole (M22)) containing rigid nonplanar conjugated tetraphenylethene (TPE) moieties with electron-deficient [1,2,5]thiadiazolo[3,4-g]quinoxaline (QTD) or BBTD as acceptors ( Figure 10). A twisted TPE had the excellent aggregation-induced emission enhancement (AIEE) and showed a higher fluorescence efficiency in the solid state than in solution [111][112][113]. So incorporation of TPE units into the chemical structures of poor fluorophores could improve their fluorescence efficiency in the solid state significantly. Nondoped OLEDs based on these fluorophores were made and exhibited EL emission spectra peaking from 706 to 864 nm. The EQE of these devices were ranged from 0.89% to 0.20% and remained fairly constant over a range of current density of 100-300 mA cm −2 . The device with the highest solid fluorescence efficiency emitter M19 showed the best performance with a maximum radiance of 2917 mW Sr −1 m −2 and EQE of 0.89%. In 2016, Ledwon et al. [114] synthesised a novel organic material (E,E)4,7-Bis(5-(2-(9-ethylcarbazol-3yl)ethenyl)-4-hexylthien-2-yl)-benzo-2,1,3-thiadiazole (M23) with the structure D-π-A-π-D.
Carbazole was utilized as the electron donor and BT as the electron acceptor unit ( Figure 10). The choice of different, electron-rich and electron-poor units along the π-conjugated, organic backbone of the molecule could control the material functionality for organic electronic applications through the push-pull effect. Futhermore, the substitution pattern of carbazole modified the molecule properties [115][116][117] because carbazole has fine optical and electronic properties and high chemical stability. OLEDs based on M23 presented efficient emission in red and infrared spectral ranges, with an EQE of 3.13%. Electroluminescence is not strongly affected by quenching in the solid state, which is commonly observed for other D-A compounds. In 2020, Promarak et al. [118] also designed and synthesized two NIR fluorophores M24 and M25 with hole-transporting, molecular bulky tercarbazole ( Figure 10). The two isomeric NIR chromophores, based on thiadiazole [3,4-c]pyridine derivatives, achieved a high Φ PL of 34% with an excellent electrical and morphological properties. The nondoped OLED ( Figure 11) based on M24 displayed NIR color emission peaked at 726 nm with high EQE of 1.51%, demonstrating that the bulky tercarbazole terminuses not only improved holetransporting property, but also build in a highly steric conformation to the molecules. And then they reported [119] a new D-A-D structure type NIR emitter 4,9-Bis(3-hexyl-5-(4-(1,2,2triphenylvinyl)phenyl)thiophen-2-yl)naphtho[2,3-c][1,2,5]thiadiazole (M26) bearing naphthothiadiazole and flanked with tetraphenylethene (TPE) (Figure 10), which utilized the aggregation-induced emission (AIE) as a new approach to obtain efficient NIR solid emitter. A non-doped device fabricated with M26 emitted a bright NIR emission at 754 nm with a high EQE of 1.48% and high efficiency stability. Moreover, they studied two NIR fluorescent emitters 4,4 - with the oligo(3-hexylthiophene) (Figure 10), which provided the tuning of the emission colour and solubility [120]. The optimized OLEDs exhibited strong emission in the NIR range (702-723 nm) with a high maximum EQE of 1.52% (M27).
In addition to adjust the groups and structures of fluorescent molecules, researchers have also turned their attention to the host of the OLED device, utilizing the transfer of energy between the host and the emitter to improve the efficiency. In 2018, Yang et al. [131] achieved triplet-singlet energy transfer by the Förster mechanism. The NIR-OLEDs based on N 4 ,N 4 ,N 9 ,N 9 -tetra-p-tolylnaphtho[2,3-c][1,2,5]thiadiazole-4,9-diamine (M38) were optimized with a sensitizer (Figure 12), where triplet excitons could be utilized via Förster energy transfer process due to the better overlap between the sensitizer emission and the dopant absorption. As a result, the optimized device achieved an EQE max of 0.77% with electroluminescent peak at 786 nm. In 2022, Wong et al. [132] revealed that the good spectral overlap between the emissions of exciplex co-host and the absorption of emitter (M39) ensured an efficient Förster resonance energy transfer for good NIR emission ( Figure 12). The optimal NIR-OLED device achieved a maximum EQE of 5.3% with the EL peaked at 774 nm, which was the best EQE ever reported based on the exciplex co-host with an organic fluorescent dopant.
The photophysical and electroluminescent properties of polymers from M1 to M39 are summarized in Table 2. Most of the NIR organic fluorophores are D-A type or flat π-conjugated molecules. Extending conjugated system, strengthening the D-A interaction and utilizing the energy transfer from the host to emitter are the main methods to construct oligomer fluorescent materials with NIR emitting, which can also guide further design and optimization of NIR emitters for biomedical, security, and communication applications.

NIR Phosphorescent Materials Based on Small Molecules
In general, holes and electrons injected from electrodes to emitters generate excitons, and the excitons are classified into singlet and triplet excitons that are formed at a ratio of 1:3. In the case of fluorescent emitting materials, only singlet excitons can be transformed into photons, and so only 25% internal quantum efficiency (QE) is theoretically possible, where the remaining 75% of non-radiation energy is lost. Therefore, breaking spin statistics to utilize the other 75% triplet energy is the key factor to improving OLED efficiency.

Conclusions
Shifting the spectral range of OLEDs/PLEDs from the visible to the NIR region of the electromagnetic spectrum is of great interest. To date, much efforts have been made to develop NIR phosphorescent OLEDs/PLEDs using transition metal complexes. However, high costs, limited resources of phosphorescent materials, and efficiency roll-offs at high current densities remain challenges for their applications in long-term mass production. To reduce cost and improve environmental sustainability, the development of highly efficient OLEDs/PLEDs that does not rely on heavy metal-containing compounds remains an important need. As an alternative material system, the "heavy metal-free" NIR fluorophores have been widely investigated for their cost advantage and versatility in tuning molecules. However, the EQE of traditional organic near-infrared fluorescent OLEDs is generally about 0.1% or even lower due to low exciton utilization rate and low fluorescence quantum yield in solid state, which has become an almost insurmountable obstacle for their further development. Therefore, several strategies have been proposed to realize high quantum efficiency in pure organic dyes by utilizing triplet energy. Nevertheless, this research field is still in its infancy, and while many examples harvesting triplet excitons are reported, only a few studies have focused on their NIR emission, particularly in terms of OLEDs/PLEDs applications.
This review summarized the development of NIR emission materials based on organic fluorophores, and their applications for OLEDs/PLEDs. Conjugated polymers and traditional organic small molecules just use singlet excitons and present relatively low internal quantum yield. While organic fluorophores with doublet, TTA, HLCT or TADF state utilize triplet excitons directly and theoretical internal quantum yield can reach to 100%. The most remarkable is the NIR TADF material. Devices based on the molecules with TADF properties have achieved a maximum EL wavelengths of more than 1000 nm, and the OLED based on the TADF molecule has achieved a maximum EQE of more than 13%, so TADF channels with triplet states in the near infrared region are expected to be the next research topic. It is hoped that this review will contribute to the singularity of TADF emitter design, which will lead to practical device performance through efficient triplet utilization.
On the other hand, these research results provide us some points to design NIRemitting organic fluorophores like constructing twisted D-A structure rather than just enlarging π-conjugated system to minimize the ∆E ST value, and attaching cyano groups to the aromatic ring or utilizing boron bonding units rather than just inserting heteroatoms to enhance the electron-withdrawing capability of acceptor.
Some excellent works have been done the field of NIR OLEDs, the quantum yield of the material, the electroluminescence wavelength, stability and EQE of the device have been greatly improved. However, the types of high-performance organic NIR luminescence materials are still few, and the problems of low device efficiency and efficiency roll-down are still the bottlenecks in the development and application. The future directions may include the following fields: (1) develop new and efficient organic near infrared luminescence materials; (2) study the relationship between molecular structure and electroluminescence properties; (3) reveal the relationship between device properties and device processing technology, and host and guest energy levels; (4) realize the application of organic NIR luminescence materials and devices in military, optical fiber communication, biomedical imaging and other fields.