Iridium(III) complexes are one of themost important electrophosphorescent dyes with the features of high photoluminescence quantum yield (PLQY), short microsecond-level emission lifetime (τ), desired versatility in excited state modulation through ligand engineering for designable emission color in full range from visible to near infrared lights and favorable thermo-depositability and solvability for device fabrications through vacuumevaporation and solution processing technologies have beenstudied intensely during the last decade due to their application in phosphorescent organic light-emitting diodes (PHOLEDs) [1-3]. Nevertheless, similar to most of electrophosphorescent emitters, the small-molecularridium(III) complexes commonly suffer from serious quenching effects, e.g. triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ), which are ascribed to the long lifetime of triplet excitons and the unbalanced charge carrier fluxes in emitting layers (EML) of their PHOLEDs. On account of the collision-induced intrinsic characteristics forTA andPQ effects, in 1999, Baldo et al. initiatively reported the first example of efficientr3+ complexesbased PHOLED employing EML of tris(2-phenylpyridine)iridium(III) (Ir(ppy)3)doped4, 40-N, N0-dicarbazole-biphenyl (CBP)matrix, which achieved the peak efficiencies of 8.0% for external quantum efficiency (EQE), 8 cd A-1 for current efficiency (CE) and 31 lmW-1 1 for power efficiency (PE) .With this, doping EML configuration of emitter dispersing in host matrix is widely adopted Ir3+ complexes-based PHOLEDs, and indeed endows the state-of-the-art performance to the full-color devices [5-10].
It is known that hostmaterials in PHOLEDs should possess two functions, namely supporting efficient energy transfer to dopant and optimizing charge carrier flux balance for unitary exciton recombination (Fig. 1a) [5-9]. Nevertheless, in recent study, besides of the potential host-dopant phase separation-induced device degradation, it is noticed that the host-host and host- dopant interactions would also induce the efficiency decrease, making device optimization become complicated [10, 11].n this case, a strategy named “host-dopant integration” (HDI) would be promising to solve these intrinsic drawbacks of doping systems, in which host-featured groups and emissive cores are chemically bonded through either conjugated (Fig. 1b) or aliphatic (Fig. 1c) linkages. HDI systems can undoubtedly enhance the host-dopant compatibility and dispersity at the molecular level. Significantly, HDI systems are superior in (i) adjustable host density and spatial distribution for effective encapsulation of emissive core; (ii) controllable combination of functional groups with complimentary electrical properties for charge flux balance; (iii) thorough suppression of host-dopant and dopant-dopant interaction induced quenching; (iv) great potential in constructing singlemolecular nano-size emitters for super-resolution displays.
|Figure 1. Functions of host matrixes in doping systems (a) and the so-called “Host-opantntegration (HDI)” functionalization strategies of Ir3+ complexes through conjugated (b) and aliphatic linkages (c).|
This review briefly introduces the recent progress of functionalized Ir3+ complexes for emission color tuning, optoelectronic enhancement and quenching suppression, and so on.he emphasis will be focused on ligand functionalization and the device performance improvement.his review seeks to provideperspectives on molecular design strategies, function integration approaches and future directions for electrophosphorescent Ir3+ complexes.2. Functionalized Ir3+ complexes through conjugated linkages
After the pioneer works by Baldo andorrest aboutr(ppy)3  and acetylacetonato-bis-(2-phenylpyridine)-iridium(III) (Ir(ppy)2acac) , the superiority of theseridium complexes in electroluminescence (EL) encourages the further modification of these cyclometalated iridium(III) complexes with functional substituents, which can dramatically optimize their phase and optoelectronic properties . Cao et al. showed that just simple introducing a tert-butyl inr(ppy)3 to affordr(Bu-ppy)3 can improve host-dopant compatibility and thereby enhance the efficiency stability of its devices [14, 15]. Electrically inertial arylsilyl with big hindrance and good solubility was also incorporated in red electrophosphorescent tris(2-(2′-benzo[b]- thienyl)-5-(4′-triphenylsilylphenyl) pyridinato-N, C3’) iridium(II) to improve phase homogeneity of EML .n most of the works, the functionalization was focused on enhancing the opto- and electro-activity of the complexes through introducing groups with the excellent sensitizability and/or carrier transporting characteristics.2.1. Ir3+ complexes functionalized with hole-transporting groups
Arylamine groups are the most important electron-donating groups utilized to improve the hole-transporting ability of Ir3+ complexes (Fig. 2). Zhou and Wong et al. firstly introduced diphenylamine (DPA) groups at para-position of ligands in redemitting tris(1-phenylisoquinolinolato-C2, N)iridium(III) (Ir(piq)3) to afford a maximum EQE beyond 10% .hen, they further extended the method to Ir3+ complexes bearing functionalized ligand as a combination of thiazolyl and triphenylamino (TPA) groups for the balance of charge carrier injection and transportation, in which 1 (IrTZ1) endowed its devices with yellow emission peaked at 568 nm and the maximum EQE of about 15% . With the same strategy, Mi et al. developed tris(N, N, 6-triphenylpyridazin- 3-amine)iridium(III) (2, rNPPya) with high PLQY of 0.55 and short t of 6 μs, which should be ascribed to steric hindrance of its TPA groups on suppressing intermolecular interaction . Employing as 2 yellow dopant emitter, the monocolor PHOLEDs showed the state-of-the-art efficiency with the maxima of 70.8 cd A-1 , 75.4 lm W-1 and 30.8%, accompanied by emission peak at 560 nm and Commissionnternationale de l’Eclairage (CIE) coordinates of (0.51, 0.49). Meanwhile, its complementary whiteemitting diodes also achieved the extremely high efficiencies up to 49.9 cd A-1, 55.9 lm W-1 and3.9% with CIE coordinates of (0.33, 046).hrough incorporating 1-naphthalenylphenylamine with stronger electron-donating ability at 6-position of-phenylbenzo[d]thiazole ligand framework, the highest occupied molecular orbital (HOMO) of the complex 3 was remarkably elevated by 0.15 eV to improve the hole injection and transportation.espite the relatively low PLQY of 0.09 for 3, its orange and white-emitting spin-coated devices realized the peak EQEs of 8.73% and 4.90%, accompanied with favorable CIE coordinates of (0.60, 0.40) and (0.33, 0.35), respectively.
Carbazole is a famous hole-transporting group with host characteristics, which is widely adopted in electrophosphorescent host materials. Yang group established the first example of carbazole-based Ir3+ complexes with green and orange emissions [20-22]. Except for the electrical performance, the long alkyl chain at N-position of carbazole can prevent crystallization and improve host-dopant compatibility, thereby rendering the maximal EQE of 8.51% for the green spin-coated devices.hrough displacing phenyl ofr(piq)3 with carbaozle, the resulted red-emitting complexes can also realize the improved EL performance with the peak EQE of 7.65%. Li et al. further developed two red Ir3+ complexes with- (9-phenyl-9H-carbazolyl)-benzothiazole ligand .t was showed that the introduction of carabzole group induced the bathochromic shift of emission peaks due to the conjugation extension and the significant contribution of carbazole as an electron-donating element to the HOMO of the complexes.he complex 4 was yellow emissive with PLQY of 0.42 and t as short as 2.77 μs, accompanied by the deep HOMO of -5.0 eV, which supported its devices with the impressive performance, such as peak efficiencies beyond 75 cd A-1, 48lm W-1 and3%, respectively. Mei et al. reported two disubstituted Ir3+ complexes (5, Ir(ECPC)2(pic) and 6, Ir(TPC)2(pic)) with saturated red emission peaked around 650 nm using carbazole and DPA-functionalized phthalazine derivatives as cyclometalated ligands . Both of the complexes revealed the extremely short τ of ~0.2 μs, beneficial to quenching suppression. With modified energy level matching between host and the carbazole-substituted complexes, the carrier injection and transportation were tuned for balance, giving rise to the impressive EQE as high as 16.3% from the spin-coated devices of 5 with favorable color purity of (0.68, 0.29).
Guanidinate was firstly used as ancillary ligand with- benzothiozolato-phenyl (bt) as primary ligand to form an efficient red-emitting Ir3+ complex ((bt)2Ir(dipba)) . When using holetransporting group modified guanidinate ligands to stabilize bis(2-phenylpyridinato)iridium, the electrical performance of the complexes 7-17 can be improved.heir emission colors can be tuned from green to yellow, along with the increased electron donating ability of the arylamine substituents, in which the green-emitting complex 11 with phenothiazine group exhibited the maximum CE as high as 118 cd A-1 . On contrary, changing primary ligands can remarkably tune the emission colors of the complexes from green to deep red . With (NiPr)2C(NPh2) as ancillary ligand, the complex 18 endowed its red-emitting diodes with peak CE of 53.5 cd A-1, accompanied by favorable CIE of (0.67, 0.31).urthermore, N, N’-diisopropyl-diisopropyl-guanidinate (dipig) was utilized as ancillary ligand to establish two redemitting Ir3+ complexes 19 ((bt)2Ir(dipig), BTIPG)  and0 ((bzq)2Ir(dipig), BZQPG) [30, 31] with phenylbenzothiazole (bt) and bis(7, 8-benzoquinolinato) (bzq) as primary ligands, respectively. Complex 20 exhibited the state-of-the-art EL efficiencies with maxima beyond7% for EQE and 75 lm W-1 for PE, accompanied by negligible roll-offs at 1000 and 5000 cd m-2, respectively.
Beside of arylamine, other electron-donating groups, such as phenylether  and thiophene , were also introduced in Ir3+ complexes to modulate the electronic states and electrical performance.he flexible phenoxy group can not only elevate the HOMO level, but also improve the PLQY of the complex 21 to ~0.70, thereby endowed its devices with outstanding peak efficiencies of 76.2 cd A-1, 72.8 Im W-1 and 22.5%. Meanwhile, with thiophene disubstitution, bis[2, 5-di(4-hexylthiophen-2- yl)pyridine](acetylacetonate)iridium(III) (22, Ir(ht-5ht-py)2(acac)) can support the maximum EQE of 8% to its solution-processed devices, as well as deeπ-red emission peaked at 628 nm and CIE coordinates of (0.68, 0.31).
2.2. Ir3+ complexes functionalized with electron transporting groups
The electron-withdrawing groups are widely employed to adjust the optoelectronic properties of Ir3+ complexes in two aspects: (i) bonding with the HOMO or LUMO-localized moieties for blue or red shift of emissions, respectively; (ii) enhancing the electron injection and transportation for charge flux balance in the EMLs (Fig. 3).
|Figure 3. EL Ir3+ complexes substituted with electron-withdrawing groups.|
Fluorine is the simplest and the most popular electronwithdrawing group involved in Ir3+ complexes. It was firstly introduced in bis(2-(2′-benzo[4, 5-a]thienyl)pyridinato- N, C3′)iridium(acetylacetonate) (Btp2Ir(acac)) through a trifluoromethyl substitutent, improving the EQE of the red-emitting devices to 9.6% [35, 36].he device performance of another classic yellow Ir3+ complex (bt)2Ir(acac) can be remarkably improved through introducing CF3 on benzothiazole ring, e.g. the maximum EQE beyond0% . Yang group developed a series of (bt)2Ir(acac) derivatives 23-25 with, Cl and Br atoms at 4- position of phenyl ring, respectively .he introduction of halogen atoms lowered the HOMO levels of (bt)2Ir(acac), rendering the blue shift of emission to 540 nm, 554 and 555 nm, respectively. PLQY reduction by 0.1~0.2, the maximum EQEs of 23-25-based devices reached 12.1%-17.3%, which were 0.5-1-fold of that of (bt)2Ir(acac)-based analogues.urther introduction of three fluorine atoms did not remarkably influence optical properties of 26, rendering yellow emission with peak at 543 nm, PLQY of 0.56 and short τ of only 0.37 μs . Complex 26 endowed its devices with the maximum CE beyond 50 cd A-1.2-(4-(Trifluoromethyl)- phenyl)pyridine and (2-(5-pentafluorophenyl-1, 3, 4-oxadiazol-2- yl)-phenol were utilized as primary and ancillary ligands to afford green-emitting complex 27 with PLQY of 0.34 and τ of 1.99 μs in degassed CH2Cl2, whose devices realized the maximum EQE of 19.7% .
Fluorine is incorporated in most of blue Ir3+ complexes, such asirpic , FIr6  and FCNIr .aking use ofIrpic as the basis, -CN and -SCN groups  and Phosphorus [44, 45] and pyridine azole -containing auxiliary ligands were employed to further shift the emission to true blue range. Yang et al. introduced halogen atoms on 4-position of pyridine ring inirpic .The HOMOs of the resulted complexes 28-30 were stabilized; while, their LUMOs were preserved, rendering the blue-shifted emissions peaked at 465 nm. Complex-28-based devices revealed rather high efficiencies with maxima of 29 cd A-1, 9lm W-1 and 14.6%, accompanied by of favorable CIE coordinate of (0.15, 0.28). Kim et al. developed a series of deeπ-blue Ir3+ complexes 31 ((TF)2Ir(pic)), 32 ((TF)2Ir(fptz)), 33 ((HF)2Ir(pic)) and 34 ((HF)2Ir(fptz)), incorporating2′, 4′′- difluororphenyl-3-methyl-pyridine with electron-withdrawing groups of trifluoromethyl carbonyl or heptafluoropropyl carbonyl at the 3′-position as the primary ligands and a picolinate or a trifluoromethylated-triazole as the ancillary ligand, respectively .The HOMOs of 31-34 weremainly localized on phenyls of their primary ligands, accompanied by a significant contribution fromthe d orbital of Ir3+ , giving rise to the decrease of the HOMO levels due to the strong electron-withdrawing effect of perfluorocarbonyl groups. Meanwhile, their influence on the LUMO energy levels was negligible, therefore expanding the energy gaps for deep blue emissions peaked at ~450 nm. When doped in hostmatrix, PLQY of 31 was as high as~75%.he devices based on 31 and 32 achieved the EQE up to 17.1% and 8.4%, as well as high color purity with CIE coordinates of (0.141, 0.158) and (0.147, 0.116), respectively.he authors further utilized heptafluoropropyl instead of perfluorocarbonyl to construct complexes 35 ((HFP)2Ir(pic)), 36 ((HFP)2Ir(mpic)) and 37 ((HFP)2Ir(fptz)) .hese complexes also achieved the deep blue emission with peaks at ~450 nm; while, PLQY of 36 in host matrix was further improved to -90%. As the result, 36 supported the state-of-the-art performance to its devices, including the maximum EQE of1.4% and CIE coordinates of (0.146, 0.165).
Mro et al. developed two orange and green phosphorescent heteroleptic iridium complexes 38 and 39 bearing ppy ligands functionalized with benzylsulfonyl group and fluorine atoms .he strong electron-withdrawing effect of benzylsulfonyl group doubled PLQYs of 38 and 39 as 0.6-0.7, accompanied by the red shift of emissions.he solution-processed devices of 39 realized the peak EQE of 12%. Zhou et al. further combined fluorine and sulfonyl groups as fluorinated aromatic sulfonyl groups to modify ppy ligand, establishing complex 40-43 with one, two, three and five fluorine atoms, respectively .heir electron injecting ability was enhanced through tuning the LUMO to -3.0 eV by the inductive effects of fluorine and sulfonyl groups.hese greenish yellow-emitting complexes simultaneously realized PLQY as high as ＞0.95.he state-of-the-art performance of 42 was demonstrated with the maximum efficiencies of 81.2 cd A-1, 64.5 lm W-1 and 19.3%.
Similar to sulfonyl, phosphine oxide (PO) is an electronwithdrawing group with appropriate inductive effect (Fig. 4) [52, 53].he emissions ofIrpic derivatives 44 (POFIrpic) and 45 (SOFIrpic) can be tuned to deep blue zone with peak at 460 nm through incorporating phosphoryl and sulfonyl at 5′-position of the phenyl ring, which should be attributed to the HOMO deepened by the inductive effects of the substituents . On the basis of PLQY about 0.5, 44 provided the deep blue EL emission to its solution-processed devices with CIE coordinates of (0.17, 0.29) and the maximum efficiencies of 11.1 cd A-1 and 7.1%. Zheng et al. utilized a PO compound named tetraphenylimidodiphosphinate (tpip) as the ancillary ligand to form green and sky blue emissive Ir3+ complexes 46 (Ir(tfmppy)2(tpip)) and 47 (Ir(dfppy)2(tpip)) .he polarity of P=O bond can shorten the emission lifetimes to suppress quenching effects.herefore, 46 and 47 rendered high CE of 67.95 and5.45 cd A-1 for their devices, accompanied by the reduced efficiency roll-off.hen, the authors introduced as many as six CF3 groups in the tpiπ-based greenemitting complex 48 to reduce the nonradiative deactivation and quenching effects, giving rise to the excellent device efficiencies of 115.39 cd A-1 and 113.23 lm W-1 . (2′, 6′-Bis-(trifluoromethyl)-2, 3′-bipyridine and 2′, 6′-bis(trifluoromethyl)-2, 4′-bipyridine were further adopted as primary ligands to form complexes 49 and 50 with tpip as ancillary ligand .The integration of fluorination and P=O groups successfully increased the electron mobility, resulting in the balanced charge injection and transportation. Accompanied with the reduced nonradiative deactivation and suppressed molecular packing of CF3 groups, 49 and 50 endowed their devices with the maximum efficiencies as high as 101.96 and 99.97 cd A-1 and 31.6% and 30.5%, respectively, as well as small efficiency roll-off.
2.3. Ambipolar functionalized Ir3+ complexes
It is believed that the integration of hole and electron transporting groups in a single molecular system can realize the ambipolar characteristics for charge flux balance in the devices (Fig. 5). Wong group firstly established a bifunctional Ir3+ complex with a fluorine-substituted carbazole-based ligand, which supported the maximum EQE of 11.6% to its green emitting devices . Li et al. developed a yellow-emitting Ir3+ complex 51 on the basis ofr(ppy)3 core substituted with electron-transporting diphenylphosphine oxide (DPPO) and hole-transportingPA groups, whose devices achieved the efficiencies with the maxima of 29.6 cd A-1, 15.5 lm W-1 and 8.8% . Upon the similar strategy, two saturated red-emitting Ir3+ complexes 52 and 53 with bifunctional charge-transporting peripheral groups were established through encapsulatingr(piq)3 core withPA and triphenylphosphine oxide (TPPO) groups, which can suppress intermolecular interaction-induced concentration quenching, facilitate intramolecular energy transfer and carrier injection and transportation balance .he single-layer spin-coated device based on 52 achieved the maximum EQE of 7.6% with CIE coordinates of (0.68, 0.30). Zhou et al. further developed twor(ppy)2(acac)-based asymmetric complexes 54 (Ir-SO2O) and 55 (Ir-SO2N) with ambipolar characteristics for charge carrier balance, whose two ppy ligands were substituted with electron-withdrawing sulfonyl and electron-donating phenoxy orPA, respectively .he orange-emitting 54-based devices realized the state-ofthe- art efficiencies of 69.4 cd A-1, 35.2 lm W-1 and 20.2%.
3. Functionalized Ir3+ complexes through aliphatic linkages
Carrier transporting groups can also be introduced inr3++ complexes through aliphatic linkages, namely so-called “combined carrier transporting moieties and aliphatic chains” (CTAC) modification strategy .n comparison to conjugated linkages, aliphatic chains can be readily incorporated through C-O and C-N bonds, making multi-position functionalization feasible.he combination of host-featured carrier transporting groups and flexible chains can provide the effective encapsulation of emissive Ir3+ complex cores to suppress quenching effects, without the cost of reducing electrical performance (Fig. 6) .
|Figure 6. Functionalized Ir3+ complexes on the basis of CTAC modification.|
Wang group introduced host-featured 3, 6-di-tert-butyl-carbazolyl (tBCz) groups into complexes 56-58 through butylene linkages with two, three and four peripheral groups, respectively . Along with the increase of carbazole density, the emission t of these green-emitting complexes was gradually elongated, indicating the restrained intermolecular interactions by encapsulation. Consequently, 58 endowed its nondoped solution-processed devices with the favorable EL performance, e.g. the maximum values of 23.4 cd A-1 and 16.3 lm W-1, respectively.n the same way, they further developed a self-host blue-emitting Ir3+ complex 59 with butylene-linked carbazole dendron .he excellent host characteristics of carbazole dendron unit can improve the intramolecular energy transfer and enhance the carrier injecting and transporting ability of the complex.he spin-coated host-free device of 59 realized the impressive efficiencies with the maxima of 31.3 cd A-1, 8.9 lm W-1 and 15.3%, as well as pure blue emission with CIE coordinates of (0.16, 0.29).
On the basis of CTAC modification, the electron-transporting 1, 3, 4-oxadiazole (OXD) and hole-transporting carbazole groups were introduced in phenylbenzimidazole (PBI) ligand to construct complexes 60 (IrPBICO) and 61 (IrPBIC2O2) with two and four peripheral functional groups, respectively . With larger group density, 61 achieved the doubled PLQY of 0.61 and τ of 7.4 μs in film, owing to the suppression of interaction-induced quenching effects.he bifuncitonalization of 61 improved the balance of carrier injection and transportation, which effectively limited the efficiency roll-off to ~16% at 1000 cd m-2 for its solution-processed host-free devices. Recently, we reported a series of nano-size Ir3+ complexes 62-64 (Ir(23NTCzPBI)3, r(24NTCzPBI)3 andr(34NTCzPBI)3) with three dimensional encapsulation with as many as nine tBCz groups .hrough tuning the substitution positions of tBCz groups, besides of emission variation fromyellow, bluish green to green, the spatial distribution and extension direction of the peripheral groups were purposefully adjusted to render the effective encapsulation of the emissive Ir3+ complex core and thereby high PLQY of 0.7 for 64.he spin-coated host-free bilayer devices of 64 realized the extremely low turn-on voltage of 3.5 V and the excellent maximum efficiencies of 27.7 cd A-1, 15.4 lm W-1 and 8.3%, respectively, accompanied by negligible efficiency roll-offs of 0.4%at 1000 cd m-1.4. Conclusions
This review briefly summarized the recent development of EL Ir3+ complexes with modified optoelectronic characteristics.Through classifying these materials according to their functional substituent types, the correlations between functionlization, physical properties and the device performance would be established to guidance the molecular design of high-efficiency Ir3+ complexes.his kind of materials will be doubtlessly focused in the future studies, with the great superiority in energy conservation and performance stability.he main challenge for Ir3+ complexes is still their too complicated device structures, restraining the commercial applications.urthermore, the insufficiency of highly efficient deep-blue phosphors also impeded the achievement of high-quality displays with satisfied emissive color saturation. Nevertheless, the great progress in functionalization of Ir3+ complexes shows the feasibility of simplifying device structures on the basis of host-dopant integration strategy and the multi-functional combination.or the future study, the enhancement of electrocactivity, optimization of optical properties, improvement of intramolecular energy transfer and harmonization of multi-functions should be still focused in molecular design of Ir3+ complexes with desired optoelectronic performance.Acknowledgment This project was financially supported by NSFC (Nos. 51373050 and 61176020), New Century Excellentalents Supporting Program of MOE (No. NCET-12-0706), Science andechnology Bureau of Heilongjiang Province (Nos. ZD201402 and JC2015002), Education Bureau of Heilongjiang Province (No.014CJHB005) and theok Ying-Tong Educationoundation for Youngeachers in the Higher Educationnstitutions of China (No. 141012).
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