Efficient all-solution-processed near-infrared (NIR) polymer light-emitting diode (PLED) based on the [Ir(C^N1)2(C^N2)]-heteroleptic Ir(III)-complex [Ir(iqbt)2(Br-ppy)]
Introduction
Owing to the strong spin-orbit coupling (SOC) effect [1] induced by iridium(III) center, phosphorescent cyclometalated Ir(III)-complexes characteristic of chemical inertness, rather short lifetime and high quantum efficiency, have been prevalently employed as dopants for organic/polymer light-emitting diodes (OLEDs/PLEDs) [[2], [2](a), [2](b), [2](c), [2](d)]. Associated with the ligand-centered (3LC) and metal-to-ligand charge transfer (3MLCT) excited states, the T1 level of one specific Ir(III)-complex can be flexibly modulated by the chemical structure of the ligands, endowing a wide range of tunable emission colors [[3], [3](a), [3](b)]. Nonetheless, as constrained by the so-called “energy gap law” [4], it remains a real challenge to develop new Ir(III)-complex phosphors towards their high performance vacuum-deposited [[5], [5](a), [5](b), [5](c), [5](d)] or solution-processed NIR-OLEDs/PLEDs [6,7], which are highly promising in night-vision and information security displays [[8], [8](a), [8](b)], telecommunication [[9], [9](a), [9](b)] and photo-dynamic therapies [10]. Undoubtedly, using the doping system of the Ir(III)-complex into an appropriate polymeric host as the emitting layer (EML), its reliable NIR-PLED [7] is evidently advantageous of high material utilization rate, cost effectiveness and large-area scalable production. In this perspective, for avoidance of the detrimental triplet-triplet annihilation (TTA) [11], the low-concentration doping of the Ir(III)-complex dye within the EML must be strictly adopted. Moreover, considering the inherent 103-fold higher hole mobility (10−3 cm2/V⋅s) [12] compared with electron mobility (10−6 cm2/V⋅s), the rational supplementation of an electron-transport layer (ETL) and/or hole-block layer (HBL) by vacuum-deposition is necessary to facilitate the desirable carriers’ balance. Accordingly, the certain efficiency progress appreciable for the multi-layer NIR-PLED, is paying the price of the additional casting cost. By contrast, the fabrication of all-solution-processed NIR-PLEDs based on Ir(III)-complex resources caters to a practical population with the lower cost, while the realization of their satisfactory performance, gives rise to a parallel challenge.
Up to now, the π-conjugated ligand Hiqbt (Scheme 1) linked with one electron-enriched benzo[b]thiophene and one N-heterocyclic isoquinoline, is one of the best C^N-cyclometalated main ligands for efficient NIR-emitting Ir(III)-complexes. As a matter of fact, starting from the homoleptic fac-[Ir(iqbt)3] complex (λem = 690 nm) [13], concerted efforts including our group’ work have been devoted to the [Ir(iqbt)2(L^X)]-heteroleptic (HL^X = ancillary ligand) Ir(III)-complexes. Besides the superiority of the L^X-endowed (HL^X = N^N, N^OH or O^OH, etc) structural diversity (cationic [Ir(iqbt)2(N^N)]+ [14] or neutral [Ir(iqbt)2(O^O)] [[15], [15](a), [15](b)] and [Ir(iqbt)2(N^O)] [[16], [16](a), [16](b)] forms) through the easier and higher-yield synthesis, the modification of the HL^X ancillary ligand can provide a unique channel to color-tuning within the NIR-emissive regime. For example, through the disproportional stabilization of both the HOMO and LUMO levels with an increased HOMO-LUMO gap from the N^N-ancillary incorporation, the slight blue-shifts at λem = 682–683 nm for its [Ir(iqbt)2(N^N)]+ cations [14] in relative to λem = 690 nm of the fac-[Ir(iqbt)3] [13] were observed. On the contrary, contributing from the stabilization of the HOMO level and/or the destabilization of the LUMO one for the [Ir(iqbt)2(O^O)] [[15], [15](a), [15](b)] and [Ir(iqbt)2(N^O)] [16] Ir(III)-complexes, their LUMO-HOMO gaps were narrowed, to some extent, engendering the significant bathochromatic shifts (14–20 nm) of the [Ir(iqbt)2(O^O)]-heteroleptic (λem = 704–710 nm) [[15], [15](a), [15](b)] and the slight red-shifts (2–10 nm) of the [Ir(iqbt)2(N^O)]-heteroleptic (λem = 692–700 nm) [16] Ir(III)-complexes, respectively. More importantly, benefiting from the doping into the bipolar co-host of hole-transporting PVK (PVK = Poly(N-vinylcarbazole)) and electron-transporting OXD7 (OXD7 = 1,3-bis(5-(4-tert-butyl-phenyl)-1,3,4-oxadiazol-2-yl)benzene) [15], [16](b) or PBD (PBD = 2-(4-biphenyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole) [13], their desirable NIR-PLEDs were actually realized, whereas higher efficiencies in more dependence of the additional ETL-vacuum-deposition (such as TmPyPB, etc) were achieved for the multi-layer NIR-PLEDs [16b]. Herein, in light of the [Ir(iqbt)2(C^N2)]-heteroleptic Ir(III)-complex analogues (also Scheme 1) with more robust structure and higher thermal stability, the [Ir(C^N1)2(C^N2)]-heteroleptic new Ir(III)-complex [Ir(iqbt)2(Br-ppy)] also with NIR phosphorescence is molecularly designed. Moreover, in comparison to the routine TmPyPB-assisted multi-layer NIR-PLED-I through the doping of the Ir(III)-complex [Ir(iqbt)2(Br-ppy)] into PVK-OXD7, its all-solution-processed NIR-PLED-II is developed, from which, their comparable device performance is also expected.
Section snippets
Experimental
The detailed information of starting materials and general characterization methods was provided in the Electronic Supporting Information (ESI). The C^N1 main ligand Hiqbt (1-(benzo[b]-thiophen-2-yl)-isoquinoline) was synthesized from the improved Suzuki coupling reaction of 1-chloro-isoquinoline with benzo[b]thiophen-2-ylboronic acid as the literature [16].
Synthesis and characterization of the Ir(III)-complex [Ir(iqbt)2(Br-ppy)]
Based on the improved Suzuki coupling reaction [16] of cost-effective 1-chloro-isoquinoline (instead of 1-bromo-isoquinoline [15b]) with benzo[b]thiophen-2-ylboronic acid, the C^N1 main ligand Hiqbt was obtained in 73% yield. As to the μ-chloro-bridged dimer intermediate [Ir(dpqx)2(μ-Cl)]2, it was prepared from the typical procedure [17] by Nonoyama et al. through cyclometalation of the C^N1 main ligand Hiqbt with IrCl3⋅3H2O. Also as shown in Scheme 1, further upon the cyclometalation of the μ
Conclusions
In summary, using the [Ir(C^N1)2(C^N2)]-heteroleptic NIR-emitting (λem = 692 nm; 69% of the λem ≥ 700 nm proportion; τ = 0.48 μs and ΦPL = 0.19) Ir(III)-complex [Ir(iqbt)2(Br-ppy)] as the dopant into the PVK:OXD7 co-host, its TmPyPB-assisted multi-layer NIR-PLED-I and the all-solution-processed NIR-PLED-II were realized, respectively. Saliently, the attractive performance (ηEQEMax = 3.20% and negligible (ca. 5%) efficiency-roll-off) of the NIR-PLED-II, engenders [Ir(C^N1)2(C^N2)]-heteroleptic
Authors’ statement
All of authors promise that this work is original, unpublished even not being considered elsewhere, and we declare no conflict of interest.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Ackowledgements
This work is funded by the National Natural Science Foundation (21373160, 21173165), the MOE Laboratory of Bioinorganic and Synthetic Chemistry and the State Key Laboratory of Structural Chemistry (20190026) in P. R. of China.
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Both authors contributed equally to the study.