Thienothiophenyl‐Isoquinoline Iridium Complex‐Based Deep Red to Near‐Infrared Organic Light‐Emitting Diodes with Low Driving Voltage and High Radiant Emittance for Practical Biomedical Applications

been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/adpr.202100121. This article is protected by copyright. All rights reserved New, Reliable Thienothiophenyl-isoquinoline Iridium complex-based Deep Red to Near-Infrared Organic Light-Emitting Diodes with Low Driving Voltage and High Radiant Emittance for Practical Biomedical Applications


Introduction
Organic light-emitting diodes (OLEDs) have become mainstream as the next generation of wearable displays that will outperform lasers and light-emitting diodes (LEDs). Recently, OLEDbased platforms used for light applications were introduced. [1][2][3] In particular, DR/NIR OLEDs have emerged in the past few years because they can be applied for night vision displays, optical sensors, and phototherapy [4] . However, DR/NIR emitters exhibit intrinsic defects: these emitters are prone to undesired non-radiative decay pathways owing to the narrow bandgap. According to the energy-gap law, the nonradiative decay rate (knr) is inversely proportional to the band gap. Besides this, the radiative rate (kr) has a cubic dependence on the transition energy, so that it is expected to get smaller for lower-energy emitters, implying that DR/NIR emitters have poor quantum efficiencies because of the incorporation of ground-and excited-vibrational energy states. [5][6][7][8][9] Consequently, research on high efficiency DR/NIR OLEDs has lagged behind that on visible-region OLEDs. To overcome this limitation, diverse ways of boosting the DR/NIR efficiency were introduced. In general, there are three kinds of DR/NIR emitters for high efficiency DR/NIR OLEDs: donoracceptor-donor (D-A-D) type, [10,11] thermally activated delayed fluorescence (TADF), [12][13][14][15] and transition-metal complexes. [16,17] DR/NIR fluorophores of the D-A-D type have been researched to provide DR/NIR OLEDs with cost advantage and versatility. However, most DR/NIR OLEDs based on the D-A-D type show extremely low EQE, radiance, and unexplained operational lifetime for practical application. [10,11] TADF DR/NIR emitters utilize nonradiative triplet excitons that move to a singlet region and achieve 100% internal quantum efficiency (IQE). [18] Nonetheless, there are chronic constraints with fluorescence quenching. Hence, TADF-based DR/NIR OLEDs have a low luminance value. Meanwhile, high-efficiency DR/NIR OLEDs using transition metal complexes such as osmium (Os), [19][20][21] platinum (Pt), [22][23][24][25][26] and iridium (Ir) [27][28][29][30][31][32][33][34][35][36][37][38] have been introduced recently. They exhibit better performance with regard to EQE and radiant emittance than prior methods. Among the research published on DR/NIR OLEDs, a Pt-complex-based NIR OLED was reported with the highest record of EQE and radiant emittance. [22] Despite this world-record,

Accepted Article
This article is protected by copyright. All rights reserved there is still the need to overcome reliability issues, such as driving voltage and lifetime, to achieve practical application. Even if the radiant emittance is high, problems with power consumption could occur in the actual application stage if the driving voltage is too high. Actually, there have been no cases of applying a DR/NIR OLED to the biomedical field due to these problems. In general, bioapplications use wavelengths in the 600-700 nm region [39] or long wavelength regions above 780 nm. Moreover, biomedical application is important to view it from the perspective of radiant emittance, not from that of device efficiency. Based on previous works that reported good synergetic effects at specific wavelengths, we designed the novel DR/NIR emitter.
In Figure 1, we report a novel Ir(Ⅲ) complex designed using thieno [3,2-b]thiophen-isoquinolinebased chelating ligands. The thienothiophene moiety, containing a fused thiophene structure, has a more rigid structure and extended conjugation length, which could be suitably employed to lower the band gap required for DR/NIR emitters. [40][41][42] In addition, the high electron density of thienothiophene strengthens the covalent bond between the chelating ligand and iridium, which could improve the operational lifetime and lower the driving voltage. [40][41][42][43] Also, by employing an isoquinoline group instead of a pyridine ring, we can expect an additional red shift of over 100 nm. [32,44] Furthermore, a heteroleptic iridium complex using 2,2,6,6-tetramethyl-3,5-heptanedione as an ancillary ligand was developed to increase the quantum efficiency for high horizontal orientation. [45] Hence, we can effectively design a DR/NIR OLED that has low driving voltage and high radiant emittance for practical biomedical applications.

Synthesis and Characterization
The iridium complex was synthesized using the synthetic route shown in Scheme 1.

Accepted Article
This article is protected by copyright. All rights reserved chloroisoquinoline. A -chloro-bridged dimer was synthesized through the well-known Nonoyama reaction and base-meditated ligand exchange from iridium chloride hydrates(IrCl3 nH2O) and 1- (2). Then, (ttiq)2Ir(tmd) was prepared from Ir(Ⅲ) dimer [(ttiq)2Ir( -Cl)]2 and 2,2,6,6-tetramethyl-3,5-heptanedione. This pure iridium complex was characterized via 1 H-NMR, 13 C-NMR spectroscopy, and high-resolution mass spectroscopy. All intermediate compounds were also confirmed by identical methods (see Support Information Figures S1-10). The thermal stability of the new (ttiq)2Ir(tmd) complex was studied using TGA thermogram and DSC. The new (ttiq)2Ir(tmd) complex showed thermal stability up to 325 °C and did not show any transition up to 250 °C. This supports the notion that the complex shows sufficient thermal stability for use in OLEDs. The electrochemical property of (ttiq)2Ir(tmd) was measured using cyclic voltammetry with 0.1 M tetrabutylammonium perchlorate in dichloromethane solution.
The HOMO level was calculated to be -4.95 eV, and the shallow HOMO energy level may have resulted from the electron-rich thienothiophene unit.

Photophysical Properties
The UV-visible absorption and photoluminescence spectra of Ir(ttiq)2tmd in solution and as vacuum-deposited thin-film are both shown in Figure 2. The intense absorption band below 450 nm can be assigned to the spin-allowed π-π* transitions. The relatively weak absorption band in the range of 500-620 nm in both conditions corresponds to mixed 1 MLCT and 3 MLCT (singlet and triplet metal-to-ligand charge transfer) with an 1 ILCT (inter-ligand charge transfer). It is notable that the MLCT bands of Ir(ttiq)2tmd represent red-shifted absorption. The band gap was calculated and found to be about 2.13 eV at the band edge. The photoluminescence (PL) spectra of Ir(ttiq)2tmd in THF at 77 K shows an emission peak of 704 nm. Meanwhile, the emission peaks in CH2Cl2 solution and in vacuum-deposited thin-film at room temperature (298 K) were observed at 690 nm with a shoulder at 742 and at 696 nm with a shoulder at 750 nm. The emission peak of Ir(ttiq)2tmd was red-shifted by about 60 nm compared to that of the well-known red-emitter Ir(piq)2(2,2,6,6-

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This article is protected by copyright. All rights reserved tetramethylheptane-3,5-dione), [46] which may due to increased electron density as well as effective extension of the conjugative structure of the cyclometalating ligand, 1-(thieno[3,2-b]thiophen-2yl)isoquinoline. The absolute photoluminescence quantum yields (PLQY) of solution and thin-film were 15% and 24%, respectively. The PL decay curve of Ir(ttiq)2tmd in DCM solution is shown in Figure S11; radiative lifetime ( ) is 0.66 μs. The radiative ( ) and nonradiative ( ) transition rates are obtained from PLQY and radiative lifetime ( ). All quantities are summarized in Table 1.

Electroluminescence (EL) Performance
Based on the photoluminescence of the novel Ir(Ⅲ) complex, devices were fabricated in the

Accepted Article
This article is protected by copyright. All rights reserved All devices have the same light distribution (i.e., Lambertian) and we could easily calculate the total radiant emittance of each device.
The mCP and Bebq2 have been widely used for host material to transfer energy from host to dopant. [47] However, Device 1 performed poorly owing to triplet-triplet annihilation between host and dopant and exciton quenching on the grounds of high triplet energy in the mCP compared to the Bebq2 (T1 of mCP : 2.9 eV, Bebq2 : 2.5 eV). Therefore, Devices 2 and 3 gave outperformed Device   5b) and device reliability such as Von (Figure 5c) and operational lifetime (Figure 4b) for

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This article is protected by copyright. All rights reserved biomedical application, we obtained the competitive DR/NIR OLED. Figure 5d shows the DR/NIR OLED operating at high radiant emittance.

Cell-based In-vitro Experiment
Our group has been using OLED to irradiate cells and conducting research on its effects in various ways. [2,39,48] Likewise, we verified the cell proliferation effect using our Ir(Ⅲ)-based DR/NIR OLED in diverse conditions. Normal human fibroblasts were sprinkled on 96-well plates. An OLED jig designed by our group was used for the cell experiment in order to irradiate the cells with light. It was designed to provide irradiation 1.5 cm from the fibroblast cells and was used to irradiate in a 96-well plate under three conditions including the control. Through this in-vitro experiment, irradiation at the radiant emittance of 1.5 or 3 mW cm -2 was applied for 10, 20, and 30 min, respectively, and with the control, to verify cell cytotoxicity. According to ISO 10993-5 standards, cell viability exceeded 70% and passed the standard under all experimental conditions, as shown in Figure 6a. After that, DR/NIR OLEDs were irradiated in the same manner as in the existing cytotoxicity test to confirm cell proliferation. As a result, as the exposure time increases, the cell proliferation effect rises in each condition. In particular, a cell proliferation effect of 24% compared to the control group was observed with irradiation for 30 min at 1.5 mW cm -2 , as shown in Figure 6b.

Conclusion
In summary, novel Ir(Ⅲ)-based DR/NIR OLED were designed and synthesized to produce a DR/NIR emission and low driving voltage for biomedical application. The newly-developed thieno[3,2-b]thiophen-isoquinoline-based chelating ligand, which enhanced the electron density and extended the conjugation length, is expected to narrow the energy gap in the longer wavelength region of DR/NIR emission. The maximum EQE of the our newly-fabricated DR/NIR OLED was 2.75%, which is the best performing among the reported DR/NIR OLEDs based on the Ir(Ⅲ) Accepted Article complex. Moreover, the new Ir(Ⅲ)-based DR/NIR OLED has proven reliable by optimizing the structure to ensure sufficient lifetime at actual bio-applicable radiant emittance. We confirmed a cell proliferation effect of up to 24% after irradiating our new Ir(Ⅲ)-based DR/NIR OLED. It can accurately irradiate the target wavelength even when applied to actual human skin because our device has no angle dependence. Moreover, there are ample possibilities and room for its practical application in biomedical fields.

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General information and Materials:
All starting materials were purchased from Aldrich and Alfa Aesar. Tetrakis(triphenylphosphine)palladium(0) was purchased from Boom King. All reagents purchased commercially were used without further purification. Tetrahydrofuran (THF) was dried over sodium and benzophenone, and thieno [3,2-b]thiophen-2-ylboronic acid (1) was prepared according to the literature. [49] Measurement: The 1 H NMR and 13 C NMR spectra were recorded using a Bruker 300 MHz

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.

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