Impacts of methyl substituents on luminescence properties of NIR phosphorescent iridium(III) complexes with 2-phenylquinoxaline- based cyclometalated ligands Keima Yoneda 1 , Ryuta Shikura 1 , Naoya Suzuki 1 , Shintaro Kodama 1 , Takeshi Maeda 1 , Shigeyuki Yagi 1 , Hideki Fujiwara 2 , Seiji Akiyama 3 1 Graduate School of Engineering, Osaka Metropolitan University, Japan, 2 Graduate School of Science, Osaka Metropolitan University, Japan, 3 Science & Innovation Center, Mitsubishi Chemical Corporation, Japan Organic light-emitting diodes (OLEDs) have so far been applied to flat panel displays for mobile phones and TV sets, and thus, electroluminescence (EL) in the visible region has been focused on. 1 Nowadays, near infrared (NIR) OLEDs are attracting increasing attention from the viewpoint of next-generation technologies. NIR light over 700 nm is invisible to the naked eye and penetrates living tissues. Thus, NIR light-emitting devices are expected as light sources in the fields of security technology and medical science. In the case of OLED, phosphorescent materials are superior to traditional fluorescent ones in terms of the exciton generation efficiency. 2 Few examples of practical NIR phosphorescent materials have, however, been reported so far. We recently reported NIR phosphorescent tris-cyclometalated iridium(III) complexes with 2-(benzo[ b ]furan-2-yl)quinoline- and 2-(benzo[ b ] thiophen-2-yl)quinoline-based cyclometalated ligands, the photoluminescence (PL) spectral bands of which reach the NIR region (PL maxima; 690 and 696 nm, respectively) in dichloromethane at room temperature, respectively. 3 Towards development of highly emissive NIR phosphorescent materials with further red-shifted PL maxima, here we report the synthesis and photoluminescent properties of novel bis-cyclometalated iridium(III) complexes with 2-phenyl-3-(trifluoromethyl)quinoxaline-based ligands. We discuss the impact of the peripheral methyl substituents of the phenyl group, on their PL properties, especially focusing on elongation of the emission wavelength. We found that the introduction of methyl groups at the 3’- and 5’-positions of each of the cyclometalated ligands are effective to induce a red shift of the emission band: for example, the 3’,5’-dimethyl substituted complex exhibited NIR emission with a PL maximum at 771 nm in toluene (PL quantum yield; 0.05), 58 nm red-shifted in comparison with the unsubstituted complex. Using the present NIR phosphorescent iridium(III) complexes as an emitting dopant, NIR phosphorescent OLEDs were successfully fabricated by solution processing. Upon application of electric voltage, NIR emission was emitted from the fabricated device, the EL spectrum of which corresponded to the PL spectrum of the emitting dopant. References 1. S. Yagi, in Progress in the Science of Functional Dyes , eds. Y. Ooyama and S. Yagi, Springer, Singapore, 2021, pp.561−601. 2. C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest, J. Appl. Phys. , 2001, 90 , 5048−5051. 3. S. Ikawa, S. Yagi, T. Maeda, H. Nakazumi, H.Fujiwara, S. Koseki and Y. Sakurai, Inorg. Chem. Commun. , 2013, 38 , 14−19.
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