2,3-Diarylquinoline -Based Iridium (III) Complexes: Synthesis, Photophysical Properties and DFT studies

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Phosphorescent transition–metal complexes have received considerable interest in the development of efficient organic light-emitting diodes (OLEDs). Iridium complexes derived from heterocyclic ligands have brought a paradigm shift in the optoelectronic materials. Quinoline-based molecular architectures offer excellent materials for such applications due to their exceptional electronic properties, extended π-conjugation and strong ligand–metal binding. Iridium complexes of 2,3-diaryl quinolines with electron rich and electron deficient substituents were synthesized and their electronic properties were studied. Iridium complexes Ir1 and Ir2 bearing the electron withdrawing (-CF3) and electron donating (–OCH3) groups, respectively at the para position of the 2-phenyl ring of 2,3-diarylquinoline exhibited prominent absorption bands in the UV range. Further, the complex Ir3 with unsubstituted 2,3-diarylquinoline ligand and CF3 containing Ir1 showed intensive red phosphorescence near 603 and 611 nm, respectively due to metal-to-ligand and ligand-to-ligand (MLCT/LLCT) charge transfer characteristics. The photoluminescence (PL) spectra of the Ir4 complex derived from 2-phenylquinoline ligand exhibited a bright yellow phosphorescent emission at 589 nm. An additional C3 aryl group on the quinoline ring resulted in a red shift in the emission wavelength of Ir1 and Ir3 complexes compared to Ir4. The PL spectra of Ir2 on the other hand was weak, and blue-shifted to 489 nm, which can be attributed to a ligand-centered (LC) transition. The complexes showed photoluminescence quantum yield (PLQY) in the range of 0.004 to 0.17 and fluorescence lifetime ranged from 1.5 ns to 399 ns, implying the significant effect of the ligand structure on the properties of the complexes. Photophysical investigation of the complexed affirms these complexes to be potent materials for OLEDs fabrication. Cyclic voltammetry was used to investigate the electrochemical characteristics of Ir1-Ir4 complexes in acetonitrile. All of the complexes Ir1-Ir4 exhibited irreversible oxidation waves at 0.44 V, 0.23 V, 0.25 V, and 0.24 V respectively. Ir1-Ir4 complexes showed a reduction potential at −0.98 V, −0.96 V, −1.02 V and −0.97 V respectively. The DFT calculations were used to explore HOMO-LUMO energies, and NBO characteristics by B3LYP level of theory using ECP type double zeta (DZ) quality basis set LanL2DZ for metal and 6-311G(d) basis sets for non-metal atoms. The theoretical calculations were in significant agreement with the experimental studies of these complexes.