Synthesis and Spectroscopic Characterization of a Homologous Series of Iridium(III) Cyclometalated Complexes

This work investigates the synthesis, structural characterization, and photophysical properties of a homologous series of cyclometalated iridium(III) complexes of the general form [Ir(C^N)2(N^N)+, where C^N represents 2,3-diphenylquinoxaline based ligands and N^N is 1,10-phenanthroline. A systematic ligand design strategy was employed to evaluate the effect of para-substituted halogen groups (F, Cl, Br) on the electronic structure and emission behavior of the resulting complexes while maintaining a constant coordination environment. The complexes were synthesized using a microwave-assisted methodology optimized for efficiency and reproducibility, followed by purification and comprehensive characterization using MALDI-TOF mass spectrometry and multidimensional NMR techniques. A combined approach utilizing 1H, 13C, COSY, HSQC, HMBC, IDR-HSQC-TOCSY, and ROESY experiments enabled complete structural assignment and provided detailed insight into ligand connectivity, spatial orientation, and electronic communication within the complexes. Notably, evidence of through-metal heteronuclear coupling between ligands was observed, highlighting a previously unreported interaction mediated by the iridium center. Photophysical properties were investigated using UV-visible absorption and emission spectroscopy, quantum yield measurement, and excited-state lifetimes analysis. All complexes exhibited characteristic phosphorescent emission arising from triplet-excited states, with systematic blue shifts observed upon the incorporation of the electron-withdrawing halogen substituents. The structure-property relationships revealed that ligand substitution significantly influences metal-to-ligand charge transfer (MLCT) character, emission energy, and excited-state dynamics. Overall, this study established clear correlations between ligand design and photophysical behavior in iridium(III) complexes, providing fundamental insight into their electronic structure and demonstrating their potential as tunable emissive materials for applications in organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs).