Ligand-Driven Structural and Photophysical Modulation in Iridium(III) Complexes: Design, Synthesis, and Applications in Optoelectronics

This study presents the rational design and synthesis of six novel iridium(III) complexes (Ir-1 to Ir-6) incorporating piperazine-functionalized pyridine ligands (L1 and L2), aiming to systematically investigate ligand-induced structural and photophysical modifications. Single-crystal X-ray diffraction analyses revealed distinct packing patterns: Ir-1–Ir-3 form flexible zigzag structures with tight 3D stacking facilitated by CH−π and hydrogen-bonding interactions, whereas Ir-4–Ir-6 exhibit rigid frameworks due to methyl substitution on L2, resulting in looser packing stabilized by anion-mediated hydrogen bonds. UV–vis and fluorescence spectroscopy demonstrated ligand-dependent electronic properties, with Ir-6 displaying the strongest emission intensity (∼300,000 au at 330 nm) and a red-shifted absorption (λ_abs = 395 nm), attributed to enhanced π-conjugation from naphthoquinone ancillary ligands. Emission maxima progressively red-shifted from Ir-1 (280 nm) to Ir-6 (330 nm), correlating with increased ligand rigidity and metal-to-ligand charge transfer (MLCT) character. The tunable photophysical properties, combined with excellent thermal stability and scalable synthesis, highlight the potential of these complexes for UV-driven optoelectronic applications, such as photocatalysis and organic light-emitting diodes (OLEDs).