Tailoring Photophysics and DNA Binding through Ligand Engineering in Ir(III) Complexes

Abstract

This thesis explores the design, synthesis, and functional evaluation of cyclometallated Iridium(III) complexes, focusing on their photophysical properties and applications in energy upconversion and DNA binding. The work presents systematic ligand modifications as a strategy to control excited-state behaviour, aiming to enhance triplet- triplet annihilation upconversion (TTA-UC) efficiency, aqueous solubility and DNA-binding properties.

Chapter 2 investigates benzimidazole-based Ir(III) complexes bearing electron-donating and electron-withdrawing substituents. Structural tuning revealed a direct correlation between substitution pattern and the balance of 3MLCT/3LC character, affecting triplet- state lifetime and TTA-UC performance. Complexes containing electron-withdrawing groups demonstrated extended triplet lifetimes and superior TTA-UC efficiency in the presence of diphenylanthracene (DPA).

Chapter 3 focuses on the development of water-soluble Ir(III) luminophores using benzothiazole-based ligands and ethylenediamine co-ligands. These complexes exhibited visible green emission and favourable photophysical properties. Binding interactions with DNA were studied through UV-visible titrations, fluorescence, isothermal titration calorimetry (ITC), and docking simulations, indicating a groove-binding mechanism influenced by ligand electronics and steric effects.

Chapter 4 presents a series of quinoline-based Ir(III) complexes incorporating thiophene and amide functional groups, specifically designed to explore emission tuning toward the red region. Systematic variation of electron-donating and electron-withdrawing substituents on the quinoline framework allowed modulation of the emission properties, with the most red-shifted complex emitting at 642 nm. Photophysical and electrochemical data showed that electronic and structural modifications strongly affected emission behaviour.

Collectively, these studies demonstrate a modular approach to designing Ir(III) complexes with tunable excited-state and DNA-binding properties, offering potential for applications in energy upconversion, molecular recognition, and bioimaging.