Octahedral d6 iridium(III) complexes possess rich photophysical properties. The most distinct phtotphysical properties of the Ir(III) complexes are their high triplet excited-state formation quantum yields, long-lived triplet excited states, and feasile structural modifications. To better understand the impact of ligand -conjugation on the photophysics and reverse saturable absorption (RSA) or PDT of the Ir(III) complexes, six series of Ir(III) complexes bearing various bidentate or terdentate ligands were designed and synthesized in this dissertation. In Chapter 1, the photophysical principles, typical electronic transitions in Ir(III) complexes, the prototypes of the tris-bidentate and bis-terdentate Ir(III) complexes, the state-of-art on exploring Ir(III) complexes for RSA and PDT, and the materials design criteria are reviewed. In Chapters 2 and 3, sixteen cyclometalated cationic Ir(III) complexes were synthesized and investigated to understand how the benzannulation site on diimine ligands influences the characteristics of the excited states of these complexes. The site−dependent benzannulation influenced the spectral feature and intensity of the triplet transient absorption (TA) and T1 lifetimes, and their RSA strength. In Chapter 4, ten 2-phenylpyridine based Ir(III) complexes with varied degrees of π-conjugation and sites of benzannulation were synthesized. Benzannulation at the different sites of 2-phenylpyridine exerted a different effect on the energies of the S1 and T1 excited states, the TA spectral features, and the RSA performances of the complexes. In Chapter 5, the synthesis, photophysics, and RSA of three Ir(III) complexes with different degrees of -conjugation on the diimine ligands were discussed. The impact of this structural variation on the RSA at 532 nm was demonstrated. In Chapter 6, five Ir(III) complexes bearing terpyridine-capped fluorenyl bridging ligands and different terminal terdentate ligands were investigated to reveal the effects of different terminal ligands on the S1 and T1 excited states. Their in vitro theranostic PDT effects toward the SKMEL28 cells were evaluated. In Chapter 7, the photophysics and in vitro PDT studies of five neutral Ir(III) complexes incorporating BODIPY-substituted N-heterocyclic carbene (NHC) ligands were studied. The attachment position of the BODIPY substituent did not alter the photophysical properties significantly but changed the dark- and photo- cytotoxicity of these complexes toward SKMEL28 cells.