Photoactive transition metal complexes are known to serve as valuable platforms for studying photochemical energy and electron transfer processes and have extensive applications in the areas of sensing, photocatalysis, and photodynamic therapy for cancer. Our research program focuses on the rational synthesis of multimetallic phosphorescent cyclometalated iridium(III) and platinum(II) complexes for various applications including (A) organelle specific cellular imaging and photodynamic therapy; (B) use as sensing materials within the cellular environment.

A. A major challenge in the field of cellular imaging is the selective staining of specific cell organelles or sub-cellular compartments. The cyclometalated heteroleptic iridium(III) complexes of polypyridyl ligands are highly efficient in imaging of cells and cellular organelles, and are important tools for studying cellular functions. The primary criteria for designing a cell-organelle specific imaging agent involve the provision of interactions between the sub-cellular microenvironment and the potential imaging agent, resulting in specific localization and efficient fluorescence emission characteristics and they have also been reported to act as efficient photosensitizing agents and induce cell death by photoirradiation.

B. The strong emissive backbone of cyclometalated iridium(III) connected to the Pt(II)-amines can be used for sensing amino acids such as histidine, cystine, nucleic bases such as guanine or other various nitrogen or sulphur containing biomolecules. The adduct formation could easily be monitored through the change in their emission profile and the extensive photophysical study by our group reveals that a) the non-emissive metal centre connected to emissive Ir(III) centre do not affect the emissive behaviour much, b) The HOMO-LUMO is monitored mainly by the ligand group that holds the emissive metal centre. Though ligand group plays an expected role in determining HOMO-LUMO energy, they retain their redox character intact within the complexes. The strategic synthesis of mixed-metal complexes involves proper combination of the emissive centre (that does not directly interact with the biomolecule) and biologically relevant metal centre. The emissive centre thus plays a significant role in understanding the interaction without being exposed to the biomolecules.