The design and synthesis of economically viable homogeneous and heterogeneous transition metal complexes for photocatalytic hydrogen generation using visible light irradiation and as OLED’s are under intense investigation. Transition metal complexes are drawing great interest as they offer highly efficient room temperature phosphorescence. In particular iridium (III) complexes are considered to be the most promising since they exhibit: (1) good photo and thermal stabilities, (2) high phosphorescence quantum efficiency, (3) very large values for excited state lifetimes, (4) facile colour tuning through ligand structure control and (5) large cross section for exciton formation. Such characterstic features are attributed to the efficient spin-orbit coupling provided by the Ir metal as well as the strong structural or electronic interactions between the Ir metal and ligands. Chapter 1 starts up with the general introduction to various technologies used at present for H2 production followed by its storage, distribution and utilisation. Basic mechanism for natural photosynthesis is described along with different artificial photosynthetic systems giving special attention to intermolecular and intramolecular photocatalyts. Effort has been taken to include most of the recent publications in these catagories. This is followed by a complete insight into the iridium chemistry starting with the iridium complexes using (N^N) co-ordinating ligands and multimetal complexes derived from it followed by Ir complexes with the cyclometallating ligands which then moves along the synthetic aspects and photophysics of the tris complexes and then to the importance of iridium terpyridine complexes. A brief history on the basic excited state properties of iridium metal complexes is described after that. The chapter ends up with the application of the iridium complexes mainly concentrating on the OLED’s. Chapter 2 details the basic synthetic procedures and instruments employed in the studies presented in later chapters. Various techniques used for the characterisation of the complexes including 1D and 2D Nuclear Magnetic Resonance Spectroscopy (NMR), Mass Spectroscopy, Elemental Analysis (CHN), High Performance Liquid Chromatography (HPLC), Absorption and Emission spectroscopy, Time Corelated Single Photon Counting (TCSPC), Laser Flash Photolysis and Gas Chromatography (GC) are briefly described in this chapter. Chapter 3 deals with the synthesis of iridium polypyridyl complexes with various cyclometallating ligands used for intermolecular and intramolecular photocatalytic H2 generation from H2O. A series of heteroleptic iridium monomers and hetrodinuclear Ir-Pt/Pd dimers, which are potential candidates for photocatalytic H2 generation, are described along with preliminary photophysics measurements and photocatalytic H2 production results as Turn Over Numbers (TON’s) measured using gas chromatography. Chapter 4 describes the detailed excited state photophysics of novel iridium complexes. Absorption and emission spectra of these novel complexes are taken in different solvents in order to examine any solvatochromic effects. Deaeration was done using freeze pump thaw method in order to remove any oxygen present in the samples that can cause quenching. Temperature dependent emission measurements were carried out in temperature range 77 K to 298 K in every 10 K temperature interval. Temperature dependent lifetime measurements were also carried out in temperature range 77 K to 298 K in every 10 K temperature interval using TCSPC and a cryostat. Chapter 5 gives an account on the novel high yield synthesis, characterisation, reaction mechanism and excited state photophysics of Ir(polypyridyl)2Cl2 complexes and their deuteriated analogous. Taking [Ir(bpy)2Cl2]PF6 as the standard complex significant effort has been made to figure out the synthetic mechanism of the reaction with the aid of NMR and HPLC done in a concomitant way. This section also explains the novel synthetic method for removing the inner sphere chlorides as triflate complexes and binding the triflate intermediates to other polypyridyl ligands. Detailed study has been carried out on probing the excited state photophysics of these complexes. Excited state lifetimes measured using both TCSPC and laser flash photolysis which provided promising evidence on the effect of deuteriation on lifetimes and the role these complexes can play in OLED applications. Effort has taken in order to study intermolecular photocatalysis with these complexes. Chapter 6 gives an overview of all the work that has been carried out up to date and a future research plan is also included.