There are several strategies in modifying the electronic structure of molecules. Functional group modification is perhaps the most used approach. In this thesis, I explore a different approach through the expansion of ligand π-systems by fusing benzene rings (benzannulation). Benzannulation introduces physicochemical and photophysical changes in molecules without significant changes in the parent framework. In this work, I use computational analysis to investigate the impact of benzo-fused pyridines, quinoline and phenanthridine. Phenanthridine introduces a lower-lying unoccupied molecular orbital (LUMO) compared with quinoline, red-shifting the absorption spectra of complexes. Combining phenanthridines with π-donor ligands enabled the isolation of iron complexes with panchromatic absorption. In addition, benzannulation enables more efficient mixing between the excited singlet and triplet states facilitating the direct Tn←S0 transition. Interestingly, the luminescent properties of the complexes supported by phenanthridine exhibit blue-shifted emission compared with the quinoline-supported analogues. The enhanced rigidity provided by phenanthridine contributes to the observed higher energy shift. Building on these findings, a series of new phenanthridine-containing ligands and their boron, zinc and platinum complexes were then pursued. Some of these complexes exhibit long-live emissive excited states originating from triplet states. In addition, new phenanthridine precursors bearing water-soluble and surface-anchoring groups were introduced. Finally, the antineoplastic activity of Pt(II) complexes supported by N^N^O ligands was explored, leading to the discovery of complexes with enhanced cytotoxicity compared with cisplatin both in dark and upon exposure to light.