Square planar d8 platinum(II) complexes and octahedral d6 iridium(III) complexes were synthesized. Their photophysics were studied in detail. Structure-property relationship was studied by varying the substitution on the ligands or the π-conjugation extent of the ligands. In Chapter 2, bipyridyl platinum(II) bisstilbenylacetylide complexes (2-1 – 2-6) with different auxiliary substituents on the stilbenylacetylide ligands were synthesized. While the substitution of H on the 4'-position of stilbene by Br and OMe groups does not alter the photophysical properties of the complexes eminently, the photophysical properties are significantly tuned by the CHO, NO2 and NPh2 substituents. In Chapter 3, platinum(II) complexes (3-1 – 3-6) containing 6-[7-R-9,9-di(2-ethylhexyl)-9H-fluoren-2-yl]-2,2'-bipyridine (R = NO2, CHO, benzothiazol-2-yl, n-Bu,carbazol-9-yl, NPh2) ligands were synthesized. It is found that electron-withdrawing substituents (NO2, CHO, BTZ) and electron-donating substituents (n-Bu, CBZ, NPh2) exert distinct effects on the photophysics of the complexes. In chapter 4, platinum(II) complexes (4-1 – 4-6) containing 6-[9,9-di(2-ethylhexyl)-7-R-9H-fluoren-2-yl]-2,2'-bipyridine ligands (R = 4-R'-phenylethynyl with R'= NO2, BTZ, H and OCH3 or R = 4'-BTZ-phen-1-yl or BTZ) were synthesized. The effects of terminal substituents and the different π-conjugated linkages between the BTZ component and the C^N^N core on the photophysics of these ligands and complexes were systematically investigated. In Chapter 5, iridium(III) complexes (5-1 – 5-5) featuring 7-(benzothiazol-2-yl)-9,9-di(2-ethylhexyl)-9H-fluoren-2-yl attachment to the 2-phenylpyridine was synthesized and studied. The effects of the extent of the π conjugation was studied in by the comparison between 5-1 and 5-2, and the effect of the number of the 7-(benzothiazol-2-yl)-9,9-di(2-ethylhexyl)-9H-fluoren-2-yl unit was compared in 5-3 – 5-5. In Chapter 6, bypyridyl iridium(III) complexes (6-1 – 6-7) with different cylometallated arylpyridyl ligands were synthesized. The effects of π-conjugation extension and direction were systematically investigated. Most complexes showed moderate to strong ns transient absorption from visible to near-IR region, indicating stronger excited-state absorption than ground-state absorption in the corresponding region and potential application as reverse saturable absorption materials. Thus, their application as nonlinear absorption materials was demonstrated by reverse saturable absorption (RSA) upon 532 nm ns laser. The RSA trend can be deciphered by the absorption cross section ratio between the excited states and ground states (σex/σ0).