Theoretical studies on the structural and optical properties of a series of Os(II) diimine complexes [Os(N∧N)(CO)2I2] (N∧N = 2,2′-bipyridine(bpy), 4,4′-di-tert-butyl-2,2′-bipyridine(dbubpy), 4,4′-dichlorine-2,2′-bipyridine(dclbpy))