Synthesis and photophysical properties of luminescent platinum(II) complexes with terdentate polypyridine ligands: [Pt(bpqb)X] and [Pt(tbbpqpy)X](PF6) (bpqb-H = 1,3-bis(4′-phenyl-2′-quinolinyl) benzene; tbbpqpy = 4-tert-butyl-1,3-bis(4′-phenyl-2′-quinolinyl) pyridine; X = Cl, CCC6H5, CCC6H4NMe2, CCC