New Pt(II) diimine bis(acetylide) complexes where the diimine is a substituted bipyridine or phenanthroline and the arylacetylide is 4-ethynylbenzaldehyde have been prepared in good to excellent yields. Spectroscopic characterization supports a square planar coordination geometry with cis-alkynyl ligands, and the crystal structure of one of the complexes, Pt(phen)(CdropCC(6)H(4)CHO)(2) (1), confirms the assignment. The new diimine bis(acetylide) complexes exhibit an absorption band ca. 400 nm that corresponds to a Pt(d) --> pi* diimine charge transfer transition and are brightly emissive in fluid solution, with excited state lifetimes in the range 100-800 ns. Correlation of diimine substituent with lambda(max) for the 400 nm absorption band gives strong support to the MLCT assignment. Complex 1 undergoes electron transfer quenching, showing good Stern-Volmer behavior with a variety of oxidative and reductive quenchers. Quenching studies conducted with DNA nucleosides (A, T, C, G) were also investigated. Silyl-protected adenosine and guanosine were found to quench the luminescence of 1 better than similarly protected cytidine or thymidine. Since the former are the more easily oxidized bases, the results suggest that the Pt(II) diimine bis(acetylide) complexes are more powerful photooxidants than photoreductants with regard to electron transfer to DNA bases.