Ruthenium(II) complexes of 6,7-dicyanodipyridoquinoxaline: Synthesis, luminescence studies, and DNA interaction

The hexafluorophosphate and chloride salts of a series of ruthenium(II) com plexes incorporating a new dipyridophenazine-based ligand, dicnq (6,7-dicya nodipyrido[2,2-d:2',3'-f]quinoxaline synthesized in good-to-moderate yields . These mono ([Ru(phen)(2)(dicnq)](2+); phen = 1,10-phenanthroline), bis ([ Ru(phen)(dicnq)(2)](2+)), and tris ([Ru(dicnq)(3)](2+)) complexes are fully characterized by elemental analysis, infrared, FAB-MS, H-1 NMR, and cyclic voltammetric methods. Results of absorption titration and thermal denatura tion studies reveal that these complexes are moderately strong binders of c alf-thymus (CT) DNA, with their binding constants spanning the range (1-3) x 10(4) M-1. On the other hand, under the identical set of experimental con ditions of light and drug dose, the DNA (pBR 322)-photocleavage abilities o f these ruthenium(II) complexes follow the order [Ru(phen)(2)(dicnq)](2+) > [Ru(phen)(dicnq)(2)](2+) much greater than [Ru(dicnq)(3)](2+), an order wh ich is the same as that observed for their MLCT emission quantum yields. St eady-state emission studies carried out in nonaqueous solvents and in aqueo us media with or without DNA reveal that while [Ru(dicnq)(3)](2+) is totall y nonemissive under these solution conditions, both [Ru(phen)(2)(dicnq)](2) and [Ru(phen)(dicnq)(2)](2+) are luminescent and function as "molecular l ight switches" for DNA. Successive addition of CT DNA to buffered aqueous s olutions containing the latter two complexes results in an enhancement of t he emission in each case, with the enhancement factors at saturation being approximately 16 and 8 for [Ru(phen)(2)(dicnq)](2+) and [Ru(phen)(dicnq)(2) ](2+), respectively. These results are discussed in light of the relationsh ip between the structure-specific deactivations of the MLCT excited states of these metallointercalators and the characteristic features of their DNA interactions, and attempts are made to compare and contrast their propertie s with those of analogous dipyridophenazine-based complexes, including the ones reported in the preceding paper.