Enhanced DNA photocleavage properties of Ru(II) terpyridine complexes upon incorporation of methylphenyl substituted terpyridine and/or the polyazine bridging ligand dpp (2,3-bis(2-pyridyl)pyrazine)

Abstract

The heteroleptic complexes, [(MePhtpy)RuCl(dpp)](PF 6) and [(tpy)RuCl(dpp)](PF 6), have been synthesized, characterized, and investigated with respect to their photophysical, redox, and DNA photocleavage properties (where MePhtpy = 4′-(4-methylphenyl)-2,2′:6′,2′′-terpyridine and dpp = 2,3-bis(2-pyridyl)pyrazine, tpy = 2,2′:6′,2′′-terpyridine). The X-ray crystal structure confirms the identity of the new [(MePhtpy)RuCl(dpp)](PF 6) complex. These heteroleptic complexes were found to photocleave DNA in the presence of oxygen, unlike the previously studied complex, [Ru(tpy) 2](PF 6) 2. The photophysical, redox, and DNA photocleavage properties of the heteroleptic complexes were compared with those of the homoleptic complexes, [Ru(MePhtpy) 2](PF 6) 2 and [Ru(tpy) 2](PF 6) 2. The heteroleptic complexes showed intense metal to ligand charge transfer (MLCT) transition at lower energy ([(MePhtpy)RuCl(dpp)](PF 6), 522 nm; [(tpy)RuCl(dpp)](PF 6), 516 nm) and longer excited state lifetimes as compared to the homoleptic complexes. The [Ru(MePhtpy) 2] 2+ complex was found to photocleave DNA in contrast to [Ru(tpy) 2] 2+. The introduction of a methylphenyl group on the tepyridine ligand not only enhances the 3MLCT excited state lifetime but also increases the lipophilicity and thereby the DNA binding ability of the molecule. An increase in lipophilicity upon addition of a methylphenyl group on the 2,2′:6′,2′′-terpyridine ligand was confirmed by determination of the partition coefficient ([(MePhtpy)RuCl(dpp)](PF 6), log P = +1.16; [(tpy)RuCl(dpp)](PF 6), log P = −1.27). The heteroleptic complexes photocleave DNA more efficiently than the homoleptic complexes, with the greatest activity being observed for the newly prepared [(MePhtpy)RuCl(dpp)](PF 6) complex.