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)

The heteroleptic complexes, [(MePhtpy)RuCl(dpp)](PF6) and [(tpy)RuCl(dpp)1(PF6), 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)](PF6) complex. These heteroleptic complexes were found to photocleave DNA in the presence of oxygen, unlike the previously studied complex, [Ru(tPY)(2)](PF6)(2). The photophysical, redox, and DNA photocleavage properties of the heteroleptic complexes were compared with those of the homoleptic complexes, [Ru(MePhtpy)2](PF6)(2) and [Ru(tPY)21(PF6)2. The heteroleptic complexes showed intense metal to ligand charge transfer (MLCT) transition at lower energy ([(MePhtpy)RuCl(dpp)](PF6), 522 nm; [(tpy)RuCl(dpp)](PF6), 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 (MLCT)-M-3 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)](PF6), logP = +1.16; [(tpy)RuCl(dpp)](PF6), logP = -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)](PF6) complex. (c) 2008 Elsevier Inc. All rights reserved.