Lighting Up DNA in the Near-Infrared: An Os(II)–pydppn Complex with Light-Switch Behavior

The osmium(II) polypyridyl complex [Os(tpy)(pydppn)]²⁺ (tpy = 2,2':6',2″-terpyridine; pydppn = 3-(pyrid-2'-yl)-4,5,9,16-tetraaza-dibenzo[a,c]naphthacene) was synthesized and characterized to evaluate the effect of an extended planar π-system on photophysical properties and DNA interactions. This complex represents the π-expanded analog of the previously studied [Os(tpy)(pydppz)]²⁺ system. Electrochemical studies revealed a reversible Os(II)/Os(III) oxidation at +0.99 V vs. SCE and five ligand-centered reductions, generally less negative than those of the smaller pydppz analog, consistent with enhanced electron-accepting ability. In acetonitrile, the complex exhibits UV absorption bands at 328 and 473 nm and near-infrared emission at 840 nm, assigned to a long-lived ³MLCT state (<i>τ</i> = 110 ns, <i>Φ</i> = 0.02). Upon titration with calf-thymus DNA, [Os(tpy)(pydppn)]²⁺ shows a pronounced light-switch effect, hypochromism, red-shifted MLCT bands, induced circular dichroism, and an increase in DNA melting temperature (Δ<i>T</i>_m = 8.9 ± 0.5 °C), consistent with intercalative binding. Viscometric titrations further support intercalation, with a binding constant <i>K</i>_B ≈ 1.2 × 10⁶ M^-1. Transient absorption spectroscopy indicates that DNA binding prolongs the excited-state lifetime and modifies vibrational relaxation pathways. These results highlight how π-system extension in Os(II) complexes modulates photophysical behavior and DNA affinity, offering insights for the rational design of NIR-emitting, DNA-targeted luminescent probes and potential phototherapeutic agents.