Near-Infrared Absorbing and Emitting RuII−PtII Heterodimetallic Complexes of Dpdpz (Dpdpz = 2,3-Di(2-pyridyl)-5,6-diphenylpyrazine)

The reaction of 2,3-di(2-pyridyl)-5,6-diphenylpyrazine (dpdpz) with K2PtCl4 in a mixture of acetonitrile and water afforded mono-Pt complex (dpdpz)PtCl2 4 in good yield, with two lateral pyridine nitrogen atoms binding to the metal center. Two types of RuII−PtII heterodimetallic complexes bridged by dpdpz, namely, [(bpy)2Ru(dpdpz)Pt(CCC6H4R)]2+ (7−9, R = H, NMe2, or Cl, respectively) and [(tpy)Ru(dpdpz)Pt(CCPh)] + (12), were then designed and prepared, where bpy = 2,2′-bipyridine and tpy = 2,2′;6′,2′′-terpyridine. In both cases, the platinum atom binds to dpdpz with a C∧N∧N tridentate mode. However, the coordination of the ruthenium atom with dpdpz could either be noncyclometalated (N∧N bidentate) or cyclometalated (C∧N∧N tridentate). The electronic properties of these complexes were subsequently studied and compared by spectroscopic and electrochemical analyses and theoretical calculations. These complexes exhibit substantial absorption in the visible to NIR (near-infrared) region because of mixed MLCT (metal-to-ligand-charge-tranfer) transitions from both the ruthenium and the platinum centers. Complexes 7 and 9 were found to emit NIR light with higher quantum yields than those of the mono-Ru complex [(bpy)2Ru(dpdpz)]2+ (5) and bis-Ru complex [(bpy)2Ru(dpdpz)Ru(bpy)2]4+ (13). However, no emission was detected from complex 8 or 12 at room temperature in acetonitrile.