Pyrimidoquinxoalinophenanthroline opens next chapter in design of bridging ligands for artificial photosynthesis

Using a dehydrogenative chemistry on the complex approach, a new polypyridine bridging ligand that bridges the gap of already existing systems is synthesized. By the usage of versatile cross-coupling reactions two different coordination spheres are included in the ligand architecture. Due to the twisted geometry of the novel ditopic ligand, the resultant division of the ligand in two distinct subunits leads to steady state as well as excited state properties of the corresponding mononuclear Ru(II) polypyridine complex resembling those of prototype [Ru(bpy)3]2+ (bpy = 2,2´-bipyridine). The localization of the initially optically excited and the nature of the long-lived excited states on the Ru-facing ligand spheres is evaluated by resonance Raman and fs-TA spectroscopy, respectively, and supported by DFT and TDDFT calculations. Coordination of a second metal (Zn or Rh) to the available bis-pyrimidyl-like coordination sphere strongly influences the frontier molecular orbitals apparent by e.g., luminescence quenching. Thus, the new bridging ligand motif offers electronic properties which can be adjusted by the nature of the second metal center. Using the heterodinuclear Ru-Rh complex, visible light-driven reduction of NAD+ to NADH was achieved, highlighting the potential of this system for photocatalytic applications.