Polycationic Ru(II) Luminophores: Syntheses, Photophysics, and Application in Electrostatically Driven Sensitization...

A series of photoluminescent Ru(II) polypyridine complexes have been synthesized in a manner that varies the extent of the cationic charge. Two ligand systems (L1 and L2), based upon 2,2'-bipyridine (bipy) mono- or difunctionalized at the 5- or 5,5'-positions using N-methylimidazolium groups, were utilized. The resulting Ru(II) species therefore carried +3, +4, +6, and +8 complex moieties based on a [Ru(bipy)₃]²⁺ core. Tetra-cationic [Ru(bipy)₂(L2)][PF₆]₄ was characterized using XRD, revealing H-bonding interactions between two of the counteranions and the cationic unit. The ground-state features of the complexes were found to closely resemble those of the parent unfunctionalized [Ru(bipy)₃]²⁺ complex. In contrast, the excited state properties produce a variation in emission maxima, including a bathochromic 44 nm shift of the ³MLCT band for the tetra-cationic complex; interestingly, further increases in overall charge to +6 and +8 produced a hypsochromic shift in the ³MLCT band. Supporting DFT calculations suggest that the trend in emission behavior may, in part, be due to the precise nature of the LUMO and its localization. The utility of a photoactive polycationic Ru(II) complex was then demonstrated through the sensitization of a polyanionic Yb(III) complex in free solution. The study shows that electrostatically driven ion pairing is sufficient to facilitate energy transfer between the ³MLCT donor state of the Ru(II) complex and the accepting ²F_5/2 excited state of Yb(III).