Mono- and dinuclear ruthenium(II) and osmium(II) polypyridine complexes built around spiro-bridged bis(phenanthroline) ligands: Synthesis, electrochemistry, and photophysics

Two new dyads have been synthesized in which terminal Ru(II) and Os(II) pol ypyridine complexes are separated by sterically constrained spiro bridges. The photophysical properties of the corresponding mononuclear complexes ind icate the importance of the decay of the lowest-energy triplet states local ized on the metallo fragments through the higher-energy metal-centered exci ted states. This effect is minimized at 77 K, where triplet lifetimes are r elatively long, and for the Os(II)-based systems relative to their Ru(II)-b ased counterparts. Intramolecular triplet energy transfer takes place from the Ru(II)-based fragment to the appended Os(II)-based unit, the rate const ant being dependent on the molecular structure and on temperature. In all c ases, the experimental rate constant matches surprisingly well with the rat e constant calculated for Forster-type dipole-dipole energy transfer. As su ch, the disparate rates shown by the two compounds can be attributed to ste reochemical factors. It is further concluded that the spiro bridging unit d oes not favor through-bond electron exchange interactions, a situation conf irmed by cyclic voltammetry.