Luminescence and spectroscopic studies of organometallic rhodium and rhenium multichromophore systems carrying polypyridyl acceptor sites and phenylethynyl antenna subunits

The synthesis, structure, photophysics, and spectroscopic characterization of two novel organometallic rhodium and rhenium multichromophore photosensitizers carrying a central 2,2′-bipyridyl acceptor moiety are reported. Due to the presence of rigid phenylethynyl substituents in their 5,5′-position acting as intramolecular antenna groups, these compounds display perturbed excited state properties and a modified luminescence behaviour compared to the unsubstituted parent systems. The rhodium derivative is characterized by a weak blue-green intraligand (IL) phosphorescence occurring only at low temperatures. In the rhenium complex both a blue-green ligand-based luminescence and a red phosphorescence of metal-to-ligand charge transfer (MLCT) origin is observed at room temperature in solution. In rigid 77 K matrix the lowest-lying excited state levels approximate in energy and a structured luminescence band occurs which is ascribed to a ligand phosphorescence with admixed MLCT character.