Oxidative quenching within photosensitizer–acceptor dyads based on bis(bidentate) phosphine-connected osmium(II) bipyridyl light absorbers and reactive metal sites

For the first time oxidative quenching of OsP₂N₄ chromophores by reactive Pt^II or Pd^II sites containing cis, trans, cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane (dppcb) is directly observed despite the presence of a saturated cyclobutane backbone "bridge". This dramatic effect is measured as a sudden temperature-dependent onset of a reduction in phosphorescence lifetime in [Os(bpy)₂(dppcb)MCl₂](SbF₆)₂ (M = Pt, 1; Pd, 2). The appearance of this additional energy release is not detectable in [Os(bpy)₂(dppcbO₂)](PF₆)₂ (3), where dppcbO₂ is cis, trans, cis-1,2-bis(diphenylphosphinoyl)-3,4-bis(diphenylphosphino)cyclobutane. Obviously, the square-planar metal centers in 1 and 2 are responsible for this effect. In line with these observations, the emission quantum yields at room temperature for 1 and 2 are drastically reduced compared with 3. Since this luminescence quenching implies strong intramolecular interaction between the Os^II excited states and the acceptor sites and depends on the metal⋯metal distances, also the single crystal X-ray structures of 1-3 are given.