A pre-organised truxene platform for phosphorescent [Ru(bpy)2] and [Os(bpy)2] metal centres: a clear-cut switch from Förster- to Dexter-type energy-transfer mechanism.

We report on the synthesis, optical properties and energy-transfer features of a series of transition-metal-containing complexes and dyads, based on a pre-organised truxene scaffold. In this series, the [Ru(bpy)3]2+ and [Os(bpy)3]2+ photoactive terminals are coupled to the bridging aromatic truxene core through rigid ethynyl linkers. The photoinduced energy-transfer processes taking place from the Ru- to the Os-based levels, and from the truxene bridging ligand to the terminal-metal chromophores are studied and the pathways and mechanisms are discussed. The photoinduced energy-transfer process observed in the dyad proceeds rapidly through: i) an efficient 1L→1Os direct energy transfer followed by intersystem crossing to 3Os, and ii) a fast 1L→1Ru energy-transfer step and subsequent intersystem crossing to 3Ru followed by a 3Ru→3Os energy-transfer process. The first 1L→1Os and 1L→1Ru steps are controlled by a dipole–dipole interaction (Forster mechanism), whereas the subsequent 3Ru→3Os step proceeds by means of a long-range (≈24 A) through-bond mediated Dexter mechanism, facilitated by the conjugation along the bpy-truxene-bpy molecular axis.