The multichromophore approach: A case of temperature controlled switching between single and dual emission in Ru(II) polypyridyl complexes

A new series of ruthenium(II) complexes, based on 4′-(9-anthryl)-2,2′:6′,2″-terpyridine (an-tpy), have been synthesized from the Ru(III) precursor (an-tpy)RuCl₃ (4). These new Ru(II) complexes, [(an-tpy)Ru(tpy-pm-R)] (R = H, 3a; Cl, 3b; phenyl, 3c; p-bromophenyl, 3d), have extended π-conjugation through the 5′-substituted pyrimidyl group, and have been characterized by analytical and spectroscopic methods, and X-ray single crystal structure determination for 3b and 3d. Luminescence lifetime measurements have shown that the anthryl chromophore greatly increases room temperature (r.t.) excited-state lifetime, even though it is not directly connected to the ligand involved in the metal-to-ligand charge transfer (³MLCT) emitting state. This result demonstrates that an equilibrium is established between the anthryl triplet state (³an) and the ³MLCT state even though the two chromophores are physically separated by more than one nanometer. At low temperature in rigid matrix, such equilibrium does not take place and emission arises from both ³an and ³MLCT excited states. The temperature dependence of the excited-state equilibration induces a temperature switch from single (room temperature) to dual (77 K) emission behavior.