Intramolecular triplet energy transfer in pyrene-metal polypyridine dyads:A strategy for extending the triplet lifetime of the metal complex

A series of photoactive dyads bearing pyrene and metal (M = Ru-II) or Os-II ) tris(2,2'-bipyridine) terminals bridged by an ethynylene or Pt-II bis(sig ma-acetylide) moiety has been synthesized and investigated by transient spe ctroscopy. Selective excitation into the terminal metal complex is possible in each case and generates the lowest energy, excited triplet state locali zed on that molecular fragment. For both Os-II-based dyads, the triplet sta te is unperturbed by the appended pyrene unit and the observed photophysica l properties can be understood within the framework of the energy-gap law T he triplet state localized on the metal complex in the two Ru-II-based dyad s is involved in reversible energy transfer with the triplet associated wit h the pyrene unit, which is situated at slightly lower energy. When the ter minal metal complex is a Ru-II bis(2,2':6',2"-terpyridyl) fragment, however , the triplet levels are inverted such that the pyrene-like triplet state l ies slightly above that of the metal complex. Kinetic spectrophotometry has allowed determination of the various rate constants and energy gaps and, o n the basis of nonadiabatic electron-transfer theory, it appears that the c entral Pt bis(sigma-acetylide) unit is a much inferior electronic conductor than is a simple ethynylene group. Reversible energy transfer of this type greatly prolongs the triplet lifetime of the Ru-II tris(2,2'-bipyridyl) fr agment. For example, equilibration between the triplet states is achieved w ithin 10 ps for the ethynylene-bridged dyad while the equilibrium mixture d ecays with a lifetime of about 40 mu s in deoxygenated acetonitrile at room temperature.