Energy transfer dynamics in multichromophoric arrays engineered from phosphorescent Pt(II)/Ru(II)/Os(II) centers...

A rigid star-shaped tetrachromophoric trimetallic complex engineered from a 5,5',10,10',15,15'-hexabutyltruxene platform functionalized in the 2,7,12 positions with three different metal centers, namely, a terpyridine-Pt(II) ethynylene unit and Ru(II) and Os(II) bipyridine centers, was synthesized in a controlled fashion and characterized by (1)H NMR and mass spectrometry. The protocol was devised in such a way that key mono and dinuclear model complexes and two reference truxene ligands could also be prepared. Room temperature (RT) optical absorption and RT and 77 K luminescence studies were performed on the truxene ligands, the trimetallic species, the various mono- and binuclear complexes and precursors lacking the truxene fragment; RT nanosecond transient absorption measurements were also carried out in particular cases. The electronic properties of the Ru and Os subunits in the arrays were found to be unaffected by the presence of the truxene core whereas direct linking of the Pt subunit to the truxene via the σ-alkyne bond markedly influences the spectroscopic behavior of the Pt center. Remarkably the truxene phosphorescence was clearly established in the two ligands (lifetime of 4.3 s for the mono ethynyl-bipy substituted truxene and 17.5 ms for the bis ethynyl-bipy substituted truxene) and also detected in the Pt-containing complexes PtL' (model Pt-truxene) and Pt-Os (Pt-truxene-Os dyad) at low temperature. This is attributed to the closeness in energy of the Pt (3)CT level and the truxene triplet at low temperature and to the spin-orbit coupling induced by the Pt heavy atom. Transient absorbance measurements evidenced the population of the Pt-based triplet in the Pt-truxene mononuclear complex PtL' at room-temperature. For the trimetallic complex, where the various centers exhibit an energy gradient for the local excited levels, and following an approach based on the use of selected excitation of the components, an initial energy transfer was found to occur from the central truxene unit toward the peripheral Pt, Ru, and Os metal-based centers. Subsequent Pt-based and Ru-based excited state depletion contributes to the final sensitization of the low-lying Os triplet excited state; the excited state dynamics for these multicascade processes are examined in detail.