d→f energy transfer in Ir(III)/Eu(III) dyads: use of a naphthyl spacer as a spatial and energetic "stepping stone"

A series of luminescent complexes based on {Ir(phpy)2} (phpy = cyclometallating anion of 2-phenylpyridine) or {Ir(F2phpy)2} [F2phpy = cyclometallating anion of 2-(2',4'-difluorophenyl)pyridine] units, with an additional 3-(2-pyridyl)-pyrazole (pypz) ligand, have been prepared; fluorination of the phenylpyridine ligands results in a blue-shift of the usual (3)MLCT/(3)LC luminescence of the Ir unit from 477 to 455 nm. These complexes have pendant from the coordinated pyrazolyl ring an additional chelating 3-(2-pyridyl)-pyrazole unit, separated via a flexible chain containing a naphthalene-1,4-diyl or naphthalene-1,5-diyl spacer. Crystal structures show that the flexibility of the pendant chain allows the naphthyl group to lie close to the Ir core and participate in a π-stacking interaction with a coordinated phpy or F2phpy ligand. Luminescence spectra show that, whereas the {Ir(phpy)2(pypz)} complexes show typical Ir-based emission--albeit with lengthened lifetimes because of interaction with the stacked naphthyl group--the {Ir(F2phpy)2(pypz)} complexes are nearly quenched. This is because the higher energy of the Ir-based (3)MLCT/(3)LC excited state can now be quenched by the adjacent naphthyl group to form a long-lived naphthyl-centered triplet ((3)nap) state which is detectable by transient absorption. Coordination of an {Eu(hfac)3} unit (hfac = 1,1,1,5,5,5-hexafluoro-pentane-2,4-dionate) to the pendant pypz binding site affords Ir-naphthyl-Eu triads. For the triads containing a {Ir(phpy)2} core, the unavailability of the (3)nap state (not populated by the Ir-based excited state which is too low in energy) means that direct Ir→Eu energy-transfer occurs in the same way as in other flexible Ir/Eu complexes. However for the triads based on the{Ir(F2phpy)2} core, the initial Ir→(3)nap energy-transfer step is followed by a second, slower, (3)nap→Eu energy-transfer step: transient absorption measurements clearly show the (3)nap state being sensitized by the Ir center (synchronous Ir-based decay and (3)nap rise-time) and then transferring its energy to the Eu center (synchronous (3)nap decay and Eu-based emission rise time). Thus the (3)nap state, which is energetically intermediate in the {Ir(F2phpy)2}-naphthyl-Eu systems, can act as a "stepping stone" for two-step d→f energy-transfer.