d→f Energy Transfer in Ir(III)/Eu(III) Dyads: Use of a Naphthyl Spacer as a Spatial and Energetic “Stepping Stone”

Abstract

A series of luminescent complexes based on {Ir(phpy) ₂} (phpy = cyclometallating anion of 2-phenylpyridine) or {Ir(F ₂phpy) ₂} [F ₂phpy = 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 ³MLCT/ ³LC 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 F ₂phpy 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(F ₂phpy) ₂(pypz)} complexes are nearly quenched. This is because the higher energy of the Ir-based ³MLCT/ ³LC excited state can now be quenched by the adjacent naphthyl group to form a long-lived naphthyl-centered triplet ( ³nap) 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 ³nap 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(F ₂phpy) ₂} core, the initial Ir→ ³nap energy-transfer step is followed by a second, slower, ³nap→Eu energy-transfer step: transient absorption measurements clearly show the ³nap state being sensitized by the Ir center (synchronous Ir-based decay and ³nap rise-time) and then transferring its energy to the Eu center (synchronous ³nap decay and Eu-based emission rise time). Thus the ³nap state, which is energetically intermediate in the {Ir(F ₂phpy) ₂}–naphthyl–Eu systems, can act as a “stepping stone” for two-step d→f energy-transfer.

Abstract

If the Ir-based ³MLCT/ ³LC excited state is high enough in energy, in Ir−naphthyl−Eu triads the triplet excited state of the naphthyl group ( ³nap) acts as a spatial and energetic intermediate for two-step Ir→ ³nap and then ³nap→Eu energy-transfer; the grow-in and decay of the ³nap state as it receives and then passes on the excitation energy can be followed by transient absorption spectroscopy.