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 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 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 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(F2phpy)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(F2phpy)2}–naphthyl–Eu
systems, can act as a “stepping stone” for two-step
d→f energy-transfer.