REPORT
◥INORGANIC CHEMISTRY
Luminescence and reactivity of a
charge-transfer excited iron complex
with nanosecond lifetime
Kasper Skov Kjær^1 , Nidhi Kaul^2 , Om Prakash^3 *, Pavel Chábera^1 , Nils W. Rosemann^1 ,
Alireza Honarfar^1 , Olga Gordivska^3 , Lisa A. Fredin^4 †, Karl-Erik Bergquist^3 ,
Lennart Häggström^5 , Tore Ericsson^5 , Linnea Lindh^1 , Arkady Yartsev^1 ,
Stenbjörn Styring^2 , Ping Huang^2 , Jens Uhlig^1 , Jesper Bendix^6 , Daniel Strand^3 ,
Villy Sundström^1 ‡, Petter Persson^4 ‡, Reiner Lomoth^2 ‡, Kenneth Wärnmark^3 ‡
Iron’s abundance and rich coordination chemistry are potentially appealing features for
photochemical applications. However, the photoexcitable charge-transfer states of most
iron complexes are limited by picosecond or subpicosecond deactivation through low-lying
metal-centered states, resulting in inefficient electron-transfer reactivity and complete lack of
photoluminescence. In this study, we show that octahedral coordination of iron(III) by two
mono-anionic facialtris-carbene ligands can markedly suppress such deactivation.The resulting
complex [Fe(phtmeimb) 2 ]+, where phtmeimb is {phenyl[tris(3-methylimidazol-1-ylidene)]
borate}−, exhibits strong, visible, room temperature photoluminescence with a 2.0-nanosecond
lifetime and 2% quantum yield via spin-allowed transition from a doublet ligand-to-metal
charge-transfer (^2 LMCT) state to the doublet ground state. Reductive and oxidative electron-
transfer reactions were observed for the^2 LMCT state of [Fe(phtmeimb) 2 ]+in bimolecular
quenching studies with methylviologen and diphenylamine.
P
hotoactive transition metal complexes play
an important role in processes ranging
from solar light harvesting ( 1 – 3 )andlight-
emitting technology ( 4 ) to photocatalysis
( 5 ) and photodynamic therapy ( 6 ). Such
applications almost always rely on charge-
transfer (CT) excited states with sufficient life-
time and energy to drive electron transfer and
visible light emission. Iron complexes provide
an earth-abundant and environmentally benign
alternative to noble metal systems ( 7 )buthave
until recently been limited by subpicosecond de-
activation of their CT states ( 8 ) to low-energy
metal-centered (MC) states ( 9 – 12 ). These dynamics
arise from the moderate ligand field splitting
of Fe complexes with commonly used oligopyridyl
ligands ( 8 , 11 , 12 ). Early work on Fe-centered
oligopyridyl systems suggested the involvement
of MC states with nanosecond lifetime in electron-
transfer reactions ( 13 ). However, these results were
later shown to be incompatible with the excited-
state dynamics of the systems ( 14 ); the MC states
are now generally considered too low in energy to
participate in photochemistry of interest. Efforts to
develop Fe-centered photofunctional systems have
therefore focused on inhibiting the ultrafast CT→
MC transitions ( 8 ).
We recently showed that strongly electron
donatingN-heterocyclic carbene (NHC) ligands
raise the energy of MC states relative to CT states
of iron complexes, thereby increasing the life-
time of the excited CT states ( 15 ). For FeIIcom-
plexes with four NHC moieties and two pyridine
moieties, we and others have recently demon-
strated triplet metal-to-ligand charge-transfer
(^3 MLCT) state lifetimes of a few tens of pico-
seconds ( 15 – 17 ), thereby crossing the threshold for
efficient interfacial electron injection from surface-
bound Fe photosensitizers to a TiO 2 electrode ( 17 ).
To further increase the lifetime of the CT
states in iron complexes, we very recently sat-
urated the iron center with six coordinating
NHC moieties, leading to the [Fe(btz) 3 ]2+/3+
complex [btz, 3,3′-dimethyl-1,1′-bis(p-tolyl)-
4,4′-bis(1,2,3-triazol-5-ylidene)]. This complex fea-
tured order-of-magnitude-higher charge-transfer
lifetimes in both its FeIII[100-ps ligand-to-metal
charge-transfer (^2 LMCT)] and FeII(528-ps^3 MLCT)
states ( 18 , 19 ). Moreover [Fe(btz) 3 ]3+exhibited
room temperature photoluminescence (PL) froma CT state in the visible regime, albeit with an
extremely low quantum yield (0.03%). The pico-
second CT lifetimes still preclude most light-
harvesting and light-emitting applications, but
these results suggested that further improvements
of FeIIand FeIIIcomplexes are notable in the
broader context of development of photoactive
and photoluminescent 3d^6 and 3d^5 complexes,
respectively ( 20 , 21 ).
We identified maximal ligand field strength
and an optically allowed lowest CT excited state
( 18 , 19 ) as key design elements for extending
lifetimes and further increasing PL. For superior
ligand field strength, we targeted anions for even
more pronounceds-donor ability. Near-perfect
octahedral coordination capability ( 22 – 24 ) was
another factor that drew us to the tridentate
facial NHC ligand {phenyl[tris(3-methylimidazol-
1-ylidene)]borate}–(phtmeimb–)( 25 ). This ligand
has very recently been shown to support weak
low-temperature solid-state LMCT and d-d emis-
sion in the d^3 complex [MnIV(phtmeimb) 2 ](OTf) 2
(OTf, triflate) ( 26 ).
In this study, we demonstrate that the com-
bination of a^2 LMCT lowest excited state with
the exceptional electronic and steric properties
of the phtmeimb–ligand results in a [FeIII
(phtmeimb) 2 ]PF 6 complex featuring a CT state
with nanosecond lifetime. [FeIII(phtmeimb) 2 ]PF 6
was efficiently synthesized from FeIIBr 2 and in
situ generatedtris-NHC-carbene (phtmeimb)−
as illustrated in Fig. 1A. During the workup
procedure in air, FeIIis spontaneously oxidized
to FeIII, resulting in an analytically pure product
(see supplementary materials). Counterion metath-
esis with NaBPh 4 provided access to the correspond-
ing BPh 4 −salt (Ph, phenyl). The oxidation state
of iron in both complexes was confirmed by
single-crystal x-ray diffraction analysis. The cation
in both [FeIII(phtmeimb) 2 ]X [X = PF 6 −(Fig. 1B) or
BPh 4 −] salts displays a near-perfect octahedral geo-
metry (table S4) in contrast to [Fe(btz) 3 ](PF 6 ) 3 ( 19 ).
Mößbauer spectroscopy and magnetometry
identified the ground state of the isolated sample
of [FeIII(phtmeimb) 2 ]PF 6 as low spin (S=½),
containing <1% FeII(figs. S12 to S14). Surprisingly,
but similar to some low-spin FeIII-porphyrin
complexes ( 27 ), the^1 H nuclear magnetic res-
onance (NMR) spectrum of the paramagnetic
[FeIII(phtmeimb) 2 ]PF 6 in CD 3 CN shows narrow
peaks (<15 Hz at 298 K) (table S1), whereas the
X-band electron paramagnetic resonance (EPR)
spectrum in frozen solvent glasses atT=4to
20 K displays no distinct assignable bands (see
the supplementary materials section).
Cyclic voltammetry of [FeIII(phtmeimb) 2 ]+
(Fig. 2A) illustrates that both reduction to FeII
and oxidation to FeIVare reversible, with half-
wave potentials ofE½=−1.16 and 0.25 V versus
Ferrocene (Fc), respectively. The pronounced
shift toward negative potentials compared with
potentials of previously reported Fe-NHC complexes
( 15 , 17 , 19 ) illustrates the exceptionally strong
electron donor properties of the negatively charged
tris-NHC ligands. Further oxidation of the FeIV
complex at 1.67 V is irreversible (fig. S17). Pre-
vious observation of irreversible oxidation ofRESEARCH
Kjæret al.,Science 363 , 249–253 (2019) 18 January 2019 1of5
(^1) Division of Chemical Physics, Department of Chemistry,
Lund University, Box 124, SE-22100 Lund, Sweden.
(^2) Department of Chemistry, Ångström Laboratory, Uppsala
University, Box 523, SE-75120 Uppsala, Sweden.^3 Center for
Analysis and Synthesis (CAS), Department of Chemistry,
Lund University, Box 124, SE-22100 Lund, Sweden.^4 Division
of Theoretical Chemistry, Department of Chemistry, Lund
University, Box 124, SE-22100 Lund, Sweden.^5 Department of
Physics and Astronomy, Ångström Laboratory, Uppsala
University, Box 516, SE-751 20 Uppsala, Sweden.
(^6) Department of Chemistry, University of Copenhagen,
Universitetsparken 5, DK-2100 Copenhagen, Denmark.
*These authors contributed equally to this work.
†Present address: Department of Chemistry, Lehigh University, 6 E
Packer Avenue, Bethlehem, PA 18015, USA.
‡Corresponding author. Email: [email protected]
(V.S.); [email protected] (P.P.); reiner.lomoth@kemi.
uu.se (R.L.); [email protected] (K.W.)
on January 22, 2019^
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