[Fe(btz) 3 ]3+( 19 ) at very similar potential indicates
the assignment of this process to ligand oxidation
as the potentials of metal-centered couples are
lowered substantially by the phtmeimb ligand.
With this assignment, the electrochemical po-
tentials agree consistently with the energies of
the LMCT transitions found for [Fe(btz) 3 ]3+( 19 )
and the FeIIIand FeIVstates of [FeIII(phtmeimb) 2 ]
(see fig. S18 and associated discussion). Reduction
of the FeIIcomplex does not occur within the
solvent-electrolyte potential window, demonstrat-
ing that the ligand reduction potential is below
−3.3 V, which is again consistent with the inter-
pretation of the electronic spectra (see supple-
mentary materials).
The visible absorption spectrum of
[Fe(phtmeimb) 2 ]+in acetonitrile (Fig. 2B,
left curve) is dominated by a single band
peaking at 502 nm (molar decadic absorption
coefficientemax=2950M−^1 cm−^1 )withaminor
shoulder around 545 nm. This band is bleached
upon oxidation and reduction of the metal
center, and its energy matches relatively well
the difference in electrochemical potential be-
tween the FeIII-FeIIcouple and ligand oxida-
tion. The lowest-energy absorption band of
[Fe(phtmeimb) 2 ]+is therefore assigned to a
LMCT transition.
Excitation of [Fe(phtmeimb) 2 ]+in acetonitrile
with visible light below 600 nm results in strong
orange PL (Fig. 2C). The spectral profile of the PL
shown in Fig. 2B, right, peaks at 655 nm and has
no appreciable structurebesides a broad shoulder
around 620 nm, which mirrors the LMCT absorp-
tion band of the complex. The PL intensity tracks
the absorption cross section throughout the vis-
ible region, as illustrated by the superimposable
absorption and excitation spectra (red circles
in Fig. 2B).
The measured emission quantum yield of
[Fe(phtmeimb) 2 ]+in air-saturated dry acetonitrile
at room temperature was 2.1% (Fe= 0.021 ±
0.002). Notably, this value is a factor of 70 higher
than the quantum yield measured for [Fe(btz) 3 ]3+
( 19 ) and is even slightly higher than the 1.8%
quantum yield of the prototypical transition metal
photosensitizer [Ru(bpy) 3 ]2+(bpy, 2,2′-bipyridine)
under air-saturated conditions, used as reference
for the quantification ( 28 ). The emission decay
kinetics measured by time-correlated single-
photon counting (TCSPC) (Fig. 3A) show a single
exponential with a lifetime oft=1.96±0.04ns,a
factor of 20 longer thantobserved in [Fe(btz) 3 ]3+
( 19 ). The photophysical properties of [Fe(phtmeimb) 2 ]+,
[Fe(btz) 3 ]3+, and [Ru(bpy) 3 ]2+are compared in
Table 1.
Taken together, the emission quantum yield
and excited-state lifetime provide a radiative rate
constant ofkr=Fe/t= 1.1 ± 0.2 × 10^7 s−^1. The good
agreement of this value with the approximate
radiative rate constantkr=1.5×10^7 s−^1 estimated
from the integrated extinction coefficientA
(Table 1) of the LMCT band via the Strickler-Berg
relationship strongly suggests that the emission
occurs directly from the^2 LMCT state. Thus,
the intersection of the normalized absorption
and emission bands at 582 nm provides the
energy of the^2 LMCT excited state,E0-0=2.13eV
(17,200 cm−^1 ). From the photophysical param-
eters, the 70-fold increase in emission quan-
tum yield from 0.03% for [Fe(btz) 3 ]3+to 2.1% for
[Fe(phtmeimb) 2 ]+can be rationalized in terms
of a 20-fold slower nonradiative decay (knr=
[1−Fe]/t) and a 3.5-fold faster radiative rate
constant (kr=Fe/t).
The transient absorption (TA) spectra of
[Fe(phtmeimb) 2 ]+recorded after excitation of the
LMCT band at 500 nm are dominated by excited-state absorption (ESA) below 450 nm and exhibit
a clear stimulated emission band between 600
and 800 nm. The ESA can be attributed to the
transiently reduced iron center of the^2 LMCT
state. The stimulated emission indicates that the
ground-state recovery is spin allowed, offer-
ing further support for the^2 LMCT assignment of
the excited state (see supplementary mate-
rials). The TA decay kinetics of [Fe(phtmeimb) 2 ]+
(Fig. 3A, green) are in perfect agreement with the
TCSPC results.
The long-term photostability of [Fe(phtmeimb) 2 ]+
was measured and compared with that of
[Ru(bpy) 3 ]2+by irradiating aerated acetonitrile
solutions of both complexes with an 11-W com-
pact fluorescent lamp for a total of 156 hours
and measuring the absorption and emission spectra
at intervals during this time period. Whereas clear
signs of degradation set in for [Ru(bpy) 3 ]2+after
48 hours, the [Fe(phtmeimb) 2 ]+sample was virtu-
ally unchanged throughout the 156-hour exper-
iment (figs. S21 and S22).
Density functional theory (DFT) revealed the
minimum energies of the^4 MC and^6 MC states of
[Fe(phtmeimb) 2 ]+(Fig. 3B) that are destabilized
by 13 and 23% with respect to the ground-state
minimum as compared with the previously re-
ported [Fe(btz) 3 ]3+( 19 ). The increased energies
of the MC states together with the fact that the(^2) LMCT states are isoenergetic within the exper-
imental uncertainty for the two systems suggest
that the increase in the experimentally observed
lifetime is related to an effective increase of the
activation barrier for the decay of the^2 LMCT state
into the^4 MC state.
Temperature-dependent emission lifetime
measurements (fig. S23) show that the excited-
state lifetime of [Fe(phtmeimb) 2 ]+increases by a
factor of 4 (from 2.0 to 7.8 ns) upon decreasing
Kjæret al.,Science 363 , 249–253 (2019) 18 January 2019 2of5
Fig. 1. Synthesis and structure of
[Fe(phtmeimb) 2 ]PF 6 .(A)Synthetic
route: 1, precipitation with tetra-n-butyl-
ammonium bromide in acetone; 2,
dissolution in water and precipitation
with ammonium hexafluorophosphate;
3, dissolution in tetrahydrofuran
under N 2 , then cooling to–78°C and
addition oftert-butoxide; 4, addition
of FeBr 2 , stirring under N 2 at room
temperature for 24 hours. (B) X-ray
crystal structure. Thermal ellipsoids
are shown at 50% probability with
the six Fe–C bonds highlighted in
black and gray stripes. Hydrogen atoms,
counterions, and solvent molecules
are omitted for clarity. Fe, orange;
B, purple; N, blue; C, black.
RESEARCH | REPORT
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