Monitoring Fe(II) Spin-State Equilibria via Eu(III) Luminescence in Molecular Complexes: Dream or Reality?

Article abstract of DOI:10.1021/acs.inorgchem.9b02713

The modulation of light emission by Fe(II) spin-crossover processes in multifunctional materials has recently attracted major interest for the indirect and noninvasive monitoring of magnetic information storage. In order to approach this goal at the molec

Full text of DOI:10.1021/acs.inorgchem.9b02713

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Monitoring Fe(II) Spin-State Equilibria via Eu(III) Luminescence in
Molecular Complexes: Dream or Reality?
Lathion, Alexandre Furstenberg, Cel
̈
́ ́
ine Besnard, Andreas Hauser,†
†
†
,†
‡
Timothee
Azzedine Bousseksou, and Claude Piguet*
#
†Department of Inorganic and Analytical Chemistry, and of Physical Chemistry, University of Geneva, 30 quai E. Ansermet,
CH-1211 Geneva 4, Switzerland
‡
Laboratory of Crystallography, University of Geneva, 24 quai E. Ansermet, CH-1211 Geneva 4, Switzerland
#
Laboratory of Coordination Chemistry (LCC), CNRS & Universite
1077 Cedex 4, France
́
de Toulouse (UPS, INP), 205 route de Narbonne, Toulouse
3
*
S Supporting Information
ABSTRACT: The modulation of light emission by Fe(II) spin-crossover processes in
multifunctional materials has recently attracted major interest for the indirect and noninvasive
monitoring of magnetic information storage. In order to approach this goal at the molecular
level, three segmental ligand strands, L4−L6, were reacted with stoichiometric mixtures of
divalent d-block cations (M(II) = Fe(II) or Zn(II)) and trivalent lanthanides (Ln(III) =
La(III) or Eu(III)) in acetonitrile to give C -symmetrical dinuclear triple-stranded helical
3
5
+
[
LnM(Lk)3] cations, which can be crystallized with noncoordinating counter-anions. The
divalent metal M(II) is six-coordinate in the pseudo-octahedral sites produced by the facial
wrapping of the three didentate binding units, the ligand field of which induces variable Fe(II)
spin-state properties in [LnFe(L4)3]5 (strictly high-spin), [LnFe(L5) ] (spin-crossover
+
5+
3
5
+
(
SCO) around room temperature), and [LnFe(L6)3] (SCO at very low temperature). The
5
+
introduction of the photophysically active Eu(III) probe in [EuFe(Lk)3] results in
europium-centered luminescence modulated by variable intramolecular Eu(III) → Fe(II)
energy-transfer processes. The kinetic analysis implies Eu(III) → Fe(II) quenching efficiencies close to 100% for the low-spin
configuration and greater than 95% for the high-spin state. Consequently, the sensitivity of indirect luminescence detection of
Fe(II) spin crossover is limited by the resulting weak Eu(III)-centered emission intensities, but the dependence of the
luminescence on the temperature unambiguously demonstrates the potential of indirect lanthanide-based spin-state monitoring
at the molecular scale.
1
3
INTRODUCTION
aquo ions) also make them good candidates for the
alternative sensing mechanism, where they act as donors. In
this case, their luminescence is quenched by energy transfers
toward neighboring d-block acceptors in Eu-M and Tb-M
■
Open-shell trivalent lanthanides, Ln(III), are famous for their
atomlike optical and magnetic properties, which can be further
tuned in a rational way by their specific chemical environments
in molecular complexes or macroscopic materials. In this
context, Eu(III) and Tb(III) have been exhaustively used as
luminescent probes because of their strong, easily detectable,
14
15
12
molecular pairs where M = Cr(III), Fe(II), Fe(III),
1
16
17
Cu(II), and Ru(II). From a quantitative point of view, the
probability WD,A that a resonant energy-transfer process occurs
from a donor D (Eu or Tb) to an acceptor A (d-block cation)
2
long-lived, and well-understood visible emission. For instance,
18
is given by Fermi’s golden rule.
Eu(III) and Tb(III) stains have been used for probing point
3
4
mutations in DNA, labeling proteins, improving contrast in
2π
ℏ
2
*
*
5
6
WD,A
=
|⟨DA |H|D A⟩| Ω
D,A
optical microscopy, sensing local chiral environments,
(1)
7
quantifying analytes in complex media, and designing colored
8
phosphors. In inorganic and coordination chemistry, the latter
H is the interaction Hamiltonian that mediates energy transfer
from the excited donor, D*, to the ground-state acceptor, A,
and ΩD,A is the spectral overlap integral (eq 2) where g (E)
and g (E) are the normalized line shape functions for the
homogeneous lines of the donor (emission spectrum) and
acceptor (absorption spectrum), respectively.
visible lanthanide probes were intensively exploited for
monitoring intermetallic d−f communications operating in
supra)molecular assemblies. When associated in dyads with
D
9
(
A
10
11
high-energy sensitizers such as Ir(III), Pt(II), or Fe(II) in
19
1
2
ferrocene, Eu(III) and Tb(III) probes act as acceptors which
ultimately convert ultraviolet excitation into bright visible light.
However, the relatively high energies of their emissive levels
Received: September 10, 2019
−
1
5
−1
5
(
17 277 cm for Eu( D ) and 20 462 cm for Tb( D ) in the
0
4
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02713
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
molecules, (ii) strong emission self-quenching due to the high
concentration of luminophores in the solid state, and (iii)
drastic luminescence extinction due to the close proximity of
donor and acceptor in the complex. All these aspects are
ΩD,A
=
g (E)·g (E)dE
A D
∫
(2)
Ω
ensures energy conservation and intimately depends on
D,A
the location of the excited states of the two partners implied in
the energy transfer, while the matrix element |⟨DA*|H|D*A⟩|
21b,25
2
detrimental for short-term technological applications,
but
fundamental mechanistic rationalization and pertinent pro-
gramming for future materials may greatly benefit from the
preparation and analysis of heterometallic (supra)molecular
complexes where stoichiometry, structure, and electronic
pathway can be strictly controlled.
relies on (i) the interaction mechanism (through-space
electrostatic or double electron exchange) and (ii) the distance
separating the donor (D) and the acceptor (A). Programming
the spectral overlap is rather obvious because it results from
the simple choice of a pair of d-block and f-block metals
embedded in specific chemical environments, but an
unambiguous and rational tuning of the interaction term can
be envisioned only in molecular systems where structural
defaults and statistical distributions are excluded, while
The three didentate benzimidazole-2-yl-pyridine units L1−
L3 have been designed for this purpose, because they give
2
+
stable pseudo-octahedral mononuclear [Fe(Lk)3] complexes,
the spin-state behavior of which (low-spin, high-spin, or spin-
crossover) can be controlled by (i) the location of the methyl
group on the pyridine ring (Scheme 1) and (ii) the facial/
20
intermetallic contact distances are fixed. In this context, the
use of spin-crossover Fe(II) working as an adjustable acceptor
for Ln = Eu(III), Tb(III) is rather obvious because its
absorption spectrum strongly varies with its spin state (Figure
Scheme 1. Chemical Structures of Ligands L1−L6
Highlighting the Positions of the Methyl Groups Bound to
the Terminal Pyridine Rings
2
1
1
). For many pseudo-octahedral Fe(II)N chromophores,
6
Figure 1. Room-temperature electronic absorption spectra recorded
5
+
for Fe(II)N chromophores in [LaFe(L5) ] (low spin, purple trace)
6
3
5
+
and in [LaFe(L4)3] (high-spin, orange trace). The emission
5
+
spectrum recorded for Eu(III) in [EuZn(L5)3] in acetonitrile
corresponds to the red trace. Adapted from ref 15. Copyright 2001
Wiley.
22
which are famous for inducing SCO behavior, the absorption
spectrum of the low-spin configuration is dominated by an
intense metal-to-ligand charge-transfer band (MLCT) covering
the visible part of the electromagnetic spectrum (purple trace
in Figure 1) together with weaker, and usually masked spin-
1
1
1
1
allowed d−d transitions ( A → T and A → T in O
1
1
1
2
h
symmetry) on the high-energy side. For high-spin Fe(II) in the
same environment, the MLCT band is usually much less
intense and shifted toward higher energy (orange trace in
meridional organization of the chelate binding units around the
2
6
central cation. The connection of the facial FeN
chromophores to luminescent pseudotricapped trigonal
prismatic EuN reporters in the triple-stranded helicates
(Lk = L4−L6, Scheme 1) is reported and
6
−
1
−1
Figure 1), while a single weak (10 ≤ ε ≤ 30 M ·cm ) spin-
5
5
allowed transition ( T → E in O symmetry) occurs on the
O
6 3
2
h
1
5,21a
5+
low-energy side (around 900 nm).
[EuFe(Lk) ]
3
The latter change in the Fe(II)N absorption spectra has
discussed in this contribution together with what we believe to
be the first comprehensive photophysical characterization of
molecular EuFe pairs, where Fe(II) displays low-spin, high-
spin, or spin-crossover properties.
6
been exploited for the variable quenching of light emission
provided by appended high-energy polyaromatic bound
ligands, external organic emitters, or fluorescent counter-
anions, thus initiating a novel topic which combines SCO and
21
RESULTS AND DISCUSSION
Synthesis and Characterization of the Dinuclear
Triple-Stranded Helicates [LnM(Lk) ] (Ln = La(III),
luminescence in materials science. However, only little
interest has been focused on related Fe(II) spin-state
■
23
24
5+
modulations using high-energy d-block or f-block donors
3
operating within isolated heterometallic FeM or FeLn
Eu(III); M = Fe(II), Zn(II); Lk = L4−L6). The segmental
2
4a,b
15
27
entities.
It should be emphasized here that these molecular
ligands L4 and L5 were synthesized according to published
procedures, which were extended for the preparation of the
novel ligand L6 (Scheme 2).
systems suffer from intrinsic limitations such as (i) total lack of
cooperativity for the SCO transition operating in isolated
B
DOI: 10.1021/acs.inorgchem.9b02713
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 2. Multistep Synthesis of the Segmental Ligand L6
Figure 2. Molecular structures of the triple-helical cations [EuFe-
5
+
(
Lk)3] in the crystal structures of their triflate salts recorded at 180
K. Hydrogen atoms, ionic counter-anions, and solvent molecules have
been omitted for clarity. The reported uncertainties refer to the
standard deviations estimated when calculating average values. The α
angles correspond to the intrastrand dihedral angle between the
connected pyridine and benzimidazole rings bound to Fe(II). The
methyl groups attached to the pyridine rings are highlighted with
green spheres.
into acetonitrile solutions (Figure 2, right; Tables S2 and S3;
and Figures S13 and S14). For the sake of completeness, ligand
L5 was previously used for preparing hexa-cationic triple-
6
+
stranded helicates [LnCr(L5) ] (Ln = Nd, Eu, and Yb) and a
3
5
+
penta-cationic helicate [EuRu(L5) ] , the crystal structures of
3
which are similar (but not isostructural) with those of the
28
LnFe and LnZn analogues discussed in this contribution.
Commercially available 3-methylpicolinonitrile 1 was hydro-
lyzed under basic conditions to give the corresponding
carboxylic acid 2 in good yield (91%). Subsequent activation
as its acyl chloride followed by coupling with picoline-amide
The molecular structures of the latter two triple-helical
cations are superimposable (Figure S15) and unveil the head-
to-head-to-head (HHH) arrangement of the three ligand
strands leading to pure facial organization of the three di-imine
moieties around Zn(II) or Fe(II) (Figure 2). According to
2
7
3
gave 4 in good yield (86%). The final steps involved a
29
Bechamp reduction of the aromatic nitro groups into their
corresponding anilines using powdered iron in acidic
conditions followed by a double Philips condensation of the
aforementioned aniline groups with the amide moieties to give
L6 in 57% yield. Stoichiometric reactions of the segmental
ligands L4−L6 in CH Cl (3.0 equiv) with equimolar
SHAPE scores, the coordination sphere around the d metal
cation is systematically close to the perfect octahedron and the
intermolecular intermetallic distance dM(II)···Ln(III) = 8.8(3) Å is
globally insensitive to the choice of the d−f pairs (Table S4
and Figure 2). The steric constraints induced by the methyl
groups bound to the terminal pyridine rings merit further
comment. Upon connection to the 5-position in [LnFe-
2
2
acetonitrile solutions of Ln(CF SO ) (1.0 equiv) and
3
3 3
5
+
M(CF SO ) (1.0 equiv) quantitatively afforded the desired
(L5) ] , the methyl groups bring no special intra- or
3
3 2
3
[
LnM(Lk) ](CF SO ) (Ln = La(III), Eu(III); M = Fe(II),
interstrand interactions. The Fe−N bonds in the FeN
3
3
3 5
6
Zn(II); Lk = L4−L6) helicates in solution as ascertained by
their diagnostic diamagnetic (LaZn) and paramagnetic (EuZn,
LaFe, and EuFe) C -symmetrical H NMR spectra (Figures
S1−S12 in the Supporting Information). Diffusion of diethyl
ether into concentrated acetonitrile solutions provided fair
yields (68−90%) of microcrystalline powders with various
amounts of cocrystallized solvent molecules (Table S1 in the
Supporting Information). In line with the crystal structures
chromophore can be thus easily shrunk upon high-spin to
low-spin transition, thereby leading to around 80% of low-spin
1
conformation at 180 K with Fe−N bond lengths smaller than
3
26
24b,26
2.0 Å (Figure 2, center and Table S4).
Connection of the
5
+
methyl groups at the 3-position in [LnFe(L6)3] results in no
specific interstrand interaction, but the chelating pyridine-
benzimidazole ring of each didentate binding unit cannot
adopt coplanar arrangements (Figure 2, right and Table S4).
The considerable dihedral angles α = 37.4(7)° move away the
previously reported at 180 K for high-spin [LnFe(L4) ]-
3
2
6
30
(
CF SO ) (Ln = La(III), Eu(III), Lu(III); Figure 2, left),
two coordinating nitrogen atoms, which prevents Fe−N
3
3 5
low-spin [LnFe(L5) ](CF SO ) (Ln = La(III), Eu(III);
contraction and stabilizes the high-spin (≥99%) configuration
3
3
3 5
2
4b,26
Figure 2, center)2,
and [EuZn(L5) ](CF SO ) triple-
at 180 K characterized by long d
= 2.12 Å bond distances.
3
3
3 5
Fe−N
7
stranded helicates, monocrystals suitable for X-ray diffraction
Finally, upon attachment of the methyl groups at the six-
5
+
studies could be also obtained for the complexes [EuZn-
(
(
position in [LnFe(L4) ] , the intrastrand constraints found in
3
5
+
L6) ](CF SO ) ·4MeCN·1.75H O and [EuFe(L6) ]-
[LnFe(L6)3] are replaced with marked interstrand ones in
3
3
3
5
2
3
t
5+
CF SO ) ·4MeCN·1.75H O by slow diffusion of BuOMe
[LnFe(L4) ] , and the FeN chromophores cannot be
3
3 5
2
3
6
C
DOI: 10.1021/acs.inorgchem.9b02713
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
compressed to produce short Fe−N bond length compatible
moiety. Upon decreasing the temperature in going from 400 to
5+
with low-spin configurations. The [LnFe(L4) ] complexes
250 K, the χ T versus T plot corresponds to pure high-spin
3
M
thus display incompressible dFe−N = 2.13(2)−2.34(7) Å bond
behavior (blue trace in Figure 3) and can be successfully fitted
3
−1
−6
3
distances and strict high-spin configuration at any accessible
to C = 3.58(1) cm ·K·mol and TIP = 175(2)·10 cm ·
hs
hs
15,26
−1
temperatures (Figure 2, left and Table S4).
mol with eq 3, a trend similar to that previously reported for
the mononuclear [Fe(L3) ](CF SO ) analogue (Table S5).
26
Monitoring the magnetic susceptibilities with the help of a
SQUID magnetometer reveals, as previously reported for the
3 3 3 2
Below 50 K, the abrupt decrease of χ T observed for
M
15
perchlorate analogue [LaFe(L4) ](ClO ) , that the magnetic
[LaFe(L6) ](CF SO ) can be attributed mainly to the axial
3
4 5
3
3
3 5
Fe(II) center in [LaFe(L4) ](CF SO ) adopts exclusively a
zero-field splitting, but the experimental smooth variation
between 250 and 50 K cannot be modeled with the only resort
3
3
3 5
high-spin configuration (S = 5/2) over the whole accessible
temperature range (red trace in Figure 3). The Curie constant
to zero-field splitting. The latter decrease of the χ T versus T
M
plot is therefore attributed to a spin-crossover phenomenon
taking place at very low temperature. Because it proved to be
difficult to separate zero-field splitting effects from the SCO
transition by simple magnetic measurements, the Fe(II) spin
state in [LaFe(L6) ](CF SO ) was probed by Mossbauer
̈
3
3
3 5
spectroscopy (Figure 4 and Table 1).
Figure 3. Molar magnetic susceptibilities recorded for the dinuclear
complexes [LaFe(Lk) ](CF SO ) (Lk = L4−L6) in the solid state.
3
3
3 5
3
−1
Chs = 3.97(1) cm ·K·mol and paramagnetic independent
3
−1
paramagnetism TIPhs ≈ 0 cm ·mol were fitted using eq 3
II
within the 150−400 K range and fell within the range for Fe
31
Figure 4. Mo
for [LaFe(L6) ](CF SO ) at 80 K. High-spin sites I and II, orange
traces; low-spin site, purple trace.
̈
ssbauer spectra (black circles) and fitted data (red trace)
high-spin complexes (Table S5).
3
3
3 5
χMT = C + T·TIPhs
hs
(3)
At lower temperature, the slight decrease of χ T can be
M
Table 1. Mo
in the Solid State at 80 K
̈
3 3 3 5
satisfyingly fitted using axial zero-field splitting (E/D = 0, eq
3
2
4
1
=
)
and the van Vleck equation to give g = 2.31(2) and D =
−1
a
b
.29(1) cm for [LaFe(L4) ](CF SO ) , which are close to g
2.31(1) and D = 0.77(1) cm found for the mononuclear
T
δ
ΔE
Q
3
3
3 5
−
1
K) (mm·s−1)
(mm·s−1)
(
model complex [Fe(L1) ](CF SO ) (Table S5).
80
1.115(2)
1.854(4)
3
3
3 2
i
S(S + 1)y
0
j
2
z
2
2
1.048(5)
2.97(1)
0.173(9)
14.2(8) high spin,
site II
21.7(5) low spin
E = D·jS −
z + E·(S − S )
n
z
̂
x
̂
y
̂
j
k
z
{
3
(4)
0
.191(5)
0.512(9)
0.205(6)
a
b
c
By implementing the methyl group at the 5-position of the
pyridine ring, where it no longer hinders the complexation of
the di-imine moiety to Fe(II), the expected spin-crossover
Isomer shift. Quadrupole splitting. Line width.
2
6
properties are restored and the χ T versus T plot for
LaFe(L5) ](CF SO ) displays a gradual spin transition
between 100 K and room temperature (black trace in Figure
). Above 300 K, the spin transition becomes more marked,
and at 400 K, the high-spin molar fraction computed with the
However, the rather sophisticated multistep synthesis of
M
[
ligand L6 followed by self-assembly processes limits the
quantity of available crystallized helicates to a few tenths of
milligrams for performing all the analysis and characterization.
3
3
3 5
3
For Mossbauer spectroscopy, the faint quantity of iron (∼2%
̈
3
−1
help of eq 5 and using C = 3.6(1) cm ·K·mol , C = 0, and
of a few tenths of milligrams) is challenging, and several weeks
of γ irradiation are required with our setup. We have thus
limited this study to a single temperature (80 K) where the
existence or the absence of high-spin/low spin mixture is
diagnostic for the operation of spin crossover for rationalizing
the SQUID data recorded on the same sample of complex. At
hs
ls
TIP ≈ 0 amounts to about x ≈ 0.85. Below 200 K, a small
hs
fraction (x ≈ 0.2) of the complex remains high-spin, as
hs
2
6
previously reported for [LaFe(L5) ](ClO ) .
3
4 5
χ T = x ·(C + T·TIP ) + (1 − x )·(C + T·TIP )
M
hs
hs
hs
hs
ls
ls
(5)
8
0 K, the Mossbauer spectrum of [LaFe(L6) ](CF SO )
̈
3 3 3 5
Finally, the shift of the methyl group to the 3-position of the
pyridine ring in [LaFe(L6) ](CF SO ) also forces the Fe(II)
center to adopt a high-spin configuration at room temperature
by twisting the aromatic planes and increasing the distance
between the two coordinating nitrogen atoms of the di-imine
finally showed that a low-spin fraction of 21.7(5)% indeed
coexists with 78.2(6)% of high-spin complexes partitioned
between two sites with molar fractions of xhs = 64(1)% and
14.2(8)% (Figure 4 and Table 1). This is likely due to the
presence of two random crystallographic sites because the
3
3
3 5
D
DOI: 10.1021/acs.inorgchem.9b02713
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Inorganic Chemistry
Article
3
4
−1
complex was isolated as a microcrystalline powder. The
ship (eq 7) for spontaneous emission, where AJ′,J (in s )
stands for Einstein’s probability of spontaneous J′ → J
emission; h is Planck’s constant, ν the energy gap (in
frequency unit) between the two indicated J and J′ states,
Mo
of two spin states for [LaFe(L6) ](CF SO ) at low temper-
̈
ssbauer spectrum confirms unambiguously the coexistence
3
3
3 5
ature in complete agreement with magnetic data and the
operation of a spin crossover process in this complex.
Eu(III) as a Luminescent Probe for Monitoring Fe(II)
Spin State in Dinuclear Triple-Stranded Helicates
and c the speed of light; g and g are the degeneracies of states
J
J′
J and J′, respectively.
3
gJ
5
+
8πhν
rad
Eu
[
EuFe(Lk)3]
(Lk = L4−L6): A Theoretical Approach.
k
= AJ′,J
=
B
J,J′
3
To our surprise, we were unable to find in the literature a
straightforward analysis of the kinetic mechanism summarized
in Scheme 3 which combines Fe(II) spin-crossover behavior
c
g
J′
(7)
The last crucial term, B , is Einstein’s coefficient giving the
probability per unit time and per unit spectral energy density of
J,J′
the radiation field that an electron in state J absorbs a photon
Scheme 3. (a) Kinetic Scheme for the Luminescent
Monitoring of the Fe(II) Spin State Using Eu(III) in a
Dinuclear EuFe Complex and (b) Associated Kinetic
and jumps to state J′. B is proportional to the square of the
J,J′
2
̂
transition dipole moment ⟨ϕ |H |ϕ ⟩ , where ϕ and ϕ are the
J
p
J′
J
J′
a
wave functions of the J and J′ states and H is the
̂
Matrix
p
nonrad
electromagnetic-induced perturbation Hamiltonian. k
is
Eu
an Arrhenius-type temperature-dependent nonradiative pho-
nonrad,0
non-assisted deactivation (eq 8), in which kEu
the nonradiative relaxation at T → ∞ and E
stands for
nonrad
Eu
is the
activation energy of the phonon-assisted relaxation pathway,
whereby low-temperature tunnelling, being much smaller than
18a,35
q
q
the radiative decay, is neglected.
Finally, k and k are
LS HS
the first-order quenching rate constants produced by intra-
molecular Eu(III) → Fe(II) energy transfers toward low-spin
and high-spin Fe(II), respectively.
nonrad
i
y
j E
z
nonrad
Eu
nonrad,0
j
Eu
z
k
= kEu
·expj−
z
j
j
z
z
RT
(
8)
k
{
The time-dependent evolution of the normalized population
densities N are given by the differential matrix rate eq 9.
|
i⟩
t
i
|0⟩
y
i
|0⟩y
jdN /dtz
jN z
j
t
z
j
t z
j
z
j
z
j
z
j
z
j
z
j
z
j
z
j
z
j
|1⟩
z
j
|1⟩z
jdN /dtz
jN z
j
t
z
j
t z
j
z
j
z
j
z
j
z
j
z = Mj
z
j
z
j
z
j
|2⟩
z
j
|2⟩z
j
z
j
z
jdN /dtz
jN z
j
t
z
j
t z
j
z
j
z
a
j
z
j
z
j
z
j
z
Dashed upward arrows, excitation; full downward arrows, relaxation;
j
j
z
z
j
j
z
|3⟩
|3⟩z
dN /dt
N
and straight diagonal arrows, low-spin → high-spin transformations.
k
t
{
k
t
{
(9)
Under pulsed excitation, the mathematical resolution of eq 9
predicts a double exponential decay expressed in eq 10 for the
total Eu(III)-centered emission I (t) accompanying the
(kLH and kHL are the first-order rate constants for low-spin →
high-spin and high-spin → low-spin transitions, respectively)
with neighboring metal-centered luminescent detection,
exemplified here by a trivalent europium considered in its
ground state, Eu, and its excited state, Eu*. The excitation
Eu
depopulation of the normalized excited population densities
|
2⟩
|3⟩
N
and Nt (see Appendix 2 in the Supporting Information
t
for a detailed mathematical treatment).
process (continuous or pulsed) is modeled by the rate constant
rad
|2⟩
|3⟩
exc
Eu
IEu(t) = kEu (Nt + N )
t
k
given in eq 6, where λ is the pump wavelength, P the
P
rad
Eu
−k1t
−k2t
incident pump intensity, h Planck constant, c the speed of light
=
k
((C + D )e
+ (C + D )e
)
(10)
1
1
2
2
in vacuum, and σ the absorption cross section (dashed traces
Eu
3
3
5+
The characteristic decay rate constants k are calculated with
in Scheme 3). Please note that in molecular [EuFe(Lk)3]
1,2
relax
q
LS
the help of eq 11, where k = k + k + k and k = k +
cations, indirect sensitization is obtained via absorption
through the ligand-centered excited states followed by energy
transfer onto the Eu(III) center (antenna effect). This justifies
the neglect of stimulated emission processes in the associated
kinetic scheme.
A
LH
Eu
B
HL
relax
q
k
+ k , while C , C , D , and D are coefficients that can be
Eu
HS 1 2 1 1
deduced from the initial conditions (see Appendix 2).
ik + k y
1
2
j A
B z
2
k1,2 = j
z ±
(k − k ) + 4k k
j
k
z
{
A
B
HL LH
2
(11)
λp
hc
exc
Eu
k
=
PσEu
Under continuous-wave excitation, the steady-state population
are obtained by numerically solving eq 9 for
dN /dt = 0 (Appendix 3 in the Supporting Information). The
(6)
|i⟩
densities N
S−S
|
i⟩
The decay rate constants of the Eu(III) excited states, i.e.,
t
5
Eu
Eu
the Eu( D ) spectroscopic level, can be partitioned between
associated intrinsic quantum yield ϕ , which is correlated to
the global quantum yield ϕ via the sensitization process
0
relax
rad
nonrad
L
Eu
radiative and nonradiative contributions kEu = k + k
.
Eu
Eu
rad
1a,b
k
is temperature-independent and obeys Einstein’s relation-
ηsens
,
can be easily deduced as soon as the ratio of the
Eu
E
DOI: 10.1021/acs.inorgchem.9b02713
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
|
3⟩
steady-state population of the excited emissive levels N
/
S−S
N| is available (eq 12, Appendix 3).
2⟩
S−S
L
ϕ
Eu
Eu
Eu
ϕ
=
ηsens
rad
Eu
|3⟩
|2⟩
k
(1 + (N /N ))
S−S S−S
= k
(1 + (N
relax
Eu
|3⟩
/N )) + k qH S(N /N ) + kq
|2⟩
|3⟩
|2⟩
LS
S−S
S−S
S−S
S−S
(12)
Under weak continuous excitation so that k eE xu c ≪ k , k , k ,
HL LH
A
|2⟩
k , the steady-state excited-state population densities N and
B
S−S
|
3⟩
N
S−S
are negligible compared with ground-state population
densities, and the following approximations hold:
|
1⟩
|1⟩
0
N
N
kLH
= KSCO = e−(ΔG
/RT)
S−S
0
SCO
|0⟩
|1⟩
≈ N
=
and N + N
=
|
0⟩
|0⟩
S−S
S−S
N
kHL
S−S
0
|
i⟩
N . With this in mind, solving eq 9 for dN /dt = 0 provides
0
t
analytical equations for the steady-state population densities,
from which the total emitted intensity IS−S is given in eq 13
(Appendix 3).
rad
|2⟩
|3⟩
relax
Eu
I
= kEu (N
+ N
)
Figure 5. Temperature dependence of (a) k
= 1/τobs in
S−S
S−S
S−S
5
+
q
relax
[
EuZn(L5)3] (acetonitrile, 1 mM) and (b) k = (1/τobs) − k
HS
Eu
i
y
k (k + k ) + k (k + k )
5+
exc radj LH
A
HL
HL
B
LH z
j
z
in [EuFe(L4)3] (acetonitrile, 15 mM). The dashed traces were
computed with eqs 14 and 16 using the kinetic parameters reported in
the figure.
=
N k k j
z
0
Eu Eu j
z
z
j
(
kHL + k )(k k − k k )
LH
A B
HL LH
{
(
13)
k
5
+
[
EuZn(L5) ] as a Model for Extracting Experimental
3
L
rad
Eu
ϕ
k
Radiative and Nonradiative Eu-Centered De-excitation
Rate Constants in Solution. The determination of the rate
constants pertinent to isolated dinuclear helicates in
acetonitrile solution (see kinetic Scheme 3) starts with the
photophysical study of [EuZn(L5)3]5 at millimolar concen-
Eu
Eu
Eu
ϕ
=
=
rad
Eu
nonrad
ηsens
k
+ k
Eu
rad
k
Eu
= k
+ kEu
·exp(−EEu /RT)
+
rad
Eu
nonrad,0
nonrad
(15)
27
tration, where chemical dissociation is negligible. In the latter
Eu
helicate, Fe(II) is replaced by 3d10 closed-shell Zn(II) which
The room-temperature intrinsic quantum yield ϕ = 0.82
can be compared with the global quantum yield ϕ = 4.2 ×
10 previously recorded upon ligand-based excitation for this
Eu
L
Eu
exhibits neither spin-crossover (k = kHL = 0) nor energy
feeding from Eu(III) (k = k = 0). The observed decay of
Eu(III)-centered luminescence thus corresponds to kEu
summarized in eq 14 for its temperature dependence
LH
−2
q
q
HS
LS
37
complex. The associated sensitization process thus amounts
relax
L
Eu
Eu
Eu
−2
5+
to ηsens = ϕ /ϕ = 5 × 10 for [EuZn(L5)3] in acetonitrile
at room temperature.
(
combination of eqs 7 and 8).
5+
[
EuFe(L4)3] as a Model for Extracting Experimental
Eu → FeHS Energy-Transfer Rate Constants in Solution.
nonrad
Eu
i
y
5+
j E
z
z
relax
Eu
rad
nonrad
rad
nonrad,0
j
Upon 355 nm ligand-centered excitation, [EuFe(L4) ]
3
k
= kEu + kEu
= kEu + kEu
·expj−
z
j
j
z
z
5
7
RT
exhibits weak, but standard Eu-based D → F (J = 0−6)
luminescence (Figure S18) characterized by biexponential
decays of the Eu( D ) excited level (Figures S19). Because of
0 J
k
{
(
14)
5
0
Upon pulsed 355 nm ligand-centered excitation using the
the weaker affinity of L4 with respect to L5 for the central
1
5
metallic cations in the associated dinuclear triple-helicates,
a
third harmonic of a Nd:YAG laser, an acetonitrile solution of
5
+
5
+
5
7
concentrated 15 mM acetonitrile solution of [EuFe(L4)3]
[
6
EuZn(L5)3] exhibits standard Eu-based D → F (J = 0−
0
J
was used for the photophysical measurements in order to
minimize the amount of free Eu(III) in solution. Despite this
precaution, biexponential rate laws were systematically
required for fitting the experimental decays. The minor long-
) luminescence (Figure S16) characterized by millisecond
range monoexponential decays of the population of the
Eu( D ) excited level (Figure S17 and Table S6). The
characteristic lifetime τobs = 1/kEu = 2.43(1) ms measured
5
0
relax
lived contributions (pre-exponential factors = 0−10%, τ
≈
obs
at 293 K is coherent with the previously reported value of τ
obs
3
33 μs at 300 K) probably originate from traces of solvated
3
=
2.89(2) ms at 295 K for a 3.7 × 10− M acetonitrile solution
Eu(III) released in solution, while the major short-lived
1
5
of the analogous [EuZn(L4) ](ClO ) helicate. As predicted
3
4 5
component (pre-exponential factors = 90−100%, τ = 46(1)
obs
by eq 14, the temperature dependence of the relaxation rate
μs at 300 K) can be assigned to Eu(III) centers bound in
relax
5+
constant kEu = 1/τobs monitored in [EuZn(L5) ] obeys an
5+
3
[EuFe(L4)3] (Figure S20). The latter short lifetime is
rad
Eu
−1 nonrad,0
8
Arrhenius-type law with k = 339(2) s , k
= 8(2) × 10
= 39.4(6) kJ·mol (Figure 5a) leading to
almost quantitative intrinsic quantum yields at low temperature
eq 15 and Table S7).
Eu
consistent with τ = 59(1) μs previously reported for 3.7 ×
obs
−
1
nonrad
−1
−3
15
s
and EEu
10 M solution of [EuFe(L4) ](ClO ) at 295 K. Only this
3 4 5
fast component was further considered as pertinent to Eu(III)
5
+
6
(
in [EuFe(L4) ] . Because Fe(II) adopts a pure high-spin 3d
3
F
DOI: 10.1021/acs.inorgchem.9b02713
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
5
+
5
electronic configuration at all temperatures in [EuFe(L4) ]
determined for the analogous [LaFe(L5) ] helicate in
3
3
1
5,26
5
−1
(
kHL = 0),
the decay of the Eu( D ) excited level combines
in eq 14) with a
supplementary thermally activated intramolecular Eu(III) →
acetonitrile (ΔH
= 29.6(2) kJ·mol
SCO
−1 26
and ΔS
=
0
SCO
5+
relax
Eu
−1
the one found for [EuZn(L5)3] (k
86.2(5) J·K ·mol ), eq 19 predicts that these dinuclear
helicates are essentially low-spin at 233 K (KSCO = 7 × 10−
x /(1 − x ) ⇒ x = 0.6%) while at room temperature xHS
3
=
q
HS
Fe(II)HS energy transfer modeled with k
≠ 0 and
HS
HS
HS
summarized in eq 16 (Scheme 3, right part).
≈ 16%. The maximum high-spin fraction that can be reached
in acetonitrile at 333 K is x = 40% (Figure 6).
q
HS
i E
y
obs
Eu
relax
q
relax
q,0
j
HS z
j
z
k
= kEu + k = kEu + kHS·expj−
z
z
HS
j
k
5+ k
LH
5+
RT
(16)
{
[EuFe (L5) ] XooY [EuFe (L5) ]
LS
3
HS
3
k
HL
Taking into account the isostructural [EuN O ] coordina-
6
3
5
+27
kLH
kHL
iΔS
ΔH
y
j
j
j
SCO
SCO z
tion spheres found in the crystal structures of [EuZn(L5) ]
z
z
3
KSCO
=
= exp
−
5
+
26
relax
and [EuFe(L4) ] , we can reasonably assume that k
,
k
R
RT {
(19)
3
Eu
5
+
previously obtained for [EuZn(L5) ] , also holds for
EuFe(L4) ] . With this in mind, the temperature depend-
ence of the differences in the observed rate constants can be
3
5
+
[
3
obs
obs
q
q,0
q
HS
fitted to 1/τ
− 1/τ
= k = k ·exp(−E /RT),
EuZnL5 HS
6 −1 nonrad
EuFeL4
q,0
HS
from which k = 8(1) × 10 s and EEu
= 14.7(4) kJ·
HS
−
1
mol are deduced using a linear least-squares fit (Figure 5b).
q
HS
−1
At room temperature, we calculate that k = 18 855 s for the
intramolecular Eu(III) → Fe(II) energy transfer operating in
5
+
[
η
EuFe(L4) ] , which leads to an energy-transfer efficiency of
3
q
ET
q
relax
= k /(k + k ) = 97.8% (Table S7). According to
HS HS Eu
Eu→FeHS
36
the dipole−dipole energy-transfer theory, the latter ratio is
related to the distance between the energy donor and energy
acceptor R , taken as the intramolecular Eu···Fe = 9.0 Å
5+
DA
HS
Figure 6. Spin-crossover properties of [LaFe(L5)3] in acetonitrile
intermetallic separation (Figure 2), and the so-called critical
radius R for 50% energy transfer.
−1
computed using eq 19, ΔH
= 29.6(2) kJ·mol , and ΔS
=
SCO
SCO
−
1
−1 26
0
86.2(5) J·K ·mol .
ET
6
−1
η
= [1 + (RDA/R ) )]
(17)
Eu→Fe
0
According to Scheme 3, the Eu(III)-centered emission in the
5
+
Application of eq 17 to the Eu/FeHS pair in [EuFe(L4)3]5+
spin-crossover helicate [EuFe(L5)
]
3
may arise from both
Fe −Eu* and Fe −Eu* excited states, which are inter-
converted via k and k kinetic rate constants. Taking a
reasonable Arrhenius-type relationship for the high-spin to
LS
HS
leads to R = 17.5 Å, which corresponds to a considerable
0
LH
HL
distance if molecular design is envisioned for limiting
rad
intramolecular Eu(III) quenching. Using the values of k ,
Eu
0
0
8
6
nonrad
, and kq (Table S7), an Eu(III) intrinsic quantum yield
low-spin relaxation kHL = kHL·exp(−ELH/RT) with kHL = 10
k
Eu
HS
−1
−1
38
Eu
Eu
−2
s
s
and EHL = 10 kJ/mol, a typical value of kHL ≈ 1.7 × 10
of ϕ = 1.8 × 10 can be calculated with eq 18 for
EuFe(L4)3] in acetonitrile at room temperature (Table S7).
5+
5
+
is computed for [EuFe(L5)
]
3
at room temperature, from
[
5
−1
which kLH = KSCO·kHL = 3.3 × 10 s is obtained with eq 19.
L
rad
Eu
q
LS
The quenching constant k , which corresponds to the rate of
ϕ
k
Eu
Eu
Eu
ϕ
=
= k
=
intramolecular Eu(III) → Fe(II)LS energy transfer, thus
remains the only unknown parameter in Scheme 3.
rad
Eu
nonrad
q
HS
ηsens
+ kEu
+ k
rad
Assuming k qL S > kHS = 18 655 s (Table S7) because of the
q
−1
k
Eu
rad
Eu
nonrad,0
nonrad
q,0
q
HS
much larger spectral overlap integral ΩD,A expected for the
k
+ kEu
·exp(−EEu /RT) + kHS·exp(−E /RT)
Eu−Fe pair (see eq 1 and Figure 1), the biexponential decay
LS
(18)
curves following laser-pulsed excitation (eq 10, Figure 7) and
the global quantum yields obtained under continuous-wave
Compared with ϕ = 0.82 observed for [EuZn(L5)3]5+ in
Eu
Eu
−
2
the absence of Eu(III) → Fe(II) energy transfer, we conclude
xenon lamp (eq 12; P ≤ 1 W·cm ) have been simulated
that the large majority of the Eu-centered emission is quenched
(Table 2).
5
+
by the neighboring high-spin Fe(II) partner in [EuFe(L4) ] .
The opening of the additional Eu(III) → Fe(II) energy-
3
LS
Because a global quantum yield ϕL = 3 × 10 was previously
−4
transfer quenching channel using plausible 10 ≤ k ≤ 10 s
6
q
8 −1
Eu
LS
37
reported for the latter complex in acetonitrile solution, the
values for the first-order quenching rate constant dramatically
affects the global quantum yield, which drops by 2−3 orders of
associated sensitization process can be easily computed as η
sens
L
Eu
−2
5+
L
=
[
ϕ /ϕ = 2 × 10 , and it fairly matches that obtained for
magnitude by going from pure high-spin [EuFe(L4) ] (ϕ =
5 Eu
Eu
Eu
5
+
−2
−4
5+
−7
L
Eu
EuZn(L5)5] (η = 5 × 10 ).
EuFe(L5)3] as a Model for the Luminescence
3 × 10 ) to spin-crossover [EuFe(L5) ] (7.0 × 10 ≤ ϕ
sens
5
5
+
−6
[
= 9 × 10 , entry 6 in Table 2) at room temperature. This
Monitoring of Fe(II) Spin State in Dinuclear Helicates
drastic decrease compromises the detection of any lumines-
5
+
in Solution. Having in hand the kinetic behavior of the
cence arising from Eu(III) in the spin-crossover [EuFe(L5)5]
helicate. Furthermore, the two characteristic lifetimes shorten
to such an extent (τ < 1 μs) with increasing k that there is no
chance to introduce sufficient delay between the strong pulse
and the detection procedure to be able to catch the (very)
weak end of the decay traces (Figure 7b,c). Tentative excited-
state lifetime measurements were however carried out on
Eu(III) probe when it is isolated in [EuZn(L5)3]5+ (k ) and
relax
Eu
5
+
q
LS
when it communicates with high-spin Fe(II) in [EuFe(L4) ]
k ), the last step of the analysis focuses on the combination
3
i
q
HS
(
of the Eu(III) probe with a spin-crossover Fe(II) center in
5
+
[
EuFe(L5) ] . Taking into account that the thermodynamic
3
parameters of the spin-state equilibrium (eq 19) have been
G
DOI: 10.1021/acs.inorgchem.9b02713
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Inorganic Chemistry
Article
Table 2. Characteristic Decay Lifetimes τ = 1/k and Initial
i
i
Relative Intensities A = C + D (i = 1, 2; eq 10), Global
i
i
i
L
Eu
Quantum Yields ϕ (eq 12), Energy-Transfer Efficiency
, and Critical Radius for 50% Energy Transfer R
Computed for [EuFe(L5)3] as a Function of Eu(III) →
ET
Eu→Fe
η
0
5+
q
LS
Fe(II)LS Energy-Transfer Rate Constants k in Acetonitrile
Solution at Room Temperature
q
LS
(s−1
k
)
1
06
107
108
τ = 1/k (ns)
A1 (%)
432
5.7
1320
94
0.28
8.6 × 10
96
73
597
23
1.11
9
i
1
83
577
16
9.38
7.0 × 10
97.9
τ = 1/k (ns)
i
2
A (%)
2
3⟩
N| /N
|2⟩
a
S−S
S−S
L
b
−6
−6
−7
ϕ
1.4 × 10
97.9
Eu
ET
η
(%)
97.9
Eu→FeHS
R (Eu→FeHS) (Å)
17.5
>99.9
17.5
>99.9
17.5
>99.9
0
ET
η
(%)
Eu→FeLS
R (Eu→Fe ) (Å)
33.5
49.3
72.3
0
LS
a
Ratio of the steady-state excited population densities obtained for
−
2
b
incident pump intensities P ≤ 1 W·cm (see Scheme 3). Computed
−
2
5+
using η = 2 × 10 observed for [EuFe(L4) ] .
sens
5
→
Fe(II)LS is close to quantitative, which corresponds to a
q
8
−1
quenching rate of k ≥ 10 s (column 4 in Table 2).
LS
Solid [EuFe(L5) ](CF SO ) as a Model for Monitoring
3
3
3 5
the Fe(II) Spin State in Dinuclear Helicates. Because (i)
the total amount of emitted photons increases with dye
concentration and (ii) the large molecular size of the
5
+
[
LnFe(L5)5] cations prevents close contact in the crystalline
state so that intramolecular intermetallic contact distances
24b
26
(
dLa···Fe = 9.19 Å and dEu···Fe = 9.17 Å ) remain shorter than
intermolecular ones (the minimum intermetallic contact
24b
distance is found between iron atoms dFe···Fe = 9.7 Å),
solid-state samples of [EuFe(L5) ](CF SO ) (total concen-
3
3
3 5
27
−
23
tration C = Z/(Vmol·N ) = 8/(28 426 × 10 ·6.02 × 10 ) =
.47 M) were tentatively irradiated at 355 nm for monitoring
A
0
5
Fe(II) spin transition via the modulation of Eu(III)
luminescence. While the emitted intensity induced by pulsed
excitation was clearly too weak to be exploited in time-resolved
experiments, continuous-wave excitation provided detectable
and reproducible emission spectra characterized by standard
Figure 7. Eu( D ) decay traces predicted with eq 10 and associated
0
5
+
quantum yields (eq 12) computed for [EuFe(L5)5] for various
q
6
−1
Eu(III) → Fe(II) energy-transfer rate constants: (a) k = 10 s ,
LS
LS
7
−1
q
8
−1 rad
nonrad
Eu
(
b) kq = 10 s , and (c) k = 10 s . k and k
are those found
LS
LS
Eu
5+
q
−1
for [EuZn(L5) ] , and k = 18 655 s corresponds to that
measured for [EuFe(L4) ] and kHL = 1.7 × 10 s and kLH = KSCO
kHL = 3.3 × 10 s (see text and Table S7).
5
HS
5
+
6
−1
5
7
·
Eu-based D → F (J = 0−6) transitions (Figure S22). Their
5
0
J
5
−1
temperature dependence shows an unusual increase in
intensity within the 200−300 K domain, which is diagnostic
for the optical monitoring of the spin-state equilibrium because
strong Fe(II)LS quenchers are replaced with weaker accepting
Fe(II)HS centers upon heating (dashed black trace in Figure
8a).
twice-recrystallized [EuFe(L5) ](PF ) samples dissolved in
ultradry distillated acetonitrile. Detectable Eu(III)-centered
luminescence can be recorded only for concentrations between
5
6 5
−
4
−3
5+
4
× 10 and 2 × 10 M when [EuFe(L5) ] undergoes
5
24b
partial dissociation (Figure S21).
The rather long 200 μs
Taking into account all the kinetic rate constants pertinent
to Scheme 3 and previously determined in Figures 5 and 6,
characteristic lifetime measured in these conditions unambig-
uously points to the existence of traces of solvated Eu(III) as
the unique detectable emitter. For concentrations of [EuFe-
q
q,0
q
together with the assumption k = k ·exp(−E / RT) with
LS
LS
LS
q,0
= 10 s and Eq = 2 kJ/mol, so that k ≈ 5 × 10 s at
room temperature, eq 13 indeed predicts the observed unusual
dependence of the total emitted intensity on the temperature
for the spin-crossover [EuFe(L5) ](CF SO ) complex
9
−1
q
8 −1
k
LS
LS
LS
5
+
−3
−2
(
L5)5] in the 4 × 10 to 2 × 10 M range where
dissociation is removed, the emitted intensity suddenly
decreases in line with the exclusive existence of the poorly
emissive [EuFe(L5)5]5 species in solution (Figure S21).
Taking into account that the sensitivity of our optical detection
system has been recently optimized for recording visible
3
3
3 5
+
(Figure 8b). Similar emission spectra recorded for pure high-
spin [EuFe(L4) ](CF SO ) in the solid state (Figure S23)
3
3
3 5
exhibit the expected and usual decrease of emitted intensity
with increasing temperature (red dashed trace in Figure 8a), a
−7
39
quantum yields down to 10 , we conclude that the Eu(III)
H
DOI: 10.1021/acs.inorgchem.9b02713
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
component (120−240 μs, 70%; Figure S24). Following the
recent successful analysis of multiexponential decay traces
characterizing Tb(III) emission modulated by Fe(II) spin-
24b
crossover in doped solid materials,
average lifetimes, τavg,
were similarly computed in Table 3 by using the simple
weighting scheme summarized in eq 20, where A are the pre-
n
exponential factors and τ are the associated characteristic
n
lifetimes.
∑n A ·τn
∑n An
n
τavg
=
(20)
Depending on the d metal used as potential quencher for the
Eu(III) luminescent center in [EuM(Lk) ]X helicates, the
3
5
5
average Eu( D ) lifetime varies from 1 to 2.5 ms at room
0
temperature for photophysically inactive Zn(II) (Table 3,
entries 1 and 2) to 90−180 μs for high-spin Fe(II) energy
acceptor (Table 3, entries 3−5).
For strict Eu(III)−Fe(II) pairs in [EuFe(L4) ](CF SO ) ,
HS
3
3
3 5
the temperature dependence of the average lifetime τ in the
avg
solid state (dashed red trace in Figure 9) fairly mirrors the
Figure 8. (a) Molar paramagnetic susceptibility χ T (left axis, full
M
traces) and normalized emission (right axis, circles and dashed traces)
for high-spin [LnFe(L4) ](CF SO ) (red, Ln = La(III) for
3
3
3 5
magnetism, Ln = Eu(III) for luminescence) and spin-crossover
LnFe(L5) ](CF SO ) (black, Ln = La(III) for magnetism, Ln =
[
3
3
3 5
Eu(III) for luminescence) helicates in the solid state. (b) Simulated
steady-state emission intensity profile using eq 13 and the
thermodynamic (Figure 6) and kinetic rate constants (Figure 5)
5
+
5+
determined for [EuZn(Lk) ] and [EuFe(Lk) ] (Lk = L4, L5) in
Figure 9. Molar paramagnetic susceptibility (left axis, full traces) and
3
3
solution together with kq = k ·exp(−E /RT) (k = 10 s and Eq
q,0
LS
q
LS
q,0
LS
9
−1
5
average Eu( D
) characteristic lifetimes (right axis, circles and dashed
](CF SO (red, Ln = La(III) for
magnetism, Ln = Eu(III) for luminescence) and spin-crossover
LnFe(L6) ](CF SO ) (black, Ln = La(III) for magnetism, Ln =
LS
LS
0
=
2 kJ/mol; see text).
traces) for high-spin [LnFe(L4)
3
3
)
3 5
[
3
3
3 5
Eu(III) for luminescence) helicates in the solid state.
trend in line with the operation of standard thermally activated
nonrad
q
quenching processes affecting kEu (eq 8) and k (eq 16).
HS
In the absence of low-spin Fe(II), as found in pure high-spin
standard trend illustrated by the decrease in intensity recorded
for the related steady-state emission (dashed red trace in
Figure 8a). Moreover, it further matches the 200 μs (233 K) to
29 μs (323 K) range observed for the same complex in solution
(Table S6).
[
EuFe(L4) ](CF SO ) and [EuFe(L6) ](CF SO ) around
3 3 3 5 3 3 3 5
room temperature, the luminescence is strong enough to allow
the reliable determination of characteristic lifetimes after
pulsed excitation (Figure S24). Because of the existence of
structure defects and additional intermolecular communica-
tions in solids, multiexponential fits are required for satisfyingly
Moving to [EuFe(L6) ](CF SO ) shows the opposite
3
3
3 5
trend with a global increase of τavg with increasing temperature
in the 10−300 K range (dashed black trace in Figures 9 and
S25). Again, this counterintuitive behavior can be assigned to a
reproducing the experimental decay traces arising from the
5
Eu( D ) level. For solid samples of pure high-spin [EuFe-
0
(
L4) ](CF SO ) , a biexponential behavior is observed with a
smooth spin-crossover process operating in [EuFe(L6) ]-
3
3
3 5
3
minor short contribution (40−70 μs; 30%) and a major longer
(CF SO ), where the 21.7% trapped low-spin configuration at
3
3
5
Table 3. Average Lifetimes τavg of the Eu( D ) Excited Level (eq 20) for [EuM(Lk) ]X Helicates (M = Zn(II), Fe(II); Lk = L4,
0
3
5
L6 and X = ClO4− or CF SO ) Measured for the D → F Transition at 10 and 300 K
−
5
7
3
3
0
2
complex
Fe(II) spin state
λexc (nm)
τavg at 10 K (ms)
τavg at 300 K (ms)
reference
a
b
[
[
[
[
[
EuZn(L4) ](ClO )
378
355
355
308
355
2.63(1)
2.53(1)
15
3
4 5
EuZn(L6) ](CF SO )
3 5
1.186(1)
0.1732(3)
0.277(7)
0.1216(4)
0.993(1)
0.0902(1)
0.129(1)
0.1810(3)
this work
this work
15
3
3
EuFe(L4) ](CF SO )
3 5
high-spin
high-spin
spin-crossover
3
3
a
b
EuFe(L4) ](ClO )
4 5
3
EuFe(L6) ](CF SO )
3 5
this work
3
3
a
b
T = 13 K. T = 295 K.
I
DOI: 10.1021/acs.inorgchem.9b02713
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
0 K (see Figure 4) is stepwise replaced by less-quenching
Article
8
pure high-spin Fe(II) acceptors upon heating.
CONCLUSIONS
■
After more than 80 years of intense activity in spin-crossover d-
block complexes, with a special emphasis on Fe(II) centers, for
which the magnetic response can be switched from
diamagnetic (low-spin Fe(II)) to paramagnetic (high-spin
Fe(II)), myriads of local environments have been screened for
optimizing the thermodynamic characteristics of their spin-
state equilibria (enthalpy, entropy, cooperativity, and transition
2
1,22,30,40
temperature).
In this context, the didentate binding
units 1-methyl-2-(x-methylpyridin-2-yl)-1H-benzo[d]-
imidazole (x = 6, L1; x = 5, L2; x = 3, L3) exploit rather
standard intramolecular constraints for modulating ligand field
strength around Fe(II) in FeN6 chromophores and thus
controlling Fe(II) spin-state equilibria. The resort to
L
Figure 10. Global quantum yields ϕ (%) computed for [EuFe-
Eu
5
+
(L5)
2
] in acetonitrile using the kinetic parameters gathered in Table
and k = 10 s as a function of the mole fraction of high-spin
3
q
LS
8
−1
Fe(II).
“
extended” coordination chemistry, i.e., metallosupramolecular
5+
chemistry, provides the dinuclear [LnFe(Lk)3] (Lk = L4−
L6) complexes, in which the trivalent lanthanide partner has a
indirect lanthanide-based optical detection of Fe(II) spin
crossover in molecular complexes. Considering eqs 10, 12, and
double function: (1) to constrain the FeN chromophores to
6
adopt pure facial arrangements and (2) to act as an innocent
reporter for probing Fe(II) spin-state equilibrium. To the best
of our knowledge, the kinetic diagram depicted in Scheme 3,
even if it seems straightforward, has no precedent for
deciphering Ln → Fe(II) communication operating in
multimetallic spin-crossover complexes. Moreover, the virtues
of metallosupramolecular chemistry allow that open-shell
1
3, major improvements for inducing detectable Fe(II)
modulation of Ln(III)-centered emission in isolated dinuclear
assembly under pulsed excitation require a massive decrease of
the critical radius, R , for 50% energy transfer in the Ln/Fe
0
LS
pair so that molecular dimensions become accessible (see the
last entry in Table 2). Because both through-bond (Dexter-
type) or through-space (Forster-type) mechanisms are known
̈
(
Eu(III) and Fe(II)) or closed-shell (La(III) and Zn(II))
to be efficient for promoting intermetallic d−f energy transfer
9b
metallic centers can be voluntarily introduced into the various
over several nanometers, i.e. the perturbation term |⟨DA*|H|
5+
2
dinuclear [LnM(Lk)3] helicates without major alteration of
their molecular structures. The La/Zn pair is thereby exploited
for the easy structural characterization of these complexes in
solution with the help of diamagnetic NMR spectroscopy. The
La/Fe pair gives then fair access to the Fe(II) spin-crossover
D*A⟩| in eq 1 is systematically large, an obvious option
consists of reducing the spectral overlap integral Ω for both
D,A
Ln/Fe and Ln/Fe pairs through a judicious choice of the
HS
LS
Ln(III) donor. Clearly, Ω for the Eu(III)/Fe(II) pair is so
D,A
large that europium-based luminescence is drastically
quenched as soon as traces of Fe(II)LS are present. Because
of the numerous excited levels produced by electron−electron
process (KSCO = k /k ) via magnetic measurements without
LH HL
any perturbation brought by the diamagnetic lanthanide. The
Eu/Zn pair provides a simple estimation of the de-excitation
41
repulsion in 4f-block cations, it appears rather straightfor-
rad
nonrad
5
+
rate constants kEu and kEu
by using time-resolved emission
ward to replace Eu(III) in [LnFe(Lk)3] with alternative NIR
emitters (Ln = Nd, Ho, Er) for designing a new generation of
molecular Ln/Fe helicates, in which luminescence could be
exploited for monitoring Fe(II) spin crossover not only in pure
solids but also in diluted solutions.
spectra, while similar studies focused on the Eu/FeHS pair
q
HS
reveal the rate constant k pertinent to intramolecular Eu(III)
→
Fe(II)HS energy transfers. Given that solution data are
available for intact complexes in diluted solutions (i.e.
nondissociated), energy migrations are limited to intra-
pair found in
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b02713.
SCO
■
5
+
q
LS
*
S
5
+
Experimental section (Appendix 1) and mathematical
resolution of the matrix differential eq 9 (Appendices 2
and 3); Tables S1−S7 collecting elemental analyses,
crystallographic, magnetic, and photophysical data;
Figures S1−S25 showing H NMR spectra, crystal
structures, and luminescence spectra recorded under
various conditions (PDF)
1
−6
5
+
escapes time-resolved detection and is therefore limited to
continuous wave excitation of solid-state samples, where the
Eu(III) concentration is maximum. The resulting anomalous
temperature dependence of Eu(III)-centered emission ob-
served for [EuFe(L6) ](CF SO ) (Figure 9) and [EuFe-
Accession Codes
CCDC 1947454−1947455 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
3
3
3 5
(
L5) ](CF SO ) (Figure 8a) and its modeling with eq 13
Figure 8b) represent unambiguous clues for the successful
3 3 3 5
(
J
DOI: 10.1021/acs.inorgchem.9b02713
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Beta-diketonate-Eu(III) Displacement Assay. Chem. Commun. 2007,
AUTHOR INFORMATION
Corresponding Author
E-mail: Claude.Piguet@unige.ch.
■
129−131. (b) Akiba, H.; Sumaoka, J.; Komiyama, M. Binuclear
Terbium(III) Complex as a Probe for Tyrosine Phosphorylation.
Chem. - Eur. J. 2010, 16, 5018−5025. (c) Shuvaev, S.; Starck, M.;
Parker, D. Responsive, Water-Soluble Europium(III) Luminescent
Probes. Chem. - Eur. J. 2017, 23, 9974−9989.
*
ORCID
Alexandre Fu
̈
rstenberg: 0000-0002-6227-3122
(8) (a) Gao, F.; Luo, F.; Tang, L.; Dai, L.; Wang, L. Preparation of a
Cel
́
ine Besnard: 0000-0001-5699-9675
Novel Fluorescence Probe of Terbium-Europium Co-luminescence
Composite Nanoparticles and its Application in the Determination of
Proteins. J. Lumin. 2008, 128, 462−468. (b) Abdallah, A.;
Daiguebonne, C.; Suffren, Y.; Rojo, A.; Demange, V.; Bernot, K.;
Calvez, G.; Guillou, O. Microcrystalline Core-Shell Lanthanide-Based
Coordination Polymers for Unprecedented Luminescent Properties.
Inorg. Chem. 2019, 58, 1317−1329.
Claude Piguet: 0000-0001-7064-8548
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank J. Meunier for recording Mo
Financial support from the Swiss National Science Foundation
is gratefully acknowledged.
■
̈
ssbauer spectra.
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Sensitized Luminescence from Lanthanides in d-f Bimetallic
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G.-T.; Chen, Z.-N. Recent Advances in Lanthanide Luminescence
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