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

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Reference

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

LATHION, Timothee, et al.

Abstract

The modulation of light emission by Fe(II) spin-crossover processes in multifunctional materials has recently attracted major interest for the indirect and non-invasive 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), Eu(III)) in acetonitrile to give C3-symmetrical dinuclear triplestranded helical [LnM(Lk)3]5+ cations, which can be crystallized with non-coordinating counteranions. 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)3]5+ (spin-crossover (SCO) around room temperature) and [LnFe(L6)3]5+ (SCO at very low temperature). The introduction of the photophysically active Eu(III) probe in [EuFe(Lk)3]5+ results in europium-centered luminescence modulated by variable intramolecular [...]

LATHION, Timothee, et al. Monitoring Fe(II) Spin-State Equilibria via Eu(III) Luminescence in Molecular Complexes: Dream or Reality? Inorganic Chemistry, 2020, vol. 59, no. 2, p.

1091-1103

DOI : 10.1021/acs.inorgchem.9b02713

Available at:

http://archive-ouverte.unige.ch/unige:129611

Disclaimer: layout of this document may differ from the published version.

1 / 1

Publication: Inorg. Chem. 2020, 59, 1091-1103. DOI: 10.1021/acs.inorgchem.9b02713

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

__________________________________________________________________

Timothée Lathion,^† Alexandre Fürstenberg,^† Céline Besnard,^‡ Andreas Hauser,^† 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 & Université de Toulouse (UPS, INP), 205 route de Narbonne, Toulouse 31077 Cedex 4 (France).

__________________________________________________________________

Abstract

The modulation of light emission by Fe(II) spin-crossover processes in multifunctional materials has recently attracted major interest for the indirect and non-invasive 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), Eu(III)) in acetonitrile to give C³-symmetrical dinuclear triple- stranded helical [LnM(Lk)³]⁵⁺ cations, which can be crystallized with non-coordinating 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)³]⁵⁺ (spin-crossover (SCO) around room temperature) and [LnFe(L6)3]⁵⁺ (SCO at very low temperature). The introduction of the

photophysically active Eu(III) probe in [EuFe(Lk)³]⁵⁺ 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 larger 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.

Introduction

Open-shell trivalent lanthanides, Ln(III), are famous for their atom-like optical and magnetic properties, which can be further tuned in a rational way by their specific chemical environments in molecular complexes or macroscopic materials.¹ In this context, Eu(III) and Tb(III) have been exhaustively used as luminescent probes because of their strong, easily-detectable, long-lived and well-understood visible emission.² For instance, Eu(III) and Tb(III) stains have been used for probing point mutations in DNA,³ for labelling proteins,⁴ for improving contrast in optical microscopy,⁵ for sensing local chiral environments,⁶ for quantifying analytes in complex media⁷ and for designing colored phosphors.⁸ In inorganic and coordination chemistry, the latter visible lanthanide probes were intensively exploited for monitoring intermetallic d-f communications operating in (supra)molecular assemblies.⁹ When associated in dyads with high-energy sensitizers such as Ir(III),¹⁰ Pt(II),¹¹ or Fe(II) in ferrocene,¹² 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 (17277 cm^-1 for Eu(⁵D⁰) and 20462 cm^-1 for Tb(⁵D⁴) in the aquo ions)¹³ also make them good candidates for the alternative sensing mechanism, where they act as donor. In this case, their luminescence is quenched by energy transfers toward neighboring d-block acceptors in Eu-M and Tb-M molecular pairs where M = Cr(III),¹⁴ Fe(II),¹⁵ Fe(III),¹² Cu(II)¹⁶ and Ru(II)¹⁷. 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) is given by Fermi’s golden rule.¹⁸

* * 2

D,A D,A

2 DA D A

W _  H _

 ⁽¹⁾

H is the interaction Hamiltonian that mediates energy transfer from the excited donor D^* to the ground-state acceptor A, and ^D,A is the spectral overlap integral (eqn 2) where gD(E) and gA(E) are the normalized line shape functions for the homogeneous lines of the donor (emission spectrum) and acceptor (absorption spectrum), respectively.¹⁹

   

D,A gA E gD E dE

 



D,A ensures energy conservation and intimately depends on the location of the excited states of the

two partners implied in the energy transfer, while the matrix element DA^* H D A^{* 2} 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 since 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 only envisioned in molecular systems where structural defaults and statistical distributions are excluded, while intermetallic contact distances are fixed.²⁰ In this context, the use of spin-crossover Fe(II) working as an adjustable acceptor for Ln = Eu(II), Tb(III) is rather obvious since its absorption spectrum strongly varies with its spin-state (Figure 1).²¹ For many pseudo-octahedral Fe(II)N6

chromophores, which are famous for inducing SCO behaviour,²² 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-allowed d-d transitions (¹A1¹T1 and ¹A1¹T2 in Oh 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 Figure 1), while a single weak (10 ≤  ≤ 30 M^-1cm^-1) spin-allowed transition (⁵T2⁵E in Oh symmetry) occurs on the low-energy side (around 900 nm).^15,21a

Figure 1. Room temperature electronic absorption spectra recorded for Fe(II)N⁶ chromophores in [LaFe(L5)3]⁵⁺ (low spin: purple trace) and in [LaFe(L4)3]⁵⁺ (high-spin: orange trace). The emission spectrum recorded for Eu(III) in [EuZn(L5)3]⁵⁺ in acetonitrile corresponds to the red trace. Adapted from reference 15.

The latter change in the Fe(II)N6 absorption spectra has 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 topics which combines SCO and luminescence in materials science.²¹ However, only little interest has been focused on related Fe(II) spin-state modulations using high-energy d-block²³ or f-block²⁴ donors operating within isolated heterometallic FeM or FeLn entities .^24a,b It should be emphasized here that these molecular systems suffer from intrinsic limitations such as (i) total lack of cooperativity for the SCO transition operating in isolated 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 detrimental for short-term technological applications,^21b,25 but fundamental mechanistic rationalization and pertinent programming 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.

Scheme 1. Chemical structures of ligands L1-L6 highlighting the positions of the methyl groups bound to the terminal pyridine rings.

0 1 2 3 4 5 6

13.5 15.5 17.5 19.5 21.5 23.5

Absorption / M-1cm-1

Emission/arbitrary units

Eu(⁵D₀⁷F_J)

J=0 J=1 J=2

J=3 J=4

Low-spin Fe

High-spin Fe MLCT

MLCT

500 600 550 650

700 450

Wavelength /nm

Wavenumber / 10³ cm^-1

The three didentate benzimidazole-2-yl-pyridine units L1-L3 have been designed for this purpose, since they give stable pseudo-octahedral mononuclear [Fe(Lk)3]²⁺ complexes, the spin-state behavior of which (low-spin, high-spin, spin-crossover) can be controlled by (i) the location of the methyl group on the pyridine ring (Scheme 1) and (ii) the facial/meridional organization of the chelate binding units around the central cation.²⁶ The connection of the facial FeN6 chromophores to luminescent pseudo-tricapped trigonal prismatic EuN6O3 reporters in the triple-stranded helicates [EuFe(Lk)3]⁵⁺ (Lk = L4-L6, Scheme 1) is reported and discussed in this contribution together with what we believe to be the first comprehensive photophysical characterization of molecular EuFe pairs, where Fe(II) display low-spin, high-spin or spin-crossover properties.

Results and Discussions

Synthesis and characterization of the dinuclear triple-stranded helicates [LnM(Lk)3]⁵⁺ (Ln = La(III), Eu(III); M = Fe(II), Zn(II); Lk = L4-L6). The segmental ligands L4¹⁵ and L5²⁷ were synthesized according to published procedures, which were extended for the preparation of the novel ligand L6 (Scheme 2).

L1 L2 L3

6 5

3

6 5

3

L4 L5 L6

Scheme 2. Multistep synthesis of the segmental ligand L6.

Commercially-available 3-methylpicolinonitrile 1 was hydrolyzed 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 3²⁷ gave 4 in good yield (86%). The final steps involved a 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 CH2Cl2 (3.0 eq.) with equimolar acetonitrile solutions of Ln(CF3SO3)3 (1.0 eq) and M(CF3SO3)2 (1.0 eq.) quantitatively afforded the desired [LnM(Lk)3](CF3SO3)5 (Ln = La(III), Eu(III); M = Fe(II), Zn(II); Lk = L4-L6) helicates in solution as ascertained by their diagnostic diamagnetic (LaZn) and paramagnetic (EuZn, LaFe, EuFe) C3-symmetrical ¹H NMR spectra (Figures S1-S12 in the Supporting information).²⁶ Diffusion of diethylether into concentrated acetonitrile solutions provided fair yields (68-90%) of microcrystalline powders with various amounts of co-

crystallized solvent molecules (Table S1 in the Supporting Information). In line with the crystal structures previously reported at 180 K for high-spin [LnFe(L4)³](CF³SO³)⁵ (Ln = La(III), Eu(III), Lu(III), Figure 2, left),²⁶ for low-spin [LnFe(L5)³](CF³SO³)⁵ (Ln = La(III), Eu(III), Figure 2 center)^24b,26 and for [EuZn(L5)3](CF3SO3)5 triple-stranded helicates,²⁷ monocrystals suitable for X- ray diffractions studies could be also obtained for the complexes [EuZn(L6)3](CF3SO3)5ꞏ4MeCNꞏ1.75H2O and [EuFe(L6)3](CF3SO3)5ꞏ4MeCNꞏ1.75H2O by slow diffusion of ^tBuOMe into acetonitrile solutions (Figure 2 right, Tables S2-S3 and Figures S13-S14 in the Supporting Information). For the sake of completeness, ligands L5 was previously used for preparing hexa-cationic triple-stranded helicates [LnCr(L5)3]⁶⁺ (Ln = Nd, Eu, Yb) and a penta- cationic helicate [EuRu(L5)3]⁵⁺, the crystal structures of which are similar (but not isostructural) with those of the LnFe and LnZn analogues discussed in this contribution.²⁸

Figure 2. Molecular structures of the triple-helical cations [EuFe(Lk)3]⁵⁺ 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  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.

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 SHAPE²⁹ scores, the coordination sphere around the d metal cation is systematically close to the perfect octahedron and the intermolecular intermetallic distance d^M(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 comments. Upon connection to the 5- position in [LnFe(L5)³]⁵⁺, the methyl groups bring no special intra- or inter-strand interactions. The Fe-N bonds in the FeN⁶ chromophore can be thus easily shrunk upon high-spin to low-spin transition, thereby leading to around 80% of low-spin conformation at 180 K with Fe-N bond lengths smaller than 2.0 Å (Figure 2 center and Table S4).^24b,26 Connection of the methyl groups at the 3-position in [LnFe(L6)³]⁵⁺, 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  = 37.4(7) move away the two coordinating nitrogen atoms,³⁰ which prevents Fe-N contraction and stabilizes the high-spin (≥ 99%) configuration at 180 K characterized by long dFe-N = 2.12 Å bond distances. Finally, upon attaching methyl groups at the six- position in [LnFe(L4)3]⁵⁺, the intrastrand constraints found in [LnFe(L6)3]⁵⁺ are replaced with marked interstrand ones in [LnFe(L4)3]⁵⁺, and the FeN6 chromophores cannot be compressed to produce short Fe-N bond length compatible with low-spin configurations. The [LnFe(L4)³]⁵⁺ complexes thus display incompressible d^Fe-N = 2.13(2)-2.34(7) Å bond distances and strict high-spin configuration at any accessible temperatures (Figure 2 left and Table S4).^15,26

Monitoring the magnetic susceptibilities with the help of a SQUID magnetometer reveals, as previously reported for the perchlorate analogue [LaFe(L4)³](ClO⁴)⁵,¹⁵ that the magnetic Fe(II) center in [LaFe(L4)3](CF3SO3)5 adopts exclusively a high-spin configuration (S = 5/2) over the whole accessible temperature range (red trace in Figure 3). The Curie constant Chs = 3.97(1) cm³ꞏKꞏmol^-1

and paramagnetic independent paramagnetism TIPhs  0 cm³mol^-1 were fitted using eqn (3) within the 150-400 K range and fell within the range for Fe^II high-spin complexes (Table S5).³¹

MT Chs T TIPhs

    (3)

Figure 3. Molar magnetic susceptibilities recorded for the dinuclear complexes [LaFe(Lk)³](CF³SO³)⁵ (Lk = L4-L6) in the solid state.

At lower temperature, the slight decrease of MT can be satisfyingly fitted using axial zero-field splitting (E/D = 0, eqn 4)³² and the van Vleck equation to give g = 2.31(2) and D = 1.29(1) cm^-1 for [LaFe(L4)3](CF3SO3)5 close to g = 2.31(1) and D = 0.77(1) cm^-1 found for the mononuclear model complex [Fe(L1)3](CF3SO3)2 (Table S5).

   

0 2 2 2

ˆ ˆ ˆ

z x y

1

n 3

E D S S S  E S S

     

  ⁽⁴⁾

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 properties are restored²⁶ and the MT versus T plot for [LaFe(L5)3](CF3SO3)5 displays a gradual spin transition between 100 K and room temperature (black trace in Figure 3). Above 300 K, the spin transition becomes more marked and, at 400 K, the high-spin molar fraction computed with the help of eqn (5) and using Chs = 3.6(1) cm³ꞏKꞏmol^-1, Cls = 0 and TIP  0 amounts to about xhs ≈ 0.85. Below 200 K, a small fraction (xhs ≈ 0.2) of the complex remains high-spin, as previously reported for [LaFe(L5)3](ClO4)5.²⁶

4.0 3.2 2.4 1.6 0.8 0.0

400 300 200 100 0

T /K

MT/cm3Kmol-1

[LaFe(L4)₃](CF₃SO₃)₅

[LaFe(L5)₃](CF₃SO₃)₅ [LaFe(L6)₃](CF₃SO₃)₅

     

MT xhs Chs T TIPhs 1 xhs Cls T TIPls

          ⁽⁵⁾

Finally, the shift of the methyl group to the 3-position of the pyridine ring in [LaFe(L6)3](CF3SO3)5

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 moiety. Upon decreasing the temperature in going from 400 to 250 K, the MT versus T plot corresponds to pure high-spin behavior (blue trace in Figure 3) and can be successfully fitted to C^hs

= 3.58(1) cm³ꞏKꞏmol^-1 and TIPhs = 175(2)ꞏ10^-6 cm³ꞏmol^-1 with eqn (3), a trend similar to that previously reported for the mononuclear [Fe(L3)3](CF3SO3)2 analogue (Table S5).²⁶ Below 50 K, the abrupt decrease of ^MT observed for [LaFe(L6)³](CF³SO³)⁵ can be attributed mainly to the axial zero- field splitting, but the experimental smooth variation between 250-50 K cannot be modeled with the only resort to zero-field splitting. The latter decrease of the MT versus T plot is therefore attributed to a spin-crossover phenomena taking place at very low temperature. Since 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)3](CF3SO3)5 was probed by Mössbauer spectroscopy (Figure 4 and Table 1).

Figure 4. Mössbauer spectra (black circles) and fitted data (red trace) for [LaFe(L6)3](CF3SO3)5 at 80 K. High-spin sites I & II: orange traces; low spin site: purple trace.

1.718x10⁶ 1.714 1.710 1.706 1.702 1.698

4 3 2 1 0 -1 -2

v/mms^-1 ls

hs-I hs-II

Intensity/cps

However, the rather sophisticated multistep synthesis of ligand L6 followed by self-assembly processes limits the quantity of available crystallized helicates to a few tenth of milligrams for performing all the analysis and characterization. For Mössbauer spectroscopy, the faint quantity of iron (2% of a few tenth of milligrams) is challenging and several weeks of  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 80 K, the Mössbauer spectrum of [LaFe(L6)³](CF³SO³)⁵ 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 x^hs = 64(1)% and 14.2(8)% (Figure 4 and Table 1). This is likely due to the presence of two random crystallographic sites since the complex was isolated as a microcrystalline powder. The Mössbauer spectrum confirms unambiguously the coexistence of two spin sates for [LaFe(L6)³](CF³SO³)⁵ at low temperature in complete agreement with magnetic data and the operation of a spin crossover processes in this complex.

Table 1. Mössbauer Parameters for [LaFe(L6)3](CF3SO3)5 in the Solid State at 80 K.

T /K δ ^a / mms^-1

ΔEQb

/mms^-1

Γ ^c /mms^-1

x

/% Attribution

80

1.115(2) 1.854(4) 0.236(4) 64(1) high-spin, site I 1.048(5) 2.97(1) 0.173(9) 14.2(8) high-spin, site II 0.191(5) 0.512(9) 0.205(6) 21.7(5) low spin

a Isomer shift. ^b Quadrupole splitting. ^c Line width.

Eu(III) as a luminescent probe for monitoring Fe(II) spin-state in dinuclear triple-stranded helicates [EuFe(Lk)3]⁵⁺ (Lk = L4-L6): a theoretical approach. 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 (kLH and kHL are the first-order rate constants for low- spinhigh-spin and high-spinlow-spin transitions, respectively) with neighbouring metal-centered

luminescent detection, exemplified here by a trivalent europium considered in its ground state Eu and its excited state Eu^*. The excitation process (continuous or pulsed) is modeled by the rate constant

exc

kEu given in eqn (6), where P is the pump wavelength, P is the incident pump intensity, h is the Planck constant, c is the speed of light in vacuum and Eu is the absorption cross section (dashed traces in Scheme 3).³³ Please note that, in molecular [EuFe(Lk)3]⁵⁺ 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.

exc p

Eu Eu

k P

hc

 

 ⁽⁶⁾

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 matrix. Dashed upward arrows = excitation, full downward arrows = relaxation and straight diagonal arrows = low-spin  high-spin transformations.

The decay rate constants of the Eu(III) excited states, i.e. the Eu(⁵D0) spectroscopic level, can be partitioned between radiative and non-radiative contributions k_Eu^relax k_Eu^radk_Eu^non-rad. k_Eu^rad is temperature-independent and obeys Einstein’s relationship (eqn 7) for spontaneous emission,³⁴ where

Fe_LS-Eu 0

1 2

Energy

exc

kEu

kLH

Fe_HS-Eu kHL

relax

kEu q

kLS

Fe_LS-Eu^*

3

exc

kEu

kLH

Fe_HS-Eu^* kHL

relax

kEu q

kHS

a)

b)

 

 

exc relax q

Eu LH HL Eu LS

exc relax q

LH Eu HL Eu HS

relax q

exc Eu LS

Eu HL

LH

relax q

exc Eu HS

Eu LH

HL

0 0

0

0

k k k k k

k k k k k

k k

k k

k

k k

k k

k

   

 

    

 

    

   

   

   

  

  

 

M

AJ’,J (in s^-1) stands for Einstein’s probability of spontaneous J’J emission, h is Planck’s constant,  is the energy gap (in frequency unit) between the two incriminated J and J’ states, c is the speed of light and gJ and gJ’ are the degeneracies of states J and J’, respectively.

3 rad

Eu ', 3 , '

'

8 _J

J J J J

J

g

k A h B

c g

    (7)

The last crucial term B^J,J’ is Einstein’s coefficient giving the probability per unit time and per unit spectral energy density of the radiation field that an electron in state J absorbs a photon and jumps to state J’. B^J,J’ is proportional to the square of the transition dipole moment , where ^J and

J’ are the wavefunctions of the J and J’ states and is the electromagnetic-induced perturbation

Hamiltonian. k_Eu^non-rad is an Arrhenius-type temperature-dependent non-radiative phonon-assisted deactivation (eqn 8), in which k_Eu^non-rad,0 stands for the non-radiative relaxation at T and E_Eu^non-rad is the activation energy of the phonon-assisted relaxation pathway, whereby low-temperature tunelling, being much smaller than the radiative decay, is neglected.^18a,35 Finally, k_LS^q and k_HS^q are the first-order quenching rate constants produced by intramolecular Eu(III)Fe(II) energy transfers toward low- spin, respectively high-spin Fe(II).

non-rad

non-rad non-rad,0 Eu

Eu Eu exp E

k k

RT

 

   

  (8)

The time-dependent evolution of the normalized population densities N_tⁱ are given by the differential matrix rate eqn (9).

0 0

1 1

2 2

3 3

t t

t t

t t

t t

dN dt N

dN dt N

dN dt N

dN dt N

   

   

   

   

   

   

   

M (9)

Under pulsed excitation, the mathematical resolution of eqn (9) predicts a double exponential decay expressed in eqn (10) for the total Eu(III)-centered emission IEu(t) accompanying the depopulation of

2

ˆ '

J Hp J

 

ˆp

H

the normalized excited population densities N_t² and N_t³ (see Appendix 2 in the Supporting Information for a detailed mathematical treatment).

 





 ^{ }

^{ }



Eu Eu Eu 1 1 2 2

k t k t

t t

I t k N N k C D e^  C D e^ (10)

The characteristic decay rate constants k1,2 are calculated with the help of eqn (11), where

relax q

A LH Eu LS

k k k k and k_B k_HLk_Eu^relaxk_HS^q , while C1, C2, D1 and D1 coefficients can be deduced from the initial conditions (see Appendix 2).

 

A

1,2 A HL LH

1 4

2 2

B

B

k k

k    k k  k k

  ⁽¹¹⁾

Under continuous-wave excitation, the steady-state population densities N_S-Sⁱ are obtained by numerically solving eqn (9) for dN_tⁱ dt0 (Appendix 3 in the supporting information). The associated intrinsic quantum yield _Eu^Eu, which is correlated to the global quantum yield _Eu^L via the sensitization process ^sens,^1a,b can be easily deduced as soon as the ratio of the steady-state population of the excited emissive levels N_S-S³ N_S-S² is available (eqn 12, Appendix 3).

 

 

 

  ^{ }

3 2

L Eurad S-S S-S

Eu Eu

Eu relax 3 2 HS 3 2 LS

sens Eu S-S S-S q S-S S-S q

1 1

k N N

k N N k N N k

 



  

   ⁽¹²⁾

Under weak continuous excitation so that k_Eu^exc << kHL, kLH, kA, kB, the steady-state excited-state population densities N_S-S² and N_S-S³ are negligible compared with ground-state population densities, and the following approximations hold S-S¹ 0¹ LH  ^SCO0 

0 0 SCO S-S 0 HL

G RT

N N k

K e

N N k

      _and N_S-S⁰ N_S-S¹ N₀.

With this in mind, solving eqn (9) for dN_tⁱ dt0 provides analytical equations for the steady-state population densities, from which the total emitted intensity IS-S is given in eqn (13) (Appendix 3).





_ ^

_ ^

^

_ ^

rad exc rad

S-S Eu S-S S-S 0 Eu Eu

HL LH A B HL LH

k k k k k k

I k N N N k k

k k k k k k

    

       ⁽¹³⁾

[EuZn(L5)3]⁵⁺ as a model for extracting experimental radiative and non-radiative 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 concentration, where chemical dissociation is negligible.²⁷ In the latter helicate, Fe(II) is replaced by 3d¹⁰ closed-shell Zn(II) which exhibits neither spin-crossover (k^LH = kHL = 0) nor energy feeding from Eu(III) (k_LS^q k_HS^q 0). The observed decay of Eu(III)-centered luminescence thus corresponds to k_Eu^relax summarized in eqn (14) for its temperature dependence (combination of eqns 7-8).

non-rad

relax rad non-rad rad non-rad,0 Eu

Eu Eu Eu Eu Eu exp E

k k k k k

RT

 

      

  (14)

Upon pulsed 355 nm ligand-centered excitation using the third harmonic of a Nd-YAG laser, an acetonitrile solution of [EuZn(L5)3]⁵⁺ exhibits standard Eu-based ⁵D0⁷FJ (J = 0-6) luminescence (Figure S16) characterized by millisecond range mono-exponential decays of the population of the Eu(⁵D0) excited level (Figure S17, Table S6). The characteristic lifetime _obs 1 k_Eu^relax = 2.43(1) ms measured at 293 K is coherent with the previously reported value of τ^obs = 2.89(2) ms at 295 K for a 3.7ꞏ10^-3 M acetonitrile solution of the analogous [EuZn(L4)³](ClO⁴)⁵ helicate^.15As predicted by eqn (14), the temperature dependence of the relaxation rate constant k_Eu^relax 1_obs monitored in [EuZn(L5)3]⁵⁺ obeys an Arrhenius-type law with k_Eu^rad = 339(2) s^-1, k_Eu^non-rad,0 = 8(2)10⁸ s^-1and E_Eu^non-rad

= 39.4(6) kJmol^-1 (Figure 5a) leading to almost quantitative intrinsic quantum yields at low temperature (eqn 15 and Table S7).

 

L rad rad

Eu Eu Eu Eu

Eu rad non-rad rad non-rad,0 non-rad

sens Eu Eu Eu Eu exp Eu

k k

k k k k E RT

 

  

    (15)

The room temperature intrinsic quantum yield _Eu^Eu = 0.82 can be compared with the global quantum yield _Eu^L = 4.2ꞏ10^-2 previously recorded upon ligand-based excitation for this complex.³⁷ The associated sensitization process thus amounts to _sens  _Eu^L _Eu^Eu = 5ꞏ10^-2 for [EuZn(L5)3]⁵⁺ in acetonitrile at room temperature.

Figure 5. Temperature dependence of a) k_Eu^relax 1_obs in [EuZn(L5)3]⁵⁺ (acetonitrile, 1 mM) and b)

 

q relax

HS 1 obs Eu

k   k in [EuFe(L4)³]⁵⁺ (acetonitrile, 15 mM). The dashed traces were computed with eqn (14) and (16) using the kinetic parameters reported in the Figure.

[EuFe(L4)3]⁵⁺ as a model for extracting experimental EuFeHS energy transfer rate constants in solution. Upon 355 nm ligand-centered excitation, [EuFe(L4)3]⁵⁺ exhibits weak, but standard Eu- based ⁵D0⁷FJ (J = 0-6) luminescence (Figure S18) characterized by bi-exponential decays of the Eu(⁵D⁰) excited level (Figures S19). Due to the weaker affinity of L4 with respect to L5 for the central metallic cations in the associated dinuclear triple-helicates,¹⁵ a concentrated 15 mM acetonitrile solution of [EuFe(L4)³]⁵⁺ was used for the photophysical measurements in order to minimize the amount of free Eu(III) in solution. Despite this precaution, bi-exponential rate laws were systematically required for fitting the experimental decays. The minor long-lived contributions (pre- exponential factors = 0-10%, τobs ≈ 333 µs at 300 K) probably originate from traces of solvated Eu(III) released in solution, while the major short-lived component (pre-exponential factors = 90-100%, τobs

= 46(1) µs at 300 K) can be assigned to Eu(III) centers bound in [EuFe(L4)3]⁵⁺ (Figure S20). The latter short lifetime is consistent with τobs = 59(1) µs previously reported for 3.7ꞏ10^-3 M solution of [EuFe(L4)3](ClO4)5 at 295 K.¹⁵ Only this fast component was further considered as pertinent to

3 4 5 6 7 8 9

233 253 273 293 313 333

0 1 2 3 4

233 253 273 293 313 333 T/K

a)

T/K b)

kEu/s-1relax kHS/s-1q

rad 1

Eu 339(2)s

k  ^

non-rad,0 8 1

Eu 8(2) 10 s

k   ^

non-rad

Eu 39.4(6) kJ/mol

E 

q,0 6 1

HS 8(1) 10 s

k   ^

q

HS 14.7(4) kJ/mol

E 

x 10000x 100

Eu(III) in [EuFe(L4)³]⁵⁺. Since Fe(II) adopts a pure high-spin 3d⁶ electronic configuration at all temperatures in [EuFe(L4)³]⁵⁺ (k^HL = 0),^15,26 the decay of the Eu(⁵D⁰) excited level combines the one found for [EuZn(L5)3]⁵⁺ (k_Eu^relaxin eqn 14) with a supplementary thermally-activated intramolecular Eu(III)Fe(II)HS energy transfer modeled with k_HS^q 0 and summarized in eqn (16) (Scheme 3, right part).

q

obs relax q relax q,0 HS

Eu Eu HS Eu HS exp E

k k k k k

RT

 

      

  (16)

Taking into account the isostructural [EuN⁶O³] coordination spheres found in the crystal structures of [EuZn(L5)3]⁵⁺²⁷ and [EuFe(L4)3]⁵⁺,²⁶ we can reasonably assume that k_Eu^relax, previously obtained for [EuZn(L5)³]⁵⁺, also holds for [EuFe(L4)³]⁵⁺. With this in mind, the temperature dependence of the differences in the observed rate constants can be fitted to

 

obs obs q q,0 q

EuFe EuZn HS HS HS

1 _L41 _L5 k k exp E RT , from which k_HS^q,0 = 8(1)10⁶ s^-1and E_Eu^non-rad = 14.7(4) kJmol^-1 are deduced using a linear least-square fit (Figure 5b). At room temperature, we calculate that k_HS^q = 18855 s^-1 for the intramolecular Eu(III)→Fe(II) energy transfer operating in [EuFe(L4)3]⁵⁺, which leads to an energy transfer efficiency of HS

 

ET q q relax

Eu Fe kHS kHS kEu

 _   = 97.8% (Table S7).

According to the dipole-dipole energy transfer theory,³⁶ the latter ratio is related to the distance between the energy donor and energy acceptor RDA, taken as the intramolecular Eu∙∙∙FeHS = 9.0 Å intermetallic separation (Figure 2), and the so-called critical radius R0 for 50% energy transfer.

 

ET 6

Eu Fe 1 RDA/R0)

 _  ^ ^^ (17)

Application of eqn (17) to the Eu/FeHS pair in [EuFe(L4)3]⁵⁺ leads to R0 = 17.5 Å, which corresponds to a considerable distance if molecular design is envisioned for limiting intramolecular Eu(III) quenching. Using the values of k_Eu^rad, k_Eu^non-rad and k_HS^q (Table S7), an Eu(III) intrinsic quantum yield of _Eu^Eu = 1.8ꞏ10^-2 can be calculated with eqn (18) for [EuFe(L4)³]⁵⁺ in acetonitrile at room temperature (Table S7).

   

L rad rad

Eu Eu Eu Eu

Eu rad non-rad q rad non-rad,0 non-rad q,0 q

sens Eu Eu HS Eu Eu exp Eu HS exp HS

k k

k k k k k E RT k E RT

 

  

        (18)

Compared with _Eu^Eu = 0.82 observed for [EuZn(L5)3]⁵⁺ in the absence of Eu(III)→Fe(II) energy transfer, we conclude that the large majority of the Eu-centered emission is quenched by the neighbouring high-spin Fe(II) partner in [EuFe(L4)3]⁵⁺. Since a global quantum yield _Eu^L = 3ꞏ10^-4 was previously reported for the latter complex in acetonitrile solution,³⁷ the associated sensitization process can be easily computed _sens  _Eu^L _Eu^Eu= 2ꞏ10^-2, and it fairly matches that obtained for [EuZn(L5)5]⁵⁺ (sens = 5ꞏ10^-2).

[EuFe(L5)3]⁵⁺ as a model for the luminescence monitoring of Fe(II) spin-state in dinuclear helicates in solution. Having in hand the kinetic behavior of the Eu(III) probe when it is isolated in [EuZn(L5)3]⁵⁺ (k_Eu^relax), and when it communicates with high-spin Fe(II) in [EuFe(L4)3]⁵⁺ (k_HS^q ), the last step of the analysis focuses on the combination of the Eu(III) probe with a spin-crossover Fe(II) center in [EuFe(L5)3]⁵⁺. Taking into account that the thermodynamic parameters of the spin-state equilibrium (19) have been determined for the analogous [LaFe(L5)3]⁵ helicate in acetonitrile (HSCO

= 29.6(2) kJmol^-1 and S^SCO = 86.2(5) JK^-1mol^-1),²⁶ eqn (19) predicts that these dinuclear helicates are essentially low spin at 233 K (KSCO = 710^-3 = xHS/(1-xHS)  xHS = 0.6%) while at room temperature xHS ≈ 16%. The maximum high-spin fraction that can be reached in acetonitrile at 333 K is xHS = 40% (Figure 6).

LH HL

5 5

LS 3 HS 3

[EuFe ( ) ] ^k [EuFe ( ) ]

k

 

L5 L5 _SCO ^{LH SCO SCO}

HL

exp S H

K k

k R RT

 

 

     (19)

According to Scheme 3, the Eu(III)-centered emission in the spin-crossover helicate [EuFe(L5)3]⁵⁺

may arise from both FeLS-Eu* and FeHS-Eu* excited states, which are interconverted via kLH and kHL

kinetic rates constants. Taking a reasonable Arrhenius-type relationship for the high-spin to low-spin relaxation ^{kHL kHL0 exp}





3.310⁵ s^-1 is obtained with eqn (19). The quenching constant k_LS^q , which corresponds to the rate of intramolecular Eu(III)→Fe(II)LS energy transfer, thus remains the only unknown parameters in Scheme 3.

Figure 6. Spin-crossover properties of [LaFe(L5)3]⁵⁺ in acetonitrile computed using eqn (19) and

H^SCO = 29.6(2) kJmol^-1 and S^SCO = 86.2(5) JK^-1mol^-1.²⁶

Table 2. Characteristic Decay Lifetimes _i 1 k_i and Initial Relative Intensities Ai = Ci + Di (i = 1, 2; eqn 10), Global Quantum Yields _Eu^L (eqn 12), Energy Transfer Efficiency _Eu^ET_Fe and Critical Radius for 50% Energy Transfer R0 Computed for [EuFe(L5)3]⁵⁺ as a Function of Eu(III)→Fe(II)LS

Energy Transfer Rate Constants k_LS^q in Acetonitrile Solution at Room Temperature.

q

kLS /s^-1 10⁶ 10⁷ 10⁸

1 1k1

  /ns 432 96 9

A1 /% 5.7 73 83

2 1 k2

  /ns 1320 597 577

A2 /% 94 23 16

3 2

S-S S-S

N N ^a 0.28 1.11 9.38

L

Eu ^{b 8.6∙10-6 1.4∙10-6 7.0∙10-7}

HS

ET Eu Fe

 _{ /%} ^{97.9 97.9 97.9}

R⁰(EuFe^HS) /Å 17.5 17.5 17.5

LS

ET Eu Fe

 _ /% >99.9 >99.9 >99.9 R0(EuFeLS) /Å 33.5 49.3 72.3

a Ratio of the steady-state excited population densities obtained for incident pump intensities P ≤ 1 Wcm^-2 (see Scheme 3). ^b Computed using sens = 2ꞏ10^-2 observed for [EuFe(L4)5]⁵⁺.

T/K 50

40 30 20 10 0

333 313 293 273 253 233 xHS/%

Assuming k_LS^q k_HS^q = 18655 s^-1 (Table S7) because of the much larger spectral overlap integral D,A

expected for the Eu-FeLS pair (see eqn 1 and Figure 1), the bi-exponential decay curves following laser-pulsed excitation (eqn 10, Figure 7) and the global quantum yields obtained under continuous- wave xenon lamp (eqn 12; P ≤ 1 Wcm^-2) have been simulated (Table 2).

Figure 7. Eu(⁵D0) decay traces predicted with eqn (10) and associated quantum yields (eqn 12) computed for [EuFe(L5)⁵]⁵⁺ for various Eu(III)→Fe(II)^LS energy transfer rate constant a) k_LS^q = 10⁶ s^-1, b) k_LS^q = 10⁷ s^-1 and c) k_LS^q = 10⁸ s^-1. k_Eu^rad, k_Eu^non-rad are those found for [EuZn(L5)5]⁵⁺ and k_HS^q = 18655 s^-1 corresponds to that measured for [EuFe(L4)5]⁵⁺ and kHL = 1.710⁶ s^-1 and kLH = KSCOkHL = 3.310⁵ s^-1 (see text and Table S7).

0.0 0.2 0.4 0.6 0.8 1.0

0 100 200 300 400 500

Normalized Intensity

t/ns 0.0

0.2 0.4 0.6 0.8 1.0

0 100 200 300 400 500

Normalized Intensity

t/ns

0.0 0.2 0.4 0.6 0.8 1.0

0 100 200 300 400 500

Normalized Intensity

t/ns

I_tot

I_short(₁= 432 ns) I_long(₂= 1320 ns)

I_tot

I_short(₁= 96ns) I_long(₂= 597 ns)

I_tot

I_short(₁= 9.0 ns) I_long(₂= 577 ns) a) k_LS^{q =106s-1,}Eu^L = 8.6∙10^-6

b) k_LS^{q =107s-1,}Eu^L = 1.4∙10^-6

c) k_LS^{q =108s-1,}Eu^L = 7.0∙10^-7