Bistability of Fc-PTM-Based dyads: The role of the donor strength

Article abstract of DOI:10.1021/cm400147p

Bistability of valence tautomeric donor-acceptor dyads formed by covalently linking a ferrocene-based electron-donor and the perchlorotriphenylmethyl radical, as the electron-acceptor, is tuned via a chemical modification of the ferrocene group. Specifically, the methylation of the ferrocene unit increases the strength of the donor group stabilizing the zwitterionic state in polar solvents and leading to an intriguing coexistence of neutral and zwitterionic species in solvents of intermediate polarity. Bistability in the crystalline phase is demonstrated by temperature dependent Moessbauer spectra. This complex and interesting behavior is quantitatively rationalized in the framework of a bottom-up modeling strategy. Optical spectra in solution are first analyzed to extract and parametrize an effective two-state molecular model, which is then adopted to rationalize the observed bistability in the solid state as due to cooperative electrostatic interchromophore interactions.

Full text of DOI:10.1021/cm400147p

Article
pubs.acs.org/cm
Bistability of Fc-PTM-Based Dyads: The Role of the Donor Strength
Judith Guasch,† Luca Grisanti,‡,⊥ Stefan Jung,§ Dayana Morales,† Gabriele D’Avino,‡,# Manuel Souto,†
Xavier Fontrodona,∥ Anna Painelli,‡,* Franz Renz,§ Imma Ratera,† and Jaume Veciana†,*
†Institut de Cien
Spain
̀ ̀
cia de Materials de Barcelona (ICMAB-CSIC)/CIBER-BBN, Campus de Bellaterra, E-08193 Cerdanyola del Valles,
‡Dipartimento Chimica, GIAF, Parma University/INSTM-UdR, I-4310, Parma, Italy
§Institute of Inorganic Chemistry, Leibniz University Hannover, Callinstr. 9, D-30167 Hannover, Germany
∥Serveis Científico-Tec
̀
nics, Campus de Montilivi, University of Girona, E-17071 Girona, Spain
S
* Supporting Information
ABSTRACT: Bistability of valence tautomeric donor−acceptor dyads formed by covalently linking a ferrocene-based electron-
donor and the perchlorotriphenylmethyl radical, as the electron-acceptor, is tuned via a chemical modification of the ferrocene
group. Specifically, the methylation of the ferrocene unit increases the strength of the donor group stabilizing the zwitterionic
state in polar solvents and leading to an intriguing coexistence of neutral and zwitterionic species in solvents of intermediate
polarity. Bistability in the crystalline phase is demonstrated by temperature dependent Mossbauer spectra. This complex and
̈
interesting behavior is quantitatively rationalized in the framework of a bottom-up modeling strategy. Optical spectra in solution
are first analyzed to extract and parametrize an effective two-state molecular model, which is then adopted to rationalize the
observed bistability in the solid state as due to cooperative electrostatic interchromophore interactions.
KEYWORDS:
Bistability, valence tautomers, intramolecular electron-transfer, optical spectroscopy, solvatochromism, Mossbauer spectroscopy.
̈
synthesized by Veciana and co-workers.15 In this system, a
ferrocene (Fc) moiety, acting as electron-donor (D), is linked
through a conjugated vinylene bridge to the perchlorotriphe-
nylmethyl (PTM) radical, a strong electron-acceptor (A).15−18
Absorption spectra of Fc-PTM solutions18 show a band in the
near-infrared region related to the intramolecular electron
transfer (IET) between the ferrocene donor (D) and the PTM
acceptor (A). The band shows a strong positive solvatochrom-
ism, shifting from 892 nm in n-hexane to 1003 nm in DMSO,
unambiguously pointing to a neutral (DA) ground state of Fc-
PTM in solution. Accordingly, an intense absorption at 387
nm,17−20 assigned to the neutral PTM radical, is observed in all
solvents, while the absorption feature at 490 nm, ascribed to the
PTM anion,17,18,21,22 is not present. The IET absorption of Fc-
INTRODUCTION
■
Bistability, the property of a material to be found in two
different stable states in the same external conditions,1
represents the extreme manifestation of the nonlinear response
of the material to an external stimulus and is only supported in
systems with strong cooperative interactions. Bistability is
therefore a rare phenomenon in molecular materials.2 However,
the possibility to switch a bistable molecular material between
states having strikingly different properties by the application of
(possibly tiny) external stimuli (like, e.g., small variations of
temperature or pressure, applied electric or magnetic field, or
even illumination) opens the way to the development of
molecular-based switches and memories:3−14 bistable molecular
materials are in demand to build very fundamental molecular
electronic devices.
Valence-tautomeric molecules are obvious building blocks for
bistable molecular materials. A few years ago, a new and
promising valence tautomeric compound, Fc-PTM, was
Received: January 14, 2013
Revised: February 10, 2013
Published: February 12, 2013
© 2013 American Chemical Society
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PTM locates the zwitterionic (D+A−) state, reached upon
photoexcitation in solution, at an energy more than 1.3 eV
higher than the neutral ground state,18,23 a fairly high energy
that sharply contrasts with the observed coexistence in a wide
temperature range of the neutral and the zwitterionic forms of
Fc-PTM in the crystalline phase, as demonstrated by
Scheme 1. Synthesis of Me8Fc-PTM
temperature dependent Mossbauer spectra.17
̈
This complex phenomenology was understood in a
comparatively simple and coherent picture based on a
bottom-up modeling strategy.23 Specifically, solution spectra
of Fc-PTM were analyzed based on a two-state model explicitly
accounting for the coupling between electronic and vibrational
degrees of freedom as well as for polar solvation. The resulting
molecular model was then used to describe interacting
molecules in the crystalline phase. Within this picture, the
coexistence of the neutral and zwitterionic phases in the solid-
state is quantitatively ascribed to bistability induced in the
system by cooperative electrostatic intermolecular interactions.
Fc-PTM then offered the first, and so far unique, experimental
realization of bistability in crystals of neutral DA molecules, as
theoretically predicted ten years ago.24,25 Few other bistable
molecular systems based on ferrocenyl-conjugated pyrylium
salts have been described. In these systems, electrostatic
interactions also involve couterions that play a major role, in
contrast to the neutral Fc-PTM.26
Herein, we report the synthesis of a new valence tautomeric
compound, Me8Fc-PTM (Figure 1), with the same skeleton as
Figure 2. Absorption spectra of Me8Fc-PTM in different solvents.
indicating that Me8Fc-PTM is only present in the zwitterionic form
(D+A−). Concomitantly, a very intense band shows up at 18 870 cm−1
(530 nm), as expected for a PTM subunit in its anionic form.17,18,21,22
We conclude that the strong electron-donor character of the
methylated ferrocene unit lowers the energy of the zwitterionic state
of Me8Fc-PTM, so that, at variance with Fc-PTM that stays in the
neutral (DA) form in all solvents,23,30 the ground state of Me8Fc-PTM
changes from neutral in low-polarity solvents to zwitterionic in
strongly polar solvents.
In acetone, an intermediate polarity solvent, the Me8Fc-PTM
spectrum shows both the band due to the neutral PTM radical at 25
840 cm−1 and that due to the PTM anion at 18 870 cm−1, pointing to
the coexistence of the neutral and zwitterionic species in this solvent.
Therefore, it is possible to tune the solvent polarity as to drive the
equilibrium between the neutral and the zwitterionic states of Me8Fc-
PTM, eventually forcing the system into a coexistence regime.
The weak band observed around 15 400 cm−1 (650 nm) is assigned
to a π-bridge-to-PTM electron transfers, as found for other π-
conjugated PTM radicals.18,23,30 Finally, the lowest energy band,
located around 7500 cm−1 (1300 nm) in low-polarity solvents, is
ascribed to the IET resonance between the neutral and the zwitterionic
forms of Me8Fc-PTM, and its solvent dependence will be analyzed
based on the theoretical model discussed in the next section.
●
Figure 1. Molecular structures relevant to the neutral DA (purple,
and zwitterionic D A (yellow, ) states of Me8Fc-PTM.
)
+
−
●
Fc-PTM, but with eight methyl groups attached to the
ferrocene subunit in order to increase its donor strength. An
extensive characterization of Me8Fc-PTM in solution and in the
crystalline phase is presented, demonstrating that, at variance
with Fc-PTM, Me8Fc-PTM in solution can be found in the
neutral (DA) and/or zwitterionic (D+A−) forms, depending on
the solvent polarity, while, much as with Fc-PTM, true
bistability is observed in the solid state.
EXPERIMENTAL SECTION
■
Synthesis. Me8Fc-PTM (Figure 1) was synthesized in three steps
with a good yield (83%) (Scheme 1).
The protonated phosphonate PTM derivative 127 and the 1-formyl-
2,2′,3,3′,4,4′5,5′-octamethylferrocene 228,29 yielded compound 3
through a Horner-Wadsworth-Emmons reaction. The deprotonation
of compound 3 with potassium hydroxide gave 4, which was stabilized
using 18-Crown-6. The oxidation of 4 with silver nitrate afforded
Me8Fc-PTM, which is very stable under atmospheric conditions in
solid state. Dark-brown crystals of Me8Fc-PTM were obtained by a
slow-evaporation from a solution of CH2Cl2:hexane (2:3).
̈
Mossbauer Spectra. The Mossbauer spectrum recorded at 295 K
̈
on Me8Fc-PTM crystalline powders is reported in Figure 3a and
relevant data are summarized in Table 1. It shows two doublets: the
first doublet, with an isomer shift IS = 0.418 mm/s relative to α-iron
and a quadrupole splitting Δ = 2.326 mm/s, is assigned to the Fe (II)
ion in the ferrocene unit.17 The second doublet, with a smaller Δ =
0.594 mm/s, is attributed to Fe (III) ion, in the ferrocenium cation.17
The relative area of the two signals amounts to 32% and 68%,
respectively, suggesting that at room temperature the neutral (DA)
and zwitterionic (D+A−) forms of Me8Fc-PTM coexist in the crystals,
with a predominance of the zwitterionic form. The spectrum collected
UV/vis/NIR Spectra. Optical absorption spectra of Me8Fc-PTM, in
Figure 2, show a pronounced solvent dependence in the whole spectral
range. In low polarity solvents, from n-hexane to anisole, an intense
and sharp absorption appears at 25 840 cm−1 (387 nm), corresponding
to a well-characterized transition of the neutral PTM radical.17−20 In
the strongly polar solvent dimethylformamide (DMF), the sharp band
ascribed to the neutral PTM radical at 25 840 cm−1 disappears,
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structure observed at 78 K is safely assigned to the presence of slightly
different structures of compound 3. On this basis, we rule out the
possibility that the doublet ascribed to the Fe (III) ion in Mossbauer
̈
spectra of Me8Fc-PTM is due to thermal motion of the iron ion inside
the ferrocene moiety, and confirm the coexistence of the neutral and
zwitterionic forms in the crystal.
X-ray Diffraction Analysis. Single crystal X-ray diffraction data
were collected from small dark-brown crystals of Me8Fc-PTM at 300
and 100 K (details are reported in the Supporting Information, SI).
The ferrocene and PTM subunits do not show any significant
structural change upon lowering temperature (SI Table S2 and S3).
Indeed, it is known that the geometry of the PTM group is marginally
affected when going from the radical to the anionic form.31 Moreover,
the detailed study of Fc-based materials in ref 26, similarly suggests
marginal structural variation of the Fc group with its charge. The only
relevant structural variation with temperature is seen in the vinylene-
bridge region, where the difference between the single (C−C) and
double (CC) bond lengths amounts to 0.250 Å at 300 K and
decreases to 0.095 Å at 100 K, suggesting a weaker conjugation in the
zwitterionic than in the neutral forms of Me8Fc-PTM.
RESULTS AND DISCUSSION
■
Molecular Model and Solvatochromism. The two-state
model for open and closed shell donor−acceptor dyads has
been presented in previous publications23,30,32−34 and is shortly
summarized here. The low-energy electronic structure of the
dyads, governed by the charge resonance between the neutral
(DA) and the zwitterionic (D+A−) states, is captured in its
essence by the two-state Mulliken Hamiltonian:
−
−
−
hel(z, τ) = 2z|D+A ⟩⟨D+A | − τ(|DA⟩⟨D+A |
−
+ |D+A ⟩⟨DA|)
(1)
where 2z is the energy required to turn a neutral DA pair into
the zwitterionic state and τ is the hybridization energy mixing
the two states. The diagonalization of the above Hamiltonian
leads to a mixed-valence ground state, D+ρA−ρ, characterized by
a molecular ionicity, ρ, ranging from 0 (fully neutral dyad) to 1
(fully zwitterionic dyad).
This minimal model is empirical and must be validated
against the evolution of absorption spectra with solvent
polarity. To such an aim, the model must be extended to
account for the coupling between electrons and molecular
vibrations and for the effects of polar solvation. The adiabatic
Hamiltonian, including the linear coupling to a vibrational
coordinate, q, and to an effective solvation coordinate
proportional to the orientational component of the solvent
reaction field, For, reads:23,30,32−34
Figure 3. (a) Mossbauer spectrum of a crystalline sample of Me Fc-
̈
8
PTM at 295 K; (b) the same as (a) at 85 K; (c) Mossbauer spectrum
̈
of a crystalline sample of compound 3 at 295 K; (d) the same as (c) at
78 K. Blue curves in (a), (b), and (c) are fitted spectra calculated
summing the contributions from Fe(III) (red curves) and Fe(II)
(green curves), according to the data in Table 1 (for (c) the Fe(III)
contribution vanishes). In (d) the total fit is obtained summing over
two Fe(II) contributions (green curves).
Table 1. Parameters of the Mossbauer Spectra of Crystalline
̈
Samples of Me8Fc-PTM and Its Protonated Precursor 3 at
Room and Low Temperatures
site
population
(%)
T
IS
a
b
compound (K) assignment (mm/s)
Δ (mm/s)
μ0
4εor
Me8Fc-
PTM
295
85
Fe (II)
Fe (III)
Fe (II)
Fe (III)
Fe (II)
Fe (II)
Fe (II)
0.418(35)
0.127(37)
0.517(43)
0.162(35)
0.286(21)
0.416(72)
0.412(14)
2.325(71)
0.593(68)
2.281(86)
0.493(60)
2.396(43)
2.252(24)
2.957(41)
32
68
1
2
hsol(zsol, τ) = hel(zsol, τ) + ω q2 +
Fo2r
v
(2)
88
where 2zsol = 2z − ωv(2εv)1/2q − μ0For is the q- and For-
dependent ionization energy of the DA dyad, ωv is the
frequency of the vibrational mode, εv is the vibrational
relaxation energy, μ0 is the dipole moment of the D+A− state
(following Mulliken all other matrix elements of the dipole
moment operator are neglected), and εor is the orientational
component of the solvent relaxation energy.34−36
The solvent reaction field For is treated as a classical
coordinate, hence calculated absorption spectra are the
Boltzmann-weighted average of spectra obtained for different
values of For.23,34,35 It is worth reminding that molecular
parameters, as extracted from the analysis of solution
absorption spectra, implicitly account for the effect of fast
electronic polarization of a generic organic nonpolar medium,
12
3
295
100
73
78
b
27
a
Isomer shift. Quadrupole splitting.
at 85 K (Figure 3b) shows the same features, but with a predominance
of the neutral species, amounting to 88% of the total.
Panels c and d of Figure 3 show Mossbauer spectra of 3, the
protonated precursor of Me8Fc-PTM, at 295 and 78 K, respectively. In
this compound the iron ion has exactly the same environment as in
Me8Fc-PTM, but IET is prevented due to the protonation of the PTM
̈
unit. Only the Fe (II) doublet is observed in Mossbauer spectra of 3 at
all temperatures (Figure 3c,d), with no hint of Fe (III). The complex
̈
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and can therefore be employed to model mutually interacting
molecules in bichromophoric species,37−40 in films33,41 as well
as in the solid state.23,33
Figure 4 compares experimental and calculated absorption
spectra of Me8Fc-PTM in the IET region. Calculated spectra
Figure 5. Black continuous curves: ground state energy calculated for
Me8FcPTM as a function of the reaction field, For, for (a) εor = 0.18 eV
(corresponding to CHCl3), (b) 0.35 eV (CH2Cl2), (c) 0.8 eV
(acetone), and (d) 1.2 eV (DMF). Violet and orange lines show the
energies of DA and D+A− state, respectively. Black dashed line shows
the room temperature Boltzmann probability distribution calculated
based on the ground state energy.
Figure 4. Experimental (a) and calculated (b) IET band of Me8Fc-
PTM in different solvents. Calculated spectra were obtained with
molecular parameters in Table 1 and εor values listed in the panel b.
Table 2. Molecular Parameters of Me8Fc-PTM (eV)
surface of the neutral (DA) state (violet line). In low-polarity
solvents then, the system is in a largely neutral state.
z
τ
εv
ωv
0.42
0.30
0.12
0.18
In strongly polar solvents (Figure 5d) the zwitterionic state is
greatly stabilized by the interaction with the polar solvent and
the ground state potential energy surface (black line) shows a
deep minimum corresponding to the zwitterionic state (the
energy of the D+A− state is shown as the orange line). At room
temperature, only the zwitterionic state is populated (black
dashed line showing the relevant distribution). In the
intermediate regime (Figure 5c), the potential energy surfaces
corresponding to the two neutral and zwitterionic states (violet
and orange lines, respectively) have similar energies and, since
the mixing matrix element is not too large, the ground state
potential energy surface (black line) shows two minima
corresponding to largely neutral and zwitterionic states. As
shown by the Boltzmann distribution (black dashed line), both
states are populated at ambient temperature, in agreement with
experimental results for acetone solutions.
Before closing this section, we shortly compare the
spectroscopic behavior of Me8Fc-PTM and its nonmethylated
and fully methylated analogues, Fc-PTM and Me9Fc-PTM,
respectively. As discussed before, the nonmethylated Fc group
is too poor a donor to stabilize the zwitterionic state in
solution: Fc-PTM stays neutral in all solvents.23,30 Increasing
the donor strength by methylation favors the zwitterionic state
and the fully methylated Me9Fc-PTM analogue behaves in a
similar way as Me8Fc-PTM, switching to a zwitterionic state in
strongly polar solvents, as demonstrated by solution spectra in
SI Figure S4.
are based on molecular parameters listed in Table 2 and on εor
values shown in Figure 4b. In low polarity solvents (n-hexane,
CHCl3, CH2Cl2, and anisole) Me8Fc-PTM is in a largely
neutral ground state, with calculated ρ values slightly increasing
from 0.097, in n-hexane, to 0.120, in anisole. As expected for a
largely neutral chromophore,34,36 a progressive red shift of the
IET band is calculated with increasing solvent polarity, in
excellent agreement with experimental spectra.
In DMF, the most polar solvent considered here, Me8Fc-
PTM is zwitterionic, as inferred from the absence of the sharp
absorption of neutral PTM radical at 25840 cm−1 (see Figure
2). Concomitantly, the IET absorption band ascribed to the
neutral DA specie disappears. Our model describes a largely
ionic ground state (ρ = 0.972) in strongly polar solvents (εor =
1.2 eV). In these conditions, a weak and broad IET absorption
is calculated around 14 500 cm−1 (cf Figure 4b), at higher
frequency than in less polar solvents. This feature is consistent
with the weak and broad band observed around 14 000 cm−1 in
spectra collected in DMF solutions (Figure 4a).
The evolution from a neutral to a zwitterionic ground state
with increasing solvent polarity and the coexistence of the two
states in intermediate polarity solvents is well captured by the
model. Specifically, in Figure 5, the black lines in each panel
show the ground state energy as a function of the solvent
reaction field, For, calculated for Me8Fc-PTM with the model
parameters in Table 2 and four different values of εor as to
mimic increasing solvent polarity. In low polarity solvents
(panels a and b in Figure 5, corresponding to CHCl3 and
CH2Cl2, respectively) the energy of the neutral basis state
(violet line) is much lower than the energy of the zwitterionic
basis state (orange line), so that the ground state potential
energy surface (black line) is similar to the potential energy
This behavior is rationalized in our model by a smooth
decrease of 2z, the energy difference between the neutral and
zwitterionic basis states, with increasing the number of methyl
substituents. Specifically, z ranges from 0.61 eV for Fc-PTM23
to 0.42 eV for Me8Fc-PTM, and 0.36 eV for Me9Fc-PTM,30 in
line with the increasing donating character of the ferrocene unit
with successive methylation. The other model parameters (cf
Table 2) are exactly the same for the two methylated
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compounds, in line with the strong similarity between the two
compounds.
electrostatic interactions, as measured by M, falls in the region
−0.87 eV < M < 0.67 eV.
Modeling Bistability in the Solid State. To describe the
bistability of Me8FcPTM in the solid state we adopt the same
bottom-up modeling strategy that was successfully applied to
the nonmethylated system, Fc-PTM.2,23,30 Specifically, the
molecular model parameters in Table 2, extracted from the
analysis of solution spectra of Me8FcPTM, are implemented in
a model for Me8FcPTM crystals, where electrostatic
intermolecular interactions are introduced in the mean-field
approximation. The mean-field Hamiltonian for a crystal of DA
molecules reads:2,23,25,30
To confirm our model for bistability in Me8FcPTM crystals
we calculate M, according to the following expression:23
qiqj
|rim − rjn|
1
2
M =
∑ ∑
m≠n i,j
(4)
where m and n count on the molecules in the crystal, i, j run on
the atoms within each molecule, rmi denotes the position of
atom i in molecule m, and qi is the charge on the i-th atom
relevant to the molecule in the zwitterionic D+A− state. To
estimate these charges, we perform quantum chemical
calculations using the PM6 Hamiltonian42 (MOPAC 2007
package)43 on an isolated (gas phase) Me8FcPTM subject to an
external electric field aligned along the DA axis (connecting the
central C atom of PTM to the Fe atom).23 Increasing the
applied field drives the molecule from a largely neutral ground
state (no field) to a zwitterionic state (large applied field). This
is confirmed by data in Figure 6b (black, scale on the left side)
that shows the component of the calculated dipole moment
parallel to the applied field, μz = Σizi qi, as a function of the
field. Closed and open symbols refer to results obtained for the
molecular geometry extracted from the analysis of X-ray data at
room and low temperature, respectively, and show similar
behavior. Specifically, at low fields a smooth and almost linear
μz(F) dependence is observed, safely ascribed to the molecule
in a largely neutral (ρ ≈ 0) state. With increasing field an
abrupt jump in the calculated dipole is observed at F ≈ 0.3 V/
Å, toward another region of smooth and almost linear behavior
corresponding to larger values of the molecular dipole moment.
This second region (F = 0.4 ÷ 0.80 V) is therefore ascribed to a
largely zwitterionic (ρ ≈ 1) state. Quite interestingly, the
threshold field ∼0.3 V/Å estimated for Me8FcPTM is lower
than the value 0.45 V/Å previously obtained for the
nonmethylated analogue, Fc-PTM,23 in line with the stronger
donor character of the methylated ferrocene group.
1
2
Hcry(zcry , τ) = hel(zcry , τ) + ωvq2 − Mρ2
(3)
where hel(zcry,τ) is the two state electronic Hamiltonian defined
in eq 1, but accounting for the crystalline value of the molecular
parameter z. Specifically, 2zcry is the effective energy required to
switch a single molecule from the neutral (DA) to the
zwitterionic (D+A−) state when the molecule is embedded into
a crystal of molecules in the relevant ground state. This energy
then self-consistently depends on the degree of charge transfer
of the molecules in the crystal, ρ: zcry(ρ) = 2z − 2εvρ + 2Mρ. In
these equations, the information about the crystal is fully
conveyed by M, the electrostatic (Madelung) energy of a lattice
of molecules in the zwitterionic D+A− state.2,23,25,30 For
attractive intermolecular interactions, M is negative and the
energy required to create a zwitterionic molecule is reduced in
the crystal.
The Hamiltonian in eq 3 is diagonalized for different values
of zcry to extract in a self-consistent approach the dependence of
the molecular ionicity ρ on M.2,23,30 Figure 6a shows the
We are now in the position to estimate M, the Madelung
energy of a crystal of zwitterionic molecules, as entering the
self-consistent Hamiltonian in eq 3. The red squares in the right
panel of Figure 6 show the Madelung energy calculated
inserting in eq 4 the atomic charges computed for different
values of the applied electric field (sums were performed on
crystal supercells large enough to obtain converged values
within 0.01 eV). Once again, closed and open circles refer to
results obtained at room and low temperature, respectively. In
all cases, Madelung energies estimated in the neutral regime are
negligible with respect to the values obtained in the F-range
relevant to the zwitterionic regime. The values of the Madelung
energy estimated in the F-region relevant to zwitterionic states
(the region roughly comprised between the two vertical dashed
lines in Figure 6b) then define our best estimate for M = −0.7
÷ −1.15 eV, as extracted from the room temperature structure
and M = −0.61 ÷ −0.98 eV from the low-temperature
structure. While the reduced M values estimated at low
temperature are in line with the increased volume of the unit
cell (see SI Table S1) as well as with the reduced content of
zwitterionic species (see Table 1), the large uncertainties in the
estimated values make this difference irrelevant. What is more
important is that in both cases, the estimated interval of M
values largely overlaps with the bistability window, −0.87 eV <
M < 0.65 eV, predicted for Me8FcPTM (see Figure 6a).
Figure 6. (a) Molecular ionicity as a function of M calculated based on
the Hamiltonian in eq 3 and for molecular parameters in Table 2.
Continuous and dashed lines refer to stable and unstable solutions,
respectively; (b) black points (left scale) show the component of the
dipole moment along the DA direction calculated for Me8FcPTM as a
function of the applied electric field; red squares (right scale) show the
Madelung energy calculated according to eq 4. Closed and open
symbols refer to results relevant to the room temperature and low-
temperature structure, respectively. Dipole moment and Madelung
energy are calculated with the Coulson net atomic charges obtained at
different values of F. The green dashed lines delimitate the region
where the molecule is in a largely polar state (ρ ≈ 1).
resulting ρ(M) curve, calculated adopting molecular parameters
in Table 2. The curve describes a crystal of largely neutral
molecules (ρ < 0.14) for relatively weak electrostatic
interactions (|M| < 0.67) and a crystal of zwitterions (ρ >
0.93) for strong interactions (|M| > 0.87). In the intermediate
region, the Hamiltonian in eq 3 has two stable solutions
(continuous line), corresponding to a crystal of largely neutral
or largely ionic molecules. Our model for the Me8FcPTM
crystal then supports bistability, provided the strength of
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We then conclude that the proposed model quantitatively
supports the hypothesis of bistability induced by electrostatic
interactions in crystals of Me8FcPTM, leading to a picture fully
consistent with results previously obtained for the parent Fc-
PTM system. Both molecules fulfill the requirements for
bistability as dictated by the phase diagram in ref 2, namely a
strict balance between the molecular ionization energy and the
electrostatic interaction strength (z ≈ −2M), combined with a
low hybridization energy (z > τ). Nevertheless, the similarity
between the two dyads is only formal. In fact, the valence
tautomerism of the two compounds is characterized by very
different values of ionization energy (z = 0.61 eV for Fc-PTM
against 0.42 eV for Me8Fc-PTM) in virtue of the stronger
donor character of the substituted ferrocene group. This
difference shows itself in the qualitatively different behavior of
the two molecules in solution, with Fc-PTM remaining neutral
even in strongly polar solvents,23,30 while Me8Fc-PTM turns
zwitterionic in polar solvents, showing coexistence in
intermediate polarity solvents. In the crystalline phase, the
relatively high ionization energy of Fc-PTM is counterbalanced
by strong electrostatic interactions (M was estimated in the
−1.5 to −1 eV range), while in Me8Fc-PTM the estimated M
values are considerably lower (between −1.15 and −0.75 eV,
most probably due to the spacing effect of the bulky methyl
groups), leading also in this case to a perfect matching of the
stringent conditions for bistability.
fact a fast equilibrium between the two forms is unavoidable in
solution. True bistable behavior implies long-lived (non-
interconverting) states that can only be achieved via strong
cooperative interactions.2 Indeed, according to an original
theoretical suggestion,24,25 we have recently proved that
electrostatic intermolecular interactions are a strong enough
source of cooperativity to induce bistability in Fc-PTM
crystals,23 even if, in this specific system coexistence is not
observed in solution.
Bistability is a rare phenomenon that requires a weak
conjugation between the D and A group (small τ in the two-
state model), as well as a very precise balance between the
energy required to transfer a charge from the D to the A site
(2z in the two-state model) and the electrostatic interchromo-
phore interaction, measured in the proposed model for the
crystal in terms of the Madelung energy relevant to a crystal of
fully zwitterionic chromophores, M. Here we demonstrate that
this very delicate balance, achieved for the parent Fc-PTM
compound,23 is also met for the methylated analogue, Me8Fc-
PTM: the methyl groups not only lower the energy required for
the electron transfer, but also, due to their bulky nature, reduce
intermolecular interactions, as to maintain the stringent
conditions for bistability. Unfortunately, we were not able to
collect Mossbauer spectra of the fully methylated compound
̈
(Me9Fc-PTM): this system shows in solution a similar behavior
as the octamethylated compound (see Supporting Informa-
tion), but, having a stronger donor group and hence a lower z
value, is expected to show a narrower bistability region,2 making
the requirement for the M vs z balance even more stringent.
The PTM-vinylene-bridge moiety represents a very promis-
ing building-block for valence-tautomeric molecules: a careful
choice of the strength of the donor species to be connected to
the bridge can lead to molecules whose ground state can be
switched from neutral to zwitterionic changing the polarity of
the surrounding medium, with the two species coexisting in
media of intermediate polarity. Even more interestingly, the
same systems can be designed to provide true bistability in the
solid state, provided a careful matching of on-site and intersite
interactions is achieved.
CONCLUSIONS
■
A new donor−acceptor dyad Me8Fc-PTM is synthesized and
carefully characterized in solution and in the solid state, to
provide a sound basis for a theoretical description of the
observed phenomena of coexistence and bistability. Me8Fc-
PTM has a very strong electron donating group (the
octamethylated ferrocene unit) linked through a vinylene-
bridge to a strong electron-acceptor (the perclorotriphenyl-
methyl radical). The energetic proximity of the neutral (DA)
and zwitterionic (D+A−) structures in this system is well
demonstrated by the possibility to tune the molecular charge
distribution from a largely neutral state, Dρ+Aρ− with ρ ≈ 0, in
nonpolar or mildly polar solvents, to a largely zwitterionic state,
Dρ+Aρ− with ρ ≈ 1, in strongly polar solvents. States with
intermediate charge-transfer (Dρ+Aρ−, with ρ ≈ 0.5) are not
accessible to the system because of the poor conjugation
between the D and A groups: as a result, in intermediate
polarity solvents (acetone) coexistence of largely neutral and
largely zwitterionic species is experimentally observed. This
intriguing behavior is qualitatively different from what observed
for the parent nonmethylated compound, Fc-PTM that, due to
the poorer donating character of the nonmethylated ferrocene
unit, stays in a neutral state even in the strongly polar DMSO
solvent.23,30 These features are quite naturally understood
based on an essential two-state model parametrized to
reproduce the IET band observed in the NIR region of optical
absorption spectra. Quite interestingly, coexistence of neutral
and zwitterionic species in solution has been recently observed
in similar PTM-based dyads.44 In these systems, a vinylene
bridge connects the PTM acceptor to tetrathiafulvalene (TTF),
a strong electron-donor: apparently the vinylene-bridge offers a
poor conjugation path toward the PTM group, as required to
observe coexistence of neutral and zwitterionic species in
systems with strong donors.
ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed synthesis and characterization of compounds 3, 4, and
Me8Fc-PTM. Cyclic voltammetry, single crystal structure, and
UV/vis/NIR of Me9Fc-PTM. Crystallographic information files
(CIF) for Me8Fc-PTM at 300 and 100 K. This material is
available free of charge via the Internet at http://pubs.acs.org
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: anna.painelli@unipr.it (A.P.); vecianaj@icmab.es
(J.V.).
Present Addresses
⊥Laboratory for Chemistry of Novel Materials, University of
Mons, Place du Parc 20, BE-7000 Mons, Belgium.
#Dipartimento di Chimica Industriale “Toso Montanari” and
INSTM, Universita
40136 Bologna, Italy.
̀
di Bologna, Viale del Risorgimento 4, IT-
Author Contributions
While suggestive, coexistence of neutral and zwitterionic
species in solution is not a signature of genuine bistability: in
The manuscript was written through contributions of all
authors.
813
dx.doi.org/10.1021/cm400147p | Chem. Mater. 2013, 25, 808−814

Chemistry of Materials
Article
(23) D’Avino, G.; Grisanti, L.; Guasch, J.; Ratera, I.; Veciana, J.;
Painelli, A. J. Am. Chem. Soc. 2008, 130, 12064.
Funding
This work was supported by the DGI grant (POMAs;
CTQ2010−19501), the Networking Research Center on
Bioengineering, Biomaterials, and Nanomedicine (CIBER-
BBN), and the Generalitat de Catalunya (grant
2009SGR00516).
(24) Painelli, A.; Terenziani, F. J. Am. Chem. Soc. 2003, 125, 5624.
(25) Terenziani, F.; Painelli, A. Phys. Rev. B 2003, 68, 165405.
(26) Kondo, M.; Uchikawa, M.; Namiki, K.; Zhang, W.-W.; Kume, S.;
Nishibori, E.; Suwa, H.; Aoyagi, S.; Sakata, M.; Murata, M.; Kobayashi,
Y.; Nishihara, H. J. Am. Chem. Soc. 2009, 131, 12112. Kondo, M.;
Uchikawa, M.; Zhang, W.-W.; Namiki, K.; Kume, S.; Murata, M.;
Kobayashi, Y.; Nishihara, H. Angew. Chem. Int. Ed. 2007, 46, 6271.
(27) Rovira, C.; Ruiz-Molina, D.; Elsner, O.; Vidal-Gancedo, J.;
Bonvoisin, J.; Launay, J.-P.; Veciana, J. Chem. Eur. J. 2001, 7, 240.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.G. is grateful to the Consejo Superior de Investigaciones
Científicas (CSIC) for a “JAE” fellowship.
(30) Grisanti, L.; D’Avino, G.; Painelli, A.; Guasch, J.; Ratera, I.;
Veciana, J. J. Phys. Chem. B 2009, 113, 4718.
(31) Miravitlles, C.; Molins, E.; Solans, X.; Germain, G.; Declercq, J.
ABBREVIATIONS
■
P. J. Incl. Phenom. 1985, 3, 27. Veciana, J.; Riera, J.; Castaner, J.; Ferrer,
̃
IET, intramolecular electron transfer; D, electron-donor; A,
electron-acceptor; PTM, perchlorotriphenylmethyl radical; Fc,
ferrocene; DMF, N,N-dimethylformamide; DMSO, dimethyl
sulfoxide
N. J. Organomet. Chem. 1985, 297, 131. Molins, E.; Mas, M.;
Maniukiewicz, W.; Ballester, M.; Castaner, J. Acta Cryst. C 1996, 52,
̃
2412. Guasch, J.; Fontrodona, X.; Ratera, I.; Rovira, C.; Veciana, J. Acta
Cryst. C 2013, DOI: 10.1107/S0108270113002072.
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Terenziani, F.; Painelli, A.; Comoretto, D. J. Phys. Chem. A 2000, 104,
11049.
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