Intervalence charge transfer in mixed valence compound modified by the formation of a supramolecular complex.

Article abstract of DOI:10.1021/jp207518r

The electrochemical and spectroelectrochemical properties of N,N-diphenyl-1,4-phenylenediamine (PDA) were investigated in the absence and in the presence of 18-crown-6-ether (18C6) or dibenzo 24-crown-8-ether (DB24C8), in a solution of tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile and in the presence of trifluoroacetic acid (TFA) only for 18C6. In neutral acetonitrile, PDA undergoes two reversible oxidation processes, which lead first to the formation of the cation-radical considered as mixed valence (MV) compound, and then to the dicationic species. When 18C6 is added in the medium and depending on 18C6 concentration, cyclic voltammetry shows a marked shift to more cathodic potentials of the current waves of the second redox process only. This is attributed to a strong interaction between the PDA⁺² dication and two 18C6 molecules, leading to the formation of a supramolecular complex with an association constant value K_a = 7.0 × 10⁷ M^-2. The interaction of 18C6 with PDA⁺² dication has a direct effect on the PDA^+. cation-radical corresponding to a decrease in the lifetime of the MV compound and of the intramolecular electron transfer rate when 18C6 is present. Indeed, it results in a large decrease in the intervalence charge transfer (IV-CT) between the two amine centers in the MV compound (k_th = 1.35 × 10¹⁰ s^-1 in 18C6-free neutral solution containing 5.0 × 10^-4 M PDA, and k_th = 3.6 × 10⁹ s^-1 in the same medium at [18C6]/[PDA] = 20/1). And the comproportionation constant K_co falls from 6.0 × 10⁶ in 18C6-free solution to 1.6 × 10 ³ at [18C6]/[PDA] = 20/1. In acidified acetonitrile and when TFA concentration is increased, PDA still shows the two successive and reversible oxidation processes, but both are shifted to more anodic potentials. However, when 18C6 is added, the two oxidation waves shift to more cathodic potentials, indicating an interaction of all protonated PDA redox states with 18C6, resulting in the formation of supramolecular complexes. In the presence of TFA, the value of K_co is decreased to 4.3 × 10⁴, but it remains unchanged when 18C6 is added, indicating no change in the lifetime of the MV compound. In this medium, IV-CT in the MV compound is greater with 18C6 (k_th = 2.3 × 10¹⁰ s^-1 for [18C6]/[PDA] = 20/1) than without (k_th = 1.4 × 10⁹ s^-1), which indicates a more important IV-CT rate when 18C6 is present. The results show for the first time that is it possible to control the IV-CT rate, through the lifetime and the potential range where the MV compound is the most important. This control is not obtained as usual by chemical modification of the structure of the starting molecule, but by varying either the acidity or the 18C6 concentration as external stimuli, which lead to reversible formation/dissociation of a supramolecular complex species. Moreover, we also studied the electrochemical properties of PDA in the presence of wider crown ether such as DB24C8. We showed that PDA undergoes the same electrochemical behavior with DB24C8 than with 18C6 in neutral organic medium (K_a = 2.9 × 10³ M^-1). This result suggests that the complexation between the electrogenerated PDA⁺² dication and the crown ethers may occur through face-to-face mode rather than rotaxane mode even with DB24C8 which is supposed to form inclusion complexes.

Full text of DOI:10.1021/jp207518r

ARTICLE
pubs.acs.org/JPCA
Intervalence Charge Transfer in Mixed Valence Compound Modified
by the Formation of a Supramolecular Complex.
Mourad Mechouet,† Christian Perruchot,† Franc-ois Maurel,† Salah Aeiyach,† Christophe Bucher,‡
Sylvie Chardon,‡ and Mohamed Jouini*,†
†Universitꢀe Paris Diderot, Sorbonne Paris Citꢀe, Laboratoire Interfaces, Traitements, Organisation et Dynamique des Systꢁemes,
UMR 7086, 15 rue Jean Antoine de Baif, F-75205 Paris Cedex 13, France
‡Universitꢀe Joseph Fourier Grenoble I, Dꢀepartement de Chimie Molꢀeculaire, UMR CNRS-UJF 5250, BP 53, F-38041 Grenoble Cedex 9, France
ABSTRACT: The electrochemical and spectroelectrochemical properties of
N,N-diphenyl-1,4-phenylenediamine (PDA) were investigated in the absence
and in the presence of 18-crown-6-ether (18C6) or dibenzo 24-crown-8-ether
(DB24C8), in a solution of tetrabutylammonium hexafluorophosphate (TBAPF6)
in acetonitrile and in the presence of trifluoroacetic acid (TFA) only for 18C6. In
neutral acetonitrile, PDA undergoes two reversible oxidation processes, which lead
first to the formation of the cation-radical considered as mixed valence (MV)
compound, and then to the dicationic species. When 18C6 is added in the medium
and depending on 18C6 concentration, cyclic voltammetry shows a marked shift to
more cathodic potentials of the current waves of the second redox process only. This
is attributed to a strong interaction between the PDA+2 dication and two 18C6
molecules, leading to the formation of a supramolecular complex with an association
constant value Ka = 7.0 ꢀ 107 Mꢁ2. The interaction of 18C6 with PDA+2 dication
has a direct effect on the PDA+. cation-radical corresponding to a decrease in the lifetime of the MV compound and of the intramolecular
electron transfer rate when 18C6 is present. Indeed, it results in a large decrease in the intervalence charge transfer (IV-CT) between the
two amine centers in the MV compound (kth = 1.35 ꢀ 1010 sꢁ1 in 18C6-free neutral solution containing 5.0 ꢀ 10ꢁ4 M PDA, and kth =
3.6 ꢀ 109 sꢁ1 in the same medium at [18C6]/[PDA] = 20/1). And the comproportionation constant Kco falls from 6.0 ꢀ 106 in
18C6-free solution to 1.6 ꢀ 103 at [18C6]/[PDA] = 20/1. In acidified acetonitrile and when TFA concentration is increased,
PDA still shows the two successive and reversible oxidation processes, but both are shifted to more anodic potentials. However,
when 18C6 is added, the two oxidation waves shift to more cathodic potentials, indicating an interaction of all protonated PDA
redox states with 18C6, resulting in the formation of supramolecular complexes. In the presence of TFA, the value of Kco is
decreased to 4.3 ꢀ 104, but it remains unchanged when 18C6 is added, indicating no change in the lifetime of the MV compound.
In this medium, IV-CT in the MV compound is greater with 18C6 (kth = 2.3 ꢀ 1010
s
ꢁ1 for [18C6]/[PDA] = 20/1) than without
(kth = 1.4 ꢀ 109 sꢁ1), which indicates a more important IV-CT rate when 18C6 is present. The results show for the first time that
is it possible to control the IV-CT rate, through the lifetime and the potential range where the MV compound is the most
important. This control is not obtained as usual by chemical modification of the structure of the starting molecule, but by varying
either the acidity or the 18C6 concentration as external stimuli, which lead to reversible formation/dissociation of a
supramolecular complex species. Moreover, we also studied the electrochemical properties of PDA in the presence of wider
crown ether such as DB24C8. We showed that PDA undergoes the same electrochemical behavior with DB24C8 than with 18C6
in neutral organic medium (Ka = 2.9 ꢀ 103 Mꢁ1). This result suggests that the complexation between the electrogenerated
PDA+2 dication and the crown ethers may occur through face-to-face mode rather than rotaxane mode even with DB24C8 which
is supposed to form inclusion complexes.
undergo long-range motion along a molecular “track”.15 Several
1. INTRODUCTION
groups, that is, Stoddart et al.,16 Sauvage et al.,17 Credi et al.,18
Supramolecular systems that respond to external stimuli
and Brouwer et al.,11 have used chemical, photochemical and
(potential,1,2 light,3,4 chemical,5 solvent6) by undergoing rever-
electrochemical inputs to trigger changes in supramolecular
sible complexation or translocation are useful for integration into
architectures, and have demonstrated the possibility of reversible
applications such as molecular-based logic gates,7 information
control of threading and translocation processes in these
storage8 and processing, or as the active mechanical compo-
nents in molecular devices9 and nanoscale functional materials.10
Rotaxanes,1,4,11,12 pseudorotaxanes,13 and catenanes14 have been
advertised as some of the best examples of dynamic supramole-
cular systems because they can act as molecular ‘shuttles’ and
Received: August 5, 2011
Revised: November 11, 2011
Published: December 16, 2011
r
2011 American Chemical Society
970
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Figure 1. Oxidation and interaction equilibria of PDA in neutral acetonitrile with and without 18C6.
self-assembled structures. For such uses, various macromole-
cules, able to form complexes through weak intermolecular
bounds, are employed, such as cryptands,19,20 cyclodextrins,21
hemispherands,22 or calixarenes.23
particularly organic diamines with different substituents, to under-
stand and control the IV-CT process in the MV compounds of these
molecular models. The cation-radical of these organic diamines
is described as the MV state (+1 and 0 state);35ꢁ37 where the
comproportionation constant Kco,39 which indicates the MV stabi-
lity, is measured by different techniques, such as electrochemistry
and UVꢁvis, IR, and Raman spectroscopies.35,36 In previous work,
Lambert and N€oll40 succeeded in changing the IV-CT process by
varying π-electron spacers in triarylamines. Nishiumi et al.35,36
modulated it by introducing substituents into one or other of the
phenyl moieties of N,N-diphenyl-1,4-phenylenediamine (PDA). In
these cases, modulation was obtained by chemical modification of
the structure of the starting amine derivatives.
In this work, we demonstrated that 18-crown-6-ether (18C6)
(Figure 1) can be used to modulate the stability of the cationic
species and allows a gradual control of the IV-CT process in the
MV compound generated by electrochemical oxidation of PDA
(Figure 1). The high comproportionation constant is decreased by
the presence of 18C6; this is the consequence of the formation of a
supramolecular complex between 18C6 and the PDA+2 dication. In
acetonitrile, this complex is highly stable and is formed after cation-
radical oxidation at higher potential, whereas it dissociates on the
backscan at low potential. Varying the solution acidity can modify
the potential range where the MV compound predominates.
Consequently, both 18C6 concentration and acidity can be used
as external stimuli to easy control of this range and of the lifetime of
the MV compound.
The macrocyclic crown ether compounds, first synthesized by
Pedersen24,25 are receiving increasing attention because of their
ability to form complexes with alkaliꢁmetal cations through hydro-
gen bound interactions. Nevertheless, investigations on supramole-
cular complexes of crown ethers with organic molecules26ꢁ33 are of
particular interest because the positive charge of the organic
molecule can be modified by protonation (pH variation)32a and
also by oxidation reaction.9 As an example, the interaction between
secondary ammonium salts and crown ethers is generally acidꢁbase
controlled and may occurs through different modes of supramole-
cular interactions (face-to-face28 or rotaxane27,29,30). Generally,
when phenyl moieties are present as terminal groups in secondary
dialkylammonium salt guests they can act as stoppers to trap crown
ether on the axle.31,29f The 24-crown-8-ether derivatives are the most
used macrocycles hosts to form rotaxanes;31 and for crown ethers
with less than 24 atoms in their macrorings, the rotaxane formation
is far from being accepted. Indeed, recent results suggest that smaller
cavities are supposed to form only face-to-face complex with
secondary dialkylammonium salts.28 However, it should be noted
that Zhang et al.31 have found that secondary dialkylammonium salts
can thread through the cavity of benzo-21-crown-7-ether (B21C7)
to form more strongly [2]-pseudorotaxane- and [2]-rotaxane-type
structures than the those with dicyclohexyl-24-crown-8-ether
(DC24C8).
Moreover and for comparison purpose, the results obtained
on the electrochemical behavior of PDA with dibenzo-24-crown-
8-ether (DB24C8) (Figure 1) are reported. The nature of the
interaction between each of the crown ethers and PDA+2 dication
is discussed and highlight the face-to-face interaction rather than
the rotaxane interaction in both systems.
Our approach contrasts with the chemical modification of the
structure of the diamine compounds reported before34,36,40 and
seems a promising advance for the design of new types of
switchable molecular devices.
Besides, mixed-valence species (MV),34ꢁ37 that is, systems
that contain an element which exists in more than one oxidation
state have interesting conductivity,34j magnetic34i and spectral
properties.34k Measuring the activation barriers for intervalence
charge transfer (IV-CT) in MV compounds is of particular interest,
because electron migration is a fundamental process in the operation
of many systems, including catalysts, light-activated devices, non-
linear optical materials, etc. Therefore, numerous investigations
have been devoted to purely organic Robin/Day38 molecules, and
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2. EXPERIMENTAL SECTION
to obtain a given [18C6]/[PDA] ratio. The very small volume
effect on the total volume of the solution (10 mL) was neglected.
The UVꢁvis spectroelectrochemical experiments were carried
out using a photodiode array UVꢁvisible-NIR spectrometer
MCS 501 UVNIR (Carl Zeiss). The light sources were halogen
CLH 500 20 W and deuterium CLD 500 lamps with an optical
fiber 041.002-UV SN 012105 using a 1 mm quartz cell (Hellma)
and an automatic shutter. For this experiment, we used a Pt grid
as working electrode, Ag/AgCl as reference electrode and a
platinum wire as counter-electrode.
2.1. Chemicals. N,N-Diphenyl-1,4-phenylenediamine (PDA)
was purchased from Aldrich. It was dissolved in ethanol, stirred
and recovered as slightly gray crystals. These were filtered
and dried in vacuum. 18-crown-6-ether (Aldrich) and dibenzo
24-crown-8-ether (Aldrich) were used as received. Acetonitrile
(Aldrich, HPLC grade) and the supporting electrolyte, tetrabuty-
lammonium hexafluorophosphate (TBAPF6) (Aldrich, electroche-
mical grade), were used without further purification. Trifluoroacetic
acid (TFA) (Sigma Aldrich, 99%) was used as received.
2.2. Electrochemistry. Electrochemical analyses were carried
out in a one-compartment three-electrode cell using an EGG
273A potentiostat. A 0.07 cm2 glassy carbon disk, a Pt grid, and a
saturated calomel electrode (SCE) were used as working elec-
trode, counter-electrode and reference electrode, respectively.
The working electrode was polished carefully with diamond
paste (3 and 1 μm), rinsed in acetone with ultrasonic stirring and
then air-dried immediately prior to use.
Cyclic voltammetry (CV) was carried out in acetonitrile,
containing PDA and 0.1 M TBAPF6 with or without TFA and
crown ether, at potential scan rates in the 20ꢁ200 mV sꢁ1 range.
2.3. Spectroelectrochemistry. In situ spectroelectrochemical
measurements were carried out on 18C6 at room temperature in
the same solution used for electrochemical measurements. The
necessary amount of solid crown ether was added to the solution
3. RESULTS AND DISCUSSION
3.1. Electrochemistry in Neutral Acetonitrile. Cyclic vol-
tammograms of PDA in acetonitrile containing 0.1 M TBAPF6
show (Figure 2) two reversible one-electron oxidation processes
at scan rates between 10 and 200 mV sꢁ1. The first anodic peak
corresponds to the formation of PDA+. cation-radical and the
second to the PDA+2 dication species. The electrochemical
behavior of PDA in the absence of crown ether (Figure 2) is
similar to that reported by Nishiumi et al.36,37 Under our condi-
tions, the standard potential values are 445 mV for the first transfer
and 846 mV for the second (Table 1).41 The standard potential
splitting ΔE, corresponding to the difference between the two
standard potentials and related to the interaction intensity be-
tween redox sites, is 401 mV; and the comproportionation
equilibrium constant39 (Kco) is 6.0 ꢀ 106, indicating the high
stability of the PDA+. cation-radical. This corresponds to a large
electron delocalization between the two amine centers in the MV
compound and supports the high IV-CT rate constantkth = 1.35 ꢀ
1010 sꢁ1 calculated from spectroelectrochemical data (Tables 2
and 3), according to the approach described by Lambert et al.40,42
The cyclic voltammograms of PDA in the presence of 18C6
(Figure 3) do not show any marked change in the shape and the
standard potential of the first transfer process, suggesting no
significant interaction between 18C6 and neutral or the MV
compound under these conditions.43,44 The peak potential of the
second redox process changes significantly, falling by about
210 mV at a [18C6]/[DPA] ratio of 20/1 (Table 1), indicating
that the dication forms more easily when 18C6 is present. This is
due to a thermodynamically favorable interaction with the
dication45ꢁ49 and results from the formation of a hydrogen
Figure 2. Cyclic voltammograms of 5.0 ꢀ 10ꢁ4 M PDA in acetonitrile
containing 0.1 M TBAPF6 at scan rates from 10 to 200 mV sꢁ1 and T=25°C.
Table 1. Standard Potentials of First and Second Redox Processes of PDA for 0.1 M TBAPF6 in Acetonitrile with or without TFA
or 18C6 ([18C6]/[PDA] = 20/1)
E° (mV/SCE) without 18C6
E° (mV/SCE) for[18C6]/[PDA] = 20
neutral acetonitrile
first charge transfer E1°
second charge transfer E2°
first charge transfer E1°
second charge transfer E2°
445
846
624
898
445
634
554
828
acetonitrile containing trifluoroacetic acid
Table 2. Absorption Maxima Observed for PDA Transcient Species in Different Media with or without 18C6
without 18C6
with 18C6
cation-radical
dication
cation-radical
dication
Wavelength (wavenumber) in Neutral ACN
Wavelength (Wavenumbre) in TFA-ACN
387 nm (25840 cmꢁ1
697 nm (14347 cmꢁ1
)
)
526 nm (19011 cmꢁ1
)
)
387 nm (25840 cmꢁ1
697 nm (14347 cmꢁ1
389 nm (25707 cmꢁ1
704 nm (14205 cmꢁ1
)
)
)
)
539 nm (18553 cmꢁ1
)
)
389 nm(25707 cmꢁ1
704 nm (14205 cmꢁ1
)
507 nm (19724 cmꢁ1
517 nm (19342 cmꢁ1
)
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Table 3. Band Shape Data for the IV-CT of PDA MV Compound Obtained under Different Experimental Conditions
medium
r/Å
νmax /cmꢁ1
ν1/2/cmꢁ1
ε/Mꢁ1 cmꢁ1
V/cmꢁ1
α
ΔG*/J Mꢁ1
kth/sꢁ1
Neutral acetonitrile
5.56
5.56
5.56
5.56
14347
14347
14205
14205
6.22 ꢀ 103
6.33 ꢀ 103
5.85 ꢀ 103
7.56 ꢀ 103
6.22 ꢀ 103
4.40 ꢀ 103
3.71 ꢀ 103
5.81 ꢀ 103
2.75 ꢀ 103
2.34 ꢀ 103
2.06 ꢀ 103
2.93 ꢀ 103
1.93 ꢀ 10ꢁ1
1.63 ꢀ 10ꢁ1
1.44 ꢀ 10ꢁ1
2.05 ꢀ 10ꢁ1
1.35 ꢀ 103
1.62 ꢀ 103
1.81 ꢀ 103
1.24 ꢀ 103
1.35 ꢀ 1010
3.60 ꢀ 109
1.44 ꢀ 109
2.27 ꢀ 1010
[18C6]/[PDA] = 20 in neutral acetonitrile
TFA - acetonitrile
[18C6]/[PDA] = 20 in TFA - acetonitrile
Figure 4. Cyclic voltammograms of 10ꢁ3 M PDA in acetonitrile
containing 0.1 M TBAPF6 in solutions at [18C6]/[PDA] = 10/1 ratio,
for scan rates from 20 to 200 mV sꢁ1 and T = 25 °C.
Figure 3. Cyclic voltammograms of 5.0 ꢀ 10ꢁ4 M PDA in acetonitrile
containing 0.1 M TBAPF6 with and without 18C6, at scan rate 50 mV sꢁ1
and T = 25 °C at different [18C6]/[PDA] ratios.
bound-based supramolecular complex between 18C6 and the
PDA+2 dication species. At even high 18C6 concentration (i.e.,
[18C6]/[PDA] = 200/1, results not shown) ΔE decreases
further, but the two redox processes do not overlap totally,
showing that the MV compound persists, in a more restricted
potential range. The PDA+2 dication interaction with 18C6
dramatically affects the comproportionation constant. Indeed,
at [18C6]/[PDA] = 20/1, Kco is estimated to be 1.6 ꢀ 103,
corresponding to a decrease of about 3 orders of magnitude. This
decrease indicates a large drop in the stability of the MV compound
when 18C6 is present. The same interaction results also in a
decrease in the electron delocalization between the two amine
centers, indicated by a lower IV-CT rate constant kth =3.6ꢀ 109 sꢁ1
at [18C6]/[PDA] = 20/1 (Tables 2 and 3). Consequently, when
the 18C6 concentration is increased in this medium, (i) the lifetime
of the MV compound falls, (ii) the MV compound exists only in a
more restricted potential range, and (iii) the IV-CT rate constant
decreases.
Figure 5. Plot of ΔEp = (Ep2ox(free) ꢁ Ep2ox(compl)) vs (log [18C6]) for
5 ꢀ 10ꢁ4 M PDA in acetonitrile containing 0.1 M TBAPF6 at scan rate
50 mV sꢁ1
.
Affinity Constant Calculation. The voltammograms obtained
for 10ꢁ3 M PDA at [18C6]/[PDA] = 10/1 show no change in
the position of the potential peaks when the scan rate is increased
from 20 to 200 mV sꢁ1 (Figure 4). This indicates that under
these conditions, there is no kinetic effect on complexation
between the different PDA species and 18C6. Consequently,
when CV is used, the interaction between PDA+2 and 18C6 is faster
versus total 18C6 concentration in solution (eq 1)
p
ΔEp ¼ EpðfreeÞ ꢁ Ecomplex ¼ 0:0592 logð1 þ Ka½CEꢂtotalÞ
ð1Þ
where [CE]total is the total 18C6 concentration, Ep(free) is the
oxidation peak potential of PDA+ cation-radical in 18C6-free
solution and Ecomplex is the oxidation peak potential of PDA+
cation-radical in the presence of a given 18C6 total concentration.
Ka is the association constant of equilibrium 3 in Figure 1 and p
is the stoichiometry of 18C6 in the complex:
than a sweep rate as high as 200 mV s .
ꢁ1 41,50 In order to evaluate the
affinity of 18C6 toward PDA and its oxidation products, different
18C6 concentrations were used to estimate the association con-
stants (Ka). For this purpose, we can assume a reversible EꢁC
mechanism for an electrochemical reaction (E) corresponding to
the oxidation of the MV compound followed by a chemical reaction
(C) corresponding to the interaction of the dication with 18C6
(Figure 1). For this type of mechanism, the association constant
(Ka) can be evaluated from the plot of the formal potential51 ΔEp
½ðPDAþþ : p ꢀ CEÞꢂeq
Ka ¼
p
½PDAþþꢂeq½CEꢂeq
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Figure 7. Cyclic voltammograms of 10ꢁ3 M PDA in acetonitrile
containing 0.1 M TBAPF6 with increasing TFA concentration, at scan
rate 100 mV sꢁ1 and T = 25 °C.
Figure 6. Cyclic voltammograms of 5.0 ꢀ 10ꢁ4 M PDA in acetonitrile
containing 0.1 M TBAPF6 with and without DB24C8, at scan rate 50 mV
s
ꢁ1 and T = 25 °C at different [DB24C8]/[PDA] ratios.
Usually, we operate with a large excess of 18C6, and in this case
the term “Ka ꢀ [CE]ptotal” is larger than unity; thus eq 1 can be
simplified as follows:
face-to-face mode; the charge delocalization makes it difficult for
crown ether to reach the partially charged amine groups to form
the complex. On the contrary, the two localized positive charges
in the PDA+2 dication make its affinity rather high to overcome
this barrier. This suggests that the localization of the charge on
the nitrogen atom of a secondary amine seems to be a necessary
driving force to lead to the interaction with crown ether.
This thermodynamically favorable complexation results prob-
ably from face-to-face interaction (Figure 1) and not through the
inclusion of PDA+2 dication in the cavity of the crown ether even
in the case of DB24C8, the cavity of which is sufficiently large to
generally allow rotaxane formation.30b
ΔEp ¼ EpðfreeÞ ꢁ Ecomplex ¼ p ꢀ 59:2 logðKa½CEꢂtotalÞmV
ð2Þ
In this case a 10-fold increase in [18C6] leads to a p ꢀ 59.2 mV
increase in ΔEp at 298 K. Consequently, a plot of ΔEp versus
log[CE]total should lead to a linear correlation, the slope of which
gives the stoichiometry, “p”; while the association constant (Ka)
can be calculated from the intercept.
In our case, the plot of the formal potential (ΔEp) versus
log[CE]total (Figure 5) exhibits typically the behavior described
above, and the slope is about 110 mV, which corresponds to p =
2, and means that one dication interacts with two 18C6
molecules as shown in Figure 1; and the corresponding associa-
tion constant (Ka) was calculated to be 7.0 ꢀ 107 Mꢁ2. This value
indicates clearly the formation of very stable hydrogen bound-
based supramolecular complex between the PDA+2 dication and
two 18C6 molecules, where each of the two amine groups of the
dication is surrounded by one 18C6 molecule (Figure 1).
The cyclic voltammograms of PDA in the presence of
DB24C8 (Figure 6) show the same behavior than the one
observed in the presence of 18C6 with the negative potential
shift of only the second current wave when DB24C8 is present.
For the same [crown ether]/[PDA] ratio, the potential shift is a
little higher with 18C6 than with DB24C8, which indicates a
more important interaction of PDA+2 dication with 18C6. The
value of the affinity constant of DB24C8 with PDA+2 dication
was equal to 2.9 ꢀ 103 for a 1:1 stoichiometry.
3.2. Electrochemistry in Acetonitrile Containing TFA.
When trifluoroacetic acid is added to 10ꢁ3 M PDA in acetonitrile
containing 0.1 M TBAPF6, the cyclic voltammograms of PDA
still show the same two reversible oxidation processes (Figure 7)
but both redox waves are progressively shifted to more anodic
potentials as the TFA concentration increases, leveling off at
about 1 M. At this ratio, the standard potentials are 624 and 898
mV for the first and second redox processes (Table 1), which
correspond to shifts of about 180 and 60 mV, respectively,
compared to their values in neutral acetonitrile solution. This
is not unexpected: the result suggests that the two amine groups
are protonated but that the oxidation of the MV compound is less
affected than that of neutral PDA. In this medium, the potential
splitting ΔE is 274 mV, and the corresponding comproportiona-
tion constant Kco is 4.3 ꢀ 104, which indicates that the MV
compound is less stable than in neutral acetonitrile. The IV-CT
rate constant (Tables 2 and 3) decreases to kth = 1.4 ꢀ 109 sꢁ1
,
indicating less electron delocalization between the two amine sites.
Moreover, in TFA-acetonitrile solution and in the presence of
18C6, the potentials of both redox processes are similarly shifted
cathodically (Figure 8) to 554 mV and 828 mV for the first and
second, respectively, at [18C6]/[PDA] = 20/1 (Table 1). This
indicates that 18C6 interacts with both the initial PDA and with
its oxidation products (Figure 9). The thermodynamically favor-
able interaction between 18C6 and each of the charged PDA
products is possible due to their initially protonated state.
Consequently, supramolecular complexes are formed with each
of these products in TFA-acetonitrile solution. Since the stan-
dard potential splitting ΔE does not vary with 18C6 concentra-
tion, the comproportionation constant Kco of the MV compound
remains the same whatever the [18C6]/[PDA] ratio. This
indicates clearly that the stability of the MV compound in
The fact that there is no interaction between the monocharged
MV compound and each of the crown ethers is surprising since
generally they interact with monocharged cations.26ꢁ33 The
interaction is apparently prevented even between the MV
compound and DB24C8 which has a large cavity, adapted to
thread secondary amines containing aromatic moeties such
as phenyl rings.29e,30,31 This means that an insuperable barrier
hinders the complex formation with the MV compound; where-
as, it does not exist in the interaction between PDA+2 dication
and the same crown ethers. The main difference is that a unique
positive charge is delocalized on the MV compound as proposed
above; whereas, in the PDA+2 dication the two positive charges
are located on each nitrogen atom of the two secondary amines.
As a consequence, if the interaction of the mono charged
MV compound with the crown ether occurs through a lateral
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TFA-acetonitrile is almost the same with or without 18C6, and
that 18C6 shifts the MV potential range without significantly
changing its lifetime. On the contrary, this behavior results in an
(Figure 11), observed on the forward and backward scans and
attributed to the cation-radical. The band at 697 nm is assigned to
the IV-CT excitation associated with a photoexcited electron
transfer from a neutral amine center to a cation-radical amine
center in the MV compound.36,37 When the potential is increased
from 700 to 1200 mV, both peaks disappear and a new one
appears at 526 nm, corresponding to the πꢁπ* transition in the
PDA+2 dication36,37 (Figure 11). This peak disappears during the
backward scan.
In the same medium and at ratio [18C6]/[PDA] = 20/1
(Figure 12), when the potential is scanned from ꢁ500 to
700 mV, the spectrum still shows the same absorption peaks at
almost the same wavelengths. This confirms that there is no
interaction between the MV compound and 18C6, as reported by
the electrochemical results. However, when the potential exceeds
700 mV, the peak which was at 526 nm is red-shifted by 13 to
increase in the IV-CT rate constant to kth = 2.3 ꢀ 1010
s
ꢁ1 for
[18C6]/[PDA] = 20/1 (Tables 2 and 3) indicating more
electron delocalization between the two amine centers.
This clearly indicates that, varying the acidity and/or the 18C6
concentration makes it possible to modulate the rate of the IV-
CT process, the lifetime and the potential range of the MV
compound.
3.3. In Situ Spectroelectrochemistry. 18C6 induces impor-
tant modifications in the voltammograms of PDA. It should also
cause some differences in the UVꢁvisible spectra, which allow
determination of the IV-CT band of the MV compound.
UVꢁvisible spectra were studied in situ, in the presence and in
the absence of 18C6 during potential sweep between ꢁ500 and
1200 mV versus Ag/AgCl, in neutral and acidified acetonitrile.
Absorption maxima are summerized in Table 2.
In 18C6-free acetonitrile containing 5 10ꢁ4 M PDA (Figure 10),
when the potential is scanned between ꢁ500 and 700 mV, the
spectrum shows two absorption maxima at 387 and 697 nm
Figure 8. Cyclic voltammograms of 10ꢁ3 M PDA in acetonitrile
containing 1 M TFA and 0.1 M TBAPF6 in the absence of 18C6 and
in the presence of increasing 18C6 concentration, at scan rate 100 mV sꢁ1
and T = 25 °C.
Figure 10. In situ spectroelectrovoltammogram of 5.0 ꢀ 10ꢁ4 M PDA
in acetonitrile containing 0.1 M TBAPF6 at 50 mV sꢁ1 scan rate. Full 3D
spectra at varying potential.
Figure 9. Oxidation and interaction equilibria of PDA in acetonitrile in (A) absence and (B) presence of 18C6.
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mixed valence nature of the cation radical. Furthermore, this band
matches with Gaussian shape at higher energy region, whereas it
does not overlap with a Gaussian shape on the low-energy one.
This result indicates that MV cation radical is classified as Class II
rather than Class III system. Moreover, the data listed in table 3
shows that, V , νmax in all the studied media, which confirms a
regime of relatively weak electronic coupling between the redox
sites corresponding to Class II system.34s
When 18C6 is added ([18C6]/[PDA] = 20/1), the two
absorption maxima (Table 2) of the first oxidation product are
not significantly modified. This does not mean that 18C6 does
not interact with the MV compound. Indeed, the interaction was
confirmed by electrochemical investigations, which show that all
PDA species interact with 18C6 in this medium. Probably the
MV compound, free and interacting with 18C6, has the same
absorption spectrum, and both forms are protonated with at least
three positive charges (Figure 8). This situation is very favorable
to allow strong interactions.
Nevertheless and just as observed in neutral acentonitrile, we
also observe a red shift of the absorption maximum of the second
oxidation product, from 507 nm in 18C6-free solution to 517 nm
when 18C6 is present at [18C6]/[PDA] = 20/1 (Table 2). This
red shift confirms that 18C6 interacts strongly with the second
oxidation product.
Figure 11. Spectra of 5.0 ꢀ 10ꢁ4 M PDA in acetonitrile containing
0.1 M TBAPF6. Blue line: At maximum absorption for PDA+ cation-
radical (with or without 18C6); red line: at maximum absorption for
PDA+2 dication without 18C6. Black line: At maximum absorption for
PDA+2 dication at [18C6]/[PDA] = 20/1.
The localization of the charge on each of the two nitrogen
atoms results in a favorable interaction strongly supported by the
red-shift observed for the dication absorption peak when crown
ether is present.
4. CONCLUSION
The possibility of forming supramolecular complex based on
weak intermolecular interactions is used for the first time to
successfully modify the charge distribution between a pair of
aromatic (redox) centers in a mixed-valence compound of PDA a
purely organic Robin/Day molecule. The intensity of this inter-
action can be modulated by changing either the acidity of the
solution and/or the concentration of the crown ether macrocycle
as external stimuli.
The addition of 18C6 in neutral acetonitrile does not modify
the lower limit value of the of MV potential range, but decreases
its upper limit. Consequently, the comproportionation constant
and the IV-CT rate constant are lowered, indicating a large
decrease of the MV stability and lifetime and less electron
delocalization between its two amine centers when 18C6 is
present. The same results are observed for DB24C8 with lower
amplitude indicating a less important effect with DB24C8. This
has been attributed to the formation of a reversible supramole-
cular complex between each of the crown ether and the dication,
following the oxidation of the crown ether-free MV compound.
The supramolecular complex is very stable at high potential
values (Ka = 7.0 ꢀ 107 Mꢁ2 for 18C6/PDA system and Ka =
2.9 ꢀ 103 Mꢁ1 for DB24C8/PDA system) and dissociates at low
potentials. The complex structure corresponds to face-to-face
mode and not to rotaxane one even with DB24C8.
Figure 12. In situ spectroelectrovoltammogram of 5.0 ꢀ 10ꢁ4 M PDA
and 0.1 M TBAPF6 in acetonitrile at [18C6]/[PDA] = 20/1 and 50 mV sꢁ1
scan rate. Full 3D spectra at varying potential.
539 nm in the presence of 18C6 (Figure 11). This shift is already
observed at [18C6]/[PDA] = 1/1 and remains constant for
higher ratios. This observation indicates that the dication inter-
acts with 18C6 (Figure 9), in good agreement with the previous
electrochemical results.
The in situ UVꢁvisible spectrum (not showed) of PDA in 1 M
TFA acetonitrile exhibits almost the same two absorption maxima
389 nm instead of 387 and 704 nm instead of 697 nm (Table 2),
which were attributed to the cation-radical. This indicates that
TFA does not significantly affect the IV-CT band. The absorp-
tion maximum of the second oxidation species (Table 2) is
observed at 507 nm, which corresponds to a blue shift of 20 nm
when compared to the value measured in neutral acetonitrile;
probably due to the protonation of the second oxidation product
in the presence of TFA.
When TFA is added, the MV potential range is gradually
shifted to more anodic values; whereas the addition of 18C6
gradually shifts it to more cathodic values. Both waves shift
results from a strong interaction of the oxidation products with
18C6 corresponding to the formation of hydrogen bound-based
supramolecular complexes between 18C6 molecules and each of
the PDA species: the starting molecule and its two oxidation
The large values of Kco obtained in neutral solution with and
without CE, indicate the high stability of the mixed valence cation
radical species. The presence of the absorption band at 697 nm
(Figure 11) in neutral medium with and without CE confirms the
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products. In this medium, the comproportionation constant of
the MV compound remains the same whatever the [18C6]/
[PDA] ratio, suggesting that the MV compound is stable with or
without 18C6, but the addition of 18C6 significantly increases
the IV-CT rate constant between the two amine centers.
In situ UVꢁvisible absorption spectroscopy confirms the
interaction of 18C6 with initial PDA and its oxidation products
in TFA acetonitrile medium and with only the second PDA
oxidation product in neutral acetonitrile. When 18C6 is present,
the IV-CT rate constant decreases in neutral acetonitrile,
whereas it increases in TFA-acetonitrile.
To the best of our knowledge, this work shows for the first
time the possible control of the IV-CT process, the lifetime and
the potential range of the MV compound. This control is not
obtained as usual by chemical modification of the structure of the
starting molecule, but simply by varying medium acidity or crown
ether concentration as external stimuli, which leads to a more or
less favorable formation of reversible hydrogen bound based-
supramolecular complex.
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’ AUTHOR INFORMATION
Corresponding Author
*Phone: +33(0)157277230. Fax: +33(0)157277263. E-mail: jouini@
univ-paris-diderot.fr.
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