Reversible Redox Switching of Chromophoric Phenylmethylenepyrans by Carbon-Carbon Bond Making/Breaking

Article abstract of DOI:10.1021/acs.joc.7b02199

Electrochromic organic systems that can undergo substantial variation of their optical properties upon electron stimulus are of high interest for the development of functional materials. In particular, devices based on radical dimerization are appropriate because of the effectiveness and speed of carbon-carbon bond making/breaking. Phenylmethylenepyrans are organic chromophores which are well suited for such purposes since their oxidation leads to the reversible formation of bispyrylium species by radical dimerization. In this paper, we show that the redox and spectroscopic properties of phenylmethylenepyrans can be modulated by adequate variation of the substituting group on the para position of the phenyl moiety, as supported by DFT calculations. This redox switching is reversible over several cycles and is accompanied by a significant modification of the UV-vis spectrum of the chromophore, as shown by time-resolved spectroelectrochemistry in thin-layer conditions.

Full text of DOI:10.1021/acs.joc.7b02199

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Article
Reversible redox switching of chromophoric
phenylmethylenepyrans by carbon-carbon bond making/breaking
Laurianne Wojcik, Francois Michaud, Sebastien Gauthier, Nolwenn
Cabon, Pascal Le Poul, Frederic Gloaguen, and Nicolas Le Poul
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b02199 • Publication Date (Web): 23 Oct 2017
Downloaded from http://pubs.acs.org on October 28, 2017
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Reversible redox switching of chromophoric
phenylmethylenepyrans by carbon-carbon bond
making/breaking
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Laurianne Wojcik,a François Michaud,b Sébastien Gauthier,c Nolwenn Cabon,c
Pascal Le Poul,c Frederic Gloaguen,a Nicolas Le Poula,*
a Laboratoire CEMCA, CNRS UMR 6521, Université de Bretagne Occidentale, 6 Avenue Le Gorgeu, CS 93837, 29238 Brest
Cedex , France.
b Service PIMMꢀDRX, Université de Bretagne Occidentale, 6 Avenue Le Gorgeu, CS 93837, 29238 Brest Cedex, France.
c IUT de Lannion, CNRS UMR 6226, Institut des Sciences Chimiques de Rennes, Université de Rennes 1, Rue Edouard
Branly, 22300 Lannion, France.
Keywords : redox switch; radical dimerization; spectroelectrochemistry; bistability; pyran
Abstract
Electrochromic organic systems that can undergo substantial variation of their optical properties upon electron
stimulus are of high interest for the development of functional materials. In particular, devices based on radical
dimerization are appropriate because of the effectiveness and speed of the carbonꢀcarbon bond making/breaking.
Phenylmethylenepyrans are organic chromophores which are well suited for such purpose since their oxidation
leads to the reversible formation of bispyrylium species by radical dimerization. In this paper, we show that the
redox and spectroscopic properties of phenylmethylenepyrans can be modulated by adequate variation of the
substituting group on the para position of the phenyl moiety, as supported by DFT calculations. This redox
switching is reversible over several cycles and is accompanied by a significant modification of the UVꢀVis
spectrum of the chromophore, as shown by timeꢀresolved spectroelectrochemistry in thin layer conditions.
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Introduction
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Molecular switches are compounds that can undergo reversible structural changes upon application of an
external stimuli, such as light, heat, electrons or chemical reactions. Switches displaying optical and redox
properties are particularly prone to lead to smart materials in the field of electrochromic and memory devices.1,2
Different strategies have been used to target efficient devices depending on the intraꢀ or intermolecular character
of the reactions and their associated kinetics. Supramolecular approaches involving large amplitude motion of
their inner components, such as observed in mechanically interlocked molecular (MIM) switches, have been
widely developed over the past years, leading to a wide range of functionalized systems including coordination
metal complexes and purely organic systems.3ꢀ10 Alternatively, organic chromophores displaying lowꢀamplitude
motion have also drawn interest when fast switching time is requested. Devices based on the photoactivatedꢀ
switching between two isomers, such as azobenzenes, have been well developed over the past years.11,12 Redoxꢀ
triggered switches displaying reversible π− or σꢀdimerization from electrochemically or photochemically
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generated radical anions or cations have also drawn interest.13ꢀ19, In particular, intermolecular σCꢀC dimerization
of radical cations of conjugated aromatic systems, such as stilbene,20 triarylamines,21,22 pyridine,23 cyanins,24
dialkoxyanthracene,25 spiropyran,26,27 tripyrrolinidobenzene28, bis(benzoꢀdithiolyl)bithienyl29 and dithiafulvene30ꢀ
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were reported. Noteworthy, recent investigations have focused on the immobilization of this family of redox
switches on solid surfaces for the development of smart multiple responsive materials.26,33ꢀ35
γꢀmethylenepyran compounds are organic chromophores which have been recently integrated as donors in
pushꢀpull devices for applications in nonꢀlinear optics and photovoltaic cells.36ꢀ39 Interestingly, electrochemical
studies have shown that methylenepyran compounds can undergo fast dimerization by σC−C bond making upon
oxidation at moderate potential values (Ep ~ 0.3 V vs. Fc) in organic solvents.37,38,40,41 The resulting bispyrylium
species is reduced irreversibly around −0.8 V vs. Fc leading back to the initial methylenepyran by C−C bond
breaking (Scheme 1).40ꢀ43 Bispyrylium compounds are much less sensitive to selfꢀdeprotonation than their
bisdithiolium analogues.30,40,43 Such stability is of high importance when considering prospective application in
the field of organic redox switches. Moreover, another aspect of interest of this family of compounds is their
optical properties associated to their conjugated structure. Indeed, most of these dyes display intense absorption
bands in the 350−700 nm wavelength range, depending on the degree of conjugation, in addition to the
fluorescence properties.37ꢀ39 To our knowledge, there has been no report on the use of the redox and optical
properties of this family of compounds for redoxꢀtriggered molecular switching.
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Herein, we have focused on the switching properties of different paraꢀsubstituted phenylmethylenepyrans
(RPhMP, Scheme 1) as probed by spectroelectrochemistry. In order to minimize geometric distortion of the
structure by hindering groups, substitution was carried out on the phenyl group rather than on the ethylene
R
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moiety. Indeed, the optical and redox properties of the neutral and oxidized forms of PhMP can be more easily
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controlled by simple variation of the substituent R. Electrochemical and UVꢀVis spectroscopic studies, supported
by DFT and TDꢀDFT calculations, were thus performed to investigate the effect of the substituting group on the
redox and optical properties. In addition, spectroelectrochemical experiments were carried out to characterize the
switching properties of this series of compounds by variation of the applied potential.
Scheme 1. Reversible C−C bond making/breaking by electrochemical oxidation of Rꢀsubstituted
phenylmethylenepyrans (RPhMP) and reduction of their bispyrylium derivatives.
Results and discussion
Synthesis of compounds 2a-f
Syntheses of 2a44,45, 2c44,45, 2d,41 and 2e46 were performed as previously reported. The synthesis is based on
a Wittigꢀtype reaction between the 2,6ꢀdiphenylꢀ4Hꢀpyranꢀ4ꢀyl triphenyl (1Ph) phosphonium tetrafluoroborate
R
salt and the substituted formylbenzoate PhCHO (Scheme 2). For the newly prepared compounds 2b and 2f, a
more convenient procedure was carried out using a tributyl (1n-Bu) pyranyl derivative instead of a triphenyl one
(1Ph). All compounds were characterized by 1H and 13C NMR spectroscopy, and mass spectrometry (see SI part).
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Scheme 2. Schematic pathway for the syntheses of Rꢀsubstituted phenylmethylenepyran compounds 2a-f.
Synthesis of compounds 3f2+ and 4f
Scheme 3. Schematic pathway for the synthesis of compound 3f2+ and 4f.
The twoꢀstep synthesis of the bispyran compound 4f was undertaken following a previouslyꢀdescribed
procedure (Scheme 3).47 During the first step, addition of the oxidizing agent AgBF4 to a solution of 2f in
CH2Cl2 led to the formation of a bispyrylium derivative 3f2+. Compound 3f2+ was isolated and characterized by
1H NMR, 13C NMR, mass spectrometry and cyclic voltammetry (see Experimental Section and SI part). On the
1H NMR spectrum of 3f2+, two diastereoisomers were observed in a 30/70 ratio. Then, a large excess of pH 9
solution (glycine buffer) was added to a solution of 3f2+ in CH2Cl2 and the twoꢀphases mixture was stirred for
half an hour at room temperature. At this stage, the deprotonation of the bispyrylium compound occurred and the
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bispyran derivative 4f was formed. Compound 4f was isolated and characterized by H NMR, 13C NMR, mass
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spectrometry and cyclic voltammetry (see Experimental Section and SI part). The H NMR spectrum of 4f
displayed characteristic signals for the two nonꢀequivalent heterocyclic hydrogen atoms at 6.56 ppm and 6.98
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ppm. The lack of signal for the exocyclic hydrogen atom of the pyranylidene moiety, which was observed at 5.97
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ppm for 2f, confirmed also the proposed structure.
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Structural characterisation of NMe2PhMP (2b)
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The molecular structure of compound 2b is displayed in Figure 1. The phenylmethylenepyran compound
crystallizes in the monoclinic centrosymmetric space group C2/c (Table S1). Selected bond distances and angles
are gathered in Table 1. C−C bonds lengths lie in the range of 1.374(3) to 1.390(3) Å for the three phenyl rings.
Low variation of the bond lengths are observed (< 0.03 Å), thus indicating that electron density is uniformly
delocalized in these parts of the molecule. The pyran ring displays less homogeneity between lengths since
C7−C8 (1.343(3) Å) and C10−C11 bonds (1.335(3) Å) are significantly shorter than C8−C9 (1.452(2) Å) and
C9−C10 (1.457(3) Å) bonds, O1−C11 (1.389(2) Å) and O1−C7 (1.379(1) Å) bonds being inꢀbetween. Such data
corroborate alternating single and double carbonꢀcarbon bonds in the pyran ring. The exocyclic C9−C18 bond
length is 1.362(3) Å, which suggests a weak pyrylium character.43 Comparison with the analogous
methylenepyran 2d bearing a carboxy substituting group41 is shown in Table 1. Clearly, both compounds display
similar bond lengths and angles. The Rꢀsubstituted phenyl ring and the pyran moiety are not coꢀplanar for both
compounds 2b and 2d. This is best emphasized by the torsion angle between C24ꢀC19ꢀC18ꢀC9 which are similar
for both compounds (151.49° for 2b and 149.88° for 2d).
Figure 1. Molecular structure of NMe2PhMP (2b) with the atom numbering scheme. Thermal ellipsoids are
drawn at 50% probability. One phenyl group is modelled as split into two positions (high thermal motion).
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Table 1. Selected bond distances (Å) and angles (°) for RPhMP compounds (R = NMe2 2b and R=CO2Me 2d 41
)
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according to numbering in Figure 1.
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R = NMe2 (2b)
1.379(2)
1.362(3)
1.472(2)
1.389(2)
1.466(3)
1.381(2)
128.4(2)
125.4(2)
122.1(2)
R = CO2Me (2d)
1.377(8)
C7−O1
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1.370(10)
1.483(9)
C9−C18
C11−C12
C11−O1
1.376(8)
1.441(1)
C18−C19
C22−R
1.479(1)
127.6(4)
C9−C18−C19
C8−C9−C18
C10−C9−C18
126.8(0)
120.1(7)
Electrochemical and computational studies
Cyclic voltammetric (CV) study of compounds 2a-f was performed at a Pt working electrode (0.3 cm in
diameter) in dry N2ꢀpurged CH2Cl2 containing 0.1 M NBu4PF6 as supporting electrolyte. Electrochemical data
are gathered in Table 2. As shown in Figure 2A, all RPhMP compounds displayed a similar redox behavior, i.e. a
single irreversible anodic peak (−0.18 V < Epa < 0.41 V vs. Fc at v = 0.1 V s−1) on the forward scan and an
irreversible cathodic peak on the backward scan (−0.86 V < Epc < 0.70 V vs. Fc). Variation of the scan rate (0.02
V sꢀ1 < v < 5 V sꢀ1) did not modify the redox behavior (Figure 2C). However, for v > 20 V sꢀ1, the oxidation
process became quasiꢀreversible (Figure S2). Voltammograms recorded at more negative potential did not
display any new redox signal except for the NO2 derivative 2f (E1/2 = −1.90 V vs. Fc, ꢀEp = 80 mV, not shown).
For all species, the process at Epc was no longer detected when scanning negatively on the forward scan or on
voltammograms recorded at a rotating disk electrode (RDE, see inset Figure 2D, curve a for R=H). Exhaustive
electrolysis of solutions of RPhMP at Epa (n = 1 e−) confirmed the stability of the resulting species reduced at Epc
~ −0.8 V (Figure 2D). Irreversibility of the oxidative process emphasized the occurrence of an EC mechanism
(E = Electrochemical step, C = Chemical step), for which the C step corresponds to the coupling of a radical
cation either with another radical cation (RRD mechanism) or with a neutral RPhMP species (RSD
mechanism).48,49
On the basis of previous studies with analogous methylenepyrans,40,42,43 the irreversible reduction process at
ca. Epc ~ −0.8 V can be ascribed to the reduction of the electrogenerated bispyrylium species [3a-f]2+ (see Figure
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S7 for the CV of 3f2+). As shown in Scheme 4, the whole process corresponds to a C−C bond making
(oxidation)/breaking (reduction) involving two phenylmethylenepyran units as previously observed with
ferrocenylꢀbased methylenepyrans.40 In order to correlate the redox behavior of 2a-f with the donor properties of
the R group, Epa values were plotted against the corresponding Brown coefficients σ+. As shown in Figure 2B,
plots follow a linear trend meaning that electronic effects of the Rꢀphenyl group essentially govern the oxidation
process. Conversely, the reduction potential of the bispyrylium 32+ is almost independent of the nature of R
(Figure 2A).
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R
Figure 2. A) CVs at a Pt electrode of Rꢀsubstituted phenylmethylenepyran compounds PhMP 2a-f (0.50 mM)
in CH2Cl2/NBu4PF6 0.1 M: v=0.1 V sꢀ1; R= NO2 (blue), CO2Me (red), Br (gray), H (orange), OMe (green), NMe2
(black); B) Corresponding plots of Epa vs. Brown constant σ+; C) CVs of NO2PhMP 2f (C=0.50 mM) for 0.01 V
sꢀ1 < v < 5 V sꢀ1; D) CVs and RDEVs (inset) of HPhMP 2a (C = 1.1 mM) before (black, a) and after (red, b)
exhaustive electrolysis at 0.4 V.
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Scheme 4. Schematic pathways for the C−C bond making/breaking of Rꢀsubstituted phenylmethylenepyran
RPhMP compounds upon electron/proton transfer.
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Table 2. Electrochemical data for compounds 2a-f in CH2Cl2/NBu4PF6 0.1 M.
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d
Epa
Epc
E0
R
(theor., ox.)
H
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0.14
0.34
0.29
0.41
ꢀ0.85
ꢀ0.83
ꢀ0.81
ꢀ0.77
ꢀ0.78
ꢀ0.73
0.32
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0.15
0.44
0.38
0.60
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2b
2c
2d
2e
2f
NMe2
OMe
CO2Me
Br
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NO2
a Experimental data obtained at v = 0.1 V sꢀ1 and C = 0.50 mM with ohmic drop
correction. b Irreversible oxidation peak. c Irreversible reduction peak obtained after
oxidation of compound 2a-f. d Calculated by DFT (see text).
Computational analysis was carried out in order to correlate structural features and redox properties for the
entire series of compounds 2a-f. First calculations were performed with compounds 2b and 2d for comparison
with Xꢀray structures (see SI for computational details). Bond distances and angles found from computed gasꢀ
phase structures at the B3LYP/6ꢀ31+G(d,p) level are in good agreement with experimental data (Table S2).
These results indicated that the calculation level was sufficiently accurate for allowing further analysis of the
whole RPhMP series. Hence, geometries of all compounds 2a-f were optimized in gas phase. Close inspection of
the structural properties demonstrated that distances and angles do not vary in a significant manner with R (Table
S3). Schematic representations of the charge density at the HOMO level for 2b (R = NMe2) and 2f (R = NO2) are
shown in Figure 3 (See Figure S18 for 2a, 2c, 2d, 2e). For all the compounds, electron density is mainly
localized on both the substituted phenyl and the pyran ring.
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Figure 3. Isodensity surfaces (isovalue of 0.02) of the HOMO and the LUMO of 2b and 2f, and of the SOMO of
2b+ and 2f+ at the B3LYP/6ꢀ31+G(d,p) level of theory.
Further analysis was performed on the calculated structures of the radical cations 2a-f+ in gas phase, these
species being too unstable to be isolated or characterized by spectroelectrochemistry. Modification of the CꢀC,
CꢀO and CꢀN bond lengths upon electron withdrawing was particularly investigated (Table S4, Figure S22).
Several conclusions can be drawn for this family of compounds, independently of the nature of the R substituting
group. The main effects of oxidation on the structure is an increase by ca. 0.048 Å of the acyclic C9ꢀC18 bond
close to the pyran moiety along with a decrease by ca. 0.035 Å of the adjacent C18ꢀC19 bond. Consequently,
these bonds have a similar length value of ca. 1.42 Å suggesting equal charge delocalization. The structure of the
pyran moiety is also affected by the oxidation in a similar manner (i.e. an increase of the double bonds C10ꢀC11
and C7ꢀC8 along with a decrease of the single bonds C8ꢀC9, C9ꢀC10, C11ꢀO1, C7ꢀO1). The structure of both
unsubstituted phenyl groups attached to the pyran moiety are unaffected by the oxidation. Moreover, the length
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of the CꢀC bonds in the Rꢀsubstituted phenyl group is not significantly modified. However, the C22ꢀR bond
increases or decreases significantly depending on the nature of R (i.e. by 0.03 Å for R=NMe2 and OMe).
Formation of the monopyrylium 2+• cation is also accompanied by minor changes in the angles. As depicted in
Figure S22 for 2e, the C1ꢀC6 phenyl group is slightly twisted out of plane of the pyran moiety by 3°. Also, the
pyran group is twisted towards the plane of the Rꢀsubstituted group by 7° (from 26° to 33°) upon oxidation, in
agreement with charge delocalization. Isodensity surfaces of the SOMO of the radical cations 2b+ and 2f+ are
shown in Figure 3. Interestingly, the localization of the electron density is fully Rꢀdependent for these two
cations. Electron density of the SOMO of 2f+ is mainly localized on the Rꢀsubstituted phenyl and the pyran
moieties. In contrast, it is positioned on the unsubstituted phenyl ring and the pyran moieties for the cations 2b+.
This result emphasizes that the Rꢀdependent variation of the redox potential of these two cations is likely
associated to the stabilization effect of the Rꢀsubstituting group on the radical cation. Further analysis of the spin
density clearly shows that for the cation 2f+ (R=NO2) the positive charge is strongly localized on C18, i.e. where
dimerization occurs (Figure S29). In contrast, for the cation 2b+ (R=NMe2), the positive charge is delocalized
over C18 and C9 and over C19 and C24. DFT calculations were also performed on the bispyrylium species 3a2+
and its elusive reduced form 3a. Isodensity surfaces of the LUMO of 3a2+ and of the HOMO of 3a suggest that
the electron transfer mainly occurs on the pyrylium moieties and not on the Rꢀsubstituted phenyl groups (Figure
S21). This result is consistent with electrochemical data showing a quasiꢀindependency of the reduction potential
of 32+ with R (Figure 2A).
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To gain further support for these conclusions, the formal oxidation potentials of compounds 2aꢀf in
dichloromethane were calculated by DFT using an isodesmic equation (see SI for computational details). As
shown in Table 2, there is a good agreement between the measured and calculated values (i.e. the difference
between the two values is typically below 100 mV except for R = NO2). Note also that the measured values are
peak potential values of an oxidation step coupled with a fast chemical reaction. In addition, as shown in Figure
4A, plots of measured (Epa, exp) and calculated ((E0theor) potentials vs. Brown constant (σ+) show similar trends.
Similarly, a formal reduction potential of E0red = −0.73 V vs. Fc was calculated by DFT for the reduction of 3a2+
to 3a (Table S5), in good agreement with the experimental data shown in Figure 2.
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Figure 4. Plots of experimental (black squares) and calculated (red triangles) values of A) the oxidation potential
in CH2Cl2/NBu4PF6 and B) the maximal wavelength λ[1] in CH2Cl2 of RPhMP 2a-f compounds against the
Brown constant value of the R groups.
UV-Vis spectroscopic properties
UVꢀVis spectroscopic properties of the 2a-f compounds were investigated. As shown in Figure 5 and Table
3, compounds 2a-f display three main absorption bands in the 250ꢀ500 nm optical range with high molar
extinction coefficients of about 3 × 104 to 4 × 104 Mꢀ1 cmꢀ1. The absorption band displaying a maximum at ca.
λ[1] = 400 nm is fully dependent on the nature R in contrast to the two others bands at λ[2] = 280 nm and λ[3] =
250 nm (Figure 5). The λ[1] absorption band has previously been reported for analogous methylenepyrans.37,38,41
In contrast to the value of the oxidation peak Epa, the value of λ[1] does not vary linearly with the Brown
constant value σ+ of the R groups (Figure 4B, black plots). Indeed, the NO2 substituted phenylmethylenepyran 2f
displays the highest wavelength value at 460 nm within the 2a-f series. Compounds 2b and 2d display λ[1]
values around 390 nm, whereas compounds 2a, 2c and 2e absorb around 360 nm.
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Figure 5. UVꢀVis spectra (optical path of 1 mm) of 2a-f compounds (0.25 mM in CH2Cl2): 2a (R=H, orange),
2b (R=NMe2, blue), 2c (R=Br, green), 2d (R=CO2Me, red), 2e (R=OMe, gray), 2f (R=NO2, black).
TDꢀDFT calculations were hence carried out to investigate the nonꢀlinear variation of λ[1] with σ+ (See SI
for computational details). As shown in Figure 4B, the close agreement between the calculated and experimental
wavelength values λ[1] indicates that the computational method is able to model the effect of electron
donating/withdrawing properties of the R group on the UVꢀvis spectra (Figure S24). A close inspection of the
energy level diagram shows that the absorption band at λ[1] for all these compounds mainly concerns the
HOMOꢀLUMO transition (ca. 80%). Actually, the bowlꢀshape of the λ[1] vs. σ+ plot can be analyzed according
to the respective variation of the energy level of the HOMO and the LUMO with the value of the Brown constant
(Figure S23). The energy levels of both the HOMO and the LUMO decrease with increasing values of σ+.
However, the energy level of the LUMO decreases sharply when the R substituting group becomes very
electrophilic (i.e. corresponding to a high value of σ+), which explains the bowlꢀshape of the HOMOꢀLUMO
energy difference vs. σ+ plot, and hence that of the λ[1] vs. σ+ plot.
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Table 3. UVꢀVis spectroscopic data (λ / nm and ε /Mꢀ1 cmꢀ1) for compounds 2a-f in CH2Cl2.
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6
Compound
R
λ [1] (ε)
λ [2] (ε)
λ [3] (ε)
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9
H
358 (32600) 274 (22400)
390 (47120) 269 (34000)
366 (44560) 271 (37600)
394 (40480) 285 (19600)
367 (41480) 280 (24800)
458 (34800) 302 (18400)
252 (27200)
252 (34800)
252 (35600)
250 (30000)
251 (32200)
248 (31600)
2a
2b
2c
2d
2e
2f
NMe2
OMe
CO2Me
Br
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NO2
Redox switch monitoring by spectroelectrochemistry
Electrochemical studies showed that the process of CꢀC dimerization of 2a-f leading to the corresponding
bispyrylium dication 3a-f2+ is relatively fast because a reversible CV response started being observed for v > 20
V s−1 (Figure S2). Reduction of these products induces fast CꢀC bond breaking and recovering of the initial
pyran compounds. Taking advantage of the relative pace of these two processes, UVꢀVis spectroelectrochemistry
experiments were carried out in CH2Cl2/Bu4NPF6 by applying potential steps to check the reversibility of the
switch by electrochemical input from 2 to 32+ species (see for example Figure 6 for 2f). A specifically design
thinꢀlayer cell of 0.2 mm optical path, allowed us to work in a reflectance mode as previously described for other
compounds.50 We particularly focused on the absorption band at λ[1] because of the targeted objective to use
these compounds as optical switches in the visible range. Table 4 displays the values of absorption band
recorded upon fast electrochemical potential switching between 2 and 32+. Whereas the value of λ[1] is fully
dependent on the nature of R, the absorption band at λ[4] for the electrochemically generated bispyrylium 32+ is
almost the same for all compounds (405 to 419 nm, Table 4). As a consequence, the change in applied potential
induces a bathochromic shift for 2a-e compounds, while the nitroꢀbased 2f pyran displays a hypsochromic shift.
DFT calculations performed on the Hꢀsubstituted bispyrylium 3a2+ and its doublyꢀreduced form 3a display good
matching between theoretical and experimental values of λ[4] (Table 4 and Figure S25). According to DFT
calculations, this absorption band reflects the HOMOꢀLUMO gap for the bispyrylium compounds.
In order to evaluate the stability of both two stable states (pyran and bisꢀpyrylium) upon redox switch, UVꢀ
visible monitoring was performed under thinꢀlayer conditions for 15 cycles for compounds 2a-f. As shown in
Figure 6 for 2f, a progressive decrease and broadening of the absorbance at λ[1] and λ[4] was detected, as well
as an increase of the absorption band at 250 nm. This suggests the formation of a new species upon cycling
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which absorbs in both 400ꢀ500 nm and 250 nm regions. This was confirmed by thinꢀlayer cyclic voltammetry
which showed the appearance of a new redox reversible system at ca. E0’ = ꢀ0.05 V vs. Fc, with a progressive
decrease of the anodic peak current related to the oxidation of the methylenepyran. This system kept on growing
continuously after several cycles (Figure S3). Noticeably, it did not appear when the study was operated under
diffusing conditions and at low scan rate (0.02 V sꢀ1), indicating a rather slow reaction at the electrode surface
(Figure S3). Accordingly, this new redox system can be ascribed to the reversible oxidation of the bispyran 4a-f
(Scheme 4) resulting from the deprotonation of the bispyrylium 3a-f2+, in agreement with previous studies.42,43
This was confirmed by the electrochemical studies of the chemically synthesized bispyran 4f in dichloromethane
which displays a reversible system at E0 = ꢀ0.07 V vs Fc involving two electrons (Figure S6), as clearly
demonstrated by exhaustive electrolysis of the solution. In addition, the UVꢀVis spectrum of 4f in
dichloromethane features three main bands at 484 nm, 398 nm and 259 nm (Figure S8). Similarly to compounds
2a-f and 3a2+, a formal potential was calculated by DFT for the twoꢀelectron oxidation of 4f. The value found
(E0 = 0.19 V vs Fc) is in good agreement with the experimental data (Table S5). Moreover, the UVꢀVis spectra
for 4f calculated from TDꢀDFT displays absorption bands at 455 nm, 355 nm and 273 nm (Figure S26), in good
agreement with the values found experimentally.
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Figure 6. A) Timeꢀresolved monitoring of the UVꢀVis characterization of compound 2f in CH2Cl2/NBu4PF6 0.1
M upon redox switching between 0.6 V and ꢀ0.9 V vs. Fc (15 cycles of 180 seconds) (optical path: 0.2 mm). B)
Plots of applied potential (left) and resulting current (right) against time during the monitoring.
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Table 4. Experimental and theoretical UVꢀVis spectroscopic data (λmax / nm) for compounds 2a-f and 3a-f2+ in
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CH2Cl2/NBu4PF6 0.1 M.
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R
Compound
Compound
λ [1]
Exp.
358
390
366
394
367
458
λ[4]
Exp.
405
411
405
415
407
419
Theor.a
364
Theor.a
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2a
2b
2c
2d
2e
2f
3a2+
3b2+
3c2+
3d2+
3e2+
3f2+
H
398
b
NMe2
OMe
CO2Me
Br
404
378
b
383
b
369
b
NO2
426
b
a Theoretical values found by TDꢀDFT. b Not calculated.
Hence, at the first cycle, oxidation of 2a-f leads to formation of the bispyrylium 3a-f2+ by CꢀC bond making,
which partly undergoes deprotonation and forms the 4a-f species. On application of a low redox potential of ꢀ0.9
V vs. Fc, the electrochemically generated compound 3a-f2+ is reduced into 2a-f by CꢀC bond breaking. As long
as the switching cycle is maintained, each new cycle leads to a slight decrease of the concentration in
bispyrylium, as well as an increase in concentration in bispyran within the thin layer. This induces a modification
of the UVꢀVis spectroscopic response by coalescence of the different absorption bands. The formation of this
new species 4a-f was observed by thinꢀlayer CV for all phenylmethylenepyrans (Figures S4 and S5). The formal
potential of the electrochemically generated bispyran was almost insensitive to the nature of the R substituting
group. This is in agreement with DFT calculations performed for 4f which suggest that oxidation involves
molecular orbitals situated on the central core of the bispyran (Figure S27).
Insights into the variation of the absorption maxima upon cycling showed slight discrepancies between 2a-f
compounds. For instance, a quasiꢀconstant UVꢀVis response was recorded for both neutral 2f and oxidized 3f2+
species after 5 cycles (Figure 7A). Such result suggests that deprotonation does not interfere in the 2fꢀ3f2+
switching process after a few cycles. In contrast, switching between 2c and 3c2+ induced a progressive decrease
of the band at 405 nm, as an indicator of a constant decrease of the concentration in bisꢀpyrylium 3c2+ within the
thin layer (Figure 7B). Moreover, the oxidation of the pyran 2c did not lead to a total decrease of the band at 366
nm, and reduction of 3c2+ failed at removing total absorption at 405 nm. This effect was also observed for 2b
(Figure S11), but not for 2a, 2d and 2e (Figures S10, S13 and S14). For R=OMe, it can ascribed to the formation
of the bispyran 4c and its oxidized analog 4c2+ in the thin layer, these two species likely absorbing in the 350ꢀ
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500 nm wavelength range as found for 4f and 4f2+ (Figures S8 and S17). This discrepancy is consistent with the
Bronsted basicity strength of the substituting group R. Basic moieties such as NMe2 and OMe can indeed
interact with protons of an adjacent bispyrylium compound and then favor deprotonation.
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At last, attempts were made to avoid deprotonation of the bispyrylium (3a-f)2+, by adjusting pulse length in
chronoamperometric studies or by decreasing phenylmethylenepyran concentration. Decreasing or increasing the
pulse duration did not lead to any significant improvement of the electrochemical current and spectroscopic
responses with cycling. We also investigated the effect of decreasing the concentration of phenylmethylenepyran
in solution (i.e. [2f] = 0.15 mM, Figure S16), but no significant improvement in stability could be detected.
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Figure 7. Plots of baseline ꢀcorrected absorbance vs time at different wavelengths starting from solutions of
compounds 2f (A) and 2c (B) upon redox switching between 0.6 V and ꢀ0.9 V vs. Fc (15 cycles of 180 seconds):
A) λ = 458 nm (black, characterizing 2f) and 419 nm (red, characterizing 3f2+); B) λ = 366 nm (blue,
characterizing 2c) and 405 nm (orange, characterizing 3c2+). Experiments performed in CH2Cl2/NBu4PF6 0.1 M
with concentration of 0.60 mM in methylenepyran (optical path 0.2 mm).
Conclusion
In summary, this work successfully demonstrates that tuning of the optical and redox properties of various
phenylmethylenepyrans can be achieved by simple substitution on the para position of the phenylmethylene
moiety. Voltammetric studies in organic media showed that all compounds undergo irreversible oxidation
according to a fast dimerization process leading to bispyrylium species by intermolecular carbonꢀcarbon bond
formation. The electrochemical reduction of the resulting bispyrylium compounds yields back the initial pyran
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species by CꢀC bond breaking. Electrochemical data showed that the oxidation potential increases linearly with
the electrophilic character of the substituting group, a result that is fully supported by DFT calculations.
Contrarily, the reduction potential of 32+ remains almost insensitive to the nature of the R group. UVꢀVis
spectroscopic studies emphasize a wide variation of the optical properties of differently paraꢀsubstituted
phenylmethylenepyran compounds vs. bispyrylium, whith absorption bands shifted by over 100 nm in the 360ꢀ
460 nm wavelength range. In contrast to electrochemical results, the spectroscopic features of the
phenylmethylenepyrans, which is well reproduced by TDꢀDFT calculations, do not vary linearly with the
donor/withdrawing properties of the substituting moiety R. On the basis of the CꢀC bond making/breaking
process, timeꢀresolved spectroelectrochemical experiments in thin layer conditions have been carried out.
Reversible optical changes associated to the electrochemical generation of pyran and bispyrylium species have
been clearly emphasized. Redoxꢀswitching in thinꢀlayer conditions suggests the progressive formation of a
neutral bispyran moiety resulting from deprotonation of the bispyrylium compound over cycles, but in a much
lower proportion than for dithiafulvene analogous systems.30 The deprotonation process seems being dependent
on the nature of the Rꢀsubstituting group. Noteworthy, the bispyran intermediate is only detected when cycling
under thinꢀlayer conditions.
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From these results, it appears clearly that this series of compounds display remarkable properties for
envisaging applications as redox switching devices. In particular, the nitroꢀbased methylenepyran exhibits the
most interesting properties, so far for this type of compound; that is : (i) fast chemical conversion between the
two stable states; (ii) low oxidation potential; (iii) large shift of 100 nm of the absorption band in the visible
region between stables states. Future work will aim at improving these characteristics, in particular for inhibiting
the deprotonation process by rational design of new compounds based on the same skeleton and/or by variation
of experimental conditions. Moreover, theoretical investigations by electrochemical and molecular modeling will
be performed to analyze the mechanism of dimerization (i.e. RRD vs. RSD) and the possible effect of the nature
of the R group on kinetics.
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Experimental section
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General procedures.
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All chemical manipulations were carried out under a nitrogen atmosphere using standard Schlenkꢀline
Techniques. Infraredꢀspectra were recorded on a PerkinꢀElmer spectrum1000 FTIR using KBr plates. All NMR
data are reported in ppm (δ) relative to tetramethylsilane as external reference. Coupling constants are reported
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in Hz. H and 13C NMR chemical shift assignments are supported by data obtained from Hꢀ1H cosy, Hꢀ13C
HMQC and Hꢀ13C HMBC. Spectra were recorded at room temperature in CDCl3 or acetone d6 on a Bruker
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Ascend TM 300 spectrometer. Mass spectra or elemental analyses were performed at the CRMPO (Université de
Rennes). Microanalytical data were obtained with a ThermoꢀFinnigan Flasch EA 1112 CHNS/0 analyzer. Mass
spectra were obtained with high resolution spectrometer (Waters QꢀTOFꢀ2 or Bruker MAXI 4G). Xꢀray crystal
structure of compound 2b was obtained at 100 K from a single crystal grown from solution in dichloromethane
at room temperature. Diffraction data were collected on an Oxford Diffraction XꢀCalibur diffractometer with
attached Cryojet cryostat and Crysalis software using MoKα radiation at Xꢀray diffraction facilities of “Plateꢀ
forme d’imagerie et de mesure en microscopie” (PIMMꢀDRX, Université de Bretagne Occidentale). Structure
was solved by direct methods using SHELXS97 program and leastꢀsquare refined using SHELXL97 program.51
Electrochemical studies of all compounds were performed in a glovebox (Jacomex) (O2 < 1 ppm, H2O < 1 ppm)
with a homeꢀdesigned 3ꢀelectrodes cell (WE: Pt diam. 3mm or 1mm, RE: Pt wire in a Fc+/Fc solution, CE:
graphite rod). Ferrocene was added at the end of the experiments to determine redox potential values. For all
experiments, ohmic drop was determined by the positive feedback method. The potential of the cell was
controlled by an AUTOLAB PGSTAT 100 (Metrohm) potentiostat monitored by the NOVA 1.11 software.
Extraꢀdry dichloromethane (Acros) was stored in the glovebox and used as received. The supporting salt
NBu4PF6 was synthesized from NBu4OH (Acros) and HPF6 (Aldrich). It was then purified, dried under vacuum
for 48 hours at 100° C, then kept under argon in the glovebox. Electrolyses were carried out with a homeꢀ
designed 3ꢀelectrodes cell (WE: graphite rod, RE: Pt wire, CE: graphite rod). Thin layer room UVꢀVis spectroꢀ
electrochemistry was performed with a specific homeꢀdesigned cell in a reflectance mode (WE: Pt diam 3mm,
RE: Pt wire, CE: Pt wire) with an optical path of 0.2 mm.50 The UVꢀVis optic fibre probe used was purchased
from Ocean Optics. Timeꢀresolved UVꢀVis detection was performed with QEPro spectrometer (Ocean optics).
Classical UVꢀVis spectroscopy in a 10 mm quartz cuvette was performed with the QEPro spectrometer.
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Syntheses of compounds 2a, 2c, 2d and 2e
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Syntheses of compounds 2a,44,45,55 2c,44,55 2d,41 and 2e46 were performed according to a previously published
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procedure based on a Wittigꢀtype reaction:
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Under nitrogen atmosphere, to
a degassed solution of 2,6ꢀdiphenylꢀ4Hꢀ4ꢀyl triphenylphosphonium
tetrafluoroborate 1Ph or 2,6ꢀdiphenylꢀ4Hꢀ4ꢀyl tributylphosphonium tetrafluoroborate salts 1n-Bu in THF, was
added, dropwise at ꢀ78°C, a small excess of nꢀbutylꢀlithium solution (2.5 M in hexane) (Figure 1). After 15 min,
the corresponding aldehyde was added to the dark solution and the mixture was stirred 30 min at this
temperature. The orange solution obtained was then allowed to room temperature for 3 hours. After evaporation
of THF, water and methylene chloride were added to the residue and the aqueous layer was extracted three times
with methylene chloride. The combined organic extracts were evaporated. The crude product was purified by
silica gel column chromatography. 1H NMR spectroscopy and mass spectrometry features of compounds 2a, 2c,
2d and 2e are given in references [41, 44ꢀ46, 55].
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4-(2,6-Diphenyl-pyran-4-ylidenemethyl)-dimethylaminobenzene (2b)
The dimethylaminobenzene derivative was synthesized thanks to the same procedure as used for 2a, 2c, 2d and
2e except the nature of the phosphonium salt (tributyl instead of triphenyl one).
Reactants: diphenylꢀ4Hꢀ4ꢀyl tributylphosphonium tetrafluoroborate salt (1g, 1.67 mmol), nꢀbutyllithium 2.5M in
hexanes (0.70 mL , 1.75 mmol, 1.1 eq), 4ꢀdimethylaminobenzaldehyde (0.249 g, 1.67 mmol, 1 eq).
Chromatography: (diethyl ether/ Petroleum ether, 1/1)
Product obtained (250 mg, 41%) as orange solid mp: 180ꢀ182°C IR ν(cmꢀ1) 2853; 1656; 1604; 1514; 1493; 1065;
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936; 762. H NMR (300 MHz, acetone d6) δ (ppm) 7.89 (d, 4H), 7.50 (d, 6H), 7.34 (d, J = 6.7Hz, 2H), 7.07
(d,1H), 6.78 (d, J = 8.0 Hz, 2H), 6.61 (d, 1H), 5.95 (s, 1H), 2.98 (s, 6H) 13C NMR and JMOD (125 MHz,
Acetoneꢀd6) δ (ppm) 151. 6 (C),149.2 (C), 148.2 (C), 133.4 (C), 133.5 (C) 129.2 (CH), 128.5 (CH), 126.6 (C),
125.9 (C) 125.2 (CH), 115.6 (CH), 112.3 (CH), 108.8 (CH), 101.9 (CH), 39.7 (CH). HRMS (ESI/ASAP) m/z
calculated for C26H24NO [M+H]+ 366.18579, found 366.1855 (1ppm).
4-(2,6-Diphenyl-pyran-4-ylidenemethyl)-nitrobenzene (2f)
A solution of n-BuLi (0.2 mL, 2.5 M in pentane) was added to a solution of diphenylꢀ4Hꢀ4ꢀyl
tributylphosphonium tetrafluoroborate salt (1 g, 1.92 mmol) in THF (30 mL) at -78 °C under nitrogen. The
reaction mixture was stirred at this temperature for 15 min, and 4-nitro-benzaldehyde (0.290 g, 1.92
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mmol) was added. The resulted mixture was allowed to warm to room temperature and stirred for 2 h.
The solvent was removed and the residue was purified by column chromatography (SiO2, Petroleum
Ether/CH2Cl2 50/50) to give compound 2f as red solid (630 mg, 85%).
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1H NMR (300 MHz, CDCl3), δ (ppm) 8.22 (d, J=8.0 Hz, 2H), 7.77-7.83 (m, 4H), 7.45-7.55 (m, 8H), 7.06 (d, J =
1.0 Hz, 1H), 6.51 (d, J = 1.0 Hz, 1H), 5.97 (s, 1H); JMOD (125 MHz, CDCl3) δ(ppm) 154.6 (C), 152.5 (C),
148.2 (C), 145.8 (C), 133.3 (C), 132.9 (C), 132.5 (C), 130ꢀ124 (CHPh), 111.8 (CH), 108.6 (CH), 101.6 (CH)
HRMS (ASAP/Qꢀtof 2) m/z calculated for C24H17NO3 [M]+.: 367.12084 ; found : 367.1206; m/z calculated for
C24H18NO3 [M+H]+: 368.12867; found : 368.1274.
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4,4'-(1,2-bis(4-nitrophenyl)ethane-1,2-diyl)bis(2,6-diphenylpyrylium) tetrafluoroborate (3f2+, 2BF4 )
Compound 2f (0.300 g, 0.816 mmol) and AgBF4 (0.159 g, 0.816 mmol) were stirred one hour in a degassed
solution of methylene chloride (30 mL) at room temperature. The yellow solution turns green and we observe a
black deposition of Ag residue which was then removed by filtration. The solution was then placed in a fridge (ꢀ
20°C) and after 24h a green precipitate was observed and isolated by filtration (502mg, 68%).
1H NMR (300 MHz, CD2Cl2), 2 diastereoisomers (A: Maj. 67% ; B : min. 33%) δ 9.21 (s, 2H, HA(3,5)), 9.12 (s,
1H, HB(3,5)), 8.46 (m, 4H, HA (Ph)), 8.40 (m,2H, HB (Ph)), 8.38 (d, J=9 Hz, 1H, HB(PhNO2)), 8.23 (d, J=9 Hz,
1H, HB(PhNO2)), 8.16 (d, J=9 Hz, 2H, HA(PhNO2)), 8.09 (d, J=9 Hz, 2H, HA(PhNO2)), 7.88ꢀ7.74 (m, 9H,
6HA(Ph) + 6HB(Ph)), 6.50 (s, 0.5H, HB(7)), 6.45 (s, 1H, HA(7)). JMOD (125 MHz, CD2Cl2) δ(ppm) 172.3
(C(2;6)), 172.1 (C2;6), 171.9 (C2;6), 147.9 (C11), 142.9(C4), 142.5 (C), 136.7 (CH), 130.5ꢀ124.9 (CHPh), 127.8
(C), 118.1 (CH(7)). HRMS (ESI) m/z calculated for C48H34N2O6 [M]2+: 367.1203; found : 367.1199;
1,2-bis(2,6-diphenyl-4H-pyran-4-ylidene)-1,2-bis(4-nitrophenyl)ethane (4f)
Compound 2f (0.060 g, 0.163 mmol) and AgBF4 (0.032g, 0.163 mmol) were stirred one hour in a degassed
solution of methylene chloride (10 mL) at room temperature. A pH = 9 glycine buffer was then added to the
solution and the heterogeneous mixture was stirred for half an hour. The Ag residue was then eliminated by
filtration and the organic layer was extracted 3 times using diethylether and washed 3 times with water. The
extract was dried over MgSO4. After evaporation of the solvent, the crude product was obtained without further
purification as a brown powder (0.035 mg, 30%).
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1H NMR (300 MHz, CDCl3), δ 8.12 (d, J=6.9 Hz, 2H), 7.74 (m, 2H), 7.67 (m,2H), 7.53 (d, J=6.9 Hz, 2H), 7.46
(m, 3H), 7.37 (m, 3H), 6.98 (s, 1H), 6.56 (s, 1H). JMOD (125 MHz, CDCl3) δ(ppm) 154.1 (C), 153.2 (C), 148.1
(C), 145.7 (C), 132.9 (C), 132.8 (C), 131.7 (C), 130ꢀ124 (CHPh), 121.2 (C), 105.8 (CH), 103.7 (CH). HRMS
(ESI) m/z calculated for C48H32N2O6 [M]+.: 732.22549; found : 732.2255 (0 ppm); m/z calculated for
C48H31N2O6 [M+Na]+: 755.21526; found : 755.2163 (1 ppm); m/z calculated for C48H31N2O6 [MꢀH]+:
731.21766; found : 368.2192 (2 ppm).
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Computational details.
Structures of the compounds 2a-f and 3a2+ and of their respectively oxidized and reduced forms were optimized
in gas phase at the B3LYP/6ꢀ31G(d) level of theory as implemented in the Gaussian 09 software.52 Analytic
frequency calculation was carried at the same level of theory on each optimized structures to identify the
stationary point as a minimum (zero imaginary frequency) and to obtain the Gibbs free energy correction. The
electronic energy in CH2Cl2 was derived from single point calculation at the B3LYP/6ꢀ31+G(d,p) level of theory
starting from the gasꢀphase optimized structure and using the SMD continuum model. Timeꢀdependent DFT
(TDꢀDFT) was used to compute the excited states of 2a-f and 3a2+. Calculations were carried out at the
B3LYP/def2ꢀTVZP level of theory53 as implemented in the ORCA 3.03 software.54 The isodensity surfaces were
calculated and plotted using GaussView 5.0.9.
Author Information
Corresponding author:
*Email: nicolas.lepoul@univꢀbrest.fr
Notes
The authors declare no competing financial interest.
Acknowledgments
The authors thank the “Institut Brestois de Santé Agro Matière” (IBSAM) for their helpful financial contribution
to the purchasing of the QEPro UVꢀVis spectrometer. The "Centre Régional de Mesures Physiques de l’Ouest"
(CRMPO, Université de Rennes) is thanked for mass spectrometry and elemental analyses.
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Experimental details (electrochemistry, spectroelectrochemistry and DFT calculations). Crystal data are reported
in Table S1. CCDC 1517637 contains the supplementary crystallographic data for this paper. The data can be
obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures
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