Outside rules inside: The role of electron-active substituents in thiophene-based heterophenoquinones

Article abstract of DOI:10.1039/c4cp05748a

The biradicaloid vs. quinoidal character of the ground state of thiophene-based heterophenoquinones bearing donor or acceptor groups is investigated. Keeping the conjugation length fixed, namely, the 5,5′-bis(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadiene-1-y

Full text of DOI:10.1039/c4cp05748a

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Outside rules inside: the role of electron-active
substituents in thiophene-based
heterophenoquinones†
Cite this: Phys.Chem.Chem.Phys.,
2015, 17, 10426
L. Colella,ab L. Brambilla,a V. Nardone,ac E. Parisini,c C. Castiglionia and C. Bertarelli*ac
The biradicaloid vs. quinoidal character of the ground state of thiophene-based heterophenoquinones
bearing donor or acceptor groups is investigated. Keeping the conjugation length fixed, namely, the
5,50-bis(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadiene-1-ylidene)-2,20-dihydroxy bithiophene backbone,
an opposite effect occurs depending on the donating or withdrawing nature of the substituents. The
character of the ground state depends not only on the electronic nature of the substituents but also on
their position on the molecular skeleton: donor groups on the 3,30-positions of the bithiophene central
core stabilize a quinoidal ground state, whereas a biradicaloid electronic structure results from the
introduction of the same donor groups onto the lateral phenones. Withdrawing groups behave similar to
donors, but in the opposite direction.
Received 9th December 2014,
Accepted 12th February 2015
DOI: 10.1039/c4cp05748a
www.rsc.org/pccp
species improve the non-optimal microstructure of the active
layer by affecting the crystallinity of the polymer as well as the
Introduction
Due to their particular electronic properties, conjugated quinoidal morphology of the blend.
species are attractive components in molecular electronics and
In some conjugated quinones, the occurrence of unexpected
optoelectronic devices for a wide range of applications. For electronic structures has been known for a long time. Thiele’s4
instance, due to their good transport properties, quinone-based and Chichibabin’s5 hydrocarbons are the first examples of
semiconductor channels are used in field effect transistors.1 systems characterized by a biradicaloid ground state, as proven
Although quinoidal species have emerged as promising n-channel by both spectroscopic analysis and theoretical modeling.6
actuators, some thiophene-based derivatives exhibit ambipolar Several further series of Chichibabin-type compounds with
behaviour, which meets the key requirement for complementary dicyanomethylene,7 oxocyclohexadienylidene8,9 or imidazolylidene10
circuits. For longer homologues, effective p-conjugation along end groups have then been synthesized. It has been observed that
the planar molecular skeleton leads to low energy gap values molecules with a rather short sequence of conjugated units exhibit a
with absorption in the red or near-infrared regions. In this quinoidal structure, whereas a diradical contribution occurs when
regard, 5,50-bis(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadiene-1-ylidene)- increasing the conjugation length. Kozaki et al. reported no diradical
2,20-dihydroxy bithiophene (QBT, Fig. 1) has been used as a character for 111 and diradical participation in cis–trans isomerisa-
hole donor in an organic photodetector2 for polymer optical tion for 210 (see Fig. 1).
fibers, which work at 650 nm.2 In organic photovoltaic cells,
Using Raman spectroscopy and quantum chemical modelling,
the addition of a small quantity of QBT to bulk heterojunctions Navarrete et al. demonstrated the occurrence of a diradical
of P3HT:PCBM (poly-3-hexylthiophene:fullerene) resulted in a structure in dicyanomethylene-end-capped quinoidal oligomers
47% increase in EQE with respect to the corresponding cells when the number of thiophene rings is equal to or larger than
based on a P3HT:PCBM binary blend only.3 In fact, the quinoidal four (3, Fig. 1).7 Further Raman spectroscopy and computa-
tional studies have clearly highlighted the peculiar ground-state
structure of QBT.12 In the same work, Fazzi et al. identified the
a Dipartimento di Chimica, Materiali e Ingegneria Chimica, Politecnico di Milano,
spectroscopic signature, i.e. the frequency position of a Raman
marker band, of the biradical structure for these thiophene-
based heterophenoquinone homologues.
More recently, it has been found that electron-donating alkoxy
groups on the thienylene core of the 5,50-bis-(3,5-di-tert-butyl-4-oxo-
2,5-cyclohexadiene-1-ylidene)-3,30-dipentoxy-2,20-dihydroxy bithio-
phene (QOO) provide a strong contribution to the stabilization of
p.za Leonardo da Vinci 32, Milan, Italy. E-mail: chiara.bertarelli@polimi.it;
Fax: +39 0270638173
b INAF, Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate, Italy
c Center for Nanoscience and Technology@PoliMi, Istituto Italiano di Tecnologia,
via Pascoli 70/3, Milan, Italy
† Electronic supplementary information (ESI) available. CCDC 1029034. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cp05748a
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ꢀ the 3,30-positions of the central bithienylene core (‘‘substituted
core’’, see Fig. 2, on the right). We have avoided functionalization of
the 4,40-positions since this can hamper coupling of the core with
the lateral phenols due to steric hindrance;
ꢀ
the lateral 4-oxo-2,5-cyclohexadiene-1-ylidene at the
3,5-positions, replacing the t-butyl groups (‘‘substituted wings’’,
see Fig. 2, on the left). Again, substitution in these positions is
preferred to the 2,6-positions, in order to avoid steric hindrance
during Suzuki coupling.
All the quinoidal molecules were obtained by oxidation of their
corresponding aromatic precursors. Throughout this paper, mole-
cules are labelled with acronyms where the first letter indicates an
aromatic (A) or quinoidal (Q) species. The remaining letters refer to
the substituents.
The general scheme for producing the ‘‘substituted core’’
class (Fig. 3) follows the procedure used for QBT, consisting of
Suzuki coupling between 5,50-dibromo-2,20-bithiophene and a
boron derivative of the substituted phenol (5).12
3,30-Dipentoxy-2,20-bithiophene (11) was synthesized according
to ref. 14 and further brominated with NBS in DMF. For 3-methoxy-
2,20-bithiophene, Kumada coupling between 2-bromo-3-methoxy-
thiophene with NBS and the Grignard reagent of 2-bromothiophene
was carried out (Scheme 1).
Suzuki coupling between the dibrominated cores and the
boronic derivative 5 gave aromatic compounds, which were
subsequently oxidized with potassium ferricyanide to provide QOO
and QHO in quantitative yields. As the symmetrically substituted
bithiophene AOO tends to oxidize spontaneously upon air
exposure, both aromatic and quinoidal products were obtained
from the workup of the reaction.
Fig. 1 Thiele’s and Chichibabin’s hydrocarbon analogues and hetero-
phenoquinones.
the quinoidal ground-state structure of these species.13 This
allows tuning of their electronic character without significantly
affecting the HOMO–LUMO energy gap.
In the present study, we provide evidence of the possibility of
modulating a quinoid vs. biradicaloid structure within thiophene-
based heteroquaterphenoquinones bearing electron-active sub-
stituents either on the phenoquinone or on the central core.
We take QBT as a reference molecule, as it is the shortest
molecule exhibiting a transition from a quinoidal to a biradicaloid
electronic ground state among thiophene-based heteropheno-
quinones.12 We demonstrate that the stabilisation of a given electronic
structure depends on the nature and strength of electron-
active substituents and that donor and acceptor groups provide
opposite effects. Interestingly, the quinoidal vs. biradicaloid
character is also affected by the position of the substituents.
As for electron-poor QBT derivatives, two electron-withdrawing
groups have been considered, namely 1,3-dioxolane (QXX) and
carboxylic acid (QCO). These two species were obtained in
succession via the same synthetic route. In the latter case, the
carboxylic group was obtained in the last oxidation step, starting
from aldehyde groups in the 3,30-positions. The synthetic route
is reported in Scheme 2.
Results and discussion
Materials
In oligoarylenes, as in other conjugated species, the introduction of
electron-donating or electron-withdrawing substituents is known to
affect the position of the frontier energy levels and their energy gap.
To highlight the role of functional groups in unsaturated pheno-
quinones, we have introduced different substituents onto the
backbone of 5,50-bis(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadiene-1-
ylidene)-2,20-dihydroxy bithiophene (QBT). The sites that we have
taken into consideration for the introduction of electron-donating
and electron-withdrawing groups are:
The two dibrominated bithienylene cores (16 and 18) were
synthesized almost entirely following the published procedure.15
According to the original route, 5,50-dibromo-3,30-diformylacetal-
2,20-bithiophene (16) was not isolated, but it was directly deprotected.
Herein, the cleavage of 16 was not carried out in a one-pot reaction.
Fig. 2 General structure of the two classes of molecules: ‘‘substituted
wings’’ (on the left) and ‘‘substituted core’’ (on the right). The spheres
correspond to the donating or withdrawing groups introduced onto the
pristine backbone.
Fig. 3 ‘‘Substituted core’’ class of quinoidal molecules starting from QBT:
QCO, QXX, QHO, QOO.
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Scheme 2 Synthetic route to the quinones belonging to the ‘‘substituted
core’’ class bearing withdrawing groups.
of the QBT structure with electron-active groups, leaving the
central bithienylene core unchanged (Fig. 4).
Two different synthetic strategies were followed. The first one
employed Suzuki coupling, starting from a lateral p-brominated
phenol and a diboronated bithienylene core. This route was
applied with a phenol substituted with electron-donating groups,
Scheme 1 Synthetic route to the quinones belonging to the ‘‘substituted
core’’ class bearing donor groups.
Accordingly, compound 16 was isolated before proceeding to the
next deprotection step. Incidentally, it was found that bromination
of 15 did not only provide 16, which is white-coloured, but also a
yellow product, which proved to be 5,50-dibromo-30-(1,3-dioxolan-2-
yl)-[2,20-bithiophene]-3-carbaldehyde (17), due to partial deprotection
of 16 on silica. Suzuki coupling between the dibrominated
cores (16 or 18) and the boronic derivative (5) gave the aromatic
compounds AXX and ACO. 5,50-Bis(3,5-di-tert-butyl-4-hydroxy-
phenyl)-[2,20-bithiophene]-3,30-dicarbaldehyde (ACO) was also
obtained through deprotection of the diformylacetal groups of
AXX with pyridinium tosylate (PPTS) in acetone (Scheme 2).
It is well known that introduction of electron-acceptor groups
induces an increase in oxidation potential in p-conjugated systems,
and hence hampers chemical and electrochemical oxidative
reactions. Therefore, unlike the case of AOO, where the oxidation
step proceeds spontaneously, oxidation of aromatic precursors
substituted with acceptor groups has been shown to be more
difficult. In addition to potassium ferricyanide, MnO2 has also
been tested as an oxidizing agent. The latter has allowed recovery
of the desired products in quantitative yields.
The set of ‘‘substituted wings’’ compounds is obtained by
replacement of the t-butyls in the a,a0-positions on the phenols
Fig. 4 ‘‘Substituted wings’’ class of quinoidal molecules starting from
QBT: QMeOMeO, QMeOCF3, and QCF3CF3.
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ACF3CF3). It was found that, with simultaneous introduction of
the two different phenols (Run 3 in Table 1), the asymmetrical
molecule AMeOCF3 is recovered as the main product (34%),
whereas the symmetrical species are formed in lower yields (4%
and 9.5% for AMeOMeO and ACF3CF3, respectively).
All the aromatic compounds were oxidized to the corre-
sponding quinones using either MnO2 or potassium ferri-
cyanide, the latter facilitating the following purification step.
This oxidant is in fact soluble in the reaction medium whereas
the desired products precipitate, thus allowing for their easy
separation by filtration.
Scheme 3 Suzuki coupling for the synthesis of AMeOMeO.
4-bromo-2,6-dimethoxyphenol (20), and a pinacol ester of
2,20-bithiophene (21) (Scheme 3).
Absorption and fluorescence spectroscopy of aryls (A) and
quinones (Q)
This reaction gave the desired product in very low yields,
with the monosubstituted compound (22) as the main product. UV-vis spectroscopy is a convenient method to highlight the
Since Stille coupling between 5,50-bis(trimethylstannyl)-2,20- strong change in p-delocalisation when passing from an aromatic to
bithiophene (24) and 20 (Scheme 4) provided higher yields a quinoidal structure. The peculiar absorption of quinoidal species
than the Suzuki reaction, the same route was applied to the at low energy is due to the almost double inter-ring bond that forces
synthesis of both the other molecules bearing substituted the skeleton into a more planar conformation, leading to a loss of
wings (i.e. ACF3CF3 and AMeOCF3). Runs were also repeated aromatic character and more extended p-delocalisation.
Except for QOO and QCF3CF3, the UV-vis spectra of these
quinoidal species show the same features as the reference QBT
under microwave irradiation, but no significant advantage was
observed.
The asymmetrical AMeOCF3, which contains both a donor-
substituted wing at one end and an acceptor-substituted wing
at the other end, required optimization of the experimental
conditions to avoid or limit the formation of the symmetrically
substituted molecules as main products (i.e. AMeOMeO and
Scheme 4 Synthetic route to yield ‘‘substituted wings’’ quinoidal molecules.
Table 1 Runs aimed at determining the correct timing of introduction of
the two different wings to obtain AMeOCF3
Addition timea
Yields (%)
Run
20b
23b
AMeOMeO
AMeOCF3
ACF3CF3
1
2
3
After 45
0
0
0
0
34
4
0
0
34
43
0
9.5
After 25
0
Fig. 5 Normalized absorption spectra of ‘‘substituted core’’ and ‘‘substi-
tuted wings’’ species in chloroform: (a) QBT, (b) QOO, (c) QXX, (d) QCO, (e)
QMeOMeO, (f) QMeOCF3, (g) QCF3CF3.
a
b
In minutes. Phenol derivatives used as reactants.
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Table
2
Absorption maxima of aromatic (A-molec) and quinoidal
(Fig. 5). In fact, the functionalized quinones exhibit a sharp,
very intense band assigned to the strongest dipole-allowed
transition, with a defined vibronic structure, at higher energy
and a weaker transition, which has been previously assigned to
a weak double-electron excitation, at lower energy.16
(Q-molec) compounds in chloroform solution. The extinction coefficient
values e (Mꢁ1 cmꢁ1) are reported in brackets. The red-shift going from
aromatic to quinoidal species is also highlighted
Compound A-molec lmax (nm)
Q-molec lmax (nm) D red-shift (nm)
The change in p-conjugation going from aromatic precursors to
the corresponding quinones can be highlighted by the strong red-
shift of the absorption bands (see D red shift in Table 2). This shift is
the largest for XX, which is characterized by a distorted backbone in
the aromatic form (AXX). In fact, the presence of bulky substituents
on the 3,30-positions of the bithiophene causes a deviation from
planarity through rotation about the quasi-single thiophene–
thiophene inter-ring bond. Relative to unsubstituted ABT, a
remarkable blue-shift in the p–p* transition is observed when
acceptor groups are introduced into the bithiophene (AXX, ACO).
In ACO, together with the main absorption band at 270 nm, a
weaker band at 406 nm is present, which is ascribed to an n–p*
transition (see ESI†), in analogy with other aromatic structures
containing carbonyl groups.
Conversely, the smallest shift of the main absorption band
going from an aromatic precursor to the corresponding quinone is
displayed by a ‘‘substituted core’’ species bearing an alkoxy group
(D red shift = 290 nm). Introduction of this substituent into the core
of the molecule provides a relevant donor contribution to the
aromatic backbone through a resonance effect without causing
significant distortion.17 This effect also plays a significant role in
the case of aromatic compounds, as shown by the red-shift of the
absorption maximum of AOO (406 nm) with respect to ABT
(382 nm). Planarization in both AOO and QOO is likely related
also to a stabilizing oxygen–sulphur interaction, which adds to the
contribution of the resonance forms, which are expected to coun-
terbalance any sterically induced distortion effect.18
BT
OO
XX
CO
382 (37 000)
406 (26 000)
329 (21 000)
680 (205 000)
696 (100 000)
699 (96 000)
299
290
370
316
317
316
308
270–406 (25 000–9460) 722 (111 000)
MeOMeO 385 (19 000)
MeOCF3
CF3CF3
702 (191 000)
695 (89 000)
680 (85 000)
379 (20 000)
372 (18 000)
the strongest effect in the methoxy-substituted quinones and the
weakest one in the molecule with the –CF3 acceptor groups. The
shift in absorption bands when going from aromatic to quinoidal
species (D red shift, Table 2) is similar for all three compounds and
indicates that the introduction of donating and withdrawing
groups onto the lateral phenol does not cause any significant
distortion of the backbone. This is also confirmed by the relatively
low Stokes shift values (between 0.46 and 0.50 eV, see ESI†), which
are comparable to that of ABT. These pieces of evidence indicate
that not only is the conjugated backbone not distorted by the
introduction of substituents, but also their electronic effect is
negligible.
Surprisingly, the donor–acceptor quinone (QMeOCF3) dis-
plays an intermediate effect with respect to the symmetrically
substituted QMeOMeO and QCF3CF3. This behaviour suggests
that no cross-talk occurs between the electron-active groups
and therefore no push–pull species is actually formed. None of
the quinones described here exhibit any fluorescence.
The role of functional groups in changing the electronic
character: quinoidal vs. biradicaloid ground state
An indication of the degree of planarity of the molecular
skeleton for the aromatic species is also given by the Stokes Despite controversial views regarding the correct description of
shift values. The photoluminescence spectra of ABT and AOO the ground-state electronic structure of conjugated quinones,
are characterized by a well-defined vibronic structure with an previous works have highlighted the strict relationship between
energy separation of about 0.15 eV (spectra are reported in the diradicals and p-conjugation along the backbone in the homo-
ESI†). The presence of the vibronic structure in the emission can be logous series of thiophene-based heterophenoquinones.12 In
ascribed to planarization of the molecular skeleton triggered by the this respect, Raman-active modes are a powerful tool for dis-
excitation.19 The Stokes shift values are in the typical range for a criminating between different p-conjugated compounds on
linear conjugated oligomer20 and decrease with an increase in the basis of their characteristic fingerprints. In particular, the
planarization along the conjugated skeleton, as in the case of AOO so-called R-mode, which corresponds to oscillation of the bond
(0.38 eV, see ESI†). Conversely, the photoluminescence spectrum of length alternation (BLA) parameter relative to the CC sequence
ACO is characterized by a roughly defined vibronic structure shifted along a conjugated structure,2,21 gives strong, diagnostic marker
to longer wavelength, whereas AXX does not display any vibronic bands. As reported in our previous work,12 the Raman frequency
shoulder and has a large Stokes shift (1.00 eV), which is ascribed to of the R-mode of QBT, at 1304 cmꢁ1, is unusually low with
the large variation in equilibrium geometry between the ground respect to the R frequency commonly exhibited by aromatic
and excited states.
oligothiophenes and short thienoquinoid oligomers, which is
According to all the reported data, for the ‘‘substituted core’’ observed at about 1440 cmꢁ1. This feature is a clear signature of
species planarization of the skeleton in the quinoidal structure the biradicaloid structure of QBT. Indeed, the remarkable down-
counteracts the hindrance effect of the side groups, which is ward shift of the R-mode can be rationalized only by considering
relevant in the aromatic precursors instead.
an almost ‘‘equalized’’ CC bond sequence in the bithiophene
As a result of the electronic effect of the substituents, the moiety, as predicted by density functional theory calculations
absorption maxima of QOO, QXX and QCO are red-shifted with adopting a broken-symmetry (BS) wavefunction.12 Conversely,
respect to QBT. Comparison between the three different ‘‘substituted not only does the most common closed-shell (CS) approach
wings’’ species (i.e. QMeOMeO, QMeOCF3, and QCF3CF3) highlights predict a quinoidal equilibrium structure for QBT, but it also
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provides a computed Raman spectrum showing the usual
Since the position of the R-mode is directly related to the
R-mode band at about 1400 cmꢁ1, in total disagreement with molecular structure and it is red-shifted in the presence of a
the experimental observation.
biradicaloid character, it is possible to assign a biradicaloid
According to these previous studies,12,13 we expect that fine- geometry to QXX and QCO.
tuning of the ground-state electron charge distribution by the
It turns out that modification of the electron distribution in
introduction of electroactive substituents into the QBT skeleton a thiophene-based quaterphenoquinone by means of electroactive
results in a remarkable modulation of the Raman features. substituents on the conjugated core leads to stabilization of a
Being specifically used as a diagnostic tool to determine a possible quinoidal ground state only when donor groups are introduced.
diradical contribution to the ground state of conjugated quinones, Acceptor groups likely provide the opposite effect.
Raman spectroscopy was carried out only for the Q-molecules
QBT and QMeOMeO exhibit similar Raman spectra: considering
studied here and not for the A-species which do not exhibit this the spectral region 1800–900 cmꢁ1, the most intense peak is
particular electronic character. Direct comparison of the experi- located at B1300 cmꢁ1 (Fig. 7). As demonstrated before for
mental Raman fingerprints allows us to classify these quinones QBT,12 the anomalous low frequency of the R-mode can be
without any support from theoretical modelling of the spectra, as explained only by taking into account a biradicaloid ground-state
the conjugated skeleton is the same for all the substituted species character. The similarity in the main Raman features between the
discussed here.
As previously reported, the Raman spectrum of QOO is markedly QMeOMeO has a ground state with a biradicaloid nature.
different from that of unsubstituted QBT, displaying the strongest Conversely, QCF3CF3, which is characterized by withdrawing
QBT and QMeOMeO spectra further establishes the fact that
band, which is assigned to the R-mode, at 1430 cmꢁ1, up-shifted by substituents on the phenones, strongly differs from QBT, while
more than 100 cmꢁ1 with respect to the R-mode transition of the being more similar to QOO: its Raman spectrum exhibits the
biradicaloid species (Fig. 6).13 The presence of a strong line assigned strongest band, assigned to the R-mode, at B1430 cmꢁ1, which
to the R-mode at higher energy proves that the dialkoxy-substituted indicates, for this molecule, a quinoidal ground state (Fig. 7).
phenoquinone is described by a quinoidal ground state. Given this
This evidence leads to the conclusion that the introduction
first result, the next step was to investigate whether the effect of of electron-active substituents on the lateral phenones has the
functional groups at the 3,30-positions of the bithienylene core opposite effect on the electronic behaviour of these materials,
(‘‘substituted core’’) is unequivocal in stabilizing a quinoidal ground compared to the introduction of electron-donating/withdrawing
state or depends on their specific electronic character (i.e. donor or groups onto the central core. In other words, it has been demon-
acceptor).
strated that the electronic character of functionalized QBTs in the
QXX and QCO exhibit several spectral features comparable ground state depends not only on the nature of the electron-active
to those of QBT (Fig. 6). For QXX and QCO, the strongest bands substituents, but also on their position on the main backbone. This is
are at 1230 cmꢁ1 and 1130 cmꢁ1, respectively, down-shifted likely related to diradical delocalization over the molecular structure.
by B200–300 cmꢁ1 with respect to the R-mode transition
characteristic of the quinoidal species discussed above (QOO).
Fig. 7 Raman spectra of powder samples of (see labels on the left): QOO
Fig. 6 Raman spectra of powder samples of (see labels on the left): QOO
(FT-Raman) and QBT, QXX, and QCO (excitation line: 632 nm).
(FT-Raman) and QBT, QMeOMeO, QCF3CF3, and QMeOCF3 (excitation
line: 632 nm).
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QMeOCF3 exhibits the strongest band delocalized at a lower the structure confirms the high degree of planarity of the
wavelength, B1470 cmꢁ1, suggesting higher stability of the molecule (Fig. 8).
quinoidal configuration. The donor–acceptor strategy, which
Only half a molecule is present in the asymmetric unit,
generally plays a significant role in modifying the electron the other half being generated through an inversion centre. The
distribution in aromatic oligomers, thus giving low-band-gap two phenyl rings are essentially coplanar with the core, the
materials, is not effective in the case of conjugated phenoqui- C(3)–C(4)–C(5)–C(10) torsion angle being 2.61. Further contri-
nones. As a consequence, since p-conjugation does not butions to the planarity of the system are interactions between
increase, a biradicaloid ground state is not obtained.
the oxygen and sulphur atoms within the central moiety of the
molecule. In fact, the distance between the sulphur atom and
the oxygen of the pentoxy group is 2.8 Å. The two pentoxy
groups slightly extend above and below the plane of the
molecule (see Fig. 8b). As expected, the outermost C atoms in
the pentoxy groups exhibit a large vibrational motion, suggest-
ing a low degree of packing constraint.
A close check of the bond distance distribution in the
molecule (Table 3) shows clear differences within the phenone
moiety that confirm the dearomatization process that occurs in
going from aromatic precursors to quinoidal species. Moreover,
the bond length distribution along the skeleton clearly suggests
Crystal structure
Among all the quinones presented here, it was possible to collect
a full X-ray diffraction data set only for QOO. This class of
thiophene-based heterophenoquinones does not usually provide
single crystals of sufficient quality for X-ray data collection and,
to date, only the crystal structure of 2,5-bis-(3,5-di-tert-butyl-
4-oxo-2,5-cyclohexadiene-1-ylidene)-1,5-dihydrothiophene has
been reported.22 However, for conjugated quinones, X-ray
structure investigation may also provide further validation of
the results obtained by Raman analysis. From the XRD data
of QOO, the occurrence of a mainly quinoidal molecular struc-
ture, exhibiting alternate shorter and longer CC bonds accord-
ing to the pattern established by the phenoquinone ends, is
expected.
Table 3 Bond lengths and angles in QOO
Bond distance (Å)
Small dark green needles were grown by slow evaporation
from a toluene solution of QOO at 4 1C. A summary of the major
crystallographic data is reported in the ESI.† Data collection
was carried out at room temperature. Despite the thin plate-like
morphology of the crystals, a complete X-ray diffraction data set
could be collected and the structure unambiguously deter-
mined and fully refined anisotropically. No clear diffraction
was observed beyond the 2y value at which the structure is
reported. Further attempts to improve the three-dimensional
size and quality of the crystals were unsuccessful, most likely
owing to intrinsic structural features of the molecule. Overall,
O(2)–C(8)
C(8)–C(7)
C(9)–C(8)
C(7)–C(6)
C(9)–C(10)
C(5)–C(6)
C(10)–C(5)
C(5)–C(4)
C(4)–C(3)
C(2)–C(3)
C(1)–C(1)
C(1)–C(2)
1.247 (9)
1.482 (11)
1.472 (11)
1.376 (10)
1.350 (10)
1.426 (10)
1.432 (10)
1.408 (10)
1.414 (10)
1.364 (10)
1.409 (14)
1.420 (10)
Angle
C(4) C(5) C(6)
C(3) C(4) C(5)
121.4 (8)
128.5 (7)
Torsion angle
C(3) C(4) C(5) C(10)
C(6) C(5) C(4) S(1)
177.6 (8)
(—) 178.8 (6)
Fig. 8 Molecular structure of QOO: (a) top view and (b) side view.
10432 | Phys. Chem. Chem. Phys., 2015, 17, 10426--10437
Fig. 9 Crystal structure of QOO viewed along the a-axis.
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PCCP
a quinoidal electronic character, as expected. Interestingly, the aldehyde was completely reacted (ca. 6 h). The solvent was
formal inter-ring double bonds C(1)–C(1) and C(4)–C(5) are removed under reduced pressure, diethyl ether was added
longer than the intra-ring double bonds, namely C(2)–C(3), and the crude product was washed with sodium hydrogen
C(6)–C(7) and C(9)–C(10).
carbonate solution and saturated sodium chloride solution.
No p–p stacking interaction is observed in the crystal (Fig. 9). The organic phase was dried with sodium sulfate, the solvent
Short intermolecular contact distances of 3.6 Å between was removed under reduced pressure and the residue was
t-butyl moieties belonging to nearest neighbouring molecules distilled (3 ꢃ 10ꢁ1 mbar, 55 1C) to give 10.5 g of the desired
stabilize the molecular packing arrangement in the crystal. product in 75% yield.
Further C–Hꢂ ꢂ ꢂp stabilizing contacts between t-butyls and
1H-NMR (400 MHz, CDCl3): d = 7.42 (dd, J(H,H) = 3.15 Hz,
centroids of thiophene rings are also observed. These inter- 4J(H,H) = 0.92 Hz, 1H; Th-H2), 7.32 (dd, J(H,H) = 3.15 Hz, 4J(H,H) =
actions can contribute to stabilization of the observed crystal 5.08 Hz, 1H; Th-H5), 7.18 (dd, J(H,H) = 3.15 Hz, J(H,H) = 5.08 Hz,
packing arrangement, which is characterized by the presence of 1H; Th-H4), 5.91 (s, 1H; O–CH–O), 4.08 ppm (dm; CH2–CH2).
large cavities.
3,30-Diformylacetal-2,20-bithiophene (15). n-Butyllithium
(12 mL, 30 mmol, 2.5 M in hexane) was added dropwise to a
stirred solution of diisopropylamine (2.90 g, 28.7 mmol) in 25 mL
dry THF at ꢁ78 1C under an argon atmosphere. After 120 minutes,
3-thiophenecarboxaldehyde acetal (14) (4.50 g, 28.7 mmol) in 35 mL
dry THF was added dropwise; the resulting mixture was stirred at a
low temperature for a further 30 minutes. Then 4.50 g anhydrous
CuCl2 was added in one portion and the reaction bath was allowed
to warm up to room temperature and stirred overnight. The
reaction mixture was poured into water and extracted with CH2Cl2.
The organic phase was washed, dried over MgSO4 and evaporated
under reduced pressure. The raw product was purified by flash
chromatography (silica gel, 8 : 2 petroleum ether : ethyl acetate) and
recrystallized from petroleum ether/CH2Cl2 (8 : 2), to afford 1.99 g
(45%) of the title product as a white crystalline solid.
Conclusion
New thiophene-based quaterphenoquinones with separate control
over the band gap and the quinoid vs. biradicaloid structure have
been investigated by introducing electron-active substituents both
onto the lateral phenoquinones and the central core.
Theoretical prediction of the Raman spectra allowed clear
identification and rationalization of the spectroscopic markers
that can be taken as signatures of a transition from a quinoidal
to a biradicaloid structure as a result of suitable chemical
modifications. Specifically, it was demonstrated that the
presence of donor or acceptor groups on the central moiety
produced opposite effects on the stabilization of the electronic
character of the structure in the ground state. The reverse occurs
when the substituents are placed on the lateral phenones, suggesting
that donor groups at the 3,30-positions of the bithiophene central
core stabilize a quinoidal ground state, whereas a biradicaloid
electronic structure results from introduction of the same
donor groups onto the lateral wings.
Finally, the simultaneous presence of donor and acceptor
groups at opposite ends of the molecule does not reduce the energy
gap and therefore does not significantly affect the p-conjugation. The
strength of a withdrawing trifluoromethyl group prevails over the
donation of electronic charge from an alkoxy group, thus resulting in
a quinoidal ground state.
3
1H-NMR (400 MHz, CDCl3): d = 7.36 (d, J(H,H) = 5.44 Hz,
3
0
0
2H; Th-H5,H5 ), 7.21 (d, J(H,H) = 5.44 Hz, 2H; Th-H4,H4 ), 5.72
(s, 2H; O–CH–O), 4.11 (m, 4H; acetal), 3.93 ppm (m, 4H; acetal).
5,50-Dibromo-3,30-diformylacetal-2,20-bithiophene (16). NBS
(3.57 g, 20.0 mmol) was added portionwise to a stirred solution
of 3,30-diformylacetal-2,20-bithiophene (3.11 g, 10.0 mmol) in
freshly distilled DMF (15 mL) at room temperature in the dark.
The suspension was stirred for 5 hours until the reaction was
complete, poured onto ice and extracted with diethyl ether. The
combined organic layers were dried over Na2SO4, filtered and
the solvent was removed under reduced pressure. The raw
product was purified using flash chromatography (silica gel,
8 : 2 petroleum ether : ethyl acetate) to afford 2.30 g (48% yield)
of the desired product as a pale yellow crystalline solid.
1
0
H-NMR (400 MHz, CDCl3): d = 7.16 (s, 2H; Th-H4,H4 ), 5.66
(s, 2H; O–CH–O), 4.089 (m, 4H; acetal), 3.94 ppm (m, 4H; acetal).
1.50 g of a yellow powder was also obtained, which corre-
Experimental section
Synthesis
Unless otherwise specified, all reagents, catalysts, spectroscopic- sponds to the asymmetrical deprotected product, 5,50-dibromo-
grade and reagent-grade solvents were commercial (Sigma Aldrich). 30-(1,3-dioxolan-2-yl)-[2,20-bithiophene]-3-carbaldehyde (17). 1H-
All reactions of air- and water-sensitive reagents and intermediates NMR (400 MHz, CDCl3): d = 9.70 (s, 1H; CHO), 7.49 (s, 1H; Th-H),
were carried out in dried glassware and an argon atmosphere. 7.23 (s, 1H; Th-H), 5.61 (s, 1H; O–CH–O), 4.06 (m, 2H; acetal),
Solvents were previously dried by conventional methods and stored 3.94 ppm (m, 2H; acetal).
under argon. Air- and water-sensitive solutions were transferred
with hypodermic syringes or via a cannula.
5,50-Dibromo-3,30-diformyl-2,20-bithiophene (18)
Method A. A solution of 5,50-dibromo-3,30-diformylacetal-
3-Thiophenecarboxaldehyde acetal (14). To a solution of 2,20-bithiophene (2.25 g, 4.8 mmol) in 120 mL acetone containing
3-thiophenecarboxaldehyde (13) (10.0 g, 89.17 mmol) in benzene pyridinium tosylate (PPTS) (1.05 g) was refluxed until the reaction
(100 mL) were added 1,2-ethanediol (20 mL) and pyridinium was complete (usually 2 days, thin layer test). The solvent was then
p-toluenesulfonate (4.1 g). The resulting mixture was refluxed removed under reduced pressure and the residue was dissolved in
with water separation by a Dean–Stark trap until the starting CH2Cl2 and washed with sodium hydrogen carbonate solution and
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saturated sodium chloride solution. The organic phase was dried (m, 4H; acetal), 1.42 ppm (s, 36H; t-Bu); HRMS (ESI): m/z calcd
and evaporated to give the title product in quantitative yield.
for C42H52O6S2 + H+: 717 [M + H+], found: 717.3 (M+).
5,50-Bis(3,5-di-tert-butyl-4-hydroxyphenyl)-[2,20-bithiophene]-
Method B. A solution of 5,50-dibromo-30-(1,3-dioxolan-2-yl)- 3,30-dicarbaldehyde (ACO)
[2,20-bithiophene]-3-carbaldehyde (2.00 g, 4.72 mmol) in 100 mL
Method A. To a solution of 5,50-dibromo-3,30-diformyl-2,20-
acetone containing pyridinium tosylate (PPTS) (600 mg) was refluxed bithiophene (18) (500 mg, 1.32 mmol) and Pd(PPh3)4 (151 mg,
until the reaction was complete (usually 2 days, thin layer test). 0.13 mmol) in previously degassed DME (2 : 1 with respect to
The solvent was then removed under reduced pressure and the water), tris[3,5-di-tert-butyl-4-(trimethylsilyloxy)phenyl] boroxine
residue was dissolved in CH2Cl2 and washed with sodium (5) (1.2 mg, 1.32 mmol) and Na2CO3 (1.25 g, 11.8 mmol) in
hydrogen carbonate solution and saturated sodium chloride aqueous solution (1 M) were rapidly mixed. The resulting
solution. The organic phase was dried and evaporated to give mixture was refluxed for 18 hours. After cooling to room tem-
the title product in quantitative yield.
perature, it was poured into water and extracted with diethyl
1H-NMR (400 MHz, CDCl3): d = 9.75 (s, 2H; CHO), 7.58 ppm ether. The organic layers were combined, dried over Na2SO4 and
0
(s, 2H; Th-H4,H4 ).
filtered. By solvent removal, a dark yellow powder was obtained.
4,40-(3,30-Di(1,3-dioxolan-2-yl)-[2,20-bithiophene]-5,50-diyl)-bis- The crude product was washed with hexane and then with
(2,6-di-tert-butylphenol) (AXX). To a solution of 5,50-dibromo- ethanol to yield 549 mg (87% yield) of the desired product as a
3,30-diformylacetal-2,20-bithiophene (650 mg, 1.38 mmol) and light yellow powder.
Pd(PPh3)4 (160 mg, 0.14 mmol) in previously degassed DME (2: 1
with respect to water), tris[3,5-di-tert-butyl-4-(trimethylsilyloxy)-
Method B. A solution of 4,40-(3,30-di(1,3-dioxolan-2-yl)-[2,20-
phenyl]boroxine (5) (1.27 mg, 1.38 mmol) and Na2CO3 (1.31 g, bithiophene]-5,50-diyl)-bis(2,6-di-tert-butylphenol) (AXX) (150 mg,
12.4 mmol) in aqueous solution (1 M) were rapidly added. The 0.208 mmol) in 10 mL acetone containing pyridinium tosylate
resulting mixture was refluxed for 19 hours. Then it was allowed (PPTS) (133 mg) was refluxed for 16 hours. The solvent was then
to cool to room temperature, poured into water and extracted removed under reduced pressure and the residue was dissolved
with diethyl ether. The organic layers were combined, dried over in DCM and washed with sodium hydrogen carbonate solution
Na2SO4 and filtered. After solvent removal, a yellow powder was and saturated sodium chloride solution. The organic phase
obtained. The crude product was first washed with hot ethanol was dried and solvent-evaporated to give the title product in
and then with ethyl acetate to yield 956 mg (96% yield) of the quantitative yield as a light yellow solid.
desired product as a bright yellow powder.
1H-NMR (400 MHz, CDCl3): d = 9.93 (s, 2H; CHO), 7.66
1H-NMR (400 MHz, CDCl3): d = 7.40 (s, 4H; Ph-H), 7.25 (s, 2H; Th-H4,H4 ), 7.44 (s, 4H; Ph-H), 5.77 (s, 2H; O–CH–O),
0
0
(s, 2H; Th-H4,H4 ), 5.77 (s, 2H; O–CH–O), 5.29 (s, 2H; –OH), 4.16 5.43 (s, 2H; –OH), 1.49 ppm (s, 36H; t-Bu); HRMS (ESI): m/z
(m, 4H; acetal), 3.96 (m, 4H; acetal), 1.48 ppm (s, 36H; t-Bu); calcd for C38H46O4S2 + H+: 631 [M + H+], found: 629.4 (M+).
HRMS (ESI): m/z calcd for C42H54O6S2 + H+: 741 [M + Na+],
found: 741.3 (M + Na+).
5,50-Bis(3,5-di-tert-butyl-4-oxocyclohexa-2,5-dien-1-ylidene)-
5H,50H-[2,20-bithiophenylidene]-3,30-dicarboxylic acid (QCO)
4,40-(3,30-Di(1,3-dioxolan-2-yl)-5H,50H-[2,20-bithiophenylidene]-
5,50-diylidene)-bis(2,6-di-tert-butylcyclohexa-2,5-dienone) (QXX)
Method A. To
hydroxyphenyl)-[2,20-bithiophene]-3,30-dicarbaldehyde (ACO)
a
solution of 5,50-bis(3,5-di-tert-butyl-4-
Method A. To a solution of 4,40-(3,30-di(1,3-dioxolan-2-yl)- (33 mg, 0.052 mmol) in CH2Cl2 (24 mL), K3Fe(CN)6 (172 mg,
[2,20-bithiophene]-5,50-diyl)-bis(2,6-di-tert-butylphenol) (AXX) (70 mg, 0.52 mmol) and a 0.4 M aqueous solution of KOH (293 mg,
0.097 mmol) in CH2Cl2 (24 mL), K3Fe(CN)6 (320 mg, 0.97 mmol) and 5.23 mmol) were added. The resulting mixture was stirred until
a 0.4 M aqueous solution of KOH (546 mg, 9.74 mmol) were added. the starting material was completely reacted (E 2 hours). The
The resulting mixture was stirred until the starting material was reaction mixture was extracted with methylene chloride and
completely reacted (E 2 hours). The reaction mixture was extracted washed with water and the organic solvent was removed under
with methylene chloride and washed with water and the organic reduced pressure. The raw product was washed with toluene to
solvent was removed under reduced pressure. The raw product was afford 26 mg (84% yield) of QCO as an iridescent dark green/
washed with toluene to afford 61 mg (87% yield) of QXX as an red solid.
iridescent green solid.
Method B. To a solution of 5,50-bis(3,5-di-tert-butyl-4-hydroxy-
Method B. To a solution of 4,40-(3,30-di(1,3-dioxolan-2-yl)- phenyl)-[2,20-bithiophene]-3,30-dicarbaldehyde (ACO) (30 mg,
[2,20-bithiophene]-5,50-diyl)-bis(2,6-di-tert-butylphenol) (AXX) (20 mg, 0.0475 mmol) in CH2Cl2 (10 mL), MnO2 (30 mg) was added. The
0.028 mmol) in CH2Cl2 (8 mL), MnO2 (17 mg, 0.190 mmol) was resulting mixture was stirred until the starting material was
added. The resulting mixture was stirred until the starting material completely reacted (E 2 hours 30 minutes). The reaction mixture
was completely reacted (E 2 hours 40 minutes). The reaction was filtered and the organic solvent was removed under reduced
mixture was filtered and the organic solvent was removed under pressure. The desired product was collected as an iridescent dark
reduced pressure. The desired product was collected as an iridescent green/red solid in quantitative yield.
green solid in quantitative yield.
1H-NMR (400 MHz, CDCl3): d = 9.5–7.3 (br. m), 1.48 ppm (s;
1H NMR0(400 MHz; CDCl3): d = 7.41 (br. s; Ph-H), 7.31 (br. t-Bu). HRMS (ESI): m/z calcd for C38H44O6S2 + H+: 661 [M + H+],
s; Th-H4,H4 ), 5.72 (s, 2H; O–CH–O), 4.25 (m, 4H; acetal), 4.15 found: 661.7 (M+).
10434 | Phys. Chem. Chem. Phys., 2015, 17, 10426--10437
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4-Bromo-2,6-dimethoxyphenol (20). 2,6-Dimethoxyphenol
PCCP
1H-NMR (400 MHz, CDCl3): d = 7.13 (d, J(H,H) = 3.71 Hz,
3
(1.0 g, 6.5 mmol) was dissolved in CHCl3 (10 mL). EtOH 2H; thiophene), 7.12 (d, 3J(H,H) = 3.71 Hz, 2H; thiophene), 6.82
(0.08 mL) and 50% NaH solution (3 mg, 0.13 mmol) were (s, 4H; phenyl-H), 5.55 (s, 2H; –OH), 3.96 ppm (s, 12H; –OCH3);
added and the mixture was cooled to ꢁ78 1C. NBS (1.15 g, HRMS (ESI): m/z calcd for C24H22O6S2 + Na+: 493 [M + Na+],
6.48 mmol) was added portionwise and the mixture stirred at found: 493.4 (M + Na+).
this temperature for 1 hour. Then the cooling bath was
The monosubstituted product, 4-([2,20-bithiophen]-5-yl)-2,6-
removed and the mixture stirred at room temperature for dimethoxyphenol (22), was obtained as the main product in
30 min and at 65 1C for 5 min. The reaction mixture was allowed Method C ( yield 46%), but as a by-product in Methods A and B:
to cool to room temperature and the solvent was removed under 1H-NMR (400 MHz, CDCl3): d = 7.61 (br. d, 2H; Th-H), 7.39
reduced pressure. The yellowish residue obtained was dissolved (br. m, 2H; Th-H), 7.14 (br. m, 3H; Th-H), 6.83 (s, 2H; Ph-H),
in diethyl ether and filtered. The solvent was evaporated to 5.55 (s, 1H; –OH), 3.96 ppm (s, 6H; –OCH3).
obtain a yellow solid which was stirred in heptane (12 mL) at
5,50-Bis(3,5-dimethoxy-4-oxo-2,5-cyclohexadien-1-ylidene)-5,50-
85 1C for 15 min. The yellowish-brown oil was filtered through a dihydro-2,20-bithiophene (QMeOMeO). To a solution of AMeO-
celite pad. The filtrate was allowed to recrystallize to give 1.0 g of MeO (5 mg, 0.012 mmol) in benzene (22 mL), K3Fe(CN)6 (40 mg,
the desired product as a white solid (66% yield).
0.11 mmol) and a 0.1 M aqueous solution of KOH (67 mg,
1H-NMR (400 MHz, CDCl3): d = 6.73 (s, 2H; Ph-H), 5.41 1.1 mmol) were added. The resulting mixture was stirred until
(s, 1H; –OH), 3.88 ppm (s, 6H; –OCH3).
4,40-([2,20-Bithiophene]-5,50-diyl)-bis(2,6-dimethoxyphenol)
(AMeOMeO)
the starting material was completely reacted (E 45 minutes). The
reaction mixture was filtered and the solid was washed with
toluene. The desired product was collected as a dark blue solid in
Method A. A solution of 4-bromo-2,6-dimethoxyphenol (20) quantitative yield.
(52 mg, 0.224 mmol), 5,50-bis(trimethylstannyl)-2,20-bithiophene
HRMS (ESI): m/z calcd for C24H20O6S2 + H+: 469 [M + H+],
(50 mg, 0.102 mmol), and Pd(PPh3)4 (12 mg, 0.01 mmol) in found: 469.4 (M+).
anhydrous toluene was thoroughly mixed in a process vial. The
4,40-([2,20-Bithiophene]-5,50-diyl)-bis(2-(trifluoromethyl)phenol)
vial was capped and the mixture was heated under microwave (ACF3CF3). To a solution of 4-bromo-2-(trifluoromethyl)phenol
irradiation conditions at 150 1C and 150 W for 53 minutes, then (23) (338 mg, 1.403 mmol) in anhydrous toluene (50 mL),
the process vial was allowed to cool to room temperature. The Pd(PPh3)4 (80 mg, 0.0609 mmol) and 5,50-bis(trimethylstannyl)-
reaction mixture was extracted with ethyl acetate and the organic 2,20-bithiophene (24) (300 mg, 0.6099 mmol) were rapidly mixed.
fractions were combined, dried over Na2SO4 and filtered. After The resulting mixture was refluxed for 14 hours. Then it was allowed
solvent removal, the raw product was purified using flash to cool to room temperature, poured into water and extracted with
chromatography (silica gel, 7 : 3 hexane: ethyl acetate) to afford ethyl acetate. The organic fractions were combined together, dried
15 mg (32% yield) of AMeOMeO as a yellow solid.
over Na2SO4, filtered, and then the solvent was removed under
reduced pressure. The raw product was purified using flash
Method B. A solution of 4-bromo-2,6-dimethoxyphenol (325 mg, chromatography (silica gel, 1 : 1 hexane: dichloromethane) to
1.40 mmol), Pd(PPh3)4 (70 mg, 0.061 mmol) and 5,50-bis(trimethyl- afford 160 mg (54% yield) of ACF3CF3 as a dark yellow solid.
4
stannyl)-2,20-bithiophene (300 mg, 0.61 mmol) in anhydrous toluene
1H-NMR (400 MHz, CDCl3): d = 7.72 (d, J(H,H) = 1.76 Hz,
3
4
(52 mL) was refluxed for 5 hours. Then it was allowed to cool to room 2H; phenyl-H), 7.63 (dd, J(H,H) = 8.59 Hz, J(H,H) = 1.76 Hz,
temperature, poured into water and extracted with ethyl acetate. The 1H; phenyl-H), 7.15 (s, 4H; thiophene), 7.00 (d, 3J(H,H) =
organic fractions were combined together, dried over Na2SO4, 8.39 Hz, 1H; phenyl-H), 6.82 (s, 2H; phenyl-H), 5.55 (s, 1H; –OH),
filtered, and then the solvent was removed under reduced pressure. 5.52 (s, 1H; –OH), 3.96 ppm (s, 6H; –OCH3);
The raw product was purified using flash chromatography (silica gel,
HRMS (ESI): m/z calcd for C22H12F6O2S2 ꢁ H+: 485 [M ꢁ H+],
1 : 1 hexane : ethyl acetate) to afford 88 mg (30% yield) of AMeOMeO found: 485.3 (M ꢁ H+).
as a dark yellow solid.
5,50-Bis(3-trifluoromethyl-4-oxo-2,5-cyclohexadien-1-ylidene)-
5,50-dihydro-2,20-bithiophene (QCF3CF3). To
a solution of
Method C. A solution of 4-bromo-2,6-dimethoxyphenol (20) ACF3CF3 (4 mg, 0.0082 mmol) in benzene (4.1 mL), K3Fe(CN)6
(50 mg, 0.215 mmol) and 5,50-bis(4,4,5,5-tetramethyl-1,3,2- (27 mg, 0.082 mmol) and a 0.4 M aqueous solution of KOH
dioxaborolan-2-yl)-2,20-bithiophene (41 mg, 0.09 mmol), Na2CO3 (46 mg, 0.82 mmol) were added. The resulting mixture was
(86 mg, 0.81 mmol) aqueous solution (1 M) and Pd(PPh3)4 (10 mg, stirred until the starting material was completely reacted
0.009 mmol) in previously degassed DME (2 : 1 with respect to water) (E 1 hour). The reaction mixture was filtered and the solid
were thoroughly mixed in a process vial. The vial was capped and the was washed with toluene. The desired product was collected as
mixture was heated under microwave irradiation conditions at a dark green solid in quantitative yield.
130 1C and 150 W for 42 min, then the process vial was allowed
to cool to room temperature. The reaction mixture was extracted found: 485.3 (M+).
HRMS (ESI): m/z calcd for C22H10F6O2S2 + H+: 485 [M + H+],
with ethyl acetate and the organic fractions were combined, dried
4-(50-(4-Hydroxy-3-(trifluoromethyl)phenyl)-[2,20-bithiophen]-
over Na2SO4 and filtered. After solvent removal, the raw product was 5-yl)-2,6-dimethoxyphenol (AMeOCF3). A solution of 4-bromo-2-
purified using flash chromatography (silica gel, 7 : 3 hexane : ethyl (trifluoromethyl)phenol (23) (252 mg, 1.05 mmol), 4-bromo-2,6-
acetate) to afford 3 mg (7% yield) of AMeOMeO as a yellow solid.
dimethoxyphenol (20) (244 mg, 1.05 mmol), Pd(PPh3)4 (97 mg,
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0.105 mmol) and 5,50-bis(trimethylstannyl)-2,20-bithiophene (24) Notes and references
(415 mg, 0.844 mmol) in anhydrous toluene (60 mL) was
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refluxed for 5 hours. Then it was allowed to cool to room
temperature, poured into water and extracted with ethyl acetate.
The organic fractions were combined together, dried over
Na2SO4, filtered, and then the solvent was removed under
reduced pressure. The raw product was purified using flash
chromatography (silica gel, 1 : 1 hexane: ethyl acetate) to afford
140 mg (34% yield) of AMeOCF3 as a dark yellow solid. 4,40-([2,20-
bithiophene]-5,50-diyl)-bis(2-(trifluoromethyl)phenol) (ACF3CF3)
and 4,40-([2,20-bithiophene]-5,50-diyl)-bis(2,6-dimethoxyphenol)
(AMeOMeO) were obtained as by-products, respectively, in
9.5% (39 mg) and 4% (16 mg) yields.
4
1H-NMR (400 MHz, CDCl3): d = 7.72 (d, J(H,H) = 1.37 Hz,
3
4
1H; Ph-H), 7.64 (dd, J(H,H) = 8.59 Hz, J(H,H) = 1.95 Hz, 1H;
Ph-H), 7.14 (m, 4H; Th-H), 7.00 (d, 3J(H,H) = 8.59 Hz, 1H; Ph-H),
6.82 (s, 2H; Ph-H), 5.55 (s, 1H; –OH), 5.52 (s, 1H; –OH), 3.96
ppm (s, 6H; –OCH3); HRMS (ESI): m/z calcd for C23H17F3O4S2 ꢁ
H+: 477 [M ꢁ H+], found: 477.4 (M ꢁ H+).
2,6-Dimethoxy-4-(50-(4-oxo-3-(trifluoromethyl)cyclohexa-2,5-
dien-1-ylidene)-[2,20-bithiophenylidene]-5-ylidene)cyclohexa-2,5-
dienone (QMeOCF3). To a solution of AMeOCF3 (5 mg,
0.0105 mmol) in benzene (21 mL), K3Fe(CN)6 (35 mg, 0.105 mmol)
and a 0.1 M aqueous solution of KOH (59 mg, 1.05 mmol)
were added. The resulting mixture was stirred until the starting
material was completely reacted (E 1 hour 30 minutes). The
reaction mixture was filtered and the solid was washed with
toluene. The desired product was collected as a dark green solid
in quantitative yield.
5 A. E. Chichibabin, A. E. Chichibabin, Chem. Ber., 1907, 40, 1810.
6 L. K. Montgomery, J. C. Huffman, E. A. Jurczak and M. P.
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´
´
7 R. Ponce Ortiz, J. Casado, S. Rodrıguez Gonzalez,
HRMS (ESI): m/z calcd for C23H15F3O4S2 + H+: 477 [M + H+],
found: 477.3 (M+).
´
´
´
V. Hernandez, J. T. Lopez Navarrete, P. M. Viruela, E. Ortı,
K. Takimiya and T. Otsubo, Quinoidal oligothiophenes:
towards biradical ground-state species, Chem. – Eur. J.,
2010, 16(2), 470–484.
Measurements and characterization
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1H-NMR spectra were collected using a Bruker ARX 400 NMR
spectrometer. Mass spectrometry was carried out with a Bruker
Esquire 3000 Plus mass spectrometer. UV-vis absorption spec-
tra were recorded with a Cary 5000 spectrophotometer (Varian).
Raman spectra were recorded with a Nicolet FT-Raman Nexus
NXR 9650 that employs a Nd/YAG laser with excitation at l =
1064 nm and a Horiba Jobin Yvon LabRam HR800 with excita-
tion light at 632 nm. The X-ray data were collected on a Bruker
X8 Prospector APEX-II/CCD diffractometer using Cu Ka radia-
tion (l = 1.54050 Å). The structure was solved by direct methods
and refined using the SHELX-97 package.23
11 M. Kozaki, A. Isoyama, K. Akita and K. Okada, Preparation,
properties, and reduction of heteroaromatic quinoids
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X-ray crystallographic analysis
CCDC-1029034.
12 D. Fazzi, E. V Canesi, F. Negri, C. Bertarelli and C. Castiglioni,
Biradicaloid character of thiophene-based heteropheno-
quinones: the role of electron–phonon coupling, Chem-
PhysChem, 2010, 11(17), 3685–3695.
Acknowledgements
This work was partly supported by Fondazione Cariplo through
the project INDIXI (Grant no. ref. 2011/0368). E.P. thanks the
European Union for financial support (‘‘Marie Curie’’ FP7 IRG
grant no. 268231).
13 E.
V Canesi, D. Fazzi, L. Colella, C. Bertarelli and
C. Castiglioni, Tuning the quinoid versus biradicaloid char-
acter of thiophene-based heteroquaterphenoquinones by
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