Synthesis of benzothiadiazole-based molecules: Via direct arylation: An eco-friendly way of obtaining small semi-conducting organic molecules

Article abstract of DOI:10.1039/c6nj00847j

New A-D-A-D-A unsophisticated small molecules based on benzothiadiazole and thiophene were designed and synthesized in a two-step pathway, involving the coupling of benzothiadiazole with thiophene-, or bithiophene-carboxaldehyde, followed by Knoevenagel condensation. The first step was specifically investigated in comparing different coupling methods and their related green metrics. It is established that the synthesis of benzothiadiazole-based small molecules via direct heteroarylation may be an environmentally attractive procedure. It generates a low amount of polluting waste in a cost-effective way in comparison to the classical Suzuki coupling. Moreover, the direct arylation using low loading of the palladium-catalyst without a ligand or an additive is the method of choice for the present case. The physico-chemical properties of the four target molecules were investigated using electronic absorption spectroscopy, differential scanning calorimetry, cyclic voltammetry, polarized optical microscopy and small angle X-ray scattering analysis. DFT studies have been performed to support the electronic characteristics.

Full text of DOI:10.1039/c6nj00847j

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Synthesis of Benzothiadiazoleꢀbased molecules via
direct arylation: an ecoꢀfriendly way of obtaining
small semiꢀconducting organic molecules
1
,2
1,2
1,2
3
Chunxiang Chen, Daniel Hernández Maldonado, Damien Le Borgne, Fabienne Alary,
4
5
5
1,2*
Barbara Lonetti, Benoît Heinrich, Bertrand Donnio, Kathleen I. Moineau-Chane Ching.
1
CNRS; LCC (Laboratoire de Chimie de Coordination) ; 205, route de Narbonne, Fꢀ31077
Toulouse, France.
2
Université de Toulouse; UPS, INP; LCC; Fꢀ31077 Toulouse, France.
3
CNRS, IRSAMC, Laboratoire de chimie et physique quantiques, 118 route de Narbonne, Fꢀ
31062 Toulouse, France.
4
Université de Toulouse; UPS/CNRS; IMRCP UMR 5623, 118 route de Narbonne, Fꢀ31062,
Toulouse Cedex 9, France.
5
Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), CNRS−Université de
Strasbourg, UMR 7504, 67034 Strasbourg cedex 2, France.
KEYWORDS
Ecoꢀfriendly chemistry •· ligandꢀfree direct arylation •·Synthetic methods •·organic electronics
1

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ABSTRACT
New AꢀDꢀAꢀDꢀA unsophisticated small molecules based on benzothiadiazole and thiophene
were designed and synthesized in a twoꢀstep pathway, involving the coupling benzothiadiazole
with thiopheneꢀ, or bithiopheneꢀcarboxaldehyde, followed by Knoevenagel condensation. The
first step was specifically investigated in comparing different coupling methods and their related
green metrics. It is established that the synthesis of benzothiadiazoleꢀbased small molecules via
direct heteroarylation may be an environmentally attractive procedure. It generates low polluting
wastes in a costꢀeffective way in comparison to classical Suzuki coupling. Moreover, the direct
arylation using low loading of palladiumꢀcatalyst whithout ligand nor additive is the method of
choice for the present case. The physicoꢀchemical properties of the four target molecules were
investigated using electronic absorption spectroscopy, differential scanning calorimetry, cyclic
voltammetry, polarized optical miscroscopy and small angle Xꢀray analysis. DFT studies have
been performed to support the electronic characteristics.
INTRODUCTION
In the search for new and efficient materials for organic electronics, many small molecules
1
have been designed and synthesized in the past few years. Nowadays, in the field of organic
2
photovoltaics, their efficiency in term of photoconversion (DRCN5T:10.08 %) has become
3
,4
equivalent to that of polymers (PffBT4Tꢀ2OD:10.8 %) for single layer devices. Besides, their
synthetic reproducibility and the numerous attainable structures make them very attractive for
generating materials with easy energy level control, mobility, solid packing mode and
2

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1
,5
solubility. In particular the development of DonorꢀAcceptor (D–A) small molecule structures,
like it is the case for polymers, has been proved to be a good strategy for reaching efficient
narrow bandꢀgap semiꢀconducting materials. Various central electron donor cores (thiophene,
benzodithiophene (BDT), dithienosilole (DTS), dithienocyclopenta (DTC), indacenodithiophene
(
IDT), silafluorene (SFL), fluorene (FL), carbazole (CZ), etc.), electron acceptor cores (2,1,3ꢀ
benzothiadiazole (Bzt), diketopyrrolopyrrole (DPP), etc.), terminal units (dicyanovinyl, alkyl
cyanoacetate, rhodanine, and 1,3ꢀindanedione), as well as πꢀconjugated bridges made of
thiophene rings with different conjugation lengths have been used for the design of small
6
,7
functional.
From the synthetic point of view, the drawback of this DꢀA strategy is related to the coupling
methods required for connecting these cores, bridges, and terminal units. On the one hand, DꢀA
molecules are often obtained by units coupling via polluting synthetic procedures using
8
stannylated derivatives or via reactions involving diboronic acid pinacol esters which belong to
9
,10
a class of expensive and low yield isolated intermediates. On the other hand, direct arylation
proceeds via CꢀH bond activation without need of the preparation of organometallic
intermediates. This method was proposed two decades ago in pioneering works of Ohta et al. on
1
1
direct arylation of thiophenes with aryl halides. Recently, direct arylation has appeared as a
simple method to synthesize conjugated materials from simple building blocks for use in organic
1
2ꢀ16
electronics.
This method offers simpler, shorter and cheaper reaction pathways for
intermediates and final materials synthesis, and generates less toxic waste in comparison to the
classic aboveꢀmentioned crossꢀcoupling techniques.
The motivation of the present work is to demonstrate an unsophisticated and easy preparation
of organic semiꢀconducting small molecules with a structure based on alternating DꢀA
3

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fragments. Among D and A fragments, thiophene (D), benzothiadiazole (A) and cyanoacetate
1
7ꢀ19
(A) are simple units usually associated with good conversion yields.
These fragments are
aimed at being connected together by coupling reactions. In this context, we propose here to use
an environmentally attractive coupling procedure without polluting wastes in a costꢀeffective
way in comparison to the other existing procedures, demonstrating for the first time that ligandꢀ
less direct arylation may apply in an efficient way to the coupling of benzothiadiazole core to
thiophene units.
The basic idea of this study is to provide small, as simple as possible molecules obtained after
as few synthetic steps as possible, from commercially available raw materials. The need of two
steps is believed to be the minimum reachable path, leading to the fewest waste production and
lowest cost. 4,7ꢀDibromobenzoꢀ2,1,3ꢀthiadiazole (diBrBz) acts here as the electrophile on both
sides, in reaction with thiophene (or bithiophene) carboxaldehyde which is found here to be
exclusively 5ꢀarylated, thus providing an interesting DꢀAꢀD core for building semi conducting
small molecules. The presence of two aldehyde functions at both extremities of the DꢀAꢀD core
renders possible to elongate the molecule via a double Knoevenagel condensation leading to an
AꢀDꢀAꢀDꢀA molecule, which is suitable for presenting semiꢀconducting character associated
with low band gap properties.
Herein we report the preparation of new small semiꢀconducting molecules. These molecules
are respectively named as Bz(T CAEH) , Bz(T CAO) , Bz(T CAEH) and Bz(T CAO) (as
1
2
1
2
2
2
2
2
shown in Scheme 1). It is demonstrated that the direct arylation without ligand nor PivOH is the
method of choice for obtaining active molecules in acceptable costs and with excellent green
metrics. Due to their conjugated and protomesogenic molecular structures, the electronic
4

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properties as well as the thermal and selfꢀorganization behaviors of the four final compounds
were also investigated, and are reported here.
S
N
N
CN
CN
O
S
S
O
O
O
Bz(T CAEH)
1
2
S
N
N
CN
CN
O
S
S
O
O
O
Bz(T CAO)
1
2
S
N
N
O
O
S
S
S
S
O
O
CN
Bz(T CAEH)
CN
2
2
S
N
N
O
O
S
S
S
S
O
O
Bz(T CAO)
CN
CN
2
2
Scheme 1. Chemical structures of the target molecules
RESULTS AND DISCUSSION
Synthesis.
2
0ꢀ22
Among the available terminal electronꢀwithdrawing groups, alkyl cyanoacetate,
3ꢀ
2
3ꢀ25
26,27
alkylrhodanine,
and 1,3ꢀindandione
are easily introduced by Knoevenagel condensation.
Both octyl cyanoacetate and 2ꢀethylhexyl cyanoacetate were chosen as the terminal groups
5

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because they confer a good solubility of the final molecule, promote selfꢀorganization in
2
8
nanoagregate structures, and contribute to enhanced photovoltaic performances.
Bz(T CAEH) , Bz(T CAO) , Bz(T CAEH) and Bz(T CAO) are obtained in two steps. The
1
2
1
2
2
2
2
2
first step consists in synthesis of intermediates 5,5'ꢀ(2,1,3ꢀbenzothiadiazoleꢀ4,7ꢀdiyl)bisꢀ(2ꢀ
thiophenecarboxaldehyde) (noted as 5, Scheme 2) or 5',5'''ꢀ(2,1,3ꢀbenzothiadiazoleꢀ4,7ꢀdiyl)bisꢀ
[
(2,2'ꢀbithiophene)ꢀ5ꢀcarboxaldehyde] (noted as 7, Scheme 3) by coupling a benzothiadiazole
ring with a thiophene or bithiophene carboxaldehyde unit, respectively. The second step consists
in the Knoevenagel condensation of 5 or 7, respectively, with 2ꢀethylhexylꢀ or octylꢀ
cyanoacetate. It is showed here that the coupling method may have a dramatic impact on
environmental and economical parameters of the synthetic method.
Three synthetic protocols for obtaining the intermediate molecule have been compared as
shown in Schemes 2 and 3, respectively: i) the classical Suzuki coupling (thereafter referred to as
Route a) using a boronylated intermediate which is rather expensive and generates polluting
wastes, ii) a direct heteroarylation procedure (Route b) without organometallic nor boronylated
intermediate but using palladium catalyst, phosphine ligands, and pivalic acid (PivOH) in
combination with potassium carbonate (K CO ). This method becomes very commonly used
2
3
nowadays for polymers synthesis and is identified as an ecofriendly synthetic procedures
offering new opportunity for the production of green energy from more environmentally friendly
2
9ꢀ32
materials.
And finally, iii) a direct heteroarylation procedure (Route c) without ligand nor
PivOH, but only with palladium acetate (Pd(OAc) ) as the catalyst and potassium acetate
2
(KOAc) as the base. This ligandꢀfree catalysis has many advantages: properly used, the catalysis
3
3
may be faster, and due to reduced numbers of additives and reactants, the workꢀup is easier and
costs are reduced. The proposed mechanism of this procedure involves the decomposition at
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3
4
1
00°C of Pd(OAc) with formation of solventꢀstabilized Pd(0)ꢀcolloids in a polar solvent. This
2
is very close to what happens in the Heck reaction where Pd(OAc) can be used as catalyst on
2
3
5
33
aryl iodides without additional ligands. However, if the solvent is concentrated or if the
catalyst/substrate ratio is too high, the solventꢀstabilized Pd(0)ꢀcolloids show a tendency to
agglomerate and form non active black palladium powder. To our knowledge, the direct
arylation without ligand at low catalyst loading has been used only once for πꢀconjugated
polymers synthesis, in particular the polycondensation reaction of 3,3′,4,4′ꢀ
tetramethylbithiophene with 2,7ꢀdibromoꢀ9,9ꢀdioctylfluorene in the presence of 2 mol % of
3
6
Pd(OAc) . In this case however, 30 mol % of PivOH was used.
2
The detailed synthetic routes used for obtaining 5 and 7 are documented in the Experimental
1
6
section. Their structures were confirmed by H NMR in CDCl and DMSOꢀd (see Supporting
3
Information).
Scheme 2. Classical (top) and new (bottom) routes to 5
7

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The key intermediate compound 5 was first obtained via Route a, i.e. the usual Pdꢀcatalyzed
Suzuki coupling, using 2,1,3ꢀbenzothiadiazoleꢀ4,7ꢀbis(boronic acid pinacol ester) (1) and 5ꢀ
bromoꢀ2ꢀthiophene carboxaldehyde (2). The used procedure was adapted from published
protocols of synthesis of alkylated thiophene derivatives. According to these protocols, either
3
7
three equivalents of thiophene reactant were required, or a reaction time of 3 days at 80°C was
3
8
recommended. We found that 2.5 equivalents of 2 and a reaction time of 24 h were adequate to
yield the desired molecule in quantitative yields and we did not investigate this reaction further
to improve the yield. This method offers the advantage to be highly efficient and regioselective.
3
9
However, it involves reactants that are either expensive (1) or very reactive (2). Because of the
high reactivity of 2, we used it in excess to reach quantitative yield, since it also leads to nonꢀ
desired homocoupling byproducts (although easily eliminated during the work up by simply
washing with ethyl acetate).
Then Route b was explored, a greener and lowꢀcost method based on the direct arylation
40
reaction offering the advantage to avoid metallated intermediates. Marder and coꢀworkers
demonstrated that the CꢀH bond of the benzothiadiazole ring can be functionalized by aryl
bromides under palladium acetate (10 mol %) catalysis in presence of trialkyl phosphines, such
t
as P Bu Me·HBF as the ligand, pivalic acid as the proton shuttle and potassium carbonate as the
2
4
base in toluene at reflux. Inspired by this work, we adapted the reaction conditions in using 4,7ꢀ
dibromobenzoꢀ2,1,3ꢀthiadiazole (3, 1 eq) as the arylating reagent and 2ꢀthiophene
carboxaldehyde (4, 2.2 eq) as the CꢀH active component. Without any optimization study, the
desired product was directly isolated in 98% yield. This reaction proved to be regioꢀselective
since only the 5ꢀposition of thiophene reacted in the coupling process.
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To further extend the synthetic utility of direct arylation, we finally tested via Route c, a oneꢀ
pot unprecedented synthesis of 5 starting from the same reactants and using the same ratio of 4/3
2.2:1 as for Route b, but in N,Nꢀdimethylacetamide (DMA) as the solvent, at lower palladium
concentration, and without neither ligand nor pivalic acid. This “ligandꢀfree” strategy was first
4
1
proposed by Doucet and coꢀworkers who obtained high yields using only 0.1ꢀ0.001 mol %
catalyst. Such low quantities of catalyst are impossible to weigh accurately in our conditions due
to the too low quantities of reactants, so we employed slightly higher relative amounts of
palladium acetate. In presence of 0.25 and 0.5 mol % catalyst and in presence of 2 equivalents of
potassium acetate relative to 3, the desired product was synthesized in 73% and 71% yield,
respectively. Note that once again, the workꢀup of the reaction is relatively easy, as it consists
only of filtration, then washing with water, ethyl acetate, and petroleum ether. It is worth noting
that only desirable coupling products resulting from CꢀH cleavage on the 5ꢀposition of 2ꢀ
thiophene carboxaldehyde was found. Moreover, no arylation of the acetyl function was
2
6,27
observed as this was the case for the reaction of 4 with other arylbromides.
Only
monoarylated product and homocoupling product of thiophene carboxaldehyde were detected as
side products (see Fig. S3 and Fig. S4 in ESI). In order to check the stability of the reactants
under the reaction conditions, both starting materials 3 and 4 were engaged separately in the
direct arylation conditions via Route c without any other reactant. After 24h at 150°C, each
batch was treated as previously described. Analyses of the batch conducted with 3 revealed the
absence of homocoupling byproducts and any other degradation processes, and the total recovery
of 3. It demonstrates the remarquable stability of diꢀbromobenzothiadiazole that does not
undergo undesirable reaction. In contrast, the batch with 4 resulted only in black degradation
products: thiophene carboxaldehyde is indeed unstable under direct arylation conditions and
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must be employed in such conditions that it reacts quickly and completely. This is the reason
why we tested the reaction of direct arylation of 3 with 4 in default in Route c (see Table S1).
When 4 is used in 0.9 eq and even down to 0.5 eq with regard to 3, then the yield (calculated
with respect to the limiting reactant) is quantitative, and no degradation product is produced.
Noting that the purification of the molecule is very easy, driven by simple washing with water to
eliminate KOAc and KBr, ethyl acetate to eliminate Pd(OAc) , and finally petroleum ether to
2
help in eliminating AcOEt and remaining DMA. The ease of purification is undoubtedly linked
to the cleanliness of the reaction that does not generate side products.
The two routes used for obtaining the elongated intermediate 7, documented in the
Experimental section, are summarized in Scheme 3.
Suzuki coupling
S
N
N
O
O
O
O
S
Br
B
B
CHO
S
1
6
S
N
N
S
S
S
S
Direct Heteroarylation
OHC
CHO
7
S
N
N
S
CHO
Br
Br
S
3
8
Scheme 3. Classical (top) and new (bottom) routes to e 7
As described above for the synthesis of 5, 7 was obtained in quantitative yield by Suzuki
coupling (Route a), using the same synthetic procedure. Route c was then directly tested by
reacting 8 and 3 in the ratio of 2.2:1 as for 5. In this case, 7 was obtained with an excellent yield
10

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of 92% which was not further investigated. The improved yield is attributed to the greater
reactivity of 8 towards the coupling compared to that of 4 and to the lower solubility of 7 with
respect to that of 5.
The Knoevenagel condensation is driven in a classical manner. Most of organic molecules for
organic electronics are purified by means of column chromatography. Here, it is noteworthy that
the target molecules are purified by simple recrystallization in ethyl acetate, reducing the waste
problem originated from the synthesis procedure.
All the target molecules were fully characterized and their structures are confirmed by
different analytic tools (elemental analysis, NMR, MS).
Environmental and economic aspects.
Among the aspects of importance for organic electronics (efficiency, stability, printability),
materials for largeꢀscale production should be as inexpensive as possible. For example, in the
field of organic photovoltaics (OPV), the cost trends of 20 €/g for the upscaling scenario and 10
4
2
€
/g for the industrial scenario have been recently suggested. The materials synthesis must be
realized from commercially available chemicals in few steps and with straightforward
4
3
purification procedures. Overall yields and Eꢀfactor (defined by the ratio of the mass of waste
per unit mass of product) are quantifiable green metrics used for evaluating the environmental
4
4,45
impact associated with the production of organic materials for sustainable applications.
These
metrics for the molecules elaborated in this work have been evaluated (see Table 1 and ESI for
details).
The three synthesis protocols described above (Scheme 1) have been compared for the
synthesis of 5. As can be seen in Table 1, the calculated Eꢀfactor and price of 5 (see ESI) show
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dramatic change from one route to another. Surprisingly, the Eꢀfactor calculation for Route b
affords a value very close to that calculated for Route a. This is due to the consequent amount of
t
catalyst (10 mol % Pd(OAc) ) and ligand (20 mol % P Bu Me.HBF ) used for the direct arylation
2
2
4
which is revealed here to be less ecoꢀfriendly than the Suzuki coupling. On the other hand,
Suzuki coupling involves expensive raw materials which increase the cost of 5. A consequent
improvement of the green metrics is obtained when direct arylation without ligand (Route c) is
carried out. This process involves a low amount of catalyst (0.5 mol % palladium acetate) and
the absence of any additive, resulting in a very simple protocol with an Eꢀfactor of 143g/g, which
is approximately one third of that obtained from the two other routes. Moreover, the cost of 5
becomes nearly one twelfth and one third of that resulted from Route a and Route b,
respectively. This represents a great improvement in term of waste production and cost for the
synthesis of the same intermediate.
Table 1 Reaction yields, Eꢀfactors and costs calculated for 5 obtained via the three investigated
routes
Route a
100%
441
Route b
98%
Route c
71%
143
Yield (%)
Eꢀfactor (g/g)
Cost (€/g)
495
176
38.5
14
We then examined if it was possible to improve the Route c metrics by increasing the yield of
the reaction. For achieving better yields and decreasing the amount of degradation products
associated with side reactions experienced by 4, the molecular ratio between 4 and 3 was
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lowered to 0.9, then 0.5 (instead of 2.2). The obtained values are gathered in Table 2. The
reaction yield is improved, but with much worse green metrics. This is due to the fact that the
nonꢀreacted 3 (when used in excess) is accounted as a waste, thus substantially contributes to the
increase of the calculated value of the Eꢀfactor. Due to the fact that 3 does not react by
homocoupling reaction nor decomposes through any other degradation processes under these
reaction conditions as demonstrated earlier, it may possible to recycle it. Thus, with optimized
synthesis conditions, including the recycling of 3, it is feasible to synthesize 5 in 71% yield and
more with an Eꢀfactor approaching 143 g/g and a cost of around 14 €/g as reported in last column
of Table 1.
Table 2 Reaction yields, Eꢀfactors and costs calculated for the synthesis of 5 obtained via Route
c with decreasing the 4/3 ratios (r)
r = 2.2
71%
143
r = 0.9
100%
226
r = 0.5
100%
408
Yield (%)
Eꢀfactor (g/g)
Cost (€/g)
14
22
39
For the synthesis of 7, the calculated green metrics for Route a and Route c confirmed the
previous trends (see Table 3): the Eꢀfactor calculation for Route c returned a value of 76 g/g,
which was around oneꢀsixth of that found for Route a, and the cost was reduced by a factor 5
between Route a and Route c. It is worth noting here that for Route c, the molecular ratio 4/3
ratios (r) was 2.2.
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Table 3 Reaction yields, Eꢀfactors and costs calculated for 7 obtained via Route a and Route c.
Route a
100%
497
Route c
91%
76
Yield (%)
Eꢀfactor (g/g)
Cost (€/g)
201
40
Finally, the four target molecules, Bz(T CAEH) , Bz(T CAO) , Bz(T CAEH) and
1
2
1
2
2
2
Bz(T CAO) , were obtained via Route c followed by Knoenenagel condensation. The overall
2
2
green metrics are presented in Table 4. In a general manner, the Eꢀfactor is twice higher for
molecules bearing one thiophene ring at each side of benzothiadiazole, than for those bearing
bithiophene units. This is directly linked to the Eꢀfactors for intermediate synthesis, of 143 and
76 g/g for compounds 5 and 7, respectively, and to the best Knoevenagel reaction yields obtained
with 7.
Table 4 Reaction yields for Knoevenagel condensation, and total Eꢀfactors and costs calculated
for the target molecules obtained via Route c followed by Knoevenagel condensation.
Bz(T CAO)2
Bz(T CAEH)
Bz(T CAO)
Bz(T CAEH)
1
1
2
2
2
2
2
Knoevenagel
condensation yield
51%
47%
76%
68%
Total Eꢀfactor (g/g)
Total cost (€/g)
230
45
380
37
144
50
162
47
The estimated cost per gram is constant from one molecule to another, of around 45 €/g, which
is acceptable for a fine chemical with a potential large scale application. It is worth noting that
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these numbers are maximum values deduced from (nonꢀnegociated) prices of commercial raw
materials, always more expensive at laboratory scale than for upscaling or industrial purpose.
The good figures of merit obtained for total Eꢀfactors as well as for the costs are undoubtedly
linked to the fact that the target molecules are obtained in only two steps, which do not require
column chromatography technics for their purification. The simple purification technics used in
this work, i.e. washing or recristallization, are useful tools for green methods.
Electronic properties.
The UVꢀvis absorbance and fluorescence in CH Cl solution were recorded at room
2
2
temperature (see Fig. 1 and Table 5). It is noticeable that the nature of the alkyl chain (R = octyl
or 2ꢀethylhexyl) does not influence the absorption spectrum. The large absorption band that
peaks at 497 nm and 522 nm for Bz(T CAR) and Bz(T CAR) , respectively, can be attributed
1
2
2
2
46,47
to the intramolecular charge transfer (ICT) transition.
It should be noted that the increase of
the number of thiophene units from one to two at both sides of the benzozothiadiazole induces a
red shift of 25 nm, as a result of the extent of the conjugation. The observed Stokes shifts for
ꢀ1
compounds Bz(T CAR) and Bz(T CAR) are of 2500 to 3000 cm .
1
2
2
2
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ꢀ6
ꢀ1
Fig. 1 normalised absorption and emission spectra of the four molecules (ca 1.6 10 mol.L )
under study in dichloromethane solution.
Table 5 Optical properties of the target molecules in dichloromethane solution.
opt
λ
ε
λ
onset
(nm)
Eg
λ
em
(nm)
Stokes shift
b
max
nm)
Φ
ꢀ1
ꢀ1
a
ꢀ1
(
(M cm )
51000
55000
53440
53440
(eV)
(cm )
Bz(T CAO)
498
497
522
525
575
575
635
2.15
2.15
1.95
1.95
570
570
620
620
2536
2577
3028
2919
0.24
0.24
0.27
0.27
1
2
Bz(T CAEH)
1
2
Bz(T CAO)
2
2
Bz(T CAEH)
635
2
2
a
b
Estimated from the onset absorption. Rhodamine B in EtOH as standard (Φ = 0.70 at
4
8
2
0°C)
The electrochemical behaviour of the target compounds was examined by cyclic voltammetry
CV) in dichloromethane solution (see Fig. 2). The four molecules undergo reversible reduction
(
and oxidation events in solution. This demonstrates that the reduction and oxidation species are
electrochemically stable, a positive feature for their potential operational stability in OPV
devices.
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Fig. 2 Cyclic voltammograms of Bz(T CAO) (left) and Bz(T CAO) (right) in 0,1 M PF TBA
1
2
2
1
2
6
ꢀ
dichloromethane solution; Pt electrode; scan rate = 200 mV.s
As indicated in Table 6, their electrochemical gaps are in good agreement with the optical
gaps. The energy levels of the highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) were estimated from the onset voltages of the oxidation
4
9
and reduction waves, respectively. The four molecules exhibit deep LUMO energy levels (ꢀ4.2
eV and –4.05 eV for the thiophene and bithiophene derivatives, respectively) and deep HOMO
energy level (ꢀ6.4 eV and ꢀ6.0 eV for the thiophene and bithiophene derivatives, respectively)
related to the presence of the benzothiadiazole ring. To make the parallel with wellꢀknown
materials for OPV, we may point out that these LUMO and HOMO energy levels are of 0.6 and
0.7 eV, respectively, lower than those of P3HT (ꢀ3.55 eV and ꢀ5.45 eV, respectively), and very
close to those of PC BM (ꢀ4.37 eV and ꢀ6.45 eV, respectively), as depicted in Fig. S14. Thus, if
6
0
thin films made of these molecules exhibit good electron mobility, then they would be good
acceptors candidates according to the energy levels of their frontier orbitals.
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Table 6 Electrochemical properties of the target molecules in dichloromethane solution.
oxidation
reduction
electroch
onset
onset
Eg
HOMO LUMO
E / ꢀE
E
ox
E1/2 / ꢀEp
E
red
a
b
b
1
/2
p
(
eV)
(eV)
(eV)
(
V/Fc+/Fc / mV) (V/Fc+/Fc
)
(V/Fc+/Fc / mV) (V/Fc+/Fc)
Bz(T CAO)
+1.12/107
+1.10/92
+0.61/98
+0.70/93
+1.01
+0.99
+0.56
ꢀ1.23/68
ꢀ1.22/69
ꢀ1.42/78
ꢀ1.38/68
ꢀ1.18
ꢀ1.17
ꢀ1.37
ꢀ1.34
2.35
2.32
2.03
2.08
ꢀ6.4
ꢀ6.4
ꢀ5.9
ꢀ6.0
ꢀ4.2
ꢀ4.2
1
2
Bz(T CAEH)
1
2
Bz(T CAO)
ꢀ4.0
2
2
Bz(T CAEH)
+0.57
ꢀ4.05
2
2
a
b
+
estimated from the E values. Estimated by cyclic voltammetry, with Fc/Fc at 5.39 eV
1
/2
4
9
below vacuum.
To obtain further insights into the effect of molecular structure and electron distribution on the
electronic characteristics of these molecules, calculations of the electronic structure in
fundamental and excited states were performed on the simplified model BzT1 and BzT2. In both
case, HOMO and LUMO are delocalized over the entire molecule. Fig. 3 illustrates that the
HOMO is distributed almost entirely over the conjugated molecule. The LUMO is rather
concentrated on thiadiazole unit in the two species with a quinoid character. This bond length
alternation scheme is a positive factor for electron delocalization properties. The calculated band
gaps are of 2.4 eV and 2.15 eV, for BzT1 and BzT2, respectively. These values are close to the
experimental results and exhibit similar trends.
In order to get qualitative comprehension on the nature of the electronic transitions responsible
for the absorption spectrum of the two species, the Natural Transition Orbitals (NTO) have been
calculated. They are displayed in Fig. 3. Only the strongest transition of each molecule is
indicated in Fig. 3, a more extensive list of computed transitions and the simulated TDꢀDFT
spectra being provided in the ESI. For both complexes, the excitations and absorption maxima in
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energies, calculated by BH and HLYP, match the experimental data with a good accuracy. From
both experiments and computed data, addition of one thiophene unit determines a sizable redꢀ
shift of the spectra. It appears from the NTO analysis that for the two species, the highest
transition is dominated by a HOMOꢀLUMO transition leading to an excitation slightly localized
on the acceptor part.
To complete this analysis, calculations on the redox species definitely confirm that electron
abstraction and electron addition take place on the HOMO and LUMO, respectively, of the
ground state neutral species.
BzT1 LUMO ꢀ3.89eV
Electron
BzT2 LUMO ꢀ3.64eV
BzT2 HOMO ꢀ5.79eV
Electron
NTO : 471nm
Hole
NTO : 528nm
Hole
BzT1 HOMO ꢀ6.29eV
Fig. 3 Representation of the frontier molecular orbitals of BzT and BzT in their optimized
1
2
geometries and their energy level values calculated with DFT. NTOs isodensity surface for the
strongest transition are represented and the associated wavelengths are indicated for the two
molecules in CH Cl .
2
2
Structural properties.
Differential scanning calorimetry (DSC) was performed at 10°C/min on the materials, and the
results are shown in Fig. 4. On the first heating cycle (not shown here), Bz(T CAEH) shows a
2
2
sharp melting endothermic transition at 169°C which becomes broader on the following cycles
ꢀ1
(
ꢀH = 15 kJ mol ), demonstrating a low ability to recover a perfect crystalline state after thermal
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ꢀ1
treatment, meanwhile the recrystallization occurs at 144°C (ꢀH = 12 kJ mol ). For
Bz(T CAO) , the sharp fusion peak recorded at 193°C during the first cycle (not shown here) is
2
2
ꢀ1
ꢀ1
splitted into two transitions at 173°C (ꢀH = 30 kJ mol ) and 186°C (ꢀH = 10 kJ mol ),
indicating the possible presence of conformational polymorphs due to thermal treatment. The
ꢀ1
recrystallization is characterized by a peak at 157 °C (ꢀH = 25 kJ mol ) immediately followed
by a broad and weak transition at around 142°C. Bz(T CAEH) shows a large endothermic peak
1
2
ꢀ1
at around 175°C (ꢀH = 32 kJ mol ), which corresponds to the phase transition from the
crystalline solid (S) to the isotropic liquid (IL) state. During cooling, the corresponding
ꢀ1
recrystallization peak is observed at around 157°C (ꢀH = 34 kJ mol ). Finally, Bz(T CAO)
1
2
possesses more complicated thermal properties. From the thermograms, upon heating
ꢀ1
Bz(T CAO) , a large exothermic peak (ꢀH = 15 kJ mol ) at 100°C indicates a thermally
1
2
induced crystallization phenomenon. A phase transition is also discernible at around 130°C, just
ꢀ1
before another sharp and intense peak at 141°C (total ꢀH = 26 kJ mol ) corresponding to the
transition to a liquid crystalline (LC) phase, then the transition from the LC to IL states is
ꢀ1
observed at 177°C (ꢀH = 3 kJ mol ). During the cooling process from IL, three low energetic
transitions, typical of monotropic organizations, are recorded at 178°C, 128°C, and 69°C (ꢀH =
ꢀ1
3
.0, 1.2, and 4.3 kJ mol ), respectively. This compound has been further investigated by
Polarizing Optical Microscopy (POM), and Small Angle Xꢀray Scattering (SAXS).
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(
exo down)
2
2
1
1
5
0
5
0
5
0
5
Bz(T CAEH)2
2
Bz(T CAO)2
2
Bz(T CAEH)2
1
Bz(T CAO)2
1
-
0
50
100
150
200
250
Temperature (°C)
Fig. 4 DSC thermograms of the molecules upon third heating and cooling cycles, measured at
10°C/min under nitrogen atmosphere.
SAXS patterns of Bz(T CAO) at 150°C (Fig. 5) contain a unique sharp smallꢀangle peak,
1
2
corresponding to a layer spacing of 35.3 Å, and a broad wideꢀangle scattering due to liquidꢀlike
lateral distances between segregated chains and mesogens. Besides the usual signal from molten
chains, h , located at about 4.7 Å, two further maxima are observed for the rigid parts around
ch
3
.6 (h ) and 7.2 Å (2h + h ), respectively attributed to localꢀrange faceꢀtoꢀface stacking and to
π π ⊥
overlapping contributions from faceꢀtoꢀface piled dimer and sideꢀbyꢀside lateral distances.
2
Molecular areas AMol close to 30 Å result from the ratio of estimated molecular volume (1040 ±
3
ꢀ3
4
0 Å , ρ ≈ 1.15 g.cm ) and layer spacing. These values substantially exceed the crossꢀsection of
2
stretched chains (23.3 Å at 150°C), which explains that the layer thickness is noticeably below
the length of the entirely stretched molecule conformation (ca. 40 Å).
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Fig. 5 SAXS pattern of Bz(T CAO) at 150°C, in the sanidic type smectic A phase.
1
2
SAXS shows then that Bz(T CAO) is crystalline in the pristine state (Fig. S9 top), melts
1
2
around 135°C to a smectic A phase with local stacking of the rigid parts (sanidic type smectic A
5
0
phase _ Fig. 5) and gives rise to crystalline polymorphism upon cooling (Fig. S9 bottom). Cold
crystallization however appears on the second heating in DSC and confirms that these
monotropic organizations revealed by DSC are metastable and thus not accessible to the more
timeꢀdemanding SAXS experiments which only evidenced crystalline phases (see Fig. S9).
On the other hand, the metastable monotropic phases could be observed by POM analysis (Fig.
5
1
6
) upon cooling from IL state. Bz(T CAO) revealed a batonnetꢀlike texture (Fig. 6 a)) within a
1 2
narrow temperature range from 185 to 180°C, corresponding to the formation of the monotropic
5
1
smectic A (SmA) phase. Then, a fan-shaped texture typical of a SmA phase is observed after
maintaining the temperature at a constant value of 170°C for 30 minutes (Fig. 6 b)). On cooling,
the DSC curve presents two further low energetic transitions at 135°C and 60°C (ꢀH = 1.4 and
ꢀ1
2
.2 kJ mol , respectively), the broken fanꢀshaped POM texture of the upper phase suggesting a
5
1
tilted smectic phase with inꢀplane order (Fig. 6 c)). Clearly, Bz(T CAO) shows crystalline and
1
2
ordered mesophases below the fluid SmA phase which may be helpful for morphology control
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5
2
and interesting for charge transfer. The linear alkyls chains play a central role in the appearance
6
,53
of mesomorphism, but as depicted in Yongsheng Chen’s work, they could also be helpful for
good photovoltaic performance.
Fig. 6 POM images of (a) the batonnetꢀlike texture of Bz(T CAO) at 183°C, (b) the SmA phase
1
2
of Bz(T CAO) at 170°C and (c) the smectic phase observed at 85°C.
1
2
CONCLUSION
In conclusion, four new molecules have been synthesized according to green practices. The
proposed molecules are unsophiscated and obtained via a synthetic approach that may
significantly impact the environmental factors, in term of nonꢀreacted raw materials or waste
generation. The ligandꢀless direct arylation reaction has proved to be a powerful green tool for
obtaining dithienylbenzothidiazole blocks, which are very often used as a building block for
more expanded materials used in organic electronics. Eꢀfactor values below 400 g/g associated
with molecules prices in the order of 45 €/g are promising numbers for potential large scale
applications. Their good thermal stability associated with their electronic properties and ability to
tolerate electron transfer make them interesting molecules to be tested as electron acceptors in
organic photovoltaic cells. We show that a simple replacement of the branched (ethylhexyl) with
linear (octyl) chain leads to a drastic change in their intermolecular interaction as supported by
SAXS measurements. This property will presumably be beneficial for achieving appropriate thin
film morphology (depending on the nature of the alkyl chain) for a given use. In that aim, further
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investigations on thin films of these molecules are also yet to be carried out to evaluate their
potential performance in light emitting diodes. Works are in progress to test them in these roles.
EXPERIMENTAL SECTION
Materials, equipment and methods
All used chemicals and solvents were purchased from chemical companies and used as
received, unless otherwise mentioned. All reactions were performed under ambient atmosphere
1
and under air exposition unless special indication. H experiments were recorded in the listed
deuterated solvents (internal standard) on a DPX 400 Bruker Spectrometer. Multiplicity of NMR
signal was denoted as m (massif), s (singlet), d (doublet), t (triplet).
Mass analyses were performed at the Service de Spectrométrie de Masse de la Structure
Fédérative Toulousaine en Chimie Moléculaire.
Elemental analyses (C, H, N) were carried out in the analysis service of the LCC laboratory.
Voltammetric measurements were carried out with a potentiostat Autolab PGSTAT100
controlled by GPES 4.09 software. Experiments were performed at room temperature in a
homemade airtight threeꢀelectrode cell connected to a vacuum/argon line. The reference
electrode consisted in a saturated calomel electrode (SCE) separated from the solution by a
bridge compartment. The counter electrode was a platinum wire of ca 1cm² apparent surface.
The working electrode was a Pt microdisk (0.5mm diameter) or a glassy carbon (1mm diameter).
The supporting electrolyte (nBu N)[PF ] (Fluka, 99% electrochemical grade) was used as
4
6
received and simply degassed under argon. Dichloromethane was freshly distilled over CaH2
ꢀ3
ꢀ1
prior to use. The solutions used during the electrochemical studies were typically 10 mol.L in
ꢀ1
compound and 0.1 mol.L in supporting electrolyte. Before each measurement, the solutions
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were degassed by bubbling Ar and the working electrode was polished with a polishing machine
(Presi P230).
UVꢀVISꢀNIR absorption spectra were recorded on a Perkin Elmer Lambda 35 spectrometer,
using dichloromethane as solvent. The solutions used for the electronic absorption analyses were
ꢀ5
ꢀ1
typically about 10 mol.L in molecule. Steadystate fluorescence spectroscopic studies were
performed on a HORIBA Jobin Yvon FluoroMaxꢀ4 spectrofluorometer, the solutions used for
ꢀ6
ꢀ1
emission analyses were typically around 1.6 10 mol.L in molecule (i.e. exhibiting an
absorbance ≤ 0.1). The slit width was 1 nm for both excitation and emission. Fluorescence
spectra were corrected. The relative quantum yields of the samples were obtained by comparing
the area under the corrected emission spectrum of the test sample with that of a solution of
standard. The quantum efficiencies of fluorescence were obtained from three measurements with
5
4
the following equation, including the index of refraction term:
Φ = Φ (I /I ) (A /A ) (η ²/η ²)
x
ref
x
ref
ref
x
x
ref
Where Φref is the reported quantum yield of the reference compound, I is the area under the
emission spectra, A is the absorbance at the excitation wavelength and η is the refractive index of
the solvent used.
Differential Scanning Calorimetry, DSC, was performed with a DSC 204 F1 Phoenix® system
(NETZSCH). Samples (2ꢀ5 mg) were filled in 30 ꢁl perforated aluminum pans and
measurements were conducted under nitrogen atmosphere. Three different heating/cooling cycles
were registered for each sample in temperature ranges situated between 5°C and 250°C with a
10°C/min rate. With the exception of the first thermogramme, all the cycles were reproducible.
The values reported correspond to those determined from the position of the top of the peaks. ꢁH
ꢀ1
is the enthalpy in kJ.mol associated with each phase transition.
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LC textures were acquired with an Olympus BX53 polarized light microscope equipped with a
Mettler FP82HT heating stage.
The SAXS patterns were obtained with a transmission Guinierꢀlike geometry. A linear
focalized monochromatic Cu Kα1 beam (λ = 1.5405 Å) was obtained using a sealedꢀtube
generator (600 W) equipped with a bent quartz monochromator. In all cases, the crude powder
was filled in Lindemann capillaries of 1 mm diameter and 10 ꢂm wallꢀthickness. The diffraction
patterns were recorded with a curved Inel CPS120 counter gasꢀfilled detector linked to a data
acquisition computer (periodicities up to 90 Å) and on image plates scanned by STORM 820
from Molecular Dynamics with 50 ꢂm resolution (periodicities up to 120 Å). The sample
temperature was controlled within ±0.01 °C, and exposure times were varied from 1 to 24 h.
Computational details
5
5
Theoretical calculations were performed with Orca code 3.0. Structural optimizations and
subsequent frequency calculations were carried out using an all electron triple zeta basis set with
5
6
one polarization functions on H,C,N,O and S atoms (Def2ꢀTZVP(ꢀf)). The B3LYP functional
5
7,58
together with Grimme’s D3(BJ)
model used to add molecular mechanics damped dispersion
was applied for ground states calculation. Vertical excitations were computed with TDꢀDFT
using BHandHLYP functional using the B3LYP optimized ground state structures. This hybrid
5
9
60
functional which is Becke’s half and half method with LYP correlation, is particularly well
adapted in our case due to the presence of molecular species with delocalized electron. Bulk
6
1
solvent effect, i.e. CH Cl , were included using the conductor like screening model (COSMO).
2
2
For the Bz(T CAEH) , Bz(T CAO) , Bz(T CAEH) and Bz(T CAO) , the terminal alkyl
1
2
1
2
2
2
2
2
groups were replaced by methyl groups, leading to BzT1 and BzT2, to gain computational time
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since experimental data concerning optoelectonic properties of the molecules (see Table 5 and
Table 6) has been proven to be not affected by this simplification. Absorption spectra were
simulated using the orca_mapspc program that applies Gaussianꢀtype line shape function to the
ꢀ1
calculated transition with a defined fullꢀwidthꢀhalfꢀmaximum of 1200cm . The nature of the
6
2,63
transitions was determined using natural transition orbitals.
Synthetic procedure
5,5'-(2,1,3-benzothiadiazole-4,7-diyl)bis-(2-thiophenecarboxaldehyde) _ (5)
Route a: All solvents and basic solutions were previously degassed by bubbling through
Argon for 20 min. 2,1,3ꢀBenzothiadiazoleꢀ4,7ꢀbis(boronic acid pinacol ester) (247 mg, 0.637
mmol, 1 eq) and 5ꢀbromoꢀ2ꢀthiophenecarboxaldehyde (0.196 mL, 1.64 mmol, 2.5 eq) were
placed in a threeꢀnecked flask which was brought under an inert atmosphere. Toluene (10 mL),
K CO (2.0 M, 5 mL), H O (4 mL) and Aliquat 336 (4ꢀ5 drops) were added and the mixture was
2
3
2
vigorously stirred under argon for 10 min at room temperature. Pd(PPh ) (45 mg, 2.4 mol%)
3
4
was then added and the mixture was allowed to react for 24 h at 80 °C. After cooling to room
temperature, the solid crude mixture was filtered, washed with distilled water, ethyl acetate, then
with a vol 1:1 dichloromethane:petroleum ether mixture. After drying under vacuum, 230 mg of
pure product were recovered (quantitative yield).
Route b: In a threeꢀnecked flask containing a stirring bar Pd(OAc) (7.6 mg, 0.034 mmol),
2
PtBu Me.HBF (16.9 mg, 0.068 mmol), pivalic acid (35 mg, 0.34 mmol), 4,7ꢀ
2
4
dibromobenzothiadiazole (100 mg, 0.34 mmol), potassium carbonate (141 mg, 1.0 mmol), and 2ꢀ
thiophenecarboxhaldehyde (84 mg, 0.75 mmol) were sequentially added under a flow of argon.
Dry toluene (4 mL) was added and the reaction mixture was heated at 120°C in an oil bath for
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5.5 h until judged complete by examining aliquots with TLC. After cooling to room temperature,
the solid crude mixture was filtered, washed with distilled water, ethyl acetate, then with
petroleum ether. After drying under vacuum, 119 mg of pure product were recovered (98%
yield).
Route c: In a 100 mL threeꢀnecked roundꢀbottomed flask, equipped with a thermometer,
magnetic stirrer, condenser and rubber seal, 2ꢀthiophenecarboxaldehyde (419 mg, 3.74 mmol,
2.2 eq) and N,Nꢀdimethylacetamide (6 mL) were added to a mixture of 4,7ꢀdibromobenzoꢀ2,1,3ꢀ
thiadiazole (500 mg, 1.7 mmol, 1 eq), potassium acetate (334 mg, 3.4 mmol, 2 eq), and
palladium acetate (1.9 mg, 8.4 10ꢀ3 mmol (0.5%)) in one portion under argon atmosphere
respectively. The reaction mixture was then heated to 150°C and stirred under vigorous reflux
for 24 h. The dark red solution was cooled to ambient temperature and filtered. After washing
with water, ethyl acetate, and then petroleum ether, the corresponding red diarylated product was
dried, affording 430 mg of pure product (71% yield).
Table S1 (see Electronic Supporting Information _ESI) resumes the results obtained from
various “r” molar ratios of 2ꢀthiophenecarboxaldehyde/4,7ꢀdibromobenzoꢀ2,1,3ꢀthiadiazole, and
various amounts of palladium acetate (relative to 4,7ꢀdibromobenzoꢀ2,1,3ꢀthiadiazole). The other
parameters are fixed (quantity of 4,7ꢀdibromobenzoꢀ2,1,3ꢀthiadiazole: 500 mg, KOAc: 334 mg,
DMA: 6 mL, temperature of reaction: 150°C and time of reaction: 24 h)
1
H NMR (400 MHz, CDCl ) δ 10.02 (s, 2H), 8.29 (d, J = 4 Hz, 2H), 8.08 (s, 2H), 7.90 (d, J = 4
3
6
Hz, 2H); 1H NMR (400 MHz, DMSOꢀd ) δ 10.03 (s, 2H), 8.43 (s, 2H), 8.38 (d, J = 4 Hz, 2H),
8.17 (d, J = 4 Hz, 2H).
5',5'''-(2,1,3-benzothiadiazole-4,7-diyl)bis-[(2,2'-Bithiophene)-5-carboxaldehyde] _ (7)
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Route a: All solvents and basic solutions were previously degassed by bubbling through
Argon for 20 min. 2,1,3ꢀBenzothiadiazoleꢀ4,7ꢀbis(boronic acid pinacol ester) (173 mg, 0.445
mmol, 1 eq) and 5'ꢀbromoꢀ2,2'ꢀbithiopheneꢀ5ꢀcarbaldehyde (305 mg, 1.11 mmol, 2.5 eq) were
placed in a threeꢀnecked flask which was brought under an inert atmosphere. Toluene (25 mL),
K2CO3 (2.0 M, 5 mL), H2O (4 mL) and Aliquat 336 (2ꢀ3 drops) were added and the mixture
was vigorously stirred under argon for 10 min at room temperature. Pd(PPh3)4 (38.5 mg, 3.0
mol%) was then added and the mixture was allowed to react for 24 h at 80 °C. After cooling to
room temperature, the solid crude mixture was filtered, washed with distilled water, ethyl
acetate, then with petroleum ether. After drying under vacuum, 230 mg of pure product were
recovered (quantitative yield).
Route c: In a 100 mL threeꢀnecked roundꢀbottomed flask, equipped with a thermometer,
magnetic stirrer, condenser and rubber seal, 2,2'ꢀbithiopheneꢀ5ꢀcarbaldehyde (727 mg, 3.74
mmol, 2.2 eq) and N,Nꢀdimethylacetamide (6 mL) were added to a mixture of 4,7ꢀ
dibromobenzoꢀ2,1,3ꢀthiadiazole (500 mg, 1.7 mmol, 1 eq), potassium acetate (334 mg, 3.4
mmol), and palladium acetate (1.9 mg, 8.4 10ꢀ3 mmol (0.5%)) in one portion under argon
atmosphere respectively. The reaction mixture was then heated to 150°C and stirred under
vigorous reflux for 24 h. The dark red solution was cooled to ambient temperature and filtered.
After washing with water, ethyl acetate, and then petroleum ether, the corresponding red
diarylated product was dried, affording 811 mg of pure product (92% yield).
1
H NMR (400 MHz, CDCl ) δ 9.92 (s, 2H), 8.12 (d, J = 4 Hz, 2H), 7.96 (s, 2H), 7.74 (d, J = 4
3
6
Hz, 2H), 7.50 (d, J = 4 Hz, 2H), 7.41 (d, J = 4 Hz, 2H); 1H NMR (400 MHz, DMSOꢀd ) δ 9.90
(
s, 2H), 8.32 (s, 2H), 8.03 (d, J = 4 Hz, 2H), 7.64 (d, J = 4 Hz, 2H), 7.52 (d, J = 4 Hz, 2H), 7.60
d, J = 4 Hz, 2H).
(
29