Access to small molecule semiconductors: Via C-H activation for photovoltaic applications

Article abstract of DOI:10.1039/c8cc02706d

The first methodology of ruthenium carboxylate-catalysed single step oxidative cross coupling that challenges the conventional Stille and Suzuki coupling reactions, affording BT and MFBT derivatives in the absence of protecting groups, was developed. Both mono and bi-arylated derivatives are formed in moderate to high yields (30-75%). Innately high selectivity, low catalyst loading and lack of formation of regio-isomers ensure the large-scale synthesis of various photonic and electronic materials employing this method.

Full text of DOI:10.1039/c8cc02706d

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
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Access to small molecules semiconductor via C-H activation for
photovoltaic applications
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
a,b
Sanchari Shome , Surya Prakhash Singh*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The first methodology of ruthenium carboxylate-catalysed single route to synthesize these donor-acceptor motifs includes the
step oxidative cross coupling that challenges the conventional Suzuki and Stille coupling reaction with substituted
Stille and Suzuki coupling reactions, affording BT and MFBT benzothiadiazoles and their stannyl or boronyl thiophene
6
derivatives in absence of protecting groups. Both mono and bi- counterpart. Although, the method is widely established one,
arylated derivatives are formed in moderate to high yields (30- it suffers several drawbacks. Firstly, stannyl derivatives are
7
5%). Innately high selectivity, low catalyst loading and lack of highly toxic and leave behind environmentally hazardous by-
formation of regio-isomers ensures the large-scale synthesis of products. Secondly, in case of the boronic acid derivatives
various photonic and electronic materials employing this method.
reaction procedure involves a number of steps including the
halogenation and boronylation of benzothiadiazole and its
coupling partner. It is in fact a challenging task to perform
controlled halogenation as always a mixture of mono and bi
halogenated compounds obtained that further on treatment
with the stannyl or boronyl derivative leads to different
products. Thirdly, the scope of this method is restricted mainly
to symmetrical products thereby limiting its diverse
Heteroarenes containing benzothiadiazole(BT) moieties
possess a range of unique optical properties showing
1
outstanding characteristic of functional materials. The use of
fluorinated benzothiadiazole (FBT) has gained immense
interest in the recent past because of the small size and high
electronegativity of the fluorine atom that imparts good
2
7
optoelectronic
properties
to
the
system.
The
applications.
benzothiadiazole–thiophene motif based on donor-acceptor
assembly developed for high performance optoelectronic
Recent report by Zhang et.al on the selective thienylation on
fluorobenzothiadiazole derivatives requires the blocking of
every active site of the FBT keeping only one C-H bond
3
materials . Substitution of the BT moieties with strong electron
8
withdrawing groups helps in lowering the Highest Occupied
Molecular Orbital (HOMO) energy levels and thereby
increasing power conversion efficiency (PCE). The PCE of FBT
found to increase manifold compared to that of simple
available for reaction.
The selectivity of the catalytic system not seemed to be
sufficient to react at one particular active site in presence of
others. This reaction can also be performed in absence of
blocking groups using main group catalysts along with
4
benzothiazole because of the presence of the electron
0
withdrawing fluorine atom. Several transition metal catalysed
reactions have been developed for the coupling of
benzothiadiazole molecules, where either one of the coupling
partner has been pre-functionalised with a metal and the
bis(dibenzylideneacetone)Pd as reported by Paul and co-
9
workers . However, one of the major drawbacks like synthesis
and stability of main group metal catalysts makes the process
10
less suitable for a large-scale synthesis . Grignard reagents
used in this process are highly moisture sensitive and requires
immense care while maintaining an intense inert condition. In
order to overcome these drawbacks, we have tried to use
catalysts that are moderately reactive, less sensitive but more
selective in nature as depicted in Fig 1.
5
other with halides . However, the atom economical C-C cross-
coupling reactions are attractive and sustainable approach
towards the synthesis of functional materials. The traditional
a.
Sanchari Shome, Surya Prakash Singh*
Earlier our group had studied the effect of bulky
Ru(MesCO ) p-cymene on the alkenylation of nitrogen
11
Polymers and Functional Materials Division,
CSIR- Indian Institute of Chemical Technology,
Uppal Road, Tarnaka, Hyderabad-500007, India
E-mail: spsingh@iict.res.in
2
2
directing inactivated aryl moieties and we further wanted to
explore the catalytic capability of Ru(MesCO ) p-cymene on
2
2
b.
Academy of Scientific and Innovative Research (AcSIR), New Delhi, India
Electronic Supplementary Information (ESI) available: [details of any
inactivated benzothiadiazoles. As ruthenium catalysts are less
supplementary information available should be included here]. See reactive but more selective in nature compared to other
DOI: 10.1039/x0xx00000x
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transition or main group metals generally used as catalyst, the yield of 3a (67%) probably due to catalyst deactivation as
addition of bulky acid group however enhanced its reactivity shown in ESI Table 1.
2
2
manifold. The present work reports the sp -sp coupling
between the unsubstituted benzothiadiazole and thiophene
X
X
N
N
N
N
X1
catalyst, oxidant
solvent
X1
counterpart in the presence of catalyst and oxidant.
Previous work
+
R
R1
R1
1
R
X= S, Se
R= H, F
X = S, Se, O
S
R1= Br, CHO, COOH, CN
Br
N
N
S
S
S
S
S
N
N
Pd(PPh
3
)
2
Cl
2
, DMF
89%
N
N
S
N
N
N
N
N
N
Sn
+
S
Br
CHO
S
S
0
S
S
S
75 C, 1hr
S
S
*
N
N
3a, 75%
3b, 56%
S
S
N
TMP
2
Mg 2LiCl, THF
3c, 70%
S
3
d, 32%
S
I
+
+
S
S
6
3%
S
N
ZnCl , THF
bis(dibenzylideneacetone)Pd0
S
N
N
2
N
N
N
N
Br
S
S
Br
S
S
S
N
N
S
S
F
F
N
N
Br
Pd(TFA)
2
, Ag O
2
S
S
7
2%
S
3e, 50%
3f, 62%
CN
3g, 42%
S
0
Br
DMSO, 80 C, 10hrs
S
N
S
S
N
F
F
N
N
N
N
N
N
COOH
S
S
S
Se
S
N
N
R1
R2
N
N
t
Pd(OAc)
2
, P BuMe-HBF
4
S
S
+
S
31%
Pivalic acid, K CO
2
3
3h, 50%
Se
3i, 56%
S
3j, 23%
3k, 48%
R
1
R
2
Se
Se
Se
N
N
N
N
N
N
N
N
S
Br
CHO
COOH
Our Work
S
S
S
N
N
3l, 74%
S
N
N
S
Ru(MesCO ) (p-cymene), Ag O
3m, 66%
Se
3n, 75%
S
2
2
2
3o, 50%
S
S
+
S
70-80%
DMSO
N
N
N
N
N
N
N
N
CHO
S
S
S
O
Fig.1 Summary of the previous reports in contrast to the present
F
F
3
F
work
p, 60%
3q, 65%
3r, 62%
3s, 45%
Br
S
N
S
S
N
N
N
N
N
The initial screening reaction was performed with
benzothiadiazole (1) and 2-bromothiophene (2a) with catalyst
N
S
S
S
S
N
Br
[
Ru(p-cymene)Cl
2
]
2
, oxidant AgOAc and toluene as solvent.
3t, 23%
S
3u, 60%
S
3z, 71%
N
However, the negative result for the above reaction motivated
us to change the solvent from non-polar to highly polar and
high boiling solvents like DMF, DMSO etc. (Fig 2). Change in
solvent from toluene to DMF increased the yield of 3a to 32%
and a remarkable hike to 56% was found on using DMSO as a
S
S
N
N
N
N
N
Br
S
Br
S
S
OHC
O
3
aa, 68%
3ab, 62%
3ac, 64%
1
2
solvent.
Fig.3 Representative examples of the synthetic schemes with
heteroarenes
Further rise in the yield of 3a to 74% was observed when the
oxidant loading was increased to 15 mol%. Combination of DMSO-
piperidine as a solvent leads to higher yields when piperidine a base
is added in minimal amount (1-2 drops). Addition of a catalytic
Fig.2 Representative optimisation reaction
amount of piperidine in DMSO (pk = 0.5) helps in the easy
a
1
6
The product 3a emphasizes the fact that reaction is tolerant to abstraction of the less acidic C-H bond of the BT and FBT . Scale up
bromo group and therefore the coupling involved H-H removal reaction was carried out for 3a derivative in order to affirm the
instead of H-Br removal. The yield of this product was further applicability of this methodology in commercial grounds. The
improved to 65% on changing the catalyst to Ru(p- reaction for 3a was performed on a scale of 2 mmol, 5 mmol, 8
2 2
cymene)(MesCO ) .The detailed optimization table is mmol and 10 mmol, where the yields ranged from 65% to 75% (see
mentioned in the ESI as Table 1. The carboxylic acid based ESI). This confirms the availability of this reaction on a few milligram
ruthenium catalyst sets off the attack of the coupling partner scales for the large scale synthesis. Formation of 3b, 3j and 3i
1
3
directly to the ortho-position of the directing group . The required around 12hr for completion along with residual starting
bulkiness of the acid group helps in the easy formation of a material. Further prolonging the reaction time for these derivatives
1
4
thermodynamically favoured 6-membered transition state . did not help in any improvement of their respective yields.
Although silver acetate, a renowned oxidizing agent for ortho- Substrates containing fluorine 3p, 3q, 3r and 3x required less
1
5
arylation , we found better results on using silver oxide, a reaction time (4hrs) than their non-fluorine counterparts. This may
milder oxidizing agent. Surprisingly enhanced yield of 3a (70%) depend on the lability of the C-H bond adjacent to the nitrogen
was found on increasing the catalyst loading to 20 mol%. atom. The homo-coupling product of 1 in Fig 2 was not observed
However excess catalyst loading upto 25 mol%, diminished the under the above reaction conditions thereby making this method a
2
| J. Name., 2012, 00, 1-3
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more versatile one. Coupling of a range of thiophenes and hydrogen’s on either of the coupling partner. The yields of the final
substituted thiophenes under the optimised reaction conditions products (3v-3y) dramatically improved from low to moderate on
resulted in mono-substituted derivatives with high yields. replacing Ag O with AgOAc.
2
Thiophenes with variety of substitutions like bromides, acids, The plausible mechanism of the reaction is illustrated in Fig 1-ESI,
aldehydes, cyano showed excellent functional group tolerance where it is expected that the initial hydrogen abstraction from the
resulting in products (3a, 3c, 3g-3i, 3m-3o, 3r) with moderate to
high yields as shown in Fig. 3. The products with aldehyde and acid
at the terminal end (3c, 3h, 3n, 3o, 3r) utilized as potential co-
sensitizer precursor for Dye Sensitized Solar Cells (DSSC). Similarly,
bisthiophene and thienothiophene (3b, 3e, 3j), obtained in good
yields can be useful in Bulk Heterojunction (BHJ) solar cells and field
benzothiadiazole takes place with the help of the catalytic amount
of piperidine that is added in the reaction mixture and the electron
rich benzothiadiazole species is attacked by the ruthenium complex
-
that is electron deficient thereby forming a 16e species which
undergoes oxidative addition. Earlier reports on the mechanistic
studies of ruthenium(p-cymene) catalyst by Fabre et.al have clearly
1
7
effect transistors .Compounds like 3e, 3g, 3t can be easily
synthesized in one-step using excess amount (4eqv) of the
thiophene partner. The unreacted thiophene or thiazole was
recovered from the reaction mixture through column
chromatography.
-
mentioned that the oxidative addition takes place on the 16e
2
0
ruthenium species. The addition of thiophene leads to the
formation of the complex C that is indicated in the HRMS spectra. In
the later stage the complex undergoes reductive elimination to give
the desired product and a ruthenium complex that undergoes one
electron reduction in presence of silver to regenerate the catalyst.
Upon synthesis of the above molecules we explained its possible
application in DSSC as co-sensitizer. The metal free organic
Further exploration, observed the selective synthesis of mono-
thienylated product required the use of excess amount of BT (3-4
eqvs). The tuning of the ratio of 1 and 2a (as depicted in Fig. 2) can
lead to successful formation of mono (3a) and bi coupled (3g)
products. In this process, we observed around 62% of bi-arylated sensitizers are an exemplary candidate for the co-sensitized
2
1
compound and 12% of mono arylated compound. This significantly DSSCs, because of its high molar extinction coefficients, easy
1
8
contrasts to the other traditional methods where 4,7-diiodoFBT is structural modification, and facile synthetic process. Earlier, some
1
9
reacted with the stannyl thiophene, followed by bromination . We groups have reported small organic dyes highlighting a simple
can overcome the demerits of the conventional methods like
toxicity, harsh conditions and increased number of steps using the
previously mentioned procedure. This can also be useful for the
synthesis of unsymmetrical FBT derivatives in one pot provided the
addition of the second thiophene partner was made after assuring
the complete reaction with the first thiophene (see ESI synthesis of
donor-π-acceptor structure using benzothiadiazole as a core
moiety, which successfully enhanced the incident photon-to-
current conversion efficiency (IPCE) of black dye (BD) in the UV
22
region in a co-sensitized DSSC. However, improvements in the
spectral response require to harvest the maximum light matching
the commercial needs. Therefore, synthesis of several organic dyes
employed, showed a reasonable absorbance in the UV-Vis region
3
ac). The unreacted BT if any was easily separated using column
chromatography and hence there is no loss of starting material.
Since benzothiadiazole-dibenzene, structural motifs have found
23
successfully to increase the photocurrent in co-sensitized DSSCs.
immense application as optoelectronic materials; their synthesis
was performed under the developed reaction conditions. Upon
optimisation, it was realised that the standardised reaction
conditions mentioned earlier in this paper was not favourable for
the coupling of benzothiadiazole with benzene. The reason may be
the lesser reactivity of the unsubstituted benzene in comparison to
other heterocycles.
S
S
Ru(p-cymene)(MesCO
CHO
2 2
)
N
N
N
N
S
CHO
S
+
2
Ag O, DMSO, Piperidine
R
R
R=F, H
NH
AcOH
4
OAc
S
N
N
COOH
CN
S
R
R=F, FBT-CAA
R=H, BT-CAA
Fig.5 Scheme for the synthesis of cyanoacetic acid co-sensitizers
Based on the above strategy, syntheses of a series of light
harvesting dyes were conducted for application in dye-sensitized
solar cells. Here we have shown few different applications of the
synthesized derivatives in solar cell. Co-sensitization is one of the
most prominent techniques to enhance the PCE of DSSCs. Research
Fig.4 Synthetic scheme for the reaction of BT or FBT with benzene
Another set of reactions were performed using 1.0 eqv of BT or FBT
and 4-8 eqv of benzene in 1ml of DMSO along with 0.2ml of
piperidine were found to be the facile reaction condition for mono
and di-arylation (Fig 4). Unlike thienylation, arylation using benzene
required excess of piperidine because of the unavailability of acidic
groups across the globe including ours have been working on this
24-30
technique to achieve higher PCE
. In addition to our efforts in
the coupling of FBT or BT with heteroarenes, we also derivatized
the products 3c and 3r to their respective cyanoacetic acids (FBT-
CAA and BT-CAA) (Fig 5). These molecules used as co-sensitizers
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with ruthenium complex N749 (Black dye) as a base dye to 12 (a) B. A. Steinho and S. S. Stahl, J. Am. Chem. Soc., 2006,
1
28, 4348; (b) K. Kobayashi, A. Sugie, M. Takahashi, K.
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3 L. Ackermann, R. Vicente and A. Althammer, Org. Lett.
2008, 10, 2299.
construct the DSSC devices.
7
These molecules (3c and 3r) showed very high PCE of 11.55% with
1
2
high Jsc of 23.41mA/cm (detailed in the ESI Fig. 4 table 4). It is
noteworthy to mention that our aim was not to achieve high 14 C. M. Anderson, M. Crespo and J. M. Tanski,
Organometallics 2010, 29, 2676.
efficiency but just to confirm that our catalytic protocol may be
1
5 R. Samanta and A.P. Antonchick, Angew. Chem. Int. Ed.
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utilized for more appropriate and highly efficient organic
2
3
1
develop several small molecules having thienothiophene and
15, 561.
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3
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thienogermoles moeities for their various solar cell applications.
In summary, we have accomplished highly selective one-step cross
thienylation and arylation of the benzothiadiazole and
fluorobenzothiadiazole using ruthenium as a catalyst. The reaction
pathway ended up with high regio and chemo selectivity. The
coupling with selenophene as well as benzoselenadiazole afforded
high yields, which also proves to be tolerant with diverse functional
groups in good efficacy. Even coupling with thieno[3,2b]thiophene
gave better results which otherwise is difficult to synthesis
following the conventional methods. On extending the derivatives
for further applications in DSSC, high efficiency was observed. These
inspiring results helps us to conclude that, use of the studied
methodology as a replacement for the conventional techniques
leads to synthesis of complex molecules for solar cell applications
thereby, making this method versatile for large-scale production.
2
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Conflicts of interest
2
2
2
2
3
Authors declare no conflict of interest.
Acknowledgements
SPS and SS thank DST Fast Track Young Scientist Project (CS-
8 L. G. Wei, Y. Na, Y. L. Yang, R. Q. Fan, P. Wang and L. Li,
Phys. Chem. Chem. Phys. 2015, 17, 1273.
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•
•
•
•
•
Potential precursor for photovoltaic materials
Single step synthesis
Avoids pre-functionalization unlike Suzuki and Stille
Protecting groups are not required
Protocol can be used in commercial scale
Photosensitizer
Efficiency:11.55%