Indoloquinoxaline as a terminal building block for the construction of π-conjugated small molecules relevant to organic electronics

Article abstract of DOI:10.1016/j.dyepig.2015.07.035

Abstract Herein we introduce the π-conjugated organic building block, 8-bromo-indolo[2,3-b]quinoxaline (indoloquinoxaline, IQ), that combines both electron withdrawing and donating nitrogen atoms. It is accessible from low-cost starting materials through a straightforward and high yielding condensation reaction. In terms of materials design, indoloquinoxaline is suitable for use as a terminal unit; it can act as a planar extension of the π-conjugated system and is easily functionalized with aliphatic side-chains, which can alter material solubility and self-assembly processes. The indoloquinoxaline framework is highly versatile and can be readily derivatized considering the hundreds of commercially available indole and phenylenediamine starting materials. In this case, the indoloquinoxaline building block is neither strongly electron rich or deficient which allows for material optoelectronic properties to be largely dictated by the character of the molecular core, as shown through the incorporation of three different core units, a weak thiophene donor, a strong benzodithiophene donor and an isoindigo acceptor.

Full text of DOI:10.1016/j.dyepig.2015.07.035

Accepted Manuscript
Indoloquinoxaline as a Terminal Building Block for the Construction of π-Conjugated
Small Molecules Relevant to Organic Electronics
Abby-Jo Payne, Jenny S.J. McCahill, Gregory C. Welch
PII:
S0143-7208(15)00303-4
10.1016/j.dyepig.2015.07.035
DYPI 4875
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 8 July 2015
Revised Date: 23 July 2015
Accepted Date: 25 July 2015
Please cite this article as: Payne A-J, McCahill JSJ, Welch GC, Indoloquinoxaline as a Terminal Building
Block for the Construction of π-Conjugated Small Molecules Relevant to Organic Electronics, Dyes and
Pigments (2015), doi: 10.1016/j.dyepig.2015.07.035.
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ACCEPTED MANUSCRIPT
Indoloquinoxaline as a Terminal Building Block for the Construction of π-
Conjugated Small Molecules Relevant to Organic Electronics
Abby-Jo Payne,1 Jenny S. J. McCahill2 and Gregory C. Welch*1
Department of Chemistry, Dalhousie University, 6274 Coburg Road, P.O. Box 15000, Halifax, Nova
Scotia, Canada, B3H 4R2
1 Current Address: Department of Chemistry, University of Calgary, 731 Campus Place N.W. Calgary,
AB, Canada
T2N 1N4
2 Current Address:Carbon Engineering Ltd. 3535 Research Road NW, Calgary, AB, Canada, T2L 2K8
* Corresponding Author
Email: gregory.welch@dal.ca / gregory.welch@ucalgary.ca
Phone Number: 1 (403)-210-7603

ACCEPTED MANUSCRIPT
Abstract: Herein we introduce the π-conjugated organic building block, 8-bromo-indolo[2,3-
b]quinoxaline (indoloquinoxaline, IQ), that combines both electron withdrawing and donating
nitrogen atoms. It is accessible from low-cost starting materials through a straightforward and
high yielding condensation reaction. In terms of materials design, indoloquinoxaline is suitable
for use as a terminal unit; it can act as a planar extension of the π-conjugated system and is easily
functionalized with aliphatic side-chains, which can alter material solubility and self-assembly
processes. The indoloquinoxaline framework is highly versatile and can be readily derivatized
considering the hundreds of commercially available indole and phenylenediamine starting
materials. In this case, the indoloquinoxaline building block is neither strongly electron rich or
deficient which allows for material optoelectronic properties to be largely dictated by the
character of the molecular core, as shown through the incorporation of three different core units,
a weak thiophene donor, a strong benzodithiophene donor and an isoindigo acceptor.
Keywords: Organic Electronics, Conjugated Small Molecules, Indoloquinoxaline, Isoindigo,
Benzodithiophene, Thiophene,
Highlights:
Introduction of a new indoloquinoxaline organic π-conjugated building block
Simple synthesis of functionalized indoloquinoxaline building blocks
Synthesis of three new chromophores with progressively red-shifted band gaps
1. Introduction
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Organic π-conjugated small molecules have found wide spread applications in a range of
electronic devices. One of the most well-known is their use as donor and acceptor components in
organic solar cells (OSC).[1–9] π-conjugated small molecules have also been employed as light
harvesters in metal-free dye sensitized solar cells (DSSC),[10–13] semiconducting materials in
organic field effect transistors (OFETS),[14–17] and as hole transporting materials in both DSSC
and perovskite solar cells (PSC).[18–23]
The solution-processability of these materials is key to their practical application,
enabling them to be printed or coated onto lightweight, flexible substrates, which in turn allows
for easy storage, transportation and installation measures.[24–30] While great improvements in
device performance have been realized over the past few years,[31,32] there is a need for new
and improved optoelectronic materials.[33,34] Key to the growth of organic electronics towards
commercialization is the development of cost-effective organic π-conjugated materials that can
be readily accessible from raw materials, through a sustainable synthesis with scalable
purification.[24,35,36]
The utility of low-cost building blocks for organic electronics is dictated by their
functional versatility. Specific π-conjugated organic building blocks are integrated within the
molecular framework to tune the optoelectronic properties and the self-assembly tendencies of
the material. One of the key design principles for π-conjugated materials is based on band-gap
engineering. From a molecular design perspective, this can commonly be done in three ways:
(1) increasing the conjugation length,[37,38] (2) incorporation of electron-donating/withdrawing
moieties and (3) the intramolecular donor-acceptor (D-A) approach.[39–41] From this perspective
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we have synthesized a novel highly tunable π-conjugated building block for use in organic
electronics.
The 8-bromo-indolo[2,3-b]quinoxaline (IQ) building block is a π-conjugated planar
framework accessible from inexpensive, readily available precursors. The base
indoloquinoxaline structure offers several avenues for structural versatility relevant to organic
electronic applications. The electronic and self-assembly tendencies of this building block can be
tuned by the incorporation of various functionalized o-phenylenediamines with over 500 mono-
substituted and more than 300 di-substituted commercially available derivatives. Furthermore,
the amido nitrogen can be functionalized with aliphatic side chains, facilitating adequate solution
processability and potentially dictating self-assembly propensities.
Indoloquinoxaline
derivatives are well known and actively studied in medicinal chemistry for their antiviral and
anticancer properties.[42–44] Only recently have they been explored as building blocks for use
in organic electronics including their incorporation into OLEDS and DSSCs. These
indoloquinoxaline derivatives, similar to our reported material but with functionalization
occurring through the 9-position, have been successfully incorporated into small molecule
architectures and have exhibited potential for application in organic electronics. [45–48]
In this study we utilized o-phenylenediamine with 6-bromoisatin to access a unique
building block, IQ, which is employed as an end-capping unit to effectively extend the π-
conjugation and promote a planar framework. In the context of strongly electron donating and
electron accepting building blocks, IQ can be described as electron neutral, as it contains both
electron donating and electron withdrawing nitrogen atoms. This neutral character is a result of
the pyrazene nitrogen lone pairs being orthogonal to the π-system, and therefore their
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contribution is solely an inductive one, withdrawing electron density from the π-system, which
leads to a slight electron-accepting character. The latter is countered by the weak electron
donation from the indole π-conjugated ring. These competing effects are thought to render the
building block neither strongly electron donating or accepting. To study the effect of this new
building block on the optoelectronic properties of a π-conjugated system, three core building
blocks were selected with differing electronic character, a weakly donating thiophene unit (Th),
a strongly donating benzodithiophene unit (BDT) and an electron accepting isoindigo core
(II(Th)2).
2. Results and Discussion
2.1 Synthesis
The acid catalyzed condensation of indoloquinoxaline from isatin and o-
phenylenediamine has been previously outlined in the literature (A);[42,49,50] however, upon
the introduction of electron withdrawing groups on the isatin analog, an alternative product is
observed. The reaction intermediate is shifted towards the cleavage of the isatin ring, forming
the ring opened species as depicted in B.[50] Upon heating a 1:1 mixture of 6-bromoisatin and o-
phenylenediamine at reflux in acetic acid, a yellow precipitate was formed after 30 minutes.
Presumed to be the mono-condensation product, dimethylformamide (DMF) was added to the
mixture allowing for a higher reflux temperature. This in turn, led to the re-dissolution of the
precipitate and the reaction was driven to the final ring closure resulting in the desired product.
The dark red/brown reaction mixture was cooled to room temperature and a yellow/brown
precipitate was observed. The material was collected via vacuum filtration and washed with
copious amounts of water followed by a 1:1 H2O:EtOH solution. Using this modified literature
procedure (C), the product 8-bromo-indolo[2,3-b]quinoxaline (IQ) was successfully synthesized
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and isolated as an off-white powder in high yields (< 80 %) with relatively high purity. IQ was
subsequently used without any further purification in a N-alkylation reaction with octylbromide
in the presence of base (K2CO3) in DMF to generate 8-bromo-N-octyl-indolo[2,3-b]quinoxaline
(IQ-Oct) (D). Upon reaction of all starting material, as monitored by thin layer chromatography
(TLC), the mixture was cooled to room temperature and stirred in a 1:1 solution of H2O/MeOH
and which resulted in the precipitation of IQ-Oct as a bright orange solid. The material was
recovered via suction filtration and washed with copious amounts of water followed by a 1:1
H2O/MeOH solution. The final yield of the alkylated product was 84 % with no further
1
purification required. Both new compounds IQ and IQ-Oct were characterized by H and 13C
NMR spectroscopy and identified by atmospheric pressure chemical ionization (APCI) mass
spectrometry.
The mono-brominated IQ building block is ideally suited for incorporation as a terminal
end-capping unit. Carbon-carbon bond forming reactions were accomplished similar to our
previously published methods with the use of the heterogeneous catalyst SiliaCat® DPP-Pd.[51–
53] Final compounds 1 and 2 were prepared by a Stille coupling between the bis-stannylated
cores and two equivalents of IQ-Oct under microwave irradiation, while compound 3 was
synthesized through the direct heteroarylation of 6,6’-di(thiophen-2-yl)-1,1’-di(octyl)isoindigo
with two equivalents of IQ-Oct under conventional heat (Figure 3). Conventional heating was
used for the direct heteroarylation reaction due to the result of multiple substitutions occurring on
the thiophene obtained upon microwave irradiation as seen in our previous work.[53]
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The work up for compounds 1 and 2 involved stirring the completed reaction mixtures in
methanol and collecting the resultant orange solid via suction filtration. The materials were then
dissolved in dichloromethane and solution filtered to remove the SiliaCat® DPP-Pd
heterogeneous catalyst. The solvent was removed under reduced pressure, the materials slurried
in methanol, and the resultant products were collected via suction filtration washing with ethanol
and isopropanol. For compound 3, the reaction mixture stirred in a 1:1 H2O:MeOH solution and
the resultant fine black powder was isolated by suction filtration and washed with water,
methanol, and ethanol. The product was further purified by flash column chromatography with
chloroform as the eluent. Compound 3 was isolated with 55 % yield. Compound 1 was
1
characterized by H and 13C NMR spectroscopy and identified by CHN elemental analysis.
Compounds 2 and 3 were characterized by 1H NMR spectroscopy but had insufficient solubility
to obtain reasonable 13C NMR spectra, but were also identified by CHN elemental analysis.
2.2 Optical Properties
The UV-Vis absorption spectra of the final compounds were obtained from 1% wt/v
solutions in chloroform, both solution and thin-film absorption profiles are shown in Figure 4
and the data is summarized in Table 1. The IQ terminal units are neither strongly electron
donating or accepting, thus the optical properties are predominately determined by the extended
π-conjugation and the nature of core building block. Compound 1 primarily absorbs light in the
blue region of the visible spectrum with λonset at ~492 nm in solution. A small electron
delocalization pathway and lack of D-A character limit light absorption to high energy photons.
Compared to compound 1 with a thiophene core, compound 2 with the BDT core has an
extended π-conjugated framework and leads to a red-shift in absorbance with a λonset at ~525
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solution. Compound 3 highlights the benefit of using a strongly absorbing dye as well as the D-
A approach to bandgap engineering as the D-A character of the thiophene-flanked isoindigo core
further narrows the bandgap, resulting in an absorption spectra extending beyond 700 nm. Upon
transitioning from solution to thin-film, a bathochromic shift in the absorption profile is observed
for each of the three molecules indicating intermolecular interactions in the solid-state. The
more pronounced low energy band visible in both solution and thin film of compound 2 is likely
a result of stronger intermolecular π−π interactions compared to compound 1. The
photoluminescence was also measured for compounds 1–3. Compound 1 has a small Stokes
shift of 43 nm, where as 2 and 3 have relatively larger Stokes shift of 141 nm 125 nm
respectively, a result of their increased conformational flexibility.
The electronic character of IQ is further highlighted upon comparing compounds 1-3
with reference compounds in the literature (Figure 5). Compound 1 has a very comparable
absorption profile to that of a similar compound Phth-Th-Phth, which is comprised of a
thiophene core end capped with alkylated phthalimides.[54–56] The onset of Phth-Th-Phth in
solution is 440 nm with an absorption max centered at 390 nm compared to 492 nm and 430 nm
for compound 1. Although 1 has a significantly extended π-conjugated pathway (9 units vs. 3
units) the absorption profile is only red-shifted by less than 50 nm. Only a slight red-shift is
observed for 1 with its extended conjugation as it lacks the D/A character of Phth-Th-Phth
which would lead to more dramatic shift. Phthalimide is an electron acceptor and when paired
with the donor thiophene core results in a D/A type system. Alternatively, for compound 1 the
bandgap is primarily dictated by the length of the π-conjugated backbone with no significant
D/A intramolecular interaction which would lead to a further red shift in absorption. Compound
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2 can be compared to the previously reported DO1[57], which is comprised of a BDT core with
electron accepting indandione endcaps bridged with alkyl thiophenes. Despite a similar
conjugation length to 2, the D/A character of DO1 leads to a significant red shifted (~125 nm)
absorption spectra as a result of the interaction between the strongly electron donating BDT core
and the strong terminal acceptor indandione. We can compare compound 3 with two different
isoindigo dithiophene compounds in the literature, Oct-II(ThPhth-1EP)2[53] which has
terminal electron accepting phthalimide units and II(Th2)2[58] which is end-capped with electron
donating hexyl-bithiophenes. The absorption profile of compound 3 with the IQ terminal units
lies appropriately in between the two, but slightly closer to that of II(Th2)2. The onset of
absorption for 3 is slightly red shifted from the II(ThPhth-1EP)2 with electron withdrawing
endcaps and is slightly blue shifted from II(Th2)2 with electron donating end caps. Thus, based
on these comparisons, the IQ unit can be thought of neither strongly electron donating or
withdrawing and can be used to effectively extend the size of the molecular structure without
dramatically altering the electron structure through D-A type electronic coupling.
2.3 Electrochemical Properties
The electrochemical properties of compounds 1–3 were investigated using cyclic
voltammetry (CV) and were found to be largely dominated by the nature of the core unit. The
highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO)
energy levels were measured from the onset of oxidation and reduction respectively relative to
the internal reference ferrocene. As shown in Figure 6, each compound has reversible oxidations
while only 3 has a reversible reduction, attributed to the electron accepting nature of the
isoindigo core. The lack of reversible reductions for 1 and 2 is reasonable, as a result of their
electron rich character. Considering the ionization potential (IP) of the two electron-donating
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cores, the substitution of Th for a BDT core is accompanied by a decrease in the IP of the
compound highlighting the more electron-rich nature of compound 2. On the other hand,
monitoring the change in electron affinity (EA) is more suitable for the II(Th)2 core substitution,
which demonstrates a greater EA than compounds 1 and 2, a result of the electron-withdrawing
character of the isoindigo core. Another noticeable trend for compound 1–3 is the narrowing of
the band gap, which is consistent with the increase in effective conjugation length.
2.4 Theoretical Modeling
Theoretical calculations were used to investigate optimized geometry (gas phase),
electronic structure, and electronic transitions of the three new chromophores with IQ terminal
units (Figure 7), see ESI for full computational methods. The molecular core dictates the
planarity of the structure, with II(Th)2 being the least distorted backbone, followed by BDT and
lastly Th. Although the π-conjugated backbone of compound 1 is slightly distorted, the
predicted HOMO and LUMOs are fully delocalized along the entire structure indicating good
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electronic communication between the Th and IQ units. Upon transitioning from the relatively
weakly donating Th core to the relatively strongly donating BDT core, the LUMO remains fully
delocalized; however, the HOMO is appropriately localized on the strong electron-donating core,
with limited contribution from the terminal IQ units. With the incorporation of the electron
accepting isoindigo core, the IQ terminal unit does not contribute to either the HOMO or LUMO
delocalized π-systems. The LUMO is largely localized at electron deficient isoindigo core, while
the HOMO π-system does not extend past the indole ring of the IQ end cap. These depictions of
the ground state geometry and π-system lend further evidence that the IQ unit is nether electron
rich or poor and can be consider electron neutral, and highlight the overwhelming influence of
the core on all facets of material properties. The predicted HOMO/LUMO energy levels and
absorption profile for the three small molecules follow the same trend observed experimentally
with progressively red-shifted band gaps; however, in the case of the 3 it was the higher energy
band that was more prominent, not the low energy band as predicted by TD-DFT.
3 Conclusions
In this study we reported the synthesis of 8-bromo-6H-indolo[2,3-b]quinoxaline (IQ), a
novel building block for application in organic electronics. IQ is easily accessible from
inexpensive starting materials and a sustainable high yielding synthesis. The IQ building block
was found to be useful as an end-capping unit for the construction of organic π-conjugated small
molecules. The IQ building block was found to be neither strongly electron rich nor poor and
can be considered an electron neutral unit. IQ is effective for the extension and planarization and
of π-conjugated systems and for allowing the material properties to be dictated by the nature of
the core building block. The versatility of the framework and the wealth of available substituted
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phenylenediamines and indoles can be exploited to tailor the electronic properties of this
terminal unit to the desired effect.
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4 Experimental
Full experimental details including materials and methods, instrumentation, and NMR
spectra can be found in the supporting information.
4.1 Synthesis of 8-bromo-6H-indolo[2,3-b]quinoxaline (IQ).
A solution of 6-bromoisatin (1.123 g, 4.5 mmol) and o-phenylenediamine (0.722 g 6.7
mmol) in glacial acetic acid (30 mL) was refluxed for 30 minutes when a significant amount of
yellow precipitate was observed. DMF (30 mL) was added to the mixture, the temperature was
increased to 150oC and the reaction was allowed to stir at reflux for 24 hours. The dark
red/brown solution was cooled to room temperature and a yellow/brown precipitate was
observed. The material was collected via vacuum filtration and washed with copious amounts of
water followed by ~100 mL of 1:1 H2O:Et2OH solution. The solid was dried in vacuo to yield
1
1.205 g (81 % yield) of the product as a light yellow powder. H NMR (300MHz, DMSO-d6,
1
2
ppm): δ 7.51 (dd, 1J= 9.9 Hz, 2J= 1.5 Hz, 1H), 7.71-7.85 (m, ov, 3H), 8.06 (dd, J= 9.3 Hz, J=
2
1.2 Hz 1H), 8.23 (dd,1J= 9.6 Hz, J= 1.2 Hz, 1H), 8.27 (d, J=8.1 Hz, 1H). 13C{1H} NMR
(300MHz, DMSO-d6, ppm): δ 114.9, 118.2, 124.0, 124.1, 124.5, 126.7, 127.7, 138.8, 139.1,
140.3, 144.9, 146.0. Mass Spec: Theoretical: 298.1, Experimental: 298.0
4.2 Synthesis of 8-bromo-N-octyl-indolo[2,3-b]quinoxaline (IQ-Oct).
A solution of 8-bromo-indolo[2,3-b]quinoxaline (0.40 g, 1.34 mmol), K2CO3 (0.28 g,
2.02 mmol), 1-octylbromide (0.3 mL, 1.34 mmol) in anhydrous DMF (5 mL) was heated to
100oC for 16 hours in a 20 mL pressure tube capped under N2. The reaction was cooled to room
temperature and poured into a 1:1 solution of H2O/MeOH (~100 mL) and stirred for two hours.
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A bright orange solid was recovered via suction filtration. The solid was washed with copious
amounts of water followed by ~100 mL 1:1 H2O/MeOH solution. The solid was dried in vacuo
to give 0.46 g (84% yield) of the alkylated product. 1H NMR (300MHz, CDCl3, ppm): δ 0.88 (t,
1
3H), 1.25-1.31 (m, br, 6H), 1.39-1.40 (m, br, 4H), 1.91-2.00 (m, 2H), 4.47 (t, 2H), 7.55 (dd, J=
9.8 Hz, 2J= 1.6 Hz, 1H), 7.65 (d, J= 1.5 Hz, 1H), 7.68-7.79 (m, ov, 2H), 8.15 (dd, 1J= 9.6 Hz, 2J=
1
2
1.3 Hz, 1H), 8.30 (dd, J= 9.7 Hz, J= 1.3 Hz, 1H), 8.34 (d, J= 8.3 Hz, 1H). 13C{1H} NMR
(300MHz, CDCl3, ppm): δ 14.0, 22.6, 27.0, 28.4, 29.1, 29.2, 31.8, 41.6, 112.8, 118.4, 123.8,
124.0, 125.1, 126.3, 128.0, 129.0, 129.4, 139.5, 140.7, 145.7. Mass Spec: Theoretical: 410.4,
Experimental: 410.0
4.3 Synthesis of 2,5-bis(6-octyl-6H-indolo[2,3-b]quinoxalin-8-yl)thiophene (1).
8-bromo-N-octyl-indolo[2,3-b]quinoxaline (0.15 g, 0.37 mmol), bis-trimethylstannyl
thiophene (0.73 g, 0.18 mmol), SiliaCat® DPP-Pd (0.05 g), and bench top toluene (4 mL) was
added to a 5 mL microwave vial and sealed with a Teflon® cap. The vial was heated in a
microwave reactor for at 150ºC for 30 minutes. The reaction mixture was cooled to room
temperature and poured into methanol (~300 mL). The orange solid was collected via suction
filtration, dissolved in dichloromethane and solution filtered to remove the heterogeneous
catalyst (SiliaCat® DPP-Pd). The solvent was removed under reduced pressure, the material
slurried in methanol, and the resultant product was collected via suction filtration washing with
ethanol and isopropanol. The collected solid was dried in vacuo yielding 0.21 g (77% yield) of
compound 1. 1H NMR (300 MHz, CDCl3, ppm): δ 0.86 (t, 6H), 1.21-1.36, (m, br, 12H), 1.36 –
1.55 (m, br, 8H), 1.96-2.03 (m, br, 4H), 4.52 (t, 4H), 7.53 (s, 2H), 7.62-7.75 (ov, 8H), 8.11 (dd,
2
1
2
1J= 9.5 Hz, J= 1.3 Hz, 2H), 8.27 (dd, J= 9.5 Hz, J= 1.2 Hz 2H), 8.46 (d, J= 8.0 Hz, 2H).
13C{1H} NMR (300 MHz, CDCl3, ppm): δ 14.1, 22.6, 27.1, 28.5, 29.2, 31.8, 41.5, 106.0, 118.7,
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118.9, 123.2, 125.5, 126.0, 127.9, 128.7, 129.2, 136.6, 139.5, 140.6, 144.7, 144.9, 146.2.
Elemental Analysis: Calculated for C48H50N6S: C, 77.6; H, 6.8; N, 11.3, Found: C, 76.4; H, 6.5;
N, 11.1.
4.4 Synthesis of 8,8'-(4,8-bis(octan-3-yloxy)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(6-
octyl-6H-indolo[2,3-b]quinoxaline) (2).
8-bromo-indolo[2,3-b]quinoxaline (0.22 g, 0.53 mmol), 2,6-bis(trimethyltin)-4,8-bis(2-
ethylhexyloxy)benzo[1,2-b:4,5-b’] dithiophene (0.19 g, 0.25 mmol), SiliaCat DPP-Pd (0.05 g),
and bench top toluene (4 mL) was added to a 5 mL microwave vial and sealed with a Teflon®
cap. The vial was heated in a microwave reactor at 150ºC for 30 minutes. The reaction mixture
was cooled to room temperature and poured into methanol (~300 mL). The dark orange solid
was collected via filtration, dissolved in chloroform and solution filtered to remove the
heterogeneous catalyst (SiliaCat® DPP-Pd). The solvent was removed under reduced pressure,
the material slurried in methanol, and the resultant product was collected via suction filtration
washing with ethanol and isopropanol. The collected solid was dried in vacuo yielding 0.17g (60
% yield) of compound 2. 1H NMR (300 MHz, 325 K, CDCl3, ppm) δ 0.85 (t, 6H), 1.01 (t, 6H),
1.14 (t, 6H), 1.20 – 1.36 (m, br 12 H), 1.36-1.55 (m, br, 14 H), 1.55-2.10 (m, br, 16 H), 4.32 (d,
J=5.4 Hz, 4H), 4.55 (t, 4H), 7.58 – 7.74 (m, 8H), 7.81 (s, 2H), 8.09 (d, J= 8.1 Hz, 2H), 8.25 (d,
J= 8.0 Hz 2H), 8.47 (d, J= 8.1 Hz, 2H). 13C{1H} NMR: Not able to obtain due to low solubility.
Elemental Analysis: Calculated for C70H84N6O2S2: C, 76.1; H, 7.7; N, 7.6. Found: C, 75.1; H,
7.4; N, 7.5.
4.5 Synthesis of 6,6’-di(thiophen-2-yl)-1,1’-di(octyl)isoindigo (Oct-II(Th)2):
Synthesized according to literature procedure.[53]
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4.6 Synthesis of 1,1'-dioctyl-6,6'-bis(5-(6-octyl-6H-indolo[2,3-b]quinoxalin-8-yl)thiophen-2-
yl)-isoindigo (3):
A 5 mL pressure tube was loaded with Oct-II(Th)2 (0.184 g, 0.283 mmol), IQ-Oct (0.253
g, 0.617 mmol), Pd(OAc)2 (0.005 g, 8 mol%), pivalic acid (0.014g, 48 mol%), K2CO3 (0.100g,
0.724 mmol) and anhydrous DMF (4 mL). The reaction mixture was sealed under N2 and heated
at 100ºC for 3 hours. Upon cooling to room temperature, the reaction mixture was poured into
~200 mL of a 1:1 H2O:MeOH solution and left to stir for two hours. The fine black solid was
isolated by suction filtration and washed with water, methanol, and ethanol. The product was
1
further purified by flash column chromatography with chloroform as the eluent (55% yield). H
NMR (300 MHz, 325 K, CDCl3, ppm) δ 0.87-0.97 (m, 14 H), 1.23-1.44 (m, 38H), 1.78 (t, 4 H),
1.97 (t, 4H), 3.84 (t, 4H), 4.46 (t, 4H), 6.79 (d, J= 1.4 Hz, 2H), 7.20 (dd, 1J= 9.96 Hz, 2J= 1.5 Hz
2H), 7.34 (dd, 1J= 14.2 Hz, 2J= 3.9 Hz 4H), 7.45 (s, 2H), 7.49 – 7.58 (m, 6H), 7.98 (dd, 1J= 9.7
2
1
2
Hz, J= 2.1 Hz 2H), 8.18 (dd, J= 9.8 Hz, J= 1.5 Hz 2H), 8.33 (d, J= 8.1 Hz, 2H), 9.13 (d, J=
8.4 Hz, 2H). 13C{1H} NMR: Not able to obtain due to low solubility. Elemental Analysis:
Calculated for C84H92N8O2S2: C, 77.0; H, 7.1; N, 8.6. Found: C, 76.5; H, 7.0; N, 8.1.
Acknowledgement
Dalhousie University, the Canada Research Chairs Program, and the Natural Science and
Engineering Council of Canada are acknowledged for financial support. Computational time was
provided by the Atlantic Computational Excellence Network (ACEnet) and Compute Calcul
Canada. The authors would like to acknowledge Dr. Lesley Rutledge for assistance with the
computational work and Seth McAfee for help with editing the manuscript.
15

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Supporting Information Available: Materials and methods, instrumentation, NMR Spectra,
and computational details are available.
References
[1] Li Y, Guo Q, Li Z, Pei J, Tian W. Solution processable D–A small molecules for bulk-
heterojunction solar cells. Energy Environ Sci 2010;3:1427–36. doi:10.1039/C003946B.
[2] Würthner F, Meerholz K. Systems Chemistry Approach in Organic Photovoltaics. Chem -
Eur J 2010;16:9366–73. doi:10.1002/chem.201001153.
[3] Walker B, Kim C, Nguyen T-Q. Small Molecule Solution-Processed Bulk Heterojunction
Solar Cells †. Chem Mater 2011;23:470–82. doi:10.1021/cm102189g.
[4] Lin Y, Li Y, Zhan X. Small molecule semiconductors for high-efficiency organic
photovoltaics. Chem Soc Rev 2012;41:4245–72. doi:10.1039/C2CS15313K.
[5] Mishra A, Bäuerle P. Small Molecule Organic Semiconductors on the Move: Promises for
Future Solar Energy Technology. Angew Chem Int Ed 2012;51:2020–67.
doi:10.1002/anie.201102326.
[6] Chen Y, Wan X, Long G. High Performance Photovoltaic Applications Using Solution-
Processed Small Molecules. Acc Chem Res 2013;46:2645–55. doi:10.1021/ar400088c.
[7] Roncali J, Leriche P, Blanchard P. Molecular Materials for Organic Photovoltaics: Small is
Beautiful. Adv Mater 2014;26:3821–38. doi:10.1002/adma.201305999.
[8] Kan B, Li M, Zhang Q, Liu F, Wan X, Wang Y, et al. A Series of Simple Oligomer-like
Small Molecules Based on Oligothiophenes for Solution-Processed Solar Cells with High
Efficiency. J Am Chem Soc 2015;137:3886–93. doi:10.1021/jacs.5b00305.
[9] Ni W, Wan X, Li M, Wang Y, Chen Y. A–D–A small molecules for solution-processed
organic photovoltaic cells. Chem Commun 2015;51:4936–50. doi:10.1039/C4CC09758K.
[10] Mishra A, Fischer MKR, Bäuerle P. Metal-Free Organic Dyes for Dye-Sensitized Solar
Cells: From Structure: Property Relationships to Design Rules. Angew Chem Int Ed
2009;48:2474–99. doi:10.1002/anie.200804709.
[11] Ooyama Y, Harima Y. Molecular Designs and Syntheses of Organic Dyes for Dye-
Sensitized Solar Cells. Eur J Org Chem 2009;2009:2903–34. doi:10.1002/ejoc.200900236.
[12] Yen Y-S, Chou H-H, Chen Y-C, Hsu C-Y, Lin JT. Recent developments in molecule-based
organic materials for dye-sensitized solar cells. J Mater Chem 2012;22:8734–47.
doi:10.1039/C2JM30362K.
[13] Dessì A, Calamante M, Mordini A, Peruzzini M, Sinicropi A, Basosi R, et al. Organic dyes
with intense light absorption especially suitable for application in thin-layer dye-sensitized
solar cells. Chem Commun 2014;50:13952–5. doi:10.1039/C4CC06160H.
[14] Allard S, Forster M, Souharce B, Thiem H, Scherf U. Organic Semiconductors for
Solution-Processable Field-Effect Transistors (OFETs). Angew Chem Int Ed
2008;47:4070–98. doi:10.1002/anie.200701920.
[15] Mas-Torrent M, Rovira C. Novel small molecules for organic field-effect transistors:
towards processability and high performance. Chem Soc Rev 2008;37:827–38.
doi:10.1039/B614393H.
[16] Wang C, Dong H, Hu W, Liu Y, Zhu D. Semiconducting π-Conjugated Systems in Field-
Effect Transistors: A Material Odyssey of Organic Electronics. Chem Rev 2012;112:2208–
67. doi:10.1021/cr100380z.
16

ACCEPTED MANUSCRIPT
[17] Mei J, Diao Y, Appleton AL, Fang L, Bao Z. Integrated Materials Design of Organic
Semiconductors for Field-Effect Transistors. J Am Chem Soc 2013;135:6724–46.
doi:10.1021/ja400881n.
[18] Liu J, Wu Y, Qin C, Yang X, Yasuda T, Islam A, et al. A dopant-free hole-transporting
material for efficient and stable perovskite solar cells. Energy Environ Sci 2014;7:2963–7.
doi:10.1039/C4EE01589D.
[19] Qin P, Paek S, Dar MI, Pellet N, Ko J, Grätzel M, et al. Perovskite Solar Cells with 12.8%
Efficiency by Using Conjugated Quinolizino Acridine Based Hole Transporting Material. J
Am Chem Soc 2014;136:8516–9. doi:10.1021/ja503272q.
[20] Wang J, Wang S, Li X, Zhu L, Meng Q, Xiao Y, et al. Novel hole transporting materials
with a linear π-conjugated structure for highly efficient perovskite solar cells. Chem
Commun 2014;50:5829–32. doi:10.1039/C4CC01637H.
[21] Ganesan P, Fu K, Gao P, Raabe I, Schenk K, Scopelliti R, et al. A simple spiro-type hole
transporting material for efficient perovskite solar cells. Energy Environ Sci 2015.
doi:10.1039/C4EE03773A.
[22] Swetha T, Singh SP. Perovskite solar cells based on small molecule hole transporting
materials. J Mater Chem A 2015. doi:10.1039/C5TA02507A.
[23] Xu B, Tian H, Lin L, Qian D, Chen H, Zhang J, et al. Integrated Design of Organic Hole
Transport Materials for Efficient Solid-State Dye-Sensitized Solar Cells. Adv Energy Mater
2015;5:n/a – n/a. doi:10.1002/aenm.201401185.
[24] Darling SB, You F. The case for organic photovoltaics. RSC Adv 2013;3:17633–48.
doi:10.1039/C3RA42989J.
[25] Forrest SR. The path to ubiquitous and low-cost organic electronic appliances on plastic.
Nature 2004;428:911–8. doi:10.1038/nature02498.
[26] Kaltenbrunner M, White MS, Głowacki ED, Sekitani T, Someya T, Sariciftci NS, et al.
Ultrathin and lightweight organic solar cells with high flexibility. Nat Commun
2012;3:770–7. doi:10.1038/ncomms1772.
[27] Kippelen B, Brédas J-L. Organic photovoltaics. Energy Environ Sci 2009;2:251–61.
doi:10.1039/B812502N.
[28] Li G, Zhu R, Yang Y. Polymer solar cells. Nat Photonics 2012;6:153–61.
doi:10.1038/nphoton.2012.11.
[29] Søndergaard R, Hösel M, Angmo D, Larsen-Olsen TT, Krebs FC. Roll-to-roll fabrication of
polymer solar cells. Mater Today 2012;15:36–49. doi:10.1016/S1369-7021(12)70019-6.
[30] Su Y-W, Lan S-C, Wei K-H. Organic photovoltaics. Mater Today 2012;15:554–62.
doi:10.1016/S1369-7021(13)70013-0.
[31] McCulloch I. Organic Electronics. Adv Mater 2013;25:1811–2.
doi:10.1002/adma.201205216.
[32] Anthony JE. Organic electronics: Addressing challenges. Nat Mater 2014;13:773–5.
doi:10.1038/nmat4034.
[33] Irimia-Vladu M, Głowacki ED, Voss G, Bauer S, Sariciftci NS. Green and biodegradable
electronics. Mater Today 2012;15:340–6. doi:10.1016/S1369-7021(12)70139-6.
[34] Irimia-Vladu M. “Green” electronics: biodegradable and biocompatible materials and
devices for sustainable future. Chem Soc Rev 2013;43:588–610.
doi:10.1039/C3CS60235D.
17

ACCEPTED MANUSCRIPT
[35] Marzano G, Ciasca CV, Babudri F, Bianchi G, Pellegrino A, Po R, et al. Organometallic
Approaches to Conjugated Polymers for Plastic Solar Cells: From Laboratory Synthesis to
Industrial Production. Eur J Org Chem 2014;2014:6583–614. doi:10.1002/ejoc.201402226.
[36] Osedach TP, Andrew TL, Bulović V. Effect of synthetic accessibility on the commercial
viability of organic photovoltaics. Energy Environ Sci 2013;6:711–8.
doi:10.1039/C3EE24138F.
[37] Roncali J, Thobie-Gautier C. An efficient strategy towards small bandgap polymers: The
rigidification of the π-conjugated system. Adv Mater 1994;6:846–8.
doi:10.1002/adma.19940061108.
[38] Brisset H, Thobie-Gautier C, Gorgues A, Jubault M, Roncali J. Novel narrow bandgap
polymers from sp3 carbon-bridged bithienyls: poly(4,4-ethylenedioxy-4H-cyclopenta[2,1-
b;3,4-b′]dithiophene). J Chem Soc Chem Commun 1994:1305–6.
doi:10.1039/C39940001305.
[39] Kitamura C, Tanaka S, Yamashita Y. Design of Narrow-Bandgap Polymers. Syntheses and
Properties of Monomers and Polymers Containing Aromatic-Donor and o-Quinoid-
Acceptor Units. Chem Mater 1996;8:570–8. doi:10.1021/cm950467m.
[40] Brocks G, Tol A. Small Band Gap Semiconducting Polymers Made from Dye Molecules:ꢀ
Polysquaraines. J Phys Chem 1996;100:1838–46. doi:10.1021/jp952276c.
[41] Yamamoto T, Zhou Z, Kanbara T, Shimura M, Kizu K, Maruyama T, et al. π-Conjugated
Donor−Acceptor Copolymers Constituted of π-Excessive and π-Deficient Arylene Units.
Optical and Electrochemical Properties in Relation to CT Structure of the Polymer. J Am
Chem Soc 1996;118:10389–99. doi:10.1021/ja961550t.
[42] Hari Narayana Moorthy NS, Karthikeyan C, Trivedi P. Design, synthesis, cytotoxic
evaluation, and QSAR study of some 6H-indolo[2,3-b]quinoxaline derivatives. J Enzyme
Inhib Med Chem 2010;25:394–405. doi:10.3109/14756360903190747.
[43] Shibinskaya MO, Lyakhov SA, Mazepa AV, Andronati SA, Turov AV, Zholobak NM, et
al. Synthesis, cytotoxicity, antiviral activity and interferon inducing ability of 6-(2-
aminoethyl)-6H-indolo[2,3-b]quinoxalines. Eur J Med Chem 2010;45:1237–43.
doi:10.1016/j.ejmech.2009.12.014.
[44] Narayana Moorthy NSH, Manivannan E, Karthikeyan C, Trivedi P. 6H-Indolo[2,3-
b]Quinoxalines: DNA and Protein Interacting Scaffold for Pharmacological Activities. Mini
Rev Med Chem 2013;13:1415–20.
[45] Thomas KRJ, Tyagi P. Synthesis, Spectra, and Theoretical Investigations of the
Triarylamines Based on 6H-Indolo[2,3-b]quinoxaline. J Org Chem 2010;75:8100–11.
doi:10.1021/jo1016663.
[46] Tyagi P, Venkateswararao A, Thomas KRJ. Solution Processable Indoloquinoxaline
Derivatives Containing Bulky Polyaromatic Hydrocarbons: Synthesis, Optical Spectra, and
Electroluminescence. J Org Chem 2011;76:4571–81. doi:10.1021/jo2004764.
[47] Venkateswararao A, Tyagi P, Justin Thomas KR, Chen P-W, Ho K-C. Organic dyes
containing indolo[2,3-b]quinoxaline as a donor: synthesis, optical and photovoltaic
properties. Tetrahedron 2014;70:6318–27. doi:10.1016/j.tet.2014.04.009.
[48] Qian X, Gao H-H, Zhu Y-Z, Lu L, Zheng J-Y. 6H-Indolo[2,3-b]quinoxaline-based organic
dyes containing different electron-rich conjugated linkers for highly efficient dye-sensitized
solar cells. J Power Sources 2015;280:573–80. doi:10.1016/j.jpowsour.2015.01.148.
18

ACCEPTED MANUSCRIPT
[49] Przyjazna B, Kucybała Z, Pączkowski J. Development of new dyeing photoinitiators based
on 6H-indolo[2,3-b]quinoxaline skeleton. Polymer 2004;45:2559–66.
doi:10.1016/j.polymer.2004.02.025.
[50] Dowlatabadi R, Khalaj A, Rahimian S, Montazeri M, Amini M, Shahverdi A, et al. Impact
of Substituents on the Isatin Ring on the Reaction Between Isatins with Ortho-
Phenylenediamine. Synth Commun 2011;41:1650–8. doi:10.1080/00397911.2010.491596.
[51] Areephong J, Hendsbee AD, Welch GC. Facile synthesis of unsymmetrical and π-extended
furan-diketopyrrolopyrrole derivatives through C–H direct (hetero)arylation using a
heterogeneous catalyst system. New J Chem 2015. doi:10.1039/C5NJ01150G.
[52] McAfee SM, McCahill JSJ, Macaulay CM, Hendsbee AD, Welch GC. Utility of a
heterogeneous palladium catalyst for the synthesis of a molecular semiconductor via Stille,
Suzuki, and direct heteroarylation cross-coupling reactions. RSC Adv 2015;5:26097–106.
doi:10.1039/C5RA02468D.
[53] McAfee SM, Topple JM, Payne A-J, Sun J-P, Hill IG, Welch GC. An Electron-Deficient
Small Molecule Accessible from Sustainable Synthesis and Building Blocks for Use as a
Fullerene Alternative in Organic Photovoltaics. ChemPhysChem 2015;16:1190–202.
doi:10.1002/cphc.201402662.
[54] Hendsbee AD, Macaulay CM, Welch GC. Synthesis of an H-aggregated thiophene–
phthalimide based small molecule via microwave assisted direct arylation coupling
reactions. Dyes Pigments 2014;102:204–9. doi:10.1016/j.dyepig.2013.10.046.
[55] Sun J-P, Hendsbee AD, Eftaiha AF, Macaulay C, Rutledge LR, Welch GC, et al.
Phthalimide–thiophene-based conjugated organic small molecules with high electron
mobility. J Mater Chem C 2014;2:2612–21. doi:10.1039/C3TC32497D.
[56] Hendsbee AD, Sun J-P, McCormick TM, Hill IG, Welch GC. Unusual loss of electron
mobility upon furan for thiophene substitution in a molecular semiconductor. Org Electron
2015;18:118–25. doi:10.1016/j.orgel.2014.12.033.
[57] Shen S, Jiang P, He C, Zhang J, Shen P, Zhang Y, et al. Solution-Processable Organic
Molecule Photovoltaic Materials with Bithienyl-benzodithiophene Central Unit and
Indenedione End Groups. Chem Mater 2013;25:2274–81. doi:10.1021/cm400782q.
[58] Mei J, Graham KR, Stalder R, Reynolds JR. Synthesis of Isoindigo-Based Oligothiophenes
for Molecular Bulk Heterojunction Solar Cells. Org Lett 2010;12:660–3.
doi:10.1021/ol902512x.
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Graphical Abstract:
Functional group(s) to tailor
electronics and self-assembly
Alkyl group for solubility
Planar framework
Extension of π-conjugated system
Tuneable terminal building block
20

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Figures and Tables:
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H
N
A)
B)
H
N
NH2
NH2
1.) CH3COOH, reflux
1.) CH3COOH, reflux
N
O
O
+
N
O
indolequinoxaline
Br
H
NH2
NH2
Br
+
N
N
NH2
O
N
O
H
ringꢀopened species
H
C)
D)
H
N
N
NH2
NH2
Br
1.) CH3COOH, reflux , 30 min
2.) DMF, 150 ºC, 16 hrs
N
+
O
N
Br
O
IQ
Octyl
H
N
N
N
N
N
K2CO3, BrꢀOctyl
N
ꢀ Functionalized indolequinoxaline
ꢀ Utility as terminal conjugated unit
Br DMF 100 oC 16 hrs
Br
IQ
IQꢀOct
22

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C8H17
N
N
N
SiliaCat® DPPꢀPd
N
S
Me3Sn
SnMe3
S
N
Toluene
MW 150 ºC 30 min.
N
77%
C8H17
1
C8H17
N
R1
R
1 O
O
O
C8H17
N
N
SiliaCat® DPPꢀPd
N
S
S
N
Me3Sn
SnMe3
Br
+
N
S
Toluene
MW 150 oC 30 min.
N
S
N
N
C8H17
2eq. IQꢀOct
O
2
R1
R1
60%
R2
N
R2
N
Pd(OAc)2
PivOH, K2CO3
C8H17
N
O
N
N
O
N
S
S
S
S
DMF
100 oC 2.5 hrs
N
O
55%
O
N
C8H17
N
R2
R2
N
3
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Compound
Solution
λonset (nm)
Film
λonset
(nm)
Film
λmax (nm)
Eopt
(eV)
HOMO1
(eV)
LUMO1
(eV)
Eg
(eV)
1
2
3
492
525
715
545
565
750
440
2.28 -5.45
-3.51
-3.53
-3.74
1.94
1.75
1.57
405,440, 500 2.19 -5.28
435,590
1.65 -5.31
1 Determined using cyclic voltammetry
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