Stepwise Bay Annulation of Indigo for the Synthesis of Desymmetrized Electron Acceptors and Donor-Acceptor Constructs

Article abstract of DOI:10.1021/acs.orglett.6b02504

A selective stepwise annulation of indigo has been demonstrated as a means of providing both monoannulated and differentially double-annulated indigo derivatives. Disparate substitution of the electron accepting bay-annulated indigo system allows for fine control over both the electronic properties as well as donor-acceptor structural architectures. Optical and electronic properties were characterized computationally as well as through UV-vis absorption spectroscopy and cyclic voltammetry. This straightforward method provides a modular approach for the design of indigo-based materials with tailored optoelectronic properties.

Full text of DOI:10.1021/acs.orglett.6b02504

Letter
pubs.acs.org/OrgLett
Stepwise Bay Annulation of Indigo for the Synthesis of
Desymmetrized Electron Acceptors and Donor−Acceptor Constructs
Matthew A. Kolaczkowski,†,‡ Bo He,† and Yi Liu*,†
†The Molecular Foundry, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California 94720, United States
‡Department of Chemistry, University of California, Berkeley, California 94720, United States
S
* Supporting Information
ABSTRACT: A selective stepwise annulation of indigo has been
demonstrated as a means of providing both monoannulated and
differentially double-annulated indigo derivatives. Disparate substitution of
the electron accepting bay-annulated indigo system allows for fine control
over both the electronic properties as well as donor−acceptor structural
architectures. Optical and electronic properties were characterized computa-
tionally as well as through UV−vis absorption spectroscopy and cyclic
voltammetry. This straightforward method provides a modular approach for
the design of indigo-based materials with tailored optoelectronic properties.
ndigo is known as one of the oldest dyes and has fascinating
photophysical properties for a molecule of its size.1 It has
Through this transformation, the resulting bay-annulated indigo
(BAI) 2 features reinforced planarity, enhanced conjugation
across the bay, and strong electron accepting ability owing to
the conjugated electron-withdrawing amide groups. BAI
behaves as an excellent electron acceptor, which has been
incorporated in the synthesis of panchromatic small mole-
cules,12 and low “bandgap” polymers for ambipolar charge
transport,11b,13 organic solar cells13 and electrochromic
devices.14
An unexplored approach to synthetically engineer the
optoelectronic properties of BAIs is to desymmetrize its
structure via sequential annulation of the two bay positions
(Scheme 1b). Such desymmetrization not only offers great
synthetic flexibility toward more complex donor−acceptor
constructs, but also provides fine control over the energy levels
through modular synthesis. Desymmetrized functionalization
has been explored in electron acceptors such as arene diimide.15
The commonly adapted stepwise imidation procedure has little
influence on the electronic properties of these imide-based
acceptors as the substituents are electronically isolated from the
core. In desymmetrized BAIs, the different aromatic units
attached to the opposite bay positions are expected to have a
significant impact on the electronic structures and thus the
optoelectronic output.
I
received increasing attention for its potential as a low-cost
building unit for electroactive semiconducting materials, given
that it is produced industrially at over 50 kilotons per year.2
With the advancement of the field of organic semiconductors,
established dyes and pigments such as perylene imides,3
diketopyrrolopyrrole (DPP)4 and isoindigo,5 have provided a
rich source of inspiration for new families of π-conjugated
systems and high performance materials.6 Indigo 1 (Scheme
1a) and related indigoids have shown high ambipolar charge
Scheme 1. Previous Synthesis of BAI Compounds and a
Proposed Strategy to Access Desymmetrized BAI Acceptors
Stepwise annulation of indigo however has remained
unexplored. Promisingly, recent indigo chelation chemistry
has hinted at different reactivity of the functional group clusters
on each bay position of indigo. The reported diketoimine
indigo derivative, Ninidigo, forms distinct mono- and bis-BF2
adducts,16 as well as metal complexes.17 A selective covalent
annulation protocol is yet lacking and warrants in-depth
carrier mobilities in field effect transistors,7 however their use as
optoelectronic materials is significantly limited due to low
solubility. Attempts to improve processablilty of these materials
has stimulated efforts in derivatizing the chromophore, such as
placing solublizing groups on the outer benzene rings,8 or
functionalizing the amine9 and/or ketone groups10 in the
central bay position of the indigoids.
Among the several chemical pathways to transform indigo
into practical electroactive units, a double bay-annulation
strategy has emerged as one of the most effective methods.11
Received: August 21, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02504
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Organic Letters
Letter
investigation. Herein we report a detailed synthetic study that
provides a high yielding, stepwise strategy toward desymme-
trized BAIs.
Scheme 1b depicts the desired synthesis of desymmetrized
BAI through a monoannulated indigo (MAI) intermediate. If
formed selectively and isolated, MAI could be annulated a
second time with a different substituted acetyl chloride to form
a desymmetrized BAI. Our initial attempt in selective
monoannulation employed nearly stoichiometric 2-thiophene-
acetic chloride (3a) (Scheme 2a). When a 1.1:1 mixture of 3a
anhydride at 140 °C (Scheme 2b). As predicted, 5 was
significantly more soluble than indigo in the most common
organic solvents. When 5 was subjected to reaction with 2-
thiopheneacetyl chloride (3a) in dioxane, MAI 4a was obtained
in 87% isolated yield. The reaction proceeded much more
cleanly compared to that of indigo 1, with high conversion rate
and only trace bis-annulated product (entry 3 in Table S1). The
increased solubility of precursors contributed to a significantly
higher isolated yield of the desired monoannulated product,
with the added benefit of easier tracking and purification due to
fewer sideproducts. No acylated 4a-Ac was isolated from the
reaction, suggesting a facile deacylation step following
monoannulation. Solvent also plays a role. Changing the
solvent to toluene resulted in additional side products with
depressed yields (entry 4 in Table S1). Utilizing acetic
anhydride as solvent also provided good yield but with product
isolated in its acylated form 4a-Ac (entry 5 in Table S1).
Reaction of 1 under these conditions with 2.5 equiv. acetyl
chloride (entry 2 in Table S1) showed an improvement in yield
(70%) compared to entry 1, presumably through in situ
formation of 5.
Scheme 2. Improved Synthesis of Monoannulated Indigo via
Acetylation
1H NMR experiments provided some insight into the
mechanisms of initial deacylation, which are proposed and
illustrated in Scheme S2 in SI. As revealed by 1H NMR studies
(Figure S1), the formation of acetic anhydride and acetic acid
suggested possible transformations that are commensurate with
the improvement in yield. Water, which is a byproduct of the
cyclodehydration reaction, can form deleterious byproducts
through hydrolysis of the acetyl chloride starting material. As
water is formed during the condensation, 5 can be deacylated,
yielding the less nucleophilic acetic acid, which competes less
aggressively for decomposition of the acid chloride. Addition-
ally, once the acid chloride is hydrolyzed it may react further
with acetic anhydride or N,N′-diacyl indigo to form a mixed
anhydride and re-enter the reaction cycle.
In exploring the scope of this methodology several aromatic
acetyl chlorides were employed and shown to be competent
reactants for a selective monoannulation (Scheme 2b). In
addition to phenyl, a 4-bromophenyl MAI 4c was obtained in
decent yield following the same reaction conditions. MAIs 4d−
4f bearing alkyl or glycol chains can also be obtained from the
corresponding substituted 2-arylacetyl chlorides in decent to
excellent yields.
The synthesis of desymmetrized BAIs was attempted by
reacting MAIs with a second equivalent of 2-arylacetyl chloride
in xylene at 145 °C, which proceeded smoothly to give a range
of desymmetrized BAIs (Scheme 3). This stepwise annulation
allows the introduction of amphiphilicity onto the opposite bay
positions of BAI, such as 6a, or selective tuning of conjugation
via the incorporation of a thiophene unit, such as 6b and 6c.
The amphiphilic 6a was synthesized as a good candidate for
the formation of self-assembled monolayers through a
Langmuir−Blodgett process.19 However, 6b and 6c are
acceptors that could be easily functionalized for incorporation
into more complex donor−acceptor systems. As shown in
Scheme 3b, selective bromination on thiophene units in 6b and
6c proceeds in high yield under mild conditions to produce
bromides 7b and 7c, respectively. Further reaction of these
compounds with a number of donor units via Suzuki or Stille
coupling conditions gave rise to linear or C3-symmetric donor−
acceptor systems including BAI adducts of fluorene (F) 10,
benzodithiophene (BDT) 11, and triphenylamine (TPA) 12
(Scheme 4). Molecules with similar donor−acceptor architec-
and indigo was heated in 1,4-dioxane at 110 °C, a mixture
composed of MAI 4a (NMR yield: 54%), BAI 2 (NMR yield:
6%) and residual 1 was obtained (entry 1 in Table S1 in
Supporting Information, SI). Screening of solvent, temperature,
base, Lewis acid, and coupling reagents provided little
improvement over 50% yield (Tables S2 and S3). Utilization
of amide coupling reagents with the corresponding substituted
acetic acids was also ineffective (Scheme S1). The moderate
yield of the reaction itself was depressed further by difficult
purification of MAI due to the presence of a significant amount
of unreacted indigo and side products, all of which have low
solubility in common organic solvents.
The monoannulated indigo also experiences low solubility
due to the remaining hydrogen bonding and increased
tendency to aggregate. In order to promote greater reactivity
in the MAI system, attempts were made to increase its
solubility by removing the intramolecular H-bonds. An initial
reaction of MAI 4a with acetic anhydride afforded an acetylated
product 4a-Ac with very good solubility in common organic
solvents (Scheme 2a). When 4a-Ac was subjected to acidic or
basic condensation conditions, no annulation product was
formed, instead deacylation occurred to give 4 in quantitative
yield. The facile deacylation prompted us to reason that a
preacylated indigo compound, such as N,N′-diacetylindigo 5,
would be a good alternative starting material which not only
ensures high solubility, but also retains good reactivity toward
annulation through in situ deprotection under acidic reaction
conditions.
To test this reaction route, 5 was synthesized in high yield
and purity following a modified acetylation protocol18 that
involves treating indigo 1 with acetyl chloride and acetic
B
DOI: 10.1021/acs.orglett.6b02504
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Scheme 3. Synthesis of Desymmetrized BAI Compounds
Figure 1. UV−vis spectroscopy of (a) MAIs 4a and 4b, (b) BAIs 6a,
6c and 2, and (c) donor−acceptor conjugates 10−12. (d) Cyclic
voltammetry of MAI and BAI compounds, using Fc/Fc+ as the internal
standard (2.5 μM in CHCl3, scan rate 100 mV/S).
Scheme 4. Synthesis of donor−acceptor systems using BAIs
the attached aromatic substituents. The high sensitivity of the
optoelectronic properties in response to slight structural
modifications further manifests the powerful nature of
desymmetrization chemistry. Coupling to stronger donors,
such as in the donor−acceptor conjugates 10, 12, and 11, led to
more drastic red shifts in the absorption spectra (Figure 1c),
corresponding to optical bandgaps of 1.8, 1.7, and 1.6 eV
respectively (Table S5 and Figure S3).
The electrochemical properties of MAI and BAI compounds
were investigated by cyclic voltammetry. Similar two quasi-
reversible one-electron reduction processes were observed for
all the compounds, with similar lowest unoccupied molecular
orbital (LUMO) energy levels situated within the range of −3.5
to −3.6 eV. The corresponding oxidations were mostly
irreversible or pseudoreversible; nevertheless, the highest
occupied molecular orbital (HOMO) energy levels and
electrochemical bandgaps could be estimated from the onset
of redox peaks except for compound 11, which match well with
the optical bandgaps (Table S5). The overall trend shows
consistent LUMO energies, suggesting the localization of
LUMO on the central MAI or BAI core, while the HOMO is
strongly dependent on the coupled electron donors.
DFT calculations were carried out to model the molecular
orbitals and frontier orbital energies, utilizing a B3LYP
functional with the 6-31G* basis set.21 The relative order of
the calculated HOMO−LUMO energy gap is consistent with
the experimental results (Table S5). The theoretical LUMO
frontier orbitals of both MAIs and BAI 6c indicated significant
delocalization over the AI core with a small contribution from
the attached donors (Figure 2 and S4). On the other hand, the
HOMO orbitals spread over the central diketopiperidopiper-
idine unit and extend into the conjugated donor units but with
varying degrees of delocalization onto the two benzene rings
from the parent indigo. The calculations on the dimeric and
trimeric donor−acceptor conjugates further illustrated that the
HOMOs have better delocalization along the conjugation
backbone, while the LUMOs are mainly localized on BAI units,
which is consistent with the experimental observation that they
all have very similar LUMO energies.
ture have shown great promise in engineering long-range
energy transport, an unusual property dominated by ordered
molecular self-assembly in the aggregated state.20
The desymmetrized functionalization provides a convenient
method to tune the energy levels through the incorporation of
different aromatic units on opposite sides of the bay position.
Compared to the parent indigo, the MAIs displayed a blue shift
of 40−60 nm for its lowest energy absorption peak due to
partial loss of the “H-chromophore” characteristics, as revealed
by UV−vis spectroscopy (Figure 1a and Figure S2 and Table
S4).9b A red shift, corresponding to a decrease of 0.04 eV in
optical bandgap, is observed when the flanking aromatic
substituent is changed from phenyl groups to a more donating
thiophene (Figure 1a). The electronic effect of the flanking
aromatic units on the optical properties was consistently
observed in the doubly annulated BAIs. The stacked absorption
spectra of BAIs 6a, 6c, and 2 in Figure 1b and Figure S3
indicated well-resolved and progressive red shifts of the
absorption maximum in the order of 0.05 eV, when the
aromatic substituents were changed successively from phenyl to
thienyl. This is in full agreement with the different electron-
donating ability between phenyl and thiophene groups, which
confirms good electronic coupling between BAI acceptors and
In summary, we report a highly effective method for stepwise
annulation of indigo via a latent diacetylated indigo precursor,
which enables the installation of different aromatic units to
form desymmetrized BAIs. The employment of the diacylated
C
DOI: 10.1021/acs.orglett.6b02504
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Letter
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b02504.
■
J.; Long, G.; Liu, Y.; Zhang, Q. J. Mater. Chem. C 2015, 3, 8219.
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G. K. Inorg. Chem. 2016, 55, 1208.
S
Experimental details, including synthesis and character-
ization; computational details (PDF)
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AUTHOR INFORMATION
Corresponding Author
■
*E-mail: yliu@lbl.gov.
Notes
(21) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1980,
102, 939.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was performed at the Molecular Foundry and was
partially supported by the Self-Assembly of Organic/Inorganic
Nanocomposite Materials program, both supported by the
Office of Science, Office of Basic Energy Sciences, of the U.S.
Department of Energy under Contract No. DE-AC02-
05CH11231.
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DOI: 10.1021/acs.orglett.6b02504
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