Eumelanin-inspired core derived from vanillin: A new building block for organic semiconductors

Article abstract of DOI:10.1039/c4cc09011j

An eumelanin-inspired core derived from the natural product, vanillin (vanilla bean extract) was utilized for the synthesis of eumelanin-inspired small molecules and polymer via Sonogashira cross coupling. The materials demonstrate that the methyl 4,7-dibromo-5,6-dimethoxy-N-methyl-1H-indole-2-carboxylate core can serve as a new building block for organic semiconductors.

Full text of DOI:10.1039/c4cc09011j

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Eumelanin-inspired core derived from vanillin: A new
building block for organic semiconductors
Cite this: DOI: 10.1039/x0xx00000x
Subhashini Selvaraju,a K. A. Niradha Sachinthani,a RaiAnna A. Hopson,a Frederick
M. McFarland,b Song Guo,b Arnold L. Rheingoldc and Toby L. Nelsona
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Extensive research has been done on the optical, electronic, physical,
metal chelating, and structural properties of natural and synthetic
eumelanins.3-4 McGinness and Proctor’s groundbreaking work on
electrical switching established eumelanins as amorphous organic
semiconductors.5 These eumelanins have excellent light absorption
ranging from 200 nm to 700 nm in the electromagnetic spectrum,
electrical conductivity reaching 10-5 Scm-1 and exhibit good charge
mobility as high as 2.1 x 10-3 cm2 V-1 s-1.6 Hence, it seems
appropriate and fitting to utilize the eumelanin indole moiety as a
platform for the development of new organic semiconductors.2a
An eumelanin-inspired core derived from the natural
product, vanillin (vanilla bean extract) was utilized for the
synthesis of eumelanin-inspired small molecules and polymer
via Sonogashira cross coupling. The materials demonstrate
that the methyl 4,7-dibromo-5,6-dimethoxy-N-methyl-1H-
indole-2-carboxylate core can serve as a new building block
for organic semiconductors.
Organic semiconductors have attracted considerable attention due to
the promise of low cost, lightweight, and flexible large area
electronic devices.1 Most of these materials are derived from
petroleum-based starting materials. Due to the increasing demand on
oil, it would be advantageous to seek alternative sources for building
blocks to develop new organic semiconductors.
Here, we present the synthesis of a eumelanin-inspired core
molecule from the natural product, vanillin. The eumelanin-inspired
building block was designed so that functionalization on the 4,7-
positions on the central indolic benzene ring can be feasible via
transition metal-catalyzed cross-coupling reactions. Two new
eumelanin-inspired compounds with interesting optoelectronic
properties were synthesized by Sonogashira cross-coupling.
In terms of chemical and functional diversity, nature is a great source
of new building blocks for bioinspired organic semiconductors. One
such inspiration is the biopolymer, melanin which is a class of
naturally occurring pigments found in the hair, eyes, skin, and the
brain of mammals and acts as a natural photoprotector against the
harmful effects of UV radiation.2 Eumelanin is the black-brown
variety of melanin and exist as a heterogeneous network, formed by
the oxidative polymerization of two monomers 5,6-dihydroxyindole
(DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) (Fig.
1).3
The synthesis of the methyl 4,7-dibromo-5,6-dimethoxy-N-methyl-
1H-indole-2-carboxylate (DBI) eumelanin-inspired core 7 is shown
in Scheme 1. Vanillin (1) was methylated to give
dimethoxybenzaldehyde (2), which was nitrated to yield 3.7
Bromination of 3 using N-bromosuccinimide resulted in compound
4.8 A modified procedure was used to synthesize the olefin 5 using
methyl bromoacetate in aqueous sodium bicarbonate, followed by
microwave-assisted Cadogan synthesis to afford 6.9 Finally, the
indole was N-methylated to avoid unwanted by-products during the
cross coupling reactions. Compound 7 was synthesized with bromo
groups at the 4 and 7 positions of the indole moiety so it could serve
as a universal partner for metal-catalyzed coupling reactions. This
approach allowed for the functionalization at the 4 and 7 positions
with alkynyl substituents using Sonogashira cross-coupling (scheme
2). The ethynyl group was chosen because of its ability to alter
optoelectronic properties by extended effective π-conjugation
length.10
HO
HO
HO
COOH
N
N
HO
H
H
DHICA
DHI
Fig. 1 Structures of DHI and DHICA.
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Br
Br
H3CO
H3CO
H3CO
H3CO
H3CO
H3CO
O
CH3I
O
HNO3
O
O
NBS
K2CO3
H
2SO4
HO
NO2
H3CO
NO2
18-crown-6
2
1
3
96 %
89 %
4
84 %
PPh3
Br
COOCH3
satd. NaHCO3
Br
Br
Br
Br
Br
O
PPh3
MoO2Cl
H3CO
H3CO
H3CO
H3CO
OCH3
2(dmf)2
CH3I
COOCH3
COOCH3
K2CO3
18-crown-6
µ
80 W
W,
THF
N
N
H3CO
H3CO
NO2
H
Br
DBI 7
95 %
6
71
5
%
78 %
Scheme 1 Synthesis of eumelanin-inspired core (DBI) 7 from vanillin.
reference electrode was calibrated against ferrocene/ferrocenium
(Fc/Fc+) redox couple. The HOMO and LUMO energy values were
calculated from the onset of the first oxidation and reduction
potentials from the equations EHOMO (eV) =
The coupling reaction was carried out using Pd(PPh3)4, CuI and
triethylamine (Et3N) as solvent and base to afford the products 9a
and 9b in high yields.11 X-ray quality crystals of 9a were grown
from a vapour diffusion of ether and dichloromethane solutions (see
Supporting Information for the crystallographic data).
COOCH3
N
Br
Br
COOCH3
N
H
3CO
OCH3
7
8
CuI
,
Pd(PPh3)4
Et3N
R
R
H
3CO
OCH3
R
9a; R=H
9b; R=OMe
%)
(98
%)
(76
Scheme 2 Synthesis of compounds 9a and 9b.
Fig. 2 The UV-Vis absorption spectra of 7, 9a and 9b.
- E1/2 (Fc/Fc+) + 4.8] and ELUMO (eV) = - [Ered
- E1/2
onset
onset
The UV-vis absorption (Figure 2) and photoluminescence (PL) of - [Eox
the eumelanin-inspired small molecules were investigated and (Fc/Fc+) + 4.8], where E1/2 (Fc/Fc+) was the cell correction. The
summarized in the Table 1. The 4,7-substituted eumelanin-inspired oxidation potential for DBI was -5.76 eV whereas 9a and 9b values
small molecules 9a and 9b featured red-shifted absorbance maxima correspond to -5.55 eV and -5.45 eV, respectively. Both 9a and 9b
due to extended conjugation of the system compared to the had higher LUMO energy levels than the DBI core (see Table 1).
The calculated bandgap values were 2.89 eV, 2.85 eV and 2.80 eV
for the 7, 9a and 9b, respectively, which indicated that the electron
donating methoxy group resulted in the lower bandgap. This trend
was also observed for the estimated optical bandgaps. From the
voltammograms (S18), it was evident that the compounds had
irreversible reduction and oxidation potentials.
unsubstituted core 7 (abs λmax 308 nm). While the absorption band at
lower wavelength (~300 nm) corresponds to the indole core, the
ethynyl substitution extends the conjugation through the eumelanin
inspired core which resulted in the absorption band ca. 400 nm.
Moreover, the incorporation of the methoxy group at the para
position of the phenyl ring (9b) resulted in a slight red shift in
absorption spectrum compared to 9a. The DBI core displayed very
weak fluorescence whereas 9a and 9b had PL maxima of 436 nm
and 449 nm, respectively. The PL quantum yields of the 9a and 9b
in dilute chloroform solutions were 0.82 and 0.91, respectively
(details in supporting information). Optical bandgaps were estimated
from the onset of the absorption are shown in Table 1. The
compound 7 had a bandgap of 3.25 eV and as expected the 4,7-
substitued molecules 9a and 9b with extended conjugation showed
reduced optical bandgaps of 2.94 and 2.87 eV, respectively.
Table 1 Optical and electrochemical properties of 7, 9a and 9b.
b
c
d
d
a
λabsa (nm)
λem
Φre
Eopt
Eox
Ered
Eox-red
(eV)
(eV)
-3.25
-2.94
(eV)
-5.76
-5.55
(eV)
-2.87
-2.70
(nm)
308*
7
-
-
2.89
2.85
384*, 298,
239
9a
436
0.82
390*, 353,
304, 241
9b
449
0.91
-2.87
-5.45
-2.65
2.80
*λmax, a Measured in dilute chloroform, bquantum yields measured in
dilute chloroform solutions relative to quinine sulfate, cmeasured from
onset of absorption , c calculated from onset of oxidation and reduction
potential.
Cyclic voltammetry measurements were carried out in dry degassed
acetonitrile under inert atmosphere using 0.1 M tetrabutylammonium
hexafluorophosphate as the supporting electrolyte. A Ag/Ag+
2 | J. Name., 2012, 00, 1-3
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Acknowledgements
COOCH3
N
We gratefully acknowledge support for this work from the ORAU
Ralph E. Powe Junior Faculty Enhancement Award and Oklahoma
State University Technology Business Development Program. We
also thank the financial support from National Science Foundation
(EPS 0903787) and (EPS 1006883). We thank Sadagopan Krishnan
for his help with cyclic voltammetry. This manuscript is dedicated to
my friend and colleague the late Dr. Marc S. Maynor.
Br
Br
H
OCH3
7
3CO
COOCH3
N
H
C12 25O
CuI
,
Pd(PPh3)
Et THF
3N,
n
H
C12 25O
Notes and references
H25
OC12
H
OCH3
3CO
aDepartment of Chemistry, Oklahoma State University, Stillwater,
Oklahoma-74078.
H25
OC12
bDepartment of Chemistry and Biochemistry, The University of
10
Southern Mississippi, Hattiesburg, MS 39406.
cDepartment of Chemistry, University of California, San Diego, La Jolla,
CA 92093-0358.
Scheme 3 Synthesis of the eumelanin-inspired polymer.
To further demonstrate the utility of the eumelanin inspired core 7 as
a building block for organic semiconductors, we attempted the
synthesis of a model conjugated polymer via Sonogashira cross-
Electronic Supplementary Information (ESI) available: [Details of the
synthesis and characterization of all the compounds are available.
Crystallographic
data
for
9a
is
also
available].
See
coupling conditions. The polymerization of
7
and 1,4-
DOI: 10.1039/c000000x/
bis(dodecyloxy)-2,5-diethynylbenzene (10), was selected because of
the similarity of 10 to methoxy monomer 9b. This resulted in a red
polymer with 36 % yield (Scheme 3). The polymer was soluble in
various solvents including THF, chloroform, toluene and
chlorobenzene, and the structure was confirmed by 1H NMR
spectroscopy. Gel permeation chromatography showed a number
average molecular weight (Mn) of 13.6 kDa, PDI = 1.88. The
photophysical characteristics of the polymer, both in dilute solutions
and thin films, were examined using UV−vis absorption and
fluorescence spectroscopy. The polymer exhibited an absorption
maximum at 485 nm in solution and a red-shifted absorption
maximum of 526 nm for polymer thin films. Green fluorescence was
observed for the polymer with an emission maximum at 508 nm and
the quantum yield in dilute chloroform was 0.60 (details in
supporting information). The electrochemical properties of the
polymer were investigated. The HOMO level for the polymer was
similar to 9b (-5.47 eV); however, LUMO level was deepened to -
3.44 eV. The morphology of the polymer thin film was characterized
by AFM, as shown in the Supporting Information. The polymer thin
film appears to be composed of packed small grains varying in size
and shape averaging 20 nm in diameter. Currently, work is
continuing to optimize the polymerization conditions in order to
improve yield and obtain higher molecular weight polymers.
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Conclusions
In summary, we demonstrate that an eumelanin-inspired core
molecule derived from the vanillin can serve as a building block for
the development of eumelanin-inspired organic semiconductors.
Two new eumelanin-inspired small molecules were synthesized in
good yields. These materials exhibited red-shifted absorption and
emission compared to the eumelanin-inspired core DBI. This is
attributed to the extended conjugation due to the
phenyleneethynylene linkage. Moreover, an eumelanin-inspired
polymer was synthesized which showed promise for optoelectronic
devices. Current efforts focus on the synthesis and evaluation of
optical and electronic properties of oligomers and polymers based on
the eumelanin-inspired core moieties.
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