Synthesis and peripheral substituent effects of bay-annulated indigo derivatives

Article abstract of DOI:10.1016/j.tetlet.2018.06.036

In this study, indolo-naphthyridine-6,13-diones (5a–d) with four different peripheral substituents were prepared via bay-annulation reactions of indigo. The resulting compounds (5a–d) exhibited fluorescence in the red to near-IR region, while the parent indigo molecule showed no fluorescence. Although the peripheral substituents were oriented to the exterior of the π-conjugated system, the electronic structure affected the absorption and fluorescence spectra. Moreover, calculated molecular orbitals and absorption spectra successfully reproduced the experimental absorption spectra and cyclic voltammograms.

Full text of DOI:10.1016/j.tetlet.2018.06.036

Accepted Manuscript
Synthesis and peripheral substituent effects of bay-annulated indigo derivatives
Taniyuki Furuyama, Daichi Tamura, Hajime Maeda, Masahito Segi
PII:
S0040-4039(18)30785-8
https://doi.org/10.1016/j.tetlet.2018.06.036
TETL 50074
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
21 April 2018
12 June 2018
15 June 2018
Please cite this article as: Furuyama, T., Tamura, D., Maeda, H., Segi, M., Synthesis and peripheral substituent
effects of bay-annulated indigo derivatives, Tetrahedron Letters (2018), doi: https://doi.org/10.1016/j.tetlet.
2018.06.036
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Taniyuki Furuyama*, Daichi Tamura, Hajime Maeda, Masahito Segi

1
Tetrahedron Letters
Synthesis and peripheral substituent effects of bay-annulated indigo derivatives
Taniyuki Furuyama, Daichi Tamura, Hajime Maeda, and Masahito Segi
Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
In this study, indolo-naphthyridine-6,13-diones (5a–d) with four different peripheral substituents
were prepared via bay-annulation reactions of indigo. The resulting compounds (5a–d) exhibited
fluorescence in the red to near-IR region, while the parent indigo molecule showed no
fluorescence. Although the peripheral substituents were oriented to the exterior of the -
conjugated system, the electronic structure affected the absorption and fluorescence spectra.
Moreover, calculated molecular orbitals and absorption spectra successfully reproduced the
experimental absorption spectra and cyclic voltammograms.
Available online
Keywords:
Indigo
Bay-annulation
Fluorescence
Near-infrared
2009 Elsevier Ltd. All rights reserved.
There are numerous organic dyes found in nature and their
bright colors have attracted a great deal of interest for a long
time. However, the development of artificial dyes with bright
colors has been a challenging topic in synthetic organic
chemistry in the past century.1 The bright visible color originates
from a small HOMO-LUMO gap, which is derived from the
delocalized π-electrons on the chromophore. Since delocalized π-
indigo. However, the synthesis of various BAI derivatives as
discrete molecules has been limited.11
electrons
show
unique
optical/electronic
properties,
natural/artificial organic dyes are part of many organic materials,
such as dye-sensitized solar cells (DSSCs), organic field-effect
transistors (OFETs), organic photovoltaics (OPVs), and chemical
sensors.2
Indigo is one of the most famous natural pigments and is
derived from Indigofera tinctoria and Indigofera suffruticosa
(representative). Its attractive blue color is widely used in the
textile industry on a large worldwide scale.3 The structure of the
indigo dye is a dimeric 3-indolinone (Fig. 1), which is firmly
fixed by strong hydrogen bonds and a C-C double bond at 2
position. The π-electrons on the indigo skeleton are highly
delocalized and cause an intense visible-light absorption (~ 600
nm). Recently, several groups proposed novel artificial dyes
based on the indigo skeleton that have improved solubility,
synthetic procedure, and light absorption properties in the near-
IR region.4–7 For instance, bay-annulation of indigo yields novel
structures (indolo-naphthyridine-6,13-diones, bay-annulated
indigo, BAI) that could be employed as new electron-acceptor
monomer units of D-A (Donor-Acceptor) type polymers for near-
IR light absorbing OPVs.8–10 A key step of the BAI monomer
unit formation is the direct double annulation of the indigo core
by arylacetyl chloride derivatives. The reaction may introduce
Figure 1. Representative procedure of bay-annulation reaction.
In this communication, we focused on the peripheral
substitution effect of BAIs. The wide π-deficient structure of the
BAI core is a potent electron acceptor. Although the aryl groups
at the bay position lie at the exterior of the π conjugation system,
the peripheral substitution effect was expected to control the
optical properties, as suggested by our previous work on aryl-
substituted tetraazaporphyrin phosphorus(V) complexes.12
Moreover, we performed a quantitative analysis combining
electrochemical properties and theoretical calculations.
The synthetic procedure of BAIs 5a–d is shown in Scheme 1.
Indigo derivative 3 was synthesized in a good yield according to
a literature procedure.9 Subsequent heating of a mixture of 3 and
arylacetyl chloride derivatives in xylene under reflux yielded the
BAI skeleton. The arylacetyl chloride generated by
arylcarboxylic acid and thionyl chloride reacted with 3 in a one-
pot procedure. Since the solubility of hydroxy-substituted BAIs
various aryl derivatives into the bay position of the annulated
———
 Corresponding author. Tel.: +81-76-234-4786; e-mail: tfuruyama@se.kanazawa-u.ac.jp (T.F).

2
Tetrahedron
4a–d was extremely low in various organic solvents, they were
characterize their structures by 1H NMR and HR-FAB-MS
spectroscopies. Moreover, O-alkylated indigo 6 was synthesized
according to the procedure used for the synthesis of 5a–d.
Unfortunately, single crystals of 5a–d and 6 suitable for X-ray
O-alkylated in order to improve their solubility. Similar to
previous reports,9 low O-alkylation yields were observed, which
may be due to the low solubility of 4a–d. The resulting O-
alkylated derivatives 5a–d exhibited enough solubility to
diffraction
analysis
could
not
be
obtained.
Scheme 1. Synthesis of bay-annulated indigo derivatives 5a–d. Reagents and conditions: (i) 3,4-Dihydro-2H-pyran (1.5 eq), PPTS (cat.), CH2Cl2/hexane, rt,
12 h, 84%; (ii) KOHaq (0.2 M), acetone, –10°C, 15 min, then KOHaq (0.4 M), acetone, rt, 24 h, 58%; (iii) arylacetyl chloride derivatives (6 eq), xylene, reflux,
n
24 h, then NaOH (5 eq) in MeOH, rt, 12 h, up to 58%; (iv) C4H9I or nC6H13I (20 eq), K2CO3 (30 eq), DMF, 60°C, 24 h, up to 21%; (v) H2SO4 (5 eq), MeOH, rt,
12 h, 88%; (vi) nC4H9I (3 eq), K2CO3 (9 eq), DMF, 60°C, 12 h, 10%. THP = Tetrahydropyranyl.
Fig. 2 shows the absorption and emission spectra of 5a–d and
6 in dichloromethane. The optical properties were summarlized
in Table S1, Supporting Infromation. Sharp and intense
absorption bands in the visible region appeared in the spectra of
the BAI derivatives. The positions of these peaks depended on
the substituent group at the bay position. The peaks of thienyl
derivative 5a appeared redshifted in comparison to those of the
other derivatives. In contrast, the peak positions of 5b–d were
close, indicating that the substituent effect at the phenyl group
was relatively small. Moreover, a small solvent effect was
observed in the absorption spectra of 5a in various solvents (Fig.
S1, Supporting Information). In contrast, the spectrum of 6 was
an absorption envelope of typical indigos, with broader peaks
than those of the BAI derivatives, implying that 6 had a smaller
absorption coefficient than the other examined compounds. In
addition, 5a–d exhibited an emission in the visible-to-near-IR
regions13–14 in moderate fluorescence quantum yields (ΦF
=
0.04–0.13 in dichloromethane), while indigo 6 showed no
emission.15
The first oxidation and reduction potentials of chemicals are
essential to their application to electronic materials such as
OPVs. Fig. 3 displays the cyclic voltammograms of 5a–d and 6.
Since the exact oxidation potentials could not be estimated, their
anodic peak potentials (Epa) were used as indexes of the first
oxidation potential values. The Epa values of 5a–d shifted
anodically and the close first reduction potentials (Ered1) showed
that the HOMOs of 5a–d were stabilized after the bay-
annulation. In fact, the substituent groups at the bay position
affected both the oxidation and reduction potentials. The Epa of
5a shifted cathodically compared to that of the other BAIs (5b–
d), indicating that the thienyl group destabilized the HOMO
energy. The position of the absorption bands (548–647 nm) was
also in accord with the order of the (Epa–Ered1) values.
Figure 2. (a) UV-vis absorption and (b) fluoresence spectra of 5a–d and
6 in CH2Cl2. Excited wavelength (ex) is 490 nm. No fluorescence peak of 6
was observed in this condition.

3
affected the absorption spectra only marginally (Fig. S2,
Supporting Information). Partial MOs related to the intense band
in the visible region are shown in Fig. 4, the respective
calculated transition energies, oscillator strengths (f), and
configurations are summarized in Table 1, and calculated
absorption spectra are shown in Fig. S3, Supporting Information.
The calculated intense absorption band in the visible region
(545–650 nm) could be assigned to the π–π* transition, which
was composed of both HOMOs and LUMOs. The fact that the
calculated HOMO–LUMO gaps of 5a–d’ were larger than that
of 6’ indicated that the absorption spectra of 5a–d were
blueshifted. Interestingly, the HOMO of 6’ was delocalized over
the entire indigo skeleton, while those of 5a–d’ were localized
on the central part of the BAI skeleton. In contrast, the LUMOs
of 5a–d’ were delocalized over the entire BAI skeleton. This
difference was reflected in the large changes in the HOMO
energies compared to that of 6’, causing the resulting BAIs to
have large HOMO–LUMO gaps. Although 5a–d’ have similar
HOMO–LUMO gaps, the energy levels depend on the
substituent groups at the bay position. The fact that both the
HOMO and LUMO are partially localized on the substituent
groups at this position suggested that the electronic structure at
the bay position can change the absorption and redox properties.
Moreover, the calculated absorption spectra, position of the
intense absorption band, and energies of the frontier orbitals
were reproduced and correlated well to the experimental spectra.
The exemplary relationship between the experimental and
calculated spectra can be useful in fashioning new functional
dyes based on BAIs, designed to access appropriate properties in
Figure 3. Cyclic voltammetry data for 5a–d and 6. Cyclic
voltammograms were acquired from 0.5 mM solutions of analyte in 0.1 M
nBu4NClO4/o-DCB. Ferrocene was used as an internal standard and set to 0
V.
In order to enhance the interpretation of the electronic
structures and substituent effects on the BAIs, MO calculations
of the model structures 5a–d’ and 6’ were preformed. Model
structures whose substituents on the O-alkyl groups were
replaced by a methyl group were used because these groups
various
fields.

4
Tetrahedron
Figure 4. Energy levels of frontier MOs and their contour plots obtained from calculations. Blue and red plots indicate occupied and unoccupied MOs,
respectively. Calculations were preformed at the B3LYP/6-31G* level, using the polarizable continuum model (PCM) that mimicked the solvation effect of
CH2Cl2.
Table 1. Calculated excited wavelength () and oscillator strengths (f) for components of selected transition energies.
Compound

calcd (nm)

exp (nm)a
f
Composition (%)
5a’
5b’
5c’
5d’
6’
588.4
541.2
558.8
544.8
649.6
585
548
558
553
647
0.72
0.57
0.75
0.62
0.27
HOMO–>LUMO (100%)
HOMO–>LUMO (99%)
HOMO–>LUMO (99%)
HOMO–>LUMO (99%)
HOMO–>LUMO (100%)
a The data were taken from Fig. 2.
2.
3.
Kim S-H. ed. Functional Dyes. Amsterdam, Netherlands:
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In summary, the first systematic synthesis of indigo-based
BAIs was shown herein. Various substituent groups such as
electron-donating and withdrawing groups, and heterocycles
were introduced into the bay position via a bay-annulation
reaction. Although the π–skeletons of the BAIs were larger than
that of the indigo dye, blueshifted absorption spectra were
obtained. BAIs have an emission, while the indigo has no
fluorescence. Additionally, it was found that the position of the
absorption peaks and redox potentials depended on the
substituent groups, with them being oriented to the exterior of
the π-conjugated system. Moreover, the calculated absorption
spectra and energies of the frontier orbitals successfully
reproduced the experimental properties. All in all, it was
concluded that the optical and electronic properties of a series of
BAIs can be changed via a simple synthetic procedure, which
could lead to the development of new methods to designing fine-
tuned optic materials in the visible to near-IR regions. Further
work is currently underway to expand the scope of the
substituent groups and prepare BAI-based stimuli-responsive
materials.
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Acknowledgments
This work was partly supported by a JSPS KAKENHI Grant
(No. 15K05409), Hokuriku Bank Foundation, The Murata
Science Foundation, The Kyoto Technoscience Center
Foundation, The TOBE MAKI Scholarship Foundation, The
Sumitomo Foundation and Kanazawa University SAKIGAKE
Project 2018.
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Supplementary data
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References and notes
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Perkin WH. J. Chem. Soc. 1862;14:230-255.

5
Highlights:
Bay-annulated indigos (BAIs) with four different peripheral substituents have been synthesized.
These materials exhibited fluorescence in the red to near-IR region.
The calculated absorption spectra and energies of the frontier orbitals reproduced the experimental
properties.