Encapsulated oligothiophenes having electron-affinity characteristics

Article abstract of DOI:10.1039/c1cc14994f

Oligothiophenes composed of dioxocyclopenta[c]thiophene bearing bulky bis(di-t-butylphenyl) groups were designed and synthesized to develop molecular wires having electron-transporting characteristics. Their effective conjugation, electron affinity, and encapsulation effects were investigated by photophysical and electrochemical measurements.

Full text of DOI:10.1039/c1cc14994f

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Encapsulated oligothiophenes having electron-affinity characteristicsw
Masaru Endou,a Yutaka Ieab and Yoshio Aso*a
Received 11th August 2011, Accepted 24th October 2011
DOI: 10.1039/c1cc14994f
Oligothiophenes composed of dioxocyclopenta[c]thiophene bearing
bulky bis(di-t-butylphenyl) groups were designed and synthesized to
develop molecular wires having electron-transporting characteristics.
Their effective conjugation, electron affinity, and encapsulation
effects were investigated by photophysical and electrochemical
measurements.
Structurally well-defined oligothiophenes are one of the most
successful classes of conjugated systems for electronics appli-
cation owing to their long effective conjugation and high hole-
transporting properties,1 and these molecules have been the
focus of application in optoelectronic devices.2 Furthermore,
nanometre-scale oligothiophenes have become a promising
candidate for molecular wires in the field of single-molecular
electronics.3,4 From a molecular-design perspective, we can
Fig. 1 Chemical structures of Cb, (TCbT)n, C, and h(TCmT)2h.
precisely tune the chemical structure of molecular wires in
terms of both steric circumstances and electronic states.
Recently most synthetic efforts have been devoted to the
former, whose methodology is based on the suppression of
intermolecular p–p interactions by encapsulation, leading to
development of insulated molecular wires.5 For this purpose,
several approaches such as the utilization of self-assembly with
encapsulation molecules6–8 or the introduction of covalently-
attached bulky substituents9,10 have been proposed. In recent
years, we have focused our efforts on the development of
encapsulated oligothiophenes and have disclosed that a
cyclopenta[c]thiophene repeating unit bearing bulky substituents
is advantageous to complete encapsulation as well as maintaining
effective conjugation.11,12 On the other hand, although the
energetic alignment between the work function of metal
electrodes and the highest occupied molecular orbital or the
lowest unoccupied molecular orbital (LUMO) level of the
molecular wire has been considered to influence charge injection,
the modification of electronic states in molecular wires is still
behind. Especially, to our knowledge, molecular wires for
electron transport through the LUMO orbital have been
limited to non-encapsulated p-conjugated systems.13,14 These
situations motivated us to develop encapsulated oligothiophene
molecular wires with high electron affinity. The introduction
of electron-withdrawing groups into oligothiophenes is
well known to stabilize the LUMO energy level, leading to
increasing the electron affinity.15 In this regard, we have
reported difluorodioxocyclopenta[c]thiophene (C) as a new
electronegative unit and its-based oligothiophenes as n-type
organic field-effect transistor materials (Fig. 1).16 We envisioned
that the replacement of the fluorine atoms in C with sterically
bulky di-t-butylphenyl (DTBP) methyl groups17 effectively
encapsulates the conjugated backbones with keeping the
electron-accepting properties. Here, we report the synthesis
and properties of encapsulated electronegative oligothiophenes
(TCbT)n containing Cb units (Fig. 1).
The synthesis of the target oligothiophenes is shown in
Scheme 1. To construct the key thiophene unit 2, we utilized
the alkylation reaction of 1,3-diketone.18 Thus, treatment of
119 with di-t-butylbenzyl bromide in the presence of KF/Celite
gave 2 in 67% yield. The repeating unit (TCbT)1 was synthesized
by the Stille coupling reaction of 2 with tributylstannylthiophene
in a yield of 93%. Palladium-catalyzed direct homocoupling of
(TCbT)1 afforded a mixture of homologous oligomers (TCbT)2,
(TCbT)3, and (TCbT)4.20 These oligomers are soluble in common
organic solvents, which allowed us to isolate them by preparative
gel-permeation liquid chromatography (GPC) in 27%, 23%, and
9% yields, respectively. Their structures and purity were fully
verified by 1H NMR, MS, and elemental analysis (see the ESIw).
Following similar reactions, we obtained non-encapsulated
h(TCmT)2h (Fig. 1). Note that we introduced hexyl groups at
both the terminal positions in h(TCmT)2h and (TCbT)4 to ensure
enough solubility.
a The Institute of Scientific and Industrial Research (ISIR),
Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan.
E-mail: aso@sanken.osaka-u.ac.jp
b PRESTO-JST, 4-1-8 Honcho, Kawaguchi, Saitama 333-0012, Japan
w Electronic supplementary information (ESI) available: Experimental
procedures, CV of h(TCmT)2h, DPV, UV-vis-NIR spectra of
h(TCmT)2h, NMR spectra, GPLC charts, and computational details.
See DOI: 10.1039/c1cc14994f
c
540 Chem. Commun., 2012, 48, 540–542
This journal is The Royal Society of Chemistry 2012

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Table 1 Photophysical and electrochemical properties
Compd
lmaxa/nm
Egb/eV
Ered.1/2 c/V
Eox.1/2 c/V
(TCbT)1
(TCbT)2
(TCbT)3
(TCbT)4
h(TCmT)2h
424
517
555
571
520
2.92
2.40
2.23
2.17
2.38
À1.98
+1.23d
À1.75
+0.69, +0.96
+0.61, +0.77
+0.59, +0.70
+0.65, +0.85
À1.92, À1.70
À1.80, À1.68
À1.61
c
a
b
In CH2Cl2. Eg
=
1240/lmax
Irreversible.
.
In CH2Cl2 containing 0.1
M
TBAPF6 V vs. Fc/Fc+
.
d
Scheme 1 Synthesis of (TCbT)n.
Fig. 3 CVs of oligothiophenes in CH2Cl2 containing 0.1 M TBAPF6
as a supporting electrolyte.
level (Fig. S3, ESIw). The CV of (TCbT)1 showed a quasi-
reversible reduction wave and an irreversible oxidation wave.
On the other hand, (TCbT)2, (TCbT)3, and (TCbT)4 exhibited
both reversible reduction and oxidation processes, showing that
the kinetic stability of ionic species is improved compared to that
of (TCbT)1. Importantly, their reduction potentials are shifted
positively upon elongating the chain length, indicating that the
LUMO orbital of the oligothiophenes is delocalized over the
conjugated backbones as shown in Fig. S3 (ESIw).
UV-vis-NIR measurements of radical cationic species
(polaron) were performed to investigate the encapsulation effects
of our developed oligothiophenes on intermolecular interactions,
because long oligothiophenes tend to form spontaneously p-dimers
between charged chains.23 Note that the formation of dianionic
species was spectroscopically observed by the reduction with
potassium/graphite under inert conditions (Fig. S4, ESIw). How-
ever, the spectral pattern is too unstable to investigate the encapsu-
lation effect. Fig. 4 shows the UV-vis-NIR spectra of (TCbT)2 and
h(TCmT)2h in CH2Cl2 at room temperature and 233 K. On one-
electron oxidation of (TCbT)2 with SbCl5 in CH2Cl2 at room
temperature, the p–p* transition band of the neutral species at
2.4 eV was replaced by two bands at 0.8 and 1.5 eV which is
attributed to polaron (Fig. 4(a)).24 These two polaronic bands
showed little difference in shape between the spectra at room
temperaturꢀe and at 233 K. In contrast, when the solution of
h(TCmT)2h + was cooled to 233 K, new bands at 0.9 and 1.7 eV
emerged along with decrease in absorption bands at 0.7 and 1.5 eV
(Fig. 4(b)). This high-energy shift of polaronic bands is a typical
characteristic of p-dimer formation.23 The formation of the p-dimer
was reversible, and the polaronic bands of h(TCmT)2hꢀ+ are
restored upon warming to ambient temperature (see Fig. S5, ESIw).
Fig. 2 (a) UV-vis spectra of oligothiophenes in CH2Cl2. (b) Correlations
between electronic transition energies and the reciprocal numbers of
thiophene rings (1/x) of (TCbT)n.
Fig. 2(a) shows the electronic absorption spectra of (TCbT)n
(n = 1–4) in CH2Cl2. The p–p* absorption maxima (lmax) of
(TCbT)n (n = 1–4) and h(TCmT)2h as well as their transition
energies (Eg) calculated from lmax are summarized in Table 1.
The absorption maxima of these oligomers are bathochromically
shifted with the chain extension. The plot of transition energy
against the reciprocal numbers of thiophene rings (1/x) followed
a linear dependence with an equation of Eg (eV) = 1.90 +
3.05/x (Fig. 2(b)). Although the quantitative evaluation of
this equation is rather difficult because of the electronically
non-equivalent repeating units of the oligomers,21 it is important
to mention that the plot kept good linearity even at thiophene
12-mer, indicating that effective conjugation is maintained in
spite of incorporating the bulky DTBP groups.22
The electrochemical properties of (TCbT)n (n = 1–4) and
h(TCmT)2h were studied by cyclic voltammetry. Fig. 3 and
Fig. S1 (ESIw) show cyclic voltammograms (CVs) of these
oligomers in CH2Cl2 containing 0.1 M tetrabutylammonium
hexafluorophosphate (TBAPF6), and their electrochemical data
are listed in Table 1. All these oligomers (TCbT)n showed not
only oxidation but also reduction processes. As shown in
Fig. S2 (ESIw), the number of electrons participating in each
redox wave was revealed by the differential pulse voltammetry
(DPV) measurement of (TCbT)n (n = 1–4). The appearance of
the reduction process indicates that the presence of the Cb unit
apparently contributes to increasing the electron affinity of
oligothiophenes through the participation of its carbonyl
groups in the LUMO orbital. This is supported by the represen-
tatively calculated frontier molecular orbitals of (TCbT)2 with
density functional theory (DFT) at the B3LYP/6-31G (d, p)
These results indicate that the presence of sterically bulky DTBP
+
groups disturbed the formation of the p-dimer in (TCbT)2ꢀ
Considering the multi-step oxidation curve of CV for
.
(TCbT)4 (Fig. 3), the spectroscopic observation of individual
c
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Chem. Commun., 2012, 48, 540–542 541

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In summary, we have demonstrated that the Cb unit is
effective for the creation of encapsulated electronegative
oligothiophenes: the shielding can be accomplished by the
DTBP groups, and the electron affinity can be increased by
the dioxocyclopenta[c]thiophene unit. These characteristics
were clearly exemplified by the spectroscopic and electro-
chemical measurements. It should be mentioned that we
observed no interruption of conjugation along the oligothio-
phene backbone despite the introduction of the encapsulating
units. This molecular design provides us a new possibility to
develop oligothiophene molecular wires for application in
single-molecular electronics. Further extension to encapsulated
electronegative oligothiophenes with electrode-binding func-
tional groups at both terminal positions and to single-molecular
conductivity measurement will be reported in due course.
This study was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. M. E. acknowledges Grant-
in-Aid for Research Fellow of the Japan Society for the
Promotion of Science. We thank Prof. A. Wakamiya and
Prof. Y. Murata in Kyoto University for the measurements
of dianionic species. Thanks are extended to the Elemental
Analysis Section of Comprehensive Analysis Center (CAC)
of ISIR, Osaka University, for assistance in carrying out
elemental analyses.
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24 The two weak bands at 0.9 and 1.7 eV are vibronic shoulders.
These bands are also observed in our completely encapsulated
oligothiophenes (see ref. 11 and 12).
c
542 Chem. Commun., 2012, 48, 540–542
This journal is The Royal Society of Chemistry 2012