Improvement of the redox stability of dithieno[3,2-b:2′,3′-d] thiophene derivatives by using bulky substituents

Article abstract of DOI:10.1016/j.molstruc.2013.01.002

With the goal of improving the long term redox stability of dithieno[3,2-b:2′,3′-d]thiophene (DTT), we synthesized derivatives with differently sized substituents. It is well known that substituents in the 2, 6 positions of DTT are active in this respect.

Full text of DOI:10.1016/j.molstruc.2013.01.002

Journal of Molecular Structure 1037 (2013) 256–263
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Improvement of the redox stability of dithieno[3,2-b:20,30-d]thiophene
derivatives by using bulky substituents
⇑
Shinjiro Okada , Kenji Yamada
Nanomaterials Technology Development Center, Canon. Inc., 3-30-2 Shimomaruko, Ohtaku, Tokyo, Japan
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
Substituent effects of dithieno[3,2-b:20,30-d]thiophene (DTT) derivatives using bulky phenyl substitutions
on its 2, 6 position were investigated. Dihedral angle between planes of the DTT and the phenyl affects
strongly on photochemical and electrochemical properties. Especially, the molecule which has the DTT
enfolded by the bulky substituent will be improved a long-term redox stability.
"
"
"
"
"
Highly redox stable compounds
were obtained using core-cage
system.
Bulky substituents on 2, 6 position of
DTT revealed excellent redox
stability.
A large size substituent is more
effective than a small size
substituent.
The core-cage system support
electrochromicity and long stability
at the same time.
The photophysical and
electrochemical properties were
studied using various cages.
a r t i c l e i n f o
a b s t r a c t
With the goal of improving the long term redox stability of dithieno[3,2-b:20,30-d]thiophene (DTT), we
synthesized derivatives with differently sized substituents. It is well known that substituents in the 2,
6 positions of DTT are active in this respect. We provide in this article more insight into mechanism.
The present study provides the first support for a size effect. Large size substituents in the 2, 6 positions
of the DTT are more effective than small size substituents. In these compounds, the DTT acts as a core of a
molecule where the HOMO is located and the substituents work as a cage to protect the core from various
attacks from the outside. This core-cage system is not only effective to prevent polymerization but also to
increase the long term redox stability. The compounds with large size substituents, such as (1,10:30,100-
terphenyl)-2-yl and 4,40-di-tert-butyl-(1,10-biphenyl)-2-yl, showed excellent long term redox stability.
The oxidation potential was maintained during repeated oxidation up to 17,000 and 20,000 times,
respectively. The compounds with small size substituents, such as phenyl and mesityl, showed 1/10 or
less of this durability. We show the substituent effects on repeated oxidation by investigating the elec-
trochemical and photophysical properties.
Article history:
Received 23 October 2012
Received in revised form 30 December 2012
Accepted 2 January 2013
Available online 16 January 2013
Keywords:
Substituent
Radical
Cation
Stability
Electrochromic
Redox
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
Organic materials which show electrochromicity involving con-
ductive polymer and small organic molecule have been the subject
of extensive studies for the past four decades. Especially, much
⇑
Corresponding author. Tel.: +81 3 3758 2111; fax: +81 3 3757 3019.
E-mail address: okada.shinjiro@canon.co.jp (S. Okada).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.01.002

S. Okada, K. Yamada / Journal of Molecular Structure 1037 (2013) 256–263
257
attention was paid to the fields of smart windows and paper like
displays. However, redox stability was the most serious problem
that needed to be overcome. If the materials could be made to ex-
hibit long term redox stability during repeated of switching of oxi-
dation state, and had high transparency in the uncolored state,
organic materials could become widely used in electrochromic
devices.
The most commonly studied organic materials are viologen
derivatives and polythiophene. Although, many electrochromic
materials have been investigated with extensive research effort,
there still remains the problem of their poor redox stability during
frequent switching. In case of organic materials, the reaction pro-
cess of electrochromism that consists of oxidation and reduction
has been accompanied essentially by a generation of radical cation
species and their extinction. Instability of oligothiophene radical
cations has already been reported. The electrochemical response
of terthiophene was degraded rapidly due to polymerization. How-
ever, introduction of blocking methyl substituents to the terminal
a-positions of terthiophene was successful in increasing the stabil-
ity of the radical cation and therefore reversible redox reaction
could be achieved [1,2]. There was a report regarding the electro-
chemical stability of fused oligothiophene that have silyl groups
at the terminal positions [3]. In the report, the fused ologothioph-
enes end caped by silyl groups showed reversible oxidation waves,
indicative of the significant stability of the produced radical cation
species in rigid ladder skeletons such as tetrathienoacene, hexathi-
enoacene and octathienoacene. There were many reports concern-
ing thiophene [4–11] and dithieno[3,2-b:20,30-d]thiophene (DTT)
derivatives [12–16]. A series of oligothiophenes surrounded by
bicycle [2,2,2] octane was also reported [17,18]. Another paper
pointed out that the intrinsic reactivity of oligomer radicals de-
creases rapidly as their conjugation length increases [19]. From
this viewpoint, many kinds of conductive polymers have a great
advantage in the radical cation stability owing to a long conjuga-
tion length in a long chain. Over the last two decades, better per-
formance of electrochromic organic materials using conductive
polymer has been reported [20–25].
Fig. 1. Molecular structures of DTT derivatives and dMTP-DBT.
A long conjugation length of the conductive polymer, which is
at the root of electroconductivity, consequently, induces delocal-
ization of hole and unpaired electron in the oxidative state. The
delocalization increases the chemical stability of the oxidative
molecule, but at the same time the neutral absorption band shifts
to the visible range due to the long conjugation length, which leads
to narrower band gap of the conductive polymer. Therefore, if we
need high transparency of electrochromic materials in the neutral
state, it is difficult to use conductive polymers. On the other hand,
small molecule electrochromic materials have a great advantage to
obtain high transmission in the neutral state, because it is possible
to design a molecular structure that does not have an optical
absorption in the visible wavelength range in the neutral state.
However, there is no space for delocalization of hole and unpaired
electron. Hence, the stability of cation radical species of small mol-
ecules is worse than that of the conductive polymer. Small mole-
cule materials using the viologen skeleton were investigated
energetically mainly in the 1970s. Viologen derivatives in the same
way have a short conjugation length [26–30]. Additionally, violo-
gen derivatives in many case form insoluble film adhering to the
electrode surface during a one electron reduction, which is unfa-
vorable because it is a causal factor of degradation [31].
Scheme 1. Synthesis of compounds 2–7.
using small size substituents to stabilize the radical cation. We re-
port synthesis, photophysical and electrochemical properties of
materials based on the DTT skeleton. We address the goal of devel-
oping novel electrochromic materials having the cage structure
based on DTT derivatives.
2. Experimental section
2.1. Synthesis
In this paper, we suggest that the redox stability can be im-
proved by locking the DTT core in a ‘‘cage structure’’. The cage
structure is intended to protect the radical cation, which forms
the core of the molecules in oxidation state, from nucleophilic
and/or electrophilic attacks in these oxidized molecules. The cage
structure using large size substituents on 2, 6 positions of the
DTT will be shown to be more effective than the cage structure
2.1.1. General
The outline of our synthetic routes to the 6 compounds listed in
Fig. 1 is shown in Scheme 1. They were synthesized by Suzuki–
Miyaura coupling of 2,6-dibromodithieno[3,2-b:20,30-d]thiophene
each with the corresponding boronic acid. Compounds 5 and 7
in Fig. 1 were also synthesized by Suzuki–Miyaura coupling of
3,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)dibenzo [b,d]

258
S. Okada, K. Yamada / Journal of Molecular Structure 1037 (2013) 256–263
thiophene and 2,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)dithieno[3,2-b:20,30-d] thiophene with 20-iodo-1,10:30,100-ter-
phenyl, respectively. The typical procedure is following. A mixture
13C NMR (CDCl3, 150 MHz), d: (ppm) = 150.77, 150.26, 144.98,
140.70, 138.35, 138.29, 132.99, 131.31, 131.14, 129.60, 128.09,
125.62, 125.45, 120.55, 34.94, 34.87, 31.73, 31.68;
of
2,6-dibromo-dithieno[3,2-b:20,30-d]thiophene
(174.0 mg,
MS (ESI) m/z: (M+), (found) 724.3207, (Calc.) 724.3231;
0.5 mmol), boronic acid of counterpart (1.5 mmol), tetrakis (tri-
phenylphosphine) palladium (81.5 mg, 0.08 mmol) and Cs2CO3
(2.0 mL, 2.0 M in aqueous) in 8 mL solvent of toluene: THF = 1:1
was stirred at reflux temperature for 8 h. All reactions were carried
out under nitrogen atmosphere with anhydrous solvents. The mix-
ture was poured into water (30 mL) and extracted with CH2Cl2
(20 mL ꢀ 3). The combined extract was washed with brine
(40 mL), dried (Na2SO4), and concentrated in vacuo. Column chro-
matography on silica gel eluted with CHCl3:toluene = 1:1. All com-
pounds were characterized by 1H and 13C NMR and mass
spectroscopy.
3,7-di([1,10:30,100-terphenyl]-20-yl)dibenzo[b,d]
thiophene
(dMTP-DBT), 7:
1H NMR (THF-d8, 600 MHz), d: (ppm) = 7.48 (dd, 2H), 7.40 (d,
4H), 7.36–7.38 (m, 4H), 7.02–7.08 (m, 20H), 6.88 (dd, 2H);
13C NMR (THF-d8, 150 MHz), d: (ppm) = 140.82, 140.47, 137.3,
135.72, 134.48, 133.15, 128.65, 128.14, 127.85, 125.83, 125.62,
124.41, 123.01, 119.55;
MS (ESI) m/z: (M+), (found) 640.2253, (Calc.) 640.2225.
2.2. Instrumentation
Dithieno[3,2-b:20,30-d]thiophene (DTT), 1:
1H NMR (600 MHz) and 13C NMR (150 MHz) spectra were re-
corded on a Bruker model AVANCE-600 NMR spectrometer. The
mass spectrometric analysis was carried out in positive ion mode
on a Waters MICROMASS, Q-TOF Ultima spectrometer equipped
with an ESI source. The capillary and cone voltages were set at
3000 and 80 V, respectively. The desolvation and source tempera-
tures were set to 150 and 80 °C, respectively. The cone gas flow and
desolvation gas flow were 50 and 600 L/h, respectively. Electro-
chemical measurements were performed using a three electrodes
cell with a platinum wire as a counter electrode, an Ag/Ag+ refer-
ence electrode in 0.1 M tetra butyl ammonium perchlorate (n-Bu4-
NClO4) in CH2Cl2 or CH2Cl–CH2Cl electrolyte and a glassy carbon as
a working electrode. All potentials are recorded against an Ag/Ag+
reference. A Solatron electrochemical interface model SI 1287 was
used under control of Corrware software. The electrolyte solution
was purged in 10 min with nitrogen before electrochemical mea-
surement commenced. UV–visible absorption spectra were mea-
sured using a JASCO V-560 spectrometer. Absorption spectra in
oxidized state were obtained using an Ocean Optics USB4000-
UV–vis spectrometer with a 1 mm thin layer quartz cell with three
electrodes and a Solatron electrochemical interface model SI 1287
as voltage source. Photophysical measurements in the neutral state
were performed with CH2Cl2 solution. Ab-initio calculation of the
electronic state of the compounds were performed to obtain the
minimum energy structure in the neutral and oxidized state using
the Gaussian-03 software package (Gaussian Inc.) with density
functional theory b3lyp calculation and bases set 6-31Gꢁ [32].
ALDRICH, CAS: 3593-75-7
2,6-diphenyldithieno[3,2-b:20,30-d]thiophene(dPh-DTT),
2:
[12,13];
1H NMR (CDCl3, 600 MHz), d: (ppm) = 7.66 (d, 4H), 7.52 (s, 2H),
7.43 (t, 4H), 7.33 (t, 2H);
13C NMR (CDCl3, 150 MHz), d: (ppm) = 145.46, 142.04, 134.86,
130.65, 129.44, 128.25, 126.05, 116.92;
MS (ESI) m/z: (M+), (found) 348.0103, (Calc.) 348.0101;
2,6-bis(2,5-dimethoxyphenyl)dithieno[3,2-b:20,30-d]thiophene
(dMeO-DTT), 3:
1H NMR (CDCl3, 600 MHz), d: (ppm) = 7.74 (s, 2H), 7.24 (d,2H),
6.95 (d, 2H), 6.84 (dd, 2H), 3.94(s, 6H), 3.84 (s, 6H);
13C NMR (CDCl3, 150 MHz), d: (ppm) = 154.19, 150.56, 141.48,
140.50, 131.80, 124.56, 119.47, 113.93, 113.90, 113.61, 56.76,
56.20;
MS (ESI) m/z: (M+), (found) 468.0489, (Calc.) 468.0524;
2,6-dimesityldithieno[3,2-b:20,30-d]thiophene (dMS-DTT), 4:
1H NMR (CDCl3, 600 MHz), d: (ppm) = 7.02 (s, 2H), 6.98 (s, 4H),
2.35 (s, 6H), 2.21 (s, 12H);
13C NMR (CDCl3, 150 MHz), d: (ppm) = 142.37, 140.38, 138.92,
138.84, 131.55, 131.23, 128.53, 120.36, 21.49, 21.11;
MS (ESI) m/z: (M+), (found) 432.1071, (Calc.) 432.1040;
2,6-di([1,10:30,100-terphenyl]-20-yl)dithieno[3,2-b:20,30-d]thio-
phene (dMTP-DTT), 5:
1H NMR (CDCl3, 600 MHz), d: (ppm) = 7.49 (dt, 2H), 7.41 (d, 4H),
7.21 (br, 20H), 6.50 (s, 2H);
13C NMR (CDCl3, 150 MHz), d: (ppm) = 143.63, 141.94, 141.47,
139.63, 131.93, 131.66, 130.20, 129.73, 128.68, 128.22, 127.05,
123.21;
3. Results and discussion
MS (ESI) m/z: (M+), (found) 652.1412, (Calc.) 652.1353;
2,6-bis(4,40-di-tert-butyl-[1,10-biphenyl]-2-yl)dithieno[3,2-
b:20,30-d]thiophene(dBBP-DTT), 6:
3.1. Electrochemical properties
The electrochemical properties of the compounds shown in
Fig. 1 were investigated. To examine the redox behavior of newly
synthesized compounds 3–6, cyclic voltammetric (CV) measure-
ments were performed in methylene chloride at room
1H NMR (CDCl3, 600 MHz), d: (ppm) = 7.56 (d, 2H), 7.43 (dd,
2H), 7.33 (d, 2H), 7.30 (d, 4H), 7.21 (d, 4H), 6.81 (s, 2H), 1.39 (s,
18H), 1.31 (s, 18H);
Table 1
Electrochemical dataa for DTT derivatives and dibenzothiophene derivative.
Compounds
Eox (V)
Ered (V)
Eox1/2 (V)
dE (mV)
HOMOb (eV)
hc (°)
1
2
3
4
5
6
7
DTT
1.12
0.85
0.53
1.01
0.84
0.85
1.39
–
Irreversible
0.82
0.47
0.98
0.81
ꢂ5.58
ꢂ5.17
ꢂ4.95
ꢂ5.36
ꢂ5.12
ꢂ5.01
ꢂ5.55
dPh-DTT
dMeO-DTT
dMS-DTT
dMTP-DTT
dBBP-DTT
dMTP-DBT
0.79
0.42
0.94
0.79
0.78
–
63
117
70
59
72
28
28
90
60
41
61
0.82
Irreversible
a
Scan rate of cyclic volutammetry was 25 mV/s, The counter electrode was Pt wire and the reference was Ag/Ag+ with a glassy carbon as a working electrode, all potentials
were referenced versus the Ag/Ag+ in CH2Cl2 with an electrolyte 0.1 M TBAP.
b
HOMO is calculated by Gaussian-03.
Dihedral angle is calculated by Gaussian-03.
c

S. Okada, K. Yamada / Journal of Molecular Structure 1037 (2013) 256–263
259
temperature. The resulting oxidation and rereduction potentials
are summarized in Table 1 together with those for compound 1
(DTT), known DTT derivatives compound 2 and compound 7 for
comparison. The electrochemical oxidation and reduction reaction
of compounds 2–6 is reversible, so that the CVs show comparable,
symmetrical anodic and cathodic waves. While compounds 2–6
exhibit reversible first oxidation waves, compound 1 (DTT) shows
irreversible redox waves due to easy polymerization [19,33–35].
In this experiment, electro polymerization occurred on the work-
ing electrode as soon as potential was applied to the electrode.
Consequently, the working electrode was coated with black
deposits. It was necessary to protect the 2, 6 positions of the
DTT core using some kind of substituent to obtain redox revers-
ibility even in for a few cycles only.
Fig. 2. Voltage application sequence to measure long term redox stability.
1.25
(a)
dPh-DTT
dMS-DTT
1.2
dMTP-DTT
dBBP-DTT
1.15
1.1
1.05
1
0.95
0
500
1000
1500
2000
2500
3000
Oxidation times
2.5E-06
2.0E-06
1.5E-06
1.0E-06
5.0E-07
0.0E+00
-5.0E-07
-1.0E-06
-1.5E-06
-2.0E-06
1
100
500
1000
1500
2000
2500
(c)
(e)
(b)
1500
2000
3000
5000
7000
0
0.5
1
0
0.5
1
Potential (Volt)
Potential (Volt)
2.5E-06
2.0E-06
1.5E-06
1.0E-06
5.0E-07
0.0E+00
-5.0E-07
-1.0E-06
-1.5E-06
-2.0E-06
1
100
5000
17000
19000
1000
(d)
1500
2000
10000
17000
20000
15000
18000
20000
0
0.5
1
0
0.5
1
Potential (Volt)
Potential (Volt)
Fig. 3. Variation of oxidation potential during repetitive oxidation.(a) oxidation peak shift during repetition (b–e) are cyclic voltammograms of (b) dPh-DTT, (c) dMS-DTT, (d)
dMTP-DTT and (e) dBBP-DTT 0.1 mM in methylene chloride (b and c) in ethylene chloride (d and e) with 0.1 M TBAP, sweep rate was 25 mV/s.

260
S. Okada, K. Yamada / Journal of Molecular Structure 1037 (2013) 256–263
Table 2
The oxidation potential is affected by the conjugation length
Photophysical data for compounds listed Fig. 1.
which is linked to the energy of the HOMO. Now we defined a dihe-
dral angle as the calculated angle between the DTT plane and the
phenyl plane of the substituent as shown in Table 1. The dihedral
angles of compounds 2–6 should have much effect on each oxida-
tion potential. As expected, compound 4 that has a substituent
with large steric hindrance exhibit the higher oxidation potential
(Eox1/2) compared to the other compounds in this study. This com-
pound 4 reveals relatively short conjugation length that is attribut-
able to the large dihedral angle. However, compounds 2, 5 and 6
have similar Eox1/2 in spite of different dihedral angles among them.
Compound 3 shows lower oxidation potential than other com-
pounds. It might be derived from longer conjugation length due
to lower dihedral angle and strong electron donation owing to
methoxy substituents of phenyl attachment to DTT. Experimental
results of oxidation potential show a nearly proportionate relation-
ship to the calculated energy of the HOMO as listed in Table 1.
From these results, it seems that the difference of oxidation poten-
tials among these compounds is connected with the structural con-
figuration of the molecules to some degree. We picked four
compounds, which had redox reversibility for a short period, to
investigate the long term redox stability.
Compounds Absorption peak (nm)
eb (Mꢂ1 cmꢂ1
)
Neutral state
Oxidation statea
1
2
3
4
5
6
7
DTT
282s, 292
370
386
474, 526
578
350, 429, 495
479
519
37,500
40,780
46,255
18,559
20,350
27,919
13,631
dPh-DTT
dMeO-DTT
dMS-DTT
dMTP-DTT
dBBP-DTT
dMTP-DBT
319, 386, 408s
309, 318
243, 278s, 348
357
243, 292s, 324s
s Shoulder absorption peak.
a
Maximum absorption peak at oxidation state.
b
Molar absorption coefficient ‘‘e’’ was measured at first absorption peak in neutral
state.
and the CV current increased due to condensation of solution. Oxi-
dation peak potential of dPh-DTT 2 shifted to 5% higher potential at
around 1500 repetitions. In case of dMS-DTT 4, 5% higher shift of
potential peak occurred at 2500 repetitions as shown in Fig. 3a.
The peak shift of dMTP-DTT 5 did not occur after 3000 times and
even after 17,000 times. Furthermore, the peak shift of dBBP-DTT
6 did not occur after 20,000 repetitions. The oxidation peaks of
dMTP-DTT 5 and dBBP-DTT 6 remained clearly distinguishable
after 17,000 and 20,000 cycles without obvious generation of reac-
tion product accompanied with new oxidation peak generation.
This result shows that dMTP-DTT 5 and dBBP-DTT 6 have excellent
stability regarding oxidation peak shift during long term redox rep-
etition more than that of dPh-DTT 2 and dMS-DTT 4. Compounds 2,
5 and 6 have similar oxidation potential potential Eox1/2, but a dif-
ferent cage structure and different long term redox stability.
Lastly, we show the long term redox stability of compounds
using different cage structures in Fig. 4. Lifetime is taken from
20% decrease of oxidation peak current or a peak shift of more than
5%. To represent the size effect of the cage structure, we try to use
the projection length of the cage structure. The projection length is
defined as orthogonal projection of cage size to the orthogonal
plane of the DTT plane using DFT calculation as shown in inset in
Fig. 4. It is likely that the long term redox stability has a high cor-
relation with the oxidized molecular structure of the compound as
well as the molecular structure in neutral state. In Fig. 4, the plots
of filled triangles show projection length of the oxidation state and
the plots of opened circles show neutral state projection lengths,
both of which are calculated from the minimum energy structure
obtained from the DFT calculation program. As shown in Fig. 4,
longer projection length of oxidation and neutral state corresponds
to higher long term redox stability. Consequently, the compounds
5, 6 having longer projection length show excellent durability. The
durablility times for retaining the oxidation potential of the initial
value, of dPh-DTT 3, dMS-DTT 4, dMTP-DTT 5 and dBBP-DTT 6 as
shown in Fig. 4 are 1500, 2000, 17,000 and over 20,000. The
20,000 times is only an upper limitation of this measurement
due to volatilization of solvent of this measurement. From these re-
sults, it is clear that the large size substituent, which was used for
the cage structure, is quite effective in improving the long term re-
dox stability.
3.2. Long term redox stability
To investigate the long term stability during switching between
oxidation and neutral state, the electrochemical measurement was
reported by applying rectangular voltage pulses to compounds 2
(dPh-DTT), 4 (dMS-DTT), 5 (dMTP-DTT) and 6 (dBBP-DTT) in the
three electrodes cell as shown in Fig. 2. Rectangular voltage pulses
consisted of two 10 s pulses. The voltage of the first pulse was
beyond
ox
beyond
is the
ox
E
and that of the second pulse was 0.0 V, where E
potential which is 0.1 V higher than the potential for first oxidation
peak for each compound. The CV measurement was carried out
three times after every 100 pulses to the cell. The three electrodes
cell was composed of a platinum wire as a counter electrode,
Ag/Ag+ reference electrode and a glassy carbon electrode as a
working electrode in 0.1 M tetra butyl ammonium perchrolate
(n-Bu4NClO4) and 0.1 mM compound as an object to be measured
in CH2Cl2 or C2H4Cl2 electrolyte solution. All potentials are
reported against an Ag/Ag+ reference.
Fig. 3a shows oxidation peak shifts of compounds in long term
redox repetition between oxidation and neutral state. Fig. 3b–e
show the CV waveforms of compounds 2, 4, 5 and 6. Measurement
of long term redox stability was done with 1,2-dichroloethane for
solvent to avoid volatilization. However, the volatilization occurred
at repetition in excess of 10,000 times, as shown in Fig. 3d and e
20000
oxidation state
18000
neutral state
16000
14000
Projection length
of Cage
12000
10000
8000
6000
4000
2000
0
3.3. The speculations of underlying cause about size effect
dMTP-DTT
15
We suggest two explanations for the improvement of the long
term redox stability using large size substituents. The first is a reduc-
tion of the intermolecular interaction due to the large size substitu-
ent suggested by the photophysical properties. The photophysical
properties of the compounds shown in Fig. 1 were investigated using
UV–vis spectroscopy. The UV–vis spectroelectrochemical oxidation
0
5
10
20
Projection length of cage (0.1 nm)
Fig. 4. Lifetime under oxidation of DTT derivatives with different cage structure.
Projection length of cage was obtained by calculation with Gaussian-03.

S. Okada, K. Yamada / Journal of Molecular Structure 1037 (2013) 256–263
261
dMS-DTT
dPh-DTT
dBBP-DTT
dMTP-DTT
Fig. 5. Spectroelectrochemical oxidation of dPh-DTT, dMS-DTT, dMTP-DTT and dBBP-DTT 0.1 mM in methylene chloride with 0.1 M TBAP.
of the compounds was carried out in 1 mm thin layer quartz cell
with methylene chloride solvent as described in experimental sec-
tion. The absorption peaks in both neutral and oxidation states of se-
ven compounds in Fig. 1 in methylene chloride are listed in Table 2.
Though the compounds, dPh-DTT 3 and dMS-DTT 4, had multiple
absorption peaks in the visible range at oxidation state, dMTP-DTT
5 and dBBP-DTT 6 had only a single absorption peak as shown in
(a)
2.50
2.00
1.50
1.00
0.50
0.00
Fig. 5. It is well known that the cation radicals form
ciation with the shift in the
pꢁ absorption band [2] and the forma-
tion of dimers is temperature dependent [11]. Fig. 6a shows the
p dimers in asso-
p
ꢂ
p
temperature dependence of the intensity ratio in the oxidation pro-
cess between 474 nm and 526 nm for dPh-DTT 3 and between
350 nm, 429 nm and 495 nm for dMS-DTT 4. Fig. 6b and c shows
the spectral change of dPh-DTT 3 and dMS-DTT 4 depending on
the temperature. The ratio of peak intensity of dPh-DTT 3 was
dependent on the temperature as shown in Fig. 6. Hence, the multi-
dMS-DTT(350/495)
dMS-DTT(429/495)
dPh-DTT(474/526)
-10
0
10
20
30
40
ple peaks of dPh-DTT 3 were supposed to be induced by
p dimer for-
Temperature (ºC)
mation [11]. However, ratio of peak intensity between the multiple
peaks of dMS-DTT 4 remained unaltered by temperature change as
shown in Fig. 6a. The result means that the multiple absorption
peaks of dMS-DTT 4 are intrinsic cation peaks. From these results,
it is probable that the large size substituents on 2, 6 positions of
(b)
(c)
DTT core work to prevent pdimer formation. Therefore, it seems that
the larger size substituent will weaken intermolecular interactions.
This finding is consistent to the result of superior long term redox
stability of compounds, dMTP-DTT 5 and dBBP-DTT 6.
Another explanation of the results is a localization of the HOMO
to the core portion. Fig. 7 shows the HOMO surface of compounds
listed in Fig. 1. It seems that dPh-DTT 3 is easily attacked from out-
wards, because dPh-DTT 3 has the HOMO not only in the DTT but
also in the cage. However, it seems that the compounds of dMS-
DTT 4, dMTP-DTT 5 and dBBP-DTT 6 are protected against attack
from outwards owing to localizing the HOMO to interior of the
molecule using cage structure as shown in Fig. 7. Compounds 4
and 5, which have a relatively large dihedral angle, have large ste-
ric hindrance at both ortho positions of the phenyl attached to the
Fig. 6. (a) Visible band absorption ratio of dMS-DTT and dPh-DTT 1 mM in
methylene chloride with 0.1 M TBAP. Set of optical spectra of the first oxidation
state of (b) dMS-DTT and (c) dPh-DTT at different temperatures.

262
S. Okada, K. Yamada / Journal of Molecular Structure 1037 (2013) 256–263
1.
DTT
(b)
(a)
dMTP-DTT,
0.23mM/ 0.1M TBAP
/ dichloromethane
dMTP-DBP,
0.1mM/ 0.1M TBAP
/ dichloromethane
2. dPh-DTT
3. dMeO-DTT
4. dMS-DTT
Wavelength (nm)
Wavelength (nm)
(c)
5. dMTP-DTT
6. dBBP-DTT
I-MTP
Wavelength (nm)
Fig. 8. Spectroelectrochemical data of dMTP-DBT, dMTP-DTT and I-MTP.
the different HOMO composition. dMTP-DTT 5 has two absorption
peaks in the neutral state. During the oxidation progress of dMTP-
DTT 5, one neutral absorption peak of 348 nm diminished and a
new cationic absorption peak of 480 nm appeared. However, an-
other neutral absorption peak of 243 nm of dMTP-DTT 5 did not
change any more as shown in Fig. 8b. Though compound dMTP-
DBT 7 had a neutral absorption peak at 243 nm, the peak dimin-
ished during the oxidation process as shown in Fig. 8a. dMTP-
DTT 5 and dMTP-DBT 7 have same cage structure and almost the
same dihedral angle as seen in Table 1. However, core structures
of these two compounds are different. dMTP-DTT 5 has DTT as a
core. On the other hand dMTP-DBT 7 has dibenzothiophen (DBT)
as a core. The DTT is an electron rich fused ring structure, richer
than the DBT. dMTP-DBT 7 has a wider band gap and higher oxida-
tion potential than dMTP-DTT 5 as listed in Tables 1 and 2. The
neutral absorption peak of 243 nm of dMTP-DTT 5 and 243 nm of
dMTP-DBT 7 belongs to the cage part absorption that is commonly
found. It is obvious by making a comparison of absorption peaks
between iodine meta terphenyl and dMTP-DBT 7 as shown in
Fig. 8 (c). It is important that in the case of dMTP-DBT 7, the cage
oxidized at a first oxidation potential, 1.39 V, while in the case of
dMTP-DTT 5, only the core oxidized at a first oxidation potential,
0.84 V, in the electrochemical oxidation process. The HOMO energy
of the DTT and the DBT calculated by Gaussian 03/b3lyp/6-31Gꢁ
were ꢂ5.58 eV and ꢂ5.80 eV, respectively. In fact, DTT oxidized
at 1.12 V which was lower than the oxidation potential of dMTP-
DBT 7, 1.39 V. We acknowledge that dMTP-DBT 7 has a HOMO
leaking to the outer shell because of low HOMO energy. Further-
more, the HOMO is expanded outwards due to the substitute posi-
tions of dMTP-DBT 7 as shown in Fig. 7. However, dMTP-DTT 5 has
a HOMO mainly in the core portion because HOMO energy of DTT
is higher than the orbital energy of the cage portion. These results
explain that the compounds with the same cage structure but dif-
ferent core structure have different electrochemical properties as
seen in Table 1.
7. dMTP-DBT
Fig. 7. Distribution of HOMO on neutral state calculated by Gaussian-03. Dihedral
angle between cage and core affected to distribution of HOMO in the neutral state.
Large dihedral angle localized HOMO to the core due to orthogonal molecular
orbital between them. Compounds 4, 5 and 6 have mainly localized HOMO to the
core. (Be reproduced in color on the Web (free of charge) and in black-and-white in
print).
DTT core. Oxidation peak shift of dMS-DTT 4 is larger than that of
dMTP-DTT 5 and dBBP-DTT 6. This will be attributing to the length
of cage structure of dMS-DTT 4, which is shorter than dMTP-DTT 5
and dBBP-DTT 6. It is likely that the length is insufficient to cover
enough to prevent attack from outside. On the other hand, while
compound 6 shows medium dihedral angle, it has a quite bulky
structure. Therefore, large size substituents of dMTP-DTT 5 and
dBBP-DTT 6 are expected to work as efficient cage to protect DTT
core from attack. It is likely that the restriction of localization of
HOMO to the core portion through use of a core-cage system is
associated with the decrease of the reactivity of the corresponding
radical cation, since the radical cation is generated in the HOMO by
oxidation.
4. Conclusions
Though the dMTP-DBT 7 has the same substituent as cage as
dMTP-DTT 5, dMTP-DBT 7 showed inferior redox stability as
shown in Table 1. In our understanding, the difference derives from
The result of our experiments clearly shows that the DTT deriv-
atives having large size substituents at 2, 6 positions have excellent

S. Okada, K. Yamada / Journal of Molecular Structure 1037 (2013) 256–263
263
[9] L. Wang, Q. Chen, G. Pan, L. Wan, S. Zhang, X. Zhan, B.H. Northrop, P.J. Stang, J.
Am. Chem. Soc. 130 (2008) 13433.
[10] W. Schroth, E. Hintzsche, M. Felicetti, R. Spitzner, J. Sieler, R. Kempe, Angew.
Chem. Int. Ed. 33 (1994) 739.
[11] B. Yina, C. Jianga, Y. Wanga, M. Laa, P. Liua, W. Deng, Synth. Met. 160 (2010)
432.
[12] O.H. Omar, F. Babudri, G.M. Farinola, F. Naso, Tetrahedron 67 (2011) 486.
[13] W. Porzio, S. Destri, U. Giovanella, M. Pasini, L. Marin, M.D. Iosip, M. Campione,
Thin Solid Films 515 (2007) 7318.
[14] P. Liu, X. Wang, Y. Zhang, X. Zhou, W. Deng, Synth. Met. 155 (2005) 565.
[15] Y.M. Sun, Y.Q. Ma, Y.Q. Liu, Y.Y. Lin, Z.Y. Wang, Y. Wang, C.A. Di, K. Xiao, X.M.
Chen, W.F. Qiu, B. Zhang, G. Yu, W.P. Hu, D.B. Zhu, Adv. Funct. Mater. 16 (2006)
426.
[16] P. Baeuerle, U. Segelbacher, A. Maier, M. Mehring, J. Am. Chem. Soc. 115 (1993)
10217.
[17] T. Nishinaga, A. Wakamiya, D. Yamazaki, K. Komatsu, J. Am. Chem. Soc. 126
(2004) 3163.
[18] A. Wakamiya, D. Yamazaki, T. Nishinaga, T. Kitagawa, K. Komatsu, J. Org. Chem.
68 (2003) 8305.
[19] Z. Xu, D. Fichou, G. Horowitz, F. Garnier, J. Electroanal. Chem. 267 (1989) 339.
[20] A.L. Dyer, M.R. Craig, J.E. Babiarz, K. Kiyak, J.R. Reynolds, Macromolecules 43
(2010) 4460.
[21] P.M. Beaujuge, S. Ellinger, J.R. Reynolds, Nat. Mater. 7 (2008) 795.
[22] G. Sonmez, H. Meng, Q. Zhang, F. Wudl, Adv. Funct. Mater. 13 (2003) 726.
[23] B. Sankaran, J.R. Reynolds, Macromolecules 30 (1997) 2582.
[24] M. Li, A. Patra, Adv. Mater. 21 (2009) 1707.
[25] P.H. Aubert, A.A. Argun, A. Cirpan, D.B. Tanner, J.R. Reynolds, Chem. Mater. 16
(2004) 2386.
[26] T. Kawata, M. Yamamoto, M. Yamana, M. Tajima, T. Nakano, Jpn. J. Appl. Phys.
14 (1975) 725.
long term redox stability. Although the high reactivity of the 2, 6
positions of DTT due to high spin density is well known, we would
like to focus attention to the size of substituents. It is notable that a
large size substituent is more effective to stabilize the redox state
than a small size substituent. It is likely that the large size cage
structure protects the core in which the HOMO exists and prevents
degradation. In other words, it is suggested that the reactivity of
the radical cation would be reduced by use of the large size cage
structure, therefore the long term redox stability of a small mole-
cule increased in spite of a short conjugation length relative to con-
ductive polymers.
The compounds with large size substituents such as 4,40-di-tert-
butyl-(1,10-biphenyl)-2-yl and (1,10:30,100-terphenyl)-2-yl show
good electrochromicity and excellent long term redox stability of
17,000 times and over 20,000 times in repeated oxidation, respec-
tively. Long term reversible behavior is a notable feature for the
present core-cage system that promises the potential use as an
electrochromic and electroconductive material in organic electron-
ics. Further studies on the core-cage structural design of dithie-
no[3,2-b:20,30-d]thiophene (DTT) derivatives as well as their
application in organic electorochromic devices are now in progress
in our group.
[27] C.J. Schoot, J.J. Ponjee, H.T. van Dam, R.A. van Doorn, P.T. Bolwijn, Appl. Phys.
Lett. 23 (1973) 64.
Appendix A. Supplementary material
[28] M. Yamana, Jpn. J. Appl. Phys. 15 (1976) 2469.
[29] J. Bruinink, P. van Zanten, J. Electrochem. Soc. 124 (1977) 1232.
[30] D.J. Barclay, B.F. Dowden, A.C. Lowe, J.C. Wood, Appl. Phys. Lett. 42 (1983) 911.
[31] R.J. Mortimer, Chem. Soc. Rev. 26 (1997) 147.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2013.01.
002.
[32] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Rob, J.R. Cheeseman,
J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar,
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox,
H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E.
Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y.
Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.
Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J.
Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M.
Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A.
Pople, Gaussian 03, Gaussian, Inc., Wallingford, CT, 2003.
[33] F. Garnier, G. Tourillon, M. Gazard, J.C. Dubois, J. Electroanal. Chem. 148 (1983)
299.
References
[1] M.G. Hill, J.F. Penneau, B. Zinger, K.R. Mann, L.L. Miller, Chem. Mater. 4 (1992)
1106.
[2] M.G. Hill, K.R. Mann, L.L. Miller, J.F. Penneau, J. Am. Chem. Soc. 114 (1992)
2728.
[3] T. Okamoto, K. Kudoh, A. Wakamiya, S. Yamaguchi, Chem. Eur. J. 13 (2007) 548.
[4] K. Takimiya, N. Niihara, T. Otsubo, Synthesis 10 (2005) 1589.
[5] S. Zhang, Y. Guo, H. Xi, C. Di, J. Yu, K. Zheng, R. Liu, X. Zhan, Y. Liu, Thin Solid
Films 517 (2009) 2968.
[6] N. Metri, X. Sallenave, L. Beouch, C. Plesse, F. Goubard, C. Chevrot, Tetrahedron
Lett. 51 (2010) 6673.
[7] C. Kima, M. Chenb, Y. Chiangb, Y. Guob, J. Youna, H. Huanga, Y. Liangb, Y. Linb,
Y. Huangb, T. Hub, G. Leed, A. Facchettia, T.J. Marks, Org. Electron. 11 (2010)
801.
[8] W. Schroth, E. Hintzsche, H. Jordan, T. Jende, R. Spitzner, I. Thondorf,
Tetrahedron 53 (1997) 7509.
[34] A. Smie, A. Synowczyk, J. Heinze, R. Alle, P. Tschuncky, G. Götz, P. Bäuerle, J.
Electroanal. Chem. 452 (1998) 87.
[35] P. Tschuncky, J. Heinze, Synth. Met. 55 (1993) 1603.