Synthesis and electrochemical properties of symmetric squarylium dyes containing diarylamine

Article abstract of DOI:10.1016/j.dyepig.2013.09.047

We studied the synthesis and electrochemical properties of symmetric squarylium dyes (SQDs) containing diarylamine with various structures. It was shown that several of our synthesized SQDs had improved electrochemical properties and might be a promising and an easy modifiable π-conjugate core block of active materials for electroactive and photoactive devices.

Full text of DOI:10.1016/j.dyepig.2013.09.047

Dyes and Pigments 101 (2014) 261e269
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and electrochemical properties of symmetric squarylium
dyes containing diarylamine
,
a
,
b
,
,b
Yutaka Ohsedo a , Misao Miyamoto 1, Akihiro Tanaka b, Hisayuki Watanabe a
*
a Advanced Materials Research Laboratory, Collaborative Research Division, Art, Science and Technology Center for Cooperative Research, Kyushu University,
4-1 Kyudaishinmachi, Nishi-ku, Fukuoka 819-0388, Japan
b Nissan Chemical Industries, Ltd., 2-10-1 Tsubonishi, Funabashi, Chiba 274-8507, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
We studied the synthesis and electrochemical properties of symmetric squarylium dyes (SQDs) con-
taining diarylamine with various structures. It was shown that several of our synthesized SQDs had
improved electrochemical properties and might be a promising and an easy modifiable p-conjugate core
Received 17 May 2013
Received in revised form
22 September 2013
Accepted 30 September 2013
Available online 19 October 2013
block of active materials for electroactive and photoactive devices.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Squarylium dyes
Electrochemical properties
Radical cation
Dication
1. Introduction
[15] and capacity for supramolecular modification in high dura-
bility applications in the field of medicine [16] have been explored.
Although the previously discussed SQDs have been extensively
studied, they absorb primarily in the visible to near-infrared re-
gions. New SQDs that absorb in the blue laser region (w405 nm) are
needed for new applications in which blue lasers are used, such as
active materials for optical memory or sensitizers for photochem-
ical reactions.
Squaraine dyes or squarylium dyes (SQDs) are a well-known
family of functional dyes to be used as colouring materials [1],
photoconductive materials in photocopiers [2] and the active layer
in optical memory such as digital versatile discs [3]. In this decade,
SQDs have been studied as a core block of active materials for new
types of organic electroactive and photoactive devices such as
electroluminescent devices [4], bulk heterojunction organic
photovoltaic cells [5], and dye-sensitized solar cells [6]. In the field
of medicinal science, SQDs with near-infrared absorption proper-
ties have been studied as a fluorophore for sensing materials [7]
and a sensitizer for photodynamic therapy [8]. Moreover, SQDs
Recently, for their usefulness in well-defined molecular struc-
ture designs,
tion as candidates for active materials of organic electroactive and
photoactive devices. Better electrochemical properties of the
p-conjugate compounds have garnered much atten-
p
-
conjugate compounds, such as electrochemical reversibility, might
be needed in the active materials for rechargeable batteries [17]. On
the other hand, better electrochemical reversibility of p-conjugate
compounds, both transporting materials and emitting materials, in
electroluminescent devices might be considered essential for
have attracted attention as unique colourants of
for supramolecular ion sensing [9], foldarmer formation [10],
extended -conjugate compounds [11] such as low bandgap poly-
mers for organic conductors [12], -extended systems such as bis-
squarylium compounds [13], and squarylium oligomers [14]. Thus,
SQDs are considered unique units of -conjugate dyes for the
p-conjugate units
p
p
designing high-performance
p-conjugate compounds [18]. The
p
electrochemical reversibility may become necessary as better
design criteria emerges for electroactive and photoactive materials.
To develop SQDs as potential blue-laser absorption dyes and as
promising candidates for the core block of functional dyes, we have
studied the synthesis and electrochemical properties of symmetric
SQDs containing diarylamine with blue light absorption (Scheme 1).
These compounds can be synthesized in a one-step reaction with
various applications mentioned above, and their unique reactivity
* Corresponding author. Tel.: þ81 92 400 4381; fax: þ81 92 400 4382.
E-mail addresses: ohsedo@astec.kyushu-u.ac.jp, twopolaron@yahoo.co.jp
(Y. Ohsedo).
1
Current address: Nippon Phosphoric Acid Co., JP, 14 Kitasode, Sodegaura, Chiba,
Japan.
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.09.047

262
Y. Ohsedo et al. / Dyes and Pigments 101 (2014) 261e269
Scheme 1. Chemical structures of symmetric SQDs.
easy workup from squaric acid and secondary arylamine. It was
shown that several of our extended SQDs have and better electro-
chemical reversibility than primitive SQDs and might be a prom-
of a quartz glass cell (concentration of a measured SQD solution:
w1 ꢂ 10ꢀ6 M).
ising p-conjugate core block of an active material for electroactive
2.2. Synthesis of SQDs
and photoactive devices using blue laser or other lasers for appli-
cations mentioned above. Further modification of arylamine as a
raw reagent will offer the versatility for creating new symmetric
SQDs with high performance based on this study.
Related to this paper, the synthesis [19] and electrochemical
properties [20] of primitive SQDs containing diarylamine (MePhA-
SQ and DPhA-SQD) were reported. The previous studies have pri-
marily focused on analysis of the chemical structure of primitive
SQDs, including dialkylamine and diarylamine. The electrochemical
properties such as reversibility of other extended symmetric-type
SQDs are reported here for the first time.
2.2.1. MePhA-SQD
Under N2 atmosphere, n-butanol/toluene [1:1 (v/v), 5 mL], N-
methylaniline (3.00 g, 27.4 mmol), and squaric acid (1.28 g,
11.2 mmol) were combined and stirred at 110 ꢁC for 21 h, then cooled
to room temperature. The obtained precipitate was filtered, washed
with methanol and dried in vacuo to yield an orange powder (2.95 g,
yield 90.8%). 1H NMR (500 MHz, DMSO-d6, TMS,
d
, ppm): 7.22 (m,
8H, AreH), 7.29 (m, 2H, AreH at the para position), 3.84 (s, 6H, Ne
CH3).13C NMR (125 MHz, CDCl3, TMS,
, ppm): 177.47,168.52, (141.24,
d
141.09), 129.24, 127.27, 123.00, (39.19, 38.73). MALDIeTOF MS: calcd
for C18H16N2O2 (MW ¼ 292.12): m/z ¼ 292.61 [Mþ]. Elemental anal.
calcd for C18H16N2O2: C, 73.95%; H, 5.52%; N, 9.58%. Found: C, 73.93%;
H, 5.59%; N, 9.66%. It is worth noting that in 13C NMR spectrum of
MePhA-SQD might show peaks based on conformers as shown in the
literature describing squalyrium dyes [21].
2. Experimental section
2.1. General
Dichloromethane (super dehydrated), N,N-dimethylformamide
(SpectrosolÒ, DOJINDO Laboratories) and squaric acid were pur-
chased from Wako Pure Chemical Industries, Ltd. and used without
further purification. All other reagents and solvents were pur-
chased from Tokyo Chemical Industry Co., Ltd. and used without
further purification. Water was deionised with an Elix UV 3 Milli-Q
integral water purification system (Nihon Millipore K.K).
2.2.2. DPhA-SQD
Under N2 atmosphere, n-butanol/toluene [1:1 (v/v), 5 mL],
diphenylamine (3.00 g, 17.6 mmol), and squaric acid (809 mg,
7.09 mmol) were combined and stirred at 110 ꢁC for 21 h, then
cooled to room temperature. The obtained precipitate was filtered,
washed with methanol and dried in vacuo to yield an orange
1H NMR and 13C NMR were recorded on an AVANCE 500
(500 MHz, Bruker BioSpin K.K.) spectrometer. MALDIeTOF MS was
performed with an autoflex III (Bruker Daltonics Inc.) using sina-
pinic acid as a matrix. Elemental analysis was performed with a
Yanaco CHN corder MT-5 (Yanaco New Science Inc.). Single-crystal
X-ray diffraction data were recorded on a SMART APEX II Ultra X-
powder (1.73 g, yield 58.5%). 1H NMR (500 MHz, DMSO-d6, TMS,
d,
ppm): 7.44 (t, 8H, J ¼ 7.7 Hz, AreH), 7.35 (t, 4H, J ¼ 7.3 Hz, AreH at
the para position), 7.25 (d, 8H, J ¼ 7.6 Hz, AreH). 13C NMR (125 MHz,
CDCl3, TMS, d, ppm): 178.82, 168.09, 140.44, 128.94, 127.67, 125.43.
MALDIeTOF MS: calcd for
C28H20N2O2 (MW
¼
416.15): m/
z ¼ 416.82 [Mþ]. Elemental anal. calcd for C28H20N2O2: C, 80.75%; H,
ray diffractometer (Bruker AXS K.K.) with CuK
a
at ꢀ50 ꢁC. Crystal
4.84%; N, 6.73%. Found: C, 80.62%; H, 4.79%; N, 6.78%.
structures were visualised from CIF data using Mercury (Cambridge
Crystallographic Data Centre). Electrochemical measurements
were carried out with an electrochemical analyzer model 708c (CH
Instruments, Inc.). Electronic absorption spectra within the ultra-
violetevisible region were measured with a V-670 UVevis spec-
trophotometer (JASCO Corporation) by using a 10 mm path length
2.2.3. NpPhA-SQD
Under N2 atmosphere, n-butanol/toluene [1:1 (v/v), 5 mL], N-
methylaniline (4.81 g, 21.3 mmol), and squaric acid (1.00 g,
8.77 mmol) were combined and stirred at 110 ꢁC for 25 h, then
cooled to room temperature. The obtained precipitate was filtered,

Y. Ohsedo et al. / Dyes and Pigments 101 (2014) 261e269
263
Scheme 2. Reaction scheme of symmetric SQDs containing arylamines.
washed with methanol and dried in vacuo to yield an orange
powder (3.55 g, yield 78.4%). 1H NMR (500 MHz, DMSO-d6, TMS,
ppm): 7.82 [s (br), 6H, AreH], 7.71 [s (br), 2H, AreH], 7.48 [s (br), 4H,
AreH], 7.43 (m, 12H, AreH). 13C NMR (125 MHz, CDCl3, TMS,
d,
d
,
ppm): 179.40, 168.68, 140.93, 138.21, 133.45, 132.82, 129.41, 128.69,
128.15, 127.25, 127.07, 125.91, 124.54, 123.84. MALDIeTOF MS: calcd
for C36H24N2O2 (MW ¼ 516.18): m/z ¼ 516.93 [Mþ]. Elemental anal.
calcd for C36H24N2O2: C, 83.70%; H, 4.68%; N, 5.42%. Found: C,
83.60%; H, 4.71%; N, 5.49%. In the 13C NMR spectrum of NpPhA-SQD,
although 16 peaks were expected on the basis of the chemical
structure, only 14 peaks were observed. When the DEPT-135 13C
NMR spectrum of NpPhA-SQD in CDCl3 was obtained, six peaks
attributable to quaternary carbons disappeared i.e., the peaks at
179.40, 168.68, 140.93, 138.21, 133.45, and 132.82 may have coin-
cided with those of quaternary carbons; therefore, the remaining
eight peaks may contain the two lost CH peaks.
Fig. 2. Electronic absorption spectra of SQDs in the DMF solution.
squaric acid (428 g, 3.75 mmol), were combined and stirred at
110 ꢁC for 17 h, then cooled to room temperature. The obtained
precipitate was filtered, washed with methanol and dried in vacuo
to yield a yellow powder (1.46 g, yield 77.2%). 1H NMR (500 MHz,
DMSO-d6, TMS,
d
, ppm): 7.41 (d, 4H, J ¼ 8.5 Hz, AreH), 7.15 (d, 4H,
J ¼ 8.8 Hz, AreH), 7.08 (d, 4H, J ¼ 8.2 Hz, AreH), 6.96 (d, 4H,
J ¼ 8.8 Hz, AreH), 3.79 (s, 6H, eOCH3), 2.33 (s, 6H, eCH3). 13C NMR
(125 MHz, CDCl3, TMS, d, ppm): 177.65, 168.89, 159.20, 138.74,
137.77, 134.18, 129.88, 127.01, 125.35, 114.49, 55.88, 21.50. MALDIe
TOF MS: calcd for C32H28N2O4 (MW ¼ 504.20): m/z ¼ 504.88 [Mþ].
Elemental anal. calcd for C32H28N2O4: C, 76.17%; H, 5.59%; N, 5.55%.
Found: C, 76.45%; H, 5.43%; N, 5.65%.
2.2.4. DTolA-SQD
Under N2 atmosphere, n-butanol/toluene [1:1 (v/v), 5 mL], p,p0-
ditolylamine (2.00 g, 9.83 mmol), and squaric acid (463 mg,
4.06 mmol) were combined and stirred at 110 ꢁC for 30 h, then
cooled to room temperature. The obtained precipitate was filtered,
washed with methanol and dried in vacuo to yield a yellow powder
2.3. Measurements
Cyclic voltammetry for N2 purged dichloromethane solutions of
(1.42 g, yield 74.1%). 1H NMR (500 MHz, DMSO-d6, TMS,
d
, ppm):
7.22 (t, 8H, J ¼ 8.2 Hz, AreH), 7.09 (t, 8H, J ¼ 8.5 Hz, AreH), 2.33 (s,
12H, eCH3). 13C NMR (125 MHz, CDCl3, TMS,
, ppm): 178.21,168.78,
the SQDs (1.0
ꢂ
10ꢀ3 mol dmꢀ3
) containing n-Bu4NClO4
(0.1 mol dmꢀ3) was conducted with a platinum disk (1.6 mm in
diameter), platinum wire, and Ag/Agþ (0.01 mol dmꢀ3) as the
working, counter and reference electrodes, respectively. Under
same conditions, the ferrocene/ferrocenium (Fc/Fcþ) potential (E1/
2) showed 0.07 V vs. Ag/Agþ (0.01 mol dmꢀ3).
d
138.62, 137.84, 129.88, 125.56, 21.49. MALDIeTOF MS: calcd for
C
32H28N2O2 (MW ¼ 472.22): m/z ¼ 472.90 [Mþ]. Elemental anal.
calcd for C32H28N2O2: C, 81.33%; H, 5.97%; N, 5.93%. Found: C,
81.30%; H, 5.94%; N, 6.00%.
2.4. Molecular modelling
2.2.5. MeOPhTolA-SQD
Under N2 atmosphere, n-butanol/toluene [1:1 (v/v), 5 mL], 4-
For the density function theory (DFT) calculations of the
absorbance wavelength of SQDs, we conducted conformational
methoxy-40-methyldiphenylamine (2.00 g, 9.19 mmol) and
Fig. 1. Crystal structures of SQDs: (a) DPhA-SQD and (b) DTolA-SQD. Small ball, hydrogen.

264
Y. Ohsedo et al. / Dyes and Pigments 101 (2014) 261e269
Table 1
(CONFLEXÔ) and structural optimizations were conducted using
B3LYP/6-31G (d) (“Firefly”).
Optical properties for SQDs in the DMF solution and results of calculations.
Dye
lmax/nm
log ε
lmax/nm (calc.)
MePh-SQD
DPhA-SQD
NpPhA-SQD
DTolA-SQD
MeOPhTolA-SQD
387
411
423
415
418
4.60
4.59
4.55
4.67
4.67
395
400
426
404
424
3. Results and discussions
Symmetric-type SQDs containing
a diarylamine moiety,
MePhA-SQD, DPhA-SQD, NpPhA-SQD, DTolA-SQD, and
MeOPhTolA-SQD (Scheme 1) were synthesized from the
dehydration reaction of squaric acid and corresponding sec-
ondary arylamine in toluene/n-butanol as solvents with reflux
(Scheme 2). Crude SQDs were filtered and washed with
methanol several times, and the resultant SQDs, obtained
without a tedious purification process such as column chro-
matography, showed sufficiently high purity in the elemental
analysis. These SQDs were soluble in CHCl3, dichloromethane,
DMF, and DMSO. In the synthesis of SQDs, we demonstrated a
searches using MMFF94s (CONFLEXÔ) and performed structural
optimizations of the ground state of SQDs using B3LYP/6-31G (d)
(“Firefly” e a freely available ab initio and DFT computational
chemistry program). Estimations of the UVevis spectra were
calculated using TD-B3LYP/6-31G(d) (“Firefly”). In the DFT calcu-
lations of the frontier molecular orbitals (HOMO and LUMO) of
SQDs, conformational searches were conducted using MMFF94s
Fig. 3. Cyclic voltammograms of SQDs in dichloromethane containing n-Bu4NClO4 (0.1 mol dmꢀ3): (a) MePh-SQD, (b) DPhA-SQD, (c) NpPhA-SQD, (d) DTolA-SQD and (e)
MeOPhTolA-SQD. Red lines, one-electron anodic oxidation process; blue lines, two-electron anodic oxidation process. The sweep rate was 0.1 V sꢀ1
.

Y. Ohsedo et al. / Dyes and Pigments 101 (2014) 261e269
265
Table 2
crystal structures, the diphenylamine moiety of DPhA-SQD and
DTA-SQD showed propeller-like conformations, as shown in the
Oxidation potentials of the SQDs [V vs. Ag/Agþ (0.01 mol dmꢀ3)].
crystal structures of
p-conjugated compounds containing triphe-
Samples
First wave
Second wave
nylamine [24] or diphenylamine moieties [25]. We supposed the
same type of arylamine connection to the cyclobutane ring in other
SQDs in this study because SQDs showed lmax in a similar wave-
length region, as in the following results. These propeller-like
connections of phenyl rings make the lmax of SQDs are shorter
compared to those of general SQDs, probably because of decreased
Epa
Epc
Ipc/Ipa
Epa
Epc
Ipc/Ipa
MePh-SQD
DPhA-SQD
NpPhA-SQD
DTolA-SQD
MeOPhTolA-SQD
0.88
0.95
0.93
0.86
0.79
0.77
0.85
0.82
0.75
0.68
0.96
0.93
0.94
1.00
0.99
1.42
1.50
1.35
1.31
1.20
e
e
e
e
e
e
1.18
1.07
1.35
1.11
p
-conjugation. On comparing the XRD results for our system with
Epa, peak anodic oxidation potential; Epc, peak anodic oxidation.
Ipc/Ipa was estimated by Nicholson’s equation [24,25] (see ESI). The sweep rate was
0.1 V sꢀ1
The sweep rate was 0.1 V sꢀ1
those of 1,3-bis(dimethylamino)-3-cyclobutene-2,4-dione, the N-
methylated analogue of our system, we observed that all atoms in
the N-methylated analogue are coplanar [26]; the distinct feature of
our SQDs containing arylamine is their propeller-like structures.
Fig. 2 shows the absorption spectra of the SQDs, and Table 1 lists
their optical properties. The differences in lmax reflect the rela-
.
.
suitable method for the synthesis and workup of SQDs using
secondary arylamines whose nucleophilic ability is hindered
by aryl groups. Related to these results, same kind of simple
purification processes on other squarylium compounds was
reported in literature [22].
To clarify the chemical structure of the SQDs, analysis of their
crystal structures was attempted by X-ray diffraction (XRD). Un-
fortunately, the crystals suitable for XRD were unable to obtain
without DPhA-SQD and DTolA-SQD. Fig. 1 shows the ball-and-stick
images of DPhA-SQD and DTolA-SQD obtained from XRD analysis
(see also Supplementary data) [23]. It was shown that the aryl-
amine moieties were connected not to the 1- and 2-positions but to
the 1- and 3-positions of the cyclobutane ring. In the obtained
tionship between the extent of
p-conjugation in SQDs and their
chemical structures. Whereas ordinary SQDs showed light ab-
sorption among the deep visible and near infrared with strong
molecular extinction coefficients, these symmetric SQDs with
arylamines exhibit absorption at
(w400 nm) because the peri-position hydrogen atoms of aryl-
amines hinder -conjugation, as indicated by crystal analysis
showing the existence of SQD propeller structures. From Table 1,
the red shift in lmax reflects the increase in -conjugation of SQDs:
on the basis of MePhA-SQD (lmax: 387 nm), substitution from
methyl to phenyl (lmax: 411 nm) and the naphthyl group (lmax
a shorter visible region
p
p
:
Fig. 4. The frontier molecular orbitals of SQDs obtained by DFT calculations.

266
Y. Ohsedo et al. / Dyes and Pigments 101 (2014) 261e269
Fig. 5. The relationship between calculated and electrochemically estimated HOMO of
SQDs.
423 nm) increased the
more -conjugation than the phenyl group), and on the basis of
DPhA-SQD (lmax: 411 nm), substitution from H- to methyl (lmax
415 nm) and the methoxy group (lmax: 418 nm) contributed to the
-conjugation by substituent effect (the methoxy group might
support better -conjugation than the methyl group by stronger
resonance effect). From these results and log ε, SQDs in this study
have similar -conjugation lengths; thus, they have symmetric
p-conjugation (the naphthyl group showed
p
:
p
p
p
structures connected with the 1- and 3-positions of the cyclo-
butane ring, as envisioned. Whereas the DFT calculations of the
lmax values of the SQDs were conducted under the assumption of
gas-phase molecules, the lmax results of the samples in DMF
coincided better with the calculated results, to within 10 nm. This
agreement between the calculated and experimental results was
Fig. 6. Cyclic voltammograms of DTolA-SQD in dichloromethane containing n-Bu4N-
ClO4 (0.1 mol dmꢀ3): (a) two-electron anodic oxidation process and (b) one-electron
anodic oxidation process. The sweep rates were from 0.05 to 0.4 V sꢀ1
.
probably because of the formation of ideal
SQDs in DMF.
p-conjugation in the
When the scan was repeated ten times in one-electron anodic
oxidation, no new peaks were observed. This suggests that the
radical cations on the SQDs were chemically stable under these
conditions. Compared with the Epa of MePh-SQD, DPhA-SQD and
NpPhA-SQD, MePh-SQD with lower lmax showed lower Epa. This
may be ascribable to the hyperconjugation of the methyl group in
MePh-SQD. The results for MePhA-SQ and DPhA-SQD corre-
sponded well to those in the literature [20]. In contrast, DTolA-SQD
and MeOPhTolA-SQD in dichloromethane underwent two electron
Next, we examine the electrochemical properties of the SQDs.
The electrochemical measurements of SQD dichloromethane so-
lutions were carried out by cyclic voltammetry. Fig. 3 shows cyclic
voltammograms for the anodic oxidation process of SQDs, and
Table 2 lists the electrochemical parameters of SQDs [27,28].
Whereas the one-electron anodic oxidation process of MePh-SQD,
DPhA-SQD, and NpPhA-SQD in dichloromethane displayed high
reversibility (Ipc/Ipa ¼ w1, but with wider Epa ꢀ Epc ¼ w100 mV),
the two-electron anodic oxidation processes for these SQDs were
irreversible. A second anodic wave ascribable to the two-electron
oxidation of the SQDs was observed, but the corresponding
cathodic waves were much weaker in intensity [Fig. 2(a)e(c)].
oxidations and showed high reversibility [Ipc/Ipa
¼
1 (for
MeOPhTolA-SQD and the first wave of DTolA-SQD), but with wider
Epa ꢀ Epc ¼ w100 mV], showing two anodic waves, and two cor-
responding cathodic waves in their cyclic voltammograms corre-
sponding to a two-electron oxidation process [Fig. 3(d) and (e)].
When the scan was repeated, no new definite anodic and cathodic
waves attributable to the formation of the corresponding SQD
dimer were observed. This suggests that the radical cation and
dication on the SQDs were chemically stable under these condi-
tions. The difference in stability improvement between electro-
chemical species of DTolA-SQD and MeOPhTolA-SQD in the second
waves (Ipc/Ipa in Table 2) might be attributable to the difference in
Table 3
HOMO, LUMO, and bandgap obtained by DFT calculations and experimental results.
Samples
HOMO/ LUMO/
Ebg
/
Eobg (onset/nm)/
HOMOec
eVc
/
eV
eV
eVa
eVb
MePh-SQD
DPhA-SQD
NpPhA-SQD
DTolA-SQD
MeOPhTolA-
SQD
ꢀ5.09
ꢀ5.00
ꢀ4.96
ꢀ4.82
ꢀ4.67
ꢀ1.68
ꢀ1.66
ꢀ1.77
ꢀ1.50
ꢀ1.38
3.41
3.34
3.19
3.32
3.29
2.86(433)
2.77(448)
2.67(465)
2.75(451)
2.71(457)
ꢀ6.11
ꢀ6.18
ꢀ6.16
ꢀ6.09
ꢀ6.02
the inductive effect between the methyl and methoxy group on p-
conjugation of SQDs, which tended to stabilize the radical cation
and dication. Because of the overestimation of the second wave
cathodic current of DTolA-SQD and MeOPhTolA-SQD due to the
difficulty of clear division from the third wave, the Ipc/Ipa value
might contain current from the third wave. These results indicate
that the stability of the SQD radical cation and dication was
HOMOec [eV] ¼ Epa ꢀ 0.07 ꢀ 5.30. “0.07” means 0.07 V vs. Ag/Agþ (0.01 mol dmꢀ3). A
rough assumption on neglecting solvent effects provides Fc/Fþc ¼ ꢀ5.30 eV from
vacuum level [28].
a
Eg (bandgap energy from calculation) was jHOMO ꢀ LUMOj.
b
Eobg (optical bandgap energy) was estimated by onset wavelength at longer
enhanced by the increased
by substituting the hydrogen atoms with the methyl and methoxy
group. Because of the extended -conjugation and resulting charge
p-conjugation length of the SQD moiety
absorption region.
c
HOMOec (electrochemically obtained HOMO) were estimated by using following
equation.
p

Y. Ohsedo et al. / Dyes and Pigments 101 (2014) 261e269
267
deviation of the HOMO, an electron may be extracted from
electron-donating N atoms (amine) rather than from the electron-
accepting cyclobutane-carbonyl moiety; therefore, the steric hin-
drance around N atoms might hamper the extraction of an electron
during electrochemical oxidation. This hindrance may explain why
the Epa of MePhA-SQD is lower than that of DPhA-SQD despite its
lower p-conjugation length, as determined from the experimental
(absorption) and calculation results. MePhA-SQD exhibited an Epa
lower than that of DPhA-SQD because of its lower steric hindrance
around N atoms, which resulted in the deviation from linearity
between the HOMOec and the calculated HOMO shown in Fig. 5.
To further investigate the electrochemical reversibility of SQDs,
the sweep rates were changed. Figs. 6 and 7 show the one- and
two-electron anodic oxidation processes of DTolA-SQD and
MeOPhTolA, respectively. It is known that linearity in the square
root of the sweep rate vs. current and constant oxidation potentials
during sweep rate changes are criteria for electrochemical revers-
ibility [27]. Whereas the increase in current was linear in the square
root of sweep rate vs. current, the oxidation potentials slightly
shifted to higher values with increasing sweep rates. In addition, it
is showed that small shoulder peaks were observed in two electron
oxidation in around 0.8 V vs. Ag/Agþ (Figs. 6(a) and 7(a)). From
these results, symmetric SQDs containing diarylamine showed
pseudo-electrochemical reversibility and chemical reversibility in
the anodic oxidation process.
4. Conclusions
We have studied the synthesis and electrochemical properties
of symmetric SQDs containing diarylamine for developing blue
laser absorption dyes and core block of functional dyes. These
SQDs were synthesized in a one-step dehydration reaction from
squaric acid and secondary arylamine with a simple workup. The
SQD chemical structures determined by XRD analysis had aryl-
amine moieties symmetrically connected to the 1- and 3-positions
of the cyclobutane ring. From electrochemical measurements in
the anodic oxidation process, these SQDs showed one- and/or
two-electron oxidation with pseudo-electrochemical reversibility
and chemical reversibility. These results suggest that symmetric
SQDs containing diarylamine might be a new family of promising
candidates of active materials for electroactive and photoactive
devices. Because secondary arylamines, which are raw materials of
SQDs in this study, are easily modified and extended by such well-
established chemical reactions as the arylamine synthesis by
BuchwaldeHartwig amination [29], symmetric SQDs containing
arylamine with further higher electrochemical performance can be
designed and easily synthesized on the basis of our results.
Fig. 7. Cyclic voltammograms of MeOPhTolA-SQD in dichloromethane containing n-
Bu4NClO4 (0.1 mol dmꢀ3): (a) two-electron anodic oxidation process and (b) one-
electron anodic oxidation process. The sweep rates were from 0.05 to 0.4 V sꢀ1
.
delocalization, the coupling reaction of the SQD radical cations that
could give rise to the corresponding dimer moieties did not take
place.
By conducting DFT calculations of the frontier molecular orbitals
of SQDs, we determined the shape of the HOMO and LUMO of SQDs
(Fig. 4). As shown in Fig. 4, the HOMO and LUMO of the SQDs are
located primarily at the cyclobutane ring and at the two adjacent N
atoms in the planar figures. This compact and planar p-conjugation
of diarylamine SQDs might make the lmax of these SQDs shorter
than that of general SQDs. This propeller-like geometry of SQDs
obtained from the DFT calculations is shown in Fig. 4 which cor-
responds well with the SQDs’ XRD results as shown in Fig. 1. In
Table 3, the bandgap energies obtained from both calculations and
absorption bands (Ebg and Eobg, respectively) and the electro-
chemically obtained HOMO (HOMOec) are listed. As shown in
Table 3, although the calculation results for the HOMOs of the SQDs
that correspond to the oxidation potentials tend to increase in value
Acknowledgements
We would like to thank Nissan Chemical Industries for financial
and technical support and Dr. Osamu Hirata and Dr. Fumiyasu Ono
for their helpful discussions and comments. We are grateful to the
Service Centre of the Elementary Analysis of Organic Compounds,
Kyushu University for the elemental analysis. A part of this work
was supported by the Nanotechnology Network Project (Kyushu-
area Nanotechnology Network) of the Ministry of Education, Cul-
ture, Sports, Science and Technology (MEXT), Japan. We would like
to thank Enago (www.enago.jp) for the English language review.
in accordance with the
p-conjugation length (lmax), the experi-
mental cyclic voltammetry results for the SQDs differ to some
extent. The Ebg values are larger than the Eobg values; however, the
Ebg values showed the same tendency as the Eobg values. The
calculated HOMOs were ca. 1 eV lower than the corresponding
HOMOec values, although both HOMOs were linearly correlated,
except for those of MePhA-SQD, as shown in Fig. 5. The deviation of
the HOMOs of MePhA-SQD from linearity is probably because of
steric hindrance around the N atoms of MePhA-SQD during the
electrochemical oxidation process. The shape of the
p-electron
orbitals of the HOMO and their surroundings, which include their
steric circumstances are considered to affect the extraction of
electrons from dyes during electrochemical processes. In the elec-
trochemical oxidation process of diarylamine SQDs, given the
Appendix A. Supplementary material
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2013.09.047.

268
Y. Ohsedo et al. / Dyes and Pigments 101 (2014) 261e269
squaraines to 1D supramolecular architectures with high molar absorptivity.
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