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T. Fukuda, N. Ishikawa / Dyes and Pigments 109 (2014) 151e154
[10]. As a consequence, reports on the liquid-phase procedures for
blue suspension was filtered by using a PTFE membrane filter to
the preparations of phthalocyanine nano- and microcrystals remain
relatively limited [11,12].
collect the formed CuPc microcrystals.
Recently, our group has successfully developed a facile synthetic
procedure for alkoxy-substituted CuPc precursors by performing
the lithium-assisted coupling reaction at lower temperature than
that employed for the typical Pc synthesis (see compound 1 in
Scheme 1 as an example) [13,14]. Importantly, these precursors
exhibit high solubilities in a variety of organic solvents, making the
wet coating of 1 onto a substrate possible. Upon heating the sub-
strate, the two methoxy groups substituted at the diagonally
(c) conc. ¼ 0.11 mM
Compound 1 (0.68 mg) was added to a pressure-proof sealed
pyrex glass tube containing 10 mL of toluene, and the solution was
ꢀ
heated at 120 C with stirring for 1 h. The resultant suspension was
centrifuged (1000 rpm, 3 min) and the supernatant liquid was
removed by decantation. The residue was washed with toluene,
and the formed CuPc microcrystals were collected by filtration.
opposed positions of the pyrrole a-carbons in 1 are eliminated to
yield thermodynamically more stable aromatic CuPc. Recently,
solution-processed small molecule organic photovoltaic devices, by
taking advantage of compound 1 as one of the key materials, have
been fabricated [15].
(d) conc. ¼ 0.92 mM
Similarly to the procedure described in (c), CuPc microcrystals
were prepared from 1 (5.84 mg) dissolved in toluene (10 mL).
In the present paper, we report procedure for the preparation of
microcrystals of CuPc by employing compound 1 as the starting
material. Since 1 is soluble and the resultant CuPc is insoluble in
general organic solvents, formation of crystalline CuPc is antici-
pated by conducting the thermal conversions of 1 in solution. In
addition, it is also expected that the rate of the crystal growth de-
pends on the initial concentration of 1, i.e. the higher concentration
results in the increased growth rate and vice versa.
3
. Results and discussion
3.1. Thermal conversion in the solution phase
The solution of 1 in toluene is almost colorless at room tem-
perature, and exhibits no characteristic band components for CuPc
in the absorption spectrum (Fig. 1(a)). At the initial solution con-
centration of 0.010 mM, the homogeneous blue solution containing
the generated CuPc was obtained after heating the solution at
2
. Material and method
ꢀ
Compound 1 was prepared according to our method [13,14].
120 C for 1 h with a vigorous stirring, (Fig. S1 in Supporting
Field emission scanning electron microscopy (FESEM, JEOL JSM-
500F) was used to obtain surface images of the obtained micro-
crystalline samples.
Information). Suction filtration of the resultant toluene solution
shortly after the heating by using a PTFE membrane filter
7
(ADVANTEC T020A025A, pore size ¼ 0.20
mm) gave no blue resi-
dues on the filter, indicating that the thermally generated CuPc in
2.1. Preparation of CuPc microcrystals starting from different initial
toluene is either dissolved or suspended as the CuPc particles less
solution concentrations (conc.)
than 0.2 mm in size. In order to promote the formation of the pre-
cipitates, an equivalent volume of cold methanol was added to the
filtrate followed by ten-fold concentration in vacuo as the follow-
up process. The obtained blue precipitate, accordingly, was exam-
ined by the FESEM experiments (see below). Attempts at centrif-
ugal separation of the crystals without the follow-up process were
unsuccessful. Although our previous thermogravimetric analyses of
1 in the crystalline solid phase indicate that 1 is thermally stable at
(
a) conc. ¼ 0.010 mM
Compound 1 (1.0 mg) dissolved in toluene (150 mL) was added
to a 200 mL three-necked round-bottom flask, and the colorless
solution was heated at 120 C with stirring for 1 h. A portion of the
ꢀ
resultant blue solution (100 mL) was filtered by using a PTFE
ꢀ
membrane filter (ADVANTEC T020A025A, pore size ¼ 0.20
mm), and
temperatures lower than 200 C [13,14], we have observed in the
the filtrate was added to cooled methanol (100 mL). The solution
was concentrated to ca. 20 mL in vacuo, and the formed blue pre-
cipitate was collected by filtration.
present paper that 1 is converted into CuPc at the temperature of as
ꢀ
low as 120 C in the liquid phase.
Upon increasing the initial concentration of 1, deeper colored
solutions predictably resulted but which contained clearly
discernible precipitates which were obtained without performing
the follow-up process. Centrifugal separation of the solutions gave
the blue CuPc precipitates and the pale yellow supernatant liquid,
supporting that the formed CuPc is practically insoluble in toluene
at higher concentrations than ca. 0.03 mM.
(
b) conc. ¼ 0.029 mM
Compound 1 (2.76 mg) dissolved in toluene (150 mL) was added
to a 200 mL three-necked round-bottom flask, and the colorless
solution was heated at 120 C with stirring for 1 h. The resultant
ꢀ
Scheme 1. Preparation of microcrystals of CuPc.