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T. Onodera and H. Oikawa
the composition ratio of electron donor to acceptor are important factors to determine physicochemical properties of charge-transfer complex [2]. On the other hand,
the nanocrystal is occupied in an intermediate state between isolated molecules
and corresponding bulk crystals. Therefore, nanocrystallization is utilized to obtain
unique properties different from both isolated molecule and bulk crystal [3]. So far,
many organic nanocrystals have been fabricated by means of the so-called reprecipitation method; for example, low molecular weight aromatic carbons, ionic dyes,
π-conjugated polymers, and so on [4–9]. The crystal size was located in the range of
several tens nanometer to sub-micrometers. In the case of crystalline charge-transfer
complex on microscale, several fabrication methods, for example, vapor deposition [10, 11], spontaneous electrolysis in organic solvents [12, 13], and chemical
and electrochemical reduction methods [14, 15] have been proposed. These fabrication methods, however, were not useful to control the size and shape of crystals
on nanoscale. Furthermore, low solubility of charge-transfer complex in common
organic solvents disabled us from employing the conventional reprecipitation method
for organic nanocrystals. Hence, novel nanocrystallization method of charge-transfer
complex should be developed.
32.2 Fabrication of Charge-Transfer Complex
Nanocrystals and Their Nanostructure
Cu-TCNQ nanocrystals were fabricated by coprecipitation method with chemical
reduction. Namely, methanol solutions of electron donor (Cu) and accepter (TCNQ)
were mixed in the presence of a reducing agent as follows [16, 17]. First, TCNQ
(0.13–0.21 mmol) was dissolved in methanol (500 mL). The methanol solution was
deoxygenated by flow of nitrogen gas and kept at a given temperature. Next, an
excess amount of NaBH 4 (0.52 mmol) was added into the solution. In this process,
the solution turned into bright green in color. TCNQ in the starting methanol solution
exists as a TCNQ radical anion (TCNQ
− ) because of the reduction with NaBH 4 ,
which was confirmed by the visible absorption spectrum assigned to TCNQ
− [18].
Next, CuSO 4 ·5H 2 O (0.11 mmol) dissolved in methanol (25–150 mL) was added
dropwise (0.83 mL min
−1 ) into the vigorously stirred methanol solution containing
TCNQ
− and NaBH 4 . After adding CuSO 4 , the mixed solution was further stirred at
a given temperature for 40 min. In this process, Cu
2+ in methanol solution of CuSO 4
was reduced into Cu
+ with NaBH 4 . At the same time, insoluble complexes of Cu
+
and TCNQ
− were formed and grew up to be nanocrystals. The resulting methanol
dispersion liquid of Cu-TCNQ nanocrystals was bright blue in color. The size of CuTCNQ nanocrystals was controlled by the changing amount of TCNQ and NaBH 4 .
On the other hand, it is also possible to prepare Cu-TCNQ nanocrystals by using
a copper salt precursor such as CuSO 4 and an alkali metal-TCNQ precursor such
as Li-TCNQ by cation exchange [16]. A methanol solution (200 µL) was injected
into a vigorously stirred solution of Li-TCNQ (0.44–2.2 mM) in methanol (10 mL).
T. Onodera and H. Oikawa
the composition ratio of electron donor to acceptor are important factors to determine physicochemical properties of charge-transfer complex [2]. On the other hand,
the nanocrystal is occupied in an intermediate state between isolated molecules
and corresponding bulk crystals. Therefore, nanocrystallization is utilized to obtain
unique properties different from both isolated molecule and bulk crystal [3]. So far,
many organic nanocrystals have been fabricated by means of the so-called reprecipitation method; for example, low molecular weight aromatic carbons, ionic dyes,
π-conjugated polymers, and so on [4–9]. The crystal size was located in the range of
several tens nanometer to sub-micrometers. In the case of crystalline charge-transfer
complex on microscale, several fabrication methods, for example, vapor deposition [10, 11], spontaneous electrolysis in organic solvents [12, 13], and chemical
and electrochemical reduction methods [14, 15] have been proposed. These fabrication methods, however, were not useful to control the size and shape of crystals
on nanoscale. Furthermore, low solubility of charge-transfer complex in common
organic solvents disabled us from employing the conventional reprecipitation method
for organic nanocrystals. Hence, novel nanocrystallization method of charge-transfer
complex should be developed.
32.2 Fabrication of Charge-Transfer Complex
Nanocrystals and Their Nanostructure
Cu-TCNQ nanocrystals were fabricated by coprecipitation method with chemical
reduction. Namely, methanol solutions of electron donor (Cu) and accepter (TCNQ)
were mixed in the presence of a reducing agent as follows [16, 17]. First, TCNQ
(0.13–0.21 mmol) was dissolved in methanol (500 mL). The methanol solution was
deoxygenated by flow of nitrogen gas and kept at a given temperature. Next, an
excess amount of NaBH 4 (0.52 mmol) was added into the solution. In this process,
the solution turned into bright green in color. TCNQ in the starting methanol solution
exists as a TCNQ radical anion (TCNQ
− ) because of the reduction with NaBH 4 ,
which was confirmed by the visible absorption spectrum assigned to TCNQ
− [18].
Next, CuSO 4 ·5H 2 O (0.11 mmol) dissolved in methanol (25–150 mL) was added
dropwise (0.83 mL min
−1 ) into the vigorously stirred methanol solution containing
TCNQ
− and NaBH 4 . After adding CuSO 4 , the mixed solution was further stirred at
a given temperature for 40 min. In this process, Cu
2+ in methanol solution of CuSO 4
was reduced into Cu
+ with NaBH 4 . At the same time, insoluble complexes of Cu
+
and TCNQ
− were formed and grew up to be nanocrystals. The resulting methanol
dispersion liquid of Cu-TCNQ nanocrystals was bright blue in color. The size of CuTCNQ nanocrystals was controlled by the changing amount of TCNQ and NaBH 4 .
On the other hand, it is also possible to prepare Cu-TCNQ nanocrystals by using
a copper salt precursor such as CuSO 4 and an alkali metal-TCNQ precursor such
as Li-TCNQ by cation exchange [16]. A methanol solution (200 µL) was injected
into a vigorously stirred solution of Li-TCNQ (0.44–2.2 mM) in methanol (10 mL).