Soluble precursors of 2,3-naphthalocyanine and phthalocyanine for use in thin film transistors

Article abstract of DOI:10.1039/b811674a

Soluble precursors of 2,3-naphthalocyanine (Nc) and phthalocyanine (Pc) were prepared and were converted into insoluble semiconducting thin films of Pc and Nc by heating after fabrication via spin-coating. The Royal Society of Chemistry.

Full text of DOI:10.1039/b811674a

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Soluble precursors of 2,3-naphthalocyanine and phthalocyanine for use
in thin film transistorsw
Atsuko Hirao,a Taiji Akiyama,a Tetsuo Okujima,*a Hiroko Yamada,ab
Hidemitsu Uno,c Yoshimasa Sakai,d Shinji Aramaki*d and Noboru Ono*a
Received (in Cambridge, UK) 9th July 2008, Accepted 18th August 2008
First published as an Advance Article on the web 5th September 2008
DOI: 10.1039/b811674a
Soluble precursors of 2,3-naphthalocyanine (Nc) and phthalo-
cyanine (Pc) were prepared and were converted into insoluble
semiconducting thin films of Pc and Nc by heating after
fabrication via spin-coating.
Nc and Pc, respectively. Although 1 was readily prepared, synth-
esis of 2 was extremely difficult17 because the retro Diels–Alder
reaction of 2 or its precursor, BCOD-2,3-dicarbonitrile, took place
during ring formation. Furthermore, the solubility of 1 was too
poor to obtain good thin films for OFETs. Herein we report the
preparation of new soluble precursors 3 and 4, which have better
solubility and can be used to make OFETs by a solution process.
Organic semiconductors have emerged as an important class of
new materials with applications, for example, in organic light
emitting diodes, organic photovoltaics (OPVs), and organic field-
effect transistors (OFETs).1–6 Various organic compounds such as
acenes, oligomeric thiophenes, and other p-conjugated molecules
have been used as semiconductors in these applications. Among
these, phthalocyanines (Pcs) and 2,3-naphthalocyanines (Ncs) are
especially attractive, for they absorb strongly in the near IR
region, and possess remarkable thermal and photochemical
stability. Hence, they have been studied widely as semiconducting
materials. The mobilities of vacuum-deposited Pc films were
generally found to be in the range 10À5–10À3 cm2 VÀ1 sÀ1 7–10
while a single-crystal CuPc-based OFET afforded the highest
mobility of 1.0 cm2 VÀ1 sÀ1 11
Langmuir–Blodgett (LB) films of
,
.
Pcs have also been used for OFETs, which showed high perfor-
mance.12,13 The red-shifted electronic absorption maxima and
photoconductive properties of Nc make it more suitable than Pc
for application in OPVs. However, there have been no reports on
thin films of Nc due to difficulties in its vaporization. In this paper
we report a new method of obtaining thin films of Pc and Nc from
soluble precursors with thermally removable groups. We have
already developed a new method of making thin films of tetra-
benzoporphyrins (TBPs) from their soluble precursors without
vacuum deposition.14 Porphyrins fused with bicyclo[2.2.2]-
octadiene (BCOD) were used as precursors for fabricating
TBP-based OFETs15 and solar cells.16 Thus, BCOD-fused tetra-
azaporphyrins 1 and 2 were expected to be soluble precursors of
a Department of Chemistry and Biology, Graduate School of Science
and Engineering, Ehime University, Matsuyama, 790-8577, Japan.
E-mail: ononbr@dpc.ehime-u.ac.jp (NO);
tetsuo@chem.sci.ehime-u.ac.jp (TO); Fax: +81 89 927 9615;
Tel: +81 89 927 9615
b PRESTO, Japan Science and Technology Agency, Kawaguchi
332-0012, Japan
c Department of Molecular Science, Integrated Center for Sciences,
Ehime University, Matsuyama, 790-8577, Japan
d Mitsubishi Chemical Group Science and Technology Research
Center, Inc., Yokohama, 227-8502, Japan.
E-mail: 1107150@cc.m-kagaku.co.jp (SA)
w Electronic supplementary information (ESI) available: Experimental
details, NMR, MS, TGA and crystallographic data. CCDC reference
numbers 692987–692988. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b811674a
Scheme 1 Synthesis of Nc. Reagents and conditions: (i) dicyanoace-
tylene, CHCl3, rt, 24 h, 46%; (ii) DDQ, 1,4-dioxane, CHCl3, reflux,
6 d, 97%; (iii) LiOBu, BuOH, reflux, 1 d, 47%; (iv) 350 1C.
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11% yield, respectively. Compounds syn- and anti-8 were
separated by column chromatography, and their structures
were determined by X-ray crystallographic analysis (CCDC
No. 692987 (anti-8) and 692988 (syn-8): see the supplementary
informationw). They were analyzed by TGA (Fig. S-7w). The
retro Diels–Alder reaction of syn-8 started at around 190 1C,
whereas that of anti-8 started at 150 1C. Thus, syn-8 is more
suitable than anti-8 for preparation of pure 4 as a mixture of
diastereomers. Compound syn-8 was treated with LiOBu in
BuOH at 110 1C to give 4 free of the retro Diels–Alder
products. Pure 4 was isolated in 2% yield after column
chromatography. The TGA curve of 4 is shown in Fig. 1, in
which it is seen that weight loss started at 180 1C and ceased
after 250 1C. The loss of weight was ca. 40%, consistent with
the calculated value of 43%. Heating 4 at 250 1C resulted in
clean formation of pure Pc. This is the first preparation of pure
Pc without vacuum technology.
Fig. 1 TGA of 1 (solid line), 3 (bold line) and 4 (broken line).
The synthesis of 3 is shown in Scheme 1. The Diels–Alder
reaction of dicyanoacetylene with 5 at room temperature gave
6 in 46% yield.18 Diels–Alder adduct 6 was oxidized with
DDQ for six days to give 7. Reaction of 7 with LiOBu in
BuOH at reflux for a day gave 3, which was purified by column
chromatography to give pure 3 as a mixture of diastereomers
in 47% yield. Subsequent heating of 3 at 350 1C gave Nc in
nearly quantitative yield. Thermogravimetric analysis (TGA)
curves of 1 and 3 are shown in Fig. 1. The weight loss of
3 started at around 280 1C and ceased after 330 1C. The retro
Diels–Alder reaction of 1 started at 250 1C and was completed
by 300 1C. Thus, the reaction temperature for 3 was higher
than that of 1.
Nc precursor 3 spin-coated on a glass plate showed a broad
Q band at 530–800 nm (Fig. 2(a)). After heating at 350 1C for
5 min, the absorption maxima showed a marked bathochromic
shift as 3 was converted to Nc. The broad Q band of the Nc
film was observed at 600–900 nm. Similarly, while a glass plate
coated with Pc precursor 4 showed four bands at 480–730 nm,
the broad Q band of Pc appeared at 520–840 nm after
treatment (Fig. 2(b)). Both Nc and Pc films exhibited a broad
absorption at around 1000 nm.
Precursors 3 and 4 were soluble in many organic solvents
such as CHCl3, CH2Cl2, THF and EtOAc. They were used as
soluble precursors for the fabrication of Nc- or Pc-based
Precursors 4 or 4-Mg were prepared by cyclization of 8 with
LiOBu or Mg(OBu)2 in BuOH as shown in Scheme 2. The
Diels–Alder reaction of cis-5,6-isopropylidenedioxycyclohexa-
1,3-diene18 with dicyanoacetylene gave a mixture of syn- and
anti-8. This mixture was treated with Mg(OBu)2 to give a
mixture of dibenzotetraazaporphyrin 9-Mg, tribenzotetraaza-
porphyrin 10-Mg and a trace of 4-Mg. Column chromato-
graphy of the products afforded 9-Mg and 10-Mg in 24% and
Scheme 2 Synthesis of Pc. Reagents and conditions: (i) dicyanoacety-
lene, CHCl3, rt, 57%; (ii) Mg(OBu)2, BuOH, rt, 1 d, 24% (9-Mg), 11%
(10-Mg); (iii) LiOBu, BuOH, 110 1C, 1 d, 2%; (iv) 250 1C.
Fig. 2 UV-Vis absorption spectra of thin films of (a) 3 (solid line),
Nc (bold line), (b) 4 (solid line) and Pc (bold line).
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Fig. 4 demonstrates the characteristics of the Pc-FET: mFET
=
6.0 Â 10À2 cm2 VÀ1
s
À1; Ion/Ioff = 1.8 Â 104. Thus, the
solution-processed Pc-based FET showed a higher perfor-
mance than was reported for vacuum deposited or LB films
of Pcs.7–10
In conclusion, we have prepared stable soluble precursors of
Nc and Pc, which were converted into Nc or Pc by heating at
350 or 250 1C, respectively. Nc and Pc films were prepared by
spin-coating their corresponding precursors followed by ther-
mal annealing. The yield of the retro Diels–Alder reaction was
nearly quantitative and the Nc film was stable up to ca. 400 1C.
OFET devices were made using these precursors; this was the
first successful fabrication of thin film devices of Nc using a
solution process. We also succeeded in the first preparation of
pure Pc without a vacuum technique and the first application
of this compound in an FET by a solution process.
This work was partially supported by Grants-in-Aid from
the Ministry of Education, Culture, Sports, Science and
Technology, Japan (No. 18550036 to NO and No. 18550037
to HY) and JGC-S Scholarship Foundation (TO). The authors
thank Venture Business Laboratory, Ehime University, for
assistance in obtaining the MALDI-TOF mass spectra.
Fig. 3 Electronic properties of an FET with Nc. (a) Id–Vd plots as a
1/2
function of Vg. (b) Id and Id versus Vg plots at Vd À60 V.
OFET devices in the same manner as for TBP.15 Nc and Pc
were readily obtained as good thin films on heavily doped
n-type Si substrates coated with 300-nm thermally grown
SiO2, respectively. The source and drain electrodes were
formed by photolithography of Au (90 nm)/Cr (10 nm), with
a channel length and width of 10 and 500 mm, respectively. Nc
precursor 3 in a 0.7 wt% CHCl3 solution was spin-coated onto
the channel region, and the obtained films were converted into
the Nc form by heating at 350 1C for 5 min. FET properties
were measured with an Agilent 4155C semiconductor analy-
zer. Electronic properties of an FET with Nc are shown in
Fig. 3. The field-effect mobility (mFET) was determined using
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