Murase and Fujita
SCHEME 1. Synthesis of Triazine-Containing
positions, resulting in efficient electron transfer in the
compound. The connection of amine (D) and triazine (A)
by a π-conjugated bridge to form the D-π-A structure
is considered to be an effective way to improve nonlinear
optical properties.5 From the viewpoint of materials
chemistry, triazine is known to serve as a core of liquid
crystals.6-9 In this way, triazine is important in many
applications.10-14
Polymersa
The method for synthesizing triazine derivatives can
be classified into two ways: one is, as mentioned above,
nucleophilic displacement of chlorine atoms, in a stepwise
manner, from cyanuric chloride; the other is trimerization
of nitriles. The latter is featured by the direct formation
of an aromatic ring (triazine) and is a simple and efficient
method for preparing symmetrical 2,4,6-trisubstituted
1,3,5-triazines.1 Various methods have been developed
for trimerization of nitriles.15-23 It is readily suspected
that, under similar conditions, dinitriles lead to triazine-
containing polymers by repeating the trimerization of
nitriles. Some papers have suggested the existence of
triazine-containing polymers.24-29 However, the charac-
terization is only the confirmation of triazine rings by
the IR spectrum, and there are no reports on the isolation
of individual oligomers from polymer mixtures. Moreover,
despite the strong electron-accepting ability of triazine
moieties, the optical properties of triazine-containing
oligomers have not been investigated at all. It is antici-
pated that fluorescent compounds with a high ΦF value
are obtained when electron-donating parts are incorpo-
rated into triazine-containing oligomers.
a Reagents and conditions: (i) p-fluorobenzonitrile, t-BuOK,
DMSO, rt, 75%. (ii) CH3I, K2CO3, DMF, 130 °C, 92%. (iii) NaOH,
neat, 200 °C.
In the present study, we report a novel one-step
preparation of triazine-amine conjugated oligomers,
composed of alternatingly arranged D (amine) and A
(triazine). The oligomers are obtained by trimerization
of aromatic dinitriles and separated by chromatography.
Then, the electronic and optical properties, including
absorption and emission, as well as redox properties, are
investigated by means of cyclic voltammetry and UV-
vis and fluorescence spectroscopy. Their properties will
be discussed, noting the structures of oligomers.
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(6) Lee, C.-H.; Yamamoto, T. Tetrahedron Lett. 2001, 42, 3993.
(7) Lee, C.-H.; Yamamoto, T. Mol. Cryst. Liq. Cryst. 2002, 378, 13.
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(9) Lee, S. J.; Chang, J. Y. Tetrahedron Lett. 2003, 44, 7493.
(10) Nishimura, N.; Kato, A.; Maeba, I. Carbohydr. Res. 2001, 331,
77.
Results and Discussion
Synthesis. The simplest amine derivative in aromatic
dinitriles is 4,4′-dicyanodiphenylamine 2. As shown in
Scheme 1, dinitrile 2 was readily prepared by the reaction
of p-aminobenzonitrile (1) with p-fluorobenzonitrile in a
moderate yield (75%).30 The melting point of 2 was high
(258 °C) despite a relatively small molecular weight, and
the solubility was very low in common organic solvents
except for aprotic polar ones. These properties are
undesirable for the synthesis of triazine-containing poly-
mers because a high-concentration condition is indispen-
sable for the synthesis of triazine derivatives via tri-
merization of aromatic nitriles. The best method is to
carry out the trimerization in the molten state at high
temperature without solvents. Moreover, good solubility
is required for triazine-containing oligomers to separate
them into their components. To remove these drawbacks,
dinitrile 2 was converted to N-methylamino derivative 3
by heating it with methyl iodide and K2CO3 in DMF. In
contrast to compound 2, dinitrile 3 was readily soluble
in common organic solvents such as chloroform, and the
melting point of 3 (156 °C) was considerably lower than
that of 2. Therefore, dinitrile 3 was chosen as a suitable
precursor for the triazine-amine conjugated polymers.
The trimerization of aromatic dinitrile 3 was conducted
in the presence of NaOH at 200 °C. To prevent 3 from
hydrolyzing into the amide, the dinitrile 3, NaOH, and
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9270 J. Org. Chem., Vol. 70, No. 23, 2005