RESULTS AND DISCUSSION
Compounds Ia, b were synthesized according to a previously suggested scheme [4], which included the following
stages: alkylation of 1,2-dioxybenzene by alkyl bromides or alkyl iodides in alcohol-alkaline media; trimerization of
3
1,2-dialkoxybenzenes under phase transfer catalysis conditions [9]; nitration with concentrated nitric acid (d = 1.52 g/cm )
in a mixture of glacial acetic acid and absolute diethyl ether [2] for homologs 5-12 or in a mixture of glacial acetic acid
and chloroform for lower homologs with varied reaction time; reduction by finely disperse tin in boiling glacial acetic acid
[2]. Compounds I-1d, 3d, If were synthesized by reaction of the amino derivative of triphenylene with a corresponding
aldehyde in dry benzene, or azomethines were obtained by fusion. We did not manage to obtain compound I-1d, since
p-bromobenzaldehyde sublimes under the reaction conditions (fusion), and at lower temperatures in benzene no synthesis
takes place because of steric hindrances. The compounds were purified chromatographically on aluminum oxide or silica
gel from organic solvents (hexane, benzene, carbon tetrachloride, and chloroform) taken in different ratios, and further
recrystallized from alcohol, alcohol-benzene mixture, or hexane (amino derivatives) to constant physicochemical
characteristics.
The structure of the synthesized compounds was confirmed by element analysis (Table 1) and also by UV and IR
spectroscopy data (Tables 2 and 3). The presence of one nitro group in the triphenylene ring was proven by NMR data
[2, 4]. The electronic spectra are broad-band (Table 2). The highest absorption intensity is observed at a wavelength of
280 nm, which refers to the π−π* transition in the conjugate system of the triphenylene ring. The other 5 or 6 maxima are
shaped as shoulders. Introduction of a nitro group into the central fragment gives rise to a new absorption band, 368 nm,
in the visible part of the spectrum, which is missing in the spectrum of the starting (unsubstituted) compound and is most
likely to be due to n−π* in the nitro group (Table 2, compare lines 4 and 11). The molar absorption coefficients also
increase due to the hyperchromic effect, resulting from the introduction of an auxochrome nitro group. Compounds from
this series have a deep yellow coloring, also indicating that the nitro group is included in the conjugate system of the
triphenylene ring. The amino derivatives are colored light beige and, accordingly, have lower absorption coefficients. For
the azomethine derivative of terephthalic aldehyde, the spectra differ in the absorption intensity, which is doubled compared
to the amino derivative; the position and the number of bands in the spectrum do not change, indicating the absence of
full conjugation in the whole molecule because of the deviation of the triphenylene rings from the plane. This is clearly
illustrated by the model of the molecule optimized by the MM method with the HyperChem program, version 5.02 (Fig. 1).
For the nitro derivatives of triphenylene, the IR spectra contain the absorption bands of the asymmetric and symmetric
−1
−1
stretching vibrations of the nitro group around 1524 and 1268 cm , respectively. In the range 3400-3100 cm , the IR
spectrum of the amino derivative of triphenylene exhibits absorption bands of the symmetric and asymmetric stretching
−1
vibrations of the NH group (3385 and 3280 cm , respectively); the deformation vibration bands are observed in the
2
−1
−1
regions 1600-1400 and 900-800 cm . In the range 800-900 cm for the azomethine derivative If, the spectrum contains
the bands of the deformation vibrations of the disubstituted and pentasubstituted benzene, 796 and 870 cm , respectively.
−1
Studies on the mesomorphous properties of the synthesized compounds have shown that all of them, except the
former two, form liquid crystals (Table 3). In all homologs, the nitro group considerably decreases the melting temperature
but increases the bleaching temperature of the liquid crystal homologs compared to their unsubstituted analogs. However,
the nitro group introduction effect, decreasing the melting point of the compound, does not promote columnar mesomorphism
in the lower homologs. Thus the nitro group substantially expands the range of existence of the mesophase in those homologs
which formed columnar structures prior to NO introduction. This experimental fact clearly correlates with the results of
2
the prediction and suggests that the M parameter is the critical factor governing the appearance of CM in this series
m
(Table 3). The introduction of the amino group instead of the nitro group produces minor effect on the values of the
parameters used to predict the existence of columnar mesophases. Again, the experiment gives good agreement with the
prediction. The range of existence of the mesophase in the amino derivatives of triphenylene is much narrower than that
in the nitro derivatives; this is explained in [4]. We have managed to prepare amino derivative samples with higher purity
compared to [4], due to the additional low-temperature crystallization from hexane (Table 3). This has made it possible to
record (on cooling) characteristic texture of homolog 7, which is an amino derivative of triphenylene (Fig. 2a). Figure 2b
shows the texture of homolog 10 of the nitro derivative of triphenylene. The photograph was taken near the phase transition
of isotropic liquid to the mesophase; one can see distorted finger-like domains, indicating hexagonal packing of columns.
For nearly all homologs of series Ia, quick cooling of samples leads to mosaic striped texture, which under shear deformation
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