Mesomorphic phase transition of a cyclotetraphosphazene containing schiff base moieties: Comparison with the corresponding cyclotriphosphazene

Article abstract of DOI:10.1039/b000497i

Newly prepared octakis{4-[N-(4'- heptyloxyphenyl)iminomethyl]phenoxy}cyclotetraphosphazene, consisting of a cyclotetraphosphazene ring backbone and Schiff base sidechain groups has been found to generate an enantiotropic smectic A phase; this is a first e

Full text of DOI:10.1039/b000497i

Mesomorphic phase transition of a cyclotetraphosphazene containing Schiff
base moieties: comparison with the corresponding cyclotriphosphazene
Keiichi Moriya,*a Yasuyuki Kawanishi,a Shinichi Yanoa and Meisetsu Kajiwarab
a Department of Chemistry, Faculty of Engineering, Gifu University, Yanagido, Gifu 501-1193, Japan.
E-mail: moriya@apchem.gifu-u.ac.jp
b School of Dentistry, Aichigakuin University, Kusumoto-cho, 1-100 Chikusa-ku 464-0074, Japan
Received (in Oxford, UK) 13th January 2000, Accepted 10th May 2000
Published on the Web 8th June 2000
Newly prepared octakis{4-[N-(4A-heptyloxyphenyl)imino-
methyl]phenoxy}cyclotetraphosphazene, consisting of a cy-
clotetraphosphazene ring backbone and Schiff base side-
chain groups has been found to generate an enantiotropic
smectic A phase; this is a first example of a mesomorphic
cyclotetraphosphazene.
Schiff bases have a strong tendency to produce mesomophic
phases.6,12,13 Therefore, for attempted generation of liquid
crystalline behavior, we synthesized octakis{4-[N-(4A-heptylox-
yphenyl)iminomethyl]phenoxy}cyclotetraphosphazene 3 (Fig.
1), with side chains containing Schiff bases with long alkoxy
tails, laterally linked to the cyclotetraphosphazene ring. We
studied its phase transitions and mesogenicity using polarizing
microscopy and DSC measurements, and the obtained results
were compared with those for the cyclotriphosphazene, hex-
akis{4-[N-(4A-heptyloxyphenyl)iminomethyl]phenoxy}cyclo-
triphosphazene 4 with similar mesogenic side groups (Fig. 1).
This paper presents the first evidence for the formation of a
mesophase in cyclotetraphosphazenes.
Cyclotetraphosphazenes are cyclic compounds of general
formula (PNX2)4, in which tetracoordinated phosphorus atoms
alternate regularly with the nitrogen atoms (Fig. 1). Several
cyclotetraphosphazenes have been synthesized by introducing
functional side chains into them.1,2 Although some examples of
mesomorphic transitions in cyclotriphosphazenes have been
described3–7 no liquid crystalline phases of cyclotetraphospha-
zenes have been observed.5,7 In particular, in a previous paper,
we reported our studies on phase transitions of cyclotetra-
phosphazenes and cyclotriphosphazenes with the same 4-octy-
loxybiphenyl mesogenic side groups.5 We found no liquid
crystalline phase for octakis[4-(4A-octyloxy)biphenoxy]-
cyclotetraphosphazene 1 but an enantiotropic smectic C phase
was observed for hexakis[4-(4A-octyloxy)biphenoxy]cyclotri-
phosphazene 2. These results were interpreted in terms of the X-
ray single crystal structure analyses of octakis(4-biphenoxy)-
Compound 1 was synthesized as follows. 4-Heptyloxy-
acetanilide 5 was prepared from 4-hydroxyacetanilide (25 g,
0.16 mol), bromoheptane (28.5 g, 0.16 mol) and KOH (11 g,
0.17 mol) in ethanol (200 ml) under reflux for 6 h. 4-Heptyloxy-
aniline 6 was synthesized from 5 (20 g, 83 mmol) and KOH (15
g, 0.23 mol) in ethanol (200 ml) under reflux for 24 h.
Hexakis(4-formylphenoxy)cyclotetraphosphazene 7 was pre-
pared from hexachlorocyclotetraphosphazene (5.0 g, 10.6
mmol) (Nihon Soda Co., Ltd.) and the sodium salt of
4-hydroxybenzaldehyde in THF (125 ml) under reflux for 3 h.
The sodium salt was prepared from 4-hydroxybenzaldehyde
(17.5 g, 0.143 mol) and NaH (5.73 g, 0.143 mol) in THF under
reflux for 2 h. Finally, compound 3 was prepared by the reaction
of 6 (2.7 g, 13 mmol) and 7 (1.0 g, 0.87 mmol) in benzene (50
ml) under reflux for 6 h. Water produced in the solution was
removed by molecular sieves in a Dean–Stark tube. The
obtained crude products were separated by filtration and
recrystallized three times from benzene and once from THF–
cyclohexane (1+1) after being separated by filtration. Com-
cyclotetraphosphazene
(OBCP)
and
hexakis-
(4-biphenoxy)cyclotriphosphazene (HBCP), which have shown
the side chains lining-up randomly in the former but in a
relatively regular fashion for the latter.8 A similar situation
might be assumed for compounds 1 and 2. In tetramer 1, the
eight side chains are directed essentially randomly so prevent-
ing the formation of a mesomorphic layer structure. However,
in trimer 2, each of the three side chains of the cyclo-
triphosphazene molecules point almost perpendicularly up-
wards and downwards from the cyclotriphosphazene ring plane,
which aids in the formation of a mesomorphic layer structure.
The molecular structure of compound 2 assuming an all trans
conformation for the octyloxy side chains based on the crystal
structure of HBOP.4 The architecture of the mesomorphic phase
is reminiscent of arrangements of the mesomorphic phase in a
cyclotetrasiloxane,9 a dendric octasilsesquioxane,10 and a
fullerene derivative.11
1
pound 3 was characterized by H and 31P NMR, IR spectros-
copy and elemental analysis.14 Phase transitions were studied
using Seiko DSC 210 and SSC 5500 systems from room
temperature to above the melting point at heating/cooling rates
of 5 K min21. The textures of the mesophase were observed
using a polarizing microscope (Nikon, Optiphot-pol XTP-11)
with a temperature controlled hot stage (Mettler, FP-80) at
heating/cooling rates of 5 K min21
.
The DSC thermograms of octakis{4-[N-(4A-heptyloxy-
phenyl)iminomethyl]phenoxy}cyclotetraphosphazene 3 for the
first cooling (1c) and second heating (2h) processes are shown
in Fig. 2. For the first cooling process, two exothermic peaks
were observed at 429 and 422 K. According to polarizing
microscopy, upon cooling from an isotropic liquid, batonnets
were observed at 429 K which grew into a fan texture at ca.
428.8 K, suggesting the presence of a smectic A phase.15 In the
first heating process, two endothermic peaks were observed at
428 and 430 K. According to polarizing microscopy, melting of
compound 3 was observed at 428 K and between 428 and 430
K a fan texture similar to that in the first cooling process was
seen, which establishes the existence of the smectic A phase.
The view become black at 430 K, which corresponds to the
SmA–I phase transition. This is the first case of a mesomorphic
phase in cyclotetraphosphazenes.
Fig. 1 Chemical formulae of octakis{4-[N-(4A-heptyloxyphenyl)imino-
methyl]phenoxy}cyclotetraphosphazene 3 and hexakis{4-[N-(4A-heptyl-
oxyphenyl)iminomethyl]phenoxy}cyclotriposphazene 4, octakis[4-(4A-
octyloxy)biphenoxy]cyclotetraphosphazene
1
and
hexakis[4-(4A-
octyloxy)biphenoxy]cyclotriphosphazene 2.
DOI: 10.1039/b000497i
Chem. Commun., 2000, 1111–1112
This journal is © The Royal Society of Chemistry 2000
1111

Table 1 Phase transition temperatures (T/K) and (in parentheses) phase transition enthalpies (DH/kJ mol21) and entropies (DS/J K21 mol21) of compound
1–4
Compound
Side chain
Backbone Cr
Sm1
SmC
SmA
I
1
2
3
4
C8H17OC6H4C6H4O
C8H17OC6H4C6H4O
C7H15OC6H4NNCC6H4O
C7H15OC6H4NNCC6H4O
P4N4
P3N3
P4N4
P3N3
·
·
·
·
411(76,185)
440(79,180)
428(77,180)
460(74,161)
—
—
—
·
—
·
—
·
—
—
·
·
·
·
·
457(15,33)
499(0.5,1)
430(28,66)
512(18,35)
482(1,3)
·
to form a mesomorphic phase because of the random orientation
of the side groups. Probably, the cyclotetraphosphazene rings
would soften and the Schiff base side chains can form a smectic
A layer structure. This behavior was observed in the SmA phase
of the cyclotriphosphazene with dodecylbiphenoxy side groups,
in which the local order parameter around the P atom obtained
by 31P NMR spectroscopy was ca. 0.44.4 This value is lower
than that in a typical sectic A phase and suggests the presence of
disorder in the cyclotriphosphazene. The difference in the
mesogenicity of tetramers and trimers is a result of their
differing molecular structure.
In conclusion, we have introduced mesogenic side groups
containing Schiff base functionalities onto the cyclotetra-
phosphazene ring and obtained the first N4P4 derivative capable
of generating a mesomorphic smectic A phase. The liquid
crystalline behavior results from assistance from the Schiff base
to the formation of a layer structure due to the favorable
intermolecular interactions of the laterally linked mesogenic
units, which may override the formation of random orientations
of the side chains typical in cyclotetraphosphazenes.
Fig. 2 DSC thermograms of octakis{4-[N-(4A-heptyloxyphenyl)iminome-
thyl]phenoxy}cyclotetraphosphazene 3 for the first cooling (1c) and second
heating (2h) processes.
Table 1 shows a comparison of the phase transition
temperatures, phase transition enthalpies and entropies for the
pairs of tetramers (1, 3) and trimers (2, 4) bearing the same
mesogenic substituents: octyloxybiphenoxy (1, 2) and 4-{N-[4A-
heptyloxyphenyl]iminomethyl}phenoxy groups (3, 4). The
thermodynamic values were obtained from the second heating
process via DSC measurements. For the tetramers, the deriva-
tive 3 with substituents containing the Schiff base (compound 3)
has a narrow enantiotropic liquid crystalline phase with a
temperature range of only 2 K, while no mesomorphic phase
was observed for the derivative with octyloxybiphenoxy side
chains (compound 1).5 The trimer with the Schiff base
substituents (compound 4) shows the enantiotropic meso-
morphic phase transition sequence Cr–Sm1–SmC–SmA–I.6 It
displays a high SmA–I transition temperature (512 K) and wide
mesomorphic phase transition region (DT = 52 K). By contrast,
the trimer with octyloxybiphenoxy side chains (compound 2)
undergoes an enantiotropic mesomorphic phase transition, Cr–
SmC–I.3–5 The SmC phase was confirmed by X-ray photo-
graphy under a magnetic field.3 The SmC–I transition tem-
perature (457 K) is lower, and the mesomorphic phase transition
region (DT = 17 K) is narrower than those for the trimer with
the Schiff base substituents (compound 4). The compounds with
the Schiff base side chains appear to have a higher thermal
stability in the mesomorphic phase and higher mesomorphic
phase generation ability than those with 4-octyloxybiphenoxy
side chains. Two possible interpretations for the observed
differences may be considered. In the first, the molecular
structure of the tetramer with Schiff base side chains is assumed
to differ significantly from that of the tetramer with octyloxy-
biphenoxy side chains. The other possible interpretation is that
the intermolecular interactions might be much stronger for the
Schiff base derivative 3 than for the octyloxybiphenoxy
derivative 1 even if the molecular structures do not differ very
much from each other. Based on this assumption, the meso-
morphic ability of the compounds with Schiff base side chains
would be high. Assuming this, and that compound 3 retains this
molecular structure after melting, it would appear to be difficult
We are grateful to Nihon Soda Co., Ltd. for providing
octachlorocyclotetraphosphazene.
Notes and references
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14 Characterization data for compound 3: mp 425 K; IR (KBr) 2932, 2856,
1
1625, 1604, 1578, 1251, 1220, 1174, 1161, 985, 846 cm21; H NMR
(CDCl3) d 0.9 (t, J 7.0 Hz, 3H), 1.3–1.8 (m, 10H), 3.9 (t, J 6.6 Hz, 2H),
6.8 (d, J 9.0 Hz, 2H), 7.0 (d, J 9.0 Hz, 2H), 7.1 (d, J 9.0 Hz, 2H), 7.7 (d,
J 8.8 Hz, 2H), 8.4 (s, 1H); 31P NMR d 212.7 (s). Anal. Calc. for
C160H192O16N4P4: C, 72.16; H, 7.27; N, 6.31 Found: C, 71.94; H, 7.08;
N, 6.25%. 1H NMR (solvent CDCl3) and 31P NMR (solvent CDCl3)
spectra were recorded on a JEOL A-400 spectrometer using TMS as the
internal standard for the former and 85% H3PO4 as the external standard
for the latter.
15 D. Demus and D. Richter, in Textures of Liquid Crystals, Verlag
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1112
Chem. Commun., 2000, 1111–1112