This paper reports on the different solid crystalline (SC) forms of 4-cyano-4'-heptyloxy biphenyl (M-21) as revealed through differential scanning calorimetric, polarizing microscopic and infrared spectroscopic investigations. There are three solid crystalline modifications, namely SCI,SCII,SCIII. The appearance and preponderance of the SC forms depend critically on the manner in which the liquid crystalline melt solidifies. Each SC phase has its CN stretching band split into two components because of Davydov or correlation splitting. The position, separation and relative intensities of the two components characterize each SC phase. The systematic study of the variations in these three features with temperature leads to interesting information about the intermolecular ordering forces and the spatial arrangement of the molecules in the unit cell. It is shown that the intermolecular interactions of the dipolar nature play a dominant role in the SC phases but contribute negligibly in stabilizing the nematic phase. The angle between the correlated molecules in the unit cell changes in the order SCI&gt; SCII &gt; SCIII &gt; nematic = 0, resulting in parallel arrangement of molecules in the nematic phase

Agnihotry, S. A.

Bhide, V. G.

Chandra, S

Jain, S. C.

English

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Solid Crystalline Polymorphism in M-21
S. C. Jaina; S. A. Agnihotrya; S. Chandraa; V. G. Bhidea
a National Physical Laboratory, New Delhi, India
First published on: 01 February 1984
To cite this Article Jain, S. C. , Agnihotry, S. A. , Chandra, S. and Bhide, V. G.(1984) 'Solid Crystalline Polymorphism in M-
21', Molecular Crystals and Liquid Crystals, 104: 1, 161 — 171
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Mol. Cryst. Liq. Cryst., 1984, Vol. 104, pp. 161-171 
0 1984 Gordon and Breach, Science Publishers, Inc 
Printed in the United States of America 
0026-8941/84/1042-0161/$18.50/0 
Solid Crystalline Polymorphism 
in M-21t 
S. C. JAIN, S. A. AGNIHOTRY, S. CHANDRA and V. G. BHIDES 
National Physical Laboratory, New Delhi- 110012, India 
( Received December 8, 1982; in final form May 28, 1983) 
This paper reports on the different solid crystalline (SC) forms of 4-cyano-4’-heptyloxy 
biphenyl (M-21) as revealed through differential scanning calorimetric, polarizing micro- 
scopic and infrared spectroscopic investigations. There are three solid crystalline modifi- 
cations, namely SC,, SCI,, SC,,, with the thermal phase diagram. 
3 3 o c  50°C 54-c 74oc 
SC, - SC,, + SC,,, - nematic - isotropic liquid 
The appearance and preponderance of the SC forms depend critically on the manner in 
which the liquid crystalline melt solidifies. Each SC phase has its CN stretching band 
split into two components because of Davydov or correlation splitting. The position, 
separation and relative intensities of the two components characterize each SC phase. 
The systematic study of the variations in these three features with temperature leads to 
interesting information about the intermolecular ordering forces and the spatial arrange- 
ment of the molecules in the unit cell. It is shown that the intermolecular interactions of 
the dipolar nature play a dominant role in the SC phases but contribute negligibly in 
stabilizing the nematic phase. The angle between the correlated molecules in the unit cell 
changes in the order SC, > SC,, > SC,,, > nematic = 0, resulting in parallel arrange- 
ment of molecules in the nematic phase. 
Liquid crystalline polymorphism i.e. the ability of a compound to exist 
in more than one form in liquid crystalline state is well known. It was 
Bernal and Crowfoot’ who in 1939 proved the presence of Solid 
Crystalline (SC) polymorphism in a number of mesogenic materials. 
Over the last few years a large number of compounds exhibiting 
TPresented at the Ninth International Liquid Crystal Conf., Bangalore, 1982. 
$Department of Physics, University of Poona, Pune-7, India. 
161 
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162 S. C.  JAIN et a/. 
different mesogenic phases such as nematic, smectic and cholesteric 
have been reported to exist in more than one solid crystalline form.*-” 
SC polymorphism has been interpreted in many ways by dif- 
ferent workers. On the basis of thermal and spectroscopic studies, 
Ogorodnik” suggested that SC polymorphism should be a necessary 
condition for a material to exhibit liquid crystalline properties. 
In our previous  publication^,'^.'^ existence of SC polymorphism was 
established in 4-n-cyano-4‘-n’-octyloxybiphenyl (M-24) through DSC, 
IR and optical studies. It was shown that SC polymorphism was due 
to conformational changes, each polymorph having a well defined 
three dimensional order. The present paper reports on the different SC 
forms of (M-21). All the three SC phases show a doublet in the CN 
stretching region with characteristic separation, intensity ratio and 
positions for the two components. These components arising because 
of “Davydov or correlation splitting” reveal information about the 
spatial arrangement of the molecules in the unit cell and also about 
the intermolecular ordering forces in various SC, mesogenic and 
isotropic liquid phases. 
The liquid crystalline sample M-21, schematically represented by 
was purchased from M/s BDH, England and was used as such 
without further purification. The material exhibits nematic phase 
between 5 4 O  -74OC. The experimental details have been described 
earlier .l 
RESULTS AND DISCUSSION 
DSC studies 
Figure l(a) is a typical DSC trace of virgin sample M-21 during 
heating (heating rate 4O C/min). The trace shows two endothermic 
peaks at 54OC and 74OC corresponding to solid-nematic and 
nematic-isotropic phase transitions. These transition temperatures 
agree very well with those reported earlier.” However, detailed DSC 
investigations of the sample subjected to a variety of thermal treat- 
ments revealed interesting results. 
Figure l(b) shows the DSC trace during heating of the solid sample 
obtained by rapid cooling of the isotropic melt (cooling rate 
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CRYSTALLINE POLYMORPHISM IN M-21 163 
0 
W 
I b i  ¶OLID O I I A I N L D  8 V  *A110 
C O O L I M Q I ~ ~ * C ~ M I N I  
I CI SOLID MTAIMCD mv NORMAL 
COOLIWO t 4 i n l M  I 
0 
X 
W 
141 SOLID OITAIMCD V I n V  
SLOW COOLIMO co.e*c/utri 
s c n  
I I I I I I I  
a4 -0 sa A 
TEMPERATURE (% 1- 
FIGURE 1 DSC traces of solid M-21 prepared under different cooling conditions. 
32"C/min). Along with the two endothermic peaks at 54°C and 
74 " C, a new endothermic peak at 50 " C and a broad exothermic dip 
between 33-38O C, are also observed. 
Figure l(c) shows the DSC trace during heating of the solid sample 
obtained by slow cooling (cooling rate 4OC/min) of the isotropic 
melt. It is qualitatively very similar to the DSC trace of the rapidly 
cooled sample in Figure l(b). However, the intensity of the 50 O C peak 
is much larger and it is accompanied by a small exothermic dip. 
Figure l(d) shows the DSC trace during heating of the solid sample, 
obtained by very slow cooling (cooling rate 0.5 O C/min). This trace is 
very similar to the above two traces, except for the fact that the broad 
exothermic dip extending from 33°C to 37°C is completely missing. 
Secondly the height of the 50°C peak is even greater than that of 
54°C. 
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164 S .  C .  JAIN et u/. 
Above observations clearly show that M-21 exists in three different 
solid crystalline forms. In the order of increasing temperature these 
can be called SC,, SC,,, SCII,. Thus SC,,, corresponds to the virgin 
M-21 sample which transforms into nematic phase at 54°C and 
further into isotropic liquid phase at 74°C. SC, is another solid phase, 
obtained by fast cooling of the isotropic melt and transforms on 
heating to SC,, at 33°C. This is an exothermic transition. SC,, on 
further heating melts and crystallizes into SC,,, at 50°C. The three 
distinct solid phases as revealed by DSC studies and their transforma- 
tions are as follows: 
33-38 C 5OoC SC,,, - 54oc Nematic - 74oc  Isotropic liquid. sc, ___ sc,, - 
These solid forms have been further investigated and established 
through infrared and polarizing microscopic studies. 
Infrared studies 
Infrared studies, particularly in the CN stretching region (2200-2300 
cm-') bring out clearly the characteristics and differences of the 
various solid forms of M-21. The CN stretching band is selected, as it 
is isolated from all other fundamentals and not overlapped by other 
overtone or combination modes. Each solid phase, nematic and iso- 
tropic liquid phase gives CN stretching band characteristic in shape, 
position and intensity. Information about the three solid phases and 
the intermolecular ordering forces in the vicinity of the CN group, is 
contained in the features such as shape, intensity, position and width 
of this band. Of particular importance is their variation with temper- 
ature. The CN stretching band for all the three solid phases is 
observed to split into two components, their separation, relative 
intensities and positions being different in different phases. By con- 
trast, the nematic and the isotropic liquid phases have their CN 
stretching band as a singlet, identical both in frequency and shape. 
Table I shows these details and Figure 2 shows the CN stretching 
band of various phases of M-21. 
The appearance of a doublet in the spectra of various solid phases 
of M-21 is explained to arise because of interaction of translationally 
inequivalent molecules, known as Davydov, correlation or factor group 
splitting. For the two components having necessarily orthogonal tran- 
sition moments, Davydov's theory predicts splitting of a band into two 
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CRYSTALLINE POLYMORPHISM IN M-21 165 
TABLE I 
Position and the nature of the CN stretching band for different phases of M-21 
Nature of the CN stretching band 
Position of the Mean Separation of 
band maxima freq. the components Intensity 
Phase in cm-' cm-' Shape (cm-'1 ratio 
SC, 2243 2232 2237.5 doublet 11 1 
SC,, 2239.5 2231 2235.25 doublet 8.5 0.7 
SC,,, 2231.5 2230.5 2233.5 singlet 
with a 7.5 0.4 
shoulder 
Nematic 2233 - single 
- sharp - 
band 
Isotropic 2233 - single 
- - liquid sharp 
band 
components, shifted symmetrically up and down from the original 
position; the shape, structure being preserved in this splitting. The 
number of fundamental components observed in the crystal spectra 
depends on the number of molecules per unit cell, the site and the unit 
cell geometry. A pair of molecules in a unit cell gives rise to two 
components, if their motions are coupled to each other but not to 
lattice. The doublets observed for all the three solid phases have the 
following characteristics. 
The position of the components of the doublet and also their mean 
value shift to lower wavenumber in going from SC, -+ SC,, -+ SC,,, 
phase. The two components of the doublet in the phase SCIII collapse 
to a central maximum at the phase transition SC,,, -+ nematic. 
It is evident from Table I that the separation between the two 
components of the CN stretching band shows a systematic variation. 
It is the largest for SC, and decreases in going to SC,, to SC,,,. 
Further in the nematic and the isotropic liquid phase there is a single 
band and thus the separation reduces to zero. 
The intensity ratio of the two components decreases in going over 
from SC, to SC,, to SCI,,. 
The study of these features gves very interesting information about 
the two molecules in the unit cell of the three SC phases. The 
separation of the two components represents the measure of the 
coupling between the two molecules. The systematic decrease in 
the separation in the three SC phases suggests similar variation in their 
coupling. The intensity ratio of the two components represents a direct 
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166 S. C. JAIN er al. 
~~ ~~ 
2 3 0 0  22 80  2 l O O  2lSO 
c n c o u e m v  ( c w " )  
FIGURE 2 CN stretching band of different phases of M-21 
measure of the relative orientation of the two molecules in the unit 
cell. The above results suggest that the angle between the two mole- 
cules in the unit cell decreases, the two molecules becoming parallel to 
each other in the nematic phase. 
The third feature, namely the position of the two components or 
their mean and their changes reflect on the variations in the degree of 
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CRYSTALLINE POLYMORPHISM IN M-21 167 
conjugation from one solid phase to the other. The frequency of the 
nitrile group on conjugation, in aromatic compounds usually shifts to 
lower values, as is evident from extensive studies by Kitson and 
Griffith.” These changes in the degree of conjugation are being 
studied from other infrared bands and results of the analysis will be 
published separately. These variations can be thought of to arise from 
varying contribution of the polar structures such as 
+ W C = . -  
The CN stretching frequency thus depends on the nature and the 
amount of the double bond character of the CN bond. In other words 
the three SC polymorphs differ in the degree of conjugation. The 
detailed spectroscopic observations to confirm this, are underway. 
These changes in the nitrile frequency attributed to changes in the 
molecular conformation give information of the type of the inter- 
molecular forces playing important role in the SC phases. Of course in 
the experiments reported here, only one aspect of the intermolecular 
force is probed and that is in the vicinity of the CN group. The 
positions of the frequency maxima differ in the three SC polymorphs. 
By contrast, the CN stretching band of the nematic and the isotropic 
liquid phases are identical in frequency. Thus the phenomenon which 
causes the shift in the CN stretching frequency in the SC polymorphs 
is no more active in the liquid crystalline states. In other words after 
the transformation SC,,, + nematic, there seems to be no more in- 
crease in the degree of conjugation. Thus the contribution of the polar 
structures or intermolecular interactions of the dipolar nature, plays 
an important role in the SC polymorphs. However, their contribution 
is insignificant in the liquid crystalline and liquid state or they 
contribute negligibly in stabilizing the nematic phase. It is quite likely 
that at higher temperatures (after SCIII -+ nematic) the increased 
thermal movements of the molecules hinder the mutual orientations of 
the two benzene rings, resulting in no further change in the degree of 
conjugation. 
Polarizing microscopic studies 
Figures 3(a,b,c) show a typical set of micrographs of M-21 (a) the 
virgin sample, SCIII (b) solid obtained by rapid cooling of the iso- 
tropic melt, SC, and (c) solid obtained by slow cooling of the nematic 
melt, SC,, respectively. It was seen that the virgin M-21 sample melted 
directly into the nematic phase at 54°C which on subsequent heating 
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168 S. C. JAIN et al. 
FIGURE 3 A typical set of micrographs of different SC phases of M-21 (a) SCIII-the 
virgin sample; (b) SCI-solid obtained by rapid cooling of the isotropic melt; (c) SCII 
-solid obtained by slow cooling of the isotropic melt. 
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CRYSTALLINE POLYMORPHISM IN M-21 169 
FIGURE 4 A typical set of micrographs of M-21 at different temperatures (a) 
SCI -+ SCl, (36OC); (b) SCII -P SCIII (49OC); (c) SCII, + nematic (54OC). 
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1 70 S. C. JAIN er a/. 
melted into isotropic phase at 74°C. Figures 4(a,b,c) show the 
micrographs of the sample during heating cycle around SC, -, SC,,, 
SCI, -+ SC,,, and SCIII -+ nematic phase transitions. It is seen from 
micrographs in Figures 3b & 4a that there are relatively very small 
structural changes in going over from SC, to SC,,. DSC studies, 
Figure l(b,c) also indicate only small transition energy for t h s  
transition. However, the structural changes in going over from SCII to 
SC,,, seem to be on a larger scale. Detailed X-ray and ultra-violet 
spectroscopic studies will bring out clearly the difference in these SC 
states and studies are underway to examine these differences. 
CONCLUSION 
DSC, infrared and microscopic investigations have clearly shown that 
there are three solid crystalline forms of M-21 namely SC,, SC,, and 
SC,,,. The appearance and preponderance of these SC forms depends 
critically on the manner in which the liquid crystalline melt solidifies. 
Each solid phase shows a characteristic infrared absorption spectrum 
in the CN stretching region. The doublet structure of the CN band 
arises because of Davydov splitting. The variation in the intensity 
ratio, separation and position of the components of the doublet yields 
information about the intermolecular interactions and about the spa- 
tial arrangement of molecules in the unit cell for the three SC phases. 
The intermolecular interactions of the dipolar nature play a dominant 
role in the SC phases but contribute negligibly in stabilizing the 
nematic phase. The angle between the correlated molecules in the unit 
cell changes in the order SC, > SC,, > SCII, > nematic = 0, resulting 
in parallel arrangement of molecules in the nematic phase. The scheme 
of the various transitions is as follows: 
,-,slow coolin: 
3 3 o c  5OoC 74oc  
SC, - SCI, - SCIII - nem. - Iso. liq. 
fast cooling 2
Acknowledgments 
The authors gratefully acknowfedge Dr. P. C. Jain, Physics Department, Delhi Univer- 
sity for his kind permission to obtain DSC data. 
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CRYSTALLINE POLYMORPHISM IN M-21 171 
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Solid crystalline polymorphism in M-21

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Solid crystalline polymorphism in M-21

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