Role of molecular structural flexibility on mesomorphism

Article abstract of DOI:10.1080/15421406.2016.1190139

A homologous series of novel chalconyl ester derivatives: RO.C₆H₄.COO.C₆H₄.CO.CH:CH.C₆H₄.OC₆H₁₃(n) shave been synthesized and studied for its thermotropic liquid crystal (LC) properties with a view to understanding and establishing the relation between molecular structure and LC properties. The homologous series consists of eleven members whose, mesomorphism commences from C₃ of the series and continues up to the C₁₆ homologue. The C₁ and C₂ homologues are nonliquid crystals. All the LC homologues are enantiotropically nematogenic with absence of smectogenic character. Transition temperatures were determined by an optical polarizing microscope (POM) equipped with a heating stage. Textures of a nematic phase are threaded or Schlieren. Analytical and spectral data confirms the molecular structures of homologues. Transition curve Cr-N/I behaved in normal manner. N-I transition curve exhibits an odd even effect; descended at C₁₀ homologue and then again rises at C₁₂ and descended from and beyond C₁₂ homologue. Thus it partly deviates from normal behaviour. Thermal stability for nematic is 95–89°C and nematogenic upper and lower mesophase lengths are 41.0°C and 07.0°C respectively. Group efficiency order for nematic on the basis of thermal stability is –OC₈H₁₇(n)> -OC₆H₁₃(n) > -OC₁₀H₂₁(n) and it is a low order melting type series.

Full text of DOI:10.1080/15421406.2016.1190139

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Role of molecular structural flexibility on
mesomorphism
S. H. Pandya & V. R. Patel
To cite this article: S. H. Pandya & V. R. Patel (2017) Role of molecular structural
flexibility on mesomorphism, Molecular Crystals and Liquid Crystals, 642:1, 1-8, DOI:
10.1080/15421406.2016.1190139
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1190139
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Date: 09 February 2017, At: 03:41

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –

http://dx.doi.org/./..
Role of molecular structural flexibility on mesomorphism
S. H. Pandya and V. R. Patel
Chemistry Department, Sheth P. T. Arts & Science College, Gujarat University, Godhra , Gujarat, India
ABSTRACT
KEYWORDS
A
homologous series of novel chalconyl ester derivatives:
Chalconyl ester;
mesomorphism; nematic;
smectic; enantiotropic
RO.C H .COO.C H .CO.CH:CH.C H .OC H (n) shave been synthe-
6
4
6
4
6
4
6
13
sized and studied for its thermotropic liquid crystal (LC) properties
with a view to understanding and establishing the relation between
molecular structure and LC properties. The homologous series consists
of eleven members whose, mesomorphism commences from C3
of the series and continues up to the C16 homologue. The C and
1
C2 homologues are nonliquid crystals. All the LC homologues are
enantiotropically nematogenic with absence of smectogenic character.
Transition temperatures were determined by an optical polarizing
microscope (POM) equipped with a heating stage. Textures of a nematic
phase are threaded or Schlieren. Analytical and spectral data confirms
the molecular structures of homologues. Transition curve Cr-N/I
behaved in normal manner. N-I transition curve exhibits an odd even
effect; descended at C10 homologue and then again rises at C12 and
descended from and beyond C12 homologue. Thus it partly deviates
from normal behaviour. Thermal stability for nematic is 95–89°C and
nematogenic upper and lower mesophase lengths are 41.0°C and
0
7.0°C respectively. Group efficiency order for nematic on the basis of
thermal stability is –OC H (n)> -OC H (n) > -OC H (n) and it is a low
8
17
6
13
10 21
order melting type series.
Introduction
In continuation of our efforts on thermotropic LCs [1] chalconyl ester derivatives, the present
investigation is planned with a view to understanding and establishing the effects of molec-
ular structure on thermotropic LC properties of a substance [2–6]. The proposed investi-
gation may be useful for the study of binary systems which may be applicable to the man-
ufacture of LC devices to be operated at room temperature or desired temperature [7–9].
Moreover novel chalconyl derivatives being biologically active molecules may be useful for the
study of bioactivity as antibacterial, antimalarial, antifungal, anticancer compounds [10–16].
The present investigation covers the synthesis of novel substances, their characterization and
evaluation of their thermometric data. The evaluated data will be interpreted and discussed
in terms of molecular rigidity and flexibility [17–20]. Number of ester homologus series
have been reported till the date [21–33], by adding –CH=N–, –N=N–, –CH=CH–CO–,
–
CO–CH=CH–, –COO.CH –, –CH=CH–COO–CH – etc central bridges. Evaluated data
2
2
CONTACT S. H. Pandya
pandyas@gmail.com
University, Godhra  , Gujarat, India.
Chemistry Department, Sheth P. T. Arts & Science College, Gujarat
©
 Taylor & Francis Group, LLC

2
S. H. PANDYA AND V. R. PATEL
of present investigation will be compared with structurally similar analogous series to derive
group efficiency order for mesophase.
Experiments
Synthesis
4
-n-Alkoxy benzoic acids (A) were synthesized from 4-hydroxy benzoic acid using corre-
sponding suitable alkylating agent (RX) by the modified method of Dave and Vora [34]. 4-
Hexyloky benzaldehyde (B) was prepared from 4-hydroxy benzaldehyde using hexyl bro-
ꢀ
mide, by known method of G. W. Gray and B. Jones [35a]. α-4-Hydroxy benzoyl –β-4 -
Hexyloxy phenyl ethylene (C) (Chalcone) was prepared from 4-hydroxy acetophenone and
4
-Hexyloxy benzaldehyde by known condensation [35b] method. 4-n-Alkoxy benzoic acids
ꢀ
(A) and α-4-Hydroxy benzoyl –β-4 -Hexyloxy phenyl ethylene (C) were condensed in 1,3-
dicyclohexylcarbodiimide (DCC), 4-dimethylaminopyridine (DMAP) and CH Cl [36]. Syn-
2
2
thetic route to usual series is mentioned below as Scheme-1. Final products were individually
filtered, washed, dried and purified till the constant transition temperatures obtained.
Scheme . Synthetic route to the series-.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
3
Table . Elemental analysis for the hexyloxy, octyloxy and decyloxy derivatives.
Elemental % found (compared with % calculated)
Molecular formula
C HO
C
H
. (.)
.(.)
. (.)
. (.)
. (.)
. (.)


C HO


CH O

Characterization
1
Some selected members of the titled series were characterized by elemental analysis, HNMR
spectra and IR spectra, textures of mesophases were characterized by miscibility method.
Microanalysis for CHN elements was performed on Perkin Elmer PE 2400 analyzer (Table 1).
1
HNMR spectra were obtained on Bruker spectrometer using CDCl as solvent. IR spectra
3
were recorded on a Perkin- Elmer spectrum GX. Transition temperatures and liquid crystal
properties were investigated using optical polarizing microscope with heating stage.
Analytical data
1
H NMR: in ppm. For Octyloxy homologue.
Ethylenes: (200 MHZ) δ (CDCl ) (ppm) 1.09 (-CH -CH of –C H ), 1.45 (long-CH -
3
3
2
8
17
2
chain), 3.2 -OCH of (–OCH of C H ), 6.98 & 6.52, 8.60 & 8.20 (p-sub. benzene rings).
2
2
8
17
1
H NMR: in ppm. For Decyloxy homologue.
Ethylenes: (200 MHZ) δ (CDCl ) (ppm) 0.95 (-CH -CH of –C H ), 1.69 (long-CH -
3
3
2
8
17
2
chain), 3.33 -OCH of (–OCH of C H ), 6.29 & 6.55, 8.41 & 8.24 (p-sub. benzene rings).
2
2
8
17
−1
IR in Cm ,
IR Spectrum For Octyloxy homologue
−1
Ethylenes: (vmax/cm ): 2980, 2882, 1477, 1348 (-C-H, aliphatic), 1738, 1260 (ester group),
743 (>C=O group), 1568 (-C=C-, aromatic), 1033,1245 (ether group), 856.5 (p-sub. ben-
1
zene ring), 1620, 1485, 1460 (Aromatic ring).
Spectrum For Decyloxy homologue
−1
Ethylenes: (vmax/cm ): 2930, 2822, 1447, 1365 (-C-H, aliphatic), 1712, 1244 (ester group),
748 (>C=O group), 1566 (-C=C-, aromatic), 1033,1245 (ether group), 844 (p- sub. benzene
1
ring),1628,1468, 1482 (Aromatic ring),
Result and discussion
The transition temperatures of novel chalconyl ester derivatives are lower than correspond-
ing dimeric n-alkoxy benzoic acids. C to C homologues are enantiotropically nematogenic
3
18
with absence of smectogenic property. C and C homologues are nonmesomorphic. Transi-
1
2
tion and melting temperatures of novel homologues were determined by an optical polarizing
microscopy equipped with a heating stage (POM). Textures of nematic phase are threaded
or Schlieren. Analytical and spectral data confirms the molecular structures of homologues.
Transition temperatures alternated for a series. Transition temperatures (Table 2) are plot-
ted for the number of carbon atoms present in n-alkyl chain and on linking like or related
transition points, the transition curves Cr-N/I and N-I are obtained as depicted in a phase
diagram (Fig. 1). Cr-N/I transition curve adopted zigzag path of rising and falling with over-
all descending tendency and behaved in normal manner. N-I transition curve initially rises

4
S. H. PANDYA AND V. R. PATEL
Table . Transition Temperatures in °C.
Compound No
n-alkyl (C H
) group
Smectic
Nematic
Isotropic
n
n+
1
C1
—
—
—
—
—
—
—
—
—
—
—
—
—
.
.
.
.
.
.
.
.
.
.
.
2
3
4
5
6
7
8
9
C2
C3
.
.
.
.
.
.
.
.
.
C4
C5
C6
C8
C10
C12
C14
C16
1
1
0
1
and then descended at C and again rises at C and finally descended up to C homologue.
10
12
16
N-I transition curve exhibited an odd-even effect and the curve for odd and even homologues
matches into each other nearby C homologue through C homologue. Thus, N-I transition
9
7
curve partly deviated from normal descending tendency. Odd–even effect disappears from
and beyond C homologue. Thermal stability for nematic is 95.89 and the upper and lower
9
mesophase length is 41.0°C to 7.0°C at C and C5/8/16,respectively. The LC properties of novel
6
series varied from homologue to homologue in the same series.
The lowering of transition temperature of novel homologues as compared to corresponding
dimeric n-alkoxy benzoic acids is due to the breaking of hydrogen bonding between them by
esterification process. The exhibition of nematogenic LC property by linking chalconyl deriva-
tive containing conjugated double bond, increases molecular length and permanent dipole
moment across the long molecular axis and balanced suitable magnitudes of anisotropic forces
of intermolecular end to end attractions and closeness as a consequence of favorable molecu-
lar rigidity and flexibility required to induce nematogenic character by arranging molecules to
float on the surface with statistically parallal orientational order for definite range of temper-
atures as depended upon the magnitudes of thermal resistivity. The absence of smectogenic
character is attributed to the absence of lamellar packing of molecules in their preoccupied
Figure . Phase behavior of series.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
5
crystal lattices which fails to construct sliding layered molecular arrangement under float-
ing condition and the exposed thermal vibrations. The absence of LC state either smectic or
nematic is due to the high crystallizing tendency which arises due to the low magnitudes of
dispersion forces and low dipole–dipole interactions which abruptly breaks the crystal lat-
tices and sharply transforms into isotropic liquid without passing through an intermediate
LC state. However on cooling the same from and below isotropic temperature carefully, nei-
ther smectic nor nematic mesophase formation appeared in irreversible manner for C and C
1
2
homologue in monotropic condition. But, on cooling the nematogenic homologues C to C ,
3
16
which show exhibition of nematic mesophase formation exactly at a temperature at which it
had appeared in reversible manner. Thus, all the nematogenic homologues are enantiotropi-
cally nematogenic. Exhibition of odd–even effect of shorter n-alkyl chain length “R” of - OR
is attributed to the addition of methylene unit up to C and then the disappearance of effect
8
is disappeared from and beyond nearby C , because longer n-alkyl chain “R” of –OR and –
9
OC H of –OC H (n) may coil or bend or flex or couple to lye with the major axis of core
6
13
6
13
structure inducing uncertainity in the status of molecular length, rigidity and flexibility, length
to breadth ratio, permanent dipole moment across the long molecular axis, dispersion forces,
suitable magnitudes of anisotropic forces of intermolecular cohesion and closeness, etc. Thus,
odd–even effect becomes inoperative from and beyond merging of N–I transition curves for
odd and even homologues and therefore the synthesis of higher homologues C , C , C , C
9
11
13
15
is ruled out. The variations in mesomorphic properties from homologue to homologue in
same present novel series is attributed to the sequentially added methylene unit or units to
the n-alkyl chain “R” of –OR which vary the parameters related to mesophase formation. The
variations in some thermometric LC properties of present novel series-1, are compared with
the structurally similar analogous series X [37] and Y [38] as under, in Fig. 2.
Homologous series of present investigation-1 and structurally similar analogous series X
and Y chosen for comparison are identical in all respect viz. three phenyl rings, two cen-
tral bridges –COO and –CO-CH=CH- contributing to the total molecular rigidity and the
left n-alkoxy terminal(-OR) for the same homologue from series to series which partly con-
tributes to the total molecular flexibility. However they differ with respect to right handed
terminal end groups –OC H (n), -OC H (n) and –OC H (n) which partly contributes to
6
13
8
17
10 21
the molecular flexibility for series 1, X and Y, respectively. Therefore, the variations in meso-
genic properties will depend upon differing features; viz. differing magnitudes of flexibility
for the same homologue from series to series and from homologue to homologue in the same
series. Following Table 3 represents some mesogenic properties of thermotropic LC for the
series under comparative study.
Evaluated thermotropic data in Table-3 indicates that,
•
All the series under comparative study are nematogenic only without exhibition of
smectic property.
Figure . Structurally Similar analogous series.

6
S. H. PANDYA AND V. R. PATEL
Table . Average thermal stabilities in °C.
Series
Series-(-OC H )
Series-X(-OC H )
Series-Y(-OC H )




 
Sm-N or Sm-I
Commencement of smectic phase
N-I
—
.
—
.
—
.
Commencement of nematic phase
(C - C )
C
. to .
C C
(C – C )
(C – C )






C
C
Total mesophase lengths in °C
. to .
C C
. to .
C C
C
C
C
•
Thermal stability for nematic of series 1 and Y are almost equivalent differing only by
.14 unit and that of series X is little bit higher by 2.99 and 3.13 units from series-1 and
0
Y.
•
•
Mesomorphism commences from C homologue for series 1 and X but it commences
late from C homologue in series Y.
Upper mesophaselengths are in decreasing order from series 1 to X to Y whereas lower
mesophase lengths are in increasing order from series 1 to Y to X.
3
4
The exhibition of only nematogenic character by the series under comparison are
attributed to the favorable magnitudes of combined effects of fixed molecular rigidity and
varied molecular flexibility due to one or other end group which disalignes the molecules
at an angle less than 90° through end to end attractions and closeness. Thus, intermolecu-
lar cohesion of suitable magnitudes are capable enough to maintain and facilitate floating of
molecules with statically parallel orientational order under the influence of externally exposed
thermal vibrations to cause nematic mesophase formation from C or C homologue. How-
3
4
ever intermolecular cohesion by lateral to terminal due to longer n-alkoxy terminal end group
other than left –OR group is insufficient to facilitate and maintain layered molecular arrange-
ment to induce smectogenic character, due to unusual and unexpected status of n-alkoxy
terminals. Thus, length to breadth ratio for all the three series under comparative study is
almost equivalent which is confirmed from the values of thermal stabilities which differ from
negligible difference 0.14 to 3.13 units only. Thus, thermal stabilities diffence indicates very
small difference of enthalpy values ꢀH , ꢀH , ꢀH for the same homologue from series to
1
X
Y
series. Enthalpy values are nothing but indicates energy stored in a molecules which is directly
related with thermal resistivity of individual molecule towards exposed thermal vibrations
which vary for the same homologue from series to series and from homologue to homologue
in the same series in form of upper and/or lower mesophase length ranges. The commence-
ment of mesophase is depended upon the extent of molecular noncoplanarity which do not
differ much amonst for the series 1, X and Y under comparison. Thus, commencement of
mesophase takes place from C homologue for series-1 and X but it commences from C4
3
homologue of series-Y. All the series under comparison exhibited odd–even effect from C3
or C homologue to C or C homologue due to sequential addition of methylene unit at the
4
8
10
shorter n-alkyl chain “R” of –OR with its expected status. However, observed deviations in
mesogenic behavior are attributed to the unexpected and uncertainity in the status of n-alkyl
chain “R” of –OR and n-alkoxy group present at the other end of each series.
Conclusions
•
Homologous series of present investigation is nematogenic with absence of smectic prop-
erty whose, mesogenic degree is considerable and of low ordered melting type.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
7
•
•
Ester homologous series (-COO-) are generally smectogenic, but addition of chalconyl
group as one of the central groups eliminates smectogenic character and induces nematic
property.
The group efficiency order derived on the basis of (i) thermal stability (ii) commence-
ment of mesophase and (iii) total mesophase lengths for nematic are as under.
(i) Nematic
−
OC H (n) > −OC H (n) > −OC H (n)
8
17
6
13
10 21
(ii) Nematic
−
OC H (n) = −OC H (n) > −OC H (n)
6
13
8
17
10 21
(iii) Mesophase length
Upper : −OC H (n) > −OC H (n) > −OC H (n)
6
13
8
17
10 21
Lower : −OC H (n)> −OC H (n)> −OC H (n)
8
17
10 21
6
13
•
•
•
Mesomorphism is very sensitive and susceptible to molecular structure as a consequence
of molecular rigidity and flexibility.
Present study may be useful for the manufacture of LC Devices and pharmaceutical
preparation via their bioactivity.
Present study supports and raises credibility to the conclusions drawn earlier.
Acknowledgments
Authors acknowledge thanks to the Dr. N.N. Vyas, the head of chemistry department and the principal
Dr. M.B. Patel and Prof.M.L. Chauhan of the college for their supportive view and providing research
facility of present investigation. Authors are also thankful to Dr. A.V. Doshi, Ex. Principal, M.V.M. Sci-
ence and Home. Sc. College, Rajkot for his valuable suggestions, comments, co operation and support
as and when needed throughout this present investigation. Authors acknowledge thanks to L.M. college
of pharmacy, Ahmedabad for analytical and spectral series.
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