149
JournalofMolecular Liquids, 32 (1986)149-160
ElsevierSciencePublishersB.V., Amsterdam-Printed in
THE POLEY ABSORPTION
IN LIQUID
GARETH J. EVANS and Joseph
Department
of Chemistry,
The Netherlands
CRYSTALS
K. MOSCICKI,l
University
College
of Wales, Aberystwyth
SY23 1NE.
and
MYRON W. EVANS
Department
of Physics,
University
College
of Swansea,
Singleton
Park, Swansea
SA2 8PP (Great Britain)
(Received
14 February
1986)
ABSTRACT
The far infrared
torsional
crystal has been isolated
modes.
using a molecule
with a dipole moment
so that torsional
oscillation
The far infra-red
power absorption
nematic,
i.e. the torsional
sharpen
itself.
and intensify
molecular
engineered
this absorption.
dynamics
those in a regular molecular
frequency
proper
for
to the long axis
this component
is therefore
about the rigid
liquid
synthesised
perpendicular
of phase changes
in liquid crystals
librational
specially
of the liquid crystal
conditions
oscillation
The effect
that phase changes
of a nematic
about this axis modulates
solid and supercooled
molecule
absorption
for the first time free of higher
This has been achieved
this purpose
oscillatory
only.
in the isotropic,
a Poley absorption,
long axis of the liquid crystal
on a macroscopic
level is to
This is clear evidence
are cooperative
to the effect
phenomena,
the
about the long axis are little different
from
liquid.
INTRODUCTION
The first far infra-red
crystal was reported
1.
Permanent
Krakow,
power absorption
contemporaneously
address:
Institute
spectrum
by three groups
of Physics,
Jagiellonian
Poland.
0167-7322/86/$03.50
of a nematic
0 1986 Elsevier Science Publishers B.V.
liquid
in the early
University,
150
seventies
aniline
Cl-31.
The object
which
The nematic
asymmetric
computer
simulation.
produce
to
The investigation
because
broad band power absorption
of absorption.
was masked
theoretically
is roughly
in the
and with
of MBBA did not
torsional
was complicated
with time because
Poley absorption,
oscillation
by higher frequency
fact that MBBA has a dipole moment which
presence
torsional
the inter-molecular
p'n butyl
by all three groups.
into the dynamics
The interpretation
and this angle fluctuated
benzylidene
the far infra-red
be due to molecular
top, and has been reproduced
clear results,
flexibility.
p methoxy
and was investigated
of these studies was to isolate
is now well known
diffusing
modes
liquid crystal
(MBBA) was then available
oscillatory
intra-molecular
further by the
at 45' to the long axis,
of the inherent molecular
On top of this MBBA is a Schiff base and decomposes
in the
of moisture.
Subsequent
cyano biphenyl
molecular
investigations
(7CB) whose
[4-81 with nematics
dipole moment
long axis have resulted
as expected
at relatively
this has been interpreted
consistently
power absorption
molecular
proper modes,
the inter-molecular
in the isolation
high frequencies
parallel
of the Poley absorption,
[41 and Kerr effect data.
oscillator
However
in this case is again complicated
and special measures
absorption
p'
to the
in the far infra red [5,81, and
using the model of the itinerant
with dielectric
infra-red
such as p n heptyl
is accurately
of interest,
self
the far
by intra-
had to be taken C81 to identify
such as supercooling
in glassy
decalin.
This paper aims to complement
torsional
oscillation
synthezised
moment
for this purpose
accurately
the work on 7CB by looking
about the long axis of a molecule
at Krakow.
perpendicular
designed
[1,5,81
Fig. 1.
n
perpendicular
the intra-molecular
of the far and mid infra-red
to be rigid with the minimum
with the ability
to exist in a nematic
modes
the molecular
in fig. (I).
of
was
compatible
phase near room temperature.
showing
dipole.
framework
of alkyl chain flexibility
Sketch of the liquid crystal molecule,
to the long axis.
a permanent
to the long axis, as illustrated
III order to remove as far as possible
absorption
This requires
at the
specially
The
the dipole moment
151
bridging
oxygen
dipole moment
neglect
the small,
power absorption
supercooled
in mind,
the most detailed
dynamics
and follows
The position
coordinate
irrelevant
responsible
red spectrum
will provide
of phase changes
Furthermore,
described
on local molecular
EXPERIMENTAL
long axis.
for looking
dynamics
Great
if our
and if all
the far infra-
in detail at the effect
in a liquid crystal
C91.
of this kind takes place relatively
maximum
a much greater
freely,
shows up in the fifty to one hundred
as in regular molecular
liquids
dynamics
insight
care was taken to produce
accurate
Mark III, interfaced
[lo].
It is possible
relatively
straightforwardly
than with MBBA or 7CB.
far infra-red
power absorption
both on the ordinate
and the abscissa, respectively the
-1
in neper cm
and the wavenumber, ;, in
with a Research
6080 local mainframe.
were obtained
cell outside
and Fourier
configuration
transformed
"Cube"
interferometer,
3802 Microprocessor
several
and
interferograms
on this system by placing
and evacuating
the sample
as much as possible
of the
up to the Golay detector.
The sample was accurately
to within
absorption
coefficient
0.01 mm.
designed
thermostated
The limiting
is related
length used to attentuate
was specially
Machines
For each spectrum,
the interferometer
determined
aliasing
of angular
Therefore,
power absorption coefficient, a(;),
-1
cm
. This was achieved with an N.P.L./Grubb-Parsons
optical
that the
as a function
METHODS
coefficients
Honeywell
oscillatory
dipole moment means
above be successful,
an ideal method
to study the liquid crystal
and this provides
torsional
it
phases.
for its modulation
long axis libration
region,
therefore
molecular
of the far
liquid, nematic,
because
modes have indeed been eliminated,
so that the Poley absorption
wavenumber
on the molecular
the various
the molecular
engineering
intra-molecular
the determination
in the isotropic
solid is of key importance,
of the permanent
will be that about
piece of molecular
therefore,
information
molecular
long axis, if we
induced by alkyl chain dynamics.
of this molecule
these through
oscillation
that the permanent
to the molecular
dipole moments
and low temperature
provides
torsional
perpendicular
temporary
With these considerations
infra-red
in fig. (I) means
illustrated
is accurately
to the inverse
the incident
radiative
to take accurately
effects were eliminated
and the sample path length Cl01
uncertainty
with
power
machined
suitable
in the power
of the uncertainty
IO.
quartz
combinations
in the path
The sample cell
optical
flats,
of filters,
and
152
all mirrors
pumping
were kept free of oil vapour
and drying agents
The power absorption
of spurious
features
eliminated.
RESULTS
coefficient
was measured
in the transmissivity
The results
especially
and moisture
favourable
with a combination
illustrated
repeatedly
of the instrument
until all signs
were
in this paper are samples
taken under
conditions.
AND DISCUSSION
Fig.
crystal
(2) illustrates
of fig.
the far infra red power absorption
(I) in the isotropic
OI
0
Fig. 2.
Far infra-red
power absorption
2)
Supercooled
glassy solid at 295 K.
3)
Nematic
4)
Isotropic
Abscissa:
200
of the molecule
four phases.
(?) solid at 233 K.
at 311 K.
liquid at 357 K.
Power absorption
-1
cm
coefficient
:
I
I
Crystalline
Ordinate:
I
100
or possibly
of the liquid
liquid phase at 357 K, in the nematic
I
1)
of
such as silica gel.
(neper cm
-1
).
cm-'
of fig. (1) in three
153
phase at 311 K in the solid at 295 K and at 233 K.
It is clear from all
four results
characteristic
that the broad,
featureless
of the torsional
oscillatory
dynamics
in the isotropic
and nematic
phase,
solid at 233 K there is visible
means
of unresolved
that the complete
butits peak at below
interpretation
at higher
lattice modes
liquids
frequencies
El01
is well resolved
and in the solid at 295 K.
in the shape of what appears
mode absorption,
number
absorption
of regular
In the
the onset of proper
to be a
of relatively
high intensity.
This
Poley absorption is not as well resolved at 233 K,
-1
is still clearly visible.
A qualitative
100 cm
of these spectra
can therefore
be given in the following
terms.
i)
The most
dynamics
absorption
There
important
observation
of the molecule
remains
roughly
phases which
on the grounds
bandshapes
This remarkable
result
dynamics
and in consequence
anticipating
at 295 K, which
could be interpreted
to nematic
between
number
absorption
indicating
This liquid crystalline
Poley
absorption
glass.
to nematic
condition
producing
evidence
and related
intensity
and that of the solid
It seems that further
the spectrum
(sometimes known
There
Cl01 as the "cross
as the
density
is a disproportionately
temperature
phase transition
is
sources.
and this is due to the increasing
to the higher
shown
for this supposition
- the
large
solid, probably
that there is a jump, i.e. a discontinuity,
the isotropic
cooperative
terms,
that of the isotropic molecular
crystallisation
per unit volume.
from the nematic
the
and that in the solid at 295 K.
to mean that the torsional
status.
No corroborative
is decreased,
that
to the other
between
i.e. the area of the curves in fig. (2), increases
of dipoles
increase
is applicable
liquid is, in dynamical
a supercooled
yet from X ray diffraction
temperature
liquid
alone Clll.
solid, and indeed there is not much of a band-
is probably
The integrated
and
to suspect
liquid are already highly
between
the nematic
of this glass induces
section"),
reasoning
that the isotropic
in fig. (2) at 233 K.
oscillatory
in a regular
difference
is very little difference
phase for instance,
liquid and the crystalline
ii)
There
to be intermediate
shape difference
available
the isotropic
of temperature
in the isotropic
the change
is well known
cooling
Similar
in this figure.
in the nematic
oscillatory
between
from fig. (2) a 1one there would be no reason
a phase change had occurred.
two spectra
frequency
is no more than could be expected
such as dichloromethane
In other words
to the torsional
in the same place at all four temperatures.
is a slight shift to higher
nematic
of relevance
of fig. (1) is that the peak of the far infra red
temperature
in the density
c91.
at
In contrast
the
154
cross section
lowered
increases
to 233 K.
much less rapidly
This means
symmetry,
there are always present
induced absorptions
the "pure" Poley absorption
The main qualitative
in the three or possibly
dichloromethane
several
freezes
four phases
long axis dynamics
explaining
of the intermediate
permittivity
power absorption
to frequencies
frequency
beyond
to consider
work [41 on this molecular
the results
into dielectric
equations
between
similarity
in
is surely a key point in
state of matter
in future dielectric
liquid and nematogen,
between
liquid
in the terahertz.
clO,121
these spectra
to stretch
The
straightforwardly
range of dielectric
downwards
Cl01
loss vs.
to the megahertz
any quantitative
in terms of molecular
the dielectric
to see
range of that figure corresponds
The complete
Naturally
loss and
at lower frequencies.
curves of fig. (2) can be converted
loss, and the wavenumber
is well known
explaining
[lOI in
differences
This pronounced
of fig. (2) are paralleled
to lower frequencies.
reproducing
One would
solid.
It will be interesting
whether
spectral
similar
liquid such as
are available
solid.
in this type of nematogen
the existence
and crystalline
in fig. (2).
when a regular
of more pronounced
phase and crystalline
on
oscillatory
in fig. (1) are remarkably
illustrated
differences
such as these
superimposed
itself.
is that the torsional
1111, and observations
rotator molecules
liquid, rotator
field 112,131.
of the librating molecule
of the long axis of the molecule
Alternatively
in packing
in far infra red spectra
C141, and these are always
conclusion
expect much more pronounced
change.
by an increase
and this may have an effect on the internal
interaction
dynamics
by much of a density
in the solid could be accompanied
Additionally
of the solid is
that if there is a phase change as the solid
is cooled it cannot be accompanied
the cooling
as the temperature
dynamics
attempt
and
at
must be capable
loss and the power absorption
of
from the same
Only then will a clear view of the dynamics begin
of motion.
to emerge.
QUANTITATIVE
INTERPRETATION
Standard methods
in the far infra-red
of quantitative
available
in the literature
transform
the power absorption
correlation
function
interpretation
range and lower frequencies
ClOl.
C10,15,161.
The first stage is to Fourier
coefficient
Neglecting
of power absorption
are well established
into a rotational
cross correlations
velocity
Cl21 and
and
155
collision
induced
effects
L141 the Fourier
are, to a first approximation,
transforms
the auto-correlation
obtained
in this way
function:
2
<$(t).fi(o)> <!i >
i.e. of the time derivative
of the dipole moment
between
the far infra red spectrum
because
this type of time correlation
stochastic
differential
the Langevin
computer
Fig.
equation
simulation
equations
molecular
velocity
can be built up from
like the Kramers
equation
C161, and also by molecular
of three direct Fourier
transforms
nematic
solid condition.
correlation
time scale, and are similar
isotropic
and supercooled
functions
in nature
liquids
1.0
are oscillatory
or
dynamics
to their well documented
such as dichloromethane
--c
0
Direct Fourier
a)
Normalised
b)
In the nematic.
Fourier
c)
In the supercooled
Ordinate:
Normalised
Abscissa:
time in ps.
equivalents
which are not
I
I
2
4
transform
glassy
The
I
transforms
Fourier
of the data in
on a picosecond
I
Fig. 3.
contact
dynamics,
C171.
fig. (2) in the isotropic,
Cl01 in regular
function
of motion
and developments
(3) shows the results
rotational
ii. This establishes
and the ensemble
of curves
(2) to (4) of fig. 2.
in the isotropic
solid.
transform
liquid.
PS
156
known to support
point.
liquid crystalline
At longer times
are almost
identical.
equivalent
power absorption
evolution
phases before
the normal
This echoes the similarity
at intermediate
spectra
at low frequency
at low frequency
in fig.
there are two observable
equivalent
Fig.
minima
The three Fourier
at the time origin
in comparison
transforms
in normalised
frequency
is slight, and especially
As with the frequency
phases.
from the correlation
place.
In fact more pronounced
temperature
of a regular
dynamics
spectra,
functions
in the sense that
the
the difference
in time
and isotropic
it would be impossible
to
that any phase changes were taking
molecular
dynamics,
to unity
reflects
in the power spectra.
the nematic
ones are observable
isotropic
In terms of molecular
dependence
so between
domain
discern
in
with one deep inimum in the
in time dependence
(3) shows that in terms of molecular
dependence
The
in fig. (3) are all normalised
so that any difference
difference
in the
(2).
times of up to about 2 ps is distinguishable
fig. (3), that of the two liquids being more oscillatory,
solid.
freezing
(from about 3 to 5 ps) the three curves of fig. (3)
the
liquid, e.g. dichloromethane
specifically
the long axis of fig. (l), the conclusions
simply by changing
torsional
oscillation
1111.
about
to be drawn from fig. (3) are as
follo"s.
i)
In the isotropic
almost
identical
responsible
for the isotropic nematic
multi-molecule
dynamics
liquid at 357 K the torsional
to those of the nematic
level.
The minute
on the single molecule
orders of magnitude
is the ability
director
in the dielectric
involving
ii)
in observable
phenomena.
the cooperative
motion
therefore
of millions
cross-correlations
identical
to that of auto-correlations
dynamics,
evolve primarily
behaviour
of individual
It is therefore
absorption
possible
with Langevin
by six or seven
birefingence.
to attempt
equations
of the nematic
This is manifest
are therefore
phenomena
of inter-molecular
region
is very similar or
such as that of i;. In other words,
i.e. picosecond
from a consideration
molecules,
to
of this
of molecules.
in the far infra-red
of the far infra-red,
on the
acting cooperatively
[9,101, which
are
oscillatory
A well known example
that the time dependence
dynamical
the dynamics
the torsional
field to cause alignment
macroscopic
loss and permittivity
It is probable
between
or more molecules
observable
dynamics
that the factors
phase change are to be defined
difference
of a weak electric
resulting
This means
level must be amplified
by a million
produce macroscopically
oscillatory
at 311 K.
and sub-picosecond
of the torsional
such as the one illustrated
to describe
[16-201.
oscillatory
in fig. (1).
the far infra-red
power
157
DESCRIPTION
USING A SIMPLE LANGEVIN
One of the simplest
Ii;(t) +
The equivalent
at
=
Kramer-s equations
for the conditional
associated
(2)
probability
density.
(1).
friction
energy V is a "minimal"
potential
a
by Reid C181.
are the e,B and y parameters
1
so that with an approximate
the complete
spectrum
the friction
coefficient
absorption
[211.
are available,
in fig. (2) can be matched
The parameters
height Vo.
frequency
only the results
In the meantime
involved
moment
By "complete"
dielectric
of the dielectric
the power absorption
from eqn. (2) as shown in fig. (4) with the parameters
A more detailed
dielectric
dynamics
dielectric
functions
theoretical
by Moscicki
and lyotropic
with experimental
study of the complete
loss will
spectra
of fig.
(3) can
shown in
predictions
loss, and it will be interesting
these predictions
data will be available
developed
nematogens
correlation
In this figure are also shown the theoretical
of maximum
spectrum
of fig. (2) on the power
to the rotational
the frequency
I
loss and power
equivalent
future work to compare
in
:
of inertia
be matched
the figure.
velocity
The
of Raid C181, defined by
but future measurements
in due course.
[10,201.
in terms of only two free parameters,
to terahertz
At present
in two dimensions,
can be solved by standard
of the effective
B and the barrier
absorption
of
by
;
can be expressed
we mean the zero frequency
be available
knowledge
moment
described
of the true mean inter-
equation
C18,201 and the spectrum
as first described
motion
displacement
representation
g,’ ; y = 2(1>)
=
librational
and W a Wiener process
The Kramers
energy.
differencing
this comparison
Here I is the effective
@ is the angular
coefficient
potential
approximately
(1)
is:
molecular
differential
C16, 18-201 is:
-
with the two dimensional
equation
B the rotational
W(t)
equation
v’ ap
IFS=
as-
the Langevin
V'(C)
class of Langevin
-v. cos 26)(t)
=
i, a0
ap+
inertia
+
1@(t)
v
relevant
EQUATION
of
in
data.
far infra-red
and
in future work with the theory of long rod
[91.
At present,
this theory
liquid crystals with dipole moments
is designed
roughly
for
158
I
I
I
1
I
I
a
E’
C
0. 25 -
,
O-
‘5-
OO
159
parallel
to the long axis, but could be adapted
described
i.e. with
in this paper,
for the type of molecule
the dipole accurately
perpendicular
to the
long axis.
ACKNOWLEDGEMENTS
The Nuffield
Foundation
for a "Small Grant"
thanked
and Leverhulme
and a Fellowship
Trust are thanked respectively
S.E.R.C.
for the award of an Advanced
Dr. Moscicki
The University
to MWE.
for the award to MWE of the Pilcher
Senior Fellowship
Fellowship
is
and the
The visit of
to GJE.
was funded by the award from the Nuffield
of Wales
Foundation.
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M.W. Evans,
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and (in part) M. Davies
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B.J. Bulkin, Helv.Chim.Acta,
J. Sciesinski
M.W. Evans,
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~01.63,
Fig. 4.
a)
Dielectric
loss curves from the Langevin
-40
D=lOTHz;
gm cm2;
= 100 x 10
1) I
2)
Power
absorption
a) log (f/THz)
b) ;/cm-'.
Poley absorption
with the parameters
a) Dielectric
at THz frequencies.
of fig (4a, curve 1).
loss
b) Power absorption
Abscissa:
y=70THz.
loss curve at GHz frequencies.
P = peak of the far infra-red
Ordinate:
equation.
D=30THz.
D = peak of the dielectric
b)
1973.
1972.
coefficient
in neper cm
-1
.
ed.
Lyotropic,
160
M.W. Evans,
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(Wiley/Interscience,
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10
M.W. Evans, G.J. Evans, W.T. Coffey and P. Grigolini,
11
M.W. Evans, J.K. Vij, C.J. Reid, G.3. Evans,
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Ref. 10, chapt. 3.
13
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ref. 10, chapt. 11.
15
M.W. Evans and G.J. Evans in ref. 9,p. 377 ff.
16
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Approaches
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C.J. Reid, Mol. Phys., 49,(1983),331.
19
M.W. Evans, Phys.Rev.
20
W.Y. Coffey,
Motion",
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18
21
Problems
1985).
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Simulation",
to Stochastic
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1975).
14
D. Fincham
Adv.Mol.
22 (1982) 79.
12
17
"Molecular
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"Development
and Application
of the Theory
of Brownian
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