Regularities of reversible radical dissociation in liquid crystals

Article abstract of DOI:10.1023/A:1023462917109

The effect of liquid-crystalline ordering of alkylcyanobiphenyls on the thermodynamic parameters of reversible dissociation of the dimeric 2,2′-bis[2-(p-dimethylaminophenyl)indane-1,3-dione] to the 2-(p-dimethylaminophenyl)indane-1,3-diyl radicals was found. The angular dependence of the ESR spectra of the radicals in the smectic mesophase is related to their orientation along the liquid crystal director.

Full text of DOI:10.1023/A:1023462917109

Russian Chemical Bulletin, International Edition, Vol. 52, No. 2, pp. 391—393, February, 2003
391
Regularities of reversible radical dissociation in liquid crystals
A. P. Vorotnikov
N. M. Emanuel Institute of Biochemical Physics, Russian Academy of Sciences,
4 ul. Kosygina, 119991 Moscow, Russian Federation.
Fax: +7 (095) 137 4101. Eꢀmail: lqc@sky.chph.ras.ru
The effect of liquidꢀcrystalline ordering of alkylcyanobiphenyls on the thermodynamic
parameters of reversible dissociation of the dimeric 2,2´ꢀbis[2ꢀ(pꢀdimethylaminophenyl)indaneꢀ
1,3ꢀdione] to the 2ꢀ(pꢀdimethylaminophenyl)indaneꢀ1,3ꢀdiyl radicals was found. The angular
dependence of the ESR spectra of the radicals in the smectic mesophase is related to their
orientation along the liquid crystal director.
Key words: liquid crystals, radicals, ESR spectroscopy, reversible dissociation, equilibrium
constants.
Chemical reactions in liquid crystals (LC) and related
molecular organized systems are determined by the comꢀ
plicated dynamics of reacting molecules in the highly
ordered matrix because both diffusion of the reactants
and the chemical act occur in an anisotropic medium.
Liquidꢀcrystalline ordering1 results in selfꢀorganization
of molecular ensembles with correlation of conformaꢀ
tional and orientational degrees of freedom of the associꢀ
ated LC molecules. The interaction of molecules of the
liquidꢀcrystalline matrix with structurally isomorphous
molecules of the dissolved reactants on going to the
mesophase changes the thermodynamic parameters and
kinetic regularities of chemical reactions2,3 and results in
an additional anisotropic component of the spectroscopic
characteristics.4,5 "Molecular recognition" in molecular
organized systems,6 which is similar to the "host—guest"
interaction in crystals, can depend on the phase state of
the medium. This effect is of particular significance for
the equilibrium monomer—dimer process when the reacꢀ
tant molecules differ strongly in the spatial structure and
electronic state. In liquidꢀcrystalline mesophases, the
equilibrium shifts toward the dimer,7 and the efficiency of
radical escape from the cage is reduced.3
In this work, we studied the regularities of reversible
dissociation8,9 of the dimeric 2,2´ꢀbis[2ꢀ(pꢀdimethylamiꢀ
nophenyl)indaneꢀ1,3ꢀdione] (1) to two 2ꢀ(pꢀdimethylꢀ
aminophenyl)indaneꢀ1,3ꢀdiyl radicals (2) (Scheme 1) in
different liquidꢀphase states: nematic (NLC), 5CB—smecꢀ
tic (SLC), and isotropic liquid of 5CB — 4ꢀnꢀpentylꢀ4´ꢀ
cyanobiphenyl (3a) (TNI = 306 К) and 8CB — 4ꢀnꢀoctylꢀ
4´ꢀcyanobiphenyl (3b) (TNI = 315 К, TSN = 306 К).
The data obtained were compared with the results8 for
Nꢀ(pꢀmethoxybenzylidene)ꢀpꢀbutylaniline (MBBA).
Scheme 1
Experimental
Solutions with the dimer concentration [1] ≈ 10–3 mol L–1
in a quartz tube 4 mm in diameter were placed in the resonator
of a RADIOPAN SE/Xꢀ2547 radiospectrometer. ESR spectra
were detected at 293—353 K with the error in determination of
the absolute and relative concentrations of radicals ≤20 and 10%,
respectively. The accuracy of temperature maintenance in a
measuring cell was 1 K. Liquidꢀcrystalline materials were used
without additional purification.
Results and Discussion
Reversible dissociation of the dimer in the ordered LC
mesophase of 3a,b is characterized by an increase in the
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 374—376, February, 2003.
1066ꢀ5285/03/5202ꢀ391 $25.00 © 2003 Plenum Publishing Corporation

392
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 2, February, 2003
A. P. Vorotnikov
logK + 8
1
2
TNI
2.0
2
1.5
1.0
316
318
H/mT
Fig. 3. ESR spectra of radical 2 in the smectic mesophase of 3b
at 296 K. The mesophase was prepared by cooling of the sample
in the magnetic field from the isotropic phase in the orientation
parallel (1) and perpendicular (2) to the magnetic field.
TNI
1
absolute value than those for standard isotropic solutions.9
In the ordered SLC phase of 3b prepared by cooling from
the isotropic liquid and NLC phase in a magnetic field,
the formed confocal texture fixes the predominant direcꢀ
tion of LC molecular layers arrangement and also incorꢀ
porated radicals 2. A remarkable difference in their ESR
spectra for the initial and orthogonal positions of the
sample (Fig. 3) at a small width of the components of the
ESR spectra proves that the radicals with a great persisꢀ
tent length are incorporated into a system of the rigid
πꢀconjugated fragments of alkylcyanobiphenyl. The anꢀ
gular dependence of the ESR spectra of the radicals in
SLC of 3b and additional splitting of the HFC compoꢀ
nents in the nematic mesophase of 3a,b are caused by the
anisotropic character of HFC interaction on the nitrogen
atom in the radical.
Since the polarities of the terminal groups in alkylꢀ
cyanobiphenyls differ and due to the rigid aromatic fragꢀ
ments of the mesogenic molecules, strong intermolecular
interactions appear to produce the "tailꢀtoꢀhead" molecuꢀ
lar pairs.10 Orientational coordination of the molecules of
the liquidꢀcrystalline matrix with the strongly polar CN
group to form donorꢀacceptor pairs with antiparallel arꢀ
rangement of dipoles10 (indicated by the strongly anisoꢀ
tropic HFC on the N atom in the radical) plays an imporꢀ
tant role for the radical. A similar effect increasing the
temperature of the phase transition occurs in 3a when
4ꢀaminobiphenyl is introduced.7 On the contrary, at high
orders of the liquidꢀcrystalline matrix, incorporation of a
bulky dimer molecule into the uniaxial NLC phase is
sterically hindered. The "compensation effect" found in
the logK—1/T plot (see Fig. 1) for NLC phase of 3a is due
to a sharp change in the character of the reactant—matrix
interaction of the "host—guest" type in a narrow temperaꢀ
ture interval. In standard isotropic liquids, the enthalpy of
dimer dissociation is ∼80 kJ mol–1, and the K values obꢀ
tained by extrapolation of K in the isotropic LC phase for
3a should more than twofold increase in NLC. The effecꢀ
tive entropy of dissociation decreases most significantly
3.0
3.1
3.2
3.3
T –1•103/K–1
Fig. 1. Temperature dependences of the equilibrium conꢀ
stants (K ) of the radical—dimer interaction in MBBA (1)
and 3a (2).
efficiency of radical escape from the cage compared to
isotropic solutions, which dramatically differs from the
earlier7 observed effects. For the dimer in the nematic
mesophase of 3a, the equilibrium constant of dissociation
K = [2]2/[1] is two orders of magnitude higher than that
in acetonitrile (6.7•10–7 and 4.4•10–9 mol L–1 at 295
and 293 К,8 respectively) and remains almost unchanged
in the whole temperature region of NLC phase (Fig. 1).
A ∼1.5ꢀfold decrease in K, which is reliable at the 10%
relative error of determination of [2], in the region of
phase transition to the isotropic liquid is accompanied by
the disappearance of the additional anisotropic splitting
of the HFC components in the ESR spectra of the radical
(Fig. 2). Solvate stabilization of the radical is retained
even in the isotropic phase of 3a because the enthalpy and
entropy of dissociation of the dimer (46 5 kJ mol–1 and
23 10 J mol–1 K–1) to radicals are much lower by the
1
2
316
318
H/mT
Fig. 2. ESR spectra of radical 2 in the nematic at 295 K (1) and
isotropic at 335 K (2) mesophases of 3a.

The regularities of reversible radical dissociation
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 2, February, 2003
393
on going from the NLC phase to isotropic liquid due to
the disappearance of orientational correlation of the liqꢀ
uidꢀcrystalline environment, which is likely the most imꢀ
portant reason for a decrease in K with temperature. The
elevated efficiency of radical escape in the NLC phase is
explained by the mutual orientation of the radicals unfaꢀ
vorable for the anisotropic liquid environment during reꢀ
combination with formation of the bulky dimer molecule.
In standard liquids or rigid matrices, by contrast, an inꢀ
crease in the medium viscosity sharply decreases the effiꢀ
ciency of radical escape from the reaction cage during
dissociation of the molecules. For alkylcyanobiphenyls K
is more than 15ꢀfold higher8 than that in the nematic
mesophase of MBBA at 295 K, which can be attributed to
the radical solvation. Since the permittivity of LC is much
lower than that of acetonitrile, the observed effect is reꢀ
lated to the orienting influence of the LC matrix for the
donorꢀacceptor radical—molecule interaction in the LC
environment. These results demonstrate that the liquidꢀ
crystalline ordering can exert a substantial effect on the
thermodynamic parameters of processes accompanying
dissociation of the carbon—carbon covalent bond. It is
also noteworthy that the processes are reversible for both
increasing and decreasing temperature and the "temperaꢀ
ture hysteresis" 4,5 in the region of phase transition can
occur at a much higher experimental accuracy taking into
account a low heat of the NLC—isotropic liquid transiꢀ
tions.11
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The author thanks L. M. Pisarenko for kindly preꢀ
sented arylindanedione dimer.
Received March 29, 2002;
in revised form June 21, 2002