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Optical Properties of Cholesteric Liquid Crystals.

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Optical Properties of Cholesteric Liquid Crystals
By Horst S[esemever“’
Nematic and cholesteric liquids have largely similar
structures: they show the parallelism o f the molecular
axes that characterizes all liquid crystals; the molecular
centers of gravity a r e distributed statistically. Cholesteric phases, which are formed only by optically active,
formanisotropic molecules, have additionally a helical
arrangement of t h e molecular axes in relation t o the
preferred axis, which causes striking optical properties:
namely an optical rotatory dispersion 1 0 R D ) o f extremely high amplitude, selectivc reflection of light
within a narrow spectral region, and a circular dichroism ( C D ) of almost 100 ‘10.
Since chiral centers in the individual molecules are
responsible for formation of the helical structure, it
is t o be expected that chiral molecules which d o not
themselves form liquid crystals will induce a helical
structure in nematic phases. Such a phase transformation nematic + cholesteric has been demonstrated for
various systems, where the chirality of the added nonmesomorphic molecules may be caused by asymmetric
carbon atoms o r by dissymmetry of the whole molecular
skeleton“’. T h e extent of the induced optical activity
depends o n t h e strength of t h e interactions between
the dissolved molecules and the solvent molecules. T h e
position o f the induced C D bands is strongly temperature-dependent, as is the case in “normal” cholesteric
phases. T h e concentration-dependence of the O R D
amplitude reaches a maximum at CQ. 2mol-%. T h e
decrease in this effect at increasing concentrations can
be explained qualitatively as follows: Since the optically
active molecules d o not show strong anisotropy, they
disturb the parallel arrangement of t h e rod-like solvent
molecules, so that eventually the long axes are distributed statistically in space,whereby the fundamental cause
of the appearance of helical structure is abolished.T h e induced optical rotation is larger by a factor of
lo3 than the molecular rotation of the dissolved nonmesomorphic molecules in isotropic solution. Provided
that the interaction between the dissolved molecules
and the nematic solvent suffices, the effect can be used
to detect small amounts of optically active substance.
If achiral molecules are dissolved in cholesteric liquid
crystals, a Cotton effect is induced within their absorption bands, this overlying the anomalous rotatory dispersion of the cholesteric phase and having the same
sign as the latter. T h e C D is smaller than that of
the cholesteric phase ( < 10%). T h e effect disappears
above the clearing point. Further, since the molar amplitude of the O R D curves is extremely high, a chiral
distortion of the chromophores by solvation can be
excluded. T h e helical structure of the solvent molecules
is in fact transferred t o the spatial distribution of the
preferred axes o f the dissolved molecules, so that the
incident light “sees” a chiral system of coupled chromophores‘”.
Lecture at Miinster, Germany, on March 27. 1972 [VB 342 IEI
German version: Angew. Chem. 84,720 (1972)
[*I Prof. Dr. H. Stegemeyer
Stranski-Institut fur Phvsikalische Chemie
der Technischen Universitat
1 Berlin 12. Strasse des 17. Juni 135 (Germany)
[ l l H . Stegemever and K.-J. Mainusch, Naturwissenschaften 58.
599 (1971).
[21 K.-J. Mainusch and H . Stegemeyer, Z . Phys. Chem. N. F.
77, 210 (1972).
Angew. Chem. Internat. Edit. / Vol. I ! (1972) 1 No. 8
Kinetics and Mechanism of the Formation
of Transition-Metal Complexes
BY Harrmut Diebler“]
Complex formation with simple ligands in aqueous
solution occurs in general by a dissociative (or S N ~ )
mechanism in which replacement of the first H2O molecule in the innermost coordination sphere o f the hydrated metal ion represents the rate-determining step. The
rate constant k, assigned to this reaction step is characteristic of a given metal ion and can be determined
from the bimolecular rate constant kf for complex formation if the formation o f the outer-sphere complex
(stability constant KO)is taken into account“’:
F o r metal ions with closed outer electron shells the
values of ki depend mainly o n the radius and charge
o f the metal ion as well as o n the coordination number.
With transition-metal ions, however, effects due t o specific configurations o f the only partly filled 3d electron
shell a r e also observed. T h u s a contribution of crystal
field stabilization energy to the activation energy o f
t h e substitution process leads t o a decrease in the reaction rate of ions with d 3 (V2+, C r 3 + ) and d8 (Ni2+)
configurations, whereas the Jahn-Teller distortion
occurring with d4 and d9 configurations gives rise t o
extremely high ki values (ca. 1O9 s- for Cr2+ and C u 2 +).
T h e dissociative nature o f the substitution mechanism
apparently still holds for ions with a Jahn-Teller distortionc2I.
’
However, numerous reactions are known which d o
not fit into this simple scheme. In the case of trivalent
metal ions, hydrolysis often prevents an unambiguous
interpretation of the kinetics of complex formation
with basic ligands (indistinguishability of the reaction
and
MOH2+
pathways
M 3 + + L”- +ML(3-n)+
+ HL(n-1)M L(3 ?d + , where M =metal and L = l i gand)[3’. Nevertheless, there is evidence that for some
trivalent metal ions the substitution mechanism has
associative (or S N ~ character.
)
This is indicated, for
instance, by some o f the experimentally determined
activation parameters for ligand exchange and by the
ligand-dependence of the substitution rate observed
with V3+14’. It might be that the availability of a n
empty tzs-orbital in the case of V3+ (and also of Ti3+)
facilitates the formation of a transition state with an
increased coordination number.
~
Reactions with more complicated ligands frequently
lead t o deviations from “normal” behavior. Complex
formation o f @amino acids with metal ions is usually
distinctly slower than that with the corresponding acompounds. Obviously it is not the replacement of
the first water molecule but rather the ring closure
step that is rate-determining in the reactions with the
B-amino acids[51. Particularly drastic decreases in the
reaction rate a r e found with some multidentate macrocyclic ligands, which owing to a lack o f flexibility d o
not allow a rapid stepwise replacement of solvate groups
from the metal atom. Very rigid ligands (porphyrins)
probably reauire simultaneous removal of several solvate groups161.-Monoprotonated bidentate ligands
also often react slower by orders of magnitude than
the corresponding unprotonated ligands; here the rate-
[*]
Dr. H. Diebler
Max-Planck-Institut fur Biophysikalische Chemie
34 Gottinsen-Nikolausberg, Postfach 968 (Germany)
73 1
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crystals, cholesterol, properties, optical, liquid
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