Features of previtamin D cis-trans isomerization in the nematic LC matrices: Orientation and cholesteric order effects

Article abstract of DOI:10.1080/15421400590955361

For the first time 7-dehydrocholesterol (7-DHC, provitamin D₃) photoisomerization was studied in nematic LC matrices (ZLI-1695, ZhK-805) in comparison with ethanol solution using UV absorption spectroscopy. Dissolution of chiral 7-DHC molecules in the nematic LCs induces cholesteric phase, and dramatic effect of the cholesteric pitch value on the spectral kinetics has been revealed. Significant increase in trans-isomer accumulation was observed as the cholesteric pitch was reduced from 2200 to 25 mkm. Noticeable effect of the LC orientation has been observed as well.

Full text of DOI:10.1080/15421400590955361

Mol. Cryst. Liq. Cryst., Vol. 434, pp. 325=[653]–332=[660], 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590955361
Features of Previtamin D Cis-Trans Isomerization in the
Nematic LC Matrices: Orientation and Cholesteric Order
Effects
I. Gvozdovskyy
T. Orlova
I. Terenetskaya
Institute of Physics, National Academy of Sciences of Ukraine, Kiev,
Ukraine
For the first time 7-dehydrocholesterol (7-DHC, provitamin D3 ) photoisomeriza-
tion was studied in nematic LC matrices (ZLI-1695, ZhK-805) in comparison with
ethanol solution using UV absorption spectroscopy. Dissolution of chiral 7-DHC
molecules in the nematic LCs induces cholesteric phase, and dramatic effect of
the cholesteric pitch value on the spectral kinetics has been revealed. Significant
increase in trans-isomer accumulation was observed as the cholesteric pitch was
reduced from 2200 to 25 mkm. Noticeable effect of the LC orientation has been
observed as well.
Keywords: induced cholesteric; photochemistry in organized media, previtamin D cis-
trans isomerization, UV absorption spectroscopy
INTRODUCTION
Photomodulation of orientations of liquid crystals (LCs) induced by
photochemical reactions of doped photochromic molecules is of great
current interest. From the other side, employment of liquid-crystalline
media to explore the influence of anisotropic environments on thermal
and photochemical reactions of dopant molecules has attracted
considerable attention [1].
A special thanks are expressed to Dr. W. Becker (Merck) for his contribution of LC-
1695. Dr. Yu. Kuriouz is thanked for technical assistance, and Prof. Yu. Reznikov - for
helpful discussions. The STCU is acknowledged for support of this research (Project
Gr-50).
Address correspondence to I. Terenetskaya, Institute of Physics, National Academy
of Sciences of Ukraine 03028, Kiev-28, Ukraine. E-mail: teren@iop.kiev.ua
325=[653]

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I. Gvozdovskyy et al.
Here we discuss the effect of LC microenvironment on the photoi-
somerization of 7-dehydrocholesterol (provitamin D3) that underlies
the complex network of vitamin D synthesis. As is known, in solution
UV irradiation of provitamin
D within its absorption band
(240–315 nm) yields rotationally flexible previtamin D that further
undergoes side photoconversions among which cis-trans isomerization
into tachysterol is the most efficient one (ucisꢀtrans ¼ 0:48) [2]. In view
of the structural requirements, it is assumed that cZc-Pre conformers
are precursors of ring-closed products whereas tZc-Pre conformers are
predominantly undergo cis-trans photoisomerization [3,4] (Fig. 1).
Studies of provitamin D photoisomerization in heterogeneous media
established pronounced effect of microenvironment on previtamin
D cis-trans photoisomerization that is coordinated with the principle
of ground state conformation control and reflects the reaction
medium effect on the conformational equilibrium of previtamin D
molecule [5].
FIGURE 1 The reaction scheme of vitamin D synthesis including two planar
previtamin D conformers [3]: Pro – provitamin D, Pre – previtamin D,
T – tachysterol, L – lumisterol, Tox – toxysterols, D – vitamin D.
R ¼ C8H17 – Vitamin D3 series, R ¼ C9H17 – Vitamin D2 series. Numbers with
arrows represent the photoconversion quantum yields.

Features of Previtamin D Cis-Trans Isomerization
327=[655]
Significant increase of the rate constants for the previtamin D3 ,
vitamin D3 thermoreaction was observed in the cholesteric liquid-
crystalline mixture in comparison with hydrocarbon solvents [6]. A
possible explanation involved microscopic interactions which shifted
conformational equilibrium in favor of cZc-Pre conformers that are
more cholesteric-like than is tZc-Pre conformer and, therefore, may
‘fit’ better into the cholesteric mixture [6].
MATERIALS AND METHODS
Irradiation of 7-dehydrocholesterol (Fluka) in ethanol (C ꢁ 10ꢀ4 mol=l)
and in the LC matrices was carried out at t ¼ 20ꢂC using the BUV-30
lamp (kirr ¼ 254 nm). Ethanol solution of 7-DHC was irradiated in
standard quartz cuvette (d ¼ 1 cm), and the LC mixtures were irra-
diated in quartz cells (10 ꢃ 35 mm2) of different thickness.
Nematic LC-805 (1:1 mixture of 4-n-butyl- and 4-n-hexyl-trans-
cyclohexan-1-carbonic acids) and ZLI-1695 (Merck) have been selected
as host matrices because of their transparency in the UV range
(250–350 nm) and thermal stability over the interval 10–407ꢂC. The
cholesteric phase with the right-handed macrohelix was induced by
doping LCs with the chiral 7-DHC molecules, and the cholesteric
pitch, P, was changed within 25 ꢄ 2200 mkm depending on the LC cell
thickness (6 ꢄ 135 mkm) and 7-DHC concentration (1.73ꢄ 0.07 wt %) [7].
The UV absorption spectra were recorded within 230–330 nm with the
Perkin & Elmer Lambda-25 spectrophotometer before UV irradiation
and after fixed UV exposures. Further to derive the photoisomer concen-
trations in ethanol the spectra were processed with computer according
to the original program that used known individual spectra of the photo-
isomers in solution as the reference ones [8]. However, the spectrophoto-
metric analysis of the LC samples was impeded because of small shift
and distortion of the 7-DHC spectrum in the LC matrices. Nevertheless,
HPLC analysis of 7-DHC photoisomerization in the LC matrices [6,9]
showed that the same photoproducts were formed as in solution.
In ethanol under irradiation at 254 nm trans-isomer tachysterol is
formed in high concentration (ꢅ70%) and its accumulation results
in the remarkable increase of the maximum absorbance at 282 nm till
the quasi-photostationary state is established [10]. Then this increase
stops and under further irradiation the absorbance gradually
decreases due to the irreversible photoreactions.
That is why the maximum absorbance increase can serve as an
indirect measure of trans-isomer amount in the photoisomer mixture
(on the condition that slow rates of the irreversible photoreactions
remain unalterable in the LC matrix).

328=[656]
I. Gvozdovskyy et al.
RESULTS AND DISCUSSION
A number of the LC samples of different thickness but with the same
initial optical density were irradiated, and the absorption spectra
transformations under UV irradiation were recorded and compared
with those ones in ethanol (Fig. 2). It was observed that in the thick
LC cell (L ¼ 135 mkm) the maximum absorbance increased in lesser
degree than in ethanol (Fig. 2a, b). However, this increase signifi-
cantly ascended in the more thin LC cells (Fig. 2c). At the same time
under prolonged UV irradiation the absorbance gradual descent in
all the LC samples was slower than in ethanol indicating the irrevers-
ible photoreactions retardation (Fig. 3).
Dramatic effect of the cholesteric pitch value on the maximum
absorbance increase is clearly shown in Figure 4 where a dotted-line
indicates this value in ethanol.
Noticeable effect of the LC orientation on the absorbance behavior
was observed as well. As compared with a non-oriented LC sample,
the maximum absorbance elevation was remarkably higher (15%)
for the planar oriented LC cells in which the substrates were coated
with a polydimethylsiloxane (PDMS) alignment layer.
Additional experiment was undertaken with the LC cells of
135mkm thickness assembled with the PDMS coated substrates for
elucidation of the twist effect on the photoreaction kinetics. In this
case three samples were irradiated: the 90ꢂ left-twisted one, the
untwisted one and the 90ꢂ right-twisted one. The time-dependences
of maximum absorbance shown in Figure 5 clearly demonstrate that
the right-side twist induced the maximum absorbance increase in com-
parison with the untwisted one as opposed to the left-side twist. These
data on the artificial twisting and untwisting of the cholesteric macro-
helix also confirm extremely high sensitivity of the spectral kinetics to
the cholesteric pitch value.
According to the Bouguer’s law the absorbance increase might
result from both the increase of the trans-isomer extinction coefficient
e and=or its concentration. Moreover, certain correlations between con-
formation and spectroscopic data are well known: when the nuclear
geometry of a chromophore in the ground state is made more planar,
the absorption spectrum becomes more structured and red shifted,
and the values of e become larger [11]. Just the observed 1 nm red shift
of the 7-DHC absorption spectrum in the LC matrix which increases
up to 4 nm during irradiation suggests that due to specific interaction
of tachysterol molecule with the microenvironment its nuclear confor-
mation becomes more planar and linear in comparison with ethanol
solution.

Features of Previtamin D Cis-Trans Isomerization
329=[657]
FIGURE 2 Transformation of the 7-DHC initial absorption spectrum as a
result of UV irradiation at 254 nm: in ethanol (a), in the LC cells of thickness
L ¼ 135 mkm (b) and L ¼ 22 mkm (c).

330=[658]
I. Gvozdovskyy et al.
FIGURE 3 Evolution of the absorbance at 282 nm with the exposure time in
the LC cells of different thickness (ZLI-1695) in comparison with ethanol (solid
symbols).
FIGURE 4 Dependence of relative maximum absorbance increase in the LC
matrix (ZLI-1695) on the cholesteric pitch value. Dotted line shows the
maximum increase in ethanol.

Features of Previtamin D Cis-Trans Isomerization
331=[659]
FIGURE 5 Evolution of the absorbance at k ¼ 282 nm with the exposure time
in the left-twisted (curve 1), untwisted (curve 2) and right-twisted (curve 3) LC
cells of 135 mkm thickness with the same concentration of 7-DHC
(C ¼ 0.1 wt%). For the untwisted cell the cholesteric pitch P ¼ 765.5 mkm.
From the other side, the increase of tachysterol concentration due to
more efficient cis-trans photoisomerization of previtamin D in the LC
matrix may also account for the absorbance growth. However, the
increase of the cis-trans isomerization quantum yield in more viscous
LC medium is hardly understandable considering its classic mech-
anism of the one-half molecule rotation around the C¼C double bond.
We suggest that the layer structure of the nematic LC is more favor-
able for sliding motion of the two halves of the molecule between the
layers in the plane of the triene p-system.
Therefore we speculate that most likely in the LC matrix a volume-
conserving ‘Hula-Twist’ mechanism of cis-trans isomerisation [12]
becomes the dominant process when classic torsional relaxation is pro-
hibited in view of the free-volume restrictions.
CONCLUSIONS
Substantial difference in the spectral kinetics of provitamin D3 photo-
isomerization in the nematic LC matrices has been revealed as com-
pared with ethanol solution. Remarkable effect of the LC orientation
was observed in parallel with dramatic effect of the cholesteric pitch

332=[660]
I. Gvozdovskyy et al.
value. We hypothesize that specific ordering and solvent-solute spatial
interactions in the induced cholesteric LC may be responsible for the
increasing amount of the rod-like trans-isomer conformers that pos-
sess the least helical twisting power.
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