Photochromic glassy liquid crystals comprising mesogenic pendants to dithienylethene cores

Article abstract of DOI:10.1039/b810955a

Photochromic glassy liquid crystals were synthesized using dithienylethenes as the volume-excluding cores to which liquid crystalline mesogens were chemically bonded through alkyl spacers. Nematic, smectic, and cholesteric glassy liquid crystals were demonstrated with glass transition temperatures above 90 °C and clearing points up to 220 °C without traces of crystallization on cooling or crystalline melting on heating. A monodomain cholesteric glassy liquid crystalline film containing an enantiomeric 2-methylpropylene chiral spacer was characterized as a left-handed helical stack, exhibiting a selective reflection band centered at 686 nm, an orientational order parameter of 0.65 for the quasi-nematic layers, and a combination of reflective coloration with photoswitchable absorptive coloration.

Full text of DOI:10.1039/b810955a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Photochromic glassy liquid crystals comprising mesogenic pendants
to dithienylethene cores†
a
Chunki Kim, Kenneth L. Marshall, Jason U. Wallace and Shaw H. Chen
b
a
ab
*
Received 27th June 2008, Accepted 10th September 2008
First published as an Advance Article on the web 22nd October 2008
DOI: 10.1039/b810955a
Photochromic glassy liquid crystals were synthesized using dithienylethenes as the volume-excluding
cores to which liquid crystalline mesogens were chemically bonded through alkyl spacers. Nematic,
smectic, and cholesteric glassy liquid crystals were demonstrated with glass transition temperatures
above 90 ꢀC and clearing points up to 220 ꢀC without traces of crystallization on cooling or crystalline
melting on heating. A monodomain cholesteric glassy liquid crystalline film containing an enantiomeric
2-methylpropylene chiral spacer was characterized as a left-handed helical stack, exhibiting a selective
reflection band centered at 686 nm, an orientational order parameter of 0.65 for the quasi-nematic
layers, and a combination of reflective coloration with photoswitchable absorptive coloration.
photochromic glassy liquid crystal consisting of a dithienyle-
Introduction
thene core with nematogenic pendants.42 In the resultant
glassy-nematic film, the electronic transition moment of the
photochromic core was shown to be uniaxially aligned, enabling
anisotropic refractive indices to be used for nondestructive
readout. The present study was motivated to generalize the
concept of a photochromic glassy-nematic liquid crystal to both
the smectic and cholesteric types through molecular design,
material synthesis, and characterization of thermotropic and
optical properties.
Organic photochromes exist in two isomeric forms that can be
interconverted by photochemical means for potential applica-
tions in optical memory and photonic switching.1–8 To be useful
in practice, a demanding set of material requirements must be
met, including absence of thermal isomerization, independently
addressable absorption spectra, short response time, high
quantum yields, and excellent fatigue resistance. Of all the
photochromic compounds that have been explored to date,
dithienylethenes appear to be the most promising.8 Crystalline,9–11
liquid crystalline,12–14 and amorphous15,16 material systems have
been reported. To achieve nondestructive and rewritable optical
memory, numerous concepts have been demonstrated on the
basis of refractive index,16 fluorescence,17–26 chiroptical
activity,27–29 infrared absorption,30–33 and photoconductivity.34
To reduce these concepts to practice, materials should most
preferably be processed into a large-area solid film without grain
boundaries to avoid light scattering. To this end, amorphous
dithienylethenes spin-cast on a glass substrate have been repor-
ted with refractive index change serving as nondestructive and
rewritable memory.16
Experimental
Material synthesis
All chemicals, reagents, and solvents were used as received from
commercial sources without further purification except tetrahy-
drofuran (THF) which was distilled over sodium and benzophe-
none. The following intermediates were synthesized according to
the published procedures: 1,3,5-benzenetricarboxylic acid, 1-tert-
butyl ester,41 4-[3-[(tert-butyldimethyl-silyl)oxy]propoxy]benzoic
acid,39 1,2-bis[2,4-dimethyl-5-(4-hydroxy)phenylthiophene-3-
yl]perfluorocyclopentene, viz. 7 in Scheme 1,43,44 1,2-bis(6-iodo-2-
methyl-1-benzothiophene-3-yl)perfluoro-cyclopentene, viz. 8 in
Scheme 2,45 3,5-dimethoxy-phenylboronic acid,46 4-(3-hydroxy-
propoxy)benzoic acid 40-cyanobiphenyl-4-yl ester,41 and (S)-4-(3-
hydroxy-2-methylpropoxy)benzoic acid 40-cyano-biphenyl-4-yl
ester.40 The procedures for the synthesis of intermediates 1–6
along with their analytical data are presented in ESI.† Compound
I has been reported previously,42 and the procedures for the
synthesis and purification accompanied by analytical data for II–
V in addition to intermediates 9 and 10 are presented in what
follows.
Glassy liquid crystals (GLCs) comprise liquid crystalline
mesogens chemically bonded to volume-excluding cores, such as
benzene, cyclohexane, bicyclooctene, cubane, and adamantane,
to prevent crystallization in the cooling process intended for
glass transition.35–41 Because of their ability to form large-area
monodomain thin films with various modes of molecular order
frozen in the solid state, glassy liquid crystals are particularly
useful for optoelectronics. The adoption of a photochromic
volume-excluding core adds new functionalities to glassy liquid
crystals. In
a recent paper, we have reported the first
aDepartment of Chemical Engineering University of Rochester, Rochester,
New York 14627-0166, USA
bLaboratory for Laser Energetics, 250 East River Road University of
Rochester, Rochester, New York 14623-1212, USA
1,2-Bis[6-(3,5-dimethoxy)phenyl-2-methyl-1-benzothiophene-3-
yl]perfluorocyclopentene, 9. Toamixtureof8(1.1g, 1.5mmol), 3,5-
dimethoxyphenylboronic acid (0.61 g, 3.4 mmol), tetrakis
(triphenylphosphine)palladium(0) (Pd(PPh3)4) (0.18 g, 0.15 mmol),
and potassium carbonate (1.1 g, 7.6 mmol) was added degassed
† Electronic supplementary information (ESI) available: Synthesis and
analytical data for intermediates 1–6. See DOI: 10.1039/b810955a
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Scheme 1 Synthesis of compound II. DPTS, 4-(dimethylamino)pyridine/p-toluenesulfonic acid; DCC, N,N-dicyclohexylcarbodiimide; DEADC,
diethyl azodicarboxylate; and TPP, triphenylphosphine.
Scheme 2 Synthesis of compound V without altering stereochemistry at the asymmetric carbon center inherited from (S)-3-bromo-2-methylpropanol.
anhydrous N,N-dimethylformamide (15 mL). After stirring under
argon at 90 ꢀC overnight, the reaction mixture was extracted with
methylene chloride. The extract was treated with brine and dried
over anhydrous magnesium sulfate. After evaporation of solvent
under reduced pressure, the crude product was purified by gradient
column chromatography on silica gel with 0 to 3% ethyl acetate in
hexanes as the eluent to afford 9 (0.49 g, 43%). 1H NMR spectral
data (400 MHz, CDCl3): d (ppm) parallel conformer: 2.54 (s, 6H,
–CH3), 3.84 (s, 12H, –OCH3), 6.46 (s, 2H, aromatics), 6.69 (s, 4H,
aromatics), 7.43 (d, 2H, aromatics), 7.62 (d, 2H, aromatics), 7.82 (s,
2H, aromatics); antiparallel conformer: 2.28 (s, 6H, –CH3), 3.90 (s,
12H, –OCH3), 6.51 (s, 2H, aromatics), 6.78 (s, 4H, aromatics), 7.65
(d, 2H, aromatics), 7.73 (d, 2H, aromatics), 7.91 (s, 2H, aromatics);
parallel:antiparallel ¼ 34:66.
mmol) in anhydrous methylene chloride (10 mL), a 1 M solution of
boron tribromide in anhydrous methylene chloride (5.3 mL, 5.3
mmol) was added dropwise at ꢁ78 ꢀC. The reaction mixture was
warmed to room temperature and stirred under argon overnight
beforedilutionwithadditional methylene chloride for washing with
water. The organic portion was further washed with brine before
drying over anhydrous magnesium sulfate. After evaporating off
the solvent under reduced pressure, the crude product was purified
by gradient column chromatography on silica gel using 0 to 3%
methanol in methylene chloride as the eluent to yield 10 (0.40 g,
1
89%). H NMR spectral data (400 MHz, DMSO-d6): d (ppm)
parallel conformer: 2.54 (s, 6H, –CH3), 6.21 (s, 2H, aromatics), 6.44
(s, 4H, aromatics), 7.45 (d, 2H, aromatics), 7.66 (d, 2H, aromatics),
8.03 (s, 2H, aromatics), 9.34 (s, 4H, –OH); antiparallel conformer:
2.33 (s, 6H, –CH3), 6.25 (s, 2H, aromatics), 6.53 (s, 4H, aromatics),
7.65 (d, 2H, aromatics), 7.68 (d, 2H, aromatics), 8.10 (s, 2H,
aromatics), 9.38 (s, 4H, –OH); parallel:antiparallel ¼ 43:57.
1,2-Bis[6-(3,5-dihydroxy)phenyl-2-methyl-1-benzothiophene-3-
yl]perfluorocyclopentene, 10. Into a solution of 9 (0.49 g, 0.66
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1,2-Bis[2,4-dimethyl-5-[4-[3,5-bis[3-[4-[(4-(2-methylbutyl)biphenyl-
4-yl)oxycarbonyl]-phenoxy]propoxycarbonyl]phencarboxyl]phenyl]-
thiophene-3-yl]perfluorocyclopentene, II. Intermediates 6 (0.70 g,
0.69 mmol) and 7 (0.19 g, 0.33 mmol) plus DPTS (0.010 g,
0.033 mmol) were dissolved in anhydrous methylene chloride
(15 mL) into which DCC (0.15 g, 0.73 mmol) was quickly added.
The reaction mixture was stirred under argon at room temper-
ature overnight. Upon filtering off white solids, the filtrate was
diluted with additional methylene chloride. The organic solution
was washed with 1 M aqueous hydrochloric acid, water and brine
sequentially before drying over anhydrous magnesium sulfate.
The crude product was collected by evaporating off methylene
chloride under reduced pressure and then purified by gradient
column chromatography on silica gel using 0 to 1% acetone in
methylene chloride. Precipitation from a methylene chloride
solution into methanol yielded II (0.43 g, 51%). Anal. calcd: C,
72.53%; H, 5.58%. Found: C, 72.68%; H, 5.54%. 1H NMR
spectral data (400 MHz, CDCl3): d (ppm) 0.86 (d, 12H,
–CH(CH3)–),0.93(t, 12H, CH3CH2–), 1.18–1.46 (m, 8H,
CH3CH2CH(CH3)–), 1.69 (quartet, 4H, –CH(CH3)–), 2.13 (s,
3H, –CH3, antiparallel), 2.15 (s, 3H, –CH3, parallel), 2.37–2.72
(m, 22H, –CH3, –CH2Ar, –COOCH2CH2–), 4.26 (t, 8H,
–CH2OAr–), 4.66 (t, 8H, –COOCH2CH2–), 7.00 (m, 8H,
aromatics), 7.23–7.30 (m, 20H, aromatics), 7.46–7.52 (m, 12H,
aromatics), 7.62 (m, 8H, aromatics), 8.16 (m, 8H, aromatics),
8.96 (s, 2H, aromatics), 9.04 (s, 4H, aromatics); paral-
lel:antiparallel ¼ 1:1.
1,2-Bis[6-[3,5-bis[(R)-3-[4-[(40-cyanobiphenyl-4-yl)oxycarbonyl]-
phenoxy]-2-methyl-propoxy]phenyl]-2-methyl-1-benzothiophene-
3-yl]perfluorocyclopentene, V. DEADC (0.33 g, 1.9 mmol) was
added dropwise at ꢁ78 ꢀC to a solution of (S)-4-(3-hydroxy-2-
methylpropoxy)benzoic acid 40-cyanobiphenyl-4-yl ester (0.70 g,
1.8 mmol), 8 (0.29 g, 0.43 mmol), and TPP (0.50 g, 1.9 mmol) in
anhydrous THF (15 mL). The reaction mixture was warmed to
room temperature and stirred under argon overnight. The solvent
wasthenremovedunderreducedpressure, andthesolidresiduewas
purified by gradient column chromatography on silica gel with 0 to
1% acetone in methylene chloride. Precipitation from a methylene
chloride solution into methanol yielded V (0.15 g, 53%). Anal.
calcd:C,72.77%;H,4.57%;N, 2.59%.Found:C,72.59%;H,4.26%;
1
N, 2.56%. H NMR spectral data (400 MHz, CDCl3): d (ppm)
parallel conformer: 1.18 (d, 12H, –CH3), 2.51 (m, 10H,
–CH2CH(CH3)CH2–, –CH3), 4.10 (m, 16H, –CH2OAr,
–COOCH2–), 6.47 (s, 2H, aromatics), 6.70 (s, 4H, aromatics), 6.96
(d, 8H, aromatics), 7.30 (d, 8H, aromatics), 7.42 (d, 2H, aromatics),
7.60 (d, 10H, aromatics), 7.67 (d, 8H, aromatics), 7.71 (d, 8H,
aromatics), 7.79 (s, 2H, aromatics), 8.12 (d, 8H, aromatics); anti-
parallel conformer: 1.22 (d, 12H, –CH3), 2.22 (s, 6H, –CH3), 2.51
(m, 4H, –CH2CH(CH3)CH2–), 4.10 (m, 16H, –CH2OAr,
–COOCH2–), 6.52 (s, 2H, aromatics), 6.78 (s, 4H, aromatics), 7.00
(d, 8H, aromatics), 7.30 (d, 8H, aromatics), 7.60 (d, 8H, aromatics),
7.67 (d, 10H, aromatics), 7.71 (d, 10H, aromatics), 7.87 (s, 2H,
aromatics), 8.16 (d, 8H, aromatics); parallel:antiparallel ¼ 3:7.
Molecular structures and thermotropic properties
1,2-Bis[2,4-dimethyl-5-[4-[3,5-bis[3-[4-[4-[(R)-(2-methylbutyl)-
biphenyl-4-yl]oxycarbonyl]-phenoxy]propoxy-carbonyl]phencar-
1
Molecular structures were elucidated with H NMR spectros-
boxyl]phenyl]thiophene-3-yl]perfluorocyclopentene,
III.
The
copy in CDCl3 or DMSO-d6 (Avance-400, 400 MHz) and
elemental analysis (Quantitative Technologies, Inc.). Thermal
transition temperatures were determined by differential scanning
calorimetry (DSC; Perkin-Elmer DSC-7) with a continuous N2
purge at 20 mL/min. Samples were preheated to the isotropic
procedures reported for II were followed for the synthesis and
purification of III (0.68 g, 56%). Anal. calcd: C, 72.53%; H,
5.58%. Found: C, 72.40%; H, 5.66%. 1H NMR spectral data (400
MHz, CDCl3): d (ppm) 0.88 (d, 12H, –CH(CH3)–),0.95(t, 12H,
CH3CH2–), 1.20–1.46 (m, 8H, CH3CH2CH(CH3)–), 1.70
(quartet, 4H, –CH(CH3)–), 2.13 (s, 3H, –CH3, antiparallel), 2.16
(s, 3H, –CH3, parallel), 2.36–2.72 (m, 22H, –CH3, –CH2Ar,
–COOCH2CH2–), 4.27 (quartet, 8H, –CH2OAr–), 4.67 (quartet,
8H, –COOCH2CH2–), 7.01 (m, 8H, aromatics), 7.23–7.30 (m,
20H, aromatics), 7.46–7.52 (m, 12H, aromatics), 7.62 (m, 8H,
aromatics), 8.16 (m, 8H, aromatics), 8.96 (s, 2H, aromatics), 9.04
(s, 4H, aromatics); parallel:antiparallel ¼ 1:1.
ꢀ
ꢀ
state followed by cooling at ꢁ20 C/min to ꢁ30 C, furnishing
the reported second heating and cooling scans. Liquid crystalline
mesomorphism was characterized by hot stage polarizing optical
microscopy (DMLM, Leica, FP90 central processor and FP82
hot stage, Mettler, Toledo).
Preparation and characterization of cholesteric glassy liquid
crystalline films
1,2-Bis[2,4-dimethyl-5-[4-[3,5-bis[(R)-3-[4-[(40-cyanobiphenyl-4-yl)-
oxycarbonyl]phenoxy]-2-methylpropoxycarbonyl]phencarboxyl]-
phenyl]thiophene-3-yl]perfluorocyclopentene, IV. The procedures
reported for II were followed for the synthesis and purification
of IV (0.33 g, 79%). Anal. calcd: C, 70.32%; H, 4.37%; N,
2,29%. Found: C, 70.00%; H, 4.02%; N, 2.23%. 1H NMR
spectral data (400 MHz, CDCl3): d (ppm) 1.21 (m, 12H, –CH3),
2.09 (s, 3H, –CH3, antiparallel), 2.13 (s, 3H, –CH3, parallel),
2.32 (s, 3H, –CH3, parallel), 2.36 (s, 3H, –CH3, antiparallel),
2.56 (m, 4H,–CH2CH(CH3)CH2–), 4.08 (t, 8H, –CH2OAr–),
4.50 (m, 8H, –COOCH2CH2–),7.00(m, 8H, aromatics), 7.25–
7.30 (m, 10H, aromatics), 7.44 (d, 4H, aromatics), 7.59–
7.72 (m, 24H, aromatics), 8.12 (m, 8H, aromatics), 8.91(s,
2H, aromatics), 9.00 (s, 4H, aromatics); parallel:antiparallel ¼
1:1.
Optically flat fused-silica substrates (25.4 mm diameter ꢂ 3 mm
thickness, Esco Products; n ¼ 1.459 at 589.0 nm) were spin-
coated with a polyimide (PI) alignment layer (Nissan SUN-
EVER) and uniaxially rubbed. Glassy chiral-nematic films of V
were prepared between two surface-treated substrates with the
film thickness defined by glass fiber spacers as 5 mm (EM
Industries, Inc.). A sample of V was placed between two align-
ment-coated fused silica substrates for heating to 230 ꢀC fol-
lowed by cooling to 207 ꢀC, at which point shearing was applied
to induce alignment. The sample was then annealed for 1 h before
quenching in liquid nitrogen to bypass the cholesteric-to-smectic
phase transition. Transmittance at normal incidence and reflec-
tion at 6ꢀ off normal were measured with unpolarized incident
light using a UV-vis-NIR spectrophotometer (Lambda-900,
Perkin Elmer equipped with a beam depolarizer). Fresnel
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reflections from the air–glass interfaces were accounted for with
a reference cell containing an index-matching fluid (n ¼ 1.500 at
589.6 nm) between two surface-treated fused silica substrates. A
combination of a linear polarizer (HNP0B, Polaroid) and zero-
order quarter waveplate (NQM-100-738, Meadowlark Optics)
was employed to produce left- or right-handed circularly polar-
ized light. Variable angle spectroscopic ellipsometry (V-VASE, J.
A. Woollam Corp.) was performed to determine the pitch length
and refractive index of a glassy-cholesteric film of V, as reported
previously for monodisperse chiral oligofluorenes.48,49 The closed
form of V was obtained by the irradiation of 365 ꢃ 10 nm light
(UV-400, Spectroline) coupled with a glass filter (UG 11 and
NG12, Schott) to photostationary state. Ellipsometry was
employed to characterize the orientational order parameter
governing the quasi-nematic layers in the helical stack of V in
closed form. To induce ring opening, a visible light source (UVP
B-100AP, UVP LLC) coupled with a bandpass filter (577FS10-
50, Andover Corporation) was used for irradiation at 577 ꢃ 5
nm. The photon flux at 365nm was determined by a spectro-
photometer equipped with an optic fiber (USB4000, Ocean
Optics Inc.) and a UVX-36 radiometer (UVP, LLC). The photon
flux at 577 nm was quantified by comparing its intensity with that
at 365 nm, both measured with the spectrophotometer. The
quantum yields of both the ring-closing and ring-opening reac-
tions were characterized using 1H-NMR and UV-vis absorption
spectroscopic techniques for molar extinction coefficient of the
closed form and the extents of photoreactions, respectively.
determined by differential scanning calorimetry. Compound I is
as reported previously,42 and II was synthesized according to
Scheme 1, in which intermediate 7 was prepared with literature
procedures.43,44 Compound III was synthesized following the
same procedures as for II using (S)-2-methyl-1-butanol to
construct the tail group. The procedures for synthesizing the
hybrid chiral-nematic precursor to IV have been reported else-
where.47 Compound V was synthesized according to Scheme 2,
where intermediate 8 was prepared following literature proce-
dures.45
Molecular structures of glassy liquid crystals with
dithienylethene cores under investigation; phase
ꢀ
transition temperatures expressed in C. Symbols: G,
glassy; SA, smectic A; SA*, chiral smectic A; SC*,
chiral smectic C; SX, unidentified smectic; Ch,
cholesteric; I, isotropic.
The heating and cooling scans at ꢃ20 ꢀC/min of I–V preheated
to 250 ꢀC with subsequent cooling to ꢁ30 ꢀC were gathered with
differential scanning calorimetry, and the results are compiled in
Fig. 1 with mesophases identified by polarizing optical micros-
copy. The core-pendant approach to glassy liquid crystals35–41
has generated photoresponsive glassy liquid crystals without
traces of crystallization or crystalline melting upon heating or
cooling. As independent chemical entities, cores and pendants
are crystalline solids or liquid crystals that tend to crystallize, but
the chemical hybrids tend to form morphologically stable glassy
liquid crystals. This property is essential to the preparation of
self-organized solid films without grain boundaries for potential
application to optoelectronics.
Results and discussion
Chart 1 presents the molecular structures of compounds I–V
accompanied by their thermal transition temperatures as
Chart 1 Molecular structures of glassy liquid crystals with dithienylethene cores under investigation; phase transition temperatures expressed in ꢀC.
Symbols: G, glassy; SA, smectic A; SA*, chiral smectic A; SC*, chiral smectic C; SX, unidentified smectic; Ch, cholesteric; I, isotropic.
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Fig. 1 Differential scanning calorimetric heating and cooling scans of
I–V at 20 ꢀC/min of samples preheated above their clearing points
followed by cooling to ꢁ30 ꢀC. Symbols: G, glassy; SA, smectic A; SA*,
chiral smectic A; SC*, chiral smectic C; SX, unidentified smectic; Ch,
cholesteric; I, isotropic.
With a cyano group at the end of all the nematic pendants,
compound I exists as an unidentifiable smectic (SX) fluid, as
shown in Fig. 2a, from the glass transition temperature (Tg) of
102 ꢀC to the SX-to-N transition at 176 ꢀC. Upon further heating,
a nematic fluid identified by marbled textures in Fig. 2b exhibits
a clearing point (Tc) at 219 ꢀC. Substitution of the terminal cyano
groups with racemic and enantiomeric 2-methylbutyl groups
exhibit the focal conic fan textures, as shown in Fig. 2c and d,
respectively, which are characteristic of smectic A (SA) and chiral
smectic A (SA*).50 The assignment of SA and SA* to II and III,
respectively, is dictated by the stereochemical nature of the tail
group. The SA* mesophase can be further substantiated by the
electroclinic effect.51 Replacing propylene with enantiomeric 2-
ꢀ
methylpropylene spacers produced IV with a Tg at 109 C and
a Tc at 160 ꢀC, between which there is a SC*-to-Ch transition at
142 ꢀC, where SC* and Ch represent chiral smectic C and
cholesteric mesomorphism, respectively. As shown in Fig. 2f, the
Ch phase is identified by oily streaks. For this particular
compound, it was difficult to identify Sc* in its naturally
occurring state under polarizing optical microscopy because of
the high melt viscosity. Therefore, the sample was sheared at 140
ꢀC to induce alignment. In the thick area, the images shown in
Fig. 2e resemble broken focal conic fan textures, but are too
small to be definitive. In the thin area where helical rotation is
suppressed by surface forces, shear-induced alignment with point
defects was observed, as shown in Fig. 2g and h with the device
shearing direction oriented along and at 45ꢀ with respect to the
polarizer (or analyzer), respectively. Note the birefringence color
change upon rotation, indicating unidirectional alignment. The
white dots in the micrographs are the point defects, commonly
observed in Sc* samples in the bookshelf geometry which is used
in surface stabilized ferroelectric liquid crystal displays. Attempts
to process IV into chiral smectic C and cholesteric glassy liquid
crystalline films by thermal annealing at appropriate temperature
regimes failed to produce glassy liquid crystalline films without
disclinations for optical characterization.
Fig. 2 Polarizing optical micrographs of I–V between crossed polarizers
for mesophase identification.
presumably because of the improved linearity of the photo-
chromic core. Specifically, a Tg at 122 ꢀC and a Tc at 218 ꢀC were
observed with an intervening SC*-to-Ch phase transition at
206 ꢀC, as ascertained by broken focal conic fans and oily streaks
shown in Fig. 2i and j, respectively. The higher temperature range
over which cholesteric mesomorphism prevails in V than in IV
enabled a disclination-free cholesteric GLC film to be prepared
by taking advantage of the lower cholesteric fluid viscosity
during thermal annealing. The optical properties of the resultant
5 mm-thick cholesteric GLC film of V were characterized. Fig. 3a
shows both reflectance and transmittance of unpolarized incident
light close to the theoretical limit of 50%. As indicted in Fig. 3b,
the cholesteric GLC film with (S)-3-bromo-2-methylpropanol as
the chiral building block selectively reflects left-handed circularly
Fusing thiophene rings with benzene rings to form a dibenzo-
thienylethene core in IV yielded another glassy liquid crystal,
V, characterized by higher phase transition temperatures
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Fig. 3 (a) Transmission and reflection spectra with unpolarized incident light and (b) circularly polarized transmission spectra of a 5 mm-thick
cholesteric GLC film of V between rubbed polyimide films.
Fig. 5 Absorption dichroism extracted from ellipsometric analysis of a 5
Fig. 4 Demonstration of photochromism at room temperature using a 5
mm-thick cholesteric glassy liquid crystal film of V in the presence of
a selective reflection band centered at 686 nm.
mm-thick cholesteric glassy liquid crystal film of V after irradiation at 365
nm to its photostationary state.
absorptive colorations. Furthermore, the absorptive coloration
can be switched on and off by photochemical means.
The film containing the closed dithienylethene ring after UV
irradiation was further characterized by variable angle spectro-
scopic ellipsometry.48,49 This analysis resulted in a helical pitch
length of 439 nm and an average refractive index of 1.564 at
wavelengths longer than 600 nm into the near infrared region, the
product of which is consistent with the selective reflection
wavelength of 686 nm independently observed in Fig. 3.
Absorption dichroism of quasi-nematic layers comprising the
cholesteric GLC film of V also emerged as part of the ellipso-
metric analysis, as plotted in Fig. 5 in which a|| and at represent
absorption coefficients parallel and perpendicular to the quasi-
nematic director, respectively. The absorption dichroic ratio, R
¼ a||/at, is then used to calculate the orientational order
parameter, S ¼ (R ꢁ 1)/(R + 2) ¼ 0.65, a value indicative of
reasonably good order within the helical stack.
polarized incident light while transmitting the right-handed
counterpart without attenuation.
Therefore, the cholesteric GLC film of V consists of a
left-handed helical stack of quasi-nematic layers. By analogy,
a right-handed helical stack is expected of (R)-3-bromo-2-
methylpropanol as the chiral building block in V. Furthermore,
the mixtures of the two cholesteric GLC materials with opposite
handedness at varying ratios will produce films with selective
reflection wavelengths ranging from 686 nm to infinity where
these two enantiomeric cholesteric GLCs are at a 1:1 ratio.
Except for the selective reflection band centered at 686 nm, the
cholesteric GLC film of a pristine sample of V carrying an open
dithienylethene core is transparent down to 400 nm, as shown in
Fig. 4. The dithienylethene core undergoes ring closure upon
irradiation with 20 J/cm2 at room temperature using a UV source
at 365 nm with a 20 nm bandwidth, generating an absorption
peak at 555 nm with a minor perturbation on the selective
reflection band. Exposure to a visible light source at 577 nm with
a 10 nm bandwidth resulted in ring opening to restore the non-
absorbing status. The quantum yields of the ring-closing and
ring-opening photoreactions were evaluated as 0.63 and 0.02,
Conclusion
Glassy liquid crystals are comprised of liquid crystalline meso-
gens chemically bonded to volume-excluding cores. They are
intended to undergo a glass transition through cooling without
encountering crystallization, thereby permitting various modes
of long-range molecular order characteristic of liquid crystalline
fluids to be frozen in the solid state. Instead of inert volume-
excluding cores—such as benzene, cyclohexane, bicyclooctene,
cubane, and adamantane—dithienylethanes were successfully
1
respectively, using the H NMR and UV-vis absorption spec-
troscopic techniques. As demonstrated in Fig. 4, the ring-closing
and -opening cycle can be induced reversibly in the solid state. In
a nutshell, the open form provides selective coloration alone, and
the closed form furnishes a combination of reflective and
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J. Mater. Chem., 2008, 18, 5592–5598 | 5597

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incorporated to introduce photochromism into nematic, smectic,
and cholesteric glassy liquid crystals. These glassy liquid crystals
have glass transition temperatures above 90 ꢀC and clearing
points up to 220 ꢀC without traces of crystallization on cooling or
crystalline melting on heating. A monodomain cholesteric glassy
liquid crystalline film was prepared by thermal processing for the
characterization of its optical properties. With enantiomeric 2-
methylpropylene as the chiral spacer between the benzene core
and the 40-cyanobiphenyl-4-yl benzoate nematogen, a left-
handed quasi-nematic stack was identified with a selective
reflection band centered at 686 nm. In addition, ellipsometric
analysis resulted in a helical pitch length of 439 nm and an
average refractive index of 1.564 beyond 600 nm into the near
infrared region, the product of which is consistent with the value
measured by UV-vis spectrophotometry. As part of the ellipso-
metric analysis, an orientational order parameter of 0.65 was also
obtained for the quasi-nematic layers comprising the cholesteric
stack. The feasibility of selective coloration coupled with pho-
toswitchable absorptive coloration was demonstrated using the
photochromic cholesteric glassy liquid crystalline film.
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Acknowledgements
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The authors thank Stephen D. Jacobs of the Laboratory for
Laser Energetics at University of Rochester for helpful discus-
sions and technical advice. They are grateful for the financial
support provided by Eastman Kodak Company, and the New
York State Center for Electronic Imaging Systems. JUW
acknowledges a Frank J. Horton Fellowship administered by
LLE. Additional funding was provided by the Department of
Energy Office of Inertial Confinement Fusion under Cooperative
Agreement No. DE-FC52-08NA28302 with LLE and the New
York State Energy Research and Development Authority. The
support of DOE does not constitute an endorsement by DOE of
the views expressed in this article.
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