Reversible thermal and photochemical switching of liquid crystalline phases and luminescence in diphenylbutadiene-based mesogenic dimers

Article abstract of DOI:10.1021/ja061575k

The synthesis and study of the photo- and thermoresponsive behavior of a series of novel asymmetric mesogenic dimers, consisting of a cholesterol moiety linked to a diphenylbutadiene chromophore via flexible alkyl chains are reported. These mesogenic dime

Full text of DOI:10.1021/ja061575k

Published on Web 05/17/2006
Reversible Thermal and Photochemical Switching of Liquid
Crystalline Phases and Luminescence in
Diphenylbutadiene-Based Mesogenic Dimers
Shibu Abraham,† V. Ajay Mallia,‡ K. Vijayaraghavan Ratheesh,†
Nobuyuki Tamaoki,*,‡ and Suresh Das*,†
Contribution from the Photosciences and Photonics, Chemical Sciences and Technology
DiVision, Regional Research Laboratory (CSIR), TriVandrum, Kerala 695019, India, and
Molecular Smart System Group, Nanotechnology Research Institute, National Institute of
AdVanced Industrial Science and Technology (AIST), 1-1-1, Higashi, Tsukuba,
Ibaraki 305-8565, Japan
Received March 7, 2006; E-mail: sdaas@rediffmail.com; n.tamaoki@aist.go.jp
Abstract: The synthesis and study of the photo- and thermoresponsive behavior of a series of novel
asymmetric mesogenic dimers, consisting of a cholesterol moiety linked to a diphenylbutadiene chromophore
via flexible alkyl chains are reported. These mesogenic dimers possess the combined glass forming
properties of the cholesterol moiety and the photochromic and luminescent properties of the butadiene
moiety. Photoinduced cis/trans isomerization of the butadiene chromophore in these materials could be
utilized to bring about an isothermal phase transition from the smectic to the cholesteric state. By
photochemically controlling the cis/trans isomer ratio, the pitch of the cholesteric could be continuously
varied making it possible to tune the color of the film over the entire visible region, and the color images
thus generated could be stabilized by converting them to N* glasses. These materials were also polymorphic,
exhibiting two crystalline forms possessing distinctly different fluorescence properties. The ability to thermally
switch these materials from one crystalline form to the other in a reversible manner also makes them
useful for recording fluorescent images.
Introduction
come into the mainstream in optical disk technology. The fast
and rewritable amorphous to crystal phase transitions of
Low molecular mass materials containing stimuli responsive
molecules that exist in two stable and reversible chemical or
physical forms have attracted much attention1 because of their
potential applications in optical memories,2 display devices,3
and holographic data storage.4 With the recent advances in
multimedia and development of digital versatile disk random
access memory (DVD-RAM), phase change materials have
inorganic mixtures by exposure to laser beams are currently
being exploited in the fabrication of commercially available
DVDs.5 Liquid crystals (LCs) form an important class of phase
change materials wherein the supramolecular organization is
controlled by a variety of weak noncovalent forces.6 LC phases
are highly susceptible to minor changes in their microenviron-
ment, since the cooperative behavior of LC molecules and their
long-range order help to amplify relatively weak processes
occurring at the molecular level into macroscopic phenomena.7,8
Thus, light induced phase transitions can be brought about in
† Photosciences and Photonics.
‡ Molecular Smart System Group.
(1) (a) Molecular Switches. Feringa, B. L., Ed.; Wiley-VCH: Weinheim, 2001.
(b) Yagai, S.; Karatsu, T.; Kitamura, A. Chem.sEur. J. 2005, 11, 4054-
4063. (c) Irie, M.; Fukaminato, T.; Sasaki, T.; Tamai, N.; Kawai, T. Nature
2002, 420, 759-760. (d) Kato, T. Science 2002, 295, 2414-2418. (e)
Vollmer, M. A.; Clark, T. D.; Steinem, C.; Ghadiri, M. R. Angew. Chem.,
Int. Ed. 1999, 38, 1598-1601. (f) Li, J.; Schuster, G. B.; Cheon, K.-S.;
Green, M. M.; Selinger, J. V. J. Am. Chem. Soc. 2000, 122, 2603-2612.
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10.1021/ja061575k CCC: $33.50 © 2006 American Chemical Society

Thermo- and Photoresponsive Mesogenic Dimers
A R T I C L E S
LCs doped with small amounts of photoresponsive molecules,9,10
and such systems find applications in the field of molecular
electronics for development of high-speed rewritable recording
devices,11 optical data storage,12 patterned nanostructures,13
fluorescence imaging,14 and band gap materials.15 Chiral nematic
(N*) LCs are especially attractive from this point of view, since
in these systems the molecules self-organize into helically
ordered structures which lead to selective reflection of light,
depending upon the pitch of the helix. The helical pitch of N*
LCs is dependent upon various factors such as temperature,
electrical, or magnetic field and on the nature and concentration
of impurities, which makes it possible to tune their color by a
variety of external stimuli.16
Chart 1
LCs essentially consist of pure materials with well-defined
molecular weights where the problem of dilution of the
photochrome does not exist. This can result in much faster
switching times and enhanced stability of the film.8,22 Our initial
effort in the design of inherently photoactive liquid crystals for
recording color images met with only a partial success.23
Although a light induced smectic to N* phase transition was
observed in cholesterol-azobenzene linked systems, the rapid
thermal cis-trans isomerization of the azobenzene moiety
resulted in a loss of the recorded image. Moreover these
materials did not form LC stable glasses.
Our continued interest in designing an inherently photoactive
glass forming N* LCs leads us to the synthesis of a series of
novel asymmetric liquid crystal dimers, consisting of a choles-
terol moiety linked to a diphenylbutadiene chromophore (CBCn,
Chart 1) via flexible alkyl chains.24 These liquid crystal dimers
possess the combined glass forming properties of the cholesterol
moiety and the photochromic and luminescent properties of the
butadiene moiety. Photoinduced cis/trans isomerization of the
butadiene chromophore in these materials could be utilized to
bring about an isothermal phase transition from the smectic to
the cholesteric state.25 By photochemically controlling the cis/
trans isomer ratio, the pitch of the cholesteric could be
continuously varied making it possible to tune the color of the
film over the entire visible region, and the color images thus
generated could be stabilized by converting them to N* glasses.
These materials were also polymorphic, exhibiting two crystal-
line forms possessing distinctly different fluorescence properties.
The ability to thermally switch these materials from one
crystalline form to the other in a reversible manner also makes
them useful for recording fluorescent images.
Mesogenic dimers or twin mesogens, which consist of
molecules containing two mesogenic units, are currently of great
interest since they exhibit complex and novel phase behavior
not usually observed in conventional LC architectures.17 We
have recently reported on dicholesteryl liquid crystals capable
of forming stable glassy LCs in which the helical ordering of
the N* phase is retained.18 Glass forming LCs and in particular
N* glasses have been attracting increasing attention in view of
their potential application as optical filters, polarizers, and lasing
materials.19 We have shown that the dicholesteryl esters doped
with photoresponsive chromophores such as azobenzene or
diphenylbutadiene derivatives could be utilized for light induced
recording of full color images.20 Recent studies have shown that
inherently photoactive LCs possess several advantages over
doped systems.21,22 Films drawn from inherently photoactive
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Results and Discussion
Phase Transition Behavior. The mesomorphic properties of
the CBC derivatives were investigated by polarized optical
microscopy (POM), differential scanning calorimetry (DSC),
and small angled X-ray diffraction (XRD). The transition
temperatures and corresponding enthalpy values are summarized
in Table 1. With the exception of CBC, which decomposed
above its isotropization temperature, all the derivatives showed
(20) (a) Tamaoki, N.; Song, S.; Moriyama, M.; Matsuda, H. AdV. Mater. 2000,
12, 94-97. (b) Moriyama, M.; Song, S.; Matsuda, H.; Tamaoki, N. J. Mater.
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N. AdV. Funct. Mater. 2004, 14, 743-748.
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J.; Genack, A. Z. Chem. Mater. 2004, 16, 1397-1399. (c) Chen, S. H.;
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(24) Supporting Information.
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A R T I C L E S
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Table 1. Melting Points, Phase Sequences, Phase Transition Temperatures, and Phase Transition Enthalpy Changes in the Heating and
Cooling Cycles of the CBC Derivatives
sample code
heating cycle,
°
C (∆H, kJ/mol)
cooling cycle, °C (∆H, kJ/mol)
CBC
CBC8
Cr 196.2a (21.4) SmA* 239.5a (1.9) TGBA* 243.6 N* 264.4a dec
Cr1 99.5a (27.2) SmA* 112.8a (25.4) Cr2 131.9a (26.2) SmA* 134
TGBA* 138 N* 202.9a (3.3) Iso
Iso 193.5a (1.1) N* 132 TGBA* 128 SmA* 74.9a (24.8) Cr
CBC11
CBC12
Cr 120.9a (36.9) SmA* 145.5a (0.03) N* 175.0 (0.13) Iso
Cr1109.4a (15.1) SmA* 112.8a (19.5) Cr2 129.5a (41.3) N* 188.1a (1.8) Iso
Iso162.1a (2.1) N* 113.7 (2.1) SmA* 84.9a (4.8) Cr
Iso 176.6a (3.6) N* 108 SmA* 73.9a (26.2) Cr
a As observed by DSC; Cr, Crystal; SmA*, Chiral smectic A; TGBA*, Twist grain-boundary phase A*; N*, Chiral nematic; and Iso, Isotropic.
Figure 1. Polarized photomicrographs of CBC8 in (a) N* (152 °C) phase, (b) TGBA* phase (135 °C), and (c) SmA* phase (121 °C).
Figure 2. Polarized photomicrographs of CBC8 in (a) crystalline-spherulite (b) growth of crystalline fibers into homeotropic areas, and (c) fiberlike crystal.
Figure 4. XRD diagram of CBC12 in (a) Cr1 and (b) Cr2 phase.
distances (d) calculated from XRD were found to be signifi-
cantly larger than the calculated dimension of the molecule (L).
Figure 3. DSC trace of CBC8 in the cooling\heating cycle, rate 5 °C per
min.
The d/L values were observed to be ∼1.5 indicating an
interdigitated layerlike arrangement.24,27
enantiotropic (thermodynamically stable) LC properties exhibit-
An unusual behavior was observed in the phase transition of
ing smectic A* (SmA*)24 and N* phases. For example, cooling
CBC8 and CBC12 in the heating cycle.28 On heating a thin film
the isotropic liquid of CBC8 resulted in the formation of the
of CBC8, POM studies indicated that the spherulite crystalline
N* phase, which could be confirmed by the single color
texture (Cr1) observed initially (Figure 2a) melted to form an
reflecting Grandjean textures, observed using POM (Figure 1a).
SmA* phase characterized by its focal conic texture with
At 132 °C, a wormlike texture characteristic of the Twist grain-
homeotropic areas.24 Interestingly, further heating resulted in
boundary A* (TGBA*) phase was observed (Figure 1b).26 On
the unexpected formation of fibrous crystals (Cr2), which slowly
further cooling, an SmA* with focal conic patterns with
grew into the smectic homeotropic areas (Figure 2b) until
homeotropic domains was observed (Figure 1c). XRD measure-
complete formation of birefringent fibrous crystals occurred
ments of the SmA* phase show a sharp low angle peak
accompanied by a diffuse peak at a wider angle. Interlayer
(27) Attard, G. S.; Date, R. W.; Imrie, C. T.; Luckurst, G. R.; Roskilly, S. J.;
Seddon, J. M.; Taylor, L. Liq. Cryst. 1994, 16, 529-581.
(26) (a) Goodby, J. W. J. Mater. Chem. 1991, 1, 307-318. (b) Textures of Liquid
Crystals; Dierking, I., Ed.; Wiley-VCH: Weinheim, Germany, 2003.
(28) Morris, N. L.; Zimmermann, R. G.; Jameson, G. B.; Dalziel, A. W.; Reuss,
P. M.; Weiss, R. G. J. Am. Chem. Soc. 1988, 110, 2177-2185.
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A R T I C L E S
Figure 5. Polarized photomicrographs of CBC12 in (a) N* and (b) SmA* glassy LC state.
(Figure 2c). Samples rigorously purified by HPLC in order to
ensure removal of any impurities gave the same results. Similar
observations were made in repetitive heating scans.
Subsequent heating of these crystals (Cr2) resulted in the
reappearance of the SmA* phase at 132 °C. The Cr1-SmA*-
Cr2 transition which was also observed in the heating cycle of
CBC12 was reproducible and was observed both for samples
crystallized from solvents and for supercooled solids. The
transition of the relatively less ordered SmA* phase to highly
ordered crystals in the heating cycles of CBC8 and CBC12 was
confirmed by DSC, which showed clear endotherms corre-
sponding to the smectic-to-crystal transformations observed by
POM (Figure 3).24 XRD measurements indicated that the
interlayer distance (d) was the same for the SmA* phases of
CBC8 which appear before and after the Cr2 phase.
Figure 6. Transmission spectra obtained for a thin-film (10-µm thickness)
of CBC12 with decreasing temperature in the cooling cycle.
An explanation for this unusual behavior can be obtained by
studying the XRD patterns of the two crystalline samples. The
XRD pattern of Cr1 was broad, indicative of a glassy nature
(Figure 4), suggesting that Cr1, which is usually obtained by
cooling the LC melt, forms a metastable state or “frustrated
crystal”,29 where the molecules are not highly ordered. On
heating, the sample initially melts to the smectic phase, wherein
the molecules have the freedom to move and which on further
heating reorient to form highly ordered crystals. The highly
crystalline nature of this form is reflected by the sharp peaks
observed in its XRD pattern (Figure 4). The two crystalline
forms also possessed distinctly different fluorescence properties,
and this aspect is discussed later.
Formation of Glassy LCs. The dimesogens CBC8, CBC11,
and CBC12 formed stable transparent glassy LCs (Figure 5)
when they were suddenly cooled from their LC state to ∼0 °C.
The glassy LCs obtained by cooling from the LC phases were
stable for several months at room temperature. POM studies
indicated a glassy LC-to-Cr transition temperature for CBC8,
CBC11, and CBC12 at 100 °C, 97 °C, and 105 °C, respectively.
The reflection induced by the helical arrangement in CLCs
satisfies the equation λmax ) np, where λmax is the reflection
maximum, p, the pitch of the helix, and n, the refractive index.
When [dp/dt] < 0, as in the present case, the pitch length
decreases with increasing temperature. The reflection band
maximum of a CBC12 film shifts from 900 to 560 nm on
changing the temperature from 115 °C to 190 °C (Figure 6).
Similar effects were observed for the other derivatives. The
glassy LCs formed by cooling the cholesteric phase retained
the reflection band observed at the temperature from which they
were cooled.
Photoinduced Isothermal Phase Transitions. Photoisomer-
ization of the diphenyl butadiene chromophore is known to result
in the formation of stable cis isomers,25 and this process could
be utilized to bring about light induced isothermal phase
transitions in the CBC derivatives. Irradiation with 366 nm light
of an SmA* film of CBC12 held at 105 °C resulted in the
formation of an N* phase characterized by its Grandjean texture
and reflected colors (Figure 7b), while prolonged irradiation
lead to isotropization.
The pitch of the N* formed on photoirradiation was observed
to decrease with an increase in the time or intensity of irradiation
(Figure 8), making it possible to tune the color of the film over
the entire visible range. Using this procedure, color images could
be recorded in the cholesteric films and the images thus formed
could be stabilized by rapidly quenching the temperature of the
irradiated film to ∼0 °C.
Photoisomerization of the all trans isomer of donor/acceptor
(D/A) substituted alkoxy diphenyl butadienes has been earlier
reported to result in the formation of thermally stable EZ and
ZE (cis isomers), which were separated by HPLC and character-
ized.25 Similarly analysis of the irradiated samples of the
mesogenic dimers used in the present study also indicated the
formation of the EZ and ZE isomers (Figure 9). By analogy
with earlier studies the first peak in the HPLC trace may be
attributed to the formation of the EZ isomer and the second
peak to that of the ZE isomer.
The total amount of the cis isomers formed on photoirradia-
tion of CBC12 in its liquid crystalline phases at various
irradiation times was determined by HPLC in order to under-
stand its effect on the phase transition characteristics. Thin
(29) (a) Jung, H.-T.; Hudson, S. D.; Lenz, R. W. Macromolecules 1998, 31,
637-643. (b) Ye, C.; Xu, G.; Yu, Z.-Q.; Lam, J. W. Y.; Jang, J. H.; Peng,
H.-L.; Tu, Y.-F.; Liu, Z.-F.; Jeong, K.-U.; Cheng, S. Z. D.; Chen, E.-Q.;
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Figure 7. Polarized photomicrographs of CBC12 at 105 °C (a) before irradiation (SmA*), (b) after irradiation for 15 s (N*), and (c) after irradiation for 9
min (isotropic phase) with 366 nm light.
Figure 8. Changes in reflection band of the cholesteric phase of CBC12
developed on photoirradiation of its smectic film held at 105 °C. Inset shows
an image stored in a cholesteric glass, obtained by energy modulated
irradiation of a smectic film at 105 °C followed by temperature quenching
to ∼0 °C .
Figure 10. Binary phase diagram obtained by the photogeneration of cis
isomers of CBC12 in the cooling cycle
cholesteric phase decreases resulting in a blue shift in its
reflection band. At high cis concentrations, an isothermal phase
transition to the isotropic phase occurs. The isotropization
temperature was lowered by ∼90 °C, which is substantially
larger than any of the previous reports on photoresponsive
LCs.23,30
Although the photogenerated cis isomers were thermally
stable, they could be reverted back to the all trans form by
irradiating with 266 nm light. It is well-known that in polyenes
and related systems trans-cis photoisomerization is prevented
in a rigid media, whereas cis-trans photoisomerization remains
feasible, resulting in stereospecific isomerization.31 Thus 266
nm photoirradiation of film containing a mixture of the cis and
trans isomers in the solid state results in near complete recovery
of the all trans isomer (Figure 12). Using this procedure the
images recorded using 366 nm light could be erased by
irradiating them with 266 nm laser light.
Figure 9. HPLC trace of CBC12 in isotropic state formed from smectic
state by photoirradiation with 366 nm light.
smectic films of CBC12 held at 105 °C between two cover slips
separated by a 10 µm spacer were irradiated using 366 nm light
for different intervals of time. Figure 10 shows the dependence
of the phase transition of CBC12 on the amount of cis isomers
formed as a result of irradiation for different periods of time.
At 105 °C it was observed that 1% conversion to the cis isomers
was sufficient to bring about an SmA* to N* transition, whereas
21% conversion to cis isomers was required to induce isotro-
pization. The SmA* phase was not observed in samples
containing more than 5% of the cis isomers.
Solid State Fluorescence and Thermal Imaging. There is
a growing interest in the development of materials whose solid
state luminescent properties can be reversibly controlled using
external stimuli.10,32 The two polymorphic crystalline forms of
CBC8 and CBC12 possess significantly different fluorescence
properties, which could be attributed to differences in the
molecular packing of the two forms. The effect of temperature
on the solid-state optical properties was investigated in order
to gain an insight into the nature of these differences. The
absorption spectra of a film of CBC12 cast on quartz plates
A schematic representation of the photoinduced effect is
shown in Figure 11. On photoirradiation, formation of the cis
isomers initially destabilizes the SmA* phase, bringing about a
reduction in the SmA*-to-N* phase transition temperature. With
increasing concentration of the cis isomers, the pitch of the
(30) Kumaresan, S.; Mallia, V. A.; Kida, Y.; Tamaoki, N. J. Mater. Res. 2005,
20, 3431-3438.
(31) (a) Liu, R. S. H. Acc. Chem. Res. 2001, 34, 555-562. (b) Sun, Y.-P.; Saltiel,
J.; Hoburg, E. A.; Waldeck, D. H. J. Phys. Chem. 1991, 95, 10336-10344.
(c) Liu, R. S. H.; Asato, A. E.; Denny, M. J. Am. Chem. Soc. 1983, 105,
4829-4830.
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Thermo- and Photoresponsive Mesogenic Dimers
A R T I C L E S
Figure 11. Schematic representation of photoinduced isothermal phase transition from SmA* to isotropic phase via the N* phase
Figure 12. HPLC trace of CBC12 in recovered all trans state obtained by
reverse isomerization in solid state using 266 nm laser.
Figure 14. Fluorescence spectra of CBC12 in (a) solution state, (b) Cr1,
(c) Cr2, and (d) cholesteric image.
maxima at 342 and 359 nm and a shoulder at 383 nm. These
results point to the existence of spectroscopically different states.
In bulk materials, the tendency to form aggregates is very high.
Interaction between chromophores in their ground state on
aggregation has been fairly well explained by McRae and Kasha
in terms of exciton coupling theory.33 The excited state of
aggregates is split into two energy levels (Davydov splitting).
The transition to the upper state is allowed in the case of
H-aggregates and is characterized by a hypsochromically shifted
absorption band, while the transition to the lower state is allowed
for J-aggregates and is marked by a bathochromically shifted
absorption band compared with the isolated monomer. The
multiple absorption peaks observed for Cr1 indicates formation
of both H- and J-type aggregates. The glassy nature of Cr1
indicated by its X-ray diffraction pattern could therefore be
attributed to the existence of such different types of interchro-
mophore arrangements.34
Figure 13. Normalized absorption spectra of CBC12 in (a) Cr1, (b) Cr2,
and (c) liquid crystalline states.
were recorded between 100 and 120 °C, where the compound
exists in its Cr2 form. The spectrum indicates a peak at 360
nm, similar to that observed in the LC phase (Figure 13). The
absorption spectrum is also very similar to that observed in
toluene solutions,24 indicating that the main absorbing species
in the crystal and in the LC phase is the monomer. In contrast,
at lower temperatures where CBC12 exists in its Cr1 form, the
absorption spectra of the film exhibited a broad band with
Evidence for differences in the molecular packing in Cr1 and
Cr2 was also obtained from their fluorescence spectra. Figure
14 shows the normalized solid-state emission spectra of CBC12
in its Cr1, Cr2, and glassy cholesteric forms. The emission
spectrum of CBC12 in toluene solution is also shown for
comparison. The short wavelength band with its maximum
centered on 438 nm can be attributed to emission of the
monomer, while the long wavelength band observed in the 475
(32) (a) Deans, R.; Kim, J.; Machacek, M. R.; Swager, T. M. J. Am. Chem.
Soc. 2000, 122, 8565-8566. (b) Precup-Blaga, F. S.; Garcia-Martinez, J.
C.; Schenning, A. P. H. J.; Meijer, E. W. J. Am. Chem. Soc. 2003, 125,
12953-12960. (c) Mutai, T.; Satou, H.; Araki, K. Nat. Mater. 2005, 4,
685-687. (d) Kishimura, A.; Yamashita, T.; Yamaguchi, K.; Aida, T. Nat.
Mater. 2005, 4, 546-549. (e) Hulvat, J. F.; Sofos, M.; Tajima, K.; Stupp,
S. I. J. Am. Chem. Soc. 2005, 127, 366-372. (f) Lim, S.-J.; An, B.-K.;
Jung, S. D.; Chung, M.-A.; Park, S. Y. Angew. Chem., Int. Ed. 2004, 43,
6346-6350. (g) Sheikh-Ali, B. M.; Weiss, R. G. J. Am. Chem. Soc. 1994,
116, 6111-6120.
(33) (a) Dellinger, B.; Kasha, M. Chem. Phys. Lett. 1976, 38, 9-14. (b) McRae,
E. G.; Kasha, M. Physical Processes in Radiation Biology; Augenstein,
L., Mason, R., Rosenberg, B., Eds.; Academic Press: New York, 1964; p
23.
(34) (a) Robinson, M. R.; Wang, S.; Heeger, A. J.; Bazan, G. C. AdV. Funct.
Mater. 2001, 11, 413-419. (b) Fu, H.; Loo, B. H.; Xiao, D.; Xie, R.; Ji,
X.; Yao, J.; Zhang, B.; Zhang, L. Angew. Chem., Int. Ed. 2002, 41, 962-
965.
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J. AM. CHEM. SOC. VOL. 128, NO. 23, 2006 7697

A R T I C L E S
Abraham et al.
heated region was similar to that shown in Figure 14, curve d.
Using this procedure a variety of images could be recorded on
the films. Erasure of the image could be achieved by following
a heating-cooling cycle, which allowed regeneration of the Cr2
form.
Conclusion
The mesogenic dimers described here form a versatile class
of materials with multifunctional properties. These inherently
photoresponsive liquid crystals form stable, clear N* glasses
and also possess polymorphic crystalline forms with visually
distinguishable fluorescence. These properties make them useful
for photochemical and thermal recording of full color and
fluorescent images in a reversible manner. The photoinduced
drop in the N*-I transition of 90 °C is the highest reported for
photoresponsive liquid crystals.
Figure 15. Fluorescent image of CBC12 obtained by placing a hot object,
which was then removed, on the blue fluorescent crystalline material (under
365 nm illumination).
to 650 nm region can be attributed to emission of the
aggregate.35 From the figure it is clear that the contribution of
emission from the aggregate is significantly less for Cr2, whereas
for Cr1 and the glassy cholesteric film the aggregate emission
is substantial.
The differences in the fluorescence of Cr1 with that of Cr2
and the glassy cholesteric film was visually perceivable making
it possible to record fluorescent images on these materials.
Figure 15 shows the fluorescent image recorded on a thin film
of Cr2 of CBC12, by momentarily placing a hot object on the
film. The object was placed for a sufficient time to allow
transformation of the Cr2 to the N* phase in the heated regions.
Removal of the object leads to rapid cooling, resulting in the
formation of a glassy N*. The fluorescence spectrum of the
Acknowledgment. The research fellowship from the Japanese
Society for Promotion of Science (JSPS) and the travel support
from the Department of Science and Technology (DST), India
under an Indo-Japanese Scientific Collaborative Project (IJSCP)
program are gratefully acknowledged. This is Contribution No.
217 from RRLT-PPD.
Supporting Information Available: Details of synthesis,
analytical, and spectral characterization data of compounds, d
spacing from X-ray measurements, and DSC thermogram of
CBC11. This material is available free of charge via the Internet
at http://pubs.acs.org.
(35) (a) Davis, R.; Rath, N. P.; Das, S. Chem. Commun. 2004, 74-75. (b) Davis,
R.; Abraham, S.; Rath, N. P.; Das, S. New J. Chem. 2004, 28, 1368-
1372.
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