Glass-forming cholesteric liquid crystal oligomers for new tunable solid-state laser

Article abstract of DOI:10.1002/adma.200902552

"Figure Presented" A new potential utility of glass-forming cholesteric liquid crystal (G-CLC) oligomers for application in tunable solid-state laser is presented. The G-CLC is capable of tuning the photonic band gaps (PBGs) by way of the annealing temper

Full text of DOI:10.1002/adma.200902552

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Glass-Forming Cholesteric Liquid Crystal Oligomers for
New Tunable Solid-State Laser
By Seiichi Furumi* and Nobuyuki Tamaoki*
Cholesteric liquid crystals (CLCs)—supramolecular helical
assemblies of nematic liquid crystal (NLC) layers with a periodic
helical pitch—have attracted a great deal of attention owing to
their wide range of potential applications in displays, full-color
recording media, polarizers, and reflectors.[1] The CLC meso-
phase appears from intrinsically chiral compounds or mixtures,
where at least one of the components has an enantiometric chiral
center. At the CLC mesophase, the orientational order of CLC
molecules is similar to that of NLC. However, the local director of
each NLC layer spatially and periodically rotates clockwise or
counterclockwise with respect to the helical axis, resulting in the
formation of right-handed or left-handed CLCs, respectively.
When the CLC materials are sandwiched between a pair of
substrates with homogeneous anchoring surfaces, the helical axis
is spontaneously oriented to the orthogonal direction of the
substrate surface. Due to the supramolecular helical structures,
the planar CLC cell shows a negative birefringence with an
extraordinary refractive index (ne) along the helical axis. Selective
light reflection is one of the most unique and important optical
properties of the CLC helical structures. The peak wavelength of
the selective light reflection (lmax) is numerically expressed as
lmax ¼ nav p, where nav is average refractive index and p is the
helical pitch length. The reflection bandwidth (Dl) is determined
by Dl ¼ p(ne ꢀ no), where no is ordinary refractive index. Another
outstanding feature is the light reflection with a chiroptical
property. When a linearly polarized light propagates into the
planar CLC cell along the helical axis, the CLC molecules can
reflect circularly polarized (CP) light with the same CLC
supramolecular helical handedness at lmax with Dl. In contrast,
CP light with the opposite CLC helical handedness transmits
through the planar cell without light reflection.
diagram. The specific regions are nowadays known as photonic
band-gaps (PBGs). Such periodic helical structures of CLCs can
be regarded as 1D PCs. Following the archetypal report one
decade ago,[3] much effort has established a new perspective on
the CLC for optically excited laser emission from both
fundamental and technologic viewpoints. When a light-emitting
organic dye is embedded in the CLC, optical excitation gives rise
to stimulated laser-emission peak(s) at edge(s) and/or within the
CLC PBGs.[4] Among the previous studies, tunable lasers is one of
the most leading research topics with the potentials for versatile
applications. Hitherto, there have been a variety of examples of
the laser wavelength tunabilities by temperature,[5] mechanical
stress,[6] photoirradiation,[7] electric field,[8] and chiral-dopant
concentration.[9]
Currently, more emphasis in the tunable CLC-laser systems is
placed on the position-dependent tuning of the laser-emission
wavelengths by continuously gradated PBG (CG-PBG) structures
in the planar CLC cells.[10,11] In other words, the CG-PBG
structures have the 1D gradation of a CLC helical pitch. Although
the CG-PBG structures are obtained by precisely controlling
temperature[10] and chiral-dopant concentration[11] of low-
molecular-weight CLCs, these strategies might encounter some
serious drawbacks as follows. In principle, the low-
molecular-weight CLCs are very vulnerable to subtle thermal
fluctuations, so that specific temperature-controllers and hea-
ting-stages are needed to keep the CG-PBG structures in the
lasing experiment.[10] In another strategy, by the gradation of
chiral dopant concentration, the CG-PBG structures are not stable
due to the fluid diffusion of chiral dopants.[11] Therefore, the
CG-PBG structures of low-molecular-weight CLCs are imprac-
tical. Very recently, the CG-PBG structures of photopolymerized
CLCs have been applied to the position-dependent tuning of laser
emission.[12] However, these CG-PBG structures require high
threshold excitation energies or peak powers for laser action after
the photopolymerization, because the initial CLC helical
orientation or light-emitting organic dyes might be deteriorated
during radical reactions induced by UV light. In this
Communication, we report the synthesis of a glass-forming
CLC (G-CLC) and its application in a new type of tunable CLC
solid-state laser. As will be seen below, we succeeded in the facile
fabrication of robust CG-PBG structures of our light-emitting
G-CLC without any covalent bonding through a supercooling
process, leading to continuously and reversibly tunable laser
emission by optical excitation with relatively low peak powers.
In order to prepare a light-emitting G-CLC, we designed and
synthesized three kinds of compounds of CD8, 11-BP, and
DC-OPV (Scheme 1). The synthesis procedures are described in
the Experimental section. First, we used CD8 as a G-CLC host.
Previously, Tamaoki et al.[13] found unique properties of a CLC
Recently, the CLCs have garnered considerable attention from
the intriguing research field of photonic crystals (PCs).[2] The PCs
are periodically modulated structures of dielectric materials on a
scale comparable to the wavelength of light, leading to the
appearance of forbidden regions in the photon dispersion
[*] Dr. S. Furumi
National Institute for Materials Science (NIMS)
1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan)
E-mail: furumi.seiichi@nims.go.jp
Prof. Dr. N. Tamaoki[+]
National Institute of Advanced Industrial Science and Technology
(AIST)
Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565 (Japan)
E-mail: tamaoki@es.hokudai.ac.jp
[ ] Present address: Research Institute for Electronic Science (RIES),
+
Hokkaido University, N20, W10, Kita-ku, Sapporo 001-0020 (Japan)
DOI: 10.1002/adma.200902552
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Adv. Mater. 2010, 22, 886–891

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G-CLC film was adjusted using crosslinked
polymer microparticles with a diameter of
ꢁ10 mm. The solidified G-CLC film was
prepared by supercooling from the CLC
mesophase temperature to 0 8C. Polarized
optical microscopic observation showed
Grandjean texture, indicative of the perpendi-
cular orientation of G-CLC helical axis with
respect to the substrate surface. In addition,
this film showed a uniform optical domain
without any crystallization of DC-OPV.
Figure 1A shows CP transmission spectra of
a G-CLC film measured with right-handed CP
(R-CP) and left-handed CP (L-CP) probe light.
First, the G-CLC thin film was solidified by
supercooling from a CLC temperature of
78 8C. The CP-transmission spectral shape
was drastically altered by the polarization state
of the CP probing light. The R-CP trans-
mission spectrum showed a single broad
band centered at 413 nm from the electronic
absorption of DC-OPV (spectrum e). Converting
Scheme 1. Chemical structure of a G-CLC for the fabrication of a new type of tunable solid-state
CLC laser. The G-CLC was prepared by mixing ternary compounds of CD8, 11-BP, and DC-OPV
at a weight ratio of 98.4:1.6:1.0, respectively.
oligomer tethering two cholesteryl units at the termini (CD8,
Scheme 1), which can be practically applied in rewritable
full-color recording media. A thin film of CD8 exhibits a PBGwith
reflected light, that is, a selective reflection band, in a wide visible
wavelength range of 420–610 nm at the CLC mesophase
temperature of 87–115 8C. The CD8 film annealed at CLC
temperatures is rapidly cooled to 0 8C, whereupon the selective
light reflection can be preserved at room temperature. Such
supercooling treatment of CD8 results in durable formation of a
glassy CLC solid state without the aid of any covalent bonding
because of its relatively high glass-transition temperature at
ꢁ80 8C. This solidified CD8 film has unique capabilities for not
only the desired tuning of PBG by annealing temperature, but
also the on-demand preservation of the shifted PBG by the
subsequent supercooling treatment. Secondly, 11-BP was adopted
as a dopant in precisely regulating the PBG wavelength and its
tuning range. The addition of 11-BP to CD8 results in fine control
over both PBG wavelength and its thermally induced shift range,
which are dependent on the doping amount of 11-BP.[14] In this
study, 1.6 wt% of 11-BP was doped into CD8 to obtain a G-CLC
host with a PBG shift range from 400 to 600 nm. Thirdly, we used
an oligo(p-phenylene vinylene) derivative of DC-OPV possessing
two cholesteryl units at the termini as a light-emitting compound.
This DC-OPV was designed to be accommodated to the G-CLC
host of CD8 and 11-BP. In a preliminary experiment, it was
confirmed that a typical laser dye, such as Pyrromethene 597,
readily crystallizes with domains of micrometer dimension in the
G-CLC host, even when the addition amount is only 0.4 wt%
(Fig. S1, Supporting Information). In contrast, a homogeneous
light-emitting G-CLC can be prepared by mixing ꢁ1.0 wt%
DC-OPV into CD8 and 11-BP (vide infra).
from R-CP to L-CP probing light, we found not only the electronic
absorption band of the DC-OPV chromophore, but also a
characteristic reflection band at 600 nm arising from the CLC
PBG (spectrum a). The differential transmittance between R-CP
Figure 1. A) L-CP transmission spectra of a G-CLC film that was solidified
by supercooling from annealing temperatures at 78 8C (spectrum a), 83 8C
(spectrum b), 88 8C (spectrum c), and 93 8C (spectrum d). The represen-
tative R-CP transmission spectrum of G-CLC supercooled from 78 8C is
shown by the dashed curve (spectrum e). The R-CP transmission spectra
had the same appearance even when the annealing temperatures were
changed. B) Changes in the lPBG of the supercooled G-CLC film as a
function of the annealing temperature.
A light-emitting G-CLC was prepared from a homogeneous
dichloromethane solution including CD8, 11-BP, and DC-OPV at
a weight ratio of 98.4:1.6:1.0 through evaporation in vacuo to
remove the solvent. This G-CLC was placed between two thin
glass substrates (ꢁ0.15 ꢂ 18 ꢂ 9.0 mm3) with an uniaxially
rubbed poly(vinyl alcohol) film surface. The thickness of
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and L-CP light around 410 nm originates from the Borrmann
effect in absorbing CLC system.[15] The PBG wavelength in the
L-CP transmission spectrum shifted to shorter wavelengths at the
oblique-incidence angle (u) of the probe light. This behavior
follows the geometric relation of lmax ¼ np cos u. The result
suggests that this G-CLC film retains a left-handed helical
molecular sense even at a glassy CLC solid state.
The PBG of a G-CLC film, observed as a selective reflection of
L-CP light, could be precisely tuned by changing the annealing
temperatures at which the supercooling was started. When the
G-CLC film was supercooled from the CLC temperatures at 83,
88, and 93 8C, the PBG wavelengths appeared at 573, 517, and
440 nm, respectively (spectra b–d). Figure 1B compiles the
dependence of annealing temperature on the maximum PBG
wavelength (lPBG). The PBG wavelength gradually decreased as
the annealing temperature increased. The PBG tuning range was
relatively wide from 400 to 600 nm. Due to the formation of a
glassy CLC solid state, the tuned PBG wavelengths of G-CLC film
remained extremely intact at room temperature, even after being
stored for over 10 months. In this way, the supercooling process
of the G-CLC film from CLC temperatures enabled the persistent
preservation of the shifted PBG wavelengths in a widely visible
range.
Fluorescence spectral measurements provide invaluable
information on intermolecular interaction between aromatic
chromophores at electronically excited states within the micro-
scopic environment, leading to intrinsic emission species such as
monomer and excimer fluorescence. We pursued the excited-
state aggregation behavior of the G-CLC film during the course of
phase changes by the annealing temperature. Figure 2 shows the
changes in the fluorescence spectra of the G-CLC film as a
function of the annealing temperature. The fluorescence spectra
were measured by exposure to 418-nm light for selective
excitation of DC-OPV. This is because DC-OPV has an absorption
band centered at 418 nm, whereas there is no intrinsic absorption
band above 320 nm in either CD8 or 11-BP (Fig. S2).
When the G-CLC film was solidified by supercooling from
crystal phase at 65 8C, two main fluorescence bands appeared at
the maxima of 470 and 500 nm (spectrum a). The fluorescence
intensities were relatively weak due to light scattering from the
polycrystalline structure. In order to clarify the characteristic
fluorescence bands, we measured a fluorescence spectrum of a
dilute DC-OPV solution (c ¼ 1.0 ꢂ 10ꢀ5 mol/L). In the solution, a
broad fluorescence band with a maximum emission wavelength
at 470 nm and a weak shoulder band around 500 nm (Fig. S3)
were observed. By comparing the fluorescence spectra between
the crystal state and dilute solution, it can be clearly inferred
that the emission peak at 470 nm is attributed to monomer
fluorescence, while the peak around 500 nm stems from excimer
fluorescence. Subsequently, the G-CLC film was supercooled
from the CLC mesophase at 90 8C (spectrum b). The fluorescence
spectral shape was drastically altered, as compared to that of the
crystal state (spectrum a). We observed the predominant excimer
emission band in the intensity due to overlapping with the
DC-OPVchromophore at the glassy CLC state. Successively, when
the G-CLC film was prepared from isotropic temperature of
115 8C, the monomer emission was intensified rather than the
excimer (spectrum c). This probably happened from the
segregation of DC-OPV by annealing at the isotropic phase.
In order to quantitatively evaluate the microscopic interaction
of electronically excited DC-OPV, the ratio of the fluorescence
intensity at 500 nm (excimer fluorescence, ID) to that at 470 nm
(monomer fluorescence, IM) was plotted as a function of the
annealing temperature (inset of Fig. 2). Of interest is a prominent
enhancement of ID/IM in the temperature range from 80 to
100 8C, implying that DC-OPV preferentially adopts the excimer
formation even at glassy CLC solid state. Such observation is
consistent with some precedents of various liquid crystal
molecules confirmed by fluorescence measurement.[16]
Our G-CLC film has unique capabilities that not only enable
the fine tuning of the PBG wavelength by the CLC annealing
temperature, but also allow the on-demand preservation of the
shifted PBG through the subsequent supercooling treatment.
Taking advantage of these salient features, we can envisage that
the continuously gradated PGB (CG-PBG) structure is easily
inscribed inside our G-CLC film by the supercooling treatment in
order to create a new tunable CLC solid-state laser. Figure 3A
shows a photograph of the CG-PBG region in a G-CLC film,
which was solidified by the supercooling from the annealing
temperatures with continuous changes from 85 8C (right side) to
95 8C (left side). Interestingly, we found that the color reflected by
the CLC PBG alters from blue to green along the 1D gradation of
the annealing temperature. Furthermore, when the annealing
temperature was tuned again, we could obtain various PBG
structures in the G-CLC film. This tuning process of the various
PBG states was repeatedly reversible (Fig. S4).
Considering the CG-PGB structure inscribed in the G-CLC
film (Fig. 3A), we demonstrated the local-position-dependent
tuning of laser emission. For this purpose, we constructed a novel
optical system to enable spectral measurements of local reflection
and emission as well as in situ observations of the optical images
at the microscopic level. The detailed setup is described in the
Experimental section. Figure 3B shows the microscopic-reflection
(upper) and optically-excited laser-emission (lower) spectra of the
CG-PGB structure in the G-CLC film. We carried out the stepwise
Figure 2. Changes in fluorescence spectra of a G-CLC film prepared by
supercooling from annealing temperatures at 65 8C (spectrum a), 90 8C
(spectrum b), and 115 8C (spectrum c). The excitation wavelength was set
at 418 nm. The inset represents the changes in the ID/IM ratio of the G-CLC
film as a function of the annealing temperature.
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Figure 4. Changes in the emission intensity from the G-CLC film at 482 nm
(open circles) and 507 nm (closed circles) as a function of excitation energy
of 418 nm light. The observed emission wavelengths at 482 and 507 nm are
close to the monomer and excimer fluorescence wavelengths, respectively,
as shown in Figure 2.
manner, the laser-emission peak gradually shifted in a range from
475 to 550 nm. Except for this wavelength range, the laser action
could not be generated. Because the laser emission from CLCs
took place at the edge(s) of the PBG wavelengths, which overlap
with the fluorescence band of G-CLC film (spectrum b,
Fig. 2).[7b,17] Additionally, the color in the microscopy image of
the laser emission altered from blue to green (lower insets,
Fig. 3B). All such laser-emission peaks always appeared at the
longer-wavelength edges of the CLC PBGs for each local position,
probably due to the fact that the light-emitting DC-OPV
chromophores are well aligned in parallel with the local molecular
direction of the G-CLC matrix.[5b] At this lasing stage, the spectral
linewidths were as narrow as ꢁ0.7 nm. The quality factor (Q)
values were estimated to be approximately 7.5 ꢂ 102 from the
laser emission spectra. The Q values were adequately high among
other CLC systems.[3–12] Moreover, the CP spectral analysis
indicated that the laser-emission peaks from this G-CLC film have
a left-handed circular polarized (L-CP) state (Fig. S5), which was
the same handedness as the molecular helical sense of this
G-CLC matrix (Fig. 1A). Generally, the polarization degree of CP
emission is defined as the g value. The CP degree of emission (g)
is expressed as g ¼ 2(IL ꢀ IR)/(IL þ IR), where IL and IR stand for
the emission intensities of L-CP and R-CP states, respectively. For
purely single-handed CP emission, it is apparent that the absolute
value of g equals 2. Therefore, the observed laser-emission peaks
showed high g values of more than þ1.9.
Figure 3. A) Photograph of a G-CLC film with CG-PBG structure. This
G-CLC film was prepared by supercooling from annealing temperatures
with the continuous changes from 85 8C (right side) to 95 8C (left side). The
photograph was taken at room temperature. B) The local position-
dependent reflection (upper) and optically excited laser-emission spectra
(lower) of a G-CLC film with CG-PBGs. The spectra were taken upon
stepwise translation of the measuring spot. The laser-emission spectra
were obtained by optical excitation with 418 nm light with an energy of
approximately 500 nJ pulseꢀ1. The insets show the microscopic images of
reflection (upper) and laser emission (lower), and the left, middle, and right
photographs correspond to the images with PBGs centered at 470, 490,
and 540 nm, respectively. The scale bars of reflection and laser emission
images represent 50 and 20 mm, respectively.
measurements of local reflection spectra in the CG-PBG region
upon translating the G-CLC film with respect to a probing light.
We could measure the PBG wavelengths with a continuous shift
from 470 to 550 nm and simultaneously observe the gradation of
reflected color from blue to green (upper insets in Fig. 3B).
As an extension of our findings, we attempted to measure the
local laser-emission spectra in the CG-PBG region. A single
laser-emission peak appeared by optical excitation at 418 nm with
the pulse energy of approximately 500 nJ pulseꢀ1. When the local
excitation area in the CG-PBG region was translated in a stepwise
Figure 4 shows the changes in the emission intensity from
the CG-PBG structure of the G-CLC film as a function of the
excitation energy. We pursued the emission spectra changes at
482 and 507 nm, close to the monomer and excimer fluorescence
wavelengths of the G-CLC film, respectively (Fig. 2). When the
excitation energy exceeded the threshold at 500 nJ pulseꢀ1 or less,
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0.68 (s, 6H, CH3) 0.85–2.00 (m, 142H, CH, CH2, CH3), 2.42 (m, 4H, CH2),
3.96–4.05 (m, 12H, ArOCH2), 4.50–4.55 (m, 2H, OCOOCH), 5.21 (s, 4H,
CH2OCOO), 5.42 (m, 2H, CH), 6.92 (s, 2H, Ar-H), 7.11 (s, 2H, Ar-H), 7.14
(s, 2H, Ar-H), 7.45 (4H, s, CH¼CH). Matrix-assisted laser desorption
ionization time-of-flight mass spectrometry (MALDI-TOF-MS) m/z: calcd.
1767.36; found 1767.97 (Fig. S6).
the emission spectra changed to a single laser-emission peak.
Importantly, it was found that the threshold excitation energy for
the laser-emission peak at 507 nm (240 nJ pulseꢀ1) is lower than
that at 482 nm (500 nJ pulseꢀ1). This behavior probably follows
the empirical fluorescence result that the intensity of excimer
fluorescence is higher than that of monomer fluorescence at this
glassy CLC solid state (spectrum b, Fig. 2). The excitation peak
power to generate the laser action at 507 nm was calculated to be
ꢁ13 MWcmꢀ2. This threshold excitation power is an adequately
low value as compared to that in previous reports on lasing action
from the photopolymerized CLCs.[12a,18] Although the PGB
structures of CLCs can be easily preserved by photopolymeriza-
tion, they remain a serious problem for applications. It is
plausible that the photopolymerization brings about the
deterioration of the initial CLC helical orientation or decom-
position of light-emitting organic dyes by radical species, thereby
leading to the requirement of high-threshold excitation energies
for laser action.[12b] On the other hand, we succeeded in the
efficient generation of laser action with relatively low threshold
optical excitation of the CG-PBG structure of our G-CLC film,
which was preserved without any covalent bonding through the
supercooling process. Taking the overall results in account, such
technologically relevant performances might be limited to this
kind of chemically designed G-CLCs.
Optical Measurements: A novel optical system was arranged to measure
the microscopic reflection and emission spectra in addition to observing
the in situ microscopic images (Fig. S7). Reflection spectra were acquired
through a motorized illuminator for a microscope (BX-RLA2, Olympus)
equipped with a 100 W halogen lamp. Emission spectra were measured
using a pulsed laser beam at 418 nm from a optical parametric oscillator
excited by the third-harmonic light from a nanosecond pulsed Q-switched
Nd:yttrium aluminum garnet (Nd:YAG) laser beam (Surelite I-10 & OPO
Plus, Continuum). This excitation wavelength almost coincided with the
absorption band of the DC-OPV. The pulse duration was ꢁ6 ns and the
repetition frequency was 10 Hz. The excitation beam propagating along
the surface normal was focused through a microscopic objective lens
(SLMPLanN ꢂ 20, Olympus) to obtain a circular spot with a diameter of
ꢁ20 mm on the G-CLC film. The collinearly transmitted emission from the
sample was collected and focused onto the entrance of an optical fiber
connected with a spectrometer (USB4000/HR4000CG, Ocean Optics).
The microscopic reflection and emission images were taken on a
complementary metal semiconductor (CMOS) camera (Moticam2000,
Shimadzu).
In conclusion, we have developed a new potential utility of
G-CLC oligomers for continuously and reversibly tunable laser
action by low-threshold optical excitation. The G-CLC film had
dual capabilities to tune the PBGs by the annealing temperature
as well as to preserve the tuned PBGs by the subsequent
supercooling process. The supercooling procedure enabled the
facile fabrication of CG-PBG structures inside the G-CLC film. A
single laser-emission peak could be continuously tuned in a
visible range by stepwise translation of the local optical excitation
area in the CG-PBG region. We succeeded in the efficient laser
action from the CG-PBGs of our G-CLC film by optical excitation
with relatively low threshold peak powers due to the preservation
of CLC helical structures through non-covalent bonding.
Moreover, a wide variety of PBGs could be reversibly prepared
and persistently preserved through a supercooling process of the
G-CLC film. Such salient performances cannot be attained by
commercially available CLCs. Our findings are of interest in the
chemical approach to new light-emitting G-CLCs for technolo-
gical development of next-generation molecular optoelectronics
devices. To develop solid-state lasers that are tunable in the full
visible-wavelength range, the elaboration in the molecular design
and synthesis of G-CLC materials is in progress.
Acknowledgements
The authors express sincere thanks to the reviewers for their helpful
comments. This work was supported in part by the Strategic Information
and Communications R&D Promotion Programme (SCOPE) Project from
the Ministry of Internal Affairs and Communications (MIC), the Iketani
Science & Technology Foundation. S.F. is indebted to Dr. H. Akiyama and
Ms. M. Wada of the AIST and Dr. H. T. Miyazaki and Dr. T. Ikeda of the
NIMS for technical advices. Supporting Information is available online
from Wiley InterScience or from the author.
Received: July 30, 2009
Published online: December 22, 2009
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