Development of laser dyes to realize low threshold in dye-doped cholesteric liquid crystal lasers

Article abstract of DOI:10.1002/adma.201001046

Highly efficient luminescence dyes based on pyrene and anthracene derivatives (see figure) are synthesized for liquid crystal dye lasers. The threshold value of one of the pyrene derivatives is as low as 1/20 that of the commonly used DCM in cholesteric liquid crystal (CLC) distributed feedback lasers. Good optical properties such as luminous efficiency and solubility in CLCs are important factors for realizing a low threshold.

Full text of DOI:10.1002/adma.201001046

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Development of Laser Dyes to Realize Low Threshold
in Dye-Doped Cholesteric Liquid Crystal Lasers
By Makoto Uchimura, Yo Watanabe, Fumito Araoka, Junji Watanabe, Hideo Takezoe,*
and Gen-ichi Konishi*
Cholesteric liquid crystals (CLCs) have attracted much attention
for use in a distributed feedback (DFB) laser cavity and their
application to wavelength-tunable dye lasers.[1–6] For practical
applications, however, the lasing threshold must be lowered
essentially to zero. Considerable attempts to achieve this have
been made concerning 1) the cavity structure (e.g., use of the
defect mode),[7–9] 2) the excitation condition (e.g., excitation by
circularly polarized light),[10,11] and 3) CLC materials (e.g., use
of highly birefringent CLCs).[12] Syntheses of highly ordered
dye molecules along the CLC local director have also been car-
ried out successfully.[13,14] However, not much effort has been
directed toward finding ideal dye systems.[15,16] Almost all the
LC lasers so far investigated have utilized commercial laser
dyes. It is obvious that dyes with higher luminous efficiency
(high molar absorption coefficient and high quantum yield)
have to be used to increase energy efficiency and decrease
threshold. Very recently, we reported an extremely low threshold
using a highly fluorescent pyrene dye.[17] In the study reported
here, we have systematically designed and synthesized series of
pyrene and anthracene derivatives and found that one of the
pyrene derivatives shows an even lower lasing threshold,
23 nJ/pulse. Here we suggest a strategy to obtain ideal dyes in
terms of luminous efficiency and radiative decay rate.
It is very important to design a high-performance dye in
order to achieve continuous wave (cw) operation, the final goal
of LC lasers. Pyrene and anthracene derivatives have phenyl,
biphenyl, terphenyl, and naphthyl moieties, and the π-extended
pyrene with 1,3,6,8-positions and anthracene with 9,10-posi-
tions have remarkable material potentials, such as high absorp-
tion coefficient, high quantum yield, and high stability with
respect to heat and oxidation.[18–22] Therefore we focus on the
use of new pyrene and anthracene derivatives as new laser dyes.
Currently, because of these important optical properties, these
dyes are used in organic light-emitting diodes (OLEDs)[21,23]
and organic light-emitting field-effect transistors (OLEFETs).[15]
The long alkyl chains provide good solubility of dyes in organic
solvents and LCs. Moreover, it is considered that these substitu-
ents including alkyl chains have larger steric hindrance than an
unsubstituted aryl group. These alkyl groups can prevent dyes
from quenching by means of π-π stacking such as an excimer
even when a dye concentration in LCs is as high as the order of
1 × 10−2 M.
The chemical structures of 1,3,6,8-tetraarylpyrenes (P1–P6)
and 9,10-diarylanthracenes (A1–A4) are shown in Figure 1,
where a typical synthesis procedure of pyrene derivatives is
also shown. The syntheses are also described in the Experi-
mental section. For more details see also the Supporting Infor-
mation. In this study, the chain length [–(CH2)n–] of the alkyl
group was fixed at n = 6 and 7 for the application of these
dyes to an organic CLC laser. The reason for fixing the chain
length at 6 and 7 is solubility. If the chain length is too short,
dye molecules will not dissolve in liquid crystals. The most
important dye (P6), for which we report the lowest threshold,
has an ethylhexyl group and another substituent attached to a
phenyl ring. The asymmetric multiple substitution increases
disorder and enhances the solubility. In such cases, we do not
need too long alkyl chains and n = 6 and 7 are appropriate. The
optical properties of 1,3,6,8-tetraarylpyrenes (P1–P6), 9,10-
diarylanthracenes (A1–A4), and 4-dicyanomethylene-2-methyl-
6-(p-dimethylaminostyryl)-4H-pyran (DCM) are summarized in
Table 1. Remarkably, pyrene derivatives exhibit very high absorp-
tion coefficients (ε > 42000) and high quantum yields (Φ >
0.85). Lasing was observed in most of the dyes used, with the
exception of P5, A1, and A2.
Fluorescence, lasing, and reflection spectra for P2 and A3
are shown in Figures 2a and b, respectively. For lasing experi-
ments, we used a pulsed laser beam at 410 nm obtained by an
optical parametric oscillator (Continuum Surelite II) pumped
by third-harmonic generation (THG) light from a Nd:YAG laser.
The pulse width was 6 ns and the repetition frequency was
10 Hz. In all anthracene derivatives, laser oscillation occurred
at the low-energy edge of the stop band. In contrast, it occurred
at the high-energy edge in all pyrene derivatives. This difference
is attributed to the orientation of the transition dipole moment
of dye molecules with respect to the local director of the CLCs.
Red and blue lines in Figures 2c and d represent absorption
spectra of dye-doped nematic cells using linearly polarized light
parallel and perpendicular to the rubbing direction (director),
respectively. It is known that the transition dipole moment
of unsubstituted pyrene is parallel to its molecular long axis,
whereas that of unsubstituted anthracene is perpendicular to
its molecular long axis.[24,25] From these results, it is considered
that pyrene and anthracene derivatives are oriented in CLCs as
shown in the insets of Figures 2c and d, respectively; that is, the
transition dipoles of pyrene and anthracene are perpendicular
and parallel to the local director, respectively. Because of the
free rotation of the transition dipole about the local director, the
∗
[ ] M. Uchimura, Y. Watanabe, Dr. F. Araoka, Prof. J. Watanabe,
Prof. H. Takezoe, Prof. G. Konishi
Department of Organic and Polymeric Materials
Tokyo Institute of Technology
2-12-1 O-okayama, Meguro-ku, Tokyo 152–8552 (Japan)
E-mail: takezoe.h.aa@m.titech.ac.jp; konishi.g.aa@m.titech.ac.jp
DOI: 10.1002/adma.201001046
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Ar1
Br
Br
Br
Br
Ar1
Ar1
(i)
(ii)
Ar1
Synthesis of 1,3,6,8-tetraarylpyrene derivatives (P1-P6). Reagents and conditions: (i) Br2,
nitrobenzene, 120 C, (ii) P1-2: Ar1-Ph-B(OH)2, 5 mol% Pd(PPh3)4,
1M Na2CO3 or K2CO3, toluene, 110ΟC.
R
R
R
R
A2:
A4:
R = OC6H13
A1:
A3:
R = C7H15
R = CH2CH(C2H5)C4H9
R = CH2CH(C2H5)C4H9
R
R
R
R
R
R
R
R
R
R
P1
: R = OC6H13
P3
P4
: R = OC6H13
R
P2
: R = C7H15
R
: R = CH2CH(C2H5)C4H9
R1
R1
R
R
R2
R2
R2
R2
R
R
P5
: R = CH2CH(C2H5)C4H9
P6
: R1 = CH2CH(C2H5)C4H9
R2 = OCH3
R1
R1
Figure 1. Typical synthesis procedure of pyrene derivatives and the chemical structures of 9,10-diarylanthracene derivatives (A1-A4) and
1,3,6,8-tetraarylpyrene derivatives (P1-P6).
absorption anisotropy in the pyrene derivative is smaller than
that in the anthracene derivative. It is known that the polariza-
tion state of the optical eigenmode in CLCs has a linear polari-
zation E at the edges of the photonic band, that is, parallel to
the local director of the CLCs at the low-energy edge of the pho-
is the emission transition moment and the bracket stands for
spatial average. Hence the propagation mode at the low-energy
ˆ
μ
edge effectively gains emission intensity for
parallel to the
director, resulting in the occurrence of lasing.[28] This is the case
for anthracene derivatives, as shown in Figure 2b. By contrast,
the situation is different in pyrene derivatives; i.e., the transition
dipole moment perpendicular to the director leads to the laser
tonic band gap and perpendicular at the high-energy edge.[26,27]
2
ˆ
ˆ
ˆ
The emission intensity is proportional to E · :
, where μ
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T a b le 1 . Optical and lasing properties of 1,3,6,8-arylpyrene (P1–P6), 9,10-arylpyrene derivatives (A1–A4) and DCM.
UV–vis λmax [nm] (ε × 10−4
)
PL λmax [nm] [a,b]
Φ [a]
τ [ns] [a,b]
Solubility
OD
Threshold
Threshold
limit [wt%]
[nJ/pulse] [c]
[nJ/pulse] [d]
P1
302 (4.47), 392 (4.27)
309 (7.24), 393 (4.57)
316 (4.17), 396 (4.79)
317 (14.5), 394 (7.10)
315 (4.79), 398 (5.56)
314 (14.5), 394 (7.24)
435
447
451
453
453
453
428
435
432
435
565
0.90 [e]
0.91 [f]
0.85 [f]
0.85 [e]
0.91 [f]
0.90 [e]
0.79 [f]
0.58 [f]
0.78 [g]
0.55 [f]
0.10 [f]
1.82
1.43
1.60
1.53
1.40
1.39
3.40
2.66
3.24
2.66
1.27
0.8
0.2
0.6
>1.5
<0.1
>1.5
0.2
0.4
1.3
2
0.65
0.16
0.45
>1
45
100
43
44
–
180
100
120
290
–
P2
P3
P4
P5
–
P6
1
23
–
130
–
A1
A2
A3
A4
DCM
356 (0.95), 374 (1.41), 394 (0.95)
357 (1.12), 374 (1.74), 396 (1.62)
357 (1.15), 374 (1.86), 394 (1.74)
358 (1.18), 373 (1.93), 394 (1.85)
463 (3.78)
0.06
0.13
0.5
0.78
>1
–
–
135
70
440
480
300
–
–
[a] Measured in deaerated CH2Cl2. [b] Excited at absorption maxima. [c] Values for 25 μm thick cells with saturated dye concentrations except for P4 and DCM, in which
optical density (OD) was tuned to be 1 by adjusting dye concentrations. [d] Measured using cells with OD of 0.16. [e] Calculated using P2 as a standard. [f] Absolute
quantum yield. [g] Calculated using A2 as a standard.
oscillation at the high-energy edge, as shown in Figure 2a. The
orientation of the transition dipole in the pyrene derivatives is
unique but is a drawback for lasing; since the transition dipoles
are equally oriented perpendicular to the director, incident light
cannot be effectively used for dye excitation. But still, we have a
very low threshold. Our current effort is directed at aligning the
transition dipole parallel to the director by designing molecular
structures.
To determine the lasing threshold, we performed lasing
experiments and estimated the lasing emission intensity and
full width at half maximum (FWHM) as a function of input
pumping energy. The values of the thresholds are summarized
in Table 1. These dye-doped CLCs exhibit very low thresh-
olds. In particular, in a P6-doped CLC cell, the extremely low
threshold of 23 nJ/pulse was obtained, as shown in Figure 2e.
For comparison, we also performed experiments with a DCM-
doped CLC cell using the same optical setup as that used for
the other dye-doped CLC cells, and obtained the threshold
440 nJ/pulse. Thus, the threshold of P6 is approximately 1/20 that
of DCM. In order to investigate the effect of optical properties
of the molecules, we performed lasing experiments using cells
with the same thickness and the same optical density (OD =
0.16) and estimated the thresholds of the compounds in such
a condition.
Now we consider the factors that are important for lowering
the threshold. The luminous efficiency is important in order to
use pumping energy more efficiently. The luminous efficiency
is proportional to the quantum yield and the molar absorption
coefficient, so it is defined here as the product of these quanti-
ties. It is reasonable that anthracene derivatives show a higher
threshold than pyrene derivatives, since anthracene has lower
quantum yield and molar absorption coefficient (see Table 1).
In Figure 2f, red dots represent the relationship between the
luminous efficiency and threshold measured using dye-doped
CLC cells with the same OD. This figure indicates that the
threshold decreases with increasing luminous efficiency of
the dye. To obtain efficient lasing action, a reasonable OD is
necessary. To satisfy this condition, not only a high value of the
molar absorption coefficient but also good solubility in CLCs is
necessary.
As an example let us compare P1 and P2, which have sim-
ilar molar absorption coefficients, at least for the lower energy
bands. P2 has a lower threshold than P1 if we compare them
under the same condition of OD = 0.16. However, P1 can be
dissolved to a higher degree in CLCs than P2. Therefore, at the
solubility limits of the dyes, P1-doped CLC cells exhibit higher
OD than P2-doped CLC cells with the same thickness. Conse-
quently, the threshold of P1 cells is very low compared to that
of P2 cells. These experimental facts imply that high luminous
efficiency (high quantum yield and molar absorption coeffi-
cient) and good solubility are important factors in achieving a
low threshold for the LC laser. From these viewpoints, we can
state that P6 is the best dye for low threshold lasing in this
study because P6 exhibits high quantum yield (Φ = 0.90), high
absorption coefficient (ε = 7.24 × 104 at λ = 394 nm), and excel-
lent solubility in CLCs derived from its asymmetric alkyl-chain
and terphenyl moiety. By contrast, some pyrene and anthracene
derivatives (P5, A1, and A2) do not attain laser oscillation even
at high excitation power, partly because of low solubility. DCM
has relatively low quantum yield of only 0.1, although many
researchers use it as a dye for CLC lasers. In comparison, our
new dyes, particularly the derivatives, exhibit very high quantum
yield (Φ = 0.8 to 0.9) and are able to use the pumping energy
more efficiently than DCM. It is also important to note that dye
molecules have to avoid concentration quenching, even if they
have high molar absorption coefficient and quantum yield. Cao
et al. reported that in the case of DCM-doped CLCs with a high
concentration, the dye fluorescence was likely quenched by a
bimolecular process such as excimer formation, which led to
an increase in the threshold.[29] In addition, an excess amount
of dye might inhibit the formation of the cholesteric phase of
the host LCs. The use of highly efficient luminescence dyes as
developed in this work is ideal because only a small amount of
dye is sufficient to provide good optical properties.
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Figure 2. R e fl ectance (green curve), lasing emission (red curve), and emission (blue curve) spectra of a) a P2-doped CLC cell and b) an A3-doped CLC
cell. Absorption spectra of c) P2 and d) A3 in nematic LC cells using linearly polarized light parallel (blue) and perpendicular (red) to the rubbing direc-
tion (director). Red and blue arrows are directions of transition dipoles of pyrene and anthracene; the green arrow is the local director of liquid crystal
molecules. e) Pumping energy dependence of intensity and FWHM of the emission spectra for a P6-doped CLC cell. f) The dependence of threshold
on luminous efficiency (red dots) and radiative decay rate kf (blue dots). Threshold was measured using cells with OD = 0.16.
Here we summarize the relationship between lasing
threshold and radiative decay rate kf. Adachi and coworkers
reported the relationship for amplified spontaneous emis-
sion (ASE) materials. They claimed that the threshold of ASE
is inversely proportional to Einstein’s B coefficient and B is
proportional to kf, and they actually realized low-threshold
ASE.[30] We also plot the dependence of the threshold on
kf as blue dots in Figure 2f and find the relationship claimed
by Adachi and coworkers, that is, lowering of the threshold
with increasing kf. Unfortunately, however, we cannot predict
good lasing performance of each dye only by luminous effi-
ciency or kf. Actually the threshold of P4 is larger than that
of P3 by a factor of two, although their structural differences
exist only in the alkyl-chain and their optical properties are
quite similar.
conditions are different in each study. However, it is still pos-
sible to claim the superiority of pyrene derivatives as laser dyes,
because the reported threshold values for DCM-doped DFB-
CLC lasers are 200,[31] 300,[3] and 1000 nJ/pulse,[32] which are
approximately the same as those obtained in the present meas-
urement. As for the other dyes, the reported threshold values
are not high either, that is, 650 nJ/pulse for triptycene poly(p-
phenylenevinylene) (T-PPV),[9] 500–1500 nJ/pulse for quaterthi-
ophene[33–35] and 780 nJ/pulse[36] for pyrromethenes.
Finally, we want to comment on the damage threshold. It
is not easy to find a reasonable way to evaluate stability with
respect to intense laser light. Qualitatively, anthracenes defi-
nitely have a lower damage threshold than pyrene. As for
pyrene and DCM, if we use cells with the same OD, DCM
showed a slightly higher damage threshold. However, the lasing
threshold of pyrene is much lower than that of DCM. Hence if
we compare the damage in the cells showing the same lasing
intensity, the damage of pyrene is much smaller than that of
Many studies have been conducted on lasing thresholds in
CLC lasers. However, it is difficult to compare the obtained
values quantitatively because optical setups and sample
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DCM. In other words, it is very important to have dyes showing
low lasing threshold to minimize damage as well.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
In summary, we have synthesized new pyrene and anthra-
cene derivatives that have both high luminous efficiency and
high solubility in CLCs. The use of such highly efficient dyes
enables us to attain lower lasing thresholds in dye-doped DFB-
CLC lasers. The threshold values were decreased to one twen-
tieth that of DCM. A comparison of these dyes revealed that
good optical properties and solubility in CLCs are important
factors for realizing low threshold. Further, the lasing spectra
of pyrene-derivative-doped CLCs were observed at the high-
energy edge of the photonic bandgap. This is an unusual phe-
nomenon in studies so far, and is attributed to the orientation
of the transition dipole moment of the dye molecules. These
results should serve as guidelines for designing new dyes with
the objective of lowering the threshold of LC lasers.
Received: March 23, 2010
Revised: May 22, 2010
Published online: August 27, 2010
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