Highly dichroic and luminescent triphenodioxazine dyes

Article abstract of DOI:10.1246/cl.2011.573

We report herein a triphenodioxazine dye, which is highly dichroic and luminescent in liquid crystal (LC). The dye shows remarkable polarization dependence resulting from its high orientational order up to 0.88: the dichroic ratios from its absorption and that from the emission reach 22.1 and 67.7, respectively. Such high order has never been reported for luminescent dyes in LC, thus showing appreciable potential advantage for some applications.

Full text of DOI:10.1246/cl.2011.573

doi:10.1246/cl.2011.573
Published on the web April 28, 2011
573
Highly Dichroic and Luminescent Triphenodioxazine Dyes
Toshihiko Tanaka,1,2 Toru Ashida,1 and Shinya Matsumoto*2
1ASET Sumitomo Chemical Laboratory, Tsukuba Research Laboratory, Sumitomo Chemical Co., Ltd.,
6 Kitahara, Tsukuba, Ibaraki 300-3294
2Department of Environmental Sciences, Faculty of Education and Human Sciences, Yokohama National University,
79-2 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501
(Received February 25, 2011; CL-110156; E-mail: smatsu@edhs.ynu.ac.jp)
We report herein a triphenodioxazine dye, which is highly
dichroic and luminescent in liquid crystal (LC). The dye shows
remarkable polarization dependence resulting from its high
orientational order up to 0.88: the dichroic ratios from its
absorption and that from the emission reach 22.1 and 67.7,
respectively. Such high order has never been reported for
luminescent dyes in LC, thus showing appreciable potential
advantage for some applications.
Dichroic dyes have been studied in many years.1,2 They
orient in LC, thus showing large polarization dependence, called
dichroism, in their light absorption and/or luminescence. Since
the discovery of their electrooptic effects in LC,3 enormous
efforts have been devoted to the synthesis of novel dichroic dyes
to apply them to LC display applications during the 70s.1
Recently, their polarized luminescence also has attracted much
attention with respect to possible applications such as fluorescent
LC displays,4 fluorescent polarizers,5 light-emitting diodes,6 and
liquid crystal lasers.7
Hence, highly dichroic luminescent guest dye molecules
have been needed in these emerging fields. Most conventional
dichroic dyes are azo or anthraquinone derivatives and they
Figure 1. Synthetic scheme for dye 1 and dye 2.
usually do not have strong luminescence. Various other
luminescent chromophores have been investigated, including
naphthalimides,8,9 dilactones,10 thiadiazoles,11 benzanthrones,12
benzothiophenes,13 fluorophenazines,14 acenequinones,15 and
conjugated oligomers.16 However, their dichroic ratios usually
do not reach 10 and never exceed 15, thereby limiting device
performance in polarization degree. A linearly ³-conjugated
thiadiazole shows a high dichroic ratio up to 12.8.11
We found previously that some triphenodioxazines are
dichroic and luminescent in LC.17 Although triphenodioxazines
have a long history as commercial dyestuffs and pigments since
their discovery by Fischer in 1879;18 little has been known about
their dichroic derivatives with a few exceptions.19 In this report,
we have improved their dichroism and luminescence further
by removing the lateral chlorine atoms from the chromophore
having four mesogenic substituents.
consisted of two quartz glass plates about 20 ¯m apart, each
having a rubbed polyimide layer to obtain a parallel uniaxial
alignment of LC. Two host mixtures (E7 and E9) consisting of
some cyanobiphenyl derivatives were purchased from Merck
Japan Co., Ltd.22,23 Another mixture ASET-010 (AS) consisting
of 4-cyano-3-fluorophenyl 4-alkyllbenzoate derivatives was
supplied by the laboratory of Merck Japan Co., Ltd.24
Polarized absorption spectra of the cells were recorded at
23 °C on a Cary 5E spectrophotometer equipped with a highly
efficient Gran-Thomson polarizer on the optical path before the
sample. The spectra were obtained at normal incidence with
polarization parallel and perpendicular to the rubbing direction
(nematic director), respectively. The reference spectra taken with
the cells of the corresponding host without dye for each
polarization were subtracted from each sample spectrum during
data processing.
Triphenodioxazines 1 and 2 were synthesized from corre-
sponding aniline derivatives, quinone derivatives, and acid
chlorides via two-step cyclization as their schemes are shown in
Figure 1.17,20,21 The crude anilide intermediates from the first
step were used for the second step without further purification.
The structure of these intermediates was confirmed by the
corresponding mass signals as their main peaks in their TOF-MS
spectra.
Emission (EM) spectra of the cells were recorded at 23 °C on
a Jobin Yvon-Spex Fluorolog3 spectrofluorometer equipped with
two polarizers, one for the EM beam and the other for the
excitation (EX) beam at 520 nm. The EM at an oblique angle
(22.5 degrees from normal to the cell surface) was recorded with
the EX at normal incidence: the two polarizing directions are
aligned. The EM spectra were obtained with the aligned direction
parallel and perpendicular to the rubbing direction, respectively.
The dyes are oriented in host LC, thereby showing
significant dichroic ratio DA of at least 9.9. Figure 2 shows
LC mixtures containing the dyes (0.2%, w/w) were injected
into cells. The cells were purchased from EHC Co., Ltd. and
Chem. Lett. 2011, 40, 573-575
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

574
Figure 2. Polarized absorption spectra of the AS mixture containing
dye 1: (a) with the light polarization parallel to the rubbing direction;
and (b) with that perpendicular to the direction.
Figure 3. Polarized emission spectra of the AS mixture containing
dye 1: (a) with the excitation and emission polarized parallel to the
rubbing direction; and (b) with that polarized perpendicular to the
direction.
Table 1. Dichroism of the LC mixtures
Absorption
Emission
than those of dye 2. In particular, the mixture AS leads to
extreme orientation: the DA value reaches 22.1 (S = 0.88),
which is at the highest rarely shown for a few nonfluorescent
anthraquinone dyes in a patent application.25
The DA and DF dependence on the hosts accounts for the
orientation of the molecules surrounding dyes. Each dye shows
higher DA and DF values in order of nematic-isotropic transition
temperature (TNI) of the hosts; TNI values are 168 (AS), 84 (E9),
and 60 °C (E7). The host orientation at room temperature
generally depends on TNI, thus leading to the higher guest
orientation.
Dye Host
Stokes
shift/nm
-
max/nm DA
S
-
max/nm
DF
38
43.1
67.7
1
1
1
E7
E9
AS
558
559
554
13.3 0.80
15.3 0.83
22.1 0.88
578
578
570
20
19
16
2
2
2
E7
E9
AS
566
567
562
9.9 0.75
10.4 0.76
14.7 0.82
587
588
578
21
21
16
24.9
27.1
50.6
typical absorption spectra and the following values are summa-
The linearity of 6,13-hydrogenated chromophore exactly
contributes the orientation in LC. It has been known that the
orientation of guest dyes depends on their linearity in molecular
shape with respect to the steric hindrance with hosts; the
geometric ratio L/D (L: length, D: diameter) of guest molecules
correlates well with their S values in LC.26 Substituting 6,13-
chlorines with hydrogens increases L/D because of their large
difference in atom size; van der Waals radii of Cl and H are 0.18
and 0.11 nm, respectively.27
rized in Table 1. Dichroic ratios DA are defined for a peak (-max
in polarized spectra as
)
DA ¼ A== =A
ð1Þ
þ
where A// and A+ are the absorbance with the light polarization
parallel and perpendicular to the nematic director, respectively.
Order parameters S are determined from each DA values by the
following formula:1
Moreover, dye 1 also shows stronger EM than dye 2, thus
illustrating that the fluorescence intensity is also significantly
improved by the 6,13-hydrogenation. The polarized spectra also
enable us to compare the yields from 1 and 2 approximately
because their spectrum shape does not vary significantly. The
intensity F// normalized by the A// at the excitation wavelength
(A// 520) was calculated, thereby showing that the F///A// 520 values
from dye 1 are 1.7, 1.6, and 1.5 times to that from dye 2, for E7,
E9, and AS, respectively. The improvement in DF of dye 1
should slightly overestimate these values because of the decrease
both in the depolarization and in the lambertian emission. Then
the normalized EM intensity from the chloroform solution of
1 is also 2.4 times to that of 2 without polarization, thereby
confirming the yield improvement exactly. These nonpolarized
absorption and emission spectra were measured in the same
manner without the polarizers, respectively.
S ¼ ðDA ꢀ 1Þ=ðDA þ 2Þ
ð2Þ
The mixtures also exhibit large polarization in EM: typical
EM spectra are shown in Figure 3 and the following values are
summarized in Table 1. The dichroic ratio DF is also defined for
a peak in polarized EM spectra as
DF ¼ F== =F
ð3Þ
þ
where F// and F+ are the PL intensity at a peak with the
polarization (both EM and EX) parallel and perpendicular to the
director, respectively. Both the high S and the polarized excitation
accounts for the high DF values from 24.9 up to 67.7. If transition
moments do not fluctuate during the fluorescence lifetime, the
2
DF values should become approximately DA according to the
following formula for polarized excitation,8 thereby showing
appreciable depolarization by thermal fluctuation:
The slope in A// spectra is always sharp on the bathochromic
side of top peak, thereby showing rather small self-absorption
for EM in spite of the small Stokes shift down to 16 nm. The A//
decreases almost 80% from absorption peak at the EM peak
1=2
1=2
S ¼ ðDF ꢀ 1Þ=ðDF þ 2Þ
ð4Þ
Dye 1 shows higher orientation in each mixture than that of
dye 2: the DA and DF values from dye 1 are significantly larger
Chem. Lett. 2011, 40, 573-575
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

575
Rev. Prog. Color. Relat. Top. 1985, 15, 15.
19 M. Kaneko, T. Ozawa, T. Yoneyama, S. Imazeki, A. Mukoh, M.
Sato, Eur. Pat. EP 76,633, 1982.
(570 nm) and becomes negligible around the shoulder peak
(605 nm) as shown in Figure 2.
We think the remarkable orientation of dye 1 with the
improved fluorescence intensity provides appreciable advantage
in possible applications, and anticipate much attention for this
kind of dye from researchers in related fields.
20 1: 1,4-benzoquinone (0.5 g) and 3-{4-[4-(4-butylcyclohexyl)phen-
oxy]butoxy}-4-methoxyaniline (4.3 g) were added to ethanol (50
mL) and the mixture was heated to reflux for 7 h. After cooling the
mixture to room temperature, the precipitates were filtered. The
precipitates on the filter were washed with water and dried, followed
by the purification by silica gel column chromatography with
chloroform as eluents, thereby obtaining 0.8 g of crude intermediate.
The intermediate (0.1 g) and 4-butylbenzoyl chloride (1.0 g) were
added to nitrobenzene (20 mL) and the mixture was heated at 140 °C
for 7 h. After cooling the mixture to room temperature, the resulting
precipitates were filtered and washed with methanol on the filter. The
precipitates were purified 3 times by silica gel thin layer chromatog-
raphy with chloroform as eluents, thereby obtaining 10 mg of the
final dye 1 reddish powder. IR (KBr): 2956, 2924, 2856, 1741
This work was performed by Association of Super-Ad-
vanced Electronics Technologies in the Ministry of Economy
and Industry Program of Super-Advanced Electronics Technol-
ogies supported by New Energy and Industrial Technology
Development Organization. We gratefully acknowledge Dr.
Sawada (Merck Japan Co., Ltd.) and Dr. Hasegawa (IBM Japan
Co., Ltd.) for supplying the special LC mixture with large TNI.
(C=O), 1611, 1565, 1512, 1487, 1468, 1435, 1378, 1261, 1224,
¹1
References and Notes
1179, 1129, 1056, 1017, 826, 578 cm .
1H NMR (500 MHz,
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Chem. Lett. 2011, 40, 573-575
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/