Synthesis and characterization of fluorescent acenequinones as dyes for guest-host liquid crystal displays

Article abstract of DOI:10.1021/ol062999m

(Figure Presented) Syntheses and spectroscopic properties of alkoxy-substituted para-acenequinones are reported. These compounds showed excellent alignment in nematic liquid crystals as evidenced by polarized UV-vis absorption and fluorescence measurements.

Full text of DOI:10.1021/ol062999m

ORGANIC
LETTERS
2007
Vol. 9, No. 6
997-1000
Synthesis and Characterization of
Fluorescent Acenequinones as Dyes for
Guest−Host Liquid Crystal Displays
Zhihua Chen and Timothy M. Swager*
Department of Chemistry, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
tswager@mit.edu
Received December 12, 2006
ABSTRACT
Syntheses and spectroscopic properties of alkoxy-substituted para-acenequinones are reported. These compounds showed excellent alignment
in nematic liquid crystals as evidenced by polarized UV vis absorption and fluorescence measurements.
−
Guest-host liquid crystal displays (GH-LCDs) that use a
LC/dye mixture as the active material have received much
attention since their invention in the 1960s because of their
wider viewing angle, daylight readability, and high stability
in harsh environments.1-3 In GH-LCDs, the change in color
intensity is obtained by controlling the direction of dichroic
dye molecules which adsorb light more at one molecular axis
than the others. The optimal parameters of GH-LCDs depend
on the dichroic properties of dyes, their solubility in the LC
host, and their stability under various environments. An-
thraquinone and azo derivatives are the two major classes
of dyes receiving the most intensive study.3
region. Additionally, the rodlike shape of nematic LC
molecules favors alignment of elongated, rodlike dye mol-
ecules along the direction of the long molecular axis of
nematic LC molecules.
In this study, we report emissive linear para-acenequinone
dyes with large dichroic ratios in LC mixtures. We describe
the syntheses of a group of alkoxy-substituted acenequinone
derivatives, their solution absorption and emission spectra,
and the polarized absorption and emission spectra of their
LC solutions. These compounds demonstrated strong orienta-
tion properties and have excellent potential as fluorescent
dyes for GH-LCDs.
Recently, fluorescent dye based GH-LCDs, which combine
the excellent hues and high brightness levels of emissive
displays with the desirable features of LCDs, have been
proposed as a less energy consuming display for portable
electronics.4,5 For fluorescent dyes in LCs, the fluorescence
intensity can be controlled in a way similar to absorption.
Therefore, the development of fluorescent GH-LCDs requires
synthesis of fluorescent dyes with a high dichroic ratio, a
high quantum yield, and a strong emission in the visible
Synthetic methodologies for preparing linear para-acene-
quinones have been previously reported,6-8 and the Diels-
Alder (D-A) reaction between isobenzofurans (or their
(5) (a) Zhang, X.; Yamaguchi, R.; Moriyama, K.; Kadowaki, M.;
Kobayashi, T.; Ishi-I, T.; Thiemann, T.; Mataka, S. J. Mater. Chem. 2006,
16, 736. (b) Zhang, X.; Gorohmaru, H.; Kadowaki, M.; Kobayashi, T.; Ishi-
I, T.; Thiemann, T.; Mataka, S. J. Mater. Chem. 2004, 14, 1901. (c) Bojinov,
V. B.; Grabchev, I. K. Org. Lett. 2003, 5, 2185. (d) Matsui, M.; Suzuki,
M.; Mizuno, K.; Funabiki, K.; Okada, S.; Kobayashi, T.; Kadowaki, M.
Liq. Cryst. 2004, 11, 1463. (e) Iwanaga, H.; Naito, K. Liq. Cryst. 2000, 27,
115.
(1) (a) Heilmeier, G. H.; Zanoni, L. A. Appl. Phys. Lett. 1968, 13, 91.
(b) White, D. L.; Taylor, G. N. J. Appl. Phys. 1974, 45, 4718.
(2) Raj, D. Mater. Chem. Phys. 1996, 43, 204.
(3) Bahadur, B. In Liquid Crystals: Applications and Uses; Bahadur,
B., Ed.; World Scientfic: Singapore, 1992; Vol. 3, pp 65-208.
(4) Baur, G.; Stieb, A.; Meier, G. Mol. Cryst. Liq. Cryst. 1973, 22, 261.
(6) (a) Goodings, E. P.; Mitchard, D. A.; Owen, G. J. Chem. Soc., Perkin
Trans. 1 1972, 1310. (b) Meng, H.; Bendikov, M.; Mitchell, G.; Helgeson,
R.; Wudl, F.; Bao, Z.; Siegrist, C.; Kloc, C.-H. AdV. Mater. 2003, 15, 1090.
(c) Payne, M. M.; Delcamp, J. H.; Parkin, S. R; Anthony, J. E. Org. Lett.
2004, 6, 1609. (d) Payne, M. M.; Parkin, S. R.; Anthony, J. E. J. Am. Chem.
Soc. 2005, 127, 8028.
10.1021/ol062999m CCC: $37.00
© 2007 American Chemical Society
Published on Web 02/14/2007

analogues) and linear 1,4-quinones is one of the more
efficient approaches.7 In this work, we employed 3,6-di-2-
pyridyl-1,2,4,5-tetrazine (DPT) and anthracene 1,4-endoxide
3 to generate an isonaphthofuran, which is subsequently
trapped by a quinone via a D-A mechanism.9 The D-A
adducts are then easily converted into the corresponding
acenequinones. This methodology avoids direct use of
isobenzofuran/isonaphthofuran compounds, which are dif-
ficult to synthesize and are generally unstable. It has also
allowed us to prepare various substituted acenequinones.
The synthetic route to a para-pentacenequinone is outlined
in Scheme 1. The starting 6,7-dibromo-2,3-dihydroxynaph-
corresponding 1,4-anthraquinones (6 and 7) as the dieno-
philes (Scheme 2).
Scheme 2. Synthesis of Hexacenequinone 10 and 11
Scheme 1. Synthesis of Pentacenequinone 5
Dienophile 6,7,8,9-tetrafluoro-1,4-anthraquinone (7) was
synthesized previously from the oxidation of corresponding
1,4-hydroquinone.12 Here, we report a more convenient route,
as shown in Scheme 3. A Friedel-Crafts biscycloacylation
Scheme 3. Synthesis of Tetrafluoroanthraquinone 7
thalene (1), which was prepared from 2,3-dihydroxynaph-
thalene in two steps via a literature procedure,10 was
converted to 2 by a Williamson ether synthesis. Intermediate
2 was treated with phenyllithium (PhLi) in the presence of
excess furan, at low temperature, to produce 3. The genera-
tion of 4 was successfully accomplished by treating 3 with
DPT, in the presence of 1,4-naphthoquinone. Pentacene-
quinone 5 was then obtained from dehydration of 4, which
could be realized by employing acetic acid,9b p-toluene-
sulfonic acid (p-TsOH)/toluene,7c or pyridinium p-toluene-
sulfonate (PPTS)/acetic anhydride.11 We found the PPTS
approach affords clean product in a satisfactory yield by a
simple filtration. Compound 5 is an orange/red solid, with a
high melting point and excellent thermal and photochemical
stability.
of 1,4-dihydroxybenzene and 3,4,5,6-tetrafluorophthalic an-
hydride was conducted in a melt of aluminum chloride/
sodium chloride (AlCl3/NaCl) to give tetrafluoroquinizarin
12.13 Reduction of 12 with sodium borohydride (NaBH4) and
a subsequent dehydration with concentrated aqueous HCl
afforded 7 as a dark-brown solid.14
Using 1,4-benzoquinone as the limiting reagent, we
prepared a tetraalkoxy-substituted heptacenequinone (14)
A similar methodology allowed us to successfully syn-
thesize two para-hexacenequinones (10 and 11) from the
Scheme 4. Synthesis of Heptacenequinone 14
(7) (a) Smith, J. G.; Dibble, P. W.; Sandborn, R. E. J. Org. Chem. 1986,
51, 3762. (b) Smith, J. G.; Fogg, D. E.; Munday, I. J.; Sandborn, R. E.;
Dibble, P. W. J. Org. Chem. 1988, 53, 2942. (c) Miller, G. P.; Briggs, J.
Tetrahedron Lett. 2004, 45, 477.
(8) (a) Martin, N.; Behnisch, R.; Hanack, M. J. Org. Chem. 1989, 54,
2563. (b) Almlo¨f, J. E.; Feyereisen, M. W.; Jozefiak, T. H.; Miller, L. L.
J. Am. Chem. Soc. 1990, 112, 1206. (c) Thomas, A. D.; Miller, L. L. J.
Org. Chem. 1986, 51, 4160. (d) Christopfel, W. C.; Miller, L. L. J. Org.
Chem. 1986, 51, 4169.
(9) (a) Warrener, R. N. J. Am. Chem. Soc. 1971, 93, 2346. (b) Chan,
S.-H.; Yick, C.-Y.; Wong, H. N. C. Tetrahedron 2002, 58, 9413. (c) Yick,
C.-Y.; Chan, S.-H.; Wong, H. N. C. Tetrahedron Lett. 2000, 41, 5957.
(10) Cammidege, A. N.; Chambrier, I.; Cook, M. J.; Garland, A. D.;
Heeney, M. H.; Welford, K. J. Porphyr. Phthalocya. 1997, 1, 77.
(11) Chen, Z.; Amara, J. P.; Thomas, S. W.; Swager, T. M. Macromol-
ecules 2006, 39, 3202.
998
Org. Lett., Vol. 9, No. 6, 2007

(Scheme 4). Interestingly, the only detected products are exo/
exo and exo/endo isomers of 13. Dehydrating both isomers
yields heptacenequinone 14 as a yellow/orange solid.
A solution of the acenequinones possesses a characteristic
strong absorption band at 350-400 nm and a second
absorption band at 420-500 nm (Figure 1A), both of which
Table 1. Absorption and Emission Maxima and Quantum
Yields of Acenequinones in Toluene
a
compd
λmax (nm)
λem (nm)
Φrel
5
366
475
369
465
378
482
376
470
515
0.30
0.21
0.24
0.18
1
can be assigned as (π, π*) absorptions.15
10
11
14
493
519
484
a
Fluorescence quantum yield relative to coumarin 153 (Φrel ) 0.38).
The orientation properties of acenequinone dyes in LC
hosts were investigated via the measurement of polarized
absorption and fluorescent spectra in aligned test cells.
Samples were prepared by loading dye/LC fluids (0.1-0.5
wt % dye in MLC-6884) into LC test cells.16 The dye/LC
mixture forms a homogeneously aligned structure, and the
polarized UV-vis absorption and emission spectra were
determined.
The polarized absorption spectra of acenequinone 5 in LC
hosts are displayed in Figure 2A. The anisotropy of molecular
Figure 1. UV-vis absorption spectra and fluorescence spectra of
acenequinones in toluene (5, red; 10, green; 11, blue; 14, black).
When compared to their nonsubstituted hydrocarbon acene
analogues, these acenequinone compounds have broadened
absorption peaks that are red shifted by about 20-30 nm.
These differences are the result of intramolecular charge
transfer (ICT) character between the electron-with-
drawing quinoid moiety and electron-donating alkoxy side
chains.15
All the acenequinone derivatives are fluorescent in solu-
tion, with the relative quantum yields ranging from 0.18 to
0.30 (Table 1). Figure 1B displays the fluorescence spectra
of the acenequinones in toluene. The fluorescence spectra
reveal that the emission maximum (λem) of 10 has a 30 nm
blue shift as compared to that of 5. This blue shift results
from the reduced ICT character of 10 because the naphtho-
fused quinoid group has weaker electron-withdrawing
properties than the benzo-fused quinoid group in 5.5a Such
an effect was also demonstrated by the 35 nm red shift of
the λem of 11 and the 10 nm blue shift of the λem of 14, with
respect to the λem of 10. Clearly, the fluorine groups
significantly enhance the ICT character in 11.5d These
results show that photophysical properties of para-acene-
quinones can be tuned by attaching suitable functional
groups.
Figure 2. Polarized absorption (A) (solid line, A|; dotted line, A )
and fluorescence spectra (B) (solid line, I|; dotted line, I ) of 5 in
MLC-6884. The vertical dotted-dashed lines indicate the wave-
lengths of dichroic ratios reported.
absorption transition moments is in agreement with our
preliminary calculations that reveal the transitions of the two
major absorption bands are aligned with the long molecular
axis.17 Therefore, the dichroic ratio (DA) and order parameter
(SA) of a dye in LC can be calculated by using the following
equations:
(12) Swartz, C. R.; Parkin, S. R.; Bullock, J. E.; Anthony, J. E.; Mayer,
A. C.; Malliaras, G. G. Org. Lett. 2005, 7, 3163.
(13) Krapcho, A. P.; Getahun, Z.; Avery, K. J., Jr. Synth. Commun. 1990,
20, 2139.
(14) Hua, D. H.; Tamura, M.; Huang, X.; Stephany, H. A.; Helfrich, B.
A.; Perchellet, E. M.; Sperfslage, B. J.; Perchellet, J.-P.; Jiang, S.; Kyle, D.
E.; Chiang, P. K. J. Org. Chem. 2002, 67, 2907.
(16) See Supporting Information for detailed descriptions of test cells
and LCs.
(17) The computation was carried out by Gaussian, employing ZINDO
on the DFT B3LYP 6-31.G(d) optimized geometry. See Supporting
Information for details.
(15) Itoh, T. Chem. ReV. 1995, 95, 2351.
Org. Lett., Vol. 9, No. 6, 2007
999

DA ) A|/A
The reorientation of dyes under the electric field was
demonstrated in a test cell (Figure 3), containing a solution
SA ) (A| - A )/(A| + 2A )
where A| and A represent the absorbance for parallel and
perpendicular irradiation with respect to the LC director.3
As summarized in Table 2, those compounds demonstrated
Table 2. Spectral Characterizations, Dichroic Ratios, and
Order Parameters of Acenequinones in MLC-6884
Figure 3. Fluorescent images of a test cell containing 5 in E7 (A,
no applied voltage; B, 9 V applied voltage). The sample was excited
with a 365 nm light from a hand-held UV lamp.
compd λmax (nm)
DA
SA
λem (nm) DF
SF
5
10
11
14
368
478
373
470
383
485
380
477
8.1
8.8
8.8
9.6
8.9
0.70
0.72
0.72
0.74
0.72
0.75
522
504
532
9.2 0.73
10.4 0.76
10.1 0.75
of 0.2 wt % 5 in E7 (a commercial LC mixture with positive
dielectric anisotropy). Because the test cell is coated with
parallel rubbed polyimide films, the LC/dye mixture forms
a homogeneously aligned texture, with the long molecular
axis of LCs and dyes parallel with the rubbing direction.
Therefore, acenequinone dyes in this orientation give strong
emission when they are excited by a UV light (Figure 3A).
Upon application of an electric field, the LC director and
acenequinone backbones align normal to the surface of the
test cell. Consequently, the transition dipole of the dye
compound is parallel to the direction of incident light,
resulting in decreased projection of the transition dipole along
the electric vector of incident UV light and minimal
absorption and emission (Figure 3B).5e,19 The polarized
fluorescence is rapidly recovered upon removal of the
voltage.
In summary, we have reported a convenient synthesis of
substituted para-acenequinones by employing DPT and an
anthracene 1,4-endoxide. The polarized absorption and
fluorescence results demonstrate that acenequinones align
with nematic LC hosts, indicating their potential as dichroic
dyes for GH-LCDs. In addition, the sensitivity of the
emission to the substitution allows the tuning of fluorescence
by functionalization of the acenequinone chromophore.
10.0
8.7-12.8a 0.72-0.80a
7.7-8.8a 0.69-0.72a
-
-
-
b
b
b
a
The exact DA and SA of 14 could not be obtained due to the strong
interference from the test cell. b The DF and SF of 14 could not be determined
due to its low relative quantum yield and poor solubility in LCs.
dichroic ratios in the range of 8-10, indicating their excellent
alignment in the LC host. It has been reported that dyes with
a dichroic ratio greater than 8 are appropriate for practical
use.5b
Figure 2B shows the representative polarized fluorescence
spectra. The dichroic ratio (DF) and order parameter (SF)
based on polarized fluorescence can be calculated by using
the following equations:
DF ) I|/I
SF ) (I| - I )/(I| + 2I )
where I| and I represent the emission intensity for parallel
and perpendicular irradiation with respect to the LC direc-
tor.18 Order parameter values from the polarized fluorescence
are similar to those from the polarized absorption spectra
(Table 2). Compounds 10 and 11 have higher aspect ratios
and as expected have higher orientation properties than 5.
This feature agrees with the classic model that elongation
of dichroic dye molecules should enhance their orientation
in nematic LCs. The fluorescence maxima of acenequinones
in LCs were found to have a small blue shift (7-13 nm) as
compared to those in toluene solution. This feature is likely
due to the different dielectric properties of each medium.
Acknowledgment. This research has been supported by
the National Science Foundation under Grant DMR-0314421.
We gratefully acknowledge Merck Chemicals Ltd. for
supplying E7. We would like to thank Dr. Anne McNeil,
Mr. Jean Bouffard, and Ms. Inja Song (MIT) for their helpful
discussions.
Supporting Information Available: Experimental pro-
cedures and characterization data for new compounds
reported in this paper. This material is available free of charge
via the Internet at http://pubs.acs.org.
OL062999M
(18) Breen, C. A.; Deng, T.; Breiner, T.; Thomas, E. L.; Swager, T. M.
J. Am. Chem. Soc. 2003, 125, 9942.
(19) Zhu, Z.; Swager, T. M. J. Am. Chem. Soc. 2002, 124, 9670.
1000
Org. Lett., Vol. 9, No. 6, 2007