Rigidified merocyanine dyes with different aspect ratios: Dichroism and photostability

Article abstract of DOI:10.1016/j.dyepig.2015.04.041

A series of new rigidified merocyanines were investigated with regard to their optical properties as dichroic dyes. Guest/host-mixtures of the dyes were prepared using a liquid crystal and a reactive mesogen mixture. Their dichroism was studied using linearly-polarized UV/Vis-spectroscopy. A strong dependence of the dichroic ratio on the aspect ratio, the number of double bonds in the molecular structure, and on the maximum wavelength of absorption was found. A strategy to increase the aspect ratio has also been demonstrated. Additionally, the photostability was characterized using continued irradiation with polychromatic light from a xenon source. High photostability was found in all host mixtures in the absence of oxygen by alignment in a matrix. The suitability for their application as dichroic dyes in thin layer polarizers and guest/host-displays is herein discussed.

Full text of DOI:10.1016/j.dyepig.2015.04.041

Accepted Manuscript
Rigidified Merocyanine Dyes with Different Aspect Ratios: Dichroism and
Photostability
Katharina Christina Kreß, Korinna Bader, Dr. Joachim Stumpe, Prof. Dr. S.Holger
Eichhorn, Prof. Dr. Sabine Laschat, Dr. Thomas Fischer
PII:
S0143-7208(15)00171-0
10.1016/j.dyepig.2015.04.041
DYPI 4763
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 5 March 2015
Revised Date: 27 April 2015
Accepted Date: 29 April 2015
Please cite this article as: Kreß KC, Bader K, Stumpe J, Eichhorn SH, Laschat S, Fischer T, Rigidified
Merocyanine Dyes with Different Aspect Ratios: Dichroism and Photostability, Dyes and Pigments
(2015), doi: 10.1016/j.dyepig.2015.04.041.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Rigidified Merocyanine Dyes with Different Aspect Ratios:
Dichroism and Photostability
a
a
b
c
Katharina Christina Kreß, Korinna Bader, Joachim Stumpe, S. Holger Eichhorn
a
b
Sabine Laschat * and Thomas Fischer
a
K. C. Kreß, K. Bader, Prof. Dr. S. Laschat, Institut für Organische Chemie, Universität Stuttgart,
Pfaffenwaldring 55, 70569 Stuttgart, (Germany), Tel: (+49) 711-68564268, E-mail:
sabine.laschat@oc.uni-stuttgart.de
b
Dr. J. Stumpe, Dr. Th. Fischer, Universität Potsdam Transfer, Am Mühlenberg 11, 14476 Potsdam-
Golm (Germany)
c
Prof. Dr. S. H. Eichhorn, Department of Chemistry and Biochemistry, University of Windsor,
Windsor, ON, N9B 3P4 (Canada)
Highlights
•
•
•
•
We synthesized new rigidified merocyanine dyes.
Improvement of dichroic ratio of a guest/host system by varying the matrix.
Increase of the photostability by alignment in a matrix.
Proof of suitability of the dyes as thin layer polarizers and guest/host displays.
Abstract
A series of new rigidified merocyanines were investigated with regard to their optical properties as
dichroic dyes. Guest/host-mixtures of the dyes were prepared using a liquid crystal and a reactive
mesogen mixture. Their dichroism was studied using linearly-polarized UV/Vis-spectroscopy. A strong
dependence of the dichroic ratio on the aspect ratio, the number of double bonds in the molecular
structure, and on the maximum wavelength of absorption was found. A strategy to increase the
aspect ratio has also been demonstrated. Additionally, the photostability was characterized using
1

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continued irradiation with polychromatic light from a xenon source. High photostability was found in
all host mixtures in the absence of oxygen by alignment in a matrix. The suitability for their
application as dichroic dyes in thin layer polarizers and guest/host-displays is herein discussed.
Keywords
dichroic dye ∙ merocyanines ∙ aspect ratio ∙ photostability ∙ guest/host systems
1
. Introduction
Currently, thin-film-transistor liquid crystal displays (TFT-LCDs) are the most popular flat panel
displays due to their low weight, low power consumption and compact size.[1] An essential part of
TFT-LCDs is the polarizing film, which is generated by stretching a polymer film containing iodine
and/or dichroic dye molecules.[2] Polyvinyl alcohol/iodine polarizing films are the most widely used
examples of these due to their excellent optical properties with distinct and intense absorption in the
UV/Vis region.[3] However, important limitations of these polarizing films are their high sensitivity
towards humidity and towards elevated temperatures due to the sublimation of iodine.[4–6]
Presently, these deficiencies are circumvented by a special lamination of the polarizing film but this
packaging increases the overall thickness to several hundred microns, which makes its incorporation
into the stack of functional layers within the display unfeasible. Instead, the polarizing film is typically
applied on top of the display glass despite the occurrence of cross-talk by parallax that restrict fill-
factor and resolution. Therefore, so-called in-cell polarizers of less than 5 microns thickness are
widely investigated for a possible integration into the LC display. A promising approach based on the
seminal work by Broer utilizes dichroitic guest/host polarizers.[7] He reported in 1989 that selective
absorption of light could be achieved by aligning a dichroitic dye (guest) in an oriented,
polymerisable liquid crystalline host. The low viscosity of the reactive monomeric liquid crystal
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facilitates good alignment, while the in situ photopolymerization enables the selection of the
optimum phase with highest molecular order.[8] This technology has been applied in the preparation
of numerous optical devices, for example patterned retarders,[9] broadband circular polarizers,[10]
wide viewing angle films,[11] and cholesteric color filters.[12] Most important properties for an
application as polarizing film are a high dichroic ratio and polarizing efficiency. Both properties can be
improved by increasing the molecular order of the polymeric host or preceding mesophase and by
increasing the aspect ratio of the dichroic dye for a better alignment in the anisotropic host matrix.
We believe the latter approach is more promising because the use of higher ordered matrices usually
requires a change from nematic to smectic mesophases but the reliable fabrication of good quality
smectic layers is technically much more challenging.[13]
Also, new dichroic dyes have many other potential applications such as in guest/host displays. They
were invented in the late 1960s, first based on the Heilmeier/Zanoni mode[14] and later on the
White/Taylor mode.[15] The latter mode does not require additional polarizers, which promises the
fabrication of simpler device geometries with lower power consumption. The lack of effective
dichroic dyes and the success of devices based on the twisted nematic mode halted the development
of guest/host displays, but renewed interest is currently fueled by their comparatively low cost.
Potential applications include large area information displays,[16] reflective [17–19] and fluorescent
displays,[20] as well as windows and tinted goggles with adjustable transparency.[21]
All these applications rely on the availability of dichroic dyes with appropriate properties. Most
dichroic dyes that have been studied in displays are based on azobenzene and anthraquinone
derivatives[22,23] and the objective of the work presented here is to evaluate the performance of
merocyanine dyes as an alternative dichroic dye. Surprisingly few studies on the dichroism of
merocyanines have been reported[13] although they have been tested for many other applications,
such as in NLO devices,[24] dye sensitized solar cells,[25,26] medical peptide sensors,[27] and water
3

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analysis,[28] because of their strong and sharp absorption bands,[29] pronounced fluorescence,
solvatochromism and photochromism, and propensity to form aggregates.[30,31]
Presented herein is the first study on the dichroism of rigidified merocyanine dyes, which have a rod-
like shape of adjustable aspect ratio, adjustable linear optical properties,[32] and are expected to
possess high photostability. The merocyanine dyes investigated here consist of three cyanoacetates
which bear different chromophoric units (trimethine MC 1, pentamethine MC 2, heptamethine
MC 3), three cyanoacetates with tethered p-alkoxybenzoate units differing in their alkyl chain length
(
(
hexyl MC 4a, decyl MC 4b and tetradecyl MC 4c), and two other heptamethine derivatives
malononitrile MC 5, barbiturate MC 6), which carry strong acceptor groups and vary significantly in
their aspect ratio (Scheme 1). To improve general solubility of the investigated compounds a
benzoate unit was added to the alkyl chain in the case of derivative MC 4. Reported are the
photostabilities of these dyes and the dichroic ratios of their mixtures with a liquid crystal (LC) and a
reactive mesogen mixture (RMM) as host matrices to assess their suitability for applications in thin
film polarizers. Also presented is an analysis of how different aspect ratios of the merocyanine dyes
correlate with the observed dichroic ratios.
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O
O
O
C H O
N
C H O
N
6
13
6 13
C H O
N
6
13
CN
CN
E/Z = 55 : 45
MC 2
CN
E/Z = 29 : 71
E/Z = 46 : 54
MC 1
MC 3
O
O
O
4
O
N
CN
E/Z = 29 : 71
RO
MC 4a R: C6H13 MC 4b R: C10H21 MC 4c R: C14H29
O
NC
CN
N
N
N
S
N
O
MC 5
MC 6
Scheme 1
2
2
. Experimental
.1 Materials
Synthesis. All commercial reagents were used without further purification. Solvents were dried and
distilled under nitrogen prior to use and all reactions were carried out under a nitrogen atmosphere
with Schlenk-type glassware. Flash chromatography was performed on silica gel, grain size 40-63 µm
(
Fluka).
Guest/host cells. As a LC host-material the LC-Mixture M677 of Nematel was used (a mixture of
cyano- and alkoxy phenylcyclohexyl-, biphenylcyclohexyl- and bicyclohexylphenyl-derivatives). The
clearing point of this mixture is 115 °C. As reactive mesogen mixtures (RMM) were used RMM14 (Tcl=
1
54 °C) and RMM34 (70 °C) of Merck KGaA and a 1:1 mixture of RMM34 and ST01011 (153 °C) of the
Synthon GmbH.
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2
.2 Characterization
The following instruments were used for characterization of the compounds. NMR: Bruker Avance
1
13
1
13
5
00 ( H, 500 MHz; C, 125 MHz). H and C NMR spectra were referenced to TMS (Me Si
4
δ = 0.0 ppm, δ = 0.0 ppm) as an internal standard. The following abbreviations were used: s
H
C
(
singlet), d (doublet), t (triplet), m (multiplet), m (centered multiplet). All spectra were recorded at
c
room temperature. Chemical shift calculations and 2D experiments (COSY and HMBC) supported the
assignments of the signals. IR: Bruker Vector 22 FT-IR Spectrometer with MKII golden gate single
reflection Diamant ATR system. To describe the intensities following abbreviations were used: vs
(
very strong), s (strong), m (medium), w (weak). Elemental analyses: Carlo Erba Strumentazione
Elemental Analyzer, Modell 1106. HRMS (ESI): Bruker Daltonics microTOF-Q spectrometer. Melting
points: Olympus BX50 polarizing microscope combined with a Linkam TP93 central controller.
The cells were characterized by linearly polarized UV/Vis-spectroscopy. For these measurements a
λ19 spectrometer (Perkin Elmer) was used, which was equipped with two Glan-Taylor prisms rotated
by a stepper motor. Alternatively, a polarization prism equipped diode array spectrometer TIDAS (co.
⊥
ǁ
J&M) was used. The dichroic ratio was calculated as DR(λ)=A (λ)/A (λ), whereas A is the absorbance
ǁ
parallel and A
FR(λ)=I (λ)/I (λ) was determined in transmission geometry, whereas the excitation was carried out
using linearly polarized monochromatic light with the polarization plane parallel (I ) and
⊥
perpendicular to the orientation direction. The anisotropy of fluorescence
ǁ
⊥
ǁ
⊥
perpendicular (I ) to the orientation direction of the sample, respectively. Remaining transmitted
excitation light was absorbed by a combination of long pass filters.
2
.3 Computation
Density functional theory (DFT) calculations were performed using a Gaussian‘09 software package.
The geometry of the dyes was optimized using the B3LYP method with the 3-21G* basis set. The
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optimized structures have been classified as local minima on their respective potential energy
surfaces according to their vibrational frequencies. None of the vibrational frequencies in the
optimized geometries generated negative frequencies in their ground state.
2
.4 Synthesis of the dyes
Compounds 2a,[33,34] 2b,[33,35] 2c,[33,36] 4,[37], 7[32] and MC 6[24] are known in the literature
see Scheme 2). The synthesis of the molecules MC 1, MC 2, MC 3 and MC 5 was reported earlier.[32]
(
Atom numbering of compounds 5 and MC 4 does not consequently follow the IUPAC guidelines but
simplifies comparison of their NMR data.
2
.4.1 The dye precursors (5)
General procedure for the synthesis of the dye precursors 5
O
2'
1
3
1'
O
3
'
O
CN
2
4
4
'
O
RO
R = C6H13 5a R = C10H21 5b R = C14H29 5c
4
7
2
-Alkyloxybenzoic acid (4-hexyloxy benzoic acid 2a 650 mg, 2.93 mmol; 4-decyloxy benzoic acid 2b
60 mg, 2.72 mmol; 4-tetradecyloxy benzoic acid 2c 967 mg, 2.89 mmol), 4-hydroxybutyl
-cyanoacetate (for 5a 460 mg, 2.93 mmol; for 5b 470 mg, 2.99 mmol; for 5c 500 mg, 3.18 mmol),
DCC (for 5a 665 mg, 3.22 mmol) respectively EDCI (for 5b 640 mg, 3.35 mmol; for 5c 683 mg,
.56 mmol), and a catalytic amount of DMAP were dissolved in dichloromethane and stirred for 48 h
3
at room temperature. After the filtration of the insoluble components, the solvent was removed
under reduced pressure. The residue was purified by flash chromatography on silica gel (petrol
ether/EtOAc 5 : 1) to yield the pure products as colorless solids.
7

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4
-(2-Cyanoacetoxy) butyl 4-(hexyloxy) benzoate (5a)
-
1
Yield: 160 mg (15%) of 5a. Mp: 46 °C. FT-IR (ATR) (cm ): 1749 (m), 1710 (s), 1707 (s), 1606 (s), 1511
(
m), 1335 (w), 1315 (w), 1276 (s), 1250 (vs), 1190 (m), 1167 (s), 1119 (m), 1102 (m), 1026 (w), 1008
1
(
w), 848 (w), 771 (m). H-NMR (CDCl , 500 MHz) (ppm): 0.91 (m , 3H, CH ), 1.32 – 1.36 (m, 4H,
3
c
3
CH CH CH ), 1.47 (m, 2H, CH CH CH OAr), 1.80 (m , 2H, CH CH OAr), 1.86 (m , 4H, 2-H, 3-H), 3.46 (s,
3
2
2
2
2
2
c
2
2
c
2
7
2
1
3
H, CH CN), 4.00 (t, J = 6.6 Hz, 2H, CH OAr), 4.29 (m , 2H, 4-H), 4.33 (m , 2H, 1-H), 6.91 (m , 2H, 3‘-H),
2 2 c c c
1
3
.97 (m , 2H, 2‘-H). C-NMR (CDCl , 125 MHz) (ppm): 14.0 (CH ), 22.6 (CH CH ), 24.7 (CH CN), 25.2,
c
3
3
3
2
2
5.3, 25.7, 29.1, 31.5 (C-2, C-3, CH ), 63.8 (C-1), 66.5 (C-4), 68.2 (CH COAr), 112.9 (CN), 114.1 (C-3’)
2
2
+
22.2 (C-1’), 131.6 (C-2’), 162.9 (C-4’), 163.1 (C=OCH ), 166.3 (ArC=O). HRMS (ESI) found [M+Na]
2
+
84.1782, molecular formula C H NO Na requires [M+Na] 384.1787. CHN: found C, 66.70; H, 7.59;
20
27
5
N, 3.98%; molecular formula C H NO requires C, 66.46; H, 7.53; N, 3.88%.
20
27
5
4
-(2-Cyanoacetoxy) butyl 4-(decyloxy) benzoate (5b)
-
1
Yield: 130 mg (11%) of 5b. Mp: 51 °C. FT-IR (ATR) (cm ): 2919 (m), 1733 (s), 1696 (s), 1606 (s), 1341
1
(
m), 1278 (m), 1257 (vs), 1211 (s), 1189 (s), 1170 (vs), 1121 (m), 1105 (m), 1016 (m), 769 (s). H-NMR
(
CDCl , 500 MHz) (ppm): 0.88 (m , 3H, CH ), 1.21 – 1.39 (m, 12H, CH ), 1.46 (m, 2H, CH CH CH OAr),
3
c
3
2
2
2
2
1
4
.80 (m , 2H, CH CH OAr), 1.86 (m , 4H, 2-H, 3-H), 3.46 (s, 2H, CH CN), 4.00 (t, J = 6.6 Hz, 2H, CH OAr),
c 2 2 c 2 2
13
.29 (m , 2H, 4-H), 4.33 (m , 2H, 1-H), 6.91 (m , 2H, 3‘-H), 7.97 (m , 2H, 2‘-H). C-NMR (CDCl , 125
c
c
c
c
3
MHz) (ppm): 14.1 (CH ), 22.7 (CH CH ), 24.7 (CH CN), 25.2, 25.3, 26.0, 29.1, 29.3, 29.4, 29.6, 31.9 (C-
3
3
2
2
2
1
, C-3, CH ), 63.8 (C-1), 66.5 (C-4), 68.3 (CH OAr), 112.9 (CN), 114.1 (C-3’) 122.2 (C-1’), 131.6 (C-2’),
2 2
+
62.9 (C-4’), 163.1 (C=OCH ), 166.3 (ArC=O). HRMS (ESI) found [M+Na] 440.2408, molecular formula
2
+
C H NO Na requires [M+Na] 440.2413. CHN: found C, 69.26; H, 8.40; N, 3.30%; molecular formula
24
35
5
C H NO requires C, 69.04; H, 8.45; N, 3.35%.
24
35
5
8

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4
-(2-Cyanoacetoxy) butyl 4-(tetradecyloxy) benzoate (5c)
-1
Yield: 160 mg (12%) of 5c. Mp: 61 °C. FT-IR (ATR) (cm ): 2957 (m), 2915 (vs), 2850 (s), 1737 (s), 1712
(
(
vs), 1607 (m), 1472 (m), 1344 (m), 1321 (w), 1282 (m), 1254 (s), 1219 (w), 1189 (m), 1171 (m), 769
1
m). H-NMR (CDCl , 500 MHz) (ppm): 0.88 (m , 3H, CH ), 1.22 – 1.39 (m, 20H, CH ), 1.46 (m, 2H,
3
c
3
2
CH CH CH OAr), 1.80 (m , 2H, CH CH OAr), 1.86 (m , 4H, 2-H, 3-H), 3.46 (s, 2H, CH CN), 4.00 (t, J = 6.6
2
2
2
c
2
2
c
2
1
3
Hz, 2H, CH OAr), 4.29 (m , 2H, 4-H), 4.33 (m , 2H, 1-H), 6.91 (m , 2H, 3‘-H), 7.97 (m , 2H, 2‘-H). C-
2
c
c
c
c
NMR (CDCl , 125 MHz) (ppm): 14.1 (CH ), 22.7 (CH CH ), 24.7 (CH CN), 25.2, 25.3, 26.0, 29.1, 29.4,
3
3
3
2
2
2
9.57, 29.61, 29.67, 29.69, 29.70, 31.9 (C-2, C-3, CH ), 63.8 (C-1), 66.5 (C-4), 68.3 (CH OAr), 114.1
2 2
(
C-3’) 122.2 (C-1’), 131.6 (C-2’), 162.9 (C-4’), 163.1 (C=OCH ), 166.4 (ArC=O). HRMS (ESI) found
2
+
+
[
M+Na] 496.3034, molecular formula C H NO Na requires [M+Na] 496.3039. CHN: found C, 70.95;
28
43
5
H, 9.16; N, 2.94%; molecular formula C H NO requires C, 71.00; H, 9.15; N, 2.96%.
28
43
5
2
.4.2 The benzoate dyes (MC 4)
General procedure for the Knoevenagel condensation[38,39]
To an ice-cooled solution of trimethyloxonium tetrafluoroborate (for MC 4a 259 mg, 1.75 mmol; for
MC 4b 135 mg, 0.91 mmol; for MC 4c 155 mg, 1.05 mmol) in absolute dichloromethane was added
the carbonyl compound 7 (for MC 4a 179 mg, 1.00 mmol; for MC 4b 93 mg, 0.52 mmol; for MC 4c
1
07 mg, 0.60 mmol) at 0 °C and the resulting mixture was warmed to room temperature. After
stirring for 12 h the oxonium salt was precipitated by addition of n-pentane. In a second flask, the
acceptor moiety (5a 180 mg, 0.50 mmol; for 5b 110 mg, 0.26 mmol; for 5c 142 mg, 0.30 mmol) was
treated with DBU (for MC 4a 0.13 mL, 0.90 mmol; for MC 4b 0.07 mL, 0.47 mmol; for MC 4c 0.08 mL,
9

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0
.54 mmol) and the mixture was stirred for 45 min at room temperature. The above described
oxonium salt was added and the reaction mixture was heated to 80 °C for 2 h and subsequently
stirred for further 2 h at room temperature. The mixture was diluted with dichloromethane and
washed with 1 N hydrochloric acid. The aqueous layer was separated, extracted with
dichloromethane and the combined organic layers were washed with brine, dried over anhydrous
magnesium sulfate, filtered and the solvent was removed under reduced pressure. The residue was
purified by flash chromatography on silica gel (petrol ether/EtOAc 2 : 1) to yield the yellow solid
products as inseparable mixtures of double bond isomers.
4
-(2-Cyano-2-(3-(piperidin-1-yl) cyclohex-2-en-1-yliden) acetoxy) butyl-4- (hexyloxy)-benzoate
(
MC 4a)
1
Yield: 105 mg (40%) of MC 4a (isomeric ratio E/Z = 29 : 71 according to H-NMR). Mp: 61 °C. FT-IR
-
1
(
(
ATR) (cm ): 2957 (m), 2915 (vs), 2850 (s), 1737 (s), 1712 (vs), 1607 (m), 1472 (m), 1344 (m), 1321
1
w), 1282 (m), 1254 (s), 1219 (w), 1189 (m), 1171 (m), 769 (m). H-NMR (CDCl , 500 MHz) (ppm, E-
3
isomer signals labeled with *): 0.90 (m , 3H, CH ), 1.32 – 1.35 (m, 4H, CH CH CH ), 1.45 – 1.47 (m, 2H,
c
3
3
2
2
CH CH CH OAr), 1.62 – 1.68 (m, 4H, 12‘‘-H), 1.68 – 1.74 (m, 2H, 13‘‘-H), 1.79 (m , 2H, CH CH OAr),
2
2
2
c
2
2
1
.82 – 1.95 (m, 6H, 2-H, 3-H, 8‘‘-H), 2.40 (m , 0.6H, 7‘‘-H*), 2.44 (m , 1.4H, 7‘‘-H), 2.66 (m , 1.4H, 9‘‘-
c c c
H), 3.05 (m , 0.6H, 9‘‘-H*), 3.45 – 3.50 (m, 1.4H, 11‘‘-H*), 3.50 – 3.55 (m, 2.6H, 11‘‘-H), 4.00 (t,
c
J = 6.6 Hz, 2H, CH OAr), 4.17 – 4.23 (m, 2H, 4-H), 4.32 (m , 2H, 1-H), 6.03 (s, 0.3H, 5‘‘-H*), 6.89 (m ,
2
c
c
1
3
2
H, 3‘-H), 7.41 (s, 0.7H, 5‘‘-H), 7.98 (m , 2H, 2‘-H). C-NMR (CDCl , 125 MHz) (ppm, E-isomer signals
c 3
labeled with *): 14.0 (CH ), 21.8 (C-8’’), 21.9 (C-8’’*), 22.6 (C-10‘), 24.2, 25.7 (C-12’’, C-13’’), 25.5,
3
2
9
5
1
5.68, 25.74, 31.6 (C-2, C-3, CH ), 26.8 (C-9‘‘*), 27.2 (C-7‘‘*), 27.4 (C-7‘‘), 29.1 (CH CH OAr) 29.7 (C-
2 2 2
‘‘), 48.0 (C-11‘‘*), 48.3 (C-11‘‘), 63.3 (C-1), 64.3 (C-4), 68.2 (CH OAr), 81.1 (C-4‘‘), 97.6 (C-5‘‘), 98.3 (C-
2
‘‘*), 114.0 (C-3‘), 122.6 (C-1‘), 131.6 (C-2‘), 162.7 (C-6‘‘), 162.9 (C-4‘), 166.2 (C-1’’), 166.5 (ArC=O),
+
67.8 (C-3’’), 170.6 (C-2’’). HRMS (ESI) found [M+Na] 545.2986, molecular formula C H N O Na
31
42
2
5
1
0

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+
requires [M+Na] 545.2991. CHN: found C, 71.23; H, 8.09; N, 5.20%; molecular formula C H N O
5
31
42
2
requires C, 71.24; H, 8.10; N, 5.36%.
4
-(2-Cyano-2-(3-(piperidin-1-yl)cyclohex-2-en-1-yliden)acetoxy) butyl-4-(decyloxy)-benzoate
(
MC 4b)
1
Yield: 56 mg (39%) of MC 4b (isomeric ratio E/Z = 29 : 71 according to H-NMR). Mp: 72 °C. FT-IR
-
1
(
ATR) (cm ): 1538 (m), 1503 (s), 1251 (s), 1224 (s), 1195 (vs), 1166 (s), 1135 (m), 1099 (s), 1074 (m),
1
1
019 (m), 997 (m), 977 (w), 845 (w), 769 (m), 729 (s). H-NMR (CDCl , 500 MHz) (ppm, E-isomer
3
signals labeled with 0.88 (m , 3H, CH ), 1.21 – 1.39 (m, 12H, CH ), 1.45 – 1.47 (m, 2H, CH CH CH OAr),
c
3
2
2
2
2
1
2
9
4
5
.61 – 1.68 (m, 4H, 12‘‘-H), 1.68 - 1.74 (m, 6H, 13‘‘-H), 1.79 (m , 2H, CH CH OAr), 1.82 – 1.95 (m, 6H,
c 2 2
-H, 3-H, 8‘‘-H), 2.41 (m , 0.6H, 7‘‘-H*), 2.45 (m , 1.4H, 7‘‘-H), 2.66 (m , 1.4H, 9‘‘-H), 3.05 (m , 0.6H,
c
c
c
c
‘‘-H*), 3.45 – 3.50 (m, 1.4H, 11‘‘-H*), 3.50 – 3.55 (m, 2.6H, 11‘‘-H), 4.00 (t, J = 6.6 Hz, 2H, CH OAr),
2
.17 – 4.23 (m, 2H, 4-H), 4.32 (m , 2H, 1-H), 6.03 (s, 0.3H, 5‘‘-H*), 6.89 (m , 2H, 3‘-H), 7.41 (s, 0.7H,
c
c
1
3
‘‘-H), 7.98 (m , 2H, 2‘-H). C-NMR (CDCl , 125 MHz) (ppm, E-isomer signals labeled with *): 14.1
c
3
(
CH ), 21.8 (C-8’’), 21.9 (C-8’’*), 22.7 (CH CH ), 24.2, 25.7 (C-12’’, C-13’’), 25.4, 25.5, 25.68, 25.3, 26.0,
3 3 2
2
9
5
1
9.4, 29.5, 31.9 (C-2, C-3, CH ) 26.8 (C-9‘‘*), 27.1 (C-7‘‘*), 27.4 (C-7‘‘), 29.1 (CH CH OAr), 29.7 (C-
2 2 2
‘‘), 48.0 (C-11‘‘*), 48.3 (C-11‘‘), 63.2 (C-1), 64.2 (C-4), 68.2 (CH OAr), 80.8 (C-4‘‘), 97.6 (C-5‘‘), 98.3 (C-
2
‘‘*), 114.0 (C-3‘), 122.5 (C-1‘), 131.6 (C-2‘), 162.7 (C-6‘‘), 162.9 (C-4‘), 166.2 (C-1’’), 166.4 (ArC=O),
+
67.7 (C-3’’), 170.6 (C-2’’). HRMS (ESI) found [M+Na] 601.3612, molecular formula C H N O Na
35
50
2
5
+
requires [M+Na] 601.3617. CHN: found C, 72.43; H, 8.73; N, 4.71%; molecular formula C H N O
5
35
50
2
requires C, 72.63; H, 8.71; N, 4.84%.
4
-(2-Cyano-2-(3-(piperidin-1-yl)cyclohex-2-en-1-yliden)acetoxy) butyl-4-(tetradecyloxy)-benzoate
(
MC 4c)
1
Yield: 108 mg (57%) of MC 4c (isomeric ratio E/Z = 29 : 71 according to H-NMR). Mp: 63 °C. FT-IR
-
1
(
ATR) (cm ): 1538 (m), 1504 (s), 1444 (w), 1331 (w), 1277 (w), 1253 (s), 1226 (s), 1196 (vs), 1167 (s),
1
1
100 (m), 1019 (w), 906 (s), 846 (w), 770 (w), 727 (vs), 647 (w). H-NMR (CDCl , 500 MHz) (ppm, E-
3
1
1

ACCEPTED MANUSCRIPT
isomer signals labeled with *): 0.88 (m , 3H, CH ), 1.21 – 1.39 (m, 20H, CH ), 1.44 – 1.46 (m, 2H,
c
3
2
CH CH CH OAr), 1.61 – 1.68 (m, 4H, 12‘‘-H), 1.68 - 1.74 (m, 2H, 13‘‘-H), 1.79 (m , 2H, CH CH OAr),
2
2
2
c
2
2
1
.82 – 1.95 (m, 6H, 2-H, 3-H, 8‘‘-H), 2.41 (m , 0.6H, 7‘‘-H*), 2.45 (m , 1.4H, 7‘‘-H), 2.65 (m , 1.4H, 9‘‘-
c c c
H), 3.05 (m , 0.6H, 9‘‘-H*), 3.45 – 3.50 (m, 1.4H, 11‘‘-H*), 3.50 – 3.55 (m, 2.6H, 11‘‘-H), 4.00 (t,
c
J = 6.6 Hz, 2H, CH OAr), 4.17 – 4.23 (m, 2H, 4-H), 4.32 (m , 2H, 1-H), 6.02 (s, 0.3H, 5‘‘-H*), 6.89 (m ,
2
c
c
1
3
2
H, 3‘-H), 7.41 (s, 0.7H, 5‘‘-H), 7.97 (m , 2H, 2‘-H). C-NMR (CDCl , 125 MHz) (ppm, E-isomer signals
c 3
labeled with *): 14.1 (CH ), 21.7 (C-8’’), 21.9 (C-8’’*), 22.7 (CH CH ), 24.1, 25.7 (C-12’’, C-13’’), 25.4,
3
3
2
2
2
5.5, 25.68, 26.0, 29.3, 29.54, 29.57, 92.64, 29.66, 31.9 (C-2, C-3, CH ), 26.8 (C-9‘‘*), 27.1 (C-7‘‘*),
2
7.4 (C-7‘‘), 29.1 (CH CH OAr) 29.7 (C-9‘‘), 48.0 (C-11‘‘*), 48.3 (C-11‘‘), 63.2 (C-1), 64.2 (C-4), 68.2
2
2
(
CH OAr), 80.8 (C-4‘‘), 97.6 (C-5‘‘), 98.2 (C-5‘‘*), 114.0 (C-3‘), 122.5 (C-1‘), 131.6 (C-2‘), 162.6 (C-6‘‘),
2
+
1
6
62.9 (C-4‘), 166.1 (C-1’’), 166.4 (ArC=O), 167.7 (C-3’’), 170.6 (C-2’’). HRMS (ESI) found [M+Na]
+
57.4238, molecular formula C H N O Na requires [M+Na] 657.4243. CHN: found C, 73.61; H, 9.22;
39
58
2
5
N, 4.25%; molecular formula C H N O requires C, 73.78; H, 9.21; N, 4.41%.
39
58
2
5
2
.5 Preparation of the guest/host samples
Preparation of the dye guest/host mixtures. An appropriate amount (0.5 to 1.3 wt%) of the dye
material was added to the host material and dissolved in a mass equivalent of dry chloroform. In the
case of the 1:1 mixture RMM34/ST01011 a 1 wt% of the photo-initiator Irg 1700 and 0.1 wt% of
thermal inhibitors phenoxazin and 4-methoxyphenol were added while preparing these mixtures.
The solution was stirred for 1 h and then the solvent was allowed to evaporate at room temperature
in order to avoid unwanted thermal polymerization (in the case of reactive mesogen mixtures).
Subsequently, the remaining solvent was evaporated in vacuum until the remaining mass was that of
the mixture components. This procedure ensures the formation of homogenous mixtures.
Liquid crystal guest/host mixture cells. The cells used in this study were purchased from company
Instec. Cells with a gap of 8 µm were filled with the LC guest/host-mixtures by capillary forces at
1
2

ACCEPTED MANUSCRIPT
1
40 °C. After complete filling the cells were allowed to cool down to room temperature slowly.
Subsequently, the cells were sealed using an epoxy resin. This was done for the LC guest/host-
mixtures of the compounds MC 1, MC 2, MC 3, MC 4a-c, MC 5 and MC 6. In all cases a uniform macro
domain was observed between crossed polarizers.
Reactive mesogen guest/host mixture cells. For the characterization of the dichroism of reactive
mesogen g/h-mixtures cells with a gap of 10 µm were prepared. As material for the aligning layers
the polyimide SE-130 (Nissan/SunEver) was spin-coated on glass substrates. After the coating
procedure the layers were dried at 80 °C for 5 min and subsequently backed at 180 °C for 1 h to
achieve full imidization. These substrates were rubbed using a machine of company Beamco. The
substrates were fixed to form a cell with anti-parallel orientation. The gap of about 10 µm was
ensured using spacer particles of the same dimension dispersed in the epoxy resin lines at the long
edges of the substrates. The filling of the cells were carried out by capillary forces at 110 °C. The
samples were heated shortly to 160 °C and then slowly cooled down to room temperature. The
photo induced cross-linking was done by irradiation with a fluorescent tube emitting at 365 nm
2
(
Philips PL-S / 10 UV-A). The used dose was app. 250 mJ/cm . At this dose a sufficient cross-linking is
reached to fix the well oriented domain. Subsequently, the cells were sealed using an epoxy resin.
Photostability in solution. The used set-up was built up using a diode array spectrometer (XDAP, co.
Polytech) and a xenon arc lamp (XBO 75). The infrared component of the light was removed by a
water cuvette. For the irradiation procedure the light was directed to the sample in normal incidence
by using a motorized mirror. Before the UV/Vis-measurements the mirror was tilted out computer
controlled and tilted in again after. The intensity of the polychromatic light of the xenon source was
measured with a LM2 detector and a Fieldmaster powermeter (both of co. Coherent). The measured
value was correlated to the maximum of the intensity distribution at 475 nm. The power density was
2
app. 150 mW/cm . The concentration of the different merocyanine dyes in the solution was adjusted
1
3

ACCEPTED MANUSCRIPT
to be app. of absorbance 1 using acetonitrile as solvent. During irradiation and measurement the
solution was stirred with a magnetic micro bar in order to ensure a homogenous concentrated
solution.
3
3
. Results and discussion
.1 Synthesis
Scheme 2
As shown in Scheme 2, methyl 4-hydroxybenzoate 1 was alkylated using alkyl bromides in the
presence of K CO in CH CN and subsequently deprotected with KOH in ethanol giving the free acids
2
3
3
2
a-c in good yields over two steps.[40,41] The ester 4 was obtained according to Parkers procedure
from 2-cyanoacetic acid 3 and butane-1,4-diol.[42] Following Steglich esterification[43] the esters
a-c were synthesized in moderate yields, hampered by incomplete conversion. In order to obtain
dye precursor ꢀ-enamino ketone 7 the method from Greenhill[44] was applied by refluxing
5
1
4

ACCEPTED MANUSCRIPT
3
-hydroxycyclohexenone 6 with piperidine in toluene under Dean–Stark conditions for 5 h, which
yielded the product 7 in 84%. For the Knoevenagel condensation, the ketone derivative 7 was
+
-
activated with Me O BF according to the method by Kreß[32] and subsequently treated with the
3
4
cyanoacetate derivative in the presence of DBU in pyridine at 80 °C. The yellow solid dyes MC 4a-c
were obtained as inseparable E/Z-isomers (29 : 71) in yields between 39-57%. The synthesis of
compounds MC 1, MC 2, MC 3, MC 5 and MC 6 was reported earlier.[32,38]
3
.2 Calculated aspect ratios
To quantify the shape anisotropy of the rod-shaped dyes, a crucial factor for their dichroic
properties, the aspect ratios (AR) were calculated based on their minimum energy conformations in
the gas phase (DFT, B3LYP 3-21G*). Error! Reference source not found. shows the optimized
structure of E-MC 4a (all structures are shown in the supporting information). Calculations were
carried out based on the assumption that the whole molecule rotates around its long axis. Wcore was
defined as the width of the chromophoric unit and Lcore as the length of the chromophoric unit
including the piperidine moiety. The values Wcore and Lcore were determined from the calculated
optimized geometries. Following a reported procedure Lchain can be estimated based on the
assumption that the core occupies a cylindrical volume (fee rotation about the long axis in the LC
state) and that the side chains must occupy a cylindrical volume of the same diameter.[45] The
required value of Vchain was calculated using reported increment volume values for aliphatic chains in
the liquid state.[46–48] Averaged AR values are reported for dyes that consist of mixtures of E and Z
Ø
isomers by taking into account their measured E/Z-ratios.
A comparison of the ꢁꢂ values in Table 1 clearly shows that the extension of alkyl chains increases
∅
the ARs of the benzoate derivatives MC 4 much more (40%) than for the cyanoacetates MC 1-3
1
5

ACCEPTED MANUSCRIPT
(
24%). Malodinitrile MC 5 and thiobarbiturate MC 6, being devoid of the hexylester chain, show
comparably smaller aspect ratios of 2.0 and 1.9 respectively.
3
3
.3 Dichroism of the different merocyanines
.3.1 Dichroism in a liquid crystalline matrix
For applications of dichroic dyes in guest/host displays or functional dichroic layers, the miscibility of
the dye with the LC host is of importance. The commercially available LC mixture M677 with a
relatively high clearing point of 115 °C was chosen as a benchmark host for the compatibility studies.
All merocyanines studied here showed a good solubility in the host over a concentration range of
0
.5 – 1.3 wt% except for MC 5 that started to separate upon cooling below 80 °C. The dichroism of
the anisotropic solutions was studied by linear polarized UV/Vis-spectroscopy utilizing thin film cells
which displayed homogeneous macro domains (except for MC 5) (see Table 2).
Figure 2 shows the absorption spectra of compound MC 2 and MC 6 for parallel and perpendicular
polarized light revealing a distinct dichroism (all absorption spectra are shown in the supporting
information). Similar results were obtained for the other merocyanine derivatives. The dichroic ratios
DR, i.e. the ratio of the absorbance at maxima parallel and perpendicular to the orientation direction
Aǁ/A⊥ꢃare summarized in Table 2 together with the aspect ratio.
Due to the poor reproducibility caused by partial crystallization, the DR values of compound MC 5
were not considered. Presumably, the host mixture M677 is not suitable for a highly polar guest such
as the malononitrile MC 5. As shown in Table 2, the dichroic ratio grows with the increasing length of
the conjugated π-system and the wavelengths of the absorption maxima. This is observed most
prominently within the homologous series of mono-, bi- and tricyclic merocyanines MC 1, MC 2 and
MC 3 as illustrated in Figure 3a. Both the increased number of double bonds and thus the improved
delocalization of the π-system, as well as increasing the strengths of the donor/acceptor moieties
1
6

ACCEPTED MANUSCRIPT
lead to a higher polarizability and dipole moment resulting in an increased dichroic ratio.
Furthermore, the dichroic ratio appears to directly depend on the aspect ratio (Figure 3b), which
means an increase in shape anisotropy of the chromophoric system significantly enhances its
alignment within the anisotropic host and, consequently, increases the measured dichroism.
In order to study the two parameters separately, merocyanines MC 4a-c were chosen, where the
chromophoric unit was kept constant, while the aspect ratio increased from 5.1 (for MC 4a) to 8.1
(
for MC 4c). For all three compounds the same dichroic ratio of 2.5 was found, which is somewhat
smaller as compared to the related merocyanine MC 1 (DR = 2.9). This is probably due to the fact
that the longitudinal molecular axis of MC 4a-c is no longer parallel to the transition dipole moment
as it was in the case of MC 1. Although with an increasing shape anisotropy of the dye molecule a
much better degree of order within the LC host should be expected. The macroscopic dichroism is
driven by the dichroism of the chromophoric system, often characterized as the degree of order of
the transition moment.[49] The macroscopic dichroism is a product of the substitution pattern,
lengths and anisotropy of the π-system, the angle between the longitudinal molecular axis and the
direction of the transition dipole moment of the chromophore, the geometric aspect ratio of the dye
molecule, and the order of the anisotropic host matrix.[49–51]
Our calculations reveal that the increased lengths of the benzoate units in MC 4a-c do indeed lead to
an increase of the aspect ratio, which was found beneficial for the solubility in the LC host. However,
in order to improve the dichroic ratio, the mesogenic (or mesogen-like) unit of the chromophore,
such as p-alkoxybenzoate, should not change the orientation of the longitudinal molecular axis, as
compared to the non-mesogenic counterpart MC 1. By maintaining a subtle balance between these
different parameters an improvement of the dichroic ratio should be achieved.
1
7

ACCEPTED MANUSCRIPT
3
.3.2 Dichroism in a RMM matrix
To investigate the suitability of the merocyanines for in-cell polarizers or anisotropic emitters a
mixture of the merocyanine dye MC 3 with the commercially available reactive mesogen host
RMM34 (0.6 wt%) was filled into 2 µm thin-film cells, annealed, and photo-crosslinked via acrylate
groups at room temperature to stabilize the nematic domains. The homogeneous appearance of the
thin-film cells indicates a good miscibility of the dye in the host matrix. A strong dichroism of tricyclic
cyanoacetate MC 3 was detected in the linear polarized UV/Vis spectrum accompanied by a high
anisotropy of the fluorescence intensity (Figure 4).
The fluorescence spectrum of compound MC 3 obtained by excitation with linear polarized light at
5
94 nm parallel and perpendicular to the orientation direction displayed a strong emission at 644 nm
with a Stokes shift of 47 nm and a pronounced fluorescence anisotropy I /I = 6.0. These results
⊥
ǁ
indicate that the emission is not disturbed by additional interactions causing depolarization.
However, the experimentally observed dichroic ratio (DR = 6.0) of the MC 3/RMM34 mixture is
significantly smaller than the value obtained for the MC 3/M677 mixture (DR = 7.9). This might be
due to the lower clearing point of the reactive mesogen mixture RMM34 (T = 70 °C) as compared to
cl
the clearing transition of the LC mixture M677 at T = 115 °C. This means the host RMM34 is less
cl
ordered at room temperature then the LC mixture M677 expressed by the higher reduced
temperature (RT/T ).[52] In order to increase the clearing temperature of the reactive mesogen
cl
mixture, a mixture of RMM34 and the reactive mesogen ST01011 (1 : 1) with a clearing point of
1
53 °C was used for all subsequent experiments.
The values of the dichroic ratio of the various merocyanines using this particular host are
summarized in Figure 5 together with photos of the thin-film cells. The photos were taken using
linear polarized light, left part perpendicular and right parallel to the orientation direction, visualizing
the extent of dichroism and presence of homogeneous macro domains. The guest/host-systems
1
8

ACCEPTED MANUSCRIPT
containing the reactive mesogen mixture display a similar trend concerning the dichroism, as was
found for the LC host. Thus, DR values increase with increasing wavelengths of the absorption
maxima due to the increase of the extension and delocalization of the π-system. Tricyclic
thiobarbiturate MC 6 possessing the most bathochromicly shifted absorption (λmax = 628 nm) shows
the largest dichroic ratio (DR = 7.3). We anticipated that a further improvement of the DR value
should be possible by employing a more highly ordered matrix. In a proof-of-principle experiment the
reactive mesogen mixture RMM14 possessing an additional week smectic A phase was used as a host
and combined with tricyclic malodinitrile MC 5. Gratifyingly, over the whole absorption range
improved dichroism was found and the DR value at λmax = 644 nm increased to 12.6, while
fluorescence measurements revealed a fluorescence anisotropy I /I = 9.7 (λexc = 622 nm). These
⊥
ǁ
values are in the range of best reported results for nematic hosts.[18,20] This increase of the dichroic
ratio using higher ordered hosts lets expect that the tricyclic merocyanines would give in smectic B
hosts a strong dichroism suitable for high performance thin-film polarizers comparable to the best
results in the literature.[8]
3
3
.4 Photostability
.4.1 Photostability in solution
Error! Reference source not found. shows the spectra of MC 3 in CH CN upon irradiation with a
3
xenon lamp. Significant photodegradation was observed upon ongoing irradiation, while
simultaneously the absorption in the UV-region increased.
A similar behavior was found for all investigated merocyanine derivatives, although MC 1 was the
most stable of this series. This might be due to the fact that its absorption maximum (λmax = 397 nm)
does not coincide with the intensity maximum of the polychromatic light source. Thus, the amount of
absorbed light is reduced in comparison to the merocyanines with a bathochromicly shifted
1
9

ACCEPTED MANUSCRIPT
absorption maximum, e.g. MC 3 (λmax = 590 nm). In order to reduce the influence of the intensity
distribution the xenon light dose was corrected by applying wavelength dependent factors taken
from the literature.[53] The correlation between the normalized absorbance at the absorption
maxima and the corrected light dose for the various dyes is shown in Figure 7.
In general, the photostability decreased with increasing number of double bonds in the
chromophoric system (e.g. MC 1 > MC 2 > MC 3). A possible reason for this partial photodegradation
might be photooxidation, as was previously described in the literature for unbranched
merocyanines.[54] In this work especially the merocyanine MC540 was investigated as a potential
photochemotherapeutic agent and a photooxidation mechanism was discussed.[55–57] In order to
evaluate this mechanism further for our stability experiments, the cuvette was flushed with dry N2
for 30 min before use and subsequently sealed in order to attempt to significantly reduce the
presence of oxygen during future measurements. Error! Reference source not found. shows the
absorption behavior with untreated (oxygen-containing) and N -treated cuvettes. After initial
2
photodegradation in the N -treated solution a steady-state was reached without further decrease of
2
the absorbance upon prolonged irradiation.
This experiment might prove that the dominating mechanism of the photodegradation is indeed
caused by a photooxidation. The photodegradation at the beginning of the experiment may be due
to remaining traces of dissolved oxygen in the system and similar results have been found in
literature.[58]
3
.4.2 Photostability in a LC and in a RMM matrix
Regarding the suitability of the merocyanines for guest/host-displays their photostability in the LC
and RMM thin-film cells was investigated. The first experiments were performed with the 10 µm
thin-film cells under similar conditions as previously described for the solution experiments. Error!
2
0

ACCEPTED MANUSCRIPT
Reference source not found. shows the spectra of bicyclic cyanoacetate MC 2 as a function of
irradiation time. In contrast to the solution spectra (Figure 6), the absorption bands did not change
except for the appearance of a weak band at ca. 550 nm, which develops with increasing irradiation
time, presumably due to a photo-induced process generated product. A similar phenomenon was
observed for MC 3 and MC 6, where photoinduced products always appeared with a bathochromic
shift of 50 nm relative to their absorption maxima. Error! Reference source not found. shows the
normalized extinctions at the absorption maximumꢃas a function of the irradiation time.
For all studied merocyanines the absorbance remains almost constant irrespective of the irradiation
time. The slight increase of the curves of MC 3 and MC 6 might be caused the presence of trace
amounts of photoinduced byproducts. All investigated merocyanines showed a much higher
photostability upon irradiation in the LC matrix as compared to the solution. This could be reasoned
to the lower solubility and diffusion rate of oxygen in the LC mixture in comparison.
In order to investigate the photostability of the RMM matrix, thin-film cells containing 0.6wt% of
MC 3 in RMM34 were irradiated in a similar fashion and the time-dependent fluorescence intensity
parallel to the orientation alignment of the cell was studied (Figure 11). Pleasingly, the emission
intensity remained almost constant even after 30 min of irradiation time (λexc = 590 nm) and no
degradation could be detected. Similar results were observed for all subsequently studied
merocyanines, demonstrating the high photochemical stability of the reactive mesogen mixture as a
host.
Conclusion
In this study we have investigated a series of known and new merocyanines, differing in their number
of rings, donor and acceptor moieties and degrees of functionalization of the donor unit, with regard
to their dichroism and photostability in different matrices. UV/Vis-spectroscopy with linearly-
polarized light revealed that the dichroic ratio increases with increasing wavelengths of the
2
1

ACCEPTED MANUSCRIPT
absorption maxima irrespective of the host. In other words, upon comparison of the LC-mixture
MM67 with the reactive mesogen mixture RMM34/ST01011 (1 : 1) as hosts, similar DR values were
obtained for a certain chromophoric guest. The highest dichroic ratios of up to 7.9 were obtained for
tricyclic merocyanines MC 3, MC 5 and MC 6. By using a reactive mesogen mixture (RMM14) with a
higher degree of order, the dichroic ratio could be further improved to 12.6 for MC 6. Furthermore,
fluorescence anisotropy correlates well with the dichroism, e.g I /I = 9.7 (λexc = 622 nm) for MC 6.
⊥
ǁ
Although attachment of mesogen-like substituents to the donor moiety increased the aspect ratio
considerably according to DFT calculations, the extent of π-delocalization, together with the
bathochromic shift of the absorption maximum, contributes to the dichroism to a much larger
extent. The attachment of aspect ratio increasing substituent must not change the coincidence of
transition moment and long molecular axis.
In order to assess the photostability of the merocyanines, time-dependent UV/Vis- and fluorescence
spectra were performed in solution, in an LC-matrix, and in a reactive mesogen matrix. While
experiments in solution revealed considerable photodegradation, which could be partially
suppressed by flushing the solution with N , experiments conducted in the LC and RMM
2
demonstrated a considerably high photostability of the investigated dyes. In particular, the use of the
reactive mesogen matrix as the host yielded almost constant fluorescence intensities even after
prolonged irradiation times. Thus, we have shown that the tricyclic merocyanines MC 3, MC 5 and
MC 6 are well suited for applications as dichroic dyes in thin-film polarizers and guest/host displays.
Acknowledgements
General financial support by the Carl-Zeiss-Stiftung (fellowship for K.C.K.), the International Max-
Planck-Research-School on Advanced Materials (fellowship for K.C.K.), the Fonds der Chemischen
Industrie, the Ministerium für Wissenschaft, Forschung und Kunst des Landes Baden-Württemberg
2
2

ACCEPTED MANUSCRIPT
and the GDCh (travel grant for K.C.K.) is gratefully acknowledged. Thanks also goes to Dr. U. Lawrentz
for the synthesis of MC 6 and Dr. Johannes for supplying us with the dye as well as to Mr. S.
Beardsworth for proof reading the manuscript.
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Figure 1 Optimized geometry of E-MC 4a.
Table 1 Calculated aspect ratios basing on the DFT calculation (B3LYP 3-21G*) of the merocyanines.
a
2 b
Lchain = Vchain/π(0.5Wchain) , aspect ratio AR = (L
+ Lchore)/Wchore.
chain
Table 2 Absorption maxima, dichroic ratio and averaged aspect ratio of the investigated
merocyanines.
Figure 2 Linearly-polarised absorption spectra of MC 2 and MC 6 in LC cells parallel and
perpendicular to the orientation
Figure 3 Observed dichroic ratio of MC 1, MC 2, MC 3 in a LC cell as a function the wavelength of
their absorption maxima (a) and on their calculated aspect ratio (b, Table 1).
Figure 4 Absorbance and emission spectra of MC 3 in a RMM matrix upon parallel and perpendicular
excitation.
Figure 5 Observed dichroic ratio of merocyanines in a RMM34 matrix and their corresponding
absorption maxima.
Figure 6 UV/Vis-spectra of MC 3 in CH CN upon irradiation with a xenon lamp as a function of time.
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Figure 7 Normalized absorbance at ꢀ(ꢁ3 in CH CN as a function of the corrected light dose.
3
Figure 8 Normalized absorbance at 593 nm of MC 3.
Figure 9 UV/Vis-spectra of MC 2 in a LC matrix upon irradiation with a xenon lamp as a function of
time.
Figure 10 Normalized absorbance at ꢀ(ꢁ3 in a LC matrix as a function of the irradiation time.
Figure 11 Emission intensity of MC 3 in a RMM matrix as a function of time upon excitation at
5
90 nm.