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Prof. Dr. Claus D. Eisenbach
Institut für Angewandte Makromolekulare Chemie, Universität Stuttgart, Pfaffenwaldring
55, D-70569 Stuttgart
1HZ�GRQRU�DFFHSWRU�FKURPRSKRUHV�IRU�SKRWRUHIUDFWLYH
FRPSRVLWHV��VWUXFWXUH�SURSHUW\�UHODWLRQVKLSV
Kai Ewert,a)d) Heidi Hayen,b) Stefan Schloter,c) Dietrich Haarer,c) Claus D.
Eisenbach*a)
a) Institut für Angewandte Makromolekulare Chemie, Universität Stuttgart, Pfaffenwaldring 55,
D-70569 Stuttgart (Germany), [email protected]
b) Makromolekulare Chemie I, Universität Bayreuth, D-95440 Bayreuth (Germany)
c) Experimentalphysik IV, Universität Bayreuth, D-95440 Bayreuth (Germany)
d)current address: Materials Research Lab, University of California, Santa Barbara CA 93106-
5121 (USA)
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$EVWUDFW
A series of donor-acceptor-type electrooptic (NLO) chromophores for use in
photorefractive guest-host materials has been synthesized and characterized.
Donor substituents (alkoxy or dialkylamino) as well as chromophore type
(azo-, stilbene-, tolane- and dicyanovinylbenzene-type) were varied
systematically to improve both the sample stability and index modulation,
and to investigate structure-property relationships. The optical, thermal and
electrochemical properties of the chromophores were studied by using
UV/VIS-spectroscopy, differential scanning calorimetry (DSC) and cyclic
voltammetry (CV). The chromophore’s donor substituents and bridging
groups have a strong influence on the spectral characteristics and the
oxidation and reduction potentials as well as on the melting behavior. Key
parameters were found to be the donor strength, the ability of the bridging
group to effect conjugation between the aromatic rings, and the bulkiness of
the donor groups. Some important chromophore properties such as
electrochemical properties and crystallinity can be tailored independently
from each other. Ternary composites of these chromophores with photo-
conducting poly(methyl-(3-(9-carbazolyl)propyl)-siloxane)siloxane and
2,4,7-trinitro-fluorenone were prepared and their photorefractive properties
studied using two-beam coupling and degenerate four-wave mixing. The
composites show high index modulations (up to 10-2) and photorefractive
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gain. The influence of chromophore structure on these properties and the
holographic response times as well as the long-term stability are discussed.
_____________________________________________________________
,QWURGXFWLRQ
Organic photorefractive materials have attracted a rapidly increasing interest
since the first description of this type of compounds in 1991.1,2,3,4 This is
due to the unique features of the photorefractive effect, which distinctively
differs from all other types of light-induced modulation of a material’s
refractive index (e.g. photochromism, thermochromism)5 and allows for
numerous applications.6 In photorefractive systems, photogenerated charges
drift under the influence of an external field and get trapped, building up a
space charge field which in turn modulates the material’s refractive index
via the Pockels and the Kerr effect. This means that the refractive index
modulation is spatially shifted with respect to the light intensity pattern,
resulting in an asymmetric energy transfer between incident laser beams.
Other features of the photorefractive effect that are of great interest
concerning potential applications are its sensitivity (making the use of diode
lasers possible) and its reversibility (allowing real-time applications). 5,7
From the mechanism of grating formation, it follows that photorefractive
materials must combine photoconducting and electrooptic properties. This is
the case for a number of inorganic crystals, e.g. lithium niobate LiNbO3,
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where the photorefractive effect was first observed in 1966.8 In organic
materials, different approaches exist for incorporating the essential
functions. In guest-host or composite systems, an amorphous matrix is
provided by a polymer which may be inert9 or bearing either
photoconducting units10,11,12,13,14,15,16 or nonlinear-optical (NLO)
chromophores.1,17 Photoconducting low molecular weight glasses have also
been used as the amorphous matrix.18 In contrast to guest-host systems,
incorporating more than one functional unit into either a polymer19 or a low
molar weight glass20 results in bifunctional or multifunctional
photorefractive materials. These do not suffer from phase separation or
chromophore crystallization, but they require higher synthetic efforts,
especially when the aim is to vary individual components. To encounter this
problem, we have developed and successfully applied a building block
approach for photorefractive polymers.21
The advent of organic photorefractive materials has renewed the interest in
NLO chromophores, a field that has been studied extensively.22 For the use
in photorefractive composites, new demands have to be met by electrooptic
dyes. Contributions to the modulation of the refractive index following the
buildup of a modulated space charge field in the material5 not only stem
from the electrooptical or Pockels effect but also from a contribution of the
chromophores’ birefringence after they reorient in the field (Kerr effect).
The term “orientational enhancement effect” has been coined for the latter
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phenomenon,23 which frequently dominates the refractive index modulation
in photorefractive composites.16 Taking both effects into account, a tentative
figure-of-merit for photorefractive chromophores
N700)
αµµβ ∆+=229
has been proposed.24,25 Here ∆α is the polarization anisotropy, k is
Boltzmann’s constant and T the temperature; M is the chromophores’
molecular weight, β its first hyperpolarizability and µ its ground-state
dipole moment.
While the model used to derive the figure of merit is incomplete even for the
description of the refractive index modulation since it neglects the field-
induced dissociation of dimeric chromophore aggregates,26 chromophores
with a large figure-of-merit are likely to exhibit large index modulations.
This has been shown experimentally, e.g. for chromophores of the
merocyanine type.27,28,24,20e However, the high dipole moments of
chromophores with optimized FOM lead to strong aggregation which is
detrimental for photorefractive performance, and chromophores with the
highest reported FOMs were found to be inactive in photorefractive
composites.26,29,30 On the other hand, chromophores with comparatively low
FOM such as 2,5-dimethyl-4-(4-nitrophenylazo)-anisole (DMNPAA),10,11 its
substituted analog EHDMNPAA,13,31 a 4-cyano-4'-alkoxy-tolane32 or amino-
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dicyanovinylbenzenes16,37,42 give composites of remarkable performance,
especially when the response times are taken into consideration. Other
important characteristics of photorefractive composites such as sample
stability, optical absorption and the speed of the photorefractive response are
also influenced strongly by the chromophores. They are, however, not
addressed by the above figure-of-merit.33 Investigations on the influence of
chromophore photoisomerization39 revealed this to be another property of
importance for photorefractive performance. Early studies on guest-host
materials in which the chromophore was varied16,34,35 did not compare series
of different chromophore type or did not investigate the influence of single
chromophore properties on the performance of guest-host photorefractive
materials. More recently, the importance of the electrochemical properties of
the chromophores has been realized and assessed.16,36,37 and structure
property relationships of newly designed chromophore types have been
investigated more systematically.27,30,38
While the understanding of structure property relationships for
chromophores used in photorefractive composite materials is still
incomplete, development of new improved materials has proceeded rapidly.
This has yielded composites showing total internal diffraction,11 refractive
index modulations (∆n) around 0.0127,39,40 and holographic rise times faster
than 10 ms.14,15,32,41,42
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In this paper, we present our approach to systematically and independently
vary important chromophore characteristics (e.g. dipole moment, HOMO-
energy, crystallization enthalphy) to determine their influence on the
performance of photorefractive guest-host materials. For this purpose, we
have synthesized a series of new NLO-chromophores, which include the
well-known chromophore DMNPAA10,11 for comparison. We have prepared
chromophores of different type (π-system) and donor substitution and
characterized them using UV/VIS spectroscopy, differential scanning
calorimetry (DSC) and cyclic voltammetry (CV). Photorefractive
composites prepared from the chromophores, a photoconducting siloxane
polymer and 2,4,7-trinitrofluorenone (TNF) were characterized using DSC,
their stability towards chromophore recrystallization was assessed and their
photorefractive properties were measured. While the chromphores in this
study have not been optimized with regard to their photorefractive figure of
merit, their facile synthesis allowed the investigation of other properties
relevant to photorefractive composites by systematic structural variations.
Nevertheless, the stationary the performance of these chromophores is
impressive with index modulations as high as 10-2.
([SHULPHQWDO�6HFWLRQ
*HQHUDO� 0HWKRGV� Nuclear magnetic resonance (NMR) spectra were
obtained using a Bruker AC 250 spectrometer with tetramethylsilane as the
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internal standard. Infrared (IR) spectra were recorded on a DIGILAB-
DIVISION BIO-RAD 3240-SPC FTS-40 instrument. Absorption spectra
were measured on a Perkin Elmer LAMBDA 15 spectrometer in
tetrahydrofurane (THF) of spectrophotometric quality. Mass spectrometry
(MS) was performed using a Finnigan MAT 8500 mass spectrometer. A
Perkin Elmer DSC 7 apparatus was used for thermal analysis. Samples of
photorefractive composites were heated to well above their glass transition
temperature and quenched in liquid nitrogen before the measurement. Flash
chromatography was performed using silica gel 60 from Merck (particle size
40 - 63 µm).
&\FOLF� YROWDPPHWU\� A three electrode cell and a potentiostat assembly
from EG&G Princeton Applied Research were used. The measurements
were performed at a sweep rate of 50 mV/s using a glassy carbon working
electrode in dry THF or 1,2-propylene carbonate (both purchased from
Aldrich) containing 0.1 M Bu4N+ PF6
-. The reference electrode was Ag/0.1 M
AgNO3 in acetonitrile and the redox standard was ferrocene (-4.80 eV
HOMO). Reversible redox processes were evaluated according to standard
procedures.43 For irreversible reactions, the potential used for the HOMO-
energy calculation was approximated as 1/2 (Epeak, anodic + Epeak, cathodic) if
there was both an anodic and a cathodic peak. If there was only one peak,
half the difference in potential of the peaks of ferrocene was added to /
subtracted from the peak potential for the irreversible reduction / oxidation.
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The aim of this procedure was to allow comparison of reversibly and
irreversibly reacting chromophores.
3KRWRUHIUDFWLYH� FKDUDFWHUL]DWLRQ� The techniques for photorefractive
sample preparation as well as the detailed experimental procedures for the
characterization of the photorefractive composites have been given
elsewhere.39 In brief, solutions of the compounds in 1,4-dioxane were mixed
at appropriate ratios and freeze-dried. The resulting powder was melted and
filled into ITO glass cells (“Test cells for LC”, 20-40 µm, EHC, Japan) by
capillary action. The setup for the degenerate four-wave mixing (DFWM)
and two-beam coupling experiments consisted of two diode-laser beams
with an intensity of 1 W/cm2 and a wavelength of 670 nm which were
intersected at an angle of 17.2° in the sample. The bisector of the beams was
tilted 45° with respect to the sample normal. Thus, a grating spacing of 4 µm
resulted.
0DWHULDOV� 4-Dimethylamino-4’-nitrostilbene (DANS, ��H) was purchased
from Kodak. Ethylcarbazole and Poly(N-vinylcarbazole) (PVK) were
purchased from Aldrich. The other starting materials were purchased from
Fluka, Aldrich or Merck. TNF (Aldrich) was recrystallized from ethanol. 4-
Nitroaniline (Merck) was recrystallized from ethanol-water. All other
materials were used as received. DMNPAA (��H) was prepared as described
by v. Auwers44, and 4-Dimethylamino-4’-nitrotolane (11,11-dimethyl-4-[2-
(4-nitrophenyl)-1-ethynyl]aniline, DANT, ��I) was synthesized according to
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the method of Akiyama et al.45 The photoconducting poly(methyl-(3-(9-
carbazolyl)propyl)-siloxane) (PSX) was prepared as described by
Strohriegl.46 The cyclic siloxane 1,3,5,7-Tetrakis-(3-(9-carbazolyl)propyl)-
1,3,5,7-tetramethyl-cyclotetrasiloxane (D4-4CZ, cf. Fig. 2) was synthesized
by hydrosilylation of N-allylcarbazole with 1,3,5,7-Tetramethyl-
cyclotetrasiloxane (Gelest) as described for PSX.46
6\QWKHVLV�� Representative experimental procedures are given below.
Procedures and spectral data for all compounds are given in the
supplementary information.
1�����HWK\OKH[\O��1��PHWK\ODQLOLQH� ���E��� To a stirred mixture of
117.9 mL (116.6 g, 1.088 mol) N1-methylaniline and 3.08 g tetrabutyl-
ammonium iodide, 108.6 mL (100 g, 0.518 mol) 2-ethylhexyl bromide were
added slowly under an inert atmosphere. After stirring for 24 hours at 85 °C,
the mixture was poured into 700 mL water. The resulting solution was made
alkaline by adding potassium hydroxide while cooling and extracted three
times with diethyl ether. The combined organic layers were washed with
water, dried (Na2SO4) and the solvent was evaporated. The residue was
fractionally distilled in vacuo (10-3) to yield 83.7 g (74%) of a colorless
liquid, bp. 90 °C (1.7⋅10-2 mbar).
�+�105 (CDCl3): δ / ppm = 0.89 ( ”t”, J = 8 Hz, 6 H, CH-CH2-C+3, CH-
(CH2)3-C+3), 1.20-1.50 (m, 8 H, CH-C+2-CH3, CH-(C+2)3-CH3), 1.65-1.85
(m, 1 H, CH2-C+(Et)(Bu)), 2.93 (s, 3 H, N-C+3), 3.16 (”d”, J = 7 Hz, 2 H,
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N-C+2-C(H)(Et)Bu), 6.55-6.80 (m, 3 H, +ar R-NRR’, +ar S-NRR’), 7.10-
7.30 (m, 2 H, +ar P-NRR’); ��&�105 (CDCl3): δ / ppm = 10.7, 14.1 (CH-
CH2-&H3, CH-(CH2)3-&H3), 23.2, 24.0 ( CH-&H2-CH3, CH-(CH2)2-&H2-
CH3), 28.7, 30.7 (CH-(&H2)2-CH2-CH3), 37.8, 39.3 (N-&H3, CH2-
&H(Et)(Bu)), 57.1 (N-&H2), 111.8 (H&ar R-NR2), 115.5 (H&ar S-NR2),
128.9 (H&ar P-NR2), 149.7 (&ar LSVR-NR2); ,5 (Film): ν / cm-1 = 3094 (w),
3063 (w), 3027 (w), 2959 (s), 2929 (s), 2873 (s), 2839 (s), 1600 (s).
��>���HWK\OKH[\O��PHWK\O�DPLQR@EHQ]DOGHK\GH ���E��47� Under an argon
atmosphere, 24.7 mL (27.0 g, 200 mmol) N-methylformanilide were mixed
with 18.2 mL (30.6 g, 200 mmol) phosphoroxy chloride and kept for 45
minutes at room temperature. Keeping the temperature below 25 °C, 43.9 g
(200 mmol) ��E were added within one hour with stirring. The mixture was
kept for 15 hours at room temperature and poured into 500 mL of cold
water. After extraction with ether (twice), combining and drying (Na2SO4)
the organic layers followed by removal of the solvent gave a colorless oil.
The crude product was fractionally distilled in vacuo (10-3 mbar) to yield
32.7 g (66%) of a colorless oil, bp. 130 °C (4⋅10-2 mbar).
�+�105 (CDCl3): δ / ppm = 0.80-1.00 (m, 6 H, CH-CH2-C+3, CH-(CH2)3-
C+3), 1.20-1.50 (m, 8 H, CH-C+2-CH3, CH-(C+2)3-CH3), 1.65-1.85 (m, 1
H, CH2-C+(Et)(Bu)), 3.06 (s, 3 H, N-C+3), 3.30 (”d”, J = 7 Hz, 2 H, N-
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C+2-C(H)(Et)Bu), 6.70 (”d”, J = 9 Hz, 2 H, +ar R-N), 7.71 (”d”, J = 9 Hz, 2
H, +ar P-N), 9.72 (s, 1 H, C+O).
�1�������(WK\O��KH[\O��1�PHWK\O�DPLQR���¶�QLWUR�D]REHQ]HQH ���E�� A
cooled suspension of 6.45 g (46.9 mmol) 4-nitroaniline in 14 mL water and
14 mL conc. hydrochloric acid was diazotized with a solution of 3.3 g
(47 mmol) sodium nitrite in 3.5 mL water at 0 to 5 °C. Excess nitrite was
removed by addition of little amidosulfuric acid. The resulting solution was
added dropwise to a cooled solution of 8.76 g (40.0 mmol) ��E in 23 mL
conc. hydrochloric acid at 0 to 5 °C. After stirring for one hour, the solution
was neutralized using saturated sodium acetate solution. Filtration gave a
crude product which was purified by recrystallization from methanol and
subsequent flash chromatography using cyclohexane (CyH)-ethyl acetate
(EAc) (9:1, v/v) as the eluent to yield 5.1 g (35%) red crystals, mp. 96.5 °C.
�+�105 (CDCl3): δ / ppm = 0.85-1.00 (m, 6 H, CH-CH2-C+3, CH-(CH2)3-
C+3), 1.20-1.50 (m, 8 H, CH-C+2-CH3, CH-(C+2)3-CH3), 1.65-1.85 (m, 1
H, CH2-C+(Et)(Bu)), 3.10 (s, 3 H, N-C+3), 3.35 (”d”, J = 7 Hz, 2 H, N-
C+2-C(H)(Et)Bu), 6.74 (”d”, J = 9 Hz, 2 H, +ar R-NRR’), 7.80-7.95 (m, 4
H, +ar P-NRR’, +ar P-NO2), 8.31 (”d”, J = 9 Hz, 2 H, +ar R-NO2); ,5
(KBr): ν = 2957 (m), 2931 (m), 2871 (w), 2857 (m), 1602 (s), 1585 (s),
1519 (m), 1507 (s), 1379 (m), 1327 (vs), 1307 (s), 1137 (s), 1102 (s), 857
(m); 06 (EI, 70 eV): m/z (%) = 368 (18, M+·), 270 (31, M - C7H14), 269
(40, M - C7H15), 223 (9, M - C7H15, - NO2), 119 (18, C8H9N).
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1�����HWK\OKH[\O��1��PHWK\O���>�(�������QLWURSKHQ\O����HWKHQ\O@DQLOLQH
���E��48� To 3.0 g (12.2 mmol) ��E and 2.21 g (12.2 mmol) 4-
nitrophenylacetic acid, 3 droplets of piperidine were added and the mixture
was stirred at 160 °C for 6 hours.. After cooling, recrystallization of the
crude product from methanol followed by flash chromatography using CyH-
EAc (4:1, v/v) as the eluent yielded 3.1 g (70 %) orange crystals, mp.
121 °C.
�+�105 (CDCl3): δ / ppm = 0.85-1.00 (m, 6 H, CH-CH2-C+3, CH-(CH2)3-
C+3), 1.20-1.50 (m, 8 H, CH-C+2-CH3, CH-(C+2)3-CH3), 1.65-1.85 (m, 1
H, CH2-C+(Et)(Bu)), 3.01 (s, 3 H, N-C+3), 3.24 (”d”, J = 8 Hz, 2 H, N-
C+2-C(H)(Et)Bu), 6.67 (”d”, J = 9 Hz, 2 H, +ar R-NRR’), 6.90 (d, 3Jtrans =
16 Hz, 1 H, =C+-C6H4-NO2), 7.20 (d, 3Jtrans = 16 Hz, 1 H,
+C=CH-C6H4-NO2), 7.42 (”d”, J = 9 Hz, 2 H, +ar P-NRR’), 7.55 (”d”, J =
9 Hz, 2 H, +ar P-NO2), 8.17 (”d”, J = 9 Hz, 2 H, +ar R-NO2); ,5 (KBr): ν =
2959 (m), 2930 (m), 2872 (w), 1606 (m), 1585 (s), 1524 (m), 1507 (s), 1337
(s), 1188 (m), 836 (m); 06 (EI, 70 eV): m/z (%) = 366 (11, M+·), 267 (40,
M - C7H15), 221 (9, M - C7H15, - NO2).
1�����HWK\OKH[\O��1��PHWK\O���LRGRDQLOLQH� ���E���To a stirred mixture of
10 g (45.6 mmol) ��E, 5.7 g (68.4 mmol) sodium hydrogencarbonate and
50 mL water, 11.7 g (45.6 mmol) iodine were added in portions. After
stirring for 3 h, the mixture was extracted three times with dichloromethane.
The combined extracts were washed with water, dried (Na2SO4) and the
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solvent was evaporated. The residue was purified by flash chromatography
using CyH-dichloromethane (15:1, v/v) as the eluent (Rf = 0.49) to yield
8.6 g (55%) of a colorless oil.
�+�105 (CDCl3): δ / ppm = 0.80-1.00 (m, 6 H, CH-CH2-C+3, CH-(CH2)3-
C+3), 1.20-1.50 (m, 8 H, CH-C+2-CH3, CH-(C+2)3-CH3), 1.65-1.85 (m, 1
H, CH2-C+(Et)(Bu)), 2.88 (s, 3 H, N-C+3), 3.12 (”d”, J = 7 Hz, 2 H, N-
C+2-C(H)(Et)Bu), 6.42 (”d”, J = 9 Hz, 2 H, +ar R-N), 7.41 (”d”, J = 9 Hz, 2
H, +ar P-N); ��&�105 (CDCl3): δ / ppm = 10.7, 14.1 (CH-CH2-&H3, CH-
(CH2)3-&H3), 23.1, 23.9 ( CH-&H2-CH3, CH-(CH2)2-&H2-CH3), 28.7, 30.7
(CH-(&H2)2-CH2-CH3), 37.6, 39.3 (N-&H3, CH2-&H(Et)(Bu)), 56.7 (N-
&H2), 76.1 (&ar LSVR-I), 114.1 (H&ar R-NR2), 137.4 (H&ar P-NR2), 149.1
(&ar LSVR-N).
1�����HWK\OKH[\O��1��PHWK\O���>�����QLWURSKHQ\O����HWK\Q\O@DQLOLQH
���E�� To a solution of 4 g (11.6 mmol) ��E and 1.69 g (11.7 mmol) 1-(1-
ethynyl)-4-nitro-benzene (�) in 44 mL diethylamine, a mixture of 0.16 g
(0.23 mmol) bis(triphenylphosphine)palladium(II) chloride and 0.02 g
(0.11 mmol) copper(I) iodide was added in small portions under an argon
atmosphere. After stirring for 18 hours, the diethylamine was distilled off.
Water was added to the residue, and the mixture was extracted three times
with dichloromethane. The combined organic layers were dried (Na2SO4)
and the solvent was evaporated. The crude product was purified by
recrystallization from ethanol and subsequent flash chromatography using
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CyH-dichloromethane (3:1, v/v) as the eluent (Rf = 0.29) to yield 1.8 g
(43%) orange crystals, mp. 95 °C.
�+�105 (CDCl3): δ / ppm = 0.80-1.00 (m, 6 H, CH-CH2-C+3, CH-(CH2)3-
C+3), 1.20-1.50 (m, 8 H, CH-C+2-CH3, CH-(C+2)3-CH3), 1.65-1.85 (m, 1
H, CH2-C+(Et)-CH2), 3.00 (s, 3 H, N-C+3), 3.24 (”d”, J = 7 Hz, 2 H, N-
C+2-C(H)(Et)Bu), 6.63 (”d”, J = 9 Hz, 2 H, +ar R-NRR’), 7.40 (”d”, J = 9
Hz, 2 H, +ar P-NRR’), 7.58 (”d”, J = 9 Hz, 2 H, +ar R-NO2), 8.17 (”d”, J = 9
Hz, 2 H, +ar P-NO2); ��&�105 (CDCl3): δ / ppm = 10.7, 14.1 (CH-CH2-
&H3, CH-(CH2)3-&H3), 23.1, 24.0 ( CH-&H2-CH3, CH-(CH2)2-&H2-CH3),
28.7, 30.7 (CH-(&H2)2-CH2-CH3), 37.9, 39.4 (N-&H3, CH2-&H(Et)(Bu)),
56.7 (N-&H2), 86.4, 97.4 (-&≡&-), 107.7 (H&ar S-NRR’), 111.4 (H&ar
R-NRR’), 123.6 (&ar R-NO2), 131.5, 133.2 (&ar S-NO2, H&ar P-NRR’��H&ar
P-NO2), 146.1 (&ar LSVR�NRR’), 150.8 (&ar LSVR-NO2); ,5 (KBr): ν = 2957
(w), 2931 (w), 2872 (w), 2857 (w), 2202 (m), 2183 (m), 1610 (m), 1584 (s),
1503 (m), 1334 (s), 1137 (m), 1105 (m), 850 (w); 06 (EI, 70 eV): m/z (%)
= 364 (52, M+·), 266 (40, M - C7H14), 265 (100, M - C7H15), 219 (17, M -
C7H15, - NO2).
�����>���HWK\OKH[\O��PHWK\O�DPLQR@SKHQ\OPHWK\OHQH�PDORQRQLWULOH
���E��34 Following the addition of 10 droplets of piperidine, a stirred solution
of 6.00 g (24.3 mmol) ��E and 3.23 g (48.6 mmol) malononitrile in THF-
methanol 1:1, v/v was warmed to 40 °C. After stirring for 4 hours, the
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solvent was evaporated. The resulting residue was dissolved in
dichloromethane and washed with aqueous sodium chloride solution, 0.1 N
hydrochloric acid and again aqueous sodium chloride solution. The organic
layer was dried (Na2SO4) and the solvent evaporated. Purification of the
crude product by flash chromatography using diethyl ether-petrol ether (40-
60 °C) (1:9, v/v) as the eluent yielded 7.5 g (63%) orange crystals, mp.
59 °C.
�+�105 (CDCl3): δ / ppm = 0.80-1.00 (m, 6 H, CH-CH2-C+3, CH-(CH2)3-
C+3), 1.20-1.50 (m, 8 H, CH-C+2-CH3, CH-(C+2)3-CH3), 1.65-1.85 (m, 1
H, CH2-C+(Et)(Bu)), 3.11 (s, 3 H, N-C+3), 3.35 (”d”, J = 8 Hz, 2 H, N-
C+2-C(H)(Et)Bu), 6.68 (”d”, J = 9 Hz, 2 H, +ar R-N), 7.44 (s, 1 H,
+C=C(CN)2), 7.79 (”d”, J = 9 Hz, 2 H, +ar P-N); ,5 (KBr): ν = 2959 (s),
2931 (s), 2873 (s), 2216 (s), 1614 (s), 1575 (s), 1565 (s), 1525 (s), 1400 (s),
1206 (s), 1191 (s), 816 (m); 06 (EI, 70 eV): m/z (%) = 295 (12, M+·), 196
(100, M - C7H15).
5HVXOWV�DQG�'LVFXVVLRQ
'HVLJQ�RI�FKURPRSKRUHV� We have synthesized a series of chromophores in
which the donor and acceptor parts were varied independently and
systematically. Thus, as shown in Table 1, we obtained azo- (��D�H),
stilbene- (��D�H), tolane- (��D�H) and dicyanovinylbenzene-chromophores
(��D�H), suited to study the structure-property relationships of donor-acceptor
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chromophores for photorefractive applications. The well known
chromophore DMNPAA (��H),11 for example, has a relatively weak donor
group (OMe), resulting in a moderate dipole moment and
hyperpolarizability.33 Its small size might be advantageous with regard to
fast orientation by an electric field in a polymer matrix, but the chromophore
is quite prone to crystallization due to its compact structure. Therefore, we
first increased the strength of the donor by replacing OMe with NMe2
(chromophore ��G) while maintaining the small size of the chromophore,
and secondly introduced bulky donor groups in chromophores ��D and ��E
(in ��E, the alkyl residue is also racemic) in order to inhibit crystallization.
The medium-sized allyl group of chromophore ��F may be used to attach this
chromophore to cyclic or polymeric siloxanes via hydrosilylation.
By varying the type of the chromophore we aimed to control
photoisomerization, which gives rise to undesired polarization gratings in
addition to the photorefractive grating.49 Azo-chromophores readily undergo
reversible photoisomerization, a phenomenon which has been widely used,
e.g. in polymer chemistry.50,51 Photoisomerization of stilbenes is not
thermally reversible, while tolanes and dicyanovinylbenzenes do not
undergo structural changes upon irradiation, thereby eliminating the
possibility of polarization gratings. As we show below, the chromophore
type also influences the electrochemical and electrooptical properties.52
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6\QWKHVLV��Efficient synthesis of the chromophore series was achieved by
using intermediates parallelly in sequences leading to different chromophore
types as shown in Schemes 1 and 2. Thus, in the first step, donor-substituted
benzene derivatives were obtained from suitable anilines. From the donor-
substituted benzenes, we prepared azo chromophores by coupling with the
diazonium salt derived from 4-nitroaniline and benzaldehydes by a
Vilsmeyer-Haack53 procedure. Those aldehydes in turn were used for both
the synthesis of the dicyanovinylbenzene and the stilbene chromophores as
also shown in Scheme 1. We applied a piperidine-catalyzed condensation
with malonodinitrile34 to obtain the dicyanovinylbenzene chromophores. To
yield WUDQV-stilbenes, the aldehydes were reacted with 4-nitro-
triphenylphosphoniumbromide54,55 or with 4-nitro-phenylacetic acid and
piperidine.48 The synthesis of the tolane chromophores (Scheme 2) applies
two Heck-type coupling reactions as key steps.56,57 First, 4-bromo-
nitrobenzene was coupled with trimethylsilylacetylene to yield 1-(1-
ethynyl)-4-nitrobenzene (�) after deprotection. Compound � was then
coupled with substituted iodobenzenes, obtained by direct iodination of the
respective donor-substituted benzenes, to yield the corresponding tolanes.
$EVRUSWLRQ�VSHFWURVFRS\� Increasing the donor strength of donor-acceptor
chromophores incresases their dipole moment as well as their
hyperpolarizability, which leads to a desirable increase in the photorefractive
FOM. On the other hand, an increase in donor strength typically effects a
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red-shift of the absorption maximum which can be undesired since
chromophore absorption at the wavelength of the lasers used should be
negligible. To investigate whether this limits the possibilities of tailoring
NLO-chromophores for photorefractive applications and to get information
on relative donor strengths, we measured the UV/VIS spectra of our
chromophore series. The absorption maxima as obtained from dilute
solutions in THF are listed in Table 2. The value for the azo parent
compound ��I was taken from the literature.58 For chromophores of different
type but identical donor substitution, the wavelength of the absorption
maxima drops in the order azo > stilbene > dicyanovinylbenzene > tolane.
The shape and structure of the absorption is quite similar for the
chromophore types, showing two maxima. For the azo chromophores, this
means that the strong π-π*-band completely overlaps with the n-π*-
absorption (“pseudo stilbene behavior”59), as otherwise two long-
wavelength absorption maxima would be expected. Only for compound ��H,
a slight shoulder resulting from the n-π*-absorption is visible. This
demonstrates the relatively small influence of substitution on the n-π*-
transition,59 implying that tuning the chromophore’s absorption in the red
region is difficult for azo-chromophores. This is different for the other
chromophore types, as the donor substitution has a pronounced effect on the
π-π*-transition. The chromophores with dialkylamino-donors absorb at the
longest wavelengths, while a strong blue-shift occurs on changing the donor-
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group from NMe2 to OMe. Methyl-substitution on the aromatic ring confers
a blue-shift of the absorption maximum for both azo- and stilbene-
chromophores, probably by increasing the dihedral angle between the
aromatic rings. In contrast, no similar blue-shift is observed for
dicyanovinylbenzene chromophores (e.g. comparing compounds ��G and
��E). Likely, the dicyanovinyl-acceptor is twisted out of the plane of the
phenyl ring even for unsubstituted derivatives due to sterical interactions
with the ring hydrogen atoms as shown in Figure 1.60 Therefore, no further
reduction of the conjugation occurs with methyl substitution of the phenyl
ring.
7KHUPDO� FKDUDFWHUL]DWLRQ� RI� WKH� FKURPRSKRUHV� The high chromophore
content of photorefractive composites frequently gives rise to phase
separation and chromophore crystallization.10, 11 This leads to strong
scattering of light or dielectric breakdown of the samples and is
unacceptable for devices.
Different approaches towards improving the stability of photorefractive
composites and to prevent crystallization-driven segregation processes have
been pursued. Hendrickx et al.61 used a random isomeric mixture of
chromophores, while Meerholz et al.62 applied an eutectic mixture. Liquid
chromophores have also been used successfully.63,12 As a more general
approach, the use of plasticizers can serve to inhibit crystallization and
simultaneously lower the glass transition temperature (Tg) of the
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composite,11,14,16,27,28,41 but the plasticizer can reduce the concentration of
functional components. Another widely applicable approach, the
introduction of a bulky alkyl-groups, was first used by Cox et al. with a
DMNPAA-like chromophore,31 and has been employed frequently in recent
work.13,27,30,40
To rationalize these different approaches, we can consider a photorefractive
composite material in a simplifying approach as a two component system
with no miscibility in the solid state. For the material to be stable,
miscibility of chromophor and photoconductor in the melt or
“compatibility” (which is increased by alkyl substituents) and lowering of
the melting point of the chromophore to below the temperature of operation
by the mixing are required. This shows why a low melting point of the
chromophore is crucial, a criterion that also holds if the goal is to inhibit
recrystallization kinetically, i.e. to achieve a very slow crystallization
velocity at room temperature (as in most DMNPAA composites). In the
latter case, a low melting enthalpy also is important, as it increases the
critical size of a crystallization nucleus and thus decreases the probability of
such a nucleus being formed.
The melting points of the chromophores and their melting enthalpies as
determined by DSC are listed in Table 5. Unsubstituted chromophores such
as DANS (��I) or the corresponding tolane derivative DANT (��I) do not
melt below 220 °C; as expected, we were not able to prepare photorefractive
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composites from these chromophores due to their facile crystallization.
Increasing the number or the bulk of substituents lowers the melting point
very effectively: e.g. compare ��I� with ��G, where unsymmetric methyl
substitution on the donor ring has been introduced; ��H�with ��G, where the
methyloxy-substituent has been replaced by a dimethylamino group; ��F
with ��E, where the allyl group has been replaced by the 2-ethyl-hexyl
group. Introducing bulky donor groups as in the series D and E is
synthetically easier than adding substituents on the phenyl ring and is a very
efficient means of reducing the melting point and, in most cases, the melting
enthalpy. The effect of a given donor substituent, however, is not identical
for chromophores of different type: a comparison of compounds ��D/E and
��D/E shows that it can not be predicted a priori which donor group is more
effective in reducing the melting point, even if the overall structure of the
chromophores is very similar.
&\FOLF� YROWDPPHWU\� The relative HOMO and LUMO energies of the
chromophore and the photoconducting compound (corresponding to their
oxidation and reduction potentials, respectively) are an important means to
understand trapping in photorefractive composites.36,37,38,64,65 For the
chromophores to act as traps - either directly by trapping holes or indirectly
by providing the compensator for radical-anions of the sensitizer which act
as traps37 - their HOMO-energy must be higher than that of the
photoconductor.
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In Table 3, the HOMO- and LUMO-energies of the chromophores are given
as determined by CV using ferrocene as the standard.66 Additionally, it is
indicated whether the observed oxidation / reduction is reversible or
irreversible. For illustration, the cyclic voltammograms of the oxidation of
chromophore ��G� and the reduction of chromophore ��H are shown in
Figure 2. The plots are typical examples of an irreversible oxidation and a
reversible reduction, respectively.
We have measured the LUMO-energy of the sensitizer TNF under identical
conditions as those of the chromophores and found it to be –3.90 eV
(reversible reaction). The chromophores’ LUMO-energies are much higher
and thus no reduction of the chromophores will take place in the composites.
The HOMO-energies are strongly affected by the donor substitution for all
chromophore types. The weaker methoxy donor group (series H) leads to a
much lower HOMO-energy than the dialkylamino groups. Variation of the
HOMO-energies with the structure of the dialkylamino donor is smaller, but
correlates roughly with the donor strengths as obtained from the analysis of
the UV/VIS-spectra. For example, the chromophores �-��D exhibit the
lowest HOMO-energies in the respective chromophore series. The decrease
of electron withdrawing properties of the acceptor / bridging group in the
order dicyanovinylbenzene > azo > stilbene > tolane parallels an increase in
the HOMO-energy for chromophores of different type but with identical
donor substitution.
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If the chromophores contribute to the trap manifold, reversibility of the
oxidation is important to avoid unwanted side effects like the formation of
permanent gratings. For azo- and dicyanovinylbenzene-chromophores,
reversibility of the oxidation reaction depends on the donor-substitution.
The reaction is irreversible for allyl-methyl-amino and methoxy-substituted
derivatives. The double bond in the allyl-residue of chromophores ��F and
��F is a probable cause for the irreversibility while in the case of the
compounds ��H and ��H, the high reactivity of the radical cations formed by
oxidation (evidenced by their high oxidation potential) is the likely cause.
For all stilbene- and tolane-chromophores, the oxidation reaction is
irreversible, even though their oxidation potentials are lower than those of
the other chromophores.
For comparison with the chromophore oxidation potentials, we have
measured the HOMO-energies of PVK and the model compounds
ethylcarbazole and D4-4CZ. No reduction of these compounds was observed
in the solvent window of THF. Their structures are depicted in Figure 3
together with that of the polymeric siloxane PSX used in the photorefractive
composites. Ethylcarbazole has been used by others as a model compound
for PVK in electrochemical measurements.37
The HOMO-energies of the carbazole derivatives are given in Table 4. To
illustrate the typical behavior, the voltammogram of ethylcarbazole is shown
in Figure 4 a). As for all carbazole compounds investigated, the oxidation is
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irreversible. In the second cycle of the measurements, a reversible oxidation
is observed at a significantly lower potential. We suggest that the carbazole
units undergo a coupling reaction after oxidation to the radical cation in
analogy to the electrochemical coupling reaction of triphenylamines.67 The
role of this reaction in the bulk is not clear. We note, however, that dimers
produced by the reaction would be able to act as deep traps in pure PVK,
decreasing the hole mobility. The voltammogram of PVK is shown in
Figure 4 b). The coupling reaction here leads to crosslinking and therefore
experimental difficulties. As there is no oxidation peak, the HOMO-energy
of PVK can only be estimated, taking the sharp rise in current at 824 mV as
peak onset.
Comparing the HOMO energies of the chromophores with those of the
carbazole derivatives, it is evident that the HOMO-energy of the
chromophore can be varied in a range is large enough to cover both a regime
where the chromophores’ HOMO-energy is significantly lower than that of
the photoconducting matrix (alkyloxy donor and not stilbene type) and one
where the HOMO-energy is low enough to be close to that of carbazole
dimers (stilbenes with dialkylamino donors). Chromophores with alkoxy
donor groups are the best choice for electrochemically inert dopants, and the
irreversibility of their oxidation is of no concern since their HOMO energies
are below that of the photoconductor.
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As a final consideration regarding the electrochemical properties of the
chromophores, the injection of holes at the electrode interfaces will become
more probable when the HOMO-energy of chromophore or photoconductor
are close that of the ITO-electrodes (ca. –4.8 eV) unless there is an
insulating layer between the ITO and the composite. This increases the
likelihood of dielectric breakdown of the samples. Of the chromophores
investigated, stilbenes are the most likely to exhibit this problem.
&KDUDFWHUL]DWLRQ� RI� SKRWRUHIUDFWLYH� FRPSRVLWHV�� We have prepared
photorefractive guest-host materials from the photoconducting polysiloxane
PSX,46 TNF and a number of the chromophores. Their composition, glass
transition temperature, shelf lifetime and important physical characterization
data are compiled in Table 6. In the context of shelf lifetimes, we refer to a
sample as stable if upon storage no processes are observed that are
detrimental for its use in photorefractive experiments, e.g. crystallization of
the chromophore or phase separation leading to opaque samples.
7KHUPDO�FKDUDFWHUL]DWLRQ� To achieve optimized specimen characteristics,
the Tg of the composites as measured by DSC was adjusted to room
temperature where all measurements of the photorefractive properties were
performed. In composites with a higher glass transition temperature,
chromophore orientation is very slow, while a low Tg made samples prone
to dielectric breakdown. The adjustment of the glass transition temperature
was achieved by varying the ratio of chromophore and photoconductor.
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Azo- and stilbene-chromophores of identical donor substitution, e.g. � D and
� D or ��H and ��H, had to be mixed with PSX in approximately the same
weight portion, reflecting their very similar structure and molecular weight.
A much smaller weight portion of the tolane- and dicyanovinylbenzene-dyes
was needed to lower Tg to room temperature. Such a smaller weight fraction
of the chromophore typically corresponds to greater stability of the
composite. This is evident in the stability of the composites made from
chromophores ��H and ��H, where ��H even has a higher melting point than
DMNPAA. The composites made from the corresponding azo and stilbene
chromophores DMNPAA (��H) and ��H are only stable for a limited time
before crystallization occurs. Thus, for composites with chromophore
contents of more than 40% by weight, only chromophores with bulky donor
substituents yield stable samples. The donor substituent approach to create
chromophores that yield morphologically stable samples is therefore
superior over substitution directly on the aromatic ring.
3KRWRUHIUDFWLYH� &KDUDFWHUL]DWLRQ� Evaluating the photorefractive
properties of the composites compiled in Table 6, we find that all but one
are “high performance materials” according to the definition by Moerner et
al..3 Only the composite based on chromophore � H fails to satisfy these
criteria (diffraction efficiency > 5%, gain coefficient > 50 cm-1) due to its
relatively small gain coefficient of 23 cm-1. This demonstrates that for
common donor-acceptor type NLO-chromophores, several properties (e.g.
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HOMO-energy, ability to photoisomerize) may be varied without severely
compromising the photorefractive refractive index modulation.
For a detailed discussion of the photorefractive performance, we will focus
on the composites made from compounds �-�� H, where the chromophore
type was varied while maintaining identical substitution. For compounds
��H, ��D and ��E, the results of extensive investigations have been published
elsewhere.39
We investigated whether a polarization grating, stemming from
photoisomerization of the chromophores, was present in the composites.
This grating is best observed for s-polarized reading beams, because the
writing beams in the DFWM-measurements are s-polarized as well.68 To
measure the small refractive index modulation ∆npolariz. resulting from
photoisomerization, the grating translation technique69,70,71 was used at E0 =
0 V/µm. The values determined for ∆npolariz. are given in Table 6. Due to
their facile and thermally reversible photoisomerization, azo chromophores
show the largest refractive index modulations at zero field. The stilbenes
exhibit a small ∆npolariz., which is larger for ��D than for ��H. This may be
attributed to the higher absorption coefficient (13 cm-1) of ��D compared to
��H�(3 cm-1). Finally, the use of tolane chromophores, which are not able to
undergo structural changes upon irradiation, prevents the formation of a
polarization grating. Thus, variation of the chromophore type can be used to
control the extent of polarization gratings.39
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The phase shift Φ between the photorefractive grating and the light intensity
pattern was also determined using the grating translation technique. The
results for the chromophores �-��H are plotted against the field E0 in
Figure 6. At small external fields (E0 ≤ 20 V/µm), the phase shift strongly
depends on E0. In this regime, Φ rises with E0 for the tolane chromophore
while it drops for the azo- and the stilbene-chromophore (compounds ��H
and ��H). The initial high phase shift is due to the polarization grating for
which Φ = 180 °. The phase shift drops to that of the photorefractive grating
once the refractive index modulation of the later becomes dominant. Since
there is no polarization grating in the composite with the tolane �� H, Φ
steadily rises due to the increasing strength of the drift field, which enables
the charges to propagate further before being trapped.
The effective trap density Neff in the composites was calculated based on
results from the standard model for organic photorefractive materials7 as
previously reported.39 The trap densities as listed in Table 6 rise with
decreasing HOMO energy. This has been found by others37 and is expected
both if the chromophores themselves were to act as traps and if they
provided a compensating charge for sensitizer radical anions. Higher trap
densities yield smaller values for the phase shift Φ at high field E0, which
thus also is influenced by the chromophores HOMO energy.
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All materials exhibit net-gain. The gain coefficients à for the series �-��H are
plotted against the external drift-field E0 in Figure 5. The dependence of Γ
on E0 is quadratic and the gain coefficients (and therefore the refractive
index modulations) for s- and p-polarized writing beams have opposite sign.
Both these results are predicted for materials in which the refractive index
modulation is predominantly due to the orientational enhancement effect.23
The gain coefficients span an order of magnitude for the chromophores
investigated, with the highest values observed for DMNPAA and its stilbene
analog ��H. Since index modulations are high in our materials, the main
factor limiting Γ is the phase shift, which in turn is related to the trap
densities and thus to the HOMO energies of the chromophores.
As a final quantity to compare the steady state performance of the
composites, the refractive index modulations ∆n for both s- and p-polarized
reading beams were obtained from the respective diffraction efficiencies in
DFWM experiments. We use the refractive index modulation rather than the
diffraction efficiency for comparison of the chromophores since ∆n is a
quantity that is less dependent on experimental parameters (e.g. sample
thickness). The dependence of ∆n on E0 is quadratic for all composites,
again demonstrating the dominance of the orientational enhancement
effect.72 The index modulations ∆n observed for the tolane composites
(chromophores ��E and ��H) are comparatively small, but this must in part be
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attributed to their lower chromophore content. The very high ∆np of the
composite with chromophore ��D, which leads to total internal diffraction at
a sample thickness of only 40 µm and a field of ca. 75 V/µm, is presumably
due to the strong dialkylamino donor group which leads to a higher
hyperpolarizability52 and thus to a larger contribution by the Pockels-effect
to the refractive index modulation.
The ratio of contributions from birefringence and Pockels effect to the index
modulation, ABR/AEO (given in Table 6) allow us to evaluate this more
quantitatively.23 These values drop as the donor strength increases, e.g
comparing chromophores ��D and ��H or ��E and ��H. Since ABR/AEO = -1/3
∆α/β µ/kT, we can conclude that an increase in donor strength increases β
significantly more than ∆α·µ for our chromophores. For compound ��H
(DMNPAA), our value of –5.5 for ABR/AEO agrees well with published
results (ABR/AEO = -5.3) on a DMNPAA/PVK composite.73
The characteristic response times the photorefractive grating were
determined by fitting the early decay of the diffraction efficiency to a single
exponential η = η0·exp(-2t/τ) as performed by other groups.39,41,36,74 The
factor two in this equation takes the quadratic dependence of the diffraction
efficiency on the refractive index modulation into account. For the
chromophore series �-��H, the decay times are plotted against the field E0 in
Figure 7. For all chromophores, the decay times decrease with increasing
field strength. For high field strengths (> 40-50 V/µm), this incremental
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decrease becomes small. Thus, the decay times τ at high field strength are
compiled in Table 6. They are quite similar for the chromophores �-��H, but
significantly longer for the other compounds. The response times are quite
long and the main limiting factor for applications of our composites. It is
therefore important to try and identify the process or processes that limit τ,
which could be any of the processes charge generation, charge transport,
charge trapping and chromophore orientation (As the time required for the
electrooptic response is less than a microsecond, we do not consider it here).
The response time rises as the size of the chromophore increases. However,
experiments in which the rotational mobility of the chromophores was
probed by in-situ measurements of second-harmonic generation (SHG)
showed that larger donor groups do not reduce the speed of orientation.75
Furthermore, Kippelen et al. found that even at a response time of 4 ms,
orientation was not limiting for a tolane-based composite.32 Therefore,
chromophore orientation is not limiting τ for our composites. It is also
unlikely that the time required for charge trapping limits the response times
since they increase with the trap density in our composites (cf. Table 6). On
the other hand, this correlation between trap density and response times may
indicate that charge transport is the limiting process, as the charge carrier
mobility will be decreased by the introduction of deep traps. This reasoning
is supported by the observation that the response times also rises with
increasing dipole moment of the chromophores. As the presence of dipoles
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in organic photoconductors leads to a broadening of the trap energy
distribution and thus to an increased energetic disorder76 in the model of
Bässler et al.,77 the charge carrier mobility is also decreased. The dipole
moments of the chromophores may be estimated from those of model
compounds with similar substitution given in Table 7.52 For chromophores
�-��H and � E the dipole moment will be quite similar to that of the model
compounds, but it will be higher for compound ��D due to the additional
ester groups.60
To investigate the influence of charge generation and corroborate that of
charge mobility, further experiments using different sensitizers and
photoconductors, respectively, are required. Improving the photoconducting
component has a strong effect on the response time of photorefractive
composites.13,14
&RQFOXVLRQV
To study their structure property relationships, we have synthesized and
characterized a number of new NLO-chromophores specifically designed for
the use in photorefractive composites. Donor substitution and chromophore
type were varied systematically using an efficient synthetic scheme.
The absorption spectra of the chromophores give information on donor
strengths and the tunability of light absorption through structural
modifications. Chromophore melting points and melting enthalpies were
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greatly lowered by introducing bulky alkyl substituents, which allowed the
preparation of photorefractive composites with long-term stability towards
chromophore recrystallization. These modifications do not compromise
chromophore orientation or tuning of the HOMO levels. By varying donor
strength and chromophore type, the HOMO-energies of the chromophores
can be tuned to cover a range of ±0.5 eV around the HOMO of carbazole-
based photoconducting host polymers. Fluorination of the chromophores38 is
another way of tuning the HOMO energy which we did not explore in this
study. This might constitute an especially promising approach for stilbene
chromophores which showed the highest HOMO energies. The trap density
in the photorefractive composites rises with the HOMO-energies of the
chromophores. It has a significant influence on both the photorefractive
phase shift and the charge mobility, which in turn determines the
holographic response time. Whether photocharge generation also limits the
response times remains to be investigated by variation of the sensitizer.
While the response times require improvement, possibly by use of modified
photoconducting matrix, all chromophores investigated allow for high
refractive index modulations and the formation of polarization gratings can
be inhibited by using chromophores which are not capable of
photoisomerization reactions.
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(OHFWURQLF� 6XSSOHPHQWDU\� ,QIRUPDWLRQ� DYDLODEOH� Experimental details
for the preparation of all compounds and their spectroscopic
characterization; absorption spectra and normalized DSC-traces for selected
compounds (PDF, 19 pages). This material is available free of charge via the
Internet or from the authors.
$FNQRZOHGJHPHQWV
We thank A. Göpfert and W. Joy for technical assistance and I. Otto for
preparation of the photorefractive samples. We are thankful to Dr.
Mukundan Thelakkat (MC I, University of Bayreuth) for an introduction to
CV measurements and helpful discussions and to Prof. H.-W. Schmidt for
his supportive interest in this work. Financial support by the Bayerische
Forschungsstiftung within FOROPTO II is gratefully acknowledged.
5HIHUHQFHV
1 S.Ducharme, J. C. Scott, R. J. Twieg and W. E. Moerner, 3K\V��5HY��/HWW�, 1991, ��, 1846.
2 W. E. Moerner and S. M. Silence, &KHP��5HY�, 1994, ��, 127.
3 W. E. Moerner, A. Grunnet-Jepsen and C. L. Thompson, $QQX��5HY��6FL., 1997, ��, 58.
4 S. J. Zilker, &KHP3K\V&KHP, 2000, �, 72.
5 L. Solymar, D. J. Webb and A. Grunnet-Jepsen, 7KH�3K\VLFV�DQG�$SSOLFDWLRQV�RI
3KRWRUHIUDFWLYH�0DWHULDOV; Claredon: Oxford, 1996.
6 a) B. L. Volodin, B. Kippelen, K. Meerholz, B. Javidi, N. Peyghambarian, 1DWXUH, 1996,
���, 58; b) C. Poga, P. M. Lundquist, V. Lee, R. M. Shelby, R. J. Twieg and D. M.
PRIVILEGED DOCUMENT FOR REVIEW PURPOSES ONLY
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Burland,�$SSO��3K\V��/HWW�,�1996, ��, 1047; c) A. Grunnet-Jepsen, C. L. Thompson and W.
E. Moerner, 6FLHQFH,�1997, ���, 549; d) A. Goonesekera, D. Wright, W. E. Moerner, $SSO�
3K\V��/HWW�, 2000, ��, 3358.
7 P. Günter and J.-P. Huignard, 3KRWRUHIUDFWLYH�0DWHULDOV�DQG�WKHLU�DSSOLFDWLRQV�,��,,;
Springer: Berlin, 1988.
8 A. Ashkin, G. D. Boyd, J. M. Dziedzic, R. G. Smith, A. A. Ballman, J. J. Levinstein and
K. Nassau, $SSO��3K\V��/HWW., 1966, �, 72.
9 K. Yokoyama, K. Arishima, T. Shimada and K. Sugekawa, -SQ��-��$SSO��3K\V�, 1994, ��,
1029.
10 O. Zobel, M. Eckl, P. Strohriegl and D. Haarer, $GY��0DWHU�, 1995, �, 911.
11 K. Meerholz, B. L. Volodin, J. F. Sandalophon, B. Kippelen and N. Peyghambarian,
1DWXUH, 1994, ���, 497.
12 H. J. Bolink, V. V. Krasnikov, P. H. J. Kouwer and G. Hadziioannou, &KHP��0DWHU�,
1998, ��, 3951.
13 M. Thelakkat, J. Ostrauskaite, A. Leopold, R. Bausinger and D. Haarer, &KHP��3K\V�,
2002, ���, 133.
14 U. Hofmann, A. Schreiber, D. Haarer, S. J. Zilker, A. Bacher, D. D. C. Bradley, M.
Redecker, M. Inbasekaran, W. W. Wu and E. P. Woo, &KHP��3K\V��/HWW�, 1999, ���, 41.
15 K. Ogino, T. Nomura, T. Shichi, S.-H. Park, H. Sato, T. Aoyama and T. Wada, &KHP�
0DWHU�, 1997, �, 2768.
16 M. A. Diaz-Garcia, D. Wright, J. D. Casperson, B. Smith, E. Glazer, W. E. Moerner, L. I.
Sukhomlinova and R. J. Twieg, &KHP��0DWHU�, 1999, ��, 1784.
17 M. Liphardt, M. Goonesekera, A. Jones, S. Ducharme and B. E. Takacs, 6FLHQFH, 1994,
���, 367.
PRIVILEGED DOCUMENT FOR REVIEW PURPOSES ONLY
37
18 U. Hofmann, S. Schloter, A. Schreiber, K. Hoechstetter, G. Bäuml, S. J. Zilker, D.
Haarer, M. Thelakkat, H.-W. Schmidt, K. Ewert and C, D. Eisenbach, 3URF��63,(, 1998,
����, 124.
19 a) B. Kippelen, K. Tamura, N. Peyghambarian, A. B. Padias and H. K. Hall, Jr., -��$SSO�
3K\V�� 1993, ��, 3617; b) C. Zhao, C.-K. Park, P. N. Prasad, Y. Zhang, S. Ghosal and R.
Burzynski, &KHP��0DWHU�, 1995, �, 1237; c) L. Yu, W. K. Chan, Z. Peng and A. Gharavi,
$FF��&KHP��5HV�, 1996, ��, 13; d) S. Schloter, U. Hofmann, K. Hoechstetter, D. Haarer,
K. Ewert and C.-D. Eisenbach, -��2SW��6RF��$P��%, 1998, ��, 2560; e) M. S. Bratcher, M.
S. DeClue, A. Grunnet-Jepsen, D. Wright, B. R. Smith, W. E. Moerner and J. S. Siegel,
-��$P��&KHP��6RF�,�1998, ���, 9680; f) M. Döbler, C. Weder, P. Neuenschwander, U. W.
Suter, S. Follonier, C. Bosshard and P. Günter, 0DFURPROHFXOHV, 1998, ��, 6184.
20 a) Y. Zhang, T. Wada, L. Wang and H. Sasabe, &KHP��0DWHU�, 1997, �, 2798; b) C.
Hohle, U. Hofmann, S. Schloter, M. Thelakkat, P. Strohriegl, D. Haarer and S. J. Zilker, -�
0DWHU��&KHP�, 1999, �, 2205; c) K. Ogino, S.-H. Park and H. Sato, $SSO��3K\V��/HWW�, 1999,
��, 3936; d) H. J. Bolink, C. Arts, V. V. Krasnikov, G. G. Malliaras and G. Hadziioannou,
&KHP��0DWHU��1997, �,�1407; e) P. M. Lundquist, R. Wortmann, C. Geletneky, R. J. Twieg,
M. Jurich, V. Y. Lee, C. R. Moylan and D. M. Burland,�6FLHQFH,�1996, ���, 1182; f) O.
Ostroverkhova, D. Wright, U. Gubler, W. E. Moerner, M. He, A. Sastre-Santos and R. J.
Twieg, $GY��)XQFW��0DWHU�, 2002, ��, 621.
21 K. Ewert, S. Schloter, U. Hofmann, K. Hoechstetter, D. Haarer and C. D. Eisenbach,
3URF��63,(, 1998, ����,�134.
22 a) C. Bosshard, K. Sutter, P. Prêtre, J. Hullinger, M. Flörsheimer, P. Kaatz and P. Günter,
2UJDQLF�1RQOLQHDU�2SWLFDO�0DWHULDOV��$GYDQFHV�LQ�1RQOLQHDU�2SWLFV���9RO���; Gordon and
Breach: Amsterdam, 1995; b) J. J. Wolff and R. Wortmann, $GY��3K\V��2UJ��&KHP�, 1999,
��, 121; c) T. Verbiest, S. Houbrechts, M. Kauranen, K. Clays and A. Persoons, -��0DWHU�
&KHP�, 1997, �, 2175.
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38
23 W. E. Moerner, S. M. Silence, F. Hache and G. C. Björklund, -��2SW��6RF��$P��%, 1994,
��, 320.
24 R. Wortmann, C. Poga, R. J. Twieg, C. Geletneky, C. R. Moylan, P. M. Lundquist, R. G.
DeVoe, P. M. Cotts, H. Horn, J. E. Rice and D. M. Burland,�-��&KHP��3K\V�,�1996, ���,
10637.
25 B. Kippelen, F. Meyers, N. Peyghambarian and S. R. Marder, -��$P��&KHP��6RF�, 1997,
���, 4559.
26 F. Würthner, S. Yao, T. Debaerdemaeker and R. Wortmann, -��$P��&KHP��6RF�, 2002,
���, 9431.
27 F. Würthner, S. Yao, J. Schilling, R. Wortmann, M. Redi-Abshiro, E. Mecher, F.
Gallego-Gomez and K. Meerholz, -��$P��&KHP��6RF�, 2001, ���, 2810.
28 B. Kippelen, S. R. Marder, E. Hendrickx, J. L. Maldonado, G. Guillernet, B. L. Volodin,
D. D. Steele, Y. Enami, Sandalphon, Y. J. Yao, J. F. Wang, H. Röckel, L. Erskine and N.
Peyghambarian, 6FLHQFH, 1998, ���, 54.
29 S. Beckmann, K.-H. Etzbach, P. Krämer, K. Lukaszuk, R. Matschiner, A. J. Schmidt, P.
Schuhmacher, R. Sens, G. Seybold, R. Wortmann and F. Würthner, $GY��0DWHU�, 1999, ��,
536.
30 F. Würthner, R. Wortmann, and K. Meerholz, &KHP3K\V&KHP, 2002, �, 17.
31 A. M. Cox, R. D. Blackburn, D. P. West, T. A. King, F. A. Wade and D. A. Leigh, $SSO�
3K\V��/HWW., 1996, ��, 2801.
32 J. A. Herlocker, K. B. Ferrio, E. Hendrickx, B. D. Guenther, S. Mery, B. Kippelen and N.
Peyghambarian, $SSO��3K\V��/HWW�, 1999, ��, 2253.
33 C. R. Moylan, R. Wortmann, R. J. Twieg and I. McComb, -��2SW��6RF��$P��%,�1998, ��,
92.
34 S. M. Silence, M. C. J. M. Donckers, C. A. Walsh, D. M. Burland, R. J. Twieg and W. E.
Moerner, $SSO��2SW�, 1994, ��, 2218.
PRIVILEGED DOCUMENT FOR REVIEW PURPOSES ONLY
39
35 S. M. Silence, J. C. Scott, J. J. Stankus, W. E.; Moerner, C. R. Moylan, G. C. Bjorklund
and R. J. Twieg, -��3K\V��&KHP�,�1995, ��, 4096.
36 G. G. Malliaras, V. V. Krasnikov, H. J.; Bolink and G. Hadziioannou, $SSO��3K\V��/HWW�,
1995, ��, 1038.
37 A. Grunnet-Jepsen, D. Wright, B. Smith, M. S. Bratcher, M. S. DeClue, J. S. Siegel and
W. E. Moerner, &KHP��3K\V��/HWW�, 1998, ���, 553.
38 E. Hendrickx, Y. Zhang, K. B. Ferrio, J. A. Herlocker, J. Anderson, N. R. Armstrong, E.
A. Mash, A. P. Persoons, N. Peyghambarian and B. Kippelen, -��0DWHU��&KHP�, 1999, �,
2251.
39 S. Schloter, U. Hofmann, P. Strohriegl, H. W. Schmidt and D. Haarer, -��2SW��6RF��$P��%,
1998, ��, 2473.
40 U. Gubler, M. He, D. Wright, Y. Roh, R. J. Twieg and W. E. Moerner, $GY��0DWHU�,
2002, ��, 313.
41 A. Grunnet-Jepsen, C. L. Thompson, R. J. Twieg and W. E. Moerner, $SSO��3K\V��/HWW�,
1997, ��, 1515.
42 D. Wright, M. A. Diaz-Garcia, J. D. Casperson, M. DeClue and W. E. Moerner, $SSO�
3K\V��/HWW�,�1998, ��, 1490.
43 G. A. Mabbott, -��&KHP��(G�, 1983, ��, 697.
44 K. v. Auwers, F. Michaelis, %HU��'WVFK��&KHP��*HV�, 1914, ��, 1275.
45 S. Akiyama, K. Tajima, S. Nakatsuji, K. Nakashima, K. Abiru and M. Watanabe, %XOO�
&KHP��6RF��-SQ�, 1995,���, 2043.
46 P. Strohriegl, 0DNURPRO��&KHP��5DSLG�&RPP�, 1986, �, 771.
47 L. F. Tietze and T. Eicher, 5HDNWLRQHQ�XQG�6\QWKHVHQ�LP�RUJDQLVFK�FKHPLVFKHQ
3UDNWLNXP�XQG�)RUVFKXQJVODERUDWRULXP; Thieme: Stuttgart, 1991.
48 P. Pfeiffer and S. Sergiewskaja, %HU��'W��&KHP��*HV�, 1911, ��, 1107.
PRIVILEGED DOCUMENT FOR REVIEW PURPOSES ONLY
40
49 K. Anderle, R. Birenheide, M. Eich and J. H. Wendorff, 0DNU� &KHP��5DSLG�&RPP�,
1989, ��, 477.
50 C. D. Eisenbach, 0DNURPRO��&KHP�, 1978, ���, 2489.
51 C. D. Eisenbach, Ber. Bunsenges. Phys. Chem., 1980, 84, 680.
52 a) L. T. Cheng et al., L.-T. Cbeng, W. Tam, S. H. Stevenson, G. R. Meredith, G. Rikken
and S. R. Marder, -��3K\V��&KHP�, 1991, ��, 10631. b) L.-T. Cbeng, W. Tam, S. R. Marder,
A. E. Stiegman, G. Rikken and C. W. Spangler, -��3K\V��&KHP�, 1991, ��, 10643.53 A. Vilsmeyer, A. Haack, %HU��'WVFK��&KHP��*HV�, 1927, ��, 119.
54 G. Wittig, G. Geissler,�-XVWXV�/LHELJV�$QQ��&KHP�, 1953, ���, 44.
55 A. R. Tatchell, 9RJHO¶V�7H[WERRN�RI�3UDFWLFDO�2UJDQLF�&KHPLVWU\; John Wiley & Sons:
New York, 1989.
56 R. Zentel, D. Jungbauer, R. J. Twieg, D. Y. Yoon and C. G. Willson, 0DNURPRO��&KHP�,
1993, ���, 859.
57 J. Wong, P. Masson and J.-F. Nicoud, 3RO\PHU�%XOO�, 1994, ��, 265.
58 H. Mustroph and J. Epperlein, -��SUDNW��&KHP., 1980, ���, 305.
59 H. Rau in 3KRWRFKURPLVP (Eds.: H. Dürr and H. Bouas-Laurent); Elsevier: Amsterdam,
New York, 1990.
60 This also results from semiempirical quantum mechanical calculations performed using
the MOPAC6 package (AM1 parametrization) of cerius2 by msi.
61 E. Hendrickx, J. F. Wang, J. L. Maldonado, B. L. Volodin, Sandalphon, E. A. Mash, A.
Persoons, B. Kippelen and N. Peyghambarian, 0DFURPROHFXOHV, 1998, ��, 734.
62 K. Meerholz, R. Bittner, Y. De Nardin, C. Bräuchle, E. Hendrickx, B. L. Volodin, B.
Kippelen and N. Peyghambarian, $GY��0DWHU., 1997, �, 1043.
63 M. C. J. M. Donckers, S. M. Silence, C. A. Walsh, F. Hache, D. M. Burland, W. E.
Moerner and R. J. Twieg, 2SW��/HWW�, 1993, ��, 1044;
64 O. Ostroverkhova and K. D. Singer, -��$SSO�3K\V�, 2002, ��, 1727.
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41
65 E. Hendrickx, D. Van Steenwinckel, A. Persoons, C. Samyn, D. Beljonne and J.-L.
Brédas, -��&KHP��3K\V�, 2000, ���, 5439.
66 J. Pommerehne, H. Vestweber, W. Guss, R. F. Mahrt, H. Bässler, M. Porsch and J. Daub,
$GY��0DWHU�, 1995, �, 551.
67 M. Yano, M. Furuichi, K. Sato, D. Shiomi, A. Ichimura, K. Abe, T. Takui and K. Itoh,
6\QWK��0HW�, 1997, ��, 1665.
68 Sandalphon, B. Kippelen, N. Peyghambarian, S. R. Lyon, A.B. Padias and H. K. Hall, Jr.
, 2SW��/HWW�, 1994, ��, 68.
69 K. Sutter and P. Günter, -��2SW��6RF��$P��%, 1990, �, 2274.
70 C. A. Walsh and W. E. Moerner, -��2SW��6RF��$P��%,�1992, �, 1642.
71 C. A. Walsh and W. E. Moerner, -��2SW��6RF��$P��%, 1992, ��, 753.
72 M. Kuzyk, M. in &KDUDFWHUL]DWLRQ�7HFKQLTXHV�DQG�7DEXODWLRQV�IRU�2UJDQLF�1RQOLQHDU
2SWLFDO�0DWHULDOV (Eds: Kuzyk, M.; Dirk, C.), Vol. 60, Marcel Dekker, Inc., New York
����.
73 Sandalphon, B. Kippelen, K. Meerholz and N. Peyghambarian, $SSO��2SW�, 1996, ��,
2346.
74 Y. Zhang, S. Ghosal, M. K. Casstevens and R. Burzynski, 3RO\PHU�3UHSU�, 1994, ��, 233.
75 K. Hoechstetter, S. Schloter, U. Hofmann and D. Haarer, -��&KHP��3K\V�, 1999, ���,
4944.
76 P. M. Borsenberger and J. J. Fitzgerald, -��3K\V��&KHP� 1993, ��, 4815.
77 H. Bässler, 3K\V��6WDW��6RO��%, 1981, ���, 9.
PRIVILEGED DOCUMENT FOR REVIEW PURPOSES ONLY
42
7DEOHV
7DEOH��� 6WUXFWXUHV�DQG�QXPEHULQJ�RI�WKH�GRQRU�DFFHSWRU�FKURPRSKRUHV�
N
O
OO
O
N N N O N
← Donor part a)
Acceptor part a) ↓
��D ��E ��F ��G ��H c) �b)N NO2N
��D ��E ��F ��G ��H ��I d) CH NO2HC
��D ��E �b) �b) ��H ��I e) C NO2C
��D ��E ��F ��G ��H �b) CNCN
a) The curved line represents the single bond to the acceptor/donor part, respectively.b) Chromophore not prepared. c) commonly denominated DMNPAA. d) commonly denominated
DANS. e) commonly denominated DANT.
PRIVILEGED DOCUMENT FOR REVIEW PURPOSES ONLY
43
7DEOH��� /RQJHVW�ZDYHOHQJWK� DEVRUSWLRQ�PD[LPD�RI� WKH�GRQRU�DFFHSWRU� W\SH� FKURPRSKRUHV
LQ�7+)�VROXWLRQ��)RU�WKH�VWUXFWXUHV�RI�WKH�FKURPRSKRUHV�FI��7DEOH���
λPD[ / nm (ε / M-1cm-1)
Acceptor
Donor Nitrophenyl-azo(�)
Nitrostilbene(�)
Nitrotolane(�)
Dicyanovinyl-phenyl(�)
D 461 (3.13 × 104) 428 (3.05 × 104) 399 (2.39 × 104) 420 (4.24 × 104)
E 477 (2.84 × 104) 443 (2.91 × 104) 416 (2.23 × 104) 431 (5.16 × 104)
F 461 (2.63 × 104) 434 (2.71 × 104) - 425 (4.25 × 104)
G 429 (2.08 × 104) 397 (2.08 × 104) - 419 (2.34 × 104)
H 397 (2.36 × 104) a) 385 (2.18 × 104) 354 (2.22 × 104) 371 (2.70 × 104)
I 475 (3.16 × 104) d) 433 (2.93 × 104) b) 409 (2.49 × 104) c) -
a) chromophore commonly denominated DMNPAA. b) chromophore commonly denominated
DANS. c) chromophore commonly denominated DANT. d) From reference 29.
PRIVILEGED DOCUMENT FOR REVIEW PURPOSES ONLY
44
7DEOH��� +202�� DQG� /802�HQHUJLHV� RI� WKH� FKURPRSKRUHV� DV� GHWHUPLQHG� E\� F\FOLF
YROWDPPHWU\�� 7KH� OHWWHUV� µL¶� DQG� µU¶� LQGLFDWH� LUUHYHUVLEOH� DQG� UHYHUVLEOH� UHDFWLRQ�
UHVSHFWLYHO\��)RU�WKH�VWUXFWXUHV�RI�WKH�FKURPRSKRUHV�FI��7DEOH���
E / eVHOMO (LUMO)
Azo-Type (�) Stilbene-Type (�) Tolane-Type (�) Dicyanovinylben-zene-Type (�)
D -5.55 r (-3.36 r) -5.18 i (-3.12 r) -5.41 i (-3.20 r) -5.65 r (-2.90 i)
E -5.39 r (-3.31 r) -5.05 i (-3.13 r) -5.30 i (-3.17 r) -5.53 r (-2.87 i)
F -5.43 i (-3.33 r) -5.07 i (-3.12 r) b) -5.60 i (-2.91 i)
G -5.36 r (-3.34 r) -5.09 i (-3.15 r) b) -5.46 ic) (-3.08 i)
H -5.99a) i (-3.37 r) -5.62 i (-3.15 r) -5.87a) i (-3.21 r) -6.21a) i (-2.99 i)
a) measured in 1,2-propylene carbonate. b) chromophore not synthesized. c) irrev. at 20 mV/s.
PRIVILEGED DOCUMENT FOR REVIEW PURPOSES ONLY
45
7DEOH��� +202�HQHUJLHV�RI�FDUED]ROH�GHULYDWLYHV�DV�GHWHUPLQHG�E\�F\FOLF�YROWDPPHWU\�
Compound E / eVHOMO
E / eVHOMO Dimer
Scan rate/ mVs-1
Ethylcarbazole -5.59 -5.22 50
PVK -5.56…-5.61a) - 50
D4-4CZ -5.62 -5.26 50
a) estimated as described in the text.
PRIVILEGED DOCUMENT FOR REVIEW PURPOSES ONLY
46
7DEOH��� 0HOWLQJ� SRLQWV� �SHDN� RQVHW�� DQG� PHOWLQJ� HQWKDOSLHV� RI� WKH� GRQRU�DFFHSWRU� W\SH
FKURPRSKRUHV�DV�GHWHUPLQHG�E\�'6&��7KH�VWUXFWXUHV�RI�WKH�FKURPRSKRUHV�DUH�JLYHQ
LQ�7DEOH���
ΤPHOW / °C (∆+PHOW�/ J g-1)
Acceptor
Donor Nitrophenyl-azo(�)
Nitrostilbene(�)
Nitrotolane(�)
Dicyanovinyl-phenyl(�)
D 114 (50.1) 84 (51.5) 95 (54.7) 101 (86.7)
E 96 (103.0) 119 (71.2) 88 (96.6) 53 (69.4)
F 120 (98.6) 156 (90.4) 109 (149.6)
G 114 (92.0) 92 (87.3) 148 (150.9)
H 164 (124.4) a) 125 (93.1) 137 (109.2) 178 (166.9)
I - 250 (subl.) b), c) 220 b), d) -
a) chromophore commonly denominated DMNPAA. b) Determined using a Kofler hot stage. c)
chromophore commonly denominated DANS. d) chromophore commonly denominated DANT.
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ate
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49
7DEOH��� 'LSROH�PRPHQWV�RI�'013$$����H����DQG�PRGHO�FRPSRXQGV���
Chromophore Solvent µ /
10-18 esu
NO2N
��I
CHCl3 6.6
NO2N
��I
CHCl3 6.1
NN
OO2N
��H
CHCl3 5.5
OO2N
1,4-dioxane 4.5
OO2N1,4-dioxane 4.4
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50
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7DEOH���
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53
6FKHPHV
D R
R N N
NO2
��E�G
G R = Me, D = NMe2
H R = Me, D = OMe
D����� R = H, D =
NMe
NMe
E R = H, D =
F R = H, D =
NO2
N2+ Cl
-
NO
O 2
HC(O)NMePh
POCl3
NC CN
piperidinea) or b)
NOHOH
NHNH2
NaH , BrR’
( KI )
D
R
R
��D�H
D
O
R
R
H
��D�H
D
CN
CN
R
R
��D�H
D R
R
NO2
��D�H
Et3N
PivCl,Me2SO4
6FKHPH���
K. Ewert et al.
PRIVILEGED DOCUMENT FOR REVIEW PURPOSES ONLY
54
NO2
Br
2. KOH, MeOH
1. (Ph3P)2PdCl2 , CuI
Et2NH
I2 , base
SiMe3
H
+
(Ph3P)2PdCl2 , CuI
Et2NH
NO2
H
�
I
D
R
R
��D�E�H
D
NO2
R
R
��D�E�H
D
R
R
��D�E�H
H
R = Me,
D = OMeN
MeE
R = H,
D = N
O
O
O
O
D
R = H,
D =
6FKHPH���
K. Ewert et al.
PRIVILEGED DOCUMENT FOR REVIEW PURPOSES ONLY
55
)LJXUHV
D
HN
N
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K. Ewert et al.
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56
0 100 200 300 400 500 600 700 800
10 µA
E vs. reference electrode / mV
-1400 -1200 -1000 -800 -600 -400 -200 0
10 µA
E vs. reference electrode / mV
D�
E�
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K. Ewert et al.
PRIVILEGED DOCUMENT FOR REVIEW PURPOSES ONLY
57
O
NO2
NO2
O2N
TNF
nO Si
(CH2)3
N
Me
PSX
4O Si
(CH2)3
N
Me
D4-4CZ
n
N N
EthylcarbazolePVK
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PRIVILEGED DOCUMENT FOR REVIEW PURPOSES ONLY
58
D�
E�
-400 -200 0 200 400 600 800 1000 1200
second cycle
first cycle5 µA
E vs. reference electrode / mV
-400 -200 0 200 400 600 800 1000 1200
second cycle
first cycle10 µA
E vs. reference electrode / mV
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K. Ewert et al.
PRIVILEGED DOCUMENT FOR REVIEW PURPOSES ONLY
59
(56:43:1)
(52:47:1)
(68:31:1)
E / V/ mµ
Γ / c
m-1
Γ / c
m-1
Γ / c
m-1
0
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60
E / V/ mµ0
Φ /
degr
eep
Φ /
degr
eep
Φ /
degr
eep
PSX:4e:TNF (56:43:1)
PSX:5e:TNF (52:47:1)
PSX:6e:TNF (68:31:1)
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61
PSX:4e:TNF(56:43:1)
PSX:5e:TNF(52:47:1)
PSX:6e:TNF(68:31:1)
E / V/ mµ
τ /
s
0
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K. Ewert et al.