(E,E,E)-4,4′-distyrylstilbenes - Synthesis, photophysics, photochemistry and phase behavior

Article abstract of DOI:10.1002/ejoc.200701046

The fluorescence quantum yields φ_F(n) of oligo[2,5-bis(propyloxy)-1,4-phenylenevinylene]s (1b) reach for the trimer (n = 3) in solution a maximum of 86%, which is almost twice as high as for the monomer (n = 1) and the octamer or undecamer (n =

Full text of DOI:10.1002/ejoc.200701046

FULL PAPER
DOI: 10.1002/ejoc.200701046
(E,E,E)-4,4Ј-Distyrylstilbenes – Synthesis, Photophysics, Photochemistry and
Phase Behavior
Herbert Meier,*[a] Thorsten Lifka,[a] Ulf Stalmach,[a] Annette Oehlhof,[a] and Sabine Prehl[a]
Keywords: Condensation reaction / Fluorescence quantum yields / Liquid crystals / Oligo(phenylenevinylene)s / Photo-
reaction
The fluorescence quantum yields φF(n) of oligo[2,5-bis(prop-
yloxy)-1,4-phenylenevinylene]s (1b) reach for the trimer (n =
3) in solution a maximum of 86%, which is almost twice as
high as for the monomer (n = 1) and the octamer or unde-
camer (n = 8, 11). The latter represent the convergence limit
of 46%. 4-Mono-, 3,4-di- or 3,4,5-trisubstituted terminal
phenyl rings in the trimers 6a–f do not lead to calamitic or
phasmidic liquid crystals; however, 2,3,4-trisubstitution with
hexyloxy groups (6h) and even with OCH3 groups (6g) ef-
fects LC formation. Irradiation with energy-rich UV light pro-
vokes a crosslinking of the originally conjugated chains. In
the case of 6c, a chemo- and stereoselective [2π+2π] photo-
cycloaddition could be established as well.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
Introduction
Due to their interesting electrical, optical and optoelec-
tronic properties, oligo- and poly(1,4-phenylenevinylene)s
[OPV and PPV] are a highly attractive class of compounds
in organic chemistry and materials science.[1] Already with
n Ͼ 3, the parent system 1a is insoluble in organic solvents,
so solubilizing alkyl or better alkoxy side chains have to be
introduced. Some time ago we prepared the series 1b (n =
1–8, 11, 15) with 2,5-dipropoxy-substituted phenyl rings
(Scheme 1).[2–4] Increasing numbers n of repeating units in
conjugated oligomers lead for various properties P to limit-
ing values which can be matched by the corresponding
polymer. This approach can be monotonous, as for the λmax
values of absorption and fluorescence of 1b,[2–4] but the ap-
proach P(n) Ǟ Pϱ can also pass for a certain number
nЈ through a maximum and/or a minimum.[1l]
Scheme 1. Oligo(1,4-phenylenevinylene)s.
Results and Discussion
An important example is given for the electrolumin-
escence efficiency of 1b which reaches a maximum for n =
3.[5]
Fluorescence Quantum Yields of the OPV Series 1b
Measurements of the fluorescence quantum yields of 1b
(n = 1–8, 11) in CH2Cl2 gave the results shown in Figure 1.
(E)-2,2Ј,5,5Ј-Tetrakis(propyloxy)stilbene, the monomer
with n = 1, has a φF value of 45%. Increasing length of the
conjugated OPV chain leads to an increase of φF via 82 (n
= 2) to a maximum of 86% for the trimer (n = 3). Further
extension of the chain results in a strong decrease of φF and
finally to an approach to 46%. The flat minimum for n = 6
(φF = 43%) may be an artefact due to the margins of error
of Ϯ5%. However, the signal intensity of the electrolumi-
nescence of the series 1b measured at a current density of
1.0 mAcm–2 has also a flat minimum before it approaches
to a limiting value.[5]
Earlier studies indicated that the photoluminescence
quantum yields φF should also have a maximum for a cer-
tain repeat number n of 1b since φF = 0.82 for the dimer n
= 2 measured in CH2Cl2 is much higher than φF of a long
oligomer chain (φF ≈ 0.5 for n = 11).[6,7] We determined
now the fluorescence quantum yields φF(n) for the whole
series 1b (n = 1–8, 11) and tried to obtain liquid crystals
(LC) for those oligomers that have the highest φF value.
[a] Institute of Organic Chemistry, University of Mainz,
Duesbergweg 10–14, 55099 Mainz, Germany
Fax: +49-6131-39-25396
E-mail: hmeier@mail.uni-mainz.de
1568
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Eur. J. Org. Chem. 2008, 1568–1574

(E,E,E)-4,4Ј-Distyrylstilbenes
Synthesis of Terminally Substituted (E,E,E)-4,4Ј-
Distyrylstilbenes
The preparation of the 4,4Ј-distyrylstilbenes 6a–h was
started with the corresponding aldehydes 2a–h (Scheme 2).
Their condensation reaction with aniline (3) afforded the
azomethines (Schiff bases) 4a–h. Almost quantitative yields
could be obtained, when the process 2+3 was performed
without solvent at low pressure, so that the generated water
was continuously distilled off. The azomethines 4a–h were
then subjected to a Siegrist reaction[12,13] with (E)-4,4Ј-di-
methylstilbene (5). Compared to other condensation reac-
tions, as for example Knoevenagel, Wittig or Wittig–
Horner reactions, the yields of Siegrist reactions are often
lower, but the major advantage of the Siegrist reaction is
given by the extremely high trans selectivity of the aniline
elimination – provided that the reaction conditions guaran-
tee kinetic control.[14] Thus, the target compounds 6a–h
were obtained as pure (E,E,E)-isomers[15] in yields between
51 and 71%, except for the ortho-substituted compounds 6g
and 6h. Their low yields of 23 and 18%, respectively, were
attributed to the steric shielding of the electrophilic carbon
atom of the imino group in 4g,h.
Figure 1. Fluorescence quantum yields of 1b (n = 1–8, 11) mea-
sured in CH2Cl2 at room temperature.
The wavelengths of the absorption maxima λmax were
used as excitation wavelengths λexc and 1,4-distyrylben-
zene[6] served as standard 1. Table 1 contains the corre-
sponding φF(n) values of the series 1b. Comparable values
were obtained by applying quinine sulfate/H2SO4 as stan-
dard 2.
Table 1. Fluorescence quantum yields of the OPVs 1b.
1b
n
Absorption
λmax [nm]
Emission
λmax [nm]
Fluorescence quantum yields
φF (%)
φF (%)
Standard 2
Standard 1
1
2
3
4
5
6
7
8
358
404
435
454
460
470
475
478
481
413
460
496
521
534
543
547
550
552
45
82
86
62
53
43
45
45
46
45
79
81
57
[a]
40
40
[a]
11
41
[a] Not measured.
We interpret the behavior of φF(n) in Figure 1 in terms
of two opposite effects. Increasing length of the relatively
rigid OPV chain leads first up to n = 3 to increasing φF
values. Radiationless deactivation becomes then more and
more efficient – not only due to the strongly increasing
number of vibration modes but predominantly by the in-
creasing tendency of photodissociation to polarons, whose
stability (average lifetime) increases with increasing num-
bers n of repeat units.[8,9] The saturation value obtained here
corresponds fairly well to the value φF = 35Ϯ5%, which
was found for poly[5-(2-ethylhexyloxy)-2-methoxy-1,4-
phenylenevinylene].[10] The φF value of polymers as this
PPV chain certainly depends on the number of defects, the
OPVs studied here are free of defects.[11] Aggregates of
OPVs have much lower φF values. The energy transfer to
fluorescence traps leads to φF values of OPVs of about one
tenth of the values in solution.[6] Therefore it is even more
interesting to apply compounds with high “molecular” φF
values. Consequently we tried to obtain LC systems of 4,4Ј-
distyrylstilbenes, analogues of the trimer 1b (n = 3).
Scheme 2. Preparation of the distyrylstilbenes 6a–h.
Eur. J. Org. Chem. 2008, 1568–1574
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H. Meier, T. Lifka, U. Stalmach, A. Oehlhof, S. Prehl
FULL PAPER
The compounds 6a–h were carefully characterized by π systems in which the outer olefinic double bonds of 6 are
one- and two-dimensional NMR spectroscopy. Figure 2 replaced by CN double bonds (and the central CC double
1
shows as an example the H and 13C chemical shifts of the bond eventually by a N=N bond).[19]
conjugated chain of 6f. The molecule exists at room tem-
perature in solution – as usual for such OPV trimers[1m]
–
in a fast equilibrium between two C2v and two Cs confor-
mations with a de facto C2v symmetry, so that it is sufficient
to depict here the half molecule. The electron releasing do-
decyloxy groups induce small polarizations of the terminal
olefinic double bonds [∆δ(13C) = 1.7 ppm, ∆δ(1H) =
0.08 ppm, 3J(1H,1H) = 16.3 Hz].[16] The effect is much lower
than for related push-pull systems.[1l]
Figure 3. Texture of 6h measured at 173 °C in the polarization
microscope (scale 1:340).
The model established by Praefcke[19] for the (azo)di-
imine mentioned above describes a biaxial nematic phase
Nb. In contrast to the results found for the diimines, exten-
sion of the length of the alkoxy chains leads in our com-
pounds to a broadening of the temperature intervals, in
which the mesophases exist.
Because of the high temperature range of the LC phases
of 6g and 6h, we could not measure directly the fluores-
cence quantum yields φF of these phases, but we determined
φF of a precipitated aggregate of 6g. An emission efficiency
of 26% was found, which is a remarkably high value com-
pared to related aggregates.[6] One cannot expect values of
about 80% as in diluted solutions, where the Förster energy
transfer cannot work.
Figure 2. 1H and 13C NMR spectroscopic data of 6f (solvent
CDCl3, TMS internal standard). Dodecyloxy chains: 1H NMR:
0.88 (t, 18 H, CH3), 1.20–1.38 (m, 96 H, CH2), 1.50 (m, 12 H, γ-
CH2), 1.75 (m, 4 H, β-CH2), 1.83 (m, 8 H, β-CH2), 3.95 (t, 4 H,
α-CH2), 4.02 (t, 8 H, α-CH2). 13C NMR: 14.1 (CH3), 22.7, 26.1,
29.4, 29.4, 29.5, 29.6, 29.7, 29.7, 30.4, 31.9 (CH2, partly superim-
posed), 69.3 (m-OCH2), 73.6 (p-OCH2).
Phase Behavior of the (E,E,E)-4,4Ј-Distyrylstilbenes 6a–h
All compounds 6a–h form yellow crystals. One or two
alkoxy chains, even dodecyloxy chains (6a,b) are not suf-
ficient to obtain meltable compounds; 6a and 6b decom-
pose on heating to temperatures above 330 °C. The phas-
midic systems 6c–f with six alkoxy groups show melting
points between 229 and 91 °C.
Differential scanning calorimetry (DSC) and polariza-
tion microscopy reveal different crystalline but no liquid
crystalline (LC) phases. Heating of 6e, for example, leads in
the DSC diagram (rate: 10 Kmin–1) to a crystal Ǟ crystal
phase transition at an onset temperature of 333 K (∆H =
3.4 kJmol–1) and to the isotropic melt at an onset tempera-
ture of 359 K (∆H = 39.0 kJmol–1). In contrast to 4-styryl-
stilbene,[17,18] the rigid, conjugated π system of 4,4Ј-distyryl-
stilbene is too long for the formation of a mesogenic build-
ing block for calamitic or phasmidic LC structures.
Photophysics and Photochemistry of the (E,E,E)-4,4Ј-
Distyrylstilbenes 6
Absorption and fluorescence maxima of the (E,E,E)-
4,4Ј-distyrylstilbenes 6 depend on the number of alkoxy
chains and to a minor extent on the position of the OR
groups. Table 2 summarizes a selection of λmax values,
which were measured in CH2Cl2. The absorption and emis-
sion maxima of 6 are blue-shifted compared to 1b (n = 3)
and exhibit a higher Stokes shift.
Table 2. Absorption and fluorescence maxima of 6a, 6b, 6f and 6h
(10–5  solutions in CH2Cl2).
Compd.
Alkoxy groups
Absorption[a]
Fluorescence
The situation changes if one of the three alkoxy groups
is fixed in ortho position of the terminal phenyl rings. Even
the hexamethoxy compound 6g generates above 513 K
(240 °C) a nematic phase N. Heating above 330 °C again
provokes decomposition. Six hexyloxy chains, present in 6h,
lower significantly the temperature range of the N phase,
which then exists between 381 and 461 K (108 and 188 °C).
Figure 3 exhibits the typical “schlieren” texture of the ne-
matic phase of 6h, which was measured close to the clearing
λmax [nm]
λmax [nm]
6a
6b
4-OC12H25
3-OC12H25
4-OC12H25
3-OC12H25
4-OC12H25
5-OC12H25
2-OC6H13
3-OC6H13
4-OC6H13
373
469
395
471, 485
6f
399
397
469
6h
469
point. The situation resembles the behaviour of conjugated [a] ε values between 79 and 104ϫ105 Lcm–1 mol–1.
1570 Eur. J. Org. Chem. 2008, 1568–1574
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(E,E,E)-4,4Ј-Distyrylstilbenes
The aggregation tendency of 6a–h in solvents like CHCl3 shift of the ortho protons of the 1-phenyl group provides
or CH2Cl2 is low. We could not find a concentration depen- moreover a proof for the 1r,2c,3t,4t stereochemistry of 7,
dence of the absorption coefficient ε(λ). However, the tail- which corresponds to the major stereochemistry of (E)-stil-
ing of the fluorescence band can be taken as a hint for the bene photodimers.[22]
existence of some aggregates (Figure 4).
According to UV/Vis reaction spectra, the other trimers
exhibit analogous photooligomerization reactions. The
crosslinking processes are particularly fast when 254 nm
light is used. Longer side chains render the isolation of pure
dimers extremely difficult. Therefore we refrained from fur-
ther oligomer separations.
Conclusions
The fluorescence quantum yield increases in the OPV
series 1b (n = 1–8,11) from the monomer (φF = 45%) to the
trimer (φF = 86% for n = 3) and decreases then to a limiting
value of 46% for higher oligomers (nՆ6). Based on this
result we tried to obtain highly fluorescent liquid crystals
of (E,E,E)-4,4Ј-distyrylstilbenes 6. It turned out that usual
calamitic or phasmidic structure concepts realized in 6a–f,
which have alkoxy chains in 4-, 3,4- or 3,4,5-position of the
terminal phenyl rings, are not successful for the rather long,
conjugated mesogen of the trimer. Only the special 2,3,4-
trisubstitution of the terminal phenyl rings with alkoxy
groups (OCH3, OC6H13), present in the compounds 6 g and
6h, led to fluorescent liquid-crystalline compounds.
Figure 4. Lower curves: absorption ε(λ) and fluorescence (normal-
ized intensity) of 6a. Upper curves: absorption ε(λ) and fluores-
cence (normalized intensity) of 6d. Measurements in CH2Cl2, λexc
= 366 nm.
On irradiation with energy-rich UV light, all OPV sys-
tems undergo photoreactions. Apart from the monomers (n
= 1), these oligomers have extremely low quantum yields
for EǞZ isomerization reactions.[14] However, CC bond
formations can be observed with high efficiency.[14,20,21]
Two different reaction types have to be considered, con-
certed [π2s+π2s] cycloadditions leading to four-membered
rings and statistical crosslinking processes.
Experimental Section
A preparative photolysis of 6c (λmax = 395 nm) with
λՆ290 nm in a 1.77ϫ10–3  solution in benzene led – ac-
cording to NMR and mass spectroscopical measurements –
to a mixture of dimers, trimers and higher oligomers. Cyclo-
dimer 7 (Scheme 3) could be isolated in a pure state by a
combination of thin-layer and column chromatography.
General Remarks: UV/Vis: Zeiss MCS 320/340; CH2Cl2 as solvent.
Fluorescence: Perkin–Elmer LS 50B. IR: Beckman Acculab (reac-
tion spectra as film). 1H and 13C NMR: Bruker AC 300, AMX
400, Avance 600, CDCl3 as solvent, if not otherwise stated; TMS
as internal standard. MS: Finnigan MAT 95 (EI and FD). DSC:
Perkin–Elmer DSC 7. Melting points: Büchi apparatus and Stuart
Scientific SMP/3; uncorrected. Elemental analyses: Microanalytical
laboratory of the Institute of Organic Chemistry, University of
Mainz.
1
The H NMR spectrum of 7 shows four AAЈBBЈ spin pat-
terns and four singlets in the aromatic region, additionally
four olefinic AB spin systems (3J = 16.3 Hz) and an ABCD
spin pattern for a cyclobutane ring. Mass spectroscopic
fragmentation of 7 led not only to the starting compound,
the trimer 6c, but also to 1,4-bis(3,4,5-trimethoxystyryl)-
benzene, a dimer, and the corresponding tetramer. These
two “perpendicular” cleavage routes for the cyclobutane
The compound series 1b (n = 1–8, 11, 15) was prepared earlier.[2–4]
General Procedure for the Preparation of the Aldimines 4: The con-
densation reaction of the substituted benzaldehydes 2a–h and ani-
line (3) can be performed in many solvents (ethanol, chloroform,
toluene, etc.), but we found that the solvent-free preparation is
most convenient and normally gives quantitative yields. A mixture
of 25 mmol aldehyde 2a–h and 35 mmol aniline (3) was stirred at
80 °C and ca. 2 kPa. The reduced pressure served for the continous
removel of the generated water. The reaction came to an end after
about 3 h (IR or TLC control). Excess aniline was then removed
in a higher vacuum (0.1 kPa). The yellow residue was pure product
and could directly be used for the subsequent reaction step. Solid
products could be recrystallized from ethanol.
1
ring and the H NMR spectrum of 7 prove that an unsym-
metrical dimer was formed by a [2+2] cycloaddition of a
central and a terminal C=C bond. The strong high-field
The Schiff bases 4e and 4h, which are new compounds, were iso-
lated and additionally the deuterated compound 4cЈ, which corre-
sponds to the known compound 4c.[23]
(E)-N-[3,4,5-Tris(hexyloxy)benzylidene]aniline (4e): Yellowish oil,
yield 12.0 g (100%). 1H NMR (CDCl3): δ = 0.89 (t, 9 H, CH3),
1.34 (m, 12 H, CH2), 1.48 (m, 6 H, CH2), 1.75 (m, 2 H, CH2), 1.82
(m, 4 H, CH2), 4.02 (t, 2 H, OCH2), 4.04 (t, 4 H, OCH2), 7.10 (s,
Scheme 3. Photocyclodimer 7 formed by a [2π+2π] cycloaddition
of a central and a terminal olefinic double bond of 6c.
Eur. J. Org. Chem. 2008, 1568–1574
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H. Meier, T. Lifka, U. Stalmach, A. Oehlhof, S. Prehl
FULL PAPER
2 H, arom. H), 7.19 (m, 2 H, arom. H), 7.21 (m, 1 H, arom. H),
(100) [M + H+]. C78H120O4 (1121.8): calcd. C 83.51, H 10.78; found
7.38 (m, 2 H, arom. H), 8.31 (s, 1 H, CHN) ppm. 13C NMR C 83.17, H 10.41.
(CDCl3): δ = 13.9 (CH3), 22.5, 25.7, 29.3, 31.5 (CH2), 69.2 (OCH2,
(E,E,E)-4,4Ј-Bis{2-[3,4,5-tris(methoxy)phenyl]vinyl}stilbene (6c):
3,5-OC6H13], 14.0 (CH3), 22.6, 30.3, 31.7 (CH2), 73.5 (OCH2, 4-
OC6H13], 107.2, 120.8, 125.6, 129.0 (arom. CH), 131.1, 141.4,
152.2, 153.4 (arom. Cq), 160.1 (CHN) ppm. MS (EI): m/z (%) =
481 (61) [M+], 43 (100). C31H47NO3 (481.7): calcd. C 77.29, H 9.83,
N 2.91; found C 77.08, H 10.05, N 2.85.
Yield 2.00 g, 71 %, yellow crystals, m.p. 229 °C after recrystalli-
1
zation from toluene. H NMR (CDCl3): δ = 3.86 (s, 6 H, OCH3),
3.90 (s, 12 H, OCH3), 6.74 (s, 4 H, arom. H, outer phenyl rings),
6.99/7.05 (AB, 3J = 16.4 Hz, 4 H, outer olefin. H), 7.11 (s, 2 H,
inner olefin. H), 7.50 (“s”, 8 H, inner phenyl rings) ppm. 13C NMR
(CDCl3): δ = 56.2 (m-OCH3), 60.9 (p-OCH3), 103.9 (arom. CH,
outer phenyl rings), 126.8, 126.9 (arom. CH, inner phenyl rings),
127.7, 128.2, 128.8 (olefin. CH), 133.0, 136.6, 136.7, 138.3, 153.5
(arom. Cq) ppm. MS (EI): m/z (%) = 564 (100) [M+], 549 (14).
C36H36O6 (564.7): calcd. C 76.57, H 6.43; found C 76.68, H 6.53.
(E)-N-[2,3,4-Tris(hexyloxy)benzylidene]aniline (4h): Yellowish oil,
yield 12.0 g (100%). 1H NMR (CDCl3): δ = 0.89 (m, 9 H, CH3),
1.10–1.55 (m, 18 H, CH2), 1.78 (m, 6 H, CH2), 3.99 (t, 2 H, OCH2),
4.03 (t, 2 H, OCH2), 4.11 (t, 2 H, OCH2), 6.74/7.85 (AB, 3J =
8.8 Hz, 2 H, arom. H), 7.21 (m, 3 H, arom. H), 7.39 (m, 2 H, arom.
H), 8.78 (s, 1 H, CHN) ppm. 13C NMR (CDCl3): δ = 14.0 (CH3,
superimposed), 22.6, 25.8, 29.2, 29.3, 29.7, 30.2, 30.3, 31.5, 31.6,
31.7 (CH2, partly superimposed), 68.8, 73.7, 74.9 (OCH2), 108.5,
121.0, 122.3, 125.4, 129.0 (arom. CH), 122.9, 141.2, 152.8, 154.3,
156.4 (arom. Cq), 156.4 (CHN) ppm. MS (EI): m/z (%) = 481 (3)
[M]+, 390 (100) C31H47NO3 (481.7): calcd. C 77.29, H 9.83, N 2.91;
found C 77.59, H 9.66, N 2.94.
(E,E,E)-4,4Ј-Bis{2-deuterio-2-[3,4,5-tris(methoxy)phenyl]vinyl}-
stilbene (6cЈ): Yield 1.59 g, 56%, yellow crystals, m.p. 228 °C after
recrystallization from toluene. 1H NMR (CDCl3): δ = 3.86 (s, 6 H,
OCH3), 3.91 (s, 12 H, OCH3), 6.74 (s, 4 H, arom. H, outer phenyl
rings), 6.99 (s, 2 H, outer olefin. H), 7.11 (s, 2 H, inner olefin. H),
7.50 (“s”, 8 H, arom. H, inner phenyl rings) ppm. 13C NMR
(CDCl3): δ = 56.2 (m-OCH3), 60.9 (p-OCH3), 103.9 (arom. CH,
outer phenyl rings), 126.8, 126.9 (arom. CH, inner phenyl rings),
127.7, 128.2 (olefin. CH), 133.0, 136.6, 136.7, 138.3, 153.5 (arom.
Cq) ppm. MS (EI): m/z (%) = 566 (100) [M+], 551 (13). C36H34D2O6
(566.7): calcd. C 76.30, H/D 6.76; found C 76.42, H/D 6.82.
(E)-N-{Deuterio[3,4,5-tris(methoxy)phenyl]methylidene}aniline
(4cЈ): The general procedure was applied to 3,4,5-[D1]trimethoxy-
benzaldehyde[24] (5.0 g, 25.4 mmol) and aniline (3) (3.0 g,
32.3 mmol). The yellow product (6.8 g, 98 %) melted after
recrystallization from ethanol at 109 °C. 1H NMR (CDCl3): δ =
3.91, 3.94 (2s, 9 H, OCH3), 7.12 (s, 2 H, arom. H), 7.18 (m, 2 H,
arom. H), 7.20 (m, 1 H, arom. H), 7.39 (m, 2 H, arom. H) ppm.
13C NMR (CDCl3): δ = 56.2, 60.9 (OCH3), 106.2, 120.8, 125.8,
129.1 (arom. CH), 131.6, 141.2, 152.0, 153.5 (arom. Cq), 159.5 [t,
1J (C,D) = 25.0 Hz, CDN] ppm. MS (EI): m/z (%) = 272 (100)
[M]+, 257 (58), 77 (24). C16H16DNO3 (272.3): calcd. C 70.57, H
6.66, N 5.14; found C 70.19, H 6.31, N 4.85.
(E,E,E)-4,4Ј-Bis{2-[4-dodecyloxy-3,5-bis(methoxy)phenyl]vinyl}-
stilbene (6d): Yield 2.40 g, 55%, yellowish crystals, m.p. 148 °C af-
ter recrystallization from acetone melt. 1H NMR (CDCl3): δ = 0.88
(t, 6 H, CH3), 1.20–1.34 (m, 32 H, CH2), 1.45 (m, 4 H, CH2), 1.75
(m, 4 H, CH2), 3.89 (s, 12 H, OCH3), 3.97 (t, 4 H, OCH2), 6.72 (s,
3
4 H, arom. H, outer phenyl rings), 6.98/7.04 (AB, J = 16.2 Hz, 4
H, outer olefin. H), 7.12 (s, 2 H, inner olefin. H), 7.49 (“s”, arom.
H, inner phenyl rings) ppm. 13C NMR (CDCl3): δ = 14.1 (CH3),
22.7, 25.9, 29.3, 29.5, 29.6, 29.6, 29.6, 29.7, 30.1, 31.9 (CH2), 56.2
(OCH3), 73.7 (OCH2), 103.9 (arom. CH, outer phenyl rings), 126.7,
126.8 (arom. CH, inner phenyl rings), 127.5, 128.1, 128.7 (olefin.
CH), 132.8, 136.6, 136.7, 137.6, 153.7 (arom. Cq) ppm. MS (FD):
m/z (%) = 873 (100) [M+]. C58H80O6 (873.3): calcd. C 79.77, H
9.23; found C 79.65, H 9.29.
General Procedure for the Preparation of the (E,E,E)-4,4Ј-Distyryl-
stilbenes 6a–h: A dry, oxygen-free stream of N2 was purged for
30 min through a solution of (E)-4,4Ј-dimethylstilbene (5) (1.03 g,
5.0 mmol) in 150 mL DMF, then 2.24 g (20.0 mmol) KOC(CH3)3
was added. After 10 min vigorous stirring 10.0 mmol of the crude
(or recrystallized) aldimine 4a–h was added and the temperature
was raised to 90 °C. After 1 h the mixture was cooled to 0–5 °C
and 150 mL H2O was added dropwise. The precipitated product
was filtered off and washed with cold methanol. The products 6c–
h were then subjected to a column filtration (20 ϫ 7 cm SiO2,
CH2Cl2) and recrystallized. The products 6a and 6b have a low
solubility and were directly recrystallized.
(E,E,E)-4,4Ј-Bis{2-[3,4,5-tris(hexyloxy)phenyl]vinyl}stilbene (6e):
Yield 2.51 g (51%), yellowish crystals, m.p. 91 °C after recrystalli-
zation from 2-propanol. 1H NMR (CDCl3): δ = 0.88 (t, 6 H, CH3),
0.89 (t, 12 H, CH3), 1.35 (m, 24 H, CH2), 1.49 (m, 12 H, CH2),
1.75 (m, 4 H, CH2), 1.85 (m, 8 H, CH2), 3.97 (t, 4 H, OCH2), 4.02
(t, 8 H, OCH2), 6.71 (s, 4 H, arom. H, outer phenyl rings), 6.95/
3
7.02 (AB, J = 16.2 Hz, 4 H, outer olefin. H), 7.10 (s, 2 H, inner
(E,E,E)-4,4Ј-Bis[2-(4-dodecyloxyphenyl)vinyl]stilbene (6a): Yield
2.26 g, 60 %, yellow crystals which after recrystallization from
CHBr3 decompose above 330 °C. 1H NMR (C2D2Cl4, T = 373 K):
δ = 0.88 (t, 6 H, CH3), 1.20–1.55 (m, 40 H, CH2), 1.77 (m, 4 H,
CH2), 3.93 (t, 4 H, OCH2), 6.85/7.38 (AAЈBBЈ, 8 H, arom. H, outer
olefin. H), 7.48 (“s”, 8 H, inner arom. H) ppm. 13C NMR (CDCl3):
δ = 14.0, 14.0 (CH3), 22.6, 22.7, 25.8, 25.8, 29.4, 30.3, 31.6, 31.8
(CH2), 69.3, 73.5 (OCH2), 105.4 (arom. CH, outer phenyl rings),
126.7, 126.8 (arom. CH, inner phenyl rings), 127.3, 128.1, 128.9
(olefin. CH), 132.5, 136.5, 136.8, 138.6, 153.5 (arom. Cq) ppm. MS
(FD): m/z (%) = 985 (100) [M+]. C66H96O6 (985.5): calcd. C 80.44,
H 9.82; found C 80.59, H 9.75.
3
phenyl rings), 6.91/7.02 (AB, J = 16.3 Hz, 4 H, outer olefin. H),
7.08 (s, 2 H, inner olefin. H), 7.43 (“s”, 8 H, inner phenyl rings)
ppm.[25] MS (FD): m/z (%) = 754 (100) [M + H+]. C54H72O2
(753.2): calcd. C 86.12, H 9.64; found C 86.09, H 9.42.
(E,E,E)-4,4Ј-Bis{2-[3,4,5-tris(dodecyloxy)phenyl]vinyl}stilbene (6f):
Yield 4.84 g (65%), yellowish crystals, m.p. 92 °C after recrystalli-
zation from 2-propanol. MS (FD): m/z (%) = 1490 (100) [M+].
(E,E,E)-4,4Ј-Bis{2-[3,4-bis(dodecyloxy)phenyl]vinyl}stilbene (6b):
Yield 3.70 g, 66 %, yellow crystals which decompose after
recrystallization from CHCl3 above 330 °C. 1H NMR (CDCl3, T =
333 K): δ = 0.88 (t, 12 H, CH3), 1.20–1.60 (m, 72 H, CH2), 1.80
C102H168O6 (1490.4): calcd. C 82.20, H 11.36; found C 82.29, H
11.42.
(m, 8 H, CH2), 4.02 (, 4 H, OCH2), 4.05 (t, 4 H, OCH2), 6.83–7.15 (E,E,E)-4,4Ј-Bis{2-[2,3,4-tris(methoxy)phenyl]vinyl}stilbene (6g):
(m, 6 H, arom. H, outer phenyl rings), 6.94/7.02 (AB, 3J = 16.4 Hz,
Yield 0.65 g (23%), yellow crystals, m.p. 240 °C after recrystalli-
4 H, outer olefin. H), 7.10 (s, 2 H, inner olefin. H), 7.47 (“s”, 8 H, zation from 2-propanol (a nematic phase is formed which decom-
1
arom. H, inner phenyl rings) ppm.[25] MS (FD): m/z (%) = 1123
poses above 330 °C). H NMR (CDCl3): δ = 3.87 (s, 3 H, OCH3),
1572
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 1568–1574

(E,E,E)-4,4Ј-Distyrylstilbenes
3.89 (s, 3 H, OCH3), 3.91 (s, 3 H, OCH3), 6.69/7.31 (AB, 3J = Measurement of the Fluorescence Quantum Yields: Solutions of
8.6 Hz, 4 H, arom. H, outer phenyl rings), 7.02/7.34 (AB, 3J =
16.4 Hz, 4 H, outer olefin. H), 7.10 (s, 2 H, inner olefin. H), 7.49
(“s”, 8 H, arom. H, inner phenyl rings) ppm. 13C NMR (CDCl3):
δ = 56.2, 60.9, 61.3 (OCH3), 108.0, 120.8, 123.0, 126.7, 126.8, 127.6,
128.1 (arom. and olefin. CH), 124.6, 136.5, 137.4, 142.6, 151.8,
each 1b (n = 1–8,11) in CH2Cl2 were prepared which had the same
absorbance for the λmax values as solutions of the standard (E,E)-
1,4-distyrylbenzene in CH2Cl2. The individual λmax values served
as excitation wavelengths λexc. The intensities of the fluorescence
bands, determined by using the software of the fluorimeter, were
153.4 (arom. Cq) ppm. MS (EI): m/z (%) = 564 (100) [M+], 475 then related to the fluorescence intensity of the standard. The fluo-
(89). C36H36O6 (564.7): calcd. C 76.57, H 6.43; found 76.62, H 6.75.
rescence quantum yield of the standard amounts in CH2Cl2 at
room temperature to 90% (φF = 0.90).[6] In addition, quinine sul-
fate in 1  H2SO4 (φF = 0.55) was used as second standard.[30,31]
The results of both measurements agree within the margins of error
(Ϯ 5%).
(E,E,E)-4,4Ј-Bis{2-[2,3,4-tris(hexyloxy)phenyl]vinyl}stilbene (6h):
Yield 0.89 g (18%), yellow crystals, m.p. 108 °C after recrystalli-
zation from 2-propanol (a nematic phase is formed, clearing point
1
at 188 °C). H NMR (CDCl3): δ = 0.88 (t, 18 H, CH3), 1.34 (m,
24 H, CH2), 1.50 (m, 12 H, CH2), 1.80 (m, 12 H, CH2), 3.96 (t, 4
H, OCH2), 3.97 (t, 4 H, OCH2), 4.03 (t, 4 H, OCH2), 6.66/7.28
Preparation and measurement of aggregates of 6g were performed
according to the literature.[6]
3
3
(AB, J = 8.2 Hz, arom. H, outer phenyl rings), 6.98/7.39 (AB, J
= 16.4 Hz, 4 H, outer olefin. H), 7.10 (s, 2 H, inner olefin. H), 7.48
(“s”, 8 H, arom. H, inner phenyl rings) ppm. 13C NMR (CDCl3):
δ = 14.0, 14.0, 14.1 (CH3), 22.6, 22.7, 22.7, 25.8, 25.8, 26.0, 29.4,
30.3, 30.4, 31.6, 31.8, 31.8 (CH2), 68.9, 73.7, 74.2 (OCH2), 108.8,
120.3, 123.4, 126.8, 127.0, 128.0 (arom. and olefin. CH), 124.6,
136.3, 137.6, 142.1, 151.4, 153.2 (arom. Cq) ppm. MS (FD): m/z
(%) = 985 (100) [M+]. C66H96O6 (985.5): calcd. C 80.44, H 9.82;
found C 80.53, H 9.79.
Acknowledgments
We are grateful to the Deutsche Forschungsgemeinschaft (DFG),
the Fonds der Chemischen Industrie and the Center of Materials
Science of the University of Mainz for financial support. We thank
Prof. D. Oelkrug, University of Tübingen, and his research group
for some of the fluorescence measurements.
Irradiation of 4,4Ј-Distyrylstilbenes 6: Irradiation of 6 in diluted,
oxygen-free solution (5.10–4 ) in C6H6 with energy-rich UV light
(λ = 254 nm) led to photo-crosslinking by CC bond formation.
According to 1H and 13C NMR reaction spectra, EǞZ isomeriza-
tions could be excluded.[26,27] The long-wavelength band of 6 de-
creased steadyly. Statistical CC bond formation and [2π+2π] cyclo-
additions took place and yielded a complex mixture of oligomers.
In the case of 6c we isolated the cyclodimer 7 in a preparative
photolysis: a solution of 50 mg (0.089 mmol) 6c in 195 mL benzene
was irradiated at 25 °C with a 450 W Hanovia medium-pressure
lamp equipped with a Pyrex filter (λՆ290 nm). The 1H NMR spec-
trum of the reaction mixture showed after 3.5 h a turnover of 35%.
The mixture was first separated by preparative TLC [SiO2 (thick-
ness d = 5 mm), toluene/ethyl acetate, 2:1]. The fractions between
RF = 0.22 and 0.43 were then chromatographed on a column
(30ϫ1 cm SiO2, toluene/ethyl acetate gradient). The first fraction
contains open-chain dimers and trimers (mass spectroscopy). Yel-
lowish crystals of the cyclodimer 7 (6.5 mg, 13%) were obtained as
second fraction.
[1] Selected monographs and reviews: a) H.-H. Hörhold, M. Hel-
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Conjugated Polymers, Kluwer, Dordrecht, 1991; c) K. Müllen,
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R. R. Tykwinski), Wiley-VCH, Weinheim, 2006, p. 476–528; n)
A. C. Grimsdale in Organic Light-Emitting Devices (Eds.: K.
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[2] H. Meier, D. Ickenroth, Eur. J. Org. Chem. 2002, 1745–1749.
Cyclodimer 7: rac-[1r-[3,4,5-Tris(methoxy)phenyl]-2c,3t-bis{4-[2-
[3,4,5-tris(methoxy)phenyl)vinyl]phenyl}-4t-[4-(2-{4-[2-(3,4,5-tris-
(methoxy)phenyl]vinyl]phenyl}vinyl)phenyl]cyclobutane (7): M.p. [3] H. Meier, U. Stalmach, H. Kolshorn, Acta Polym. 1997, 48,
270 °C (dec.) 1H NMR (CDCl3):[28] δ = 3.63 (s, 6 H), 3.73 (s, 3 H),
3.82 (s, 3 H), 3.84 (s, 6 H), 3.85 (s, 6 H), 3.90 (s, 12 H) [12 OCH3],
4.35–4.49 (m, 4 H, typical ABCD spin pattern, four-membered ring
protons), 6.26 (s, 2 H), 6.66 (s, 2 H), 6.69 (s, 2 H), 6.72 (s, 2 H)
[arom. H, trimethoxy-substituted phenyl rings], 6.88/6.93 (AB, 3J
379–384.
[4] U. Stalmach, H. Kolshorn, I. Brehm, H. Meier, Liebigs Ann.
1996, 1449–1456.
[5] V. Gebhardt, A. Bacher, M. Thelakkat, U. Stalmach, H. Meier,
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83, 231–237.
[7] U. Stalmach, Dissertation Mainz, 1996.
[8] P. Brendel, A. Grupp, M. Mehring, R. Schenk, K. Müllen, W.
Huber, Synth. Met. 1991, 45, 49–57.
[9] K. S. Wong, D. D. C. Bradley, W. Hayes, J. F. Ryan, R. H.
Friend, H. Lindenberger, S. Roth, J. Phys. C, Solid State Phys.
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[10] J. D. W. Samuel, B. Crystall, G. Rumbles, P. L. Burn, A. B.
Holmes, R. H. Friend, Chem. Phys. Lett. 1993, 213, 472–478.
[11] Defects by saturated carbon atoms in the chain can be totally
excluded; according to 1H and 13C NMR measurements, de-
fects by cis configured double bonds have whenever a portion
of less than 2%.
3
= 16.3 Hz, 2 H), 6.91/6.94 (AB, J = 16.3 Hz, 2 H), 6.97/7.02 (AB,
3J = 16.3 Hz, 2 H), 7.00 (“s”, 2 H) [olefin. H], 7.11/7.32 (AAЈBBЈ,
4 H), 7.12/7.32 (AAЈBBЈ, 4 H), 7.14/7.30 (AAЈBBЈ, 4 H), 7.43/7.46
(AAЈBBЈ, 4 H) [arom. H] ppm. 13C NMR (CDCl3): δ = 47.3, 47.6,
48.1, 48.2 (CH, cyclobutane), 56.1, 56.2, 56.2, 56.2 (m-OCH3), 60.8,
60.9, 60.9, 60.9 (p-OCH3), 103.8, 103.9, 104.0, 105.9 (arom. CH,
trimethoxy-substituted phenyl rings), 126.2, 126.3, 126.7, 127.7,
127.8, 127.9, 128.0, 128.1, 128.3, 128.4, 128.5, 128.6 (arom. and
olefin. CH, partly superimposed), 133.1, 133.1, 133.2, 135.1, 135.2,
135.3, 136.2, 136.6, 136.7, 136.8, 138.2, 138.3 (arom. Cq), 140.1,
140.3, 140.3, 140.3, 152.9, 153.4, 153.5, 153.5 (arom. CqO) ppm.
MS (EI): m/z (%) = 667 (22) [M+ – C28H29O6], 556 (100) [M/2]+,
462 (18), [M+ – C44H42O6].[29]
Eur. J. Org. Chem. 2008, 1568–1574
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1573

H. Meier, T. Lifka, U. Stalmach, A. Oehlhof, S. Prehl
FULL PAPER
[12]
[13]
[23] U. Hess, R. Thiele, J. Prakt. Chem./Chem-Ztg. 1982, 324, 385–
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A. E. Siegrist, Helv. Chim. Acta 1967, 50, 906–957.
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Acta 1969, 52, 2521–2554.
H. Meier, Angew. Chem. 1992, 104, 1425–1446; Angew. Chem.
Int. Ed. Engl. 1992, 31, 1399–1420.
According to 1H and 13C NMR measurements, traces of Z
configurations could not be detected. The margins of error
amounted to 2%.
The polarity of these double bonds is so that the outer C atoms
have a lower electron density. Two-dimensional NMR measure-
ments and a selective deuteration present in 6cЈ (see Exp. Sec-
tion) prove this result.
[24] T. Chancellor, M. Quill, D. E. Bergbreiter, J. Org. Chem. 1977,
43, 1245–1246.
[14]
[15]
[25] A 13C NMR spectrum was not measured because of the very
low solubility.
[26] Apart from (E)-stilbenes, as OPV monomers, the higher OPVs
(n = 2, 3, ..) do not show EǞZ isomerization reactions in the
first excited singlet state. The extended conjugation prevents,
that the bond order of the double bonds is so much lowered
[16]
as in S1 of (E)-stilbenes.[27]
.
[27] K. Sandros, M. Sundahl, O. Wennerström, U. Norinder, J. Am.
Chem. Soc. 1990, 112, 3082–3086.
[17] W. E. Haas, K. F. Nelson, J. A. Adams, G. A. Dir, J. Electro-
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[18] H. Meier, E. Praß, G. Zerban, F. Kosteyn, Z. Naturforsch., Teil
B 1988, 43, 889–896.
[19] K. Praefcke, B. Kohne, B. Gündogan, D. Singer, D. Demus, S.
Diele, G. Pelzl, U. Bakowsky, Mol. Cryst. Liq. Cryst. 1991, 198,
393–405.
[28] The interpretation of the 1H NMR spectra is based on ad-
ditional measurements in C6D6, NOE and INDOR techniques.
[29] According to the NMR spectra, compound 7 is pure. However,
a correct elemental analysis could not be obtained – probably
due to the decomposition, which already starts on gentle heat-
ing.
[30] G. von Bünau, T. Wolf, Photochemie - Grundlagen, Methoden,
Anwendungen, VCH, Weinheim, 1987, p. 248.
[31] W. H. Melhuish, J. Phys. Chem. 1960, 64, 762–764; W. H. Mel-
huish, J. Phys. Chem. 1961, 65, 229–235.
[20] See, for example, A. Schulz, H. Meier, Tetrahedron 2007, 63,
11429–11435.
[21] M. Lehmann, B. Schartel, M. Hennecke, H. Meier, Tetrahedron
1999, 55, 13377–13394.
Received: November 6, 2007
Published Online: February 5, 2008
[22] Ref.[14] and references cited therein.
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Eur. J. Org. Chem. 2008, 1568–1574