Preparation and photochemistry of dendrimers with isolated stilbene chromophores

Article abstract of DOI:10.1002/ejoc.200500185

In addition to the model compounds 4a and 4b, the dendrimers 11 and 14 with trans-stilbene chromophores in the core and on the periphery of the dendrons were prepared and their photochemistry was studied in solution and in neat films. Due to the flexibili

Full text of DOI:10.1002/ejoc.200500185

FULL PAPER
Preparation and Photochemistry of Dendrimers with Isolated Stilbene
Chromophores
Shahid A. Soomro,[a] Reda Benmouna,[b] Rüdiger Berger,[b] and Herbert Meier*[a]
Dedicated to Professor Rolf-Christian Schulz on the occasion of his 85th birthday
Keywords: Crosslinking / Cyclization / Dendrimers / Isomerization
In addition to the model compounds 4a and 4b, the dendri-
mers 11 and 14 with trans-stilbene chromophores in the core
and on the periphery of the dendrons were prepared and
their photochemistry was studied in solution and in neat
films. Due to the flexibility of the arms, intramolecular and
intermolecular CC bonds are formed on irradiation. Thus, the
generation of nanoparticles, which represent small oligo-
mers, is much more likely than for the cross-linking of rigid,
cross-conjugated stilbenoid dendrimers. The photoreactions
in solution were studied by UV and NMR spectroscopy, the
transformation of the films was studied by AFM.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
fast and efficient statistical CC bond formation (Scheme 1)
Introduction
on irradiation with energy-rich UV light (λ
=
254 nm).[1a,3,18] This leads very soon to insoluble particles
of big size.
Cross-conjugated trans-stilbene units represent the build-
ing blocks for various dendritic systems.[1] Some time ago,
we prepared five generations of stilbenoid dendrimers with
9 to 144 alkoxy chains attached to the peripheral benzene
rings.[2,3] Two generations of related dendrimers with dibu-
tylamino groups have been obtained as well.[4] Various sys-
tems with (E,E)-1,4-distyrylbenzene or (E,E,E)-1,3,5-tri-
styrylbenzene units were studied.[5–9] Moreover, one has to
mention stilbenoid dendrimers with special cores like
N,[10,11] anthracene,[8] binaphthyl[12] or porphyrin.[8] In
some fullerodendrimers, C60 was attached to the focal posi-
tion of two-arm stilbenoid dendrimers.[13–15] Recently den-
drimers with an oligo(1,4-phenylenevinylene) core and 2, 4
or 8 fullerene groups on the periphery were studied.[16]
Irradiation of (E)-stilbene chromophores can lead to four
photoreactions, namely E/Z isomerization, [π6a] cyclization,
[π2 s + π2 s] cyclodimerization and polymerization (statisti-
cal CC bond formation, crosslinking).[17] The latter two
processes are also relevant for 1,4-distyrylbenzene chromo-
phores.[17] The crosslinking can be applied for imaging pro-
cesses and negative photoresist techniques, because the sol-
uble compounds become insoluble in organic solvents, as
soon as a certain degree of crosslinking is reached. Stil-
benoid dendrimers with many (E)-stilbene units exhibit a
Scheme 1. Photocrosslinking of stilbenoid materials.
We are trying now to diminish intermolecular CC bond
formations by competing intramolecular processes so that
nanoparticles of smaller oligomers of stilbenoid dendrimers
are formed by irradiation. The present article deals with the
investigation of dendrimers (11, 14), which have still a high
density of (E)-stilbene chromophores, but contain so flexi-
ble dendrons, that intramolecular CC bond formations can
occur.
Results and Discussion
Synthesis
In order to study the influence of benzyloxymethyl sub-
stituents on the photochemical behaviour of (E)-stilbene,
we synthesized first the compound 4a by NBS bromination
of 4,4Ј-dimethylstilbene (1) and nucleophilic replacement of
the bromine by the reaction with benzyl alcohol (3a)
(Scheme 2). The solid product had a rather low solubility in
[a] Institute of Organic Chemistry, University of Mainz,
Duesbergweg 10–14, 55099 Mainz, Germany
Fax: +49-6131-3925396
E-mail: hmeier@mail.uni-mainz.de
[b] Max Planck Institute for Polymer Research,
Ackermannweg 10, 55128 Mainz, Germany
3586
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejoc.200500185
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Preparation and Photochemistry of Dendrimers with Isolated Stilbene Chromophores
FULL PAPER
Scheme 2. Preparation of the model compounds 4a, b.
organic solvents. Therefore we took 3,4,5-trimethoxybenzyl Photochemistry
alcohol (3b) and prepared the well soluble derivative 4b.
The photochemistry of the model compound 4b and the
dendrimers 11 and 14 were studied in solution as well as in
thin films. The absorption bands of spin-coated films are
broader and somewhat red-shifted in comparison to solu-
tion measurements. A very strong bathochromic shift be-
tween solution (λmax = 363 nm) and film (λmax = 512 nm)
was observed for the fluorescence of 4b. We attribute this
result to the emission of excimers/aggregates. Dendrimer 11
exhibits this effect to a smaller extent (Δλ = 74 nm) and
dendrimer 14 to an even smaller (Δλ = 19 nm). The en-
hanced number of flexible chain segments decreases interac-
tions of the type of J aggregates. Table 1 summarizes the
observed absorption and emission maxima.
Scheme 3 summarizes the preparation of the dendrimer
11 (first generation). 3,4,5-Trimethoxybenzaldehyde (5) was
transformed by a Wittig–Horner reaction with phosphonate
6 to yield the stilbene derivative 7. Wohl–Ziegler bromina-
tion furnished monobromide 8 together with some dibro-
mide. Chemoselective alkylation of the phenolic OH groups
in 9 by the reaction with 8 in the presence of K2CO3 and
18-crown-6 gave the alcohol 10, which then was coupled to
2 by applying phase transfer conditions.[19] The crystalline
dendrimer 11 and its precursors 2, 7, 8 and 10 have trans-
configured double bonds. The limit of detection for the cis
configuration is lower than 5% in NMR spectroscopy.
The preparation of dendrimer 14 of second generation is
summarized in Scheme 4. Williamson ether synthesis of 2
and 9 yielded tetraphenol 12. Alcohol 10 was subjected to
Appel bromination with CBr4 and PPh3. The obtained bro-
mide 13 yielded the target compound 14 by fourfold coup-
ling with 12.
Table 1. Absorption and emission of the model compound 4b and
the dendrimers 11 and 14 in CH2Cl2 and in neat films.
Absorption (λmax in nm)[a] Fluorescence[b] (λmax in nm)
CH2Cl2
film
Δλ
CH2Cl2
film
Δλ
The model compounds 4a,b and the dendrimers 11 and
4b
11
14
312
314
325
319
328
341
7
14
16
363
403
405
512
477
424
149
74
19
1
14 were characterized in detail by H and 13C NMR spec-
troscopy. For example, Figure 1 shows the exact assignment
of the signals of 4b, which was obtained by the 2D tech-
niques HMQC and HMBC.
[a] Excitation and absorption spectra differ only slightly (singlet
energy transfer). [b] Excitation at the λmax values of the absorption.
The model compound 4b is a mono-stilbene which shows
on monochromatic irradiation (λ = 340 nm) in CH2Cl2 a
trans Ǟ cis isomerization. Since the absorbance of the cis
isomers is negligable at this wavelength, the photostationary
state corresponds to an almost pure cis configuration, when
diluted solutions ( Յ 10–4 m) in CH2Cl2 are irradiated.
Apart from the shift of λmax = 312 nm of the (E)-configura-
tion to about 274 nm for the (Z)-configuration, typical new
singlet signals for the olefinic protons appear at δ = 6.60 in
1
Figure 1. Assignment of the 1H and 13C chemical shifts of 4b on
the basis of HMQC and HMBC measurements.
the H NMR spectrum. Irradiations in more concentrated
solutions ( Ն 10–3 m) exhibit the disappearance of the ab-
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S. A. Soomro, R. Benmouna, R. Berger, H. Meier
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1
sorption of the stilbene chromophore and new H NMR larly when energy-rich UV light (λ = 254 nm) is used.[17,18]
signals appear at δ = 4.56 for saturated protons on a tertiary The solubility of the material decreases constantly during
carbon atom (CH). The [π2 s + π2 s]cyclodimerization is ac- such an illumination. A mass spectroscopical determination
companied by statistical cross-linking processes − particu- of the oligomers proved to be very difficult. The MALDI-
Scheme 3. Preparation of the stilbenoid dendrimer 11.
3588
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Eur. J. Org. Chem. 2005, 3586–3593

Preparation and Photochemistry of Dendrimers with Isolated Stilbene Chromophores
FULL PAPER
Scheme 4. Preparation of the stilbenoid dendrimer 14.
TOF technique is not very suitable for the identification of configured stilbene chromophores are predominantly trans-
1
the corresponding oligomers. Only the mass of the dimer formed to their (Z)-configuration. According to H NMR
could be detected when Ag+ ions were added (m/z = 1307/ measurements a photostationary state is reached which
1309 for [C72H80AgO16]+ ions).
contains less than 10% (E)-isomer.
Monochromatic irradiation (λ = 340 nm) of the dendri-
Irradiation (λ Ն 260 nm) of 11 or 14 in more concen-
mers 11 and 14 in diluted solution (10–5 m to 10–8 m) in trated solutions ( Ն 10–3 m) yielded monomers and oligo-
CH2Cl2 shows a characteristic decrease of the band which mers with broad 1H and 13C NMR signals. The odd
is typical for the inherent (E)-stilbene chromophores number of olefinic double bonds in 11 and 14 guarantees
(Table 1). Figure 2 illustrates the reaction spectra of the ir- that a full degradation of the stilbene chromophores cannot
radiation of a 3·10–8 m solution of 14 in CH2Cl2. The (E)- be achieved by exclusively intramolecular processes. Thus,
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S. A. Soomro, R. Benmouna, R. Berger, H. Meier
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Figure 2. Reaction spectra (0, 5, 10, 30 and 60 s) of the monochromatic irradiation (λ = 340 nm) of a 3 · 10–8 m solution of 14 in CH2Cl2.
the generation of oligomers is taken for granted. Typical 1H
NMR signals for methine protons appeared at 4.1 Յ δ Յ
4.6 as broad superimposed multiplets. According to earlier
results on stilbene photocrosslinking,[17,18] these signals are
owing to concerted [π2 s + π2 s] cycloadditions and to statis-
tical CC bond formations.
Intramolecular [2π + 2π] photocycloadditions can occur
between all olefinic double bonds of 11 and 14; however,
the highest probability has the formation of a four-mem-
bered ring within one dendron. Figure 3 depicts the result
of a forcefield calculation MMX of 1,3-bis(4-vinylben-
zyloxy)benzene which can serve as a model. The head-to-
head syn process (upper part) and the head-to-tail syn ad-
dition (lower part) have the lowest steric energies; the latter
one should proceed via a less favourable excimer forma-
tion.[17] Nevertheless, we assume that also all other possible
generations of four-membered rings as well as statistical
crosslinkings are involved.
The irradiation (λ Յ 340 nm) of a spin coated film of 11
was studied by atomic force microscopy (AFM). Figure 4
(part a) shows the film surface before irradiation. The sur-
face is rather smooth. A cross sectional image contains
peaks with a height below 1 nm. A short irradiation of 3 s,
however, causes unexpected roughness (Figure 4, part b).
The cross-section reveals peaks of up to 35 nm height (Fig-
ure 4, part d). This corresponds to an oligomer of at least
9 monomer units if we assume a maximum extension by
head-to-tail cycloadditions of peripheral double bonds. The
Figure 3. MMX calculation of the intramolecular [2π + 2π] cy-
cloadducts of 1,3-bis(4-vinylbenzyloxy)benzene. Upper part: syn
head-to-head adduct (ΔHf = 8.5 kcal · mol–1, strain energy SE =
47.0 kcal · mol–1); lower part: syn head-to-tail adduct (ΔHf = 9.9
kcal · mol–1, SE = 47.6 kcal · mol–1). Calculation with Serena Soft-
real number of involved monomers is certainly much higher,
since we have to postulate here a statistical CC bond forma-
tion as well. The solubility of the material decreases more
and more during the irradiation. After 1 h the film has
again a smooth surface (Figure 4c). The cross-sectional im- ware 1998 on the basis of standard bondlengths.
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Preparation and Photochemistry of Dendrimers with Isolated Stilbene Chromophores
FULL PAPER
(isobutyronitrile) (AIBN). After refluxing for 6 h (E)-4,4Ј-bis(bro-
age is similar to that before the irradiation. Some “eleva-
tions” remain, but their height is below 10 nm. This status
represents the crosslinked material.
momethyl)stilbene (2) was obtained in a yield of 55% (88%[20]) as
1
colorless crystals which melted at 176–178 °C. H NMR (CDCl3):
δ = 7.48/7.35 (AAЈBBЈ, 8 H, aromat. H), 7.07 (s, 2 H, olefin. H),
4.49 (s, 4 H, CH2). 13C NMR (CDCl3): δ = 137.3, 137.2 (aromat.
Cq), 129.5, 126.9 (aromat. CH), 128.7 (olefin. CH), 33.4 (CH2Br).
Benzyl alcohol (3a) [0.40 g, 3.7 mmol], 2 [0.60 g, 1.8 mmol], KOH
[0.23 g, 4.25 mmol] and catalytic amounts of tetrabutylammonium
fluoride (TBAF) were refluxed in 25 mL chlorobenzene for 3 h.
The mixture was filtered and the solvent evaporated under reduced
pressure. The residue was dissolved in 50 mL CH2Cl2; 25 mL H2O
and diluted HCl were added until the aqueous phase was slightly
acidic. After vigorous shaking, the organic layer was separated,
washed with 50 mL H2O and 50 mL brine, dried with MgSO4 and
filtered through SiO2 (3 × 30 cm). The product 4a (0.136 g, 40%,
light yellow wax) could be eluted with cyclohexane/ethyl acetate
80:20. 1H NMR (CDCl3): δ = 7.51 (AAЈ of AAЈBBЈ, 4 H, aromat.
H), 7.35 (m, 14 H, aromat. H), 7.10 (s, 2 H, olefin. H), 4.55 (“s”,
8 H, CH2). 13C NMR (CDCl3): δ = 138.2, 137.7, 136.8, (aromat.
Cq), 128.4, 128.3, 128.2, 127.8, 127.7, 126.5 (aromat. and olefin.
CH), 72.1, 71.8 (CH2). FD MS: m/z (%) = 392 (100, M+·).
(E)-4,4Ј-Bis(3,4,5-trimethoxybenzyloxymethyl)stilbene (4b): The
preparation of 4b was accomplished according to the procedure
described for 4a – solely the reaction temperature was kept at 60 °C
for 24 h. A yellowish glass was obtained in a yield of 45%. 1H
NMR (CDCl3): δ = 7.50 (AAЈ of AAЈBBЈ, 4 H, aromat. H), 7.35
(BBЈ, 4 H, aromat. H), 7.10 (s, 2 H, olefin. H), 6.58 (s, 4 H, aromat.
H, outer benzene rings), 4.54 (s, 4 H, inner CH2), 4.47 (s, 4 H,
outer CH2), 3.84 (s, 12 H, OCH3), 3.82 (s, 6 H, OCH3). 13C NMR
(CDCl3): δ = 153.2, 137.6, 137.6, 136.7, 133.9 (aromat. Cq), 128.3,
128.2, 126.5, 104.6 (aromat. and olefin. CH), 72.2, 72.0 (OCH2),
60.8, 56.8 (OCH3). FD MS: m/z (%) = 601 (100, [M + H+].
Figure 4. AFM topography picture of the irradiation of a neat film
of 11. (a): Smooth surface before irradiation (root mean square
roughness RMS = 0.29 nm); (b): Rough surface (RMS = 1.44 nm)
after 3 s irradiation (λ = 340 nm); (c): Surface after 1 h irradiation,
which is smoothened by photo-crosslinking (RMS = 0.43 nm). The
z-scale of each image is shown as well; (d): Line profile along the
surface of the elevation (35 nm height) indicated by the dashed
white circle in (b).
(E)-3,4,5-Trimethoxy-4Ј-methylstilbene (7): The preparation was
performed by the reaction of 5 and 6 according to the literature;[21]
yield 90% (81%[21]), m.p. 125–127 °C. 1H NMR (CDCl3): δ = 7.42,
7.31 (AAЈBBЈ, 4 H, aromat. H), 7.16, 6.97 (AB, 3J = 16.5 Hz, 2 H,
olefin. H), 3.90 (s, 6 H, OCH3), 3.85 (s, 3 H, OCH3), 2.34 (s, 3 H,
CH3).
Conclusions
The newly synthesized dendrimers 11 and 14 contain (E)-
stilbene chromophores in the core and in the dendrons. The
irradiation of 11 and 14 (as well as the irradiation of the
model compound 4b) leads to two typical photoreactions,
namely E/Z isomerizations and intra- and intermolecular
CC bond formations. The latter process, which can be used
as photocrosslinking, predominates in concentrated solu-
tions ( Ն 10–3 m) when energy-rich UV light is used.
The irradiation of spin coated films of 11 was studied
by AFM measurements. The UV light causes first a strong
increase of the roughness of the surface (peaks of up to
35 nm height), before the progressive crosslinking leads
again to a relatively smooth surface.
(E,E)-3,5-Bis{4[2-(3,4,5-trimethoxyphenyl)ethenyl]benzyloxy}benzyl
Alcohol (10): Wohl–Ziegler bromination of the methyl group of 7
was achieved according to a method described in the literature.[21]
Yield of 8 45% (66%[21]), m.p. 107 °C. 1H NMR (CDCl3): δ = 7.46,
7.36 (AAЈBBЈ, 4 H, aromat. H), 7.03, 6.97 (AB, 3J = 16.5 Hz, 2 H,
olefin. H), 6.72 (s, 2 H, aromat. H), 4.49 (s, 2 H, CH2Br), 3.90 (s,
6 H, OCH3), 3.85 (s, 3 H, OCH3).
The recrystallized compound 8 (m.p. 107 °C from toluene) was di-
rectly used for the preparation of 10. A mixture of 13.96 g
(38.4 mmol) 8, 2.45 g (17.4 mmol) of 3,5-dihydroxybenzyl
alcohol[22] 9, 5.5 g (40.0 mmol) K2CO3 and catalytic amounts of
18-crown-6 were refluxed in acetone under nitrogen for 24 h. The
filtered mixture was evaporated under reduced pressure, the residue
dissolved in CH2Cl2, washed with water (3 × 50 mL) and brine
(50 mL), dried with MgSO4 and purified by column chromatog-
raphy on SiO2 (4 × 40 cm). A mixture of cyclohexane and ethyl
acetate with a gradient from 95:5 to 10:90 served for the elution.
A yellow glass was obtained (17.6 g, 65%), which was analytically
Experimental Section
General: Melting points were determined with an SMP/3 apparatus
from Stuart Scientifique and are uncorrected. The 1H and 13C
NMR spectra were recorded with Bruker AC-300 and AMX-400
spectrometers. CDCl3 was used as solvent and TMS as internal
standard. Mass spectra were obtained with a Finnigan MAT-95
instrument with the field desorption techniques (FD). UV/Vis spec-
tra were measured with a Zeiss MCS-320/340 spectrometer.
1
pure 10. H NMR (CDCl3): δ = 7.49, 7.38 (AAЈBBЈ, 4 H, aromat.
3
H), 6.98, (AB, J = 16.5 Hz, 4 H, olefin. H), 6.70 (s, 4 H, aromat.
4
4
H), 6.61 (d, J = 2.1 Hz, 2 H, aromat. H), 6.50 (t, J = 2.1 Hz, 1
H, aromat. H), 4.99 (s, 4 H, OCH2), 4.60 (s, 2 H, CH2O), 3.87 (s,
12 H, OCH3), 3.83 (s, 6 H, OCH3). 13C NMR (CDCl3): δ = 160.0,
153.4, 143.9, 138.0, 137.0, 136.2, 133.0 (aromat. Cq), 128.9, 127.9,
(E)-4,4Ј-Bis(benzyloxymethyl)stilbene (4a): (E)-4,4Ј-Dimethylstil-
bene (1), obtained by McMurry reaction from 4-methylbenzalde-
hyde,[20] was brominated by NBS in CCl4 in the presence of azobis-
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S. A. Soomro, R. Benmouna, R. Berger, H. Meier
127.7, 126.6, 105.6, 103.7, 101.2 (aromat. and olefin. CH), 69.8, (CH2Br). FD MS: m/z (%) = 769/767 (100, [M + H+], Br isotope
FULL PAPER
64.9 (OCH2), 60.9, 56.1 (OCH3). FD MS: m/z (%) = 704 (100,
M+·). C43H44O9 (704.8): calcd. C 73.28, H 6.29; found C 73.54, H
6.34.
pattern). C43H43BrO8 (767.7): calcd. C 67.27, H 5.65; found C
67.60, H 5.70.
Dendrimer 14 (Second Generation): A mixture of tetrahydroxy com-
pound 12 (38 mg, 0.078 mmol), benzyl bromide 13 (240 mg,
0.312 mmol), K2CO3 (1100 mg, 8.0 mmol) and catalytic amounts
of 18-crown-6 were refluxed in acetone under nitrogen for 24 h.
The filtered mixture was evaporated under reduced pressure and
the residue dissolved in 1000 mL CH2Cl2. The solution was washed
with water (3 × 50 mL) and brine (50 mL), dried with MgSO4 and
concentrated. The product was purified by column chromatography
(SiO2, 4 × 40 cm, gradient cyclohexane/ethyl acetate, 90:10 to
10:90). The product was obtained as a waxy gum (200 mg, 20%).
1H NMR (CDCl3): δ = 7.52–7.30 (m, 40 H, aromat. H), 7.06 (m,
18 H, olefin. H), 6.72 (m, 16 H, aromat. H), 6.66–6.41 (m, 18 H,
aromat. H), 4.98 (s, 16 H, OCH2), 4.93 (s, 8 H, OCH2), 4.58 (s, 4
H, inner OCH2), 4.47 (s, 4 H, OCH2), 3.89 (s, 48 H, OCH3), 3.85
(s, 24 H, OCH3). 13C NMR (CDCl3): δ = 160.1, 160.0, 153.4, 143.6,
139.4, 139.2, 138.0, 137.0, 136.0, 132.9 (aromat. Cq, partly superim-
posed), 128.9, 128.0, 127.6, 126.6, 126.0 (aromat. and olefin. CH,
partly superimposed), 106.3, 105.7, 103.6, 101.6, 101.3 (aromat.
CH), 77.3, 69.9, 65.2, 60.9 (OCH2), 56.1, 54.9 (OCH3). MS
(MALDI-TOF, dithranol matrix, Ag+): m/z (%) = 3339 (100 [M +
Ag+]).[23]
Preparation of the Dendrimer 11 (First Generation) [all-(E)-4,4Ј-
Bis{3,5-bis[4-(3,4,5-trimethoxystyryl)benzyloxy]benzyloxymethyl}-
stilbene (11): A mixture of 10 (300 mg, 0.426 mmol), 2 (60 mg,
0.164 mmol), KOH (230 mg, 4.10 mmol) and catalytic amounts of
TBAF in chlorobenzene was heated to 60 °C for 24 h. The filtered
solution was evaporated under reduced pressure and the residue
dissolved in 100 mL CH2Cl2. After washing with 50 mL diluted
HCl, the organic phase was washed with 50 mL water and 50 mL
brine, dried with MgSO4 and concentrated. The product was puri-
fied by colulmn chromatography (4 × 40 cm SiO2, cyclohexane/
ethyl acetate, 80:20). Dendrimer 11 (203 mg, 35%) was obtained as
1
a light yellow wax. H NMR (CDCl3): δ = 3.85 (s, 12 H, OCH3),
3.89 (s, 24 H, OCH3), 4.49 (s, 4 H, OCH2), 4.51 (s, 4 H, OCH2),
5.03 (s, 8 H, OCH2), 6.55 (t, 4J = 1.8 Hz, 2 H, aromat. H), 6.67 (d,
4J = 1.8 Hz, 4 H, aromat. H), 6.72 (s, 8 H, aromat. H), 7.01 (AB,
3J = 16.1 Hz, 8 H, olefin. H), 7.07 (s, 2 H, olefin. H), 7.29–7.51
(m, 24 H, aromat. H). 13C NMR (CDCl3): δ = 56.1, 60.9 (OCH3),
69.8, 71.8, 72.0 (OCH2), 101.5, 103.6, 106.8, 126.5, 126.6, 127.7,
127.9, 128.2, 128.3, 128.9 (aromat. and olefin. CH), 132.9, 136.2,
136.7, 137.0, 137.6, 138.1, 140.8, 153.4, 160.0 (aromat. Cq). FD
MS: m/z (%) = 1614 (100, M+·). C102H100O18 (1613.9): calcd. C
75.91, H 6.25, O 17.84; found C 76.10, H 6.35.
Irradiation Experiments: The UV/Vis measurements were per-
formed with an AMCO LTI Xenon 1000 lamp and an interference
filter (λ = 340 nm) in a Zeiss MCS-320/340 spectrometer. The irra-
diations for the NMR measurements were done with a 450-W
Hanovia mercury medium-pressure lamp equipped with a Duran
glass filter. A slow stream of Ar purged during the illumination
through the solutions of 11 and 14 (about 10–2 m) in absolute
CH2Cl2.
Preparation of the Second Generation Dendrimer 14
(E)-4,4Ј-Bis(3,5-dihydroxybenzyloxymethyl)stilbene (12): Dibromide
2 (0.26 g, 0.72 mmol), KOH (0.23 g, 4.25 mmol) and catalytic
amounts of TBAF were added to a solution of 9 (0.45 g,
1.45 mmol) in 25 mL chlorobenzene. After 24 h at 60 °C, the mix-
ture was filtered and the solvent evaporated under reduced pres-
sure. The residue was shaked for 6 h in a mixture of 50 mL CH2Cl2
and 25 mL diluted HCL. The two layers were separated and the
aqueous phase extracted with ethyl acetate (3 × 50 mL). The com-
bined organic phases were washed with brine (50 mL), dried with
MgSO4 and concentrated. The product was purified by column
chromatography (3.5 × 40 cm SiO2, cyclohexane/ethyl acetate,
50:50). A yellowish oil was obtained (0.17 g, 25%). According to
AFM Measurements: All samples have been prepared by using con-
centrations of 4 mg/L in toluene. In total 3–4 drops have been put
on a rectangular quartz plate substrate with edge sizes of 1 cm and
4 cm. The substrate was heated to 200 °C and spin coating was
performed at 2000 rpm. All images were recorded under ambient
conditions by using a Dimension 3100 Atomic Force Microscope
(AFM), which was connected to a NanoScope V controller. The
scanner was equipped with a closed loop xy-feedback system. The
quartz substrates were attached to the wafer chuck stage by using
the vacuum fixture. For recording the topography of samples the
AFM was operated in the tapping mode. For the measurements we
used Olympus tapping mode cantilevers OMCL-AC160TS-
W2 having an Al reflex coating (nominal resonance frequency
300 kHz and spring constant 42 N/m). The tip radius of curvature
is typically less than 10 nm.
1
the H NMR spectrum, it is pure enough for the subsequent reac-
1
tion step. H NMR (CD3OD): δ = 7.55, 7.36 (AAЈBBЈ, 8 H, aro-
matic H), 7.15 (s, 2 H, olefinic H), 6.37 (“s”, 2 H, aromat. H), 6.27
(“s”, 4 H, aromat. H), 4.52 (s, 4 H, OCH2), 4.43 (s, 4 H, OCH2).
(E,E)-3,5-Bis{4[2-(3,4,5-trimethoxyphenyl)ethenyl]benzyloxy}benzyl
Bromide (13): Alcohol 10 (1.8 g, 2.55 mmol), CBr4 (1.27 g,
3.80 mmol) and triphenylphosphane (1.0, 3.83 mmol) were stirred
at room temperature in 50 mL THF for about 30 min. A colorless
precipitate was formed. The process was quenched with 1 mL H2O
and the volatile parts removed in vacuo. The residue was dissolved
in CH2Cl2, concentrated and poured into 250 mL diethyl ether. The
precipitate, which consisted of triphenylphosphane oxide was re-
moved and the organic phase concentrated. The product was puri-
fied by column chromatography (4 × 40 cm SiO2, elution gradient
of cyclohexane/ethyl acetate from 80:20 to 20:80). A yellowish solid
(1.66 g, 85%) was obtained, which started to decompose above
Acknowledgments
We are grateful to the Deutsche Forschungsgemeinschaft, the
Fonds der Chemischen Industrie and the Center of Materials Sci-
ence of the University of Mainz for financial support.
1
90 °C. H NMR (CDCl3): δ = 7.48, 7.29 (AAЈBBЈ, 8 H, aromatic
[1] a) H. Meier, M. Lehmann, in: Encyclopedia of Nanoscience and
Nanotechnology (Ed.: H. S. Nalwa), vol. 10, 95–106, American
Scientific Publishers, Stevenson Ranch, USA, 2004 and refer-
ences cited therein; b) M. Uda, A. Momotake, T. Arai, Tetrahe-
dron Lett. 2005, 46, 3021–3024; c) A. Momotake, T. Arai, Poly-
mer 2004, 45, 5369–5390; d) A. Momotake, J. Hayakawa, R.
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H), 7.01, 6.99 (AB, 3J = 16.5 Hz, 4 H, olefin. H), 6.67 (s, 4 H,
aromat. H), 6.62 (d, 4J = 2.1 Hz, 2 H, aromat. H), 6.56 (t, 4J =
2.1 Hz, 1 H, aromat. H), 4.96 (s, 4 H, OCH2), 4.47 (s, 2 H, CH2Br).
13C NMR (CDCl3): δ = 160.1, 153.4, 139.6, 138.1, 137.1, 135.9,
133.0 (aromat. Cq), 129.0, 128.0, 127.6, 126.9, 107.7, 103.7, 102.1
(aromat. and olefin. CH), 69.9 (OCH2), 60.9, 56.1 (OCH3), 46.4
3592
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2005, 3586–3593

Preparation and Photochemistry of Dendrimers with Isolated Stilbene Chromophores
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Received: March 10, 2005
Published Online: July 7, 2005
Eur. J. Org. Chem. 2005, 3586–3593
www.eurjoc.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3593