Dendrimers with peripheral stilbene chromophores

Article abstract of DOI:10.1016/j.tet.2006.06.012

Two types of dendritic nanoparticles were prepared, which contain (E)-stilbene chromophores in the terminal positions of the dendrons. The compounds showed a highly efficient photoreactivity in the course of which statistical CC bond formations led to a c

Full text of DOI:10.1016/j.tet.2006.06.012

Tetrahedron 62 (2006) 8089–8094
Dendrimers with peripheral stilbene chromophores
*
Shahid A. Soomro, Andrea Schulz and Herbert Meier
Institute of Organic Chemistry, Johannes Gutenberg University of Mainz, Duesbergweg 10-14, D-55099 Mainz, Germany
Received 6 April 2006; revised 31 May 2006; accepted 2 June 2006
Available online 27 June 2006
Abstract—Two types of dendritic nanoparticles were prepared, which contain (E)-stilbene chromophores in the terminal positions of the
dendrons. The compounds showed a highly efficient photoreactivity in the course of which statistical CC bond formations led to a crosslinking
of the particles. Finally, all stilbene chromophores reacted and the typical (E)-stilbene absorption and fluorescence disappeared completely.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
mentioned systems2–15 shows such processes, which can be
avoided, when the absorption of the core lies far in the UV.16
Amines as core are typical examples for the latter case.17 We
tried now to attach (E)-stilbene chromophores in terminal
positions to propylene imine or benzene triester cores.
The stilbene chromophore is a very useful building block for
many applications in photochemistry since E/Z isomeriza-
tion, [p6a] cyclization, [p2s+p2s] cyclodimerization, and
statistical CC bond formations (polymerization, crosslink-
ing) offer various reaction possibilities in synthetic chemis-
try as well as in materials science.1 A fast and efficient CC
bond formation of stilbenoid compounds can be applied
as basic process for imaging and photoresist techniques. A
high density of stilbene chromophores on the periphery of
dendritic particles is certainly a good precondition for this
purpose. Scheme 1 visualizes the fundamental structure of
such multi-arm or dendritic compounds in which (E)-
stilbene chromophores are linked directly or via (saturated
and possibly branched) spacers Sp to the core. Anthracene,2
binaphthyl,3 porphyrin,2 and even various stilbenoid
systems4–15 were used as core for such systems.
2. Results and discussion
The preparation of a dendrimer with (E)-stilbene chromo-
phores fixed on an oligoamine core was based on
3,4,5-tripropoxybenzaldehyde (2),12,18 N-(4-iodophenyl)-
phthalimide (3),19 and the commercially available amine
DAB-Astramol Am-4Ò (4) (Scheme 2). Aldehyde 2 was
transformed by a Wittig reaction to 3,4,5-tripropoxystyrene
(5). A Heck coupling of 5 and iodo compound 3 yielded the
stilbene derivative 6, which was carefully purified by
column chromatography. The pure (E)-isomer was then sub-
jected to a hydrazinolysis 6/7. The obtained aminostilbene
7 was finally transformed with bis(trichloromethyl)carbon-
ate [triphosgene] to the red isocyanate 8, which was directly
added to amine 4. A molar ratio 7:4 of 4.75:1 was used in
order to achieve the complete fourfold coupling of the stil-
bene chromophore via urea functionalities to the core. The
beige solid 9 was obtained in a yield of 93% related to 7.
It showed in the field desorption mass spectrum (FD MS)
the expected peak for the [M+H+] molecular ions at
m/z¼1900 (calculated m/z values for the peak group of
[C112H156N10O16+H+]: 1898–1903, maximum at 1899).
The 1H and 13C NMR spectra of 9 revealed a uniform, sym-
metrical compound. So we are sure that all four NH2 groups
of 4 were transformed to NH–CO–NH tethers for the stil-
bene units. The detection limit for a by-product with less
than four stilbene units is about 5%. Table 1 summarizes
R
R
Sp
Core
R
n
1 ( R = H, alkyl, alkoxy)
Scheme 1. Star-shaped compounds or dendrimers having (E)-stilbene
chromophores linked by (branched) spacers Sp to the core.
When the absorption of the core chromoph0ore (S0/S1)
(E)-stilbene chromophores one has to face an energy transfer
0
overlaps with the emission (deactivation) S1 /S0 of the
€
(Forster mechanism), which competes with the photochem-
istry of the stilbene units. The majority of the before-
1
1
the H and 13C NMR data of the central H and 13C nuclei
of 9, compared to 4. The signals listed in Table 1 are sharp
for 4, but, due to a restricted mobility broad for 9. The sig-
nals of the stilbene units in 9 are sharp and well resolved.
Keywords: Crosslinking; Dendrimers; Photochemistry.
* Corresponding author. Tel.: +49 6131 3922605; fax: +49 6131 3925396;
e-mail: hmeier@mail.uni-mainz.de
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.06.012

8090
S. A. Soomro et al. / Tetrahedron 62 (2006) 8089–8094
H7C3O
H7C3O
H7C3O
O
I
O
N
O
3
CH
[H2N-(CH2)3] 2N (CH2)4-N[(CH2)3-NH2]2
4
2
OC3H7
3
+
(C6H5)3P-CH3 Br-
Pd(O
P( -CH3-C6H4)3
2 ,N(C2H5)3
H7C3O
H7C3O
O
Ac)
OC3H7
N
KOC(CH3)3
2
OC3H7
O
53%
77%
H7C3O
6
5
OC3H7
OC3H7
OC3H7
.
N2H4 H2O
CO(OCCl3)2
OC3H7
OC3H7
H2N
6
N
O
C
O
61%
[N(C2H5)3]
OC3H7
7
8
H7C3O
H7C3O
OC3H7
OC3H7
O
NH
HN
H7C3O
NH
α`
4
NH
N
β
OC3H7
OC3H7
8
α
`
β
93%
H7C3O
N
NH
O
γ
NH
OC3H7
OC3H7
H7C3O
H7C3O
NH
NH
9
O
Scheme 2. Preparation of dendrimer 9.
Scheme 3 shows the preparation of dendrimers of first
generation, which have six (E)-stilbene chromophores.
NBS bromination of the stilbenes 10a,b led to 11a,b,5,20
which were used for the twofold benzylation of resorcinol
12 in the presence of K2CO3 and 18-crown-6.5 The products
13a,b, which are primary alcohols, reacted then with
1,3,5-benzenetricarboxylic acid trichloride (14) in the
presence of 4-dimethylaminopyridine (DMAP) and triethyl-
amine. High yields (90–70%) of the corresponding triesters
15a,b were obtained. The mass spectra revealed the correct
m/z values. A MALDI-TOF measurement (dithranol, Ag+)
of 15a gave m/z¼1837 (calculation for [C120H96O12+Ag+]
leads to a peak group with a maximum at m/z¼1837). The
FD MS spectroscopy of 15b showed a molecular ion at
m/z¼2268 (calculated peak group maximum: m/z¼2269).
The base peak (100%) in the latter spectrum is owing to
the doubly charged molecular ion M2+ (m/z¼1134). The
1H and 13C NMR spectra of 15a,b are in accordance with
D3h symmetry, that means with the attachment of three den-
drons to the benzene core. The NMR data reveal constitu-
tionally and configurationally pure dendrimers 15a,b. The
detection limit of a by-product with less than six stilbene
units or with a (Z)-configured double bond is about 5%.
Most typical for the threefold ester formation is the change
1
of H/13C signals (d values) of the central benzene ring
from 9.02/135.5 for 14 to 8.91/131.3 for 15a and 8.91/
131.2 for 15b.
The long-wavelength absorption of dendrimer 9 shows
a maximum at 342 nm (3¼11.48ꢀ104 L molꢁ1 cmꢁ1). The
fluorescence band of 9 has its maximum at 399 nm and over-
laps with the absorption band in the range of the 0/0 tran-
sition at 370 nm (measurements in CHCl3). Irradiation of
a 1.1ꢀ10ꢁ5 M solution of 9 with the unfiltered light of a
Xenon high-pressure lamp leads to a fast alteration of the ab-
sorption. Figure 1 shows that the band at 342 nm decreases
and changes its shape because of the primary generation of
(Z)-configured stilbene chromophores with a maximum at
around 320 nm. A low-intensity maximum appears at
460 nm. More striking is the formation of a new maximum
far in the visible range. We assign these new bands to quinoid
species with a short half-life.21,22 The band at lmax¼614 nm
reaches the highest intensity after 15 s and disappears then
with a half-life of about 100 s—too fast for an NMR study.
Finally, all stilbene chromophores have participated in
[2p+2p]photocycloaddition reactions and statistical CC
bond formations. The crosslinking of the nanoparticles23
occurs even much faster in more concentrated solutions.
Table 1. 1H and 13C NMR data of 4 and the core of 9 (d values in CDCl3,
related to TMS as internal standard)
Compound
NMR
a
b
g
a0
2.36
b0
1.38
Dendrimers 15a,b behave similar to 9 on irradiation. Figure 2
demonstrates the degradation of the (E)-stilbene chromo-
phores of 15a. The original lmax value of 316 nm decreases
and a new maximum, assigned to the (Z)-configuration,
appears at about 289 nm. Prolonged irradiation causes then
the complete degradation of all stilbene chromophores.
4
1H
2.40
51.1
1.55
28.8
2.67
39.5
13C
53.4
23.9
9
1Ha
2.8
50.7
1.6
24.3
3.2
36.6
2.7
52.2
1.5
21.3
13Ca
a
Broad signals.

S. A. Soomro et al. / Tetrahedron 62 (2006) 8089–8094
8091
R
R
R
R
CH3
CH2Br
NBS
R
R
10
a
b
11a (72%)
R
H OCH3
(66%)
11b
R
R
K2CO3
18-Crown-6
HO
O
R
R
CH2OH
11a,b
+
+
CH2OH
HO
12
O
R
R
13a (60%)
(65%)
13b
COCl
DMAP
13a,b
COCl
ClOC
N(C2H5)3
14
Figure 2. Photodegradation of 15a in a 3ꢀ10ꢁ6 M solution in CH2Cl2
performed by irradiation with monochromatic light (l¼320 nm).
R
R
R
What remains, is the band at about 250 nm, which is typical
for benzene systems. An intermediate quinoid structure
could not be detected. This difference may be due to the
fact that the stilbene chromophores in 9 have on both sides
in 4- and 40-position heteroatoms, whereas they are fixed
in 15a,b on a saturated C atom. Simultaneously with the
change of the absorption, the fluorescence of 15a
(lmax¼359 nm in CH2Cl2) disappears. The same behavior
is observed for 15b. The absorption with lmax¼310 nm
(CH2Cl2) and the fluorescence with lmax¼404 nm
(CH2Cl2, excitation at 310 nm) decrease steadily on mono-
chromatic irradiation at 340 nm.24
R
R
O
R
O
O
O
O
R
R
O
O
O
R
O
R
O
R
R
O
O
The course of the photoreactions can be nicely followed by
1H NMR spectroscopy. Signals, which appear in the range
between 6.4 and 6.8 ppm, indicate the primary generation
of (Z)-stilbene units. Upcoming signals between 4.4 and
4.8 ppm prove then tertiary protons on a saturated carbon
atom. In principle, CC bond formation in 9 and 15a,b can
occur intra- and intermolecularly. The easily soluble product
portions have still the molecular masses of the monomers.
The amount of less soluble oligomers increases with the con-
centration of the solution and the energy of the applied
UV light. The latter effect may be due to the fact that the for-
mation of four-membered rings in head-to-head [p2s+p2s]
cycloadducts is partly reversible (Scheme 4).
R
R
R
15a (90%)
R
(70%)
15b
R
R
Scheme 3. Preparation of dendrimers 15a,b.
R2
R1
i
hν
R
hν
i
i
R
R
R1
R1
i
R1
R
R2
R2
R2
(i = 1,2)
Scheme 4. Photoreactivity of the stilbenoid compounds 9 and 15a,b.
The photooligomers give 1H NMR spectra with very broad
signals. Molecular masses could not be determined by FD,
FAB or ESI mass spectroscopy.25
Figure 1. Reaction spectra of the irradiation of a 1.1ꢀ10ꢁ5 M solution of 9
in CHCl3.

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S. A. Soomro et al. / Tetrahedron 62 (2006) 8089–8094
3. Conclusion
FD MS: m/z (%)¼278 (100) [M+]. Anal. Calcd for
C17H26O3 (278.4): C, 73.35; H, 9.41. Found: C, 72.76; H,
10.01.
The synthetic procedures described in Schemes 2 and 3
afforded dendrimers 9 and 15a,b with (E)-stilbene chromo-
phores in peripheral positions of the dendrons. Since an
intramolecular energy transfer to the core can be excluded,
the excited singlet states S1 of 9 and 15a,b are deactivated
by photophysical processes (fluorescence and radiationless
internal conversion) and by photochemical processes (E/Z
isomerization and inter- and intramolecular CC bond forma-
tions). The CC bond formation can be realized in head-to-
head [p2s+p2s] cycloadditions and statistical crosslinking.
The latter photoreaction provokes a fast and complete degra-
dation of all stilbene chromophores indicated by the total
loss of the stilbene absorption and fluorescence. In the
case of compound 9, in which the stilbene units are bound
by a urea functionality, a short-lived intermediate with an
absorption maximum far in the vis region at 614 nm can
be detected. It is tentatively assigned to a quinoid species.
4.1.4. N-[4-((E)-3,4,5-Tripropoxystyryl)-phenyl]phthal-
imide (6). The Heck reaction was performed with 5
(4.14 g, 14.9 mmol), 3 (4.19 g, 12.0 mmol), Pd(OAc)2
(25 mg, 0.1 mmol), P(o-CH3–C6H4)3 (123 mg, 0.4 mmol),
and N(C2H5)3 (1.7 mL, 12.2 mmol) in 20 mL dry DMF.
The reaction mixture was degassed and kept for 12 h at
80 ꢂC. The volatile parts of the filtered solution were evapo-
rated. Repeated column chromatography (8ꢀ20 cm SiO2,
CH2Cl2) yielded 3.17 g (53%) 6, which melted at 134 ꢂC.
1H NMR (CDCl3): d¼1.01 (t, 3H, CH3), 1.05 (t, 6H, CH3),
1.76–1.84 (m, 6H, CH2), 3.95 (t, 2H, OCH2), 3.99 (t, 4H,
3
OCH2), 6.72 (s, 2H, aromat. H), 6.98 (d, J¼16.2 Hz, 1H,
3
olefin. H), 7.02 (d, J¼16.2 Hz, 1H, olefin. H), 7.42/7.48
(AA0BB0, 4H, phenylene), 7.78/7.94 (AA0BB0, 4H, phthali-
mide); 13C NMR (CDCl3): d¼10.6, 10.6 (CH3), 22.8, 23.5
(CH2), 70.8, 75.1 (OCH2), 105.4, 123.7, 126.5, 126.9,
134.4 (aromat. CH), 126.6, 129.9 (olefin. CH), 130.6,
131.8, 132.2, 137.3, 138.5, 159.5 (aromat. Cq), 167.3
(CO); EI MS: m/z (%)¼499 (100) [M+], 457 (45)
[M+ꢁC3H6]. Anal. Calcd for C23H17NO3 (499.6): C,
74.53; H, 6.66; N, 2.80. Found: C, 74.57; H, 6.63; N, 2.88.
4. Experimental
4.1. General remarks
€
Melting points were determined on a Buchi 510 melting
point apparatus and are uncorrected. The UV/vis spectra
were obtained with a Zeiss MCS 320/340 and the fluores-
cence spectra with a Perkin–Elmer LS 50B spectrometer.
The 1H and 13C NMR spectra were recorded with the Bruker
spectrometers AMX 400 and ARX 400. The mass spectra
were obtained on a Finnigan MAT-95 (FD and EI technique)
or on a Micromass TOF spec E (MALDI-TOF) spectrometer.
The elemental analyses were determined in the Micro-
analytical Laboratory of the Chemistry Department of the
University of Mainz.
4.1.5. 4-((E)-3,4,5-Tripropoxystyryl)aniline (7). Phthali-
mide 6 (3.17 g, 6.3 mmol) was treated in 70 mL ethanol
under Ar at 50 ꢂC with N2H4$H2O (8.0 mL, 8.24 g,
160 mmol). During refluxing for 1 h, a grey solid precipi-
tated. Crystallization from ethanol yielded 1.42 g (61%) of
yellowish product, which melted at 122 ꢂC and turned pink
1
on staying in the air. H NMR (CDCl3): d¼1.00 (t, 3H,
CH3), 1.04 (t, 6H, CH3), 1.75–1.83 (m, 6H, CH2), 3.93 (t,
2H, OCH2), 3.97 (t, 4H, OCH2), 6.66 (s, 2H, aromat. H),
6.82/6.86 (AB, 3J¼16.2 Hz, 2H, olefin. H), 6.73/7.31
(AA0BB0, 4H, aromat. H); 13C NMR (CDCl3): d¼10.6,
10.6 (CH3), 22.8, 23.5 (CH2), 70.7, 75.1 (OCH2), 104.9
(aromat. CH), 115.6, 127.6 (aromat. CH), 125.6, 127.6 (ole-
fin. CH), 128.6, 133.1, 137.8, 145.1, 153.2 (aromat. Cq); FD
MS: m/z (%)¼369 (100) [M+]. Above room temperature, the
decomposition of 7 is very fast so that a correct elemental
analysis could not be obtained.
4.1.1. 3,4,5-Tripropoxybenzaldehyde (2). Preparation ac-
cording to the literature.12,18
4.1.2. N-(4-Iodophenyl)phthalimide (3). Preparation ac-
cording to the literature.19 1H NMR (CDCl3): d¼7.21/7.81
(AA0BB0, 4H, phenyl), 7.78/7.93 (AA0BB0, 4H, phthali-
mide); 13C NMR (CD3SOCD3): d¼94.1 (CqI), 123.6,
129.5, 134.9, 137.8 (aromat. CH), 131.6, 131.8 (aromat.
Cq), 166.8 (CO). Anal. Calcd for C14H8INO2 (349.1): C,
48.46; H, 2.31; N, 4.01. Found: C, 48.26; H, 2.29; N, 3.98.
4.1.6. Dendrimer 9. To 418 mg (1.4 mmol) bis(trichloro-
methyl)carbonate (triphosgene) in 7 mL dry CH2Cl2, 7
(1.42 g, 3.8 mmol) and N(C2H5)3 (0.59 mL, 428 mg,
4.2 mmol) in 24 mL CH2Cl2 were added under Ar within
3 h. The temperature was kept at 0 ꢂC and the mixture vigor-
ously stirred. The volatile parts were removed at 1 Torr
(133 Pa). Two cooled flasks with propanol were put between
vessel and pump in order to remove excess amounts of phos-
gene. The red residue was then dissolved in 5 mL dry
4.1.3. 3,4,5-Tripropoxystyrene (5). Aldehyde 2 (8.24 g,
29.0 mmol) and methyltriphenyl-phosphonium bromide
(15.75 g, 44.0 mmol) were treated in 120 mL THF with
KO(CH3)3 (4.95 g, 44.0 mmol). After 12 h at 25 ꢂC, the mix-
ture was poured on ice and neutralized with HCl. Extraction
with CH2Cl2 led to the raw product, which was purified by
column filtration (13ꢀ10 cm SiO2, CH2Cl2). Yield: 6.29 g
(77%) oil. 1H NMR (CDCl3): d¼0.99 (t, 3H, CH3), 1.03 (t,
6H, CH3), 1.75–1.82 (m, 6H, CH2), 3.91 (t, 2H, OCH2),
CH2Cl2 and treated under Ar with Astramol Am-4Ò
4
(243 mg, 0.8 mmol) dissolved in 10 mL CH2Cl2. After
12 h at ambient temperature, 4 mL CH3OH were added
and the stirring continued for further 12 h. Column chroma-
tography (50ꢀ3 cm SiO2, CH2Cl2) afforded a beige solid
3
3.94 (t, 4H, OCH2), 5.16 (d, J¼10.7 Hz, 1H, olefin. H),
3
3
1
5.60 (d, J¼17.6 Hz, 1H, olefin. H), 6.59 (dd, J¼17.6 Hz,
3J¼10.7 Hz, 1H, olefin. H), 6.60 (s, 2H, aromat. H); 13C
NMR (CDCl3): d¼10.6, 10.6 (CH3), 22.7, 23.5 (CH2),
70.7, 75.1 (OCH2), 105.0 (aromat. CH), 112.7 (olefin.
CH2), 132.8, 138.4, 153.2 (aromat. Cq), 138.4 (olefin. CH);
(1.36 g, 93%), which melted at 137 ꢂC. H NMR (CDCl3):
d¼0.98 (t, 36H, CH3), 1.40–1.85 (m, 36H, CH2), 2.53–
3.30 (m, 20H, NCH2), 3.89 (t, 24H, OCH2), 6.63 (s, 8H,
aromat. H), 6.84 (m, 8H, olefin. H), 7.31/7.42 (AA0BB0,
16H, aromat. H), 8.80 (s, 4H, NH), 9.90 (s, 4H, NH); 13C

S. A. Soomro et al. / Tetrahedron 62 (2006) 8089–8094
8093
NMR (CDCl3): d¼10.5, 10.6 (CH3), 21.3, 22.7, 23.4, 24.3
(CH2), 36.7, 50.7, 52.2 (NCH2), 70.6, 75.0 (OCH2), 104.9,
118.7, 127.0 (aromat. CH), 127.0, 127.1 (olefin. CH),
131.4, 132.7, 137.9, 139.1, 153.2 (aromat. Cq), 156.5
(CO); FD MS: m/z (%)¼1900 (68) [M+], 951 (100) [M2+].27
102.1, 103.7, 107.3, 126.6, 127.6, 127.9, 128.9 (aromat.
and olefin. CH), 131.2, 132.9, 134.9, 135.9, 137.0, 137.9,
138.1, 153.4, 160.1 (aromat. Cq and CH of central benzene
ring), 164.6 (CO); FD MS: m/z (%)¼2265 (11) [M+], 1133
(100) [M2+]. Anal. Calcd for C138H132O30 (2270.6): C,
73.00; H, 5.86. Found: C, 72.84; H, 5.90.
4.1.7. (E)-4-Bromomethylstilbenes (11a,b). The NBS bro-
mination of the corresponding (E)-4-methylstilbenes was
performed according to the literature.5,20
Irradiations: The irradiations for the UV/vis reaction spectra
were performed with a xenon high-pressure lamp (LTI
LPS1000X) with or without interference filters. Preparative
samples for 1H NMR control were irradiated with a Hanovia
450-W middle-pressure lamp. CH2Cl2 or CHCl3 served as
solvent in both cases.
4.1.8. 3.5-Bis{4-[(E)-2-(phenyl)ethenyl]benzyloxy}benz-
yloxybenzyl alcohol (13a). Compound 13a was prepared
as described for 13b.5 Compound 11a (11.34 g,
41.5 mmol), 12 (2.32 g, 16.6 mmol), K2CO3 (5.5 g,
40 mmol), and catalytic amounts of 18-crown-6 yielded
13.06 g (60%) of colorless crystals, which melted (after
Acknowledgements
1
recrystallization from methanol) at 173–176 ꢂC. H NMR
(CDCl3): d¼4.62 (s, 2H, CH2OH), 5.03 (s, 4H, OCH2),
We are grateful to the Deutsche Forschungs Gemeinschaft,
the Fonds der Chemischen Industrie and the Centre of
Materials Science of the University of Mainz for financial
support.
4
4
6.54 (t, J¼2.1 Hz, 1H, aromat. H), 6.62 (d, J¼2.1 Hz,
3
2H, aromat. H), 7.08/7.10 (AB, J¼16.5 Hz, 4H, olefin.
H), 7.26–7.53 (m, 18H, aromat. H); 13C NMR (CDCl3):
d¼65.3 (CH2OH), 69.8 (OCH2), 101.3, 105.8, 126.5,
126.7, 127.7, 127.9, 128.2, 128.7, 129.0 (aromat. and olefin.
CH), 136.1, 137.1, 137.2, 143.4, 160.1 (aromat. Cq); FD MS:
m/z (%)¼524 (100, M+). Anal. Calcd for C37H32O3 (524.7):
C, 84.70; H, 6.15. Found: C, 84.80; H, 6.45.
References and notes
1. Meier, H. Angew. Chem. 1998, 104, 1425; Angew. Chem., Int.
Ed. Engl. 1992, 31, 1399.
4.1.9. all-(E)-Tris(3,5-bis{4-[2-(phenyl)ethenyl]benzyl-
oxy}benzyl)benzene-1,3,5-tri-carboxylate (15a). A solu-
tion of 13a (0.5 g, 0.95 mmol), benzene-1,3,5-tricarboxylic
acid trichloride (14)26 (0.079 g, 0.30 mmol), triethylamine
(0.67 g, 6.60 mmol), and 4-dimethylaminopyridine (0.03 g,
0.24 mmol) in 25 mL dry THF was refluxed under Ar for
6 h. The filtered mixture was concentrated and purified by
column chromatography (50ꢀ4 cm SiO2, petroleum ether
bp 40–70 ꢂC/ethyl acetate 1:1). Yield: 1.94 g (90%) of a col-
orless solid which melted at 168–170 ꢂC. 1H NMR (CDCl3):
d¼4.62 (s, 12H, OCH2), 5.30 (s, 6H, CH2OCO), 6.50 (t,
4J¼1.8 Hz, 3H, aromat. H), 6.67 (d, 4J¼1.8 Hz, 6H, aromat.
H), 7.03–7.07 (AB, 3J¼16.5 Hz, 12H, olefin. H), 7.31–7.47
(m, 54H, aromat. H), 8.91 (s, 3H, aromat. H, central benzene
ring); 13C NMR (CDCl3): d¼67.2 (CH2OCO), 69.9 (OCH2),
102.2, 107.3, 126.6, 126.7, 127.1, 127.7, 128.2, 128.7, 129.0
(aromat. and olefin. CH), 131.3, 135.0, 136.0, 137.2, 137.2,
137.9 (aromat. Cq and CH of central benzene ring), 160.2
(aromat. CqO), 164.7 (CO); MALDI-TOF: m/z (%)¼1837
(100) [M+Ag+]. Anal. Calcd for C120H96O12 (1730.1): C,
85.31; H, 5.59. Found: C, 85.45; H, 5.90.
2. Pillow, J. N. G.; Halim, M.; Lupton, J. M.; Burn, P. L.; Samuel,
I. D. W. Macromolecules 1999, 32, 5985.
´
´
3. Dıez-Barra, E.; Garcıa-Martınez, J. C.; Rodriguez-Lopez, J.;
Gomez, R.; Segura, J. L.; Martin, N. Org. Lett. 2000, 2, 3651.
´
ꢀ
ꢀ
4. Uda, M.; Momotake, A.; Arai, T. Tetrahedron Lett. 2005, 46,
3021.
5. Soomro, S. A.; Benmouna, R.; Berger, R.; Meier, H. Eur. J.
Org. Chem. 2005, 3586.
6. Momotake, A.; Arai, T. Polymer 2004, 45, 5369.
7. Meier, H.; Lehmann, M. Encyclopedia of Nanoscience and
Nanotechnology; Nalwa, H. S., Ed.; American Scientific:
Stevenson Ranch, USA, 2004; Vol. 10, p 95.
8. Momotake, A.; Hayakawa, J.; Nagahata, R.; Arai, T. Bull.
Chem. Soc. Jpn. 2004, 77, 1195.
9. Momotake, A.; Arai, T. J. Photochem. Photobiol., C 2004, 5, 1.
10. Uda, M.; Momotake, A.; Arai, T. Org. Biomol. Chem. 2003, 1,
1635.
ꢀ
11. Segura, J. L.; Gomez, R.; Martin, N.; Guldi, D. M. Org. Lett.
2001, 3, 2645.
12. Meier, H.; Lehmann, M.; Kolb, U. Chem.—Eur. J. 2000, 6,
2462.
13. Lehmann, M.; Schartel, B.; Hennecke, M.; Meier, H.
Tetrahedron 1999, 55, 13377.
14. Meier, H.; Lehmann, M. Angew. Chem. 1998, 119, 666; Angew.
Chem., Int. Ed. 1998, 37, 643.
15. Deb, S. K.; Maddux, T. M.; Yu, L. J. Am. Chem. Soc. 1997, 119,
9097.
16. Due to saturated spacers Sp or cross-conjugation of the
dendrons, the electron transitions of the core and those of the
peripheral (E)-stilbene units can be regarded separately.
17. Lupton, J. M.; Samuel, I. D. W.; Beavington, R.; Burn, P. L.;
4.1.10. all-(E)-Tris(3,5-bis{4-[2-(3,4,5-trimethoxyphenyl)-
ethenyl]benzyloxy}benzyl)benzene-1,3,5-tricarboxylate
(15b). The preparation was performed according to the
procedure described for 15a. Compound 13b (0.75 g,
1.06 mmol), 14 (0.085 g, 0.32 mmol), Et3N (0.87 g,
8.7 mmol), and DMAP (0.03 g, 0.24 mmol) afforded
1
1.68 g (70%) of 15b, a glassy material. H NMR (CDCl3):
d¼3.83 (s, 18H, OCH3), 3.87 (s, 36H, OCH3), 4.99 (s,
4
12H, OCH2), 5.30 (s, 6H, CH2OCO), 6.50 (t, J¼1.8 Hz,
4
€
Bassler, H. Synth. Met. 2001, 116, 357 and; Adv. Mater.
2001, 13, 258.
3H, aromat. H), 6.67 (d, J¼1.8 Hz, 6H, aromat. H), 6.70
(s, 12H, aromat. H), 6.94/7.00 (AB, 3J¼16.5 Hz, 12H,
olefin. H), 7.36/7.46 (AA0BB0, 24H, aromat. H), 8.91 (s,
3H, aromat. H, central benzene ring); 13C NMR (CDCl3):
d¼56.1, 60.9 (OCH3), 67.1 (CH2OCO), 69.9 (OCH2),
18. Meier, H.; Schnorpfeil, C.; Fetten, M.; Hinneschiedt, S. Eur. J.
Org. Chem. 2002, 537.
19. Wanag, G.; Veinbergs, A. Chem. Ber. 1942, 75, 1558.

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S. A. Soomro et al. / Tetrahedron 62 (2006) 8089–8094
20. Misumi, S.; Kuwana, M.; Nakagawa, M. Bull. Chem. Soc. Jpn.
1962, 35, 135.
21. Meier, H.; Kretzschmann, H.; Lang, M. J. Prakt. Chem. 1994,
336, 297.
22. See also the behavior of related stilbenoid compounds in Ref. 13.
23. According to molecular models, the diameter of 9 amounts to
5.3 nm when the dendrons are maximally stretched.
24. Measurements of thin spin-coated films of 15a,b exhibit small
bathochromic shifts of the absorption to lmax¼324 and
328 nm, respectively; the fluorescence maxima however are
strongly red-shifted to 430 and 476 nm. The photodegradation
works in the film as well.
25. AFM measurements of related oligomers revealed the partici-
pation of many monomers in the photocrosslinking.5
26. Orth, U.; Pfeiffer, H.-P.; Breitmaier, E. Chem. Ber. 1986, 119,
3507.
27. Inclusion of solvent molecules even in carefully dried samples
did not permit a correct elemental analysis.