Poly(propylene imine) dendrimers functionalized with stilbene or 1,4-distyrylbenzene chromophores

Article abstract of DOI:10.1016/j.tet.2007.08.064

Two generations of dendritic nanoparticles were prepared, which contain (E)-stilbene or (E,E)-1,4-distyrylbenzene chromophores in the 4 or 8 terminal positions of the propylene imine dendrons. The compounds show a highly efficient photoreactivity. On prol

Full text of DOI:10.1016/j.tet.2007.08.064

Tetrahedron 63 (2007) 11429–11435
Poly(propylene imine) dendrimers functionalized with
stilbene or 1,4-distyrylbenzene chromophores
*
Andrea Schulz and Herbert Meier
Institute of Organic Chemistry, University of Mainz, Duesbergweg 10-14, 55099 Mainz, Germany
Received 13 June 2007; revised 8 August 2007; accepted 21 August 2007
Available online 25 August 2007
Abstract—Two generations of dendritic nanoparticles were prepared, which contain (E)-stilbene or (E,E)-1,4-distyrylbenzene chromophores
in the 4 or 8 terminal positions of the propylene imine dendrons. The compounds show a highly efficient photoreactivity. On prolonged
irradiation all stilbenoid chromophores were destroyed by oligomerization (crosslinking) and the typical absorption and fluorescence of
the chromophores disappeared completely.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
2. Results and discussion
Dendrimers are particularly suitable for the attachment of
chromophores in the peripheral positions of the dendrons.1
Thus, a high absorption cross-section can be obtained. We
selected poly(propylene imine) [PPI] systems because the
terminal NH2 groups permit various functionalizations,
which lead to higher amines, amides, sulfonamides, carba-
mates, ureas or azomethines. In fact all these functional links
have been used for the attachment of aromatics,2 heteroaro-
matics,3 crown ethers and complexes,2b,4 polycycles,5 etc.
2.1. Synthesis
For the preparation of the target dendrimers we used the con-
densation reaction of aldehydes with the amino groups of PPIs
of first and second generation. Scheme 1 shows the formation
of stilbene aldehyde 3a by applying the Wittig–Horner re-
action of phosphonate 1 and unilaterally protected terephthal-
aldehyde 2a. The deprotection occurred directly in the acidic
work-up. The analogous Wittig–Horner reaction of 1 and
methyl 4-formylbenzoate 2b yielded stilbene derivative 3b,
which was reduced to the 4-styrylbenzyl alcohol 4. Bromina-
tion and Arbuzov reaction with P(OEt)3 yielded 5 and the
phosphonate 6. The final Wittig–Horner reaction of 6 and 2a
afforded the 4-(stilbenylvinyl)benzaldehyde 7.11k,13–15 The
three propoxy groups in 3a and 7 serve for the enhancement
of the solubility. Wittig–Horner reactions in the stilbenoid
series exhibit E/Z selectivities of about 95:5. Purification of
the raw products by crystallization furnishes pure (E)-config-
Stilbenes belong to the best studied class of compounds from
a photophysical and photochemical viewpoint.6 They ex-
hibit E/Z isomerizations, cyclizations, dimerizations, and
polymerization reactions on irradiation and can be used for
manifold applications in material science.6e,7
Therefore dendrimers with stilbenoid building blocks repre-
sent an attractive area of research.8 The Refs. 9–11 demon-
strate the increasing interest in these systems during the
recent years. Stilbene units can represent the core,9 or can
be a part of the dendrons2a,10 or both.11 We decided to study
poly(propylene imine) dendrimers [PPI] with (E)-stilbene or
(E,E)-1,4-distyrylbenzene chromophores attached to the
four NH2 groups of the first generation PPI and to the eight
NH2 groups of the second generation PPI. Apart from our
own newly synthesized compounds of this type, there ex-
ists—to our best knowledge—only a single such dendrimer,
which was investigated by Balzani et al.2d Meijer et al. stud-
ied oligo(1,4-phenylenevinylene) [OPV] terminated PPIs.12
1
ured products, whereby the detection limit in the H NMR
spectroscopy is 2% or lower for the (Z)-isomers.16
PPI 8 (first generation) was then reacted with aldehyde 3a to
the four-fold azomethine 9 (Scheme 2). It turned out that the
conjugated Schiff bases are light-sensitive (UV component
of the daylight). This sensitivity is remarkably diminished
when the C]N double bonds are hydrogenated by the re-
action with LiAlH4 (9/10). Accordingly aldehyde 7 was
reacted with PPI 8 and in situ reduced to dendrimer 11.
Without isolation of the intermediate azomethines the den-
drimers 13 and 14 were obtained (Scheme 3) by the reaction
of PPI 12 (second generation) and aldehyde 3a and 7, respec-
tively, and subsequent hydrogenation with LiAlH4.
Keywords: Crosslinking; Dendrimers; Photochemistry.
* Corresponding author. Fax: +49 6131 39 25396; e-mail: hmeier@mail.
uni-mainz.de
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.08.064

11430
A. Schulz, H. Meier / Tetrahedron 63 (2007) 11429–11435
R
R
R
N
R
O
KOC(CH3)3
(THF)
+
R'
R
CH2 P(OEt)2
O
CH
N
N
R
R
R
R
R
R
N
N
a
2
b
1
R' CH(OEt)2
CO2CH3
N
R
a
3
b
R
R'
CO2CH3
43
CHO
84
R'
R
Yield [%]
R
LiAlH4
(THF)
3b
R
Comp.
Yield [%]
R
CH2OH
12
NH2
R
4 (83%)
OC3H7
OC3H7
OC3H7
R
R
PBr3
(C6H5–CH3)
P(OEt)3
R
13
mixture
OC3H7
OC3H7
OC3H7
CH2
CH2
HN
CH2Br
5 (87%)
R
2a, KOC(CH3)3
O
R
14
45
HN
(THF)
CH2 P(OEt)2
R
R
Scheme 3. Functionalized poly(propylene imine) dendrimers: second gen-
eration.
6 (~100%)
The major problems of the synthetic procedures shown in
Schemes 2 and 3 are firstly the complete reaction of all
four or eight amino groups to generate uniform dendrimers
and secondly the preservation of the (E)-configurations of
the stilbene units. The latter point could be realized per-
fectly, but the complete reaction of all PPI amino groups
turned out to be impossible in the divergent synthesis of
13 by reaction of 12 and 3a. Even a high excess of aldehyde
3a led to a mixture (MALDI-TOF measurement). Surpris-
ingly, the analogous process with aldehyde 7 worked fairly
well. After hydrogenation with LiAlH4, the yield of uniform
14 amounted to 45%. The 1H and 13C NMR data of 3a, 8, and
9–11 are summarized in Tables 1 and 2.
R
R
CHO
7 (47%)
R = OC3H7
Scheme 1. Preparation of the aldehyde components.
R
ε
R
R
δ
γ
α
N
N
β
R
2.2. Absorption, fluorescence, and photochemistry
Comp.
Yield [%]
R
8
NH2
Figure 1 illustrates the absorption of the dendrimers 9 and 10
and their emission. Compared to stilbenylmethanol 4,
(lmax¼330 nm) dendrimer 10 shows a small hypsochromic
shift (lmax¼328 nm) in CHCl3. Extension of the conjugation
in the chromophore of dendrimer 9 causes a red shift
(lmax¼345 nm), which is notably smaller than that of the
aldehyde 3a (lmax¼358 nm). However, 3a has a stronger in-
tramolecular charge transfer effect than 9, which influences
the long-wavelength absorption.17 The fluorescence bands of
9 and 10 have their maxima at 500 and 412 nm. The Stokes
shift of 9 (8986 cmꢀ1) is much larger than the Stokes shift of
10 (6316 cmꢀ1). The close arrangement of (E)-stilbene chro-
mophores in the periphery of dendrimers can principally
have different effects. Apart from excimer formation,6e,18,19
exciplexes with the amino subunits can be formed.20 How-
ever, corresponding bathochromically shifted emissions
could not be detected. Despite the proximity of stilbene
chromophores or stilbene and amine units, the steric
OC3H7
OC3H7
OC3H7
OC3H7
OC3H7
OC3H7
ζ
9
CH
65
N
ζ
53
42
10
CH2
HN
HN
OC3H7
OC3H7
OC3H7
ζ
11
CH2
Scheme 2. Functionalized poly(propylene imine) dendrimers: first
generation.

A. Schulz, H. Meier / Tetrahedron 63 (2007) 11429–11435
11431
Table 1. 1H NMR data of the components 3a and 8 and the dendrimers 9–11 (d values in CDCl3, TMS as internal standard)
a
Compd
CH2
g
CH/CH2
Aromat. H
C6H4
Olefin. H
OC3H7
a
b
d
3
z
C6H2
(s)
CH]CH
OCH2
(t)
CH2
CH3
(m)
(t)
(t)
(m)
(t)
(s)
9.97
(AA0BB0)
(AB, 3J¼16.3ꢁ0.1 Hz)
6.99
7.14
(m)
(t)
3a
7.62
7.83
6.73
3.92
3.99
1.76
1.84
1.01
1.06
8
9
1.36
1.48
2.34
2.41
2.39
2.50
1.60
1.77
2.67
3.61
8.23
3.70
3.73
7.48
7.66
7.32
7.46
7.27
7.43
7.44
7.44
6.69
6.83
6.70
6.93
7.05
7.06
7.12
6.93
7.00
7.03
7.07
3.93
3.96
3.89
3.96
3.94
3.98
1.76
1.82
1.70
1.78
1.77
1.83
1.02
1.04
1.01
1.03
1.02
1.05
10b
11
1.28
1.36
2.31
2.36
2.42
2.43
1.78
1.63
2.61
2.62
a
The upper values belong to the p-OC3H7 chains.
Measurement in (CD3)2CO.
b
Table 2. 13C NMR data of the components 3a and 8 and the dendrimers 9–11 (d values in CDCl3, TMS as internal standard)
a
Compd
CH2
CH/CH2
z
C6H4
C6H2
Cq
C]C
OC3H7
CH2
a
b
g
d
3
CH
Cq
CH
CqO
CH
OCH2
CH3
3a
191.5
126.7
130.2
135.1
143.5
105.7
131.7
139.1
153.4
126.3
132.4
70.9
75.1
22.7
23.5
10.5
10.6
8
9
24.9
25.1
54.0
54.0
51.8
51.6
30.8
28.3
40.7
59.8
160.7
54.8
52.4
126.5
128.4
126.9
128.2
126.5
126.7
126.8
128.5
135.3
138.5
133.6
136.7
136.0
136.5
136.7
140.1
105.3
105.9
105.3
130.1
129.1
132.5
139.5
153.3
138.8
154.1
138.4
153.3
127.0
132.2
129.0
129.1
127.3
127.8
128.2
128.7
70.7
75.1
71.0
75.1
70.7
75.1
22.7
23.5
23.4
24.2
22.8
23.5
10.5
10.6
10.8
10.9
10.6
10.6
10b
11
25.9
25.1
54.8
54.1
48.5
48.2
28.3
27.5
54.1
53.9
a
The upper values belong to the two m-OC3H7 chains.
Measurement in (CD3)2CO.
b
situation seems to be not very suitable for the generation of
excimers and exciplexes. The dendrimer generation does not
affect the absorption and emission range. Monochromatic
irradiation (l¼340 nm) of 9 in 10ꢀ5 M solution in CHCl3
leads to a decrease of the long-wavelength absorption and
the fluorescence. Using the light of a Xenon high-pressure
lamp, the process becomes very fast (Fig. 2). After 1 min
the absorbance at lmax¼345 nm is reduced to almost one
third. A new absorption arises at about 425 nm, which
disappears slowly in 75 min. Dendrimer 10 behaves in the
same way. Figure 3 shows the corresponding photodegrada-
tion of 11. The extended (E,E)-distyrylbenzene chromophore
(lmax¼366 nm) is fast destroyed and a new low-intensity
absorption appears at 425 nm, which then disappears slowly.
A novel maximum around 260 nm arises on extended
irradiation. All three photoprocesses of 9, 10, and 11 are
Figure 2. Irradiation of 9 (10ꢀ5 M, absorption in CHCl3). The numbers on
the left side of the curves indicate the irradiation time in min using the
radiation of a Xenon high-pressure lamp and quartz cuvettes.
Figure 1. Absorption A and fluorescence F of 9 and 10 (CHCl3).

11432
A. Schulz, H. Meier / Tetrahedron 63 (2007) 11429–11435
oligo- and poly(1,4-phenylenevinylene)s21 and in stilbenoid
dendrimers11f,h for photoresists. Molecular models reveal
that in the stretched (all-transoid) conformation the den-
drimers discussed here have a maximum length up to
7.3 nm (valid for 14). However, the flexibility of their struc-
tures permits backfolding and coiling. In particular intra-
molecular [p2s+p2s] cycloadditions of the stilbene units
seem to be possible. Thus, the efficiency of intermolecular
C–C bond formations on irradiation is diminished in com-
parison to rigid stilbenoid dendrimers.11f,h Nevertheless,
photocrosslinking is still the major process in long-term
irradiations.
3. Conclusion
Poly(propylene imine) [PPI] dendrimers of first and second
generation were prepared by condensation reactions of the
parent PPIs and 4-styrylbenzaldehyde 3a or 4-[4-(styryl)styr-
yl]benzaldehyde 7. The obtained azomethines can be sub-
sequently or in situ hydrogenated with LiAlH4 to the
corresponding secondary amines. Thus, PPI dendrimers with
four (E)-stilbene (9, 10), four or eight (E,E)-distyrylbenzene
(11, 14) chromophores were obtained. Their long wavelength
absorption bands and their fluorescence bands disappear com-
pletely onlong-term irradiation. Apart formthe primaryE/Z
isomerization of 9 and 10, and the secondary E/Z isomeriza-
tionof11and14,thepredominantphotoreactionisacrosslink-
ing by C–C bond formations, which lead to oligomers.
Figure 3. Irradiation of 11 (10ꢀ5 M, absorption in CHCl3). The numbers
indicate the irradiation time in min using a Xenon high-pressure lamp and
quartz cuvettes.
not uniform. The reaction spectra do not exhibit isosbestic
points. In order to conceive a structural concept for the
photoreactions, we studied the corresponding 1H NMR reac-
tion spectra in CD2Cl2. The primary photoprocess of 9 and
10 is the E/Z isomerization visible by the generation of
new olefinic signals at d¼6.6ꢁ0.3 ppm. All four stilbene
units are finally involved in this complex process, which
soon after the beginning of the irradiation is accompanied
by an oligomerization. The appearance of saturated CH
groups (d¼4.3ꢁ0.2 ppm) and the broadening of all 1H
NMR signals are typical for this route, in which not only
the olefinic centers but also some benzene rings are involved.
The new absorption in the visble region at 425 nm do we
assign, as in earlier OPV studies, to intermediate quinoid
systems.11k,21 Balzani et al. found in their investigation of
a related dendrimer the secondary formation of phenanth-
renes.2d We can definitely rule that out here. The proton in
the bay region neighboring to the oxygen atom of a propoxy
group would lead to a singlet signal at d¼9.8ꢁ0.2 ppm.22,23
We do not find signals with dꢂ7.5 ppm. The detection limit
is in the range of 3%. ESI and FD mass spectra showed
various peaks with m/z values, which are higher than the
m/z values of the monomers, but no exact oligomer values.
4. Experimental
4.1. General
€
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 tech-
niques) and on a Shimadzu AXIMA-CFR (MALDI-TOF)
spectrometer. The FTIR spectra were measured with a
Perkin–Elmer GX/200 spectrometer.
The dendrimers 11 and 14 exhibit—compared to 9 and 10—
the opposite behavior on irradiation. An E/Z photoisome-
rization is in the (E,E)-distyrylbenzene chromophore not
possible.6e The extended conjugation leads in the first
excited singlet states S1 to relatively high bond orders of
the olefinic double bonds.24 That prevents the stereoisomeri-
zation by rotation. However, as soon as the oligomerization
process degradates one olefinic double bond of the chromo-
phore, the isomerization becomes possible. Prolonged
irradiation (lꢂ300 nm) leads indeed to new 1H NMR signals
at 6.4ꢁ0.2 ppm, which are typical for (Z)-stilbene confi-
gurations. Monochromatic irradiation at 366 nm does not
affect the (E,E)-distyrylbenzene chromophore. In general,
the photooligomerization of stilbenoid compounds is the
more efficient, the higher the energy of the UV light is.
The Hg emissions at 254 and 313 nm are particularly
useful for this purpose. The oligomerization can be used in
4.1.1. (E)-4-(3,4,5-Tripropoxystyryl)benzaldehyde (3a).
A slow Ar stream was purged through a suspension of
3.87 g (34.0 mmol) KOC(CH3)3 in 75 mL dry THF at
0 ꢃC, before a degassed solution of 5.81 g (14.0 mmol)
diethyl 3,4,5-tripropoxybenzylphosphonate (1) and 3.19 g
(15.0 mmol) 4-(diethoxymethyl)benzaldehyde (2a) in
45 mL dry THF was added dropwise. The formation of the
ylide was indicated by an orange color. After 12 h stirring
under Ar at ambient temperature, the mixture was refluxed
for 1 h, poured on crushed ice, and treated with 300 mL
CHCl3 and 100 mL diluted HCl. After 3 h stirring at room
temperature, the organic layer was separated, washed with
NaHCO3 and H2O, dried with MgSO4, and evaporated. Re-
crystallization from petroleum ether (bp 40–70 ꢃC) fur-
nished 4.48 g 3a. Compared to a previous procedure,11k
the yield could be improved from 66 to 84%. The pale yel-
low solid melted at 62 ꢃC and was identified by comparison

A. Schulz, H. Meier / Tetrahedron 63 (2007) 11429–11435
11433
with an authentic sample. Anal. Calcd for C24H30O4 (382.5):
C 75.36, H 7.91; found: C 75.32, H 7.84.
bromo compound 5 (3.82 g, 8.5 mmol) were heated to
150–155 ꢃC for 2 h. The generated bromoethane was contin-
uously distilled off. At the end of the quantitative reaction
4.28 g viscous yellow oil remained, which was analytically
4.1.2. Methyl (E)-4-(3,4,5-tripropoxystyryl)benzoate
(3b). According to the preparation of 3a, the reaction of 1
(11.26 g, 28.0 mmol) and methyl 4-formylbenzoate (2b)
(4.92 g, 30.0 mmol) was performed. After 1 h reflux the
crude reaction product was recrystallized from acetone.
1
pure. H NMR (CDCl3): d 1.01 (t, 1H, CH3), 1.04 (t, 6H,
CH3), 1.22 (t, 6H, CH3 of ethyl), 1.75 (m, 2H, CH2), 1.82
2
(m, 4H, CH2), 3.12 (d, 2H, J¼21.7 Hz, PCH2), 3.97 (m,
10H, OCH2 and P(O)OCH2), 6.68 (s, 2H, aromat. H),
6.92/6.95 (AB, 2H, 3J¼16.4 Hz, olefin. H), 7.26/7.41
(AA0BB0, 4H, aromat. H); 13C NMR (CDCl3): d 10.5, 10.6
1
Yield 5.04 g (43%), yellowish solid, mp 96 ꢃC. H NMR
(CDCl3): d 1.03 (t, 1H, CH3), 1.05 (t, 6H, CH3), 1.76 (m,
2H, CH2), 1.84 (m, 4H, CH2), 3.89 (s, 3H, CO2CH3), 3.95
(t, 2H, OCH2), 3.98 (t, 4H, OCH2), 6.72 (s, 2H, aromat.
3
(CH3), 16.3 (d, J(CP)¼6.8 Hz, CH3), 22.7, 23.5 (CH2),
33.6 (d, 1J(CP)¼137.9 Hz, PCH2), 62.1 (d, 3J(CP)¼6.8 Hz,
3
H), 6.98/7.09 (AB, 2H, J¼16.4 Hz, olefin. H), 7.52/7.99
P(O)OCH2), 70.7, 75.1 (OCH2), 105.3, 126.5 (d,
(AA0BB0, 4H, aromat. H); 13C NMR (CDCl3): d 10.6, 10.6
(CH3), 22.8, 23.5 (CH2), 52.0 (OCH3), 70.8, 75.1 (OCH2),
105.5, 126.1, 129.9 (aromat. CH), 126.5, 131.4 (olefin.
CH), 131.3, 131.9, 138.8, 141.9, 153.3 (aromat. Cq), 166.9
4J(CP)¼3.4 Hz), 130.5 (d, J(CP)¼6.8 Hz) (aromat. CH),
3
127.2, 128.8 (olefin. CH), 130.7, 132.5, 136.1 (d,
2J(CP)¼4.5 Hz), 138.3, 153.3 (aromat. Cq); FD MS: m/z
ꢄ
(%) 505 (100) [M+ ]. Anal. Calcd for C28H41O6P (504.6):
C 66.65, H 8.19; found: C 66.71, H 8.17.
ꢄ
(CO); FD MS: m/z (%) 412 (100) [M+ ]. Anal. Calcd for
C25H32O5 (412.5): C 72.79, H 7.82; found: C 72.67, H 7.69.
4.1.6. (E,E)-4-[4-(3,4,5-tripropoxystyryl)styryl]benzal-
dehyde (7). According to the procedure described for 3a,
4.15 g (8.2 mmol) 6, 1.84 g (8.8 mmol) 2a and 1.38 g
(12.3 mmol) KOC(CH3)3 furnished an orange solid, which
was purified by column filtration (12ꢅ7 cm SiO2, toluene/
diethyl ether gradient) and recrystallized from cyclohexane.
Yield 1.86 g (47%), mp 154 ꢃC. 1H NMR (CDCl3): d 1.03 (t,
1H, CH3), 1.05 (t, 6H, CH3), 1.74 (m, 2H, CH2), 1.84 (m, 4H,
CH2), 3.95 (m, 2H, OCH2), 3.97 (m, 4H, OCH2), 6.72 (s, 2H,
aromat. H), 6.96/7.03 (AB, 2H, 3J¼16.2 Hz, olefin. H), 7.12/
4.1.3. (E)-4-(3,4,5-Tripropoxystyryl)benzyl alcohol (4).
Ester 3b (5.01 g, 12.1 mmol), dissolved in 25 mL dry THF,
was treated with LiAlH4 (413 mg, 10.9 mmol) at 0 ꢃC. After
refluxing for 3 h, the reaction was stopped with 10 mL H2O
and the mixture neutralized with HCl (10%). The organic
layer was washed with 25 mL H2O and the water phase
was extracted with 25 mL diethyl ether. The unified organic
layers were dried with MgSO4 and evaporated. Column fil-
tration (8ꢅ20 cm SiO2, CH2Cl2) yielded 3.87 g (83%) of
1
3
a beige solid, which melted at 90 ꢃC. H NMR (CDCl3):
7.23 (AB, 2H, J¼16.2 Hz, olefin. H), 7.49/7.52 (AA0BB0,
d 1.02 (t, 1H, CH3), 1.05 (t, 6H, CH3), 1.76 (m, 2H, CH2),
1.83 (m, 4H, CH2), 3.94 (t, 2H, OCH2), 3.97 (t, 4H,
OCH2), 4.67 (s, 2H, CH2OH), 6.70 (s, 2H, aromat. H),
6.95/6.98 (AB, 2H, 3J¼16.4 Hz, olefin. H), 7.32/7.46
(AA0BB0, 4H, aromat. H); 13C NMR (CDCl3): d 10.6, 10.6
(CH3), 22.7, 23.5 (CH2), 65.1 (CH2OH), 70.8, 75.1
(OCH2), 105.3, 126.5, 127.4 (aromat. CH), 127.3, 128.9
(olefin. CH), 132.5, 136.8, 138.3, 140.1, 153.3 (aromat.
4H, aromat. H, central benzene ring), 7.63/7.84 (AA0BB0,
4H, aromat. H), 9.97 (s, 1H, CHO); 13C NMR (CDCl3):
d 10.5, 10.6 (CH3), 22.7, 23.5 (CH2), 70.7, 75.1 (OCH2),
105.3, 126.7, 126.8, 127.3, 130.2 (aromat. CH), 126.9,
127.0, 129.4, 131.7 (olefin. CH), 132.3, 135.2, 135.6,
137.6, 138.5, 143.4, 153.3 (aromat. Cq), 191.5 (CHO); FD
ꢄ
MS: m/z (%) 485 (100) [M+ ]. Anal. Calcd for C32H36O4
(484.6): C 79.31, H 7.49; found: C 79.33, H 7.58.
ꢄ
Cq); EI MS: m/z (%) 384 (100) [M+ ], 342 (71), 209 (62),
181 (44), 165 (42), 152 (60). Anal. Calcd for C24H32O4
(384.5): C 74.97, H 8.39; found: C 74.81, H 8.38.
4.2. Preparation of the dendrimers 9, 10, 11, and 14
Astramol DAB-Am-4Ò (8, 0.316 g, 1.0 mmol) or Astramol
DAB-Am-8Ò (12, 0.381 g, 0.5 mmol) and the corresponding
aldehyde 3a or 7 in a ratio of 1:6 for 8 and 1:20 for 12 were
mixed in 10 mL dry THF. The solvent was evaporated and
the mixture heated at 0.1 kP to 70 ꢃC in order to remove
completely the generated H2O. After 0.5–1 h the viscous res-
idue, which contained 9, was recrystallized from ethanol.
The hydrogenation 9/10 could be performed in situ with
the raw product, which was dissolved in 15 mL dry THF,
cooled to 0 ꢃC and treated with LiAlH4 (285 mg,
7.5 mmol). After refluxing for 8 h, 16 mL H2O was added.
The Li complex decomposed and the residue was extracted
in a Soxhlet apparatus with 250 mL diethyl ether. The vola-
tile parts were removed in the vacuum and the crude 10 pu-
rified by column filtration (8ꢅ8 cm SiO2, CH2Cl2 followed
by H3C–CO–CH3, followed by CH3OH/N(C2H5)3 10:1).
The dendrimers 11 and 14 were obtained by the one-pot pro-
cedure. The same procedure applied for 12 and 3a yielded
instead of pure 13 a mixture of dendrimers with incomplete
reaction of the eight amino groups. Due to the light sensibil-
ity of all resulting dendrimers the entire procedure had to be
carried out in the dark for each dendrimer.25
4.1.4. (E)-40-Bromomethyl-3,4,5-tripropoxystilbene (5).
To a solution of 4 (3.78 g, 9.8 mmol) in 60 mL toluene,
PBr3 (1.50 g, 5.6 mmol) was added dropwise at 0 ꢃC. After
30 min at 0 ꢃC and 2 h stirring at 25 ꢃC, 15 mL H2O was
added. The organic layer was washed with NaHCO3 and
H2O, dried with MgSO4, and evaporated. Crystallization
from toluene yielded 3.82 g (87%) of a pale yellow solid,
1
which melted at 81 ꢃC. H NMR (CDCl3): d 1.03 (t, 1H,
CH3), 1.06 (t, 6H, CH3), 1.76 (m, 2H, CH2), 1.85 (m,
4H, CH2), 3.96 (t, 2H, OCH2), 3.99 (t, 4H, OCH2), 4.50 (s,
2H, CH2Br), 6.71 (s, 2H, aromat. H), 6.95/7.01 (AB, 2H,
3J¼16.3 Hz, olefin. H), 7.36/7.45 (AA0BB0, 4H, aromat. H);
13C NMR (CDCl3): d 10.6, 10.6 (CH3), 22.8, 23.5 (CH2),
33.6 (CH2Br), 70.8, 75.1 (OCH2), 105.4, 126.7, 129.4 (aro-
mat. CH), 126.9, 129.7 (olefin. CH), 132.3, 136.8, 137.7,
138.6, 153.3 (aromat. Cq); FD MS: m/z (%) 447/449 (100)
ꢄ
[M+ , Br isotope pattern]. Anal. Calcd for C24H31BrO3
(447.4): C 64.43, H 6.98; found: C 64.45, H 7.02.
4.1.5. Diethyl (E)-4-(3,4,5-tripropoxystyryl)benzyl phos-
phonate (6). Triethyl phosphite (20.0 g, 120 mmol) and

11434
A. Schulz, H. Meier / Tetrahedron 63 (2007) 11429–11435
4.2.1. Dendrimer 9: all-(E)-4-cascade: 1,4-diaminobu-
tane[4-N,N,N0,N0]: (2-azapent-1-enylidene)-1-[5-(10,40-
Materials Science of the University of Mainz for financial
support.
phenylenevinylene-1,2,3-tripropoxybenzene)].
Yield
1.15 g (65%) yellowish crystals, which melted at 92 ꢃC.
IR (KBr): n [cmꢀ1] 3024, 2962, 2935, 2873, 1641, 1602,
1581, 1503, 1463, 1433, 1387, 1338, 1230, 1121, 959,
825. FD MS m/z (%) 1775 (75) [M+], 1353 (25), 915
(100), 861 (44), 761 (23), 550 (45), 491 (75).
References and notes
1. See for example: (a) Newkome, G. R.; Moorefield, C. N.;
€
Vogtle, F. Dendrimers and Dendrons: Concepts, synthesis,
Applications; Wiley-VCH: Weinheim, 2001; (b) Frechet,
ꢀ
4.2.2. Dendrimer 10: all-(E)-4-cascade: 1,4-diaminobu-
tane[4-N,N,N0,N0]: (2-azapentylidene)-1-[5-(10,40-phe-
nylenevinylene-1,2,3-tripropoxybenzene)]. Yield 0.94 g
(53%) yellowish oil. IR (KBr): n [cmꢀ1] 3435, 3020, 2963,
2936, 2875, 2804, 1585, 1506, 1461, 1436, 1382, 1338,
1233, 1118, 961, 829. FD MS m/z (%) 1785 (100) [M+],
893 (13) [M2+].
J. M. J.; Tomalia, D. A. Dendrimers and Other Dendritic
Polymers; Wiley: New York, NY, 2001.
2. (a) Soomro, S. A.; Schulz, A.; Meier, H. Tetrahedron 2006, 62,
8089; (b) Ong, W.; McCarley, R. L. Org. Lett. 2005, 7, 1287; (c)
€
Vogtle, F.; Fakhrnabavi, H.; Lukin, O.; Mueller, S.; Friedhofen,
J.; Schalley, C. A. Eur. J. Org. Chem. 2004, 22, 4717; (d)
Vicinelli, V.; Ceroni, P.; Maestri, M.; Lazzari, M.; Balzani,
€
V.; Lee, S.-K.; van Heyst, J.; Vogtle, F. Org. Biomol. Chem.
2004, 2, 2207; (e) Ceroni, P.; Vicinelli, V.; Maestri, M.;
4.2.3. Dendrimer 11: all-(E)-4-cascade: 1,4-diaminobu-
tane[4-N,N,N0,N0]: (2-azapentylidene)-1-[5-(10,40-phenyl-
enevinylene-100,400-phenylenevinylene-1,2,3-tripropoxy-
benzene)]. Yield 0.92 g (42%) yellowish waxy solid. FTIR
(KBr): n [cmꢀ1] 3436, 3024, 2963, 2935, 2876, 1576, 1558,
1540, 1507, 1430, 1374, 1121, 1098, 1002, 961, 864, 826,
742, 537. FD MS m/z (%) 2195 (100) [M+], 1098 (92) [M2+].
€
Balzani, V.; Gorka, M.; Lee, S.-K.; Dragut, P.; Vogtle, F.
Collect. Czech. Chem. Commun. 2003, 68, 1541; (f) Frerot,
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Ed. 2001, 40, 746; (h) Baars, M. W. P. L.; Kleppinger, R.;
Koch, M. H. J.; Yeu, S.-L.; Meijer, E. W. Angew. Chem.
2000, 112, 1341; Angew. Chem., Int. Ed. 2000, 39, 1285; (i)
Baars, M. W. P. L.; Soentjens, S. H. M.; Fischer, H. M.;
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3. (a) Smith, G.; Chen, R.; Mapolie, S. J. Organomet. Chem.
2003, 673, 111; (b) Volkova, N. N.; Tarasov, E. V.; Kodess,
M. I.; Van Meervelt, L.; Dehaen, W.; Bakulev, V. A. Org.
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M. F.; van Haare, J. A. E. H.; Baars, M. W. P. L.; Janssen,
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4.2.4. Dendrimer 14: all-(E)-8-cascade: 1,4-diaminobu-
tane[4-N,N,N0,N0]: (1-azabutylidene)[4-N00,N00,N000,N000]:
(2-azapentylidene)-1-[5-(10,40-phenylenevinylene-100,400-
phenylenevinylene-1,2,3-tripropoxybenzene)].
Yield
]
2.02 g (45%) yellowish waxy solid. FTIR (KBr): n [cmꢀ1
3434, 3024, 2962, 2935, 2876, 1636, 1577, 1506, 1431,
1374, 1121, 1003, 961, 866, 748, 538. H NMR (CDCl3):
1
d 1.02 (t, 24H, CH3), 1.03 (t, 48H, CH3), 1.41 (br m, 4H,
CH2), 1.60 (br m, 16H, CH2), 1.76 (m, 16H, CH2), 1.83
(m, 32H, CH2), 2.40 (br s, 36H, NCH2), 2.59 (br s, 16H,
NCH2), 3.70 (s, 16H, NCH2), 3.93 (t, 16H, OCH2), 3.96 (t,
32H, OCH2), 6.69 (s, 16H, aromat. H terminal benzene
ꢀ
ꢀ
4. (a) Heuze, K.; Mery, D.; Gauss, D.; Blais, J.-C.; Astruc, D.
Chem.—Eur. J. 2004, 10, 3936; (b) Jones, J. W.; Bryant,
W. S.; Bosman, A. W.; Janssen, R. A. J.; Meijer, E. W.;
Gibson, H. W. J. Org. Chem. 2003, 68, 2385; (c) Engel,
G. D.; Gode, L. H. Chem.—Eur. J. 2002, 8, 4319; (d)
Stephan, H.; Spies, H.; Johannsen, B.; Gloe, K.; Gorka, M.;
3
rings), 6.93/6.99 (AB, J¼15.8 Hz, 16H, olefin. H), 6.99/
7.05 (AB, 3J¼15.8 Hz, 16H, olefin. H), 7.23–7.50 (m 64H,
aromat. H); 13C NMR (CDCl3): d 10.6, CH3/23.4, CH2/
75.1, OCH2 (p-OC3H7), 10.6, CH3/22.8, CH2/70.7, OCH2
(m-OC3H7), 27.5, 29.7, 29.7 (CH2), 48.2, 52.3, 53.9
(NCH2, partly superimposed), 105.3, 126.5, 126.7, 126.8,
128.5 (aromat. CH), 127.2, 127.8, 128.2, 128.7 (olefin.
CH), 132.5, 136.0, 136.5, 136.7, 140.1 (aromat Cq), 138.4,
153.3 (aromat. CqO).26
€
Vogtle, F. Eur. J. Inorg. Chem. 2001, 11, 2957.
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D€urr, H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 2003;
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€
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The irradiation experiments, discussed in Section 2.2, were
performed in 10ꢀ5 M degassed solutions in CHCl3 or
CH2Cl2. For the 1H NMR measurements 10ꢀ3–10ꢀ4 M solu-
tions in CD2Cl2 were used. A Xe high-pressure lamp (Osram
XBO, 1000 W) or a Hg medium-pressure lamp (Hanovia,
450 W) was applied as radiation source. Interference filters
(366, 340, 313, 254 nm) or a Pyrex filter (lꢂ300 nm) served
for the selection of the desired wavelengths. The reaction
times, giveninthediagrams, are relatedtounfilteredradiation.
Acknowledgements
ꢁ
ꢁ
We are grateful to the Deutsche Forschungsgemeinschaft,
the Fonds der Chemischen Industrie and the Centre of
39, 4025; (f) Sanchez, C.; Villacampa, B.; Alcala, R.;
Martinez, C.; Oriol, L.; Pin˜ol, M.; Serrano, J. L. Chem. Mater.

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