Synthesis and photomodulation of rigid polyphenylene dendrimers with an azobenzene core

Article abstract of DOI:10.1021/ma021135n

A series of azobenzene-cored polyphenylene dendrimers have been synthesized. The photoresponsive properties of these dendrimers have been investigated by optical spectroscopy and gel permeation chromatography (GPC). These polyphenylene dendrimers exhibit

Full text of DOI:10.1021/ma021135n

578
Macromolecules 2003, 36, 578-590
Synthesis and Photomodulation of Rigid Polyphenylene Dendrimers
with an Azobenzene Core
Do1r th e Gr ebel-Koeh ler ,† Da oju n Liu ,‡ Steven De F eyter ,‡ Volk er En k elm a n n ,†
Ta n ja Weil,† Ch r istia a n En gels,§ Celeste Sa m yn ,§ Kla u s Mu1 llen ,*,† a n d
F r a n s C. De Sch r yver *,‡
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany, Laboratory
for Photochemistry and Spectroscopy, Department of Chemistry, Katholieke Universiteit Leuven
(KULeuven), Celestijnenlaan 200F, B-3001 Heverlee, Belgium, and Laboratory of Macromolecular and
Physical Organic Chemistry, Department of Chemistry, Katholieke Universiteit Leuven (KULeuven),
Celestijnenlaan 200F, B-3001 Heverlee, Belgium
Received J uly 16, 2002; Revised Manuscript Received November 1, 2002
ABSTRACT: A series of azobenzene-cored polyphenylene dendrimers have been synthesized. The
photoresponsive properties of these dendrimers have been investigated by optical spectroscopy and gel
permeation chromatography (GPC). These polyphenylene dendrimers exhibit a photoresponsive behavior
upon UV and visible irradiation, resulting in a reversible and appreciable change of the dendrimer
structure. Furthermore, the influence of dendrimer generation and degree of branching on the kinetic
characteristics of the photoisomerization is discussed. The crystal structures of two precursors and two
solvate structures of the first generation are presented.
In tr od u ction
tence leads to fascinating properties and applications.
For instance, attachment of chromophores to the rim
of these rigid dendrimers allows the construction of
nanoparticles with a well-defined number of chromo-
phores in a confined volume element and has stimulated
the investigation of the photophysical processes of
individual multichromophoric dendrimers.24 The rigid
molecular framework of polyphenylene dendrimers en-
sures a rather narrow variation of the voids in their
interior and allows them to be used as ideal sensors
for volatile organic solvents with a high degree of
reproducibility.25 We have also recently found that
the shape-persistence of polyphenylene dendrimers
plays an important role in the formation of self-
organized micrometer long nanofibers.23,26
In this report, we describe the synthesis, structure,
and photochemistry of polyphenylene dendrimers with
an azobenzene core. It is our aim to construct, by
combining the photoswitchability of the azobenzene unit
and the rigid structure of polyphenylene dendrimers, a
series of macromolecules in which a light stimulus
produces a well-defined and large structural perturba-
tion and to investigate their photoresponsive properties.
Molecules that are responsive to an external stimulus
in a controlled, reproducible, and reversible manner
have many applications in diverse areas of research.1
Azobenzene, due to its efficient and fully reversible
photoisomerization and photoinduced anisotropy,2,3 has
widely been investigated as a component in photore-
sponsive materials. Azobenzene functionalized mono-
layers4-8 and thin polymer films3,9,10 exhibit interesting
photoresponsive properties. Dendrimers,11,12 a type of
highly branched and structurally regular macromol-
ecules which are prepared step by step either diver-
gently or convergently, allow a controllable positioning
of photoactive moieties in their structures. Dendrimers
with azobenzene units in the exterior,13-15a interior,15b-17
or throughout the dendritic structure18-21 have been
prepared and their photoresponsive properties have
been investigated. By placing the azobenzene unit in
the core of a dendrimer, a small conformational change
of the azobenzene moiety should produce a large change
in the whole dendritic structure. However, owing to the
flexibility of the dendrimer branches such as poly(aryl
ether)s and poly(amidoamine)s, obtaining a well-defined
and large photoinduced change of the dendrimer struc-
ture remains challenging. To this end, rigid dendrimer
subunits are a better choice because of their well-defined
three-dimensional structure. To our knowledge, the
synthesis of only one type of shape-persistent dendrim-
ers (of the phenylacetylene type) with an azobenzene
core has been reported.15e However, their photorespon-
sive properties have not been investigated.
Syn th esis
A series of azobenzene-cored polyphenylene dendrim-
ers was synthesized by means of the synthetic concept
previously presented.28 The dendrimer synthesis started
from 4,4′-bis(4-ethynyl)azobenzene (3) (Figure 1), which
was prepared in three steps from 4-iodoaniline. In the
first step, oxidative coupling gave 4,4′-diiodoazobenzene
(1).15e,29 Subsequent aryl-ethynyl coupling after Heck,
Cassar, Haghihara, and Sonogashira produced 4,4′-bis-
(triisoproylsilylethynyl)azobenzene (2). The starting
compound (3) for the dendrimer buildup was then
obtained by deprotection with tetrabutylammonium
fluoride in tetrahydrofuran.
It is known that polyphenylene dendrimers have a
shape-persistent structure22,23 and this shape-persis-
* Corresponding authors. K.M. Fax: (+49)-6131-379350. E-
mail: Muellen@mpip-mainz.mpg.de. F.C.D.S. Fax: (+32)-16-
327989. E-mail: Frans.DeSchryver@chem.kuleuven.ac.be.
† Max Planck Institute for Polymer Research.
‡ Laboratory for Photochemistry and Spectroscopy, Department
of Chemistry, Katholieke Universiteit Leuven.
The divergent synthesis of the dendrimers using
Diels-Alder cycloaddition of different cyclopentadi-
enones to alkynes was carried out in refluxing o-xylene
§ Laboratory of Macromolecular and Physical Organic Chem-
istry, Department of Chemistry, Katholieke Universiteit Leuven.
10.1021/ma021135n CCC: $25.00 © 2003 American Chemical Society
Published on Web 01/04/2003

Macromolecules, Vol. 36, No. 3, 2003
Rigid Polyphenylene Dendrimers 579
Ta ble 1. Un it Cell P a r a m eter s of 1, 4, a n d 5
5 (dichloro- 5 (tetra-
1
4
methane) hydrofuran)
a (Å)
b (Å)
6.0067(6)
7.3265(6)
6.012(3)
11.4007(3) 11.4818(10)
9.5956(4) 12.3161(4) 11.4923(10)
c (Å)
28.4091(10) 18.2563(7) 27.1975(8) 12.2369(12)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
90
90
90
1250.2
8
77.783(1) 96.196(1) 103.457(1)
85.438(1) 97.992(1) 114.423(1)
75.872(1) 113.470(1) 97.427(1)
1005.9
2
2785.1
2
1382.7
2
Dx (g cm-3
)
2.306
49.520
1.263
0.689
3692
2223
1.226
1.593
10 470
7365
1.306
0.718
4257
1274
µ (cm-1
no. of reflcns 2536
no. of reflcns 1589
(obsd)
)
R (%)
w (%)
3.812
1.993
7.527
5.892
6.789
6.135
P1h
5.912
7.490
P1h
R
space group pnaa ()Pccn, P1h
no. 56)
The purity and monodispersity of the dendrimers
were established by means of NMR spectroscopy, FD,
and MALDI-TOF mass spectrometry and elemental
analysis (see Experimental Section).
Resu lts a n d Discu ssion
Str u ctu r e. The herein presented azobenzene-cored
polyphenylene dendrimers form the first extended series
of extensively studied rigid photoswitchable dendrimers,
which not only varies in dendrimer size but also in the
degree of branching. To elucidate the effect of a rigid
dendrimer scaffold on the isomerization of a central
azobenzene chromophore in solution, it is important to
know the exact structure of the dendrimers.
F igu r e 1. Synthetic route to 3: (i) KMnO4/CuSO4‚5H2O, CH2-
Cl2, room temperature, 61%; (ii) 4 equiv of triisopropylsilyl-
ethyne, (Pd(PPh3)2]Cl2, PPh3, CuI, THF/triethylamine, room
temperature, 75%; (iii) 0.4 equiv of nBu4NF, THF, room
temperature, 82%.
During various steps of the synthesis we were able
to grow crystals to determine the molecular geometry.
We here present the structures for 1, 4, and 5. Table 1
summarizes the unit cell parameters.
at 142 °C, except for the AzoG1hex (11), which was
prepared in diphenyl ether at 210 °C. The structures of
all dendrimers are shown in Figures 2-8 together with
the abbreviations used in the text.
1 was crystallized from dichloromethane by slow
evaporation at room temperature over 3 days. Eight
molecules are in an orthorhombic cell with the space
group Pnaa ()Pccn, No. 56).31 The molecule is flat and
the azo bond is coplanar with the phenyls. Figure 9
shows the projection along the iodine-iodine axis, which
reveals the molecules are packed in a herringbone
fashion.
AzoG1 (5) resulted from the Diels-Alder cycloaddition
of 3 with tetraphenylcyclopentadienone (12). The growth
of the higher generations was carried out by repetitive
Diels-Alder cycloaddition of the A2B-branching reagent
3,4-bis(4-triisopropylsilylethynylphenyl)-2,5-diphenylcy-
clopentadienone (13) and quantitative cleavage of the
protecting groups with ammonium fluoride as shown in
Figures 2 and 3.
The crystallization of trans-4,4′-iodoazobenzene (1) to
form an orthorhombic cell, is in contrast to the behavior
of 4,4′-dichloroazobenzene and 4,4′-dibromoazobenzene,
which variably display either monoclinic symmetries or
are disordered,32 depending on the crystallization condi-
tions.
Compound 4 (Figure 10) was crystallized from dichlo-
romethane by slow evaporation of the solvent over a
week. The triclinic cell contains two crystallographically
independent molecules, which are located on two dif-
ferent symmetry centers. As already observed with 1
the molecules are planar within 0.1 Å and pack in a
herringbone-like fashion. The phenylacetylene units
with torsion angles of the terminal phenyl to the central
phenyl units of 6.38 and 4.27 respectively are also rather
flat.
The addition/deprotection cycles were carried out one,
two, or three times followed by a Diels-Alder addition
of 12 to give the unsubstituted dendrimers AzoG2 (6),
AzoG3 (7) (Figure 4), and AzoG4 (8) respectively (Figure
5).
To vary the density of the phenyl rings in the second
generation an AB- and A4B-branching reagent were
applied.30 Addition of the AB-building unit 3-(4-triiso-
propylsilylethynylphenyl)-2,4,5-triphenylcyclopentadi-
enone (14) for the AzoG2AB (9) (Figure 6) and of
the A4B-building unit 2,3,4,5-tetra(4-triisopropylsilyl-
ethynylphenyl)cyclopentadienone (15) for the AzoG2A4B
(10) yielded the protected first generations of the
dendrimers, which were thereafter deprotected to yield
AzoG1ethin2 and AzoG1ethin8, respectively, which were
then capped with 12 (Figure 7).
5 could be crystallized from either dichloromethane
or tetrahydrofuran by slow evaporation at room tem-
perature over a period of 4 weeks. The two different
crystals formed under these conditions are displayed in
Figure 11. The theoretical point group of the dendrimer
is 2/m.
To produce AzoG1hex (11), 1 was reacted with phenyl-
acetylene to give 4,4′-bis(phenylethynyl)azobenzene (4),
which was then treated in diphenyl ether at 210 °C with
tetraphenylcyclopentadienone to give 11 as shown in
Figure 8.

580 Grebel-Koehler et al.
Macromolecules, Vol. 36, No. 3, 2003
F igu r e 2. Synthesis of the first generation dendrimers: (i) 3 equiv of 12/13, respectively, o-xylene, 142 °C, 92% and 89%,
respectively; (ii) 4 equiv of nBu4NF, THF, room temperature.
Both pseudopolymorphs crystallize as solvates in
triclinic structures. The dendrimer in the tetrahydro-
furan solvate is located on a center of symmetry and
contains two solvent molecules per dendrimer. The
structure obtained from dichloromethane has the den-
drimer located at a common position; i.e., it possesses
no special symmetry. The crystal contains two molecules
of dichloromethane per dendrimer.
Nevertheless, the conformations of the phenyl groups
with respect to each other are quite similar to those
observed for other 2,3,4,5-tetraphenylphenyl units re-
ported in the literature.33 The dihedral angles between
the phenyl planes (indicated in Figure 12) are sum-
marized in Table 2. The slightly higher value for plane
3 can be ascribed to the lesser hindrance from the
adjacent hydrogen in comparison to the interaction with
two neighboring phenyls, which determine the angles
of the planes 4, 5, and 6.
These values are also in good agreement with those
found in other polyphenylene dendrimers,34 showing
that 2,3,4,5-tetraphenylphenyl moieties can be regarded
as groups with fixed conformation.
A common feature in both structures is the observa-
tion that the dendrimer molecules are arranged linearly
in a preferential orientation (Figure 13). The packing
of the dendrimers is always such that the 2,3,4,5-
tetraphenylphenyl units are located above the azo bond
of the dendrimer in the layer below, so avoiding too
much sterical hindrance. Neighboring rows are sepa-
rated by the trapped solvent molecules.

Macromolecules, Vol. 36, No. 3, 2003
Rigid Polyphenylene Dendrimers 581
F igu r e 3. Synthesis of the higher generation dendrimers: (i) 6 equiv of 12/13, respectively, o-xylene, 142 °C, 91% and 86%
respectively.
The rigidity of the polyphenylene dendrimers has
been previously studied in detail. The shape persistence
found by PFM-AFM (pulsed force mode-atomic force
microscopy) measurements23 was corroborated by analy-
sis using advanced solid-state NMR spectroscopy tech-
niques,22 which have demonstrated that only restricted
reorientations of single terminal phenyl substituents
around fixed axes occur. This can also be assumed to
be true for the dendrimers presented here. In contrast
to the flexible azobenzene-containing dendrimers stud-
ied previously15-17 the only shape-nonpersistent part of
the polyphenylene dendrimers is the azobenzene unit.
Because of the rigid dendrons, the change at the center
is expressed at the periphery.
The conformation of the resulting molecule is similar
to that of the crystal structure. As a result, we have
assumed that the calculated structure for the cis-isomer
is as good an approximation as the trans model is.
Compared with one another, the trans- and the cis-
isomers display dramatic differences in volume, already
in the first generation. This change should clearly be
detectable by gel permeation chromatography (GPC).
GPC measurements of AzoG2, AzoG3, and AzoG4 reveal
that azobenzene-cored polyphenylene dendrimers ex-
hibit an increase in elution volume upon 364 nm
irradiation to the photostationary state (PSS), as shown
in Figure 15, indicating a decrease in hydrodynamic
volume of the dendrimer molecules after UV irradiation.
The GPC measurements of a dendrimer solution were
done in the dark at room temperature immediately after
the PSS was reached. Because of the slow rate of the
thermal cis-trans isomerization, the trans- and cis-
Figure 14 shows an energy-minimized structure of
AzoG1 (5) calculated in vacuo using HyperChem Pro 6.0,
MM2 (MM+) force field optimization (the model is a
local minimum, not an absolute minimum).

582 Grebel-Koehler et al.
Macromolecules, Vol. 36, No. 3, 2003
polyphenylene dendrimers with one central chromo-
phore is even larger than that achieved by McGrath’s
dendrimers15a,c with a maximal contraction of 29%. This
stresses the need for a rigid dendrimer scaffold in order
to transfer the structural change of the small central
unit to the whole nanosphere and to make use of the
whole extent of the trans-cis isomerization. In compari-
son, in a more flexible dendrimer, the spatial relation-
ship between the functional core and the periphery
cannot be absolutely defined, and thus the conforma-
tional change cannot be completely expressed at the
periphery.36
P h otoch em istr y. All the azobenzene-cored poly-
phenylene dendrimers exhibit an effective photorespon-
sive behavior. Irradiation of a dendrimer solution at 364
nm results in photoisomerization from the trans- to the
cis-isomer, as evidenced in Figure 16, parts A and B,
by a decrease in absorbance at about 365 nm and an
increase in absorbance at about 450 nm. These cor-
respond to the values for the cis- and trans-isomers
reported of other dendronized azobenzenes.15,17 The
presence of isosbestic points in the absorption spectra
during photoisomerization is clearly seen in Figure 16,
parts A and B. Under the experimental conditions used,
a photostationary state (PSS) is reached within 10 min
of irradiation. The content of cis-isomer at the PSS was
calculated by (A0 - APSS(364))/A0, where A0 and APSS(364)
are the absorbances at 365 nm of the unirradiated
solution and of the solution at the PSS under 364 nm
irradiation, respectively. This calculation is approximate
and based on the following assumptions. First, the
initial dark-incubated dendrimer solution consists of
predominantly trans-isomer.2,15b,d This is supported by
our crystallographic data in which the crystals contain
only trans-isomers. Second, compared to the trans-
isomer, the cis-isomer has a negligible absorbance at
365 nm. This has been shown for other azobenzenes and
so can also be assumed to be true for our system.15b,d,27
The absorption spectrum of a polyphenylene dendrimer,
whose structure and bulkiness are similar to the
polyphenylene arms of dendrimer AzoG4, is shown in
Figure 16B as a dotted line, from which it is reasonable
to make the third assumption that the polyphenylene
branches have a negligible absorbance at 365 nm. The
apparent first-order rate constant k1 for trans-cis
photoisomerization is determined from a plot of ln(At -
F igu r e 4. Generation 3 (7) prepared in 93% yield.
Ta ble 2. In ter p la n a r An gles (d eg) betw een th e P h en yl
Rin gs in Den d r im er 5a
planes
planes
1
3
4
5
6
7
(a) Dichloromethane Solvate
2
8
1
53.07
51.33
134.37
118.47
117.02
123.83
114.64
112.7
64.23
67.56
5.4
0
(b) Tetrahydrofuran Solvate
122.56 121.24 111.06
2
1
51.01
62.61
APSS(364)) ∼ t (Figure 16C), where At is the absorbance
at 365 nm at irradiation time t. The content of cis-isomer
at the PSS and the corresponding apparent first-order
rate constants under 364 nm irradiation for various
dendrimers are listed in Table 3.
a
Planes 1 and 7 are the azobenzene’s rings. Planes 2 and 8
denote the central planes of the 2,3,4,5-tetraphenylphenyl units.
The other planes are arranged in a clockwise manner, with plane
3 having one hydrogen and one phenylene as neighbors. For the
assignment of the planes, see also Figure 12.
The reverse cis-trans isomerization of azobenzene-
cored polyphenylene dendrimers from the PSS obtained
upon 364 nm irradiation can be induced thermally or
by visible irradiation, for example at 447 nm. Visible
irradiation leads to a photostationary state, where the
content of the trans-isomer is greatly enhanced (Figure
17A and Table 3), whereas thermal isomerization
maximizes the absorbance at about 365 nm which
corresponds to the trans-isomer (Figure 17C).2,15 The
same isosbestic points are present during the whole
process, indicating a completely reversible and repro-
ducible photoisomerization.
isomer contents of the irradiated dendrimer solution can
be considered as invariant during the GPC measure-
ments.
For dendrimer AzoG2, the peak elution volume in-
creases from 34.17 mL before irradiation to 34.64 mL
after, corresponding to an average 38% ( 4% decrease
of the hydrodynamic volume.35 The contractions for
AzoG3 and AzoG4 upon UV irradiation are 37% ( 4%
and 24% ( 2%, respectively. Such a great photomodu-
lation of molecular size clearly shows that the confor-
mation of azobenzene-cored polyphenylene dendrimers
can be substantially altered when stimulated by light.
The reduction of the hydrodynamic volume for the
The apparent first-order rate constant for the
reverse reaction induced by visible irradiation and the
first-order rate constant for thermal isomerization
were also determined by a plot of ln(APSS(447) - At) ∼ t

Macromolecules, Vol. 36, No. 3, 2003
Rigid Polyphenylene Dendrimers 583
F igu r e 5. Generation 4 (8) prepared in 74% yield.
(Figure 17B) and ln(A0 - At) ∼ t (Figure 17D), respec-
tively, where APSS(447) is the absorbance at 365 nm of
the dendrimer solution at the PSS upon irradiation at
447 nm. The cis-isomer content at the PSS, calculated
by (A0 - APSS(447))/A0, and the (apparent) first-order
rate constants for the reverse isomerization are also
listed in Table 3. The photoresponsive behavior of
polyphenylene dendrimers with an azobenzene core is
dependent on their structures, whereas the azobenzene-
containing dendrimers with Fre´chet type dendritic arms
show no such structure dependence.15a,b From Table 3,
it can be seen that the cis-isomer content at the PSS
under 364 nm irradiation decreases with increasing
dendrimer generation and degree of branching, the
effect of the latter being more profound than the former.
This can be explained as a result of the increasing steric
effect of the dendrimer branches. In comparison, the cis
content at the PSS under 447 nm irradiation depends
only slightly on the bulkiness of the dendrimer arms.
The steric effect has only a slight influence on the rate
of the trans-cis photoisomerization and the reverse
photoreaction. The rate constant of the cis-trans ther-
mal isomerization for the first and second dendrimer
generations remains almost the same. However, it
increases for the third and fourth generations and
increases rapidly with an increasing degree of branching
(dendrimer AzoG2A4B relative to dendrimer AzoG2AB
and AzoG2). It is important to note that the reaction
rate of the thermal isomerization is much slower than
that of the photoisomerization induced by visible ir-
radiation, as shown in Table 3. In contrast to the
structure-dependent photoresponsive behavior of the
azobenzene-cored polyphenylene dendrimers studied in
this contribution, McGrath and co-workers have re-
ported only a negligible influence of the nature of the
attached dendritic fragment15d on the cis-trans thermal
isomerization of the poly(aryl ether)s which we believe
is due to the low generation and the flexibility of the
dendrimer fragments used in their study.

584 Grebel-Koehler et al.
Macromolecules, Vol. 36, No. 3, 2003
Con clu sion s
A series of polyphenylene dendrimers with an azoben-
zene core have been synthesized and characterized. The
azobenzene-cored dendrimers exhibit effective, revers-
ible, and reproducible photoresponsive properties upon
UV and visible irradiation. The photoresponsive behav-
ior of these dendrimers is highly dependent on their
structures. Because of the well-defined and rigid struc-
ture of polyphenylene dendrimer arms, the greatest
photomodulation of dendrimer molecular size has been
observed by switching a single central unit. This can
clearly be ascribed to the rigidity of the polyphenylene
dendrons and indicates that these dendrimers can be
substantially modulated by a light stimulus and express
the structural change at the periphery.
Exp er im en ta l Section
Sp ectr oscop y Mea su r em en ts. A dark-incubated azoben-
zene-cored polyphenylene dendrimer solution in toluene in a
quartz cuvette of 10 mm path length was irradiated by a
300-W xenon arc light through a band-pass filter (364 ( 7 nm)
to induce the trans-cis photoisomerization. A photostationary
state will be reached upon a certain time of UV irradiation.
The solution was then irradiated with visible light through
another band-pass filter (447 ( 17 nm) or kept in the dark to
induce the cis-trans reverse isomerization. The absorption
spectra of the dendrimer solution were recorded during the
whole process on a Perkin-Elmer Lamdba 40 UV/vis spectrom-
eter using toluene as reference. All reactions were carried out
at room temperature.
Gel P er m ea tion Ch r om a togr a p h y (GP C) Mea su r e-
m en ts. GPC was performed on a system equipped with a
Waters model 610 pump at a flow rate of 1.0 mL/min and a
Waters 484 absorbance detector (254 nm). µ-Styragel columns
with porosities of 500, 103, 104, and 105 Å were employed. THF
was used as mobile phase. Toluene was used as internal
standard.
Cr ysta l Str u ctu r e. The crystal structure determinations
were carried out on a Nonius KCCD diffractometer with
graphite monochromtated Mo KR radiation at 120 K. These
structures were solved by direct methods (Shelxs97). Refine-
ment was done with anisotropic temperature factors for all
non-hydrogen atoms. The hydrogen atoms were refined with
fixed isotropic temperature factors in riding mode. Files CCDC
189260, 189261, and 189262 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html
or from the Cambridge Crystallographic Data Centre. (Ad-
dress: 12 Union Road, Cambridge CB2 1EZ, U.K. Fax: (+44)-
1223-336-033. E-mail: deposit@ccdc.cam.ac.uk.)
Ch a r a cter iza tion . 1H and 13C NMR spectra were recorded
on a Bruker AMX250 spectrometer, a Bruker AC300 spec-
trometer, and a Bruker AMX500 NMR spectrometer by using
residual proton resonance of the solvent or the carbon signal
of the deuterated solvent as the internal standard. Chemical
shifts are reported in parts per million. For the 13C j-modulated
spin-echo NMR measurements the abbreviations Cquat and
Ctert represent quaternary C atoms and CH2 as well as CH3
and CH groups, respectively. FD mass spectra were performed
with a VG-Instruments ZAB 2-SE-FDP. MALDI-TOF mass
spectra were measured with a Bruker Reflex II and dithranol
as matrix (molar ratio dithranol/sample 250:1). The elemental
analysis was carried out by the Microanalytical Laboratory of
the University of Mainz, Mainz, Germany.
F igu r e 6. Synthetic route to the AB-branching unit: (i) (Pd-
(PPh3)2]Cl2, PPh3, CuI, THF/triethylamine, room temperature,
85%; (ii) 0.5 equiv of I2, DMSO, 155 °C, 83%; (iii) (Pd(PPh3)2]-
Cl2, PPh3, CuI, toluene/triethylamine, 80 °C, 76%; (iv) nBu4-
NOH, tert-butyl alcohol, 80 °C, 75%.
Ta ble 3. Ap p a r en t F ir st-Or d er Ra te Con sta n t k1 for
Tr a n s-Cis P h otoisom er iza tion u n d er 364 n m Ir r a d ia tion ,
Ap p a r en t F ir st-Or d er Ra te Con sta n t k2 for Cis-Tr a n s
P h otoisom er iza tion u n d er 447 n m Ir r a d ia tion ,
F ir st-Or d er Ra te Con sta n t k3 for Cis-Tr a n s Th er m a l
Isom er iza tion , th e Cis-Isom er Con ten t a t th e P SS u n d er
364 n m Ir r a d ia tion (C364), a n d th e Cis-Isom er Con ten t a t
th e P SS u n d er 447 n m Ir r a d ia tion (C447
)
dendrimer C364/% C447/% k1/min-1 k2/ min-1 k3 × 104/min-1
AzoG1
88
89
81
79
53
77
75
14
11
13
13
12
12
11
0.85
0.51
0.88
0.84
0.70
0.84
0.79
1.8
1.4
1.9
1.8
2.0
2.1
2.5
2.6
1.8
2.6
2.7
7.7
2.9
3.3
AzoG1hex
AzoG2AB
AzoG2
AzoG2A4B
AzoG3
AzoG4
4,4′-Diiod oa zoben zen e (1). 4-Iodoaniline (3 g, 13.7 mmol)
was dissolved in 80 mL of degassed dichloromethane under
an argon atmosphere. To this was added a finely ground
mixture of KMnO4 (5.1 g, 3.13 mmol) and CuSO4‚5H2O (5.1 g,
20.26 mmol). The reaction mixture was stirred at room
temperature for 3 days and afterward filtered through Celite.
The collected fractions were combined and the solvent was
evaporated under reduced pressure. The crude product was
The trans-cis isomerization of the azobenzene cored
polyphenylene dendrimers was followed by UV-vis
measurements. The resulting dramatic conformational
change of the entire dendrimer structure was apparent
in the GPC results as discussed above.

Macromolecules, Vol. 36, No. 3, 2003
Rigid Polyphenylene Dendrimers 585
F igu r e 7. Synthesis of 9 and 10: (i) 3 equiv 14/15, o-xylene, 142 °C; (ii) 2 and 8 equiv of nBu4NF, respectively, THF, room
temperature; (iii) 3 and 12 equiv of 12 respectively, o-xylene, 142 °C, 96% and 73%, respectively.
purified by column chromatography (silica gel, PE) to give 1
tography (silica gel, PE/CH2Cl2 1:1) to afford 4 as an orange
as an orange solid. Yield: 1.81 g (4.17 mmol), 61%.
solid. Yield: 232 mg (0.61 mmol), 88%.
1
1
Mp: 254-255 °C. H NMR (300 MHz, CD2Cl2, 300 K): δ )
Mp: 242-244 °C. H NMR (300 MHz, CD2Cl2, 300 K): δ )
3
3
7.89 (d, 4H; 3J ) 8.52 Hz); 7.66 (d, 4H; 3J ) 8.8 Hz). 13C NMR
(75 MHz, CD2Cl2, 300 K): δ ) 151.9, 138.6, 124.5, 98.1. MS
(FD 8 kV), m/z: 434.1 (100%, M+). Anal. Calcd for C12H8I2N2
(434.02): C, 33.21; H, 1.86; N, 6.45. Found: C, 33.14; H, 2.06;
N, 6.41.
7.95 (d, 4H; J 8.77 Hz); 7.70 (d, 4H; J 8.78 Hz); 7.60-7.57
(m, 4H); 7.42-7.39 (m, 6H). 13C j-modulated spin echo NMR
(125 MHz, CD2Cl2, 303 K): δ ) 155.39, 135.9, 135.2, 132.9,
132.2, 132.0, 129.7, 126.5, 126.4, 95.5, 92.5. MS (FD 8 kV),
m/z: 381.8 (100%, M+); 190.9 (10%, M2+). Anal. Calcd for
C
28H18N2 (382.47): C, 87.93; H, 4.74; N, 7.32. Found: C, 87.78;
4,4′-Bis(p h en yleth yn yl)a zoben zen e (4). 1 (300 mg (0.69
mmol) was dissolved in a mixture of 16 mL of triethylamine
and 8 mL of tetrahydrofuran. To this were added bis(tri-
phenylphosphine)palladium(II) dichloride (102 mg, 0.14 mmol),
copper(I) iodide (53 mg, 0.28 mmol), and triphenylphosphine
(73 mg, 0.28 mmol), and then the flask was evacuated and
flushed with argon. After the reaction was stirred for 10 min,
phenylacetylene (0.2 mL, 186 mg, 1.83 mmol) was injected into
the reaction mixture. The reaction mixture was stirred over-
night under argon atmosphere and then diluted with dichlo-
romethane and extracted with water. The organic phase was
dried over MgSO4, and the solvent was removed under reduced
pressure. The crude product was purified by column chroma-
H, 4.22; N, 6.92.
AzoG1 (5). 3 (120 mg, 0.52 mmol) and 12 (473 mg, 1.23
mmol) were reacted in 10 mL of degassed o-xylene at 170 °C
for 12 h under an argon atmosphere. The cooled reaction
mixture was poured into methanol and the crude product
collected by filtration. The filter was washed with pentane
until the filtrate became colorless. The crude product was
purified by column chromatography (silica gel, PE/CH2Cl2 2:1)
to afford 5 as an yellow amorphous powder. Yield: 451 mg
(0.48 mmol), 92%.
Mp: >300 °C. 1H NMR (250 MHz, CD2Cl2, 300 K): δ ) 7.67
(d, 4H; 3J ) 8.53 Hz); 7.58 (s, 2H); 7.29 (d, 4H; 3J ) 8.53 Hz);

586 Grebel-Koehler et al.
Macromolecules, Vol. 36, No. 3, 2003
F igu r e 10. Crystallographic structure of 4,4′-bis(phenyl-
ethynyl)azobenzene (4).
F igu r e 8. Synthetic route to 11: (i) 3 equiv of phenyl-
acetylene, (Pd(PPh3)2]Cl2, PPh3, CuI, THF/triethylamine,
room temperature, 88%; (ii) diphenyl ether, 195 °C, 62%.
F igu r e 11. Crystallographic structure of AzoG1 (5) crystal-
lized from (a) dichoromethane and (b) tetrahydrofuran.
941.9 [M]+; 470.7 [M]2+. Anal. Calcd for C72H50N2 (943.21): C,
91.69; H, 5.34; N, 2.97. Found: C, 90.96; H, 5.40; N, 2.87.
AzoG1h ex (11). 4 (120 mg, 0.31 mmol) and 12 (362 mg,
0.94 mmol) were reacted in 10 mL of degassed diphenyl ether
at 210 °C for 5 days under an argon atmosphere. The cooled
reaction mixture was poured into methanol and the crude
product collected by filtration. The filter was washed with
pentane until the filtrate became colorless. The crude product
was purified by column chromatography (silica gel, PE/CH2-
Cl2 3:1) to afford 11 as an yellow amorphous powder. Yield:
213 mg (0.19 mmol), 62%.
F igu r e 9. Crystallographic structure of 4,4′-diiodoazobenzene
(1).
7.15-7.09 (m, 8H); 6.89-6.78 (m, 32H). 13C j-modulated spin
echo NMR (75 MHz, CD2Cl2, 300 K): δ ) 150.9, 144.9, 142.0,
141.8, 141.0, 140.4, 140.1, 140.0, 139.9, 139.4, (Cquat); 131.6,
131.5, 131.1, 130.8, 130.0, 127.7, 127.1, 126.9, 126.7, 126.4,
125.8, 125.7, 125.5, 122.1, 120.1 (Ctert). MS (FD 8 kV), m/z:

Macromolecules, Vol. 36, No. 3, 2003
Rigid Polyphenylene Dendrimers 587
F igu r e 12. Assignment of the planes for the dihedral angles.
F igu r e 15. GPC chromatograms for dendrimers 6, 7, and 8
before (s) and after (‚‚‚) irradiation at 364 nm to the photo-
stationary state.
547.2 (23%, M2+). Anal. Calcd for C84H58N2 (1095.41): C, 92.11;
H, 5.34; N, 2.56. Found: C, 91.79; H, 5.13; N, 2.50.
AzoG2AB (9). AzoG1ethin2 (80 mg, 0.081 mmol) and 12 (68
mg, 0.177 mmol) were reacted in 10 mL of degassed o-xylene
at 170 °C for 12 h under an argon atmosphere. The cooled
reaction mixture was poured into methanol and the crude
product collected by filtration. The filter was washed with
pentane until the filtrate became colorless. The crude product
was purified by column chromatography (silica gel, PE/CH2-
Cl2 1:1) to afford 9 as an yellow amorphous powder. Yield: 132
mg (0.078 mmol), 96%.
Mp: >300 °C. 1H NMR (250 MHz, CD2Cl2, 300 K): δH
)
7.66 (d, 4H, 3J ) 8.77 Hz); 7.55 (s, 2H); 7.35, 7.31, (2s, 2H),
7.33 (d, 4H, 3J ) 8.39 Hz); 7.16-7.12 (m, 14H); 7,02-6.58 (m,
64H). 13C j-modulated spin echo NMR (75 MHz, CD2Cl2, 300
K): δ ) 151.9, 144.9, 144.8, 141.8, 141.7, 141.0, 140.9, 140.8,
140.7, 140.3, 140.3, 140.0, 139.9, 139.5, 139.4, 139.3, 139.3,
139.2, 139.1, 138.7, 138.3, 138.0 (Cquat); 131.5, 131.2, 131.1,
130.9, 130.8, 130.0. 128.6, 128.3, 127.7, 127.6, 127.6, 127.1,
127.1, 127.0, 126.8, 126.7, 126.5, 126.4, 126.4, 126.3, 126.0,
125.8, 125.6, 125.3, 122.1, 120.1 (Ctert). MS (FD 8 kV), m/z
[ue-1]: 1705.0 (100%, M+); 852.2 (36%, M2+); 568.2 (4%, M3+).
Anal. Calcd for C132H90N2 (1704.20): C, 93.03; H, 5.32; N, 1.64.
Found: C, 92.38; H, 5.65; N, 1.46.
F igu r e 13. Packing of 5 in the crystals: (a) dichoromethane
and (b) tetrahydrofuran.
AzoG2 (6). AzoG1ethin4 (115 mg, 0.11 mmol) and 12 (190
mg, 0.49 mmol) were reacted in 10 mL of degassed o-xylene
at 170 °C for 24 h under an argon atmosphere. The cooled
reaction mixture was poured into methanol and the crude
product collected by filtration. The filter was washed with
pentane until the filtrate became colorless. The crude product
was purified by column chromatography (silica gel, PE/CH2-
Cl2 1:1) to afford 6 as an yellow amorphous powder. Yield:
248.2 mg (0.1 mmol), 91%.
Mp: >300 °C. 1H NMR (250 MHz, CD2Cl2, 300 K): δH
)
7.64 (d, 4H, 3J ) 8.53 Hz); 7.53 (s, 2H); 7.42, 7.37 (2s, 4H);
7.25-7.07 (d, 4H; 3J ) 8.53 Hz; m, 32H); 6.98-6.65 (m, 76H);
6.56 (d, 4H, 3J ) 8.53 Hz); 6.50 (d, 4H, 3J ) 8.19 Hz). 13C
j-modulated spin echo NMR (75 MHz, CD2Cl2, 300 K): δ )
150.8, 144.8, 141.8, 141.7, 140.8, 140.7, 140.6, 140.4, 140.3,
140.2, 140.0, 139.9, 139.3, 139.2, 139.1, 138.9, 138.5, 138.0,
137.8 (Cquat); 131.6, 131.5, 131.0, 130.9, 130.6, 129.8, 128.6,
128.3, 127.6, 127.4, 127.0, 126.7, 126.7, 126.4, 126.3, 126.1,
125.8, 125.5, 125.2, 121.9 (Ctert). MS (FD 8 kV), m/z [ue-1]: 2466
[M]+. Anal. Calcd for C192H130N2 (2465.19): C, 93.55; H, 5.32;
N, 1.14. Found: C, 93.84; H, 5.47; N, 1.09.
F igu r e 14. Models of 5 (A) trans-isomer top view, (B) trans-
isomer side view, (C) cis-isomer side view.
AzoG2A4B (10). AzoG1ethin8 (50 mg (0.044 mmol) and 12
(149 mg, 0.38 mmol) were reacted in 10 mL of degassed
o-xylene at 170 °C for 48 h under an argon atmosphere. The
cooled reaction mixture was poured into methanol and the
crude product collected by filtration. The filter was washed
with pentane until the filtrate became colorless. The crude
product was purified by chromatography (silica gel, PE/CH2-
Mp: >300 °C. 1H NMR (250 MHz, CD2Cl2, 300 K): δH
)
3
3
7.29 (d, 4H; J ) 8.39 Hz); 6.96 (d, 4H; J ) 8.39 Hz); 6.91-
6.83 (m, 50H). 13C j-modulated spin echo NMR (75 MHz, CD2-
Cl2, 300 K): δ ) 149.9; 144.1, 140.7, 140.7, 140.7, 140.5, 140.4,
140.2, 139.7 (Cquat); 132.2, 131.4, 126.7, 126.6, 125.5, 125.3,
121.0 (Ctert). MS (FD 8 kV), m/z [ue-1]: 1095.5 (100%, M+);

588 Grebel-Koehler et al.
Macromolecules, Vol. 36, No. 3, 2003
F igu r e 16. (A) Plots of absorption spectra of ca. 10-5 M dendrimer 5 solution in toluene vs time under 364 nm irradiation for 0,
1, 2, 3, 4, 5, 6, 8, and 10 min (from top to bottom at 365 nm), respectively. (B) Plots of absorption spectra of ca. 10-5 M dendrimer
8 solution in toluene vs time under 364 nm irradiation for 0, 1, 2, 3, 4, 5, 6, 8, and 10 min (from top to bottom at 365 nm),
respectively. The dash line shows the absorption spectrum of a pure polyphenylene dendrimer of the same generation at the
same concentration. (C) Determination of the apparent first-order rate constant of dendrimer 5 trans-cis photoisomerization
under 364 nm irradiation.
Cl2 1:1) to afford 10 as an yellow amorphous powder. Yield:
128 mg (0.032 mmol), 73%.
122.1 (Ctert). MALDI-TOF-MS, m/z [ue-1]: 5510 [M]+. Anal.
Calcd for C432H290N2 (5509.14): C, 94.19; H, 5.31; N, 0.51.
Found: C, 93.90; H, 5.39; N, 0.55.
Mp: >300 °C. 1H NMR (250 MHz, CD2Cl2, 300 K): δH
)
7.73 (d, 4H, 3J ) 8.39 Hz); 7.52, 7.51, 7.50 (3s, 6H), 7. 43(d,
AzoG4 (8). AzoG3ethin16 (70 mg, 0.012 mmol) and 12 (219
mg, 0.57 mmol) were reacted in 10 mL of degassed o-xylene
at 170 °C for 48 h under an argon atmosphere. The cooled
reaction mixture was poured into methanol and the crude
product collected by filtration. The filter was washed with
pentane until the filtrate became colorless. The crude product
was purified by chromatography (silica gel, PE/CH2Cl2 1:1) to
afford 8 as an yellow amorphous powder. Yield: 102 mg (8.9
× 10-3 mmol), 73%.
3
4H, J ) 8.40 Hz); 7.21-7.10 (m, 44H); 7.01-7.68 (m, 140H);
3
3
6.58 (d, 4H, J ) 8.39 Hz); 6.47 (d, 4H, J ) 8.01 Hz); 6.45 (d,
4H, J ) 8.01 Hz). 13C j-modulated spin echo NMR (75 MHz,
3
CD2Cl2, 300 K): δ ) 150.9, 144.9, 142.0, 141.9, 141.9, 141.8,
141.3, 140.8, 140.8, 140.7, 140.6, 140.5, 140.3, 140.2, 140.1,
139.9, 139.6, 139.4, 139.4, 139.3, 139.2, 139.1, 139.0, 138.8,
138.6, 138.0, 137.9, 137.7 (Cquat), 131.6, 131.1, 130.8, 130.0,
129.4, 129.3, 128.8, 138.5, 127.6, 126.9, 126.6, 126.3, 125.6,
125.4, 122.2 (Ctert). MALDI-TOF-MS, m/z [ue-1]: 4091 [M +
Ag]+. Anal. Calcd for C312H210N2 (3987.17): C, 93.99; H, 5.31;
N, 0.70. Found: C, 93.89; H, 5.41; N, 0.64.
AzoG3 (7). AzoG2ethin8 (125 mg, 0.047 mmol) and 12 (173.6
mg, 0.452 mmol) were reacted in 10 mL of degassed o-xylene
at 170 °C for 48 h under an argon atmosphere. The cooled
reaction mixture was poured into methanol and the crude
product collected by filtration. The filter was washed with
pentane until the filtrate became colorless. The crude product
was purified by chromatography (silica gel, PE/CH2Cl2 1:1) to
afford 7 as an yellow amorphous powder. Yield: 242.1 mg
(0.044 mmol), 93%.
Mp: >300 °C. 1H NMR (250 MHz, CD2Cl2, 300 K): δH
)
3
7.65 (d, 4H, J ) 8.39 Hz); 7.53 (s, 2H); 7.42, 7.38, 7.37, 7.32
(4s, br, 28H); 7. 24 (d, 4H; 3J ) 8.39 Hz); 7.15-6.41 (m, 572H).
13C j-modulated spin echo NMR (75 MHz, CD2Cl2, 300 K): δ
) 150.9; 144.9, 141.9, 141.8, 141.3, 140.8, 140.7, 140.5, 140.5,
140.3, 140.1, 139.3, 139.2, 139.1, 138.8, 138.7, 138.4, 138.1
(Cquat); 131.6, 131.1, 130.0, 128.6, 128.3, 127.6, 126.8, 126.5,
126.3, 125.6, 125.3 (Ctert). MALDI-TOF-MS, m/z [ue-1]:
11597 [M]+. Anal Calcd for C912H610N2 (11597.04): C, 94.46;
H, 5.30; N, 0.24. Found: C, 94.02; H, 5.27; N, 0.22.
Mp: >300 °C. 1H NMR (250 MHz, CD2Cl2, 300 K): δH
)
Ack n ow led gm en t. The authors thank the DWTC
through IUAP-IV-11 and IUAP-V-03, the FWO (Flemish
Ministry of Education), the STWW through the
IWT project “Molecular Nanotechnology”, and the ESF
SMARTON for financial support. The collaboration was
made possible thanks to the TMR project SISITOMAS.
S.D.F. is a Postdoctoral Fellow of the Fund for Scientific
Research-Flanders (Belgium). The German Ministry of
7.65 (d, 4H, 3J ) 8.39 Hz); 7.53 (s, 2H); 7.42, 7.41 (2s, 4H);
3
7.38, 7.37 (2s, 6H); 7.33 (s, 2H); 7.26-7.03 (d, 4H; J ) 8.39
Hz; m, 82H); 7.00-6.62 (m, 308H); 6.56-6.43 (m, 24H). 13C
j-modulated spin echo NMR (75 MHz, CD2Cl2, 300 K): δ )
150.9, 144.9, 142.0, 141.8, 141.6, 141.3, 141.0, 140.9, 140.8,
140.7, 140.5, 140.4, 140.3, 140.1, 139.4, 139.3, 139.2, 139.1,
138.9, 138.7, 138.4, 138.1, 137.8 (Cquat); 131.6, 131.1, 130.8,
130.0, 128.6, 128.3, 127.6, 126.8, 126.5, 126.3, 125.6, 125.3,

Macromolecules, Vol. 36, No. 3, 2003
Rigid Polyphenylene Dendrimers 589
F igu r e 17. (A) Plots of absorption spectra of ca. 10-5 M dendrimer 5 solution in toluene (started from dendrimer solution at the
PSS under 364 nm irradiation) vs time under 447 nm irradiation for 0, 15, 30, 45, 60, 90, 120, 180, and 240 s (from bottom to top),
respectively. The dash line is the absorption spectrum of the dark-incubated dendrimer 5 solution. (B) Determination of the
apparent first-order rate constant of dendrimer 5 cis-trans photoisomerization upon irradiation at 447 nm. (C) Plots of absorption
spectra of ca. 10-5 M dendrimer 5 solution in toluene (started from dendrimer solution at the PSS under 364 nm irradiation) vs
time by thermal isomerization for 0, 1, 4, 6, 19, 29, 43, 68, 95, 144, 218, 311, 384, and 477 h (from bottom to top), respectively.
(D) Determination of the first-order rate constant of dendrimer 5 cis-trans thermal isomerization.
(10) Kumar, G. S.; Neckers, D. C. Chem. Rev. 1989, 89, 1915-
Education and Science (BMBF) is gratefully acknowl-
edged for financial support through the project “Den-
dritische Polyphenylene als funktionale Nanopartikel
in Sensorik und Katalyse” (No. 03C0299/7). This work
was also supported by the Volkswagenstiftung. Last but
not least C. Beer is thanked for synthesis support and
Dr. A. C. Grimsdale for valuable discussions.
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