Design and synthesis of photoresponsive poly(benzyl ester) dendrimers with all-azobenzene repeating units

Article abstract of DOI:10.1021/ol0165591

equation presented A novel class of dendrons and dendrimers containing all-azobenzene units (up to 29 azobenzene groups) was designed and synthesized by the convergent method using a protected orthogonal AB₂ monomer (10) and tetra-functionalized core (11) as building blocks. The preparation of the key monomer (10) involved condensation of a protected nitroso compound (9) and 5-aminoisophthalic acid. These are the first examples of photoresponsive dendrimers with all-azobenzene repeating units.

Full text of DOI:10.1021/ol0165591

ORGANIC
LETTERS
2001
Vol. 3, No. 24
3831-3834
Design and Synthesis of
Photoresponsive Poly(benzyl ester)
Dendrimers with all-Azobenzene
Repeating Units
Shuangxi Wang and Rigoberto C. Advincula*
Department of Chemistry, UniVersity of Alabama at Birmingham,
Birmingham, Alabama, 35294
gobet@uab.edu
Received August 10, 2001
ABSTRACT
A novel class of dendrons and dendrimers containing all-azobenzene units (up to 29 azobenzene groups) was designed and synthesized by
the convergent method using a protected orthogonal AB2 monomer (10) and tetra-functionalized core (11) as building blocks. The preparation
of the key monomer (10) involved condensation of a protected nitroso compound (9) and 5-aminoisophthalic acid. These are the first examples
of photoresponsive dendrimers with all-azobenzene repeating units.
Azobenzene dyes have long been investigated because of
their interesting cis-trans photoisomerization phenomena.
Ultrathin films containing azo dyes are of interest for optical
applications such as inducing control in LC molecules
(command layer effects), holographic surface relief gratings,
optical storage media, nanoscale applications, etc.1 For
10.1021/ol0165591 CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/03/2001

example, Gibbons and co-workers2 have shown the potential
for anisotropic optical display control by photoinduced
alignment of azo dyes on a spin-coated polymer matrix.
Dendrimers have been the subject of intensive investiga-
tion as a result of both their unique physical properties and
structures.3 Outstanding features of dendrimers are a highly
branched structure, a monodispersed molecular weight, a
globular and symmetrical conformation, and a high density
of peripheral functionalities. As macromolecules, they can
incorporate photoactive,4 electroactive,5 and recognition
elements6 into their architectures at different positions with
properties very different from those of linear polymers or
small molecules. The incorporation of a photochromic moiety
in dendritic architectures is very attractive because of the
possibility of creating new optical materials7 and optical
devices.8 McGrath9 and others10 have prepared dendritic
macromolecules with photochromic azobenzene units in
dendrimer exterior and interior architectures to obtain photo-
switchable systems. However, only two examples of den-
drons with the azobenzene groups in and throughout the
architecture are known.11 Their primary focus was on
obtaining organic nonlinear optical materials.
shape and size can be changed upon irradiation with UV
light. We are interested in the formation of well-defined
domain structures of azobenzene dyes with unique photo-
isomerization and photoalignment properties compared to
those of individual dyes, side chain polymers, and domain
aggregates.1,8 It will be interesting to observe how chro-
mophoric groups on the dendrimer can be influenced by
cooperative motion to change the “global” shape of the
individual dendrimers to anisotropic configurations, e.g.,
spherical to ellipsoidal, etc.12
Our synthesis starts with the photochromic G-1-OH and
AB2 monomer 4, designed and synthesized as shown in
Scheme 1. G-1-OH was derived from the nitroso compound
Scheme 1. Synthetic Route for G-1-OH and the AB2
Monomer 4
In this paper, we present the first convergent synthesis of
a novel photoresponsive all-azobenzene dendrimer from
monodendrons with orthogonal azobenzene groups. Our goal
is to create an “intelligent” macromolecule whose molecular
(1) Ichimura, K. Photochromism: Molecules and Systems; Durr, H.,
Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; p 903.
(2) Gibbons, W. M.; Shannon, P. J.; Sun, B. J. Nature 1991, 351, 49.
(3) See the following reviews: (a) Tomalia, D. A.; Naylor, A. M.;
Goddard, W. A. Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (b) Frechet,
J. M. Science 1994, 263, 1710. (c) Zeng, F.; Zimmerman, S. C. Chem ReV.
1997, 97, 1681 (d) Bosman, A. W.; Janssen H. M.; Meijer, E. W. Chem.
ReV. 1999, 99, 1665.
(4) (a) Balzani, V.; Campagna, S.; Denti, G.; Juris, A.; Serroni, S.;
Venturi, M. Acc. Chem. Res. 1998, 31, 26. (b) Wang, P.-W.; Liu, Y.-J.;
Devadoss, C.; Bharathi, P.; Moore, J. S. AdV. Mater. 1996, 8, 237.
(5) (a) Miller, L. L.; Duan, R. G.; Tully, D. C.; Tomalia, D. A. J. Am.
Chem. Soc.1997, 119, 1005. (b) Tanaka, S.; Iso, T.; Doke, Y. Chem.
Commun. 1997, 2603.
(6) (a) Zimmerman, S. C.; Wang, Y.; Bharathi, P.; Moore, J. S. J. Am.
Chem. Soc. 1998, 120, 2172 and references therein. (b) Newkome, G. R.;
He, E.; Godinez, L. A. Macromolecules 1998, 31, 4382. (c) Gitsov, I.;
Lambrych, K. R.; Remnant, V. A.; Practitto, R. J. Polym. Sci., Part A:
Polym. Chem. 2000, 389, 2711. (e) Vetter, S.; Koch, A.; Schlu¨ter, A. D. J.
Polym. Sci., Part A: Polym. Chem. 2001, 39, 1940.
(7) (a) Stewart, G. M.; Fox, M. A. J. Am. Chem. Soc. 1996, 118, 4354.
(b) Adronov, A.; Gilat, S. L.; Fre´chet, J. M. J.; Ohta, K.; Neuwahl, F. V.
R.; Fleming, G. R. J. Am. Chem. Soc. 2000, 122, 1175. (c) Gilat, S. L.;
Adronov, A.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 1999, 38, 1422-
1427. Zhu, A.; Bharathi, P.; Drickamer, H. G.; Moore, J. S. J. Polym. Sci.,
Part A: Polym. Chem. 2001, 39, 2859. (f) Andronov, A.; Robello, D. R.;
Fre´chet, J. M. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 1366. (g)
Wang, J.; Jia, X.; Zhong, H.; Wu, H.; Li, Y.; Xu, X.; Li, M.; Wei, Y. J.
Polym. Sci., Part A: Polym. Chem. 2000, 38, 4174.
(8) Rochon, P.; Bissonnette, D.; Natansohn, A.; Xie, S. Appl. Opt. 1993,
32, 7277.
(9) (a) Junge, D. M.; McGrath, D. V. Chem. Commun. 1997, 857. (b)
Junge, D. M.; McGrath, D. V. J. Am. Chem. Soc. 1999, 121, 4912. (c) Li,
S.; McGrath, D. V. J. Am. Chem. Soc. 2000, 122, 6795.
(10) (a) Archut, A.; Azzellini, G. C.; Balzani, V.; De Cola, L.; Vo¨gtle,
F. J. Am. Chem. Soc. 1998, 120, 12187 and references herein. (b) Jiang,
D.-L.; Aida, T. Nature 1997, 388, 454-456. (c) Schenning, A. P. H. j.;
Elissen-Roman, C.; Weener, J.-W.; Baars, M. W. P. L.; van der Gaast, S.
j.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 8199. (c) Tsuda, K.; Dol,
G. C.; Gensch, T.; Hofkens, J.; Latterini, L.; Weener, J. W.; Meijer, E. W.;
De Schryver, F. C. J. Am. Chem. Soc. 2000, 122, 3445.
3, which was made from the diethyl ester nitro compound
2. The nitro compound was prepared by esterification of
5-nitroisophthalic acid, 1. This esterification with ethanol in
the presence of catalytic concentrated H2SO4 under reflux
afforded the diethyl ester 2 at 88% yield. Reduction of 2
was accomplished using Zn dust at 33-35 °C to form a
hydroxylamine intermediate. This was followed by oxidation
of the hydroxylamine intermediate with FeCl3 in one pot to
give the nitroso compound 3 with an overall 47% yield.
Condensation of 3 with 4-aminobenzyl alcohol in CH2Cl2
catalyzed by acetic acid gave the first generation azo-dendron
G-1-OH in a good yield of 86%. Saponification of the G-1-
OH with aqueous KOH in refluxing mixture of EtOH
afforded the AB2 monomer 4 in 84% yield. Bromination of
the G-1-OH with CBr4/Ph3P gave the bromomethyl com-
pound 5. The crystal structure of G-1-OH was determined
by X-ray diffraction (Figure 1).
The Generation 2 dendron (G-2-OH) was obtained by
refluxing of the AB2 monomer 4 and benzyl bromide 5 in
(11) Yokoyama, S.; Nakahama, T.; Otomo, A.; Mashiko, S. Chem. Lett.
1997, 1137. Yokoyama, S.; Nakahama, T.; Otomo, A.; Mashiko, S. J. Am.
Chem. Soc. 2000, 122, 3174.
(12) Adia, T.; Jiang, D. L.; Yashima, E.; Okamoto, Y. Thin Solid Films
1998, 331, 254.
3832
Org. Lett., Vol. 3, No. 24, 2001

10, which contained two activated carboxylic groups for
generation growth and a hydroxymethyl group protected by
tert-butyldiphenylsilane chloride (TBDPSCl) for next gen-
erational growth as outlined in Scheme 3. 4-Nitrobenzyl
Scheme 3. Synthesis of a Protected AB2 Monomer (10)
alcohol 7 was treated with tert-butyldiphenylsilane chloride
in the presence of imidazole in DMF to afford the protected
silyl ether compound 8 in an excellent 96% yield. Reduction
of 8 was similar to the preparation of nitroso compound 3
and followed by oxidation yielded nitroso compound 9 in a
good 82% yield.
Figure 1. The crystal structure of G-1-OH.
Condensation of the nitroso compound 9 with 5-amino-
isophthalic acid using acetic acid as solvent produced the
new AB2 monomer 10 in 90% yield as an orange solid. With
the new AB2 monomer 10, the third and fourth generation
dendrons (G-3-OH and G-4-OH) based on G-2-OH and the
monomer 10 were smoothly obtained following the conver-
gent synthetic approach developed by Fre´chet and Hawker.14
This involved an iterative and alternating sequence of DCC
(dicyclohexyl carboxycardodiimide)/DPTS (4-dimethylamino
pyridinium-4-toluenesulfonate) mediated esterifications15 and
deprotection by HF-pyridine as shown in Scheme 4. The
reactions were selected because they are known to be
extremely mild and fast and afford high conversions. Thus,
the coupling of the G-2-OH alcohol with the AB2 monomer
10 in the presence of DCC and catalytic amount of DPTS,
followed by deprotection with HF-pyridine, proceeded
cleanly and efficiently. The reaction gave the G-3-OH
dendron in a good yield of 78% after purification. For
deprotection of the alcohol group, initial attempts to desilylate
using TBAF (tetrabutylammonium fluoride) proved unsuc-
cessful. This reaction gave complicated mixtures attributable
to cleavage of the ester linkages. We also tried to use a
THF-HCl (aq) system to desilylate. Under this reaction
condition the protecting group could not be completely
removed at room temperature and the product tends to
decompose at higher temperatures. These two steps were
methodically repeated to convert the G-3-OH to G-4-OH
with a 56% overall yield. In addition, perfect all-azobenzene
dendrimers based on poly(benzyl ester) monodendrons were
prepared by coupling different generation monodendrons
with a tetra-functionalized azobenzene core, 11, using the
the presence of potassium carbonate and 18-crown-6 with
an 84% yield, forming an organic solid (Scheme 2). Unlike
Scheme 2. Synthesis of Dendron G-2-OH
the preparation of 5, the G-2-OH could not be brominated
by CBr4/PPh3, even when using an excess of CBr4/PPh3 at
higher temperature. Also, an array of brominating agents13
(PBr3,13a PPh3/NBS,13b NBS/Me2S,13c Me3SiBr,13d and Me3SiBr/
2,6-di-tert-butylpyridine) consistently gave unsatisfactory
results. It is unclear why these reactions do not work. A
similar phenomenon has been observed in other literature.13e
So we turned to design and synthesize the AB2 monomer
(13) (a) Taschner, M. J. In Encyclopedia of Reagents for Organic
Synthesis; Paquette, L A., Ed.; Wiley: New York, 1995; Vol. 8, pp 5364-
5366. (b) Bose, A. K., Lal, B. Tetrahedron Lett. 1972, 3937. (c) Firouzabadi,
H.; Ghaderi, E. Tetrahedron Lett. 1978, 839. (d) Jung, M. E.; Hatfield, G.
L. Tetrahedron Lett. 1978, 4483. (e) Laufersweile, M. J.; Rohde, J.;
Chaumette, D. S.; Parquette, J. R. J. Org. Chem. 2001, 66, 6440.
(14) Hawker, C. J.; Fre´chet, J. M. J. Am. Chem. Soc. 1990, 112, 7638.
(15) (a) Moore, J. S.; Stupp, S. I. Macromolecules 1990, 23, 65. (b)
Hawker, C. J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1992, 114, 8405.
Org. Lett., Vol. 3, No. 24, 2001
3833

Scheme 4. Synthesis of the Dendrons via a Convergent
Scheme 5. Synthesis of the all-Azobenzene Dendrimers
Approach
groups was consistently around 5.53 ppm, while those of
OCH2- and -CH3 of ethyl groups were around 4.45 and
1.44 ppm, respectively. The relative integrations for these
three areas were a useful diagnostic tool for characterizing
the generation of the dendrons and dendrimers. MALDI-
TOF mass spectra confirmed that all the dendrons and
dendrimers obtained are of high purity and essentially
monodispersed. Their m/z values are identical to the correct
molecular weights as expected for their structures.
In conclusion, all-azobenzene dendrimers bearing up to
29 azobenzene units were first successfully synthesized by
coupling synthetic monodendrons in a convergent approach.
These dendrons and dendrimers were constructed by using
orthogonal azobenzene groups as repeating units and benzyl
ester bonds as linkers. Thus, a new class of macrodyes was
synthesized. These photoresponsive dendrons and dendrimers
will be systematically investigated for their photoisomeriza-
tion, photoalignment, and photochromic changes both as thin
films and in different matrices, e.g. polymers and solution.
The progress will be reported in due course.
same coupling conditions described above, Scheme 5. This
compound 11 was obtained from the reductive coupling of
5-nitroisophthalic acid with Zn/NaOH in a refluxing mixture
of EtOH and water in 62% yield.
Acknowledgment. We wish to acknowledge financial
support from the Petroleum Research Fund, administered by
the American Chemical Society (ACS PRF 35036-G7), Prof.
Gary Gray (X-ray structure), and the Chemistry Department
Research Staff of University of Alabama at Birmingham for
technical support.
Thus, coupling dendrons G-1-OH, G-2-OH, and G-3-OH
with the tetra-functionalized core 11 in the presence of DCC/
DPTS afforded the all-azobenzene dendrimers G-1-4, G-2-
4, and G-3-4 in 82%, 54%, and 38% yields, respectively
(Scheme 5). The synthetic azobenzene dendrons and den-
drimers were characterized by NMR, MALDI-TOF MS, and
Supporting Information Available: Experimental details
and characterization data for all the compounds reported. This
material is available free of charge via the Internet at
http://pubs.acs.org.
1
elemental analysis (Supporting Information). The H NMR
shift of the ethyl groups at the peripheries of the dendrons
and dendrimers and benzyl methyl groups were useful in
the characterization. The chemical shift of benzyl methyl
OL0165591
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Org. Lett., Vol. 3, No. 24, 2001