Photoresponsive dendrimers

Article abstract of DOI:10.1039/a700292k

Benzyl aryl ether dendrimers with azobenzene central linkers undergo reversible cisltrans isomerization of the azobenzene moiety upon exposure to ultraviolet light.

Full text of DOI:10.1039/a700292k

View Article Online / Journal Homepage / Table of Contents for this issue
Photoresponsive dendrimers
Denise M. Junge and Dominic V. McGrath*†
Department of Chemistry, University of Connecticut, 215 Glenbrook Rd., Storrs, CT 06269-4060, USA
Benzyl aryl ether dendrimers with azobenzene central
linkers undergo reversible cis/trans isomerization of the
azobenzene moiety upon exposure to ultraviolet light.
by a decrease in the absorbance at 360 nm and an increase in
absorbances 314 and 450 nm (Fig. 1). A photostationary state
was reached within approximately 60 s. Thermal reversion to
the original dark-incubated spectrum was observed over the
The elegance with which nature uses light energy to alter
function in photoresponsive systems has led chemists to engage
in much mimicry.1 Progress has been made in designing
systems containing photoresponsive moieties in small mol-
ecules,2–4 polymers5–8 and peptides9–11 for the photocontrol of
ion recognition and transport,12 membrane permeability13 and
structure,14 enzyme activity2 and materials properties.5,8,15
Typically, small molecule systems are well-understood. Their
responsiveness is predictable and easily controlled. Macro-
molecular systems, however, while abundant and widely
studied, are typically ill-defined and the photoresponsive effect
is difficult to predict a priori or to control in a precise manner.
Dendrimers, a new class of macromolecule in which the
structure of the material is well-defined, are prepared in a way
which would allow precise placement of a photoactive moiety
within the dendrimer interior. More predictable control of
photoinduced configurational changes should be possible,
allowing potential reversible alteration of function. By placing
a single photoactive moiety in the centre, rather than on the
exterior,16 of a dendrimer a small configurational change will
effect a large conformational change throughout the dendrimer
structure. Here we report the synthesis of dendrimers with
azobenzene central linkers which undergo reversible configura-
tional changes in response to light energy. This represents the
first reversible topological change induced in a dendrimer
through configurational change of the central linker.
O
O
O
O
O
O
O
O
O
O
N
N
O
O
trans–3c
dark hν
O
O
N
N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
cis–3c
Photoresponsive dendrimers of the zeroth, first and second
generations were prepared by reaction of azobenzene-derivative
central linkers 117 and 2‡ with the appropriate dendritic
bromides18 in the presence of K2CO3 in acetone (Scheme 1).§
Zeroth and first generation materials 3a and b and 4a and b are
pale orange crystalline solids, while second generation materi-
als 3c and 4c are orange glasses.¶
Dendrimers 3a–c and 4a–c all exhibited the expected
photoresponsive behaviour. For example, dark incubation of a
dichloromethane solution (40 mm) of second generation den-
drimer 3c served to maximize the absorbtion at 360 nm
(emax = 32500) corresponding to the trans-azobenzene chro-
mophore. Irradiation of this solution with 350 nm light resulted
in clean photoisomerization to cis-3c (Scheme 2), as evidenced
Scheme 2
1.6
1.2
0.8
[Gn]-Br
R'O
N
HO
N
N
OR
K2CO3
N
OR''
acetone
1 R = H
0.4
0.0
3a R' = R'' = [G0]
b R' = R'' = [G1]
2 R = Me
BnO
c R' = R'' = [G2]
[G0] = Bn
BnO
4a R' = [G0]; R'' = Me
b R' = [G1]; R'' = Me
c R' = [G2]; R'' = Me
BnO
O
[G1] =
[G2] =
BnO
300
400
500
O
BnO
λ / nm
Fig. 1 UV Absorption spectra of second generation photoresponsive
dendrimer 4c (40 mm in CH2Cl2) under irradiation conditions (350 nm; 0, 5,
10, 15, 20, 25, 30 and 60 s)
BnO
Scheme 1
Chem. Commun., 1997
857

View Article Online
§ Representative dendrimer preparation (3c): a solution of 1 (54 mg, 0.25
mmol), [G2]-Br (ref. 18) (0.45 g, 0.55 mmol), K2CO3 (89 mg, 0.65 mmol)
and dry acetone (10 ml) was kept at reflux for 24 h. After removal of the
solvent by rotary evaporation, the resulting solid residue was partitioned
between H2O (20 ml) and CH2Cl2 (20 ml). The organic layer was separated,
the aqueous layer was further extracted with CH2Cl2 (3 3 20 ml) and the
combined organic layers were dried (Na2SO4) and concentrated. Flash
chromatography of the residue (SiO2, 1:1 CH2Cl2–light petroleum gradient
to CH2Cl2) gave dendrimer 3c (0.34 g, 80%) as an orange glassy solid.
¶ Satisfactory elemental analyses and spectroscopic data were obtained for
all new compounds reported. Selected data for 3c: glassy solid; 1H NMR
[400 MHz, (CD3)2CO]: d 7.83 (d, J 9.1 Hz, 4 H), 7.46–7.30 (m, 40 H), 7.13
(d, J 9.1 Hz, 4 H), 6.76 (d, J 2.3 Hz, 4 H), 6.74 (d, J 2.3 Hz, 8 H), 6.61 (t,
J 2.3 Hz, 6 H), 5.17 (s, 4 H), 5.09 (s, 16 H), 5.07 (s, 8 H); 13C NMR [100
MHz, (CD3)2CO]: d 160.6, 160.2, 160.1, 147.2, 139.2, 139.1, 139.0, 136.8,
128.6, 128.0, 127.5, 124.4, 115.1, 106.4, 101.6, 70.13, 70.11, 70.0 (Calc. for
C110H94N2O14: C, 79.20; H, 5.68; N, 1.68. Found: C, 78.83; H, 5.78; N,
1.54%). For 4c: glassy solid; 1H NMR [400 MHz, (CD3)2CO]: d 7.87 (d, J
9.1 Hz, 2 H), 7.84 (d, J 9.1 Hz, 2 H), 7.46–7.30 (m, 20 H), 7.14 (d, J 9.0 Hz,
2 H), 7.08 (d, J 9.0 Hz, 2 H) 6.80 (d, J 2.3 Hz, 2 H), 6.76 (d, J 2.3 Hz, 4 H),
6.62 (t, J 2.4 Hz, 3 H), 5.17 (s, 2 H), 5.09 (s, 8 H), 5.07 (s, 4 H), 3.89 (s, 3
H); 13C NMR [100 MHz, (CD3)2CO]: 161.6, 160.6, 160.2, 160.1, 147.2,
147.1, 139.2, 139.0, 136.8, 128.6, 128.0, 127.6, 124.4, 124.3, 115.1, 114.2,
106.4, 106.3, 101.7, 101.6, 70.1, 70.084, 70.0, 55.5 (Calc. for C62H54N2O8:
C, 77.96; H, 5.70; N, 2.93. Found: C, 78.01; H, 5.89; N, 2.92%).
1.0
0.5
0
–2
–4
A
A
0
10000
t / s
0
5000
10 000
t / s
Fig. 2 Plot of absorbance at 360 nm of a sample of 4c kept in the dark at 293
K after irradiation (10 min at 350 nm). Inset: first order rate constant plot of
In[AH 2 A] vs. time.
course of approximately 3 h at 293 K in the dark (Fig. 2). The
first-order rate constant for this process (k = 3 3 1024 s21
;
∑ Selected data for cis-3c: 1H NMR (400 MHz, CDCl3): d 7.40–7.29 (m, 20
H), 6.78–6.74 (m, 8 H), 6.64 (d, J 2.3 Hz, 4 H), 6.60 (d, J 2.0 Hz, 2 H), 6.55
(t, J 2.8 Hz, 2 H), 6.52 (t, J 2.4 Hz, 1 H), 5.01 (s, 8 H), 4.945 (s, 4 H), 4.938
(s, 2 H), 3.74 (s, 3 H).
t1/2 = 40 min) is similar in magnitude to that of other
azobenzenes.19 Exposure to bright sunlight for a period of only
several seconds also effected almost complete reversion to the
trans isomer. Second generation compound 4c serves as a steric
control for 3c. After isomerization to cis-4c (350 nm, 10 min),
thermal isomerization to trans-4c occurs with an identical first-
order rate constant (k = 3 3 1024 s21; t1/2 = 40 min). This
indicates that there is no strong steric influence on the thermal
isomerization of cis- to trans-3c.
References
1 I. Willner and S. Rubin, Angew. Chem., Int. Ed. Engl., 1996, 35, 367.
2 P. R. Westmark, J. P. Kelly and B. D. Smith, J. Am. Chem. Soc., 1993,
115, 3416.
3 K. Murata, M. Aoki, T. Nishi, A. Ikeda and S. Shinkai, J. Chem. Soc.,
Chem. Commun., 1991, 1715.
4 For leading references, see S. Shinkai, A. Yoshioka, H. Nakayama and
O. Manabe, J. Chem. Soc., Perkin Trans. 2, 1990, 1905.
5 M. Irie, Pure Appl. Chem., 1990, 62, 1495.
6 For a review, see F. Ciardelli, O. Pieroni, A. Fissi, C. Carlini and
A. Altomare, Br. Polym. J., 1989, 21, 97.
7 F. Ciardelli, C. Carlini, R. Solaro, A. Altomare, O. Pieroni, J. L. Houben
and A. Fissi, Pure Appl. Chem., 1984, 56, 329.
8 M. Mu¨ller and R. Zentel, Macromolecules, 1996, 29, 1609.
9 For leading references, see A. Fissi, O. Pieroni, G. Ruggeri and
F. Ciardelli, Macromolecules, 1995, 28, 302.
10 F. Ciardelli, D. Fabbri, O. Pieroni and A. Fissi, J. Am. Chem. Soc., 1989,
111, 3470.
11 O. Pieroni, A. Fissi, J. L. Houben and F. Ciardelli, J. Am. Chem. Soc.,
1985, 107, 2990.
The photoinduced conversion of trans- to cis-dendrimer was
also observed by 1H NMR spectroscopy. For example, the
methoxy groups in trans- and cis-4c appear as singlets at d 3.87
and 3.74, respectively, in CDCl3. Irradiation of an NMR sample
(CDCl3) of 4c resulted in a dramatic change in the ratio of these
peaks, from 87:13 to 8:92. Similar changes were observed in
the aromatic and benzylic regions of the spectrum.∑
We have demonstrated the photoresponsive properties of a
series of simple dendrimers with azobenzene central linkers.
We anticipate that switchable dendrimers of this type will have
applications in transport systems based on the reversible
perturbation of their ability to encapsulate small molecules. We
hope to demonstrate these applications in our future studies.
The present work was supported by the donors of The
Petroleum Research Fund, administered by the American
Chemical Society, and the University of Connecticut Research
Foundation. We thank B. Smith, J. Bocarsly, C. Kumar and G.
Epling for suggestions and assistance.
12 S. Shinkai, T. Nakaji, T. Ogawa, K. Shigematsu and O. Manabe, J. Am.
Chem. Soc., 1981, 103, 111.
13 T. Kinoshita, M. Sato, A. Takizawa and Y. Tsujita, Macromolecules,
1986, 19, 51.
14 J. J. Effing and J. C. T. Kwak, Angew. Chem., Int. Ed. Eng., 1995, 34,
88.
Footnotes
¨
15 T. Oge and R. Zentel, Macromol. Chem. Phys., 1996, 197, 1805.
16 For a dendrimer with azobenzene moieties on the exterior which
† E-mail: dmcgrath@nucleus.chem.uconn.edu
undergoes
a photoinduced size change, see H.-B. Mekelburger,
‡ Preparation of 2: a solution of NaNO2 (0.52 g, 7.5 mmol) in H2O
(minimum to dissolve) was added dropwise to a cold (5 °C) solution of
4-methoxyaniline (0.89 g, 7.2 mmol) and 9 m H2SO4 (4 ml). The resulting
solution was added dropwise to a cold (5 °C) solution of phenol (0.70 g, 7.4
mmol) and 2 m NaOH (10 ml). After stirring for an additional 1.5 h, the
solution was neutralized (2 m NaOH), extracted with CH2Cl2 (3 3 50 ml)
and the combined organic extracts were dried (Na2SO4) and concentrated.
Chromatography (SiO2, 1:3 ethyl acetate–light petroleum) gave 2 (1.06 g,
65%) as a yellow crystalline solid, mp 141.5–142.5 °C (lit.,20 142 °C); 1H
NMR [400 MHz, (CD3)2CO]: d 8.98 (s, 1 H), 7.85 (d, J 9.0 Hz, 2 H), 7.80
(d, J 8.9 Hz, 2 H), 7.07 (d, J 9.0 Hz, 2 H), 6.99 (d, J 8.9 Hz, 2 H), 3.88 (s,
3 H).
K. Rissanen and F. Vo¨gtle, Chem. Ber., 1993, 126, 1161.
17 E. R. Atkinson, H. J. Lawler, J. C. Heath, E. H. Kimball and E. R. Read,
J. Am. Chem. Soc., 1941, 63, 730.
18 C. J. Hawker and J. M. J. Fre´chet, J. Am. Chem. Soc., 1990, 112,
7638.
19 S. Shinkai, T. Nakaji, Y. Nishida, T. Ogawa and O. Manabe, J. Am.
Chem. Soc., 1980, 102, 5860.
20 M. Krause, Ber. Dtsch. Chem. Ges., 1899, 32, 124.
Received in Cambridge, UK, 13th January 1997; Com.
7/00292K
858
Chem. Commun., 1997