Dendritic fullerenes (C60) with photoresponsive azobenzene groups

Article abstract of DOI:10.1016/S0040-4039(02)00994-2

Fullerene-cored dendrimers bearing up to eight photoisomerizable azobenzene groups in the periphery have been synthesized as potential photoswitches and spectroscopically characterized.

Full text of DOI:10.1016/S0040-4039(02)00994-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 5053–5056
Dendritic fullerenes (C60) with photoresponsive azobenzene
groups
Kwang-Yol Kay,a,* Ki-Jong Han,a Yong-Jae Yua and Young Dong Parkb
aDepartment of Molecular Science and Technology, Ajou University, Suwon 442-749, South Korea
bDepartment of Chemistry, Ajou University, Suwon 442-749, South Korea
Received 30 April 2002; revised 24 May 2002; accepted 27 May 2002
Abstract—Fullerene-cored dendrimers bearing up to eight photoisomerizable azobenzene groups in the periphery have been
synthesized as potential photoswitches and spectroscopically characterized. © 2002 Elsevier Science Ltd. All rights reserved.
Design and synthesis of dendrimers with multifunction-
ality has attracted much attention due to their unlim-
ited possibility for fundamental new discoveries and
practical applications.1–15 Among the various electro-
and photoactive chromophores utilized for dendrimer
chemistry, both fullerene (C60) and azobenzene groups
appear to be versatile building blocks, and at present a
growing interest is developing in fullerene-
functionalized14,16–24 or azobenzene-functionalized25–29
dendrimers. In addition, Sano and Shinkai et al.
reported on synthesis and film properties of a C60–
azobenzene derivative with an ammonium group to
obtain stable monolayers.30 In line with these aspects,
we report herein the synthesis of fullerene-cored den-
dritic molecules 1, 2 and 3 bearing up to eight photoiso-
merizable azobenzene groups in the periphery as
potential photoswitches.
Photoresponsive dendrimers 1, 2 and 3 of the first,
second and third generations were prepared by a typical
Fre´chet’s convergent method31 in which the reaction of
azobenzene derivatives [Gn]-Br with the monomer 3,5-
dihydroxybenzyl alcohol repeats for the dendrons of
higher generations, and then the appropriate dendritic
azides [Gn]-N3 react with C60 in chlorobenzene at the
final step17 to give the corresponding dendritic fullere-
Dendritic fullerenes 1, 2 and 3 are brown-colored solids
and prove to be soluble in a variety of organic solvents,
unlike C60.32 They were characterized by 1H, 13C NMR,
FAB or MALDI-TOF Mass and elemental analysis.33
1H NMR spectra of 1, 2 and 3 showed very similar
patterns of the resonance lines, which shifted downfield
from the corresponding resonance lines of the precursor
azides 9, 12 and 15 due to the electron withdrawing
influence of the fullerene sphere. The resonances for the
exterior azobenzene groups occur at 6.96–7.88 ppm, the
nes 1, 2 and 3, respectively. Every step of the reaction
sequence proceeded smoothly and efficiently to give a
good or moderate yield of the product (Schemes 1–3).
Keywords: fullerene; azobenzene; dendrimer; absorption spectrum;
UV-irradiation; trans–cis isomerization; polarized optical microscopy.
* Corresponding author. Tel.: 82-31-219-2602; fax: 82-31-219-1615;
e-mail: kykay@madang.ajou.ac.kr
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00994-2

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K.-Y. Kay et al. / Tetrahedron Letters 43 (2002) 5053–5056
peaks for the aromatic protons of the monomer units
appear in the region 6.40–6.77 ppm and the resonances
for the methylene protons occur in the region 4.80–5.20
ppm (Ar-O-CH2Ar), 4.37–4.47 ppm (N-CH2) and 4.01–
4.02 ppm (azobenzene-O-CH2). Other alkyl chain protons
show the resonances in the region 0.98–1.78 ppm. 13C
NMR spectra of 1, 2 and 3 showed almost the same
chemical shifts. A set of fullerene nucleus peaks appears
in the aromatic region between 133 and 148 ppm and a
resonance line of CH2–N occurs at 68 ppm (Fig. 1). These
peaks correspond to the open annulene structure of a
monosubstituted C60 azafulleroid and agree with the
results of Prato and Wudl34 and Hawker.17 The remainder
of the resonances in the spectrum are due to the aromatic
monomer (101–108 and 114–131 ppm) and azobenzene
carbons (114–162 ppm) of the dendritic units, appearing
as distinct peaks with relatively higher intensity than the
peaks of fullerene carbons. The resonances for the
aliphatic carbons (O-CH2 and n-butyl) appear at 70, 31,
19 and 14 ppm.
Further confirmation of the hybrid dendrimer–fullerene
structure was obtained from the IR and UV–vis spectra
of 1, 2 and 3 which were found to contain absorbances
due to both the dendrimer and fullerene fragments.33
Dendrimers 1, 2 and 3 all exhibited the expected photore-
sponsive behavior. For example, dark incubation of a
chloroform solution (10−5 M) of third generation den-
drimer 3 served to maximize the absorption at 357 nm
(log m=5.20) corresponding to the trans-azobenzene
chromophore.35 Irradiation of this solution with 365 nm
light resulted in clean photoisomerization to cis-azoben-
zene, as evidenced by a decrease in the absorbance at 357
nm and an increase in absorbance at 437 nm (Fig. 2). A
photostationary state was reached within approximately
150 s. Thermal reversion to the original dark-incubated
spectrum was observed over the course of approximately
6 h at 293 K in the dark. However, exposure to bright
sunlight for a period of only several seconds also effect
almost complete reversions to the trans isomer. The first
and second generation dendrimers 1 and 2 also showed
the same trends in trans–cis reversible isomerzation
phenomena. This indicates that there is no strong steric
influence on the trans–cis reversible isomerization of 1,
2 and 3. In addition, the photoresponsive behavior of the
azobenzene is not perturbed by incorporation into
fullerene-cored dendritic structure.
Scheme 1. Reagents and conditions: (a) n-BuBr, K2CO3, DMF,
80°C; (b) CBr4, PPh3, CH2Cl2/CHCl3 (1:1), rt, 1.5 h; (c)
3,5-dihydroxybenzyl alcohol, K2CO3, 18-crown-6, acetone/
THF (1:1), reflux, 18 h; (d) NaN3, CH3CN/THF (1:1), reflux,
18 h; (e) C60, chlorobenzene, reflux, 24 h.
Scheme 2. Reagents and conditions: (a) 3,5-dihydroxybenzyl
alcohol, K2CO3, 18-crown-6, acetone/THF (1:1), reflux, 18 h;
(b) CBr4, PPh3, CH2Cl2/CHCl3 (1:1), rt, 1.5 h; (c) NaN3,
CH3CN/THF (1:1), reflux, 18 h; (d) C60, chlorobenzene, reflux,
24 h.
Figure 1. 13C NMR spectrum 3 in CDCl3 at 25°C.
Scheme 3. Reagents and conditions: (a) 3,5-dihydroxybenzyl
alcohol, K2CO3, 18-crown-6, acetone/THF (1:1), reflux, 18 h;
(b) CBr4, PPh3, toluene, 5°C, 0.5 h and then rt, 12 h; (c) NaN3,
THF, reflux, 18 h; (d) C60, chlorobenzene, reflux, 24 h.
Figure 2. UV absorption spectra of 3 under irradiation condi-
tions (365 nm: 0, 5, 10, 15, 20, 25, 30, 60, 90, 120 and 150 s).

K.-Y. Kay et al. / Tetrahedron Letters 43 (2002) 5053–5056
5055
The data of differential scanning calorimetry (DSC) for
1, 2 and 3 showed high thermal stability. Glass transi-
tion temperatures (Tg) of 1, 2 and 3 were found to be
370, 358 and 348 K, respectively. These values were 45,
33 and 23 K higher than the value 325 K of the
previously reported Hawker’s benzylether dendrimer
with fullerene core,17 which is also 13 K higher than the
Tg value (312 K36) of the corresponding dendrimer
without fullerene core indicating the strong influence
that azobenzene and fullerene portions have on the
overall properties of the hybrid molecules 1, 2 and 3.37
However, the Tgs of [G1], [G2] and [G3] dendrimers
decrease with increasing generations.
16. Wooley, K. L.; Hawker, C. J.; Fre´chet, J. M. J.; Wudl,
F.; Srdanov, G.; Shi, S.; Li, C.; Kao, M. J. Am. Chem.
Soc. 1993, 115, 9836–9837.
17. Hawker, C. J.; Wooley, K. L.; Fre´chet, J. M. J. J. Chem.
Soc., Chem. Commun. 1994, 925–926.
18. Catalano, V. J.; Parodi, N. Inorg. Chem. 1997, 36, 537–
541.
19. Nierengarten, J.-F.; Schall, C.; Nicoud, J.-F.; Heinrich,
B.; Guillon, D. Tetrahedron Lett. 1998, 39, 5747–5750.
20. Nierengarten, J.-F.; Felder, D.; Nicoud, J.-F. Tetrahedron
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21. Felder, D.; Gallani, J.-L.; Guillon, D.; Heinrich, B.;
Nicoud, J.-F.; Nierengarten, J.-F. Angew. Chem., Int. Ed.
2000, 39, 201–204.
22. Djojo, F.; Ravanell, E.; Vostrowsky, O.; Hirsh, A. Eur. J.
Polarized optical microscopy suggests that the films
derived from 1, 2 and 3 are uniform and isotropic
(nonbirefringent), showing no mesophase upon heating
or cooling between 30 and 270°C. Therefore, films can
be readily fabricated by melt processing. This study has
yielded robust, nonscattering glasses which appear
indefinitely stable with respect to crystallization. These
easily processable solid solutions are suitable materials
for further optical studies, which are now under
investigation.
Org. Chem. 2000, 1051–1059.
23. Schwell, M.; Wachter, N. K.; Rice, J. H.; Galaup, J.-P.;
Leach, S.; Taylor, R.; Bensasson, R. V. Chem. Phys. Lett.
2001, 339, 29–35.
24. Dardel, B.; Deschenaux, R.; Even, M.; Serrano, E.
Macromolecules 1999, 32, 5193–5198.
25. Jiang, D.; Aida, T. Nature 1997, 388, 454–456.
26. Junge, D. M.; McGrath, D. V. Chem. Commun. 1997,
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27. Junge, D. M.; McGrath, D. V. J. Am. Chem. Soc. 1999,
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Langmuir 2000, 16, 10569–10572.
Acknowledgements
This work was supported by the Brain Korea 21 Project
in 2001.
30. Oh-ishi, K.; Okamura, J.; Ishi-i, T.; Sano, M.; Shinkai, S.
Langmuir 1999, 15, 2224–2226.
31. Hawker, C. J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1990,
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33. Satisfactory elemental analyses and spectroscopic data
were obtained for all new compounds reported.
Selected physical data for 1: brown solid; mp >280°C
1
(dec.); H NMR (400 MHz, CDCl3) l 7.85 (m, 8H), 7.49
(m, 4H), 6.96 (m, 4H), 6.77 (s, 2H), 6.60 (s, 2H), 5.11 (s,
4H), 4.42 (s, 2H), 4.02 (m, 4H), 1.77 (m, 4H), 1.51 (m,
4H), 1.00 (m, 6H); 13C NMR (CDCl3) l 161.75, 159.95,
152.34, 146.81, 138.97, 130.50, 128.00, 124.83, 122.90,
114.69 (azobenzene and Ar. monomer C), 107.81, 101.78
(Ar. monomer C), 147.56, 146.17, 145.31, 145.04, 144.82,
144.74, 144.52, 144.44, 144.22, 144.10, 144.03, 144.00,
143.97, 143.65, 143.53, 143.24, 142.58, 142.03, 141.53,
141.46, 140.38, 139.67, 139.58, 139.35, 139.16, 139.09,
138.97, 136.80, 135.49, 134.60, 133.61, 132.70 (fullerene
carbons), 69.72 (O-CH2), 68.05 (N-CH2), 31.23 (CH2),
19.21 (CH2), 13.83 (CH3); FAB-MS m/z 1389 (M+), 1448,
720 (C60), 211 (base peak); FT-IR (KBr) w 3055, 2959,
2932, 2866, 1600, 1503, 1250, 1142, 839 cm−1; UV–vis
(CHCl3) umax (log m) 261 (5.05), 351 (4.91), 437 (sh, 4.00),
632 (3.39) nm at 10−5 M.
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Selected data for 2: brown solid; mp >279°C (dec.); 1H
NMR (400 MHz, CDCl3) l 7.87 (m, 16H), 7.45 (m, 8H),
6.96 (d, 8H), 6.40–6.70 (m, 9H), 4.80–5.20 (m, 14H), 4.01
(m, 8H), 1.77 (m, 8H), 0.98 (m, 12H); 13C NMR (CDCl3)
l 161.74, 160.07, 152.43, 146.87, 138.95, 130.51, 127.94,
124.81, 122.90, 114.74 (azobenzene and Ar. monomer C),
106.53, 106.16, 101.78 (Ar. monomer C), 147.73, 146.14,
12. Newkome, G. R.; He, E.; Moorefield, C. N. Chem. Rev.
1999, 99, 1689–1746.
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40, 74–91.

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K.-Y. Kay et al. / Tetrahedron Letters 43 (2002) 5053–5056
144.95, 144.55, 144.47, 144.39, 144.22, 143.99, 143.94,
142.58, 141.38, 140.70, 139.66, 139.55, 139.30, 139.15,
139.01, 138.93, 138.39, 138.10, 137.53, 136.91, 136.16,
135.67, 133.63 (fullerene carbons), 70.00, 69.73, 69.66
(O-CH2), 68.06 (N-CH2), 31.26 (CH2), 19.23 (CH2), 13.85
(CH3); MALDI-TOF MS did not show M+ peak; FT-IR
(KBr) w 3060, 2956, 2931, 2872, 1599, 1500, 1252, 1153,
1140, 839 cm−1; UV–vis (CHCl3) umax (log m) 252 (5.16),
357 (5.20), 437 (sh, 4.20), 633 (3.27) nm at 10−5 M.
34. Prato, M.; Li, Q. C.; Wudl, F.; Lucchini, V. J. Am.
Chem. Soc. 1993, 115, 1148–1150.
35. The molar absorptivity of 3 is relatively higher than the
values of 1 and 2. However, the reason for this synergistic
effect of eight azobenzene groups in 3 cannot be clearly
explained yet. We are still working on this question.
36. Wooley, K. L.; Hawker, C. J.; Pochan, J. M.; Fre´chet, J.
M. J. Macromolecules 1993, 26, 1514–1519.
143.77, 143.53, 143.33, 143.07, 142.86, 142.74, 142.67,
142.62, 141.42, 140.75, 139.66, 139.60, 139.29, 139.19,
139.00, 138.95, 138.44, 138.13, 137.56, 136.96, 136.21,
135.70, 133.67 (fullerene carbons), 69.70 (O-CH2), 68.08
(N-CH2), 31.26 (CH2), 19.24 (CH2), 13.85 (CH3); FAB-
MS m/z 2166 (M+), 1448, 720 (C60), 261 (base peak);
FT-IR (KBr) w 3058, 2963, 2928, 2870, 1596, 1500, 1243,
1141, 839 cm−1; UV–vis (CHCl3) umax (log m) 253 (4.50),
353 (4.96), 435 (sh, 3.82), 637 (3.21) nm at 10−5 M.
Selected data for 3: light brown solid; mp >272°C (dec.);
1H NMR (400 MHz, CDCl3) l 7.87 (m, 32H), 7.49 (m,
16H), 6.97 (m, 16H), 6.40–6.70 (m, 21H), 4.85–5.15 (m,
28H), 4.40 (m, 2H), 4.02 (m, 16H), 1.78 (m, 16H), 1.52
(m, 16H), 0.99 (m, 24H); 13C NMR (CDCl3) l 161.76,
160.03, 152.30, 146.85, 139.01, 130.54, 127.96, 124.81,
122.78, 114.70 (azobenzene and Ar. monomer C), 106.53,
106.44, 106.16, 101.79 (Ar. monomer C), 147.69, 146.07,
144.90, 144.59, 144.40, 144.35, 144.19, 144.00, 143.90,
143.72, 143.56, 143.28, 143.07, 142.78, 142.71, 142.63,
37. In this preliminary communication, further comparison
of these Tg values of 1, 2 and 3 with the precursors 9, 12
and 15 was not made, but will be reported elsewhere
together with other film properties.