A R T I C L E S
Liao et al.
CDCl3) δ 7.92 (d, J ) 8.5 Hz, 4H), 7.39 (t, J ) 1.7 Hz, 4H), 7.26 (t,
J ) 1.7 Hz, 8H), 6.72 (d, J ) 2.0 Hz, 4H), 6.64 (d, J ) 2.0 Hz, 2H),
5.09 (s, 4H), 4.99 (s, 8H), 1.32 (s, 72H); 13C NMR (62.9 MHz, CDCl3)
δ 31.5, 34.8, 70.3, 71.0, 101.6, 102.0, 106.3, 107.1, 115.1, 122.3, 124.4,
135.6, 138.8, 147.0, 151.0; MS (MALDI) m/z 1268.2015 (M + H).
1c. Following the procedure for 1a, [G-2]-Br (0.34 g, 0.27 mmol),
THF (20 mL), 4,4′-hydroxyazobenzene (28 mg, 0.13 mmol), potassium
carbonate (40 mg, 0.28 mmol), and 18-crown-6 (7 mg, 0.026 mmol)
yielded, after purification by flash column chromatography (SiO2, 9:1
hexanes-ethyl acetate), 1c as a yellow solid (0.27 g, 78%): 1H NMR
(500 MHz, CDCl3) δ 7.88 (d, 4H, J ) 8.5 Hz), 7.43 (t, 8H, J ) 2.0
Hz), 7.30 (d, 16H, J ) 2.0 Hz), 7.08 (d, 4H, J ) 8.5 Hz), 6.76 (d, 8H,
J ) 2.0 Hz), 6.74 (t, 4H, J ) 2.0 Hz), 6.67 (t, 4H, J ) 2.0 Hz), 6.64
(t, 2H, J ) 2.0 Hz), 5.10 (s, 4H), 5.03 (d, 24H, J ) 8.0 Hz), 1.31 (s,
144H); 13C NMR (62.9 MHz, CDCl3) δ 31.5, 34.8, 70.1, 71.0, 101.5,
101.7, 103.1, 106.3, 115.1, 122.3, 124.4, 135.7, 139.0, 139.2, 147.2,
151.0, 160.2, 160.4, 160.6; MS (MALDI) m/z 2565.509, 2588.581 (M
+ Na), 2604.560 (M + K).
flexible benzyl aryl ether subunits, a most interesting conse-
quence of the flexible versus rigid nature of these two classes
of structures in responding to the covalently incorporated
photochromic switch. Indeed, irradiation of a single azobenzene
in the core of a rigid dendritic structure is as effective at size
modulation as three azobenzenes in a flexible benzyl aryl ether
dendrimer.3c
Summary
A comparison of two different classes of dendrimers with
azobenzene cores reveals a difference in the properties of the
photochromic moiety upon dendritic incorporation as well as a
significant difference on the photomodulation of dendrimer
properties. The hydrodynamic volume of azobenzene-containing
dendrimers can be significantly modulated when the azobenzene
unit is subjected to irradiation, depending on dendrimer
construction.
1d. A solution of [G-3]-Br (0.30 g, 0.12 mmol), acetone (10 mL),
4,4′-hydroxyazobenzene (12 mg, 0.059 mmol), potassium carbonate
(16 mg, 0.12 mmol), and 18-crown-6 (3 mg, 0.012 mmol) was
maintained at reflux overnight. After the reaction was complete by TLC
(SiO2, 9:1 hexanes-ethyl acetate), the mixture was filtered and the
solvent was concentrated The residue was purified by flash column
chromatography (SiO2, 9:1 hexanes-ethyl acetate) to afford 1d as a
yellow solid (0.17 g, 59%): 1H NMR (500 MHz, CDCl3) δ 7.86 (d,
4H, J ) 9.0 Hz), 7.37 (t, 16H, J ) 2.0 Hz), 7.25 (d, 32H, J ) 2.0 Hz),
7.03 (d, 4H, J ) 9.0 Hz), 6.71 (d, 16H, J ) 2.0 Hz), 6.70 (t, 8H, J )
2.0 Hz), 6.69 (d, 4H, J ) 2.0 Hz), 6.63 (t, 8H, J ) 2.0 Hz), 6.61 (t,
4H, J ) 2.0 Hz), 6.59 (t, 2H, J ) 2.0 Hz), 5.28 (s, 4H), 5.01 (s, 56H),
1.29 (s, 288H); 13C NMR (62.9 MHz, CDCl3) δ 31.4, 34.8, 70.1, 71.0,
101.6, 106.5, 122.5, 135.7, 139.0, 151.0, 160.2, 160.4; MS (MALDI)
m/z 5183.338 (M + Na).
1e. Following the procedure for 1d, [G-4]-Br (0.38 g, 0.074 mmol),
acetone (10 mL), 4,4′-hydroxyazobenzene (8 mg, 0.037 mmol),
potassium carbonate (10 mg, 0.074 mmol), and 18-crown-6 (2 mg,
0.001 mmol) yielded, after purification by flash column chromatography
(SiO2, 9:1 hexanes-ethyl acetate), 1e as a yellow solid (0.17 g, 59%):
1H NMR (500 MHz, CDCl3) δ 7.85 (d, 4H, J ) 8.0 Hz), 7.36 (t, 32H,
J ) 2.0 Hz), 7.24 (d, 64H, J ) 2.0 Hz), 6.89 (d, 4H, J ) 8.0 Hz), 6.70
(m, 120H), 6.61-6.58 (m, 30H), 4.97 (s, 124H), 1.30 (s, 576H). 13C
NMR (62.9 MHz, CDCl3) δ 31.4, 34.8, 70.1, 71.0, 101.6, 106.5, 106.6,
122.227, 122.3, 135.7, 139.0, 151.0, 160.2, 160.4; MS (MALDI) m/z
10386.144 (M + Na).
Experimental Section
Materials and Methods. NMR spectroscopy and mass spectrometry
(MS) were performed on commercially available instrumentation. Gel-
permeation chromatography (GPC) was performed on a system
consisting of a Shimadzu LC-10AD pump, a Shimadzu RID-6 RI
detector, and a Rheodyne 7725I injector running CH2Cl2 at 1 mL/min
through three columns (Jordi Gel 500 Å, 1000 Å, and 10 000 Å DVB,
250 × 10 mm) in series at ambient temperature. Tetrahydrofuran (THF)
was distilled under N2 from sodium benzophenone ketyl. Acetone was
dried over crushed 3 Å molecular sieves. Potassium carbonate (granular,
J. T. Baker) was dried at 100 °C at reduced pressure and stored in a
vacuum oven. Fre´chet-type bromides9 ([G-1]Br, [G-2]Br, [G-3]Br), 3,5-
di-tert-butylbenzyl bromide, phenylacetylene dendrons [I-M3-(t-Bu)4,
I-M7-(t-Bu)8, I-M15-(t-Bu)16, and I-M31-(t-Bu)32],10 3,5-di-tert-butylphe-
nylacetylene,10 and compounds 315 and 516 were prepared according to
the literature. All other reagents were purchased from commercial
suppliers and used as received. Flash chromatography was performed
by the method of Still et al.17 using silica gel (32-63 µm, Scientific
Adsorbants, Inc., Atlanta GA). Thin-layer chromatography (TLC) was
performed on precoated plates (Silica Gel HLO, F-254, Scientific
Adsorbants, Inc.). All quantum yields were calculated relative to
potassium ferrioxalate.18 The concentrations of the dendrimer solution
were adjusted so that thir absorbance at the π-π* peak was between
0.006 and 0.008. Every measurement was repeated 3 times.
1a. A solution of 3,5-di-tert-butylbenzyl bromide (0.10 g, 0.35
mmol), THF (10 mL), 4,4′-hydroxyazobenzene (36 mg, 0.17 mmol),
potassium carbonate (49 mg, 0.35 mmol), and 18-crown-6 (9 mg, 0.035
mmol) was maintained at reflux overnight. After the reaction was
complete by TLC (SiO2, 9:1 hexanes-ethyl acetate), the mixture was
filtered and the solvent was concentrated. The residue was purified by
flash column chromatography (SiO2, 9:1 hexanes-ethyl acetate) to
afford 1a as a yellow solid (73 mg, 70%): 1H NMR (500 MHz, CDCl3)
δ 7.91 (d, 4H, J ) 9.0 Hz), 7.41 (d, 2H, J ) 2.0 Hz), 7.28 (t, 4H, J )
2.0 Hz), 7.10 (d, 4H, J ) 9.0 Hz), 5.09 (s, 4H), 1.32 (s, 36H); 13C
NMR (62.9 MHz, CDCl3) δ 31.5, 34.9, 71.1, 115.0, 122.2, 122.4, 124.3,
135.4, 147.1, 151.2, 161.0; MS (MALDI) m/z 618.3332.
1b. Following the procedure for 1a, [G-1]-Br (0.20 g, 0.33 mmol),
THF (10 mL), 4,4′-dihydroxyazobenzene (32.0 mg, 0.16 mmol), 18-
crown-6 (10 mg), and potassium carbonate (21 mg, 0.15 mmol) yielded,
after purification by flash column chromatography (SiO2, 9:1 hexanes-
ethyl acetate), 1b as a yellow solid (0.13 g, 71%): 1H NMR (250 MHz,
2a. A heavy-walled glass tube jointed to a Teflon screw valve was
charged with 4 (0.956 g, 2.0 mmol), 3,5-di-tert-butylphenylacetylene
(0.856 g, 4 mmol), Pd(dba)2 (46 mg, 0.08 mmol), CuI (15 mg, 0.08
mmol), PPh3 (105 mg, 0.4 mmol), LiCl (504 mg, 12 mmol), and Et3N
(20 mL). The mixture was freeze-pump-thaw degassed three times
at -78 °C, sealed, and heated at 80 °C for 4 days. The mixture was
filtered and the solvent was evaporated under reduced pressure.
Purification of the residue by flash chromatography (SiO2, hexane
increasing to 8:1 hexane-CH2Cl2) gave 2a as a red solid (0.47 g,
64%): 1H NMR (500 MHz, C6D6) δ 7.90 (d, J ) 6.5 Hz, 4H), 7.72
(m, 4H), 7.60 (d, J ) 6.5 Hz, 4H), 7.53 (m, 2H), 1.23 (s, 36H); 13C
NMR (62.9 MHz, C6D6) δ 31.3, 35.4, 89.0, 94.2, 122.2, 123.32, 123.6,
123.6, 124.8, 124.6, 132.8, 151.4; MS (MALDI) m/z 606.3180.
2b. A heavy-walled glass tube joined to a Teflon screw valve was
charged with 7 (0.13 g, 0.56 mmol), I-M3-(t-Bu)4 (0.74 g, 1.2 mmol),
Pd(dba)2 (13 mg, 0.023 mmol), CuI (4 mg, 0.023 mmol), PPh3 (30
mg, 0.11 mmol), and Et3N (20 mL). The mixture was degassed three
times at -78 °C, sealed, and heated at 60 °C for 17 h. The mixture was
filtered and the solvent was evaporated under reduced pressure to give
the crude product, which was purified by flash chromatography (SiO2,
hexane increasing to 4:1 hexane-CH2Cl2) to give 2b as a red solid
(0.34 g, 49%): 1H NMR (500 MHz, C6D6) δ 7.94 (d, J ) 5 Hz, 4H),
(15) Atkinson, E. R.; Lawler, H. J.; Heath, J. C.; Kimball, E. H.; Read, E. R. J.
Am. Chem. Soc. 1941, 63, 730-733.
(16) Pratt, E. F.; McGovern, T. P. J. Org. Chem. 1964, 29, 1540-1543.
(17) Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43, 2923-2925.
(18) Handbook of Photochemistry; Murov, S. L., Carmichael, I., Hug, G. L.,
Eds.; Marcel Dekker: New York, 1993; Sect. 13.
9
2184 J. AM. CHEM. SOC. VOL. 126, NO. 7, 2004