Macromolecules, Vol. 37, No. 25, 2004
Nematic Liquid Crystal Dendrimers 9393
from the Aldrich Chemical Co. All of these substances were
used without further purification. Dry dichloromethane (re-
fluxed over calcium chloride, distilled and stored over molec-
ular sieves 4 Å) was used as the reaction solvent. All
atmosphere sensitive reactions were carried out under dry
argon. Analytical TLC was performed on Kieselgel F-254
precoated silicagel plate (SDS). Visualization of the spots was
accomplished with UV light (254 nm).
CH3), 4.10 (t, J ) 6.8 Hz, ArCOOCH2CH2-), 6.95 (m, ArH),
7.05 (bs, C(O)NH), 7.23 (d, J ) 8.6 Hz, ArH), 7.42 (dd, J1 )
2.8 Hz, J2 ) 8.6 Hz, ArH), 7.86 (d, J ) 2.8 Hz, ArH), 8.12 (m,
ArH). 13C NMR (CDCl3 100 MHz): δ ) 13.81, 19.16, 24.37,
25.79, 25.91, 27.02, 28.36, 29.19, 29.43, 29.65, 31.08, 36.55,
37.68, 51.33, 52.05, 65.57, 67.96, 68.00, 114.29, 114.36, 120.91,
121.35, 124.90, 124.98, 127.05, 132.32, 132.39, 148.05, 148.28,
163.57, 163.73, 164.06, 164.51, 164.87.
Synthesis. Series 1. We give here the general procedure
for the condensation of 4-(4-alkoxybenzoyloxy)-2-hydroxybenz-
aldehydes with poly(propylene inine) dendrimers of the first
to fifth generation PPI-(NH2)n, n ) 4, 8, 16, 32, and 64. To a
stirred solution of the 4-substituted salicylaldehyde (1 mmol)
in CH2Cl2 (15 mL) was added neutral activated grade I
alumina (0.5 g) and then the corresponding PPIn (1/n mmol).
The mixture was refluxed under argon for 12 h. The alumina
was filtered off and the solvent from the filtrate evaporated
under vacuum. The resulting solid was purified by column
chromatography using CH2Cl2 as eluent or by dissolving in
CH2Cl2 and precipitating the dendrimer with methanol and
then centrifugation; this procces was repeated several times
until pure by NMR. Absence of primary amine groups was
checked by the ninhydrine test. (Yield: 55-75%.)
Series 2. General Procedure for the Preparation of
the Side-on Liquid Crystalline Dendrimers. Compound
14 (186 mg, 0.225 mmol) was dissolved in dry dichloromethane
(20 mL) under an argon atmosphere. A poly(propyleneimine)
dendrimer of the appropriate generation (90 mg, 0.026 mmol)
was dissolved in dry dichloromethane (10 mL) and added to
the stirred solution of the promesogenic units. On mixing, the
initially transparent solutions became turbid. The mixture was
stirred under reflux for 7 days, diluted with dichloromethane
(50 mL), and washed with NaOH solution (2%) (2 × 20 mL).
The combined aqueous phases were again extracted with
dichloromethane and the combined organic phases were dried
over anhydrous sodium sulfate and concentrated in vacuo. The
crude product was purified by column chromatography using
dichloromethane/hexane (1/2) as eluent. If any impurity were
detected by GPC, the product was dissolved in the minimum
volume of dichloromethane and precipitated with hexane or
ethanol. Absence of primary amine groups was checked by the
ninhydrine test. Yield: 30-50%.
Acknowledgment. This work has been supported
by the CICYT of Spain (MAT 2002-04118-C02-01 and
MAT 2003-07806-C01) and FEDER (EU). The European
Union (HPRN-CT2000-00016), and Diputacio´n General
de Arago´n (DGA) (Spain). R.M.-R. acknowledges a
fellowship from MEC (Spain). M.McK. acknowledges a
fellowship from EU and L.P acknowledges a collabora-
tion fellowship from MEC (Spain). G.R.L. thanks the
Higher Education Funding Council for England for a
grant (JR00SOLEEQ) under the 2000 JREI to cover the
cost of the Varian spectrometer. We are also grateful
to Mr. O. G. Johannessen (Southampton) for commis-
sioning the jump rotation probe. A.M. acknowledges the
Government of Malaysia and the Universiti Malaya for
the award of a Scholarship.
References and Notes
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Characterization. The purity of each compound (interme-
diate and dendrimers) was confirmed by elemental analysis,
1H NMR, 13C NMR, IR spectroscopy, FAB+ or MALDI-TOF MS,
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1
and GPC. Because of the similarity of IR, H NMR, and 13C
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NMR spectra of these materials, we only quote data for PPI-
[T(C2)]16 and PPI-[L(C4)]16 dendrimers as representative ex-
amples. Data obtained from other techniques are gathered in
Tables 1 and 2.
PPI-[T(C2)]64. IR (Nujol, cm-1): ν˜ ) 3435.0 (O-H), 1731.2
(OCdO), 1631.8 (CdN). 1H NMR (CDCl3, 400 MHz): δ ) 1.25
(s broad, 4H, -NCH2CH2CH2CH2N-) 1.39 (s broad, 192H,
-CH2CH3,), 1.48-1.82 (m, 248H, CH2CH2CH2Nd , -CH2-
(CH2N)2), 2.43 (s broad, 372H, NCH2CH2CH2N-, -NCH2CH2-
CH2CH2N-) 3.48 (s broad, 128H, CH2CH2NdCH-), 3.95-4.08
(m, 128H, - CH2CH3,), 6.56 (d, J ) 7.6 Hz, 64H, ArH), 6.69 (s
broad, 64H, ArH), 6.85-6.92 (m, 128H, ArH), 7.09 (d, J ) 7.6
Hz, 64H, ArH), 7.97-8.10 (m, 128H, ArH), 8.15 (s broad, 64H,
-NdCH-), 13.96 (s broad, 64H -OH). 13C NMR APT (CDCl3,
100 MHz): δ ) 14.6 (-CH2CH3), 24.3, 24.2 (CH2CH2Nd,
-CH2(CH2N)2), 28.4 (CH2CH2CH2Nd), 51.3, 52.0, 52.1
(-CH2N-), 56.6 (CH2NdCH-), 63.6 (-OCH2-), 110.4, 112.0,
114.1, 116.4, 121.4, 132.1, 132.2, (ArC), 154.0, 163.2, 163.4,
(C quarter), 164.1 (CHdN), 164.3 (C quarter).
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PPI-[L(C4)]16. IR (Nujol, cm-1): ν˜ ) 3294 (CON-H), 1728
(OCdO), 1640 (OC-NH); 1H NMR (CDCl3, 400 MHz): δ ) 0.96
(m, -(CH2)2CH3), 1.05-1.30 (m, -ArCOOCH2CH2(CH2)6CH2-
CH2CONH-, -NCH2CH2CH2CH2N-), 1.35-1.60 (m,
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ArCOOCH2(CH2)7CH2CH2CONH-,
ArOCH2CH2CH2CH3,
(21) Severing, K.; Saalwachter, K. Phys. Rev. Lett. 2004, 92,
-NCH2CH2CH2N-), 1.65-1.90 (m, -NCH2CH2CH2NH(O)C-,
ArOCH2CH2CH2CH3, ArCOOCH2CH2(CH2)7CH2CONH-), 2.12
(t, J ) 7.6 Hz -NH(O)CCH2CH2-), 2.35 (bs, -NCH2CH2-
CH2N-, -NCH2CH2CH2CH2N-, -NCH2CH2CH2NH(O)C-),
3.21 (bs, -NCH2CH2CH2NH(O)C-), 4.02 (m, ArOCH2(CH2)3-
125501-1-4.
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