Photopolymerization of carbohydrate-based discotic mesogens. Synthesis and phase properties of 1,2,3,4,6-penta-O-(trans-3,4-dialkoxycinnamoyl)-(D)-glucopyranoses and their oligomers

Article abstract of DOI:10.1021/ja961665x

The neat phase properties of the α and β anomers of 1,2,3,4,6-penta-O-(trans-3,4-dialkoxycinnamoyl)-(D)-glucopyranose (nαGP and nβGP; n, the number of carbon atoms in the alkyl chains, is 6-8, 10, and 14) have been investigated by optical microscopy, diff

Full text of DOI:10.1021/ja961665x

9498
J. Am. Chem. Soc. 1996, 118, 9498-9508
Photopolymerization of Carbohydrate-Based Discotic Mesogens.
Syntheses and Phase Properties of
1,2,3,4,6-Penta-O-(trans-3,4-dialkoxycinnamoyl)-(D)-
glucopyranoses and Their Oligomers
Ravindranath Mukkamala,†,‡ Charles L. Burns, Jr.,† Robert M. Catchings III,§ and
Richard G. Weiss*,†
Contribution from the Department of Chemistry, Georgetown UniVersity,
Washington, DC 20057-1227, and Department of Physics and Astronomy,
Howard UniVersity, Washington, DC 20059
ReceiVed May 17, 1996X
Abstract: The neat phase properties of the R and â anomers of 1,2,3,4,6-penta-O-(trans-3,4-dialkoxycinnamoyl)-
(D)-glucopyranose (nRGP and nâGP; n, the number of carbon atoms in the alkyl chains, is 6-8, 10, and 14) have
been investigated by optical microscopy, differential scanning calorimetry, and X-ray powder diffraction. Each of
the GP forms an enantiotropic and thermotropic discotic columnar liquid-crystalline phase. Irradiation of the 14GP
in dilute solutions leads to two intramolecular [2 + 2] cycloaddition reactions between cinnamoyl double bonds. In
the solid I (obtained by recrystallization from solvent and in which 14GP molecules are paired in each columnar
stack) and “isotropic” melt phases, only short (<10 monomer unit), probably branched oligomers form upon irradiation
due to the continued preference for intramolecular cycloadditions and the ability of each monomer to interact with
more than two neighbors. Irradiation of the solid II phase (obtained by cooling the liquid-crystalline phase) provides
unbranched oligomers with <20 monomer units. However, more than 30 14RGP monomer units have been linked
in unbranched chains upon irradiation of its liquid-crystalline phase. This is the greatest degree of columnar
polymerization reported to date from a discotic mesophase. Qualitatively similar results are obtained upon irradiation
of other GP in their various phases. Oligomerization of GP liquid-crystalline and solid I phases is intracolumnar,
and the columnar packing arrangement of the monomers is retained to ca. 100 °C aboVe their clearing temperatures;
each GP produces nanoscale cylinders of one diameter but varying lengths. The oligomers unwind and do not
retake their original form when they are dissolved and reprecipitated or heated to above their clearing temperatures
and cooled. The nature of the oligomerization processes is discussed, especially with regard to inter-/intramolecular
and inter-/intracolumnar competitions for [2 + 2] cycloaddition reactions and the possibility of an anomeric effect
on oligomer length.
Introduction
Scheme 1
The synthesis of molecular objects with persistent shapes,
nanoscale dimensions, and defined functions has become an
important aspect of materials chemistry.1,2 Polymers with
cylindrical shapes, consisting of cofacially arranged disc-like
monomer units, have been shown to possess superior mechanical
properties, high temperature, and dimensional stability; they
have also found applications in building photonic devices and
one-dimensional charge carriers.3,4 A logical route to cylindri-
cally-shaped molecular objects is to preassemble disc-like
monomers into columns and then to effect an intracolumnar-
intermolecular polymerization (Scheme 1).5 In this regard,
columnar liquid-crystals offer obvious possibilities.
produce materials having interesting thermal, mechanical,
optical, electrical, and magnetic properties.6,7 Post-synthetic
processing is not necessary because the long-range orientational
order of monomers in their mesophases can be incorporated as
a permanent part of the macroscopic network formed after
polymerization. For example, some cholesteryl carbonates and
esters in their cholesteric liquid-crystalline phases have been
irradiated to obtain polymers with cholesteric optical properties
Recent studies have established that bulk polymerization of
organized liquid-crystalline phases is an effective way to
† Georgetown University.
(5) (a) Mertesdorf, C.; Ringsdorf, H.; Stumpe, J. Liq. Crystals 1991, 9,
337. (b) Kurihara, S.; Ohta, H.; Nonaka, T. Trans. Mat. Res. Soc. Jpn. 1994,
15A, 357. (c) Baldvins, J. E. Ph.D. Thesis, Georgetown University,
Washington, DC, 1995. (d) Meier, H.; Muller, K. Angew. Chem., Int. Ed.
Engl. 1995, 34, 1437. (e) van Nostrum, C. F.; Picken, S. J.; Schouten, A.-
J.; Nolte, R. J. M. J. Am. Chem. Soc. 1995, 117, 9957. (f) Clark, T. D.;
Ghadiri, M. R. J. Am. Chem. Soc. 1995, 117, 12364.
‡ Present address: Materials Science Division, Lawrence Berkeley
National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720.
§ Howard University.
X Abstract published in AdVance ACS Abstracts, October 1, 1996.
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mensmeyer, K.; Etzbach, K. H.; Ringsdorf, H.; Haarer, D. Nature 1994,
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S0002-7863(96)01665-4 CCC: $12.00 © 1996 American Chemical Society

Photopolymerization of Discotic Mesogens
J. Am. Chem. Soc., Vol. 118, No. 40, 1996 9499
that are retained even at very high temperatures.7a,b Similarly,
polymerization of some smectic phases has produced uniaxially
ordered polymeric networks with anisotropic optical and other
physical properties.7c Recently, shape-persistent, slab-like two-
dimensional polymers have been synthesized from monomeric
smectic phases.1
Scheme 2
Attempts to stabilize the columnar order of discotic mesogens
by polymerization through intermolecular [2 + 2] cycloadditions
of cinnamoyl double bonds have had limited success.5a-c For
example, efforts to polymerize hexakis-N-(4-tetradecyloxycin-
namoyl)hexacyclen (containing a flexible azacrown core) in its
Dho (hexagonal ordered) columnar discotic phase by intraco-
lumnar-intermolecular photochemical [2 + 2] cycloadditions
among the pendant cinnamoyl groups were unsuccessful due
to a loss of liquid-crystallinity during irradiation caused by a
parallel process, trans-cis isomerization of the double bonds.5a
Similarly, photoinduced polymerization of hexakis-(4-octade-
cyloxycinnamoyloxy)triphenylene (containing a flat, rigid aro-
matic core) has been attempted:5b intermolecular coupling
through [2 + 2] cycloadditions was inefficient in both the solid
and Dho phases, leading only to dimers and some oligomers of
unspecified length; although irradiation of the ND (nematic
discotic) phase resulted in a higher degree of intermolecular
reaction, liquid-crystallinity was lost.
Previously, we found that both the R and â anomers of a
wide range of homologues of 1,2,3,4,6-penta-O-(n-alkanoyl)-
(D)-glucopyranose (GA) aggregate into several types of colum-
nar discotic phases.8 They, like molecules based upon some
inositols9 and mono-, di-, and trisaccharides,10 constitute a novel
class of discotic liquid-crystals11 with rather rigid, chiral cores.
As an extension of that work, we sought to make disc-shaped
glucopyranose molecules that are polymerizable in columnar
stacks since glucopyranose, being more rigid than a hexacyclen5a
and more flexible than a triphenylene,5b might allow pendant
cinnamoyl groups to dimerize without loss of initial phase order.
However, both the R and â anomers of the initially synthesized
molecules, 1,2,3,4,6-penta-O-(trans-cinnamoyl)-(D)-glucopyra-
nose (GC),5c were nonmesomorphic due to insufficient flex-
ibility in the pendant groups.12 In spite of this, irradiations of
the GC in their neat melt, crystalline and glass phases reveal
the decreasing importance of both trans-cis isomerization and
intramolecular [2 + 2] cycloaddition reactions among cinnamoyl
double bonds, and increasing efficiency of intermolecular [2 +
2] cycloadditions as phase order increases.5c
of a short and a long-chained homologue of nRGP and nâGP
are reported.
Experimental Section
Materials and Methods. Benzene (Fisher, ACS grade) was dried
over calcium chloride, azeotropically distilled, and stored over type
4A molecular sieves. Spectrograde benzene (Baker) was used as
received. Pyridine (Fisher) was dried by distillation from calcium
hydride and stored over type 4A molecular sieves. 3,4-Dihydroxy-
benzaldehyde (97%) from Lancaster and the 1-bromoalkanes (97-
99%), malonic acid (99%), dipropyl amine (99%), thionyl chloride
(99+%), oxalyl chloride (98%), and 4-dimethylaminopyridine (DMAP,
99%) from Aldrich were used as received. Anhydrous R-D-glucose
(with 5% â anomer) and â-D-glucose (with 3% R anomer) from Sigma
were dried under house vacuum at ca. 50 °C for 24 h before use.
Melting points (corrected) and photomicrographs were recorded on
a Leitz 585 thermostatting stage between crossed polars with a Leitz
Wetzlar (SM-LUX-POL) microscope and a mounted Pentax K1000
35 mm camera. Differential scanning calorimetry (DSC) was performed
on a TA Instruments 2910 modulated differential scanning calorimeter,
controlled by a Thermal Analyst 3100 using indium as standard for
temperature and heat calibration. The rate of heating or cooling was
2°/min at lower temperatures and 3°/min in the higher temperature
regions.
High-performance liquid chromatography (HPLC) was performed
on a Waters Associates liquid chromatograph with a 254 nm absorbance
detector and a 4.4 × 250 mm Alltech 5µ silica gel column using 5-15/
95-85 ethyl acetate/hexane mixtures as eluents. Gel permeation
chromatography was performed on either a Varian 5500 liquid
chromatograph with a Varian RI-4 refractive index detector or a Hewlett
Packard series 1050 liquid chromatograph with a Wyatt/Optilab 903
refractive index detector, equipped with two polystyrene columns (6.2
× 250 mm Zorbax PSM Bimodel-S or 6.2 × 250 mm Altex µ
Sphereogel) connected in series and calibrated with at least four different
polystyrene standards (Polysciences, Inc.). Tetrahydrofuran (THF,
HPLC grade) was the eluent at a flow rate of 1.0 mL/min.
UV/vis absorption spectra were recorded on a Perkin-Elmer Lambda
6 UV/vis spectrophotometer and were analyzed using Perkin-Elmer
PECSS software. Circular dichroism (CD) spectra were recorded on
a JASCO J-710 spectropolarimeter using a Hellma jacketed quartz cell
of 0.1 mm path length. Proton NMR spectra were recorded on a Bruker
270 MHz nuclear magnetic resonance spectrometer with a Tecmag
operating system. IR spectra were recorded on a MIDAC FT-IR
spectrometer as KBr pellets or chloroform solutions. Elemental
analyses were performed by Desert Analytics, Tucson, AZ.
Powder X-ray diffraction patterns were collected on samples in
copper discs with flat indentations using a Scintag 2000 X-ray powder
diffractometer equipped with a liquid N2 cooled solid state Ge detector
and Cu KR radiation (1.5406 Å). The copper holders were heated on
a hot plate until the samples melted, placed quickly on the heated (to
ca. 50 °C) sample holder of the diffractometer, and then allowed to
equilibrate for about 15 min. The final temperatures were usually
between 52-54 °C. Data were collected between 0.5° and 35° in 2θ
at a scan rate of 1-3°/min with an increment of 0.03°.
Here, we investigate the properties of ten R and â anomers
of 1,2,3,4,6-penta-O-(trans-3,4-dialkoxycinnamoyl)-(D)-glu-
copyranoses (nRGP and nâGP, Scheme 2) that form discotic
columnar liquid-crystalline phases. In addition to a determi-
nation of the neat GP phase structures, results from irradiations
(8) Morris, N. L.; Zimmerman, R. G.; Jameson, G. B.; Dalziel, A. W.;
Reuss, P. M.; Weiss, R. G. J. Am. Chem. Soc. 1987, 110, 2177.
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Praefcke, K.; Marquardt, P.; Kohne, B.; Stephan, W.; Levelut, A.-M.;
Wachtel, E. Mol. Cryst. Liq. Cryst. 1991, 203, 149. (d) Praefcke, K.; Blunk,
D.; Hempel, J. Mol. Cryst. Liq. Cryst. 1994, 243, 323.
(10) (a) Vill, V.; Thiem, J. Liq. Crystals 1991, 9, 451. (b) Takada, A.;
Itoh, T.; Fukuda, T.; Miyamoto, T. Bull. Inst. Chem. Res., Kyoto UniV.
1991, 69, 77. (c) Sugiura, M.; Minoda, M.; Fukuda, T.; Miyamoto, T.;
Watanabe, J. Liq. Crystals 1992, 12, 603. (d) Takada, A.; Ide, N.; Fukuda,
T.; Miyamoto, T.; Yamagata, K.; Watanabe, J. Liq. Crystals 1995, 19, 441.
(e) Zimmermann, R. G.; Jameson, G. B.; Weiss, R. G.; Demailly, G. Mol.
Cryst. Liq. Cryst. (Letters) 1985, 1, 183.
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1977, 9, 471. (b) Hinov, H. P. Mol. Cryst. Liq. Cryst. 1986, 136, 221. (c)
Chandrasekhar, S.; Ranganath, G. S. Rep. Prog. Physics 1990, 53, 57. (d)
Chandrasekhar, S. Liquid Crystals; Cambridge University Press: New York,
1992. (e) Weiss, R. In Polymer Liquid Crystals. Thermophysical and
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Vol. 3, accepted.
(12) Collings, P. J. Liquid Crystals, Princeton University Press: NJ, 1990.

9500 J. Am. Chem. Soc., Vol. 118, No. 40, 1996
Mukkamala et al.
Irradiations employed a 220 W Black-Ray long-wavelength UV lamp
(320-360 nm). Samples of known mass between Pyrex cover slips,
separated in some cases by teflon rings of known thickness, were kept
on a Thomas thermostatting stage (about 15 cm from the lamp) mounted
on a Bausch & Lomb microscope. After specific periods of irradiation,
the Pyrex cover slips were carefully broken into small pieces and
sonicated for ca. 20 min in chloroform to dissolve the samples and to
make solutions of known concentrations. Decreases in optical densities
of these solutions at 333 nm, normalized to constant weight, were used
to approximate the photoconversions.
Molecular mechanics (force field) calculations (MM+ method) were
conducted with a program available as part of the Hyperchem package13
using the Polak-Ribiere (conjugate gradient) algorithm and an RMS
gradient termination condition of 0.1 Kcal/Å-mol.
from ethanol/chloroform to give a 36% yield of a white solid, mp
124.6-127.8 °C. IR (KBr) 3300-2400 (broad, acid O-H), 1662
1
(CdO), 1622 (olefinic CdC), 1595 (aromatic) cm-1; H NMR (270
MHz, CDCl3/TMS) δ 7.7 (d, J ) 16.0 Hz, 1H, olefinic), 7.1 (m, 2H,
aromatic), 6.85 (d, J ) 8.0 Hz, 1H, aromatic), 6.3 (d, J ) 15.8 Hz,
1H, olefinic), 4.0 (m, 4H, -O-CH2-), 1.9-0.8 (m, 54H, aliphatic).
1,2,3,4,6-Pentakis-O-(trans-3,4-dialkyloxycinnamoyl)-(D)-glucopy-
ranoses (nrGP and nâGP). All manipulations were performed in
dim light to minimize photodecomposition. 3,4-Dialkyloxycinnamic
acid (3.5-4.0 mmol) and an excess of thionyl chloride (5 mL) in 5
mL of dry benzene were stirred at ca. 60 °C for 1 h in a dry atm.
Excess thionyl chloride and benzene were distilled at atmospheric
pressure. Additional dry benzene (2 × 10 mL) was added, distilled as
before, and an aspirator vacuum (protected by anhydrous CaSO4 and
sodium hydroxide) was applied to the residue for 45 min at ca. 60 °C
to remove traces of thionyl chloride. The crude acid chloride was
dissolved in 10 mL of dry pyridine, and ca. 0.5 mmol of R- or â-D-
glucopyranose and ca. 20 mg of DMAP were added to the solution.
The reaction mixture was stirred in the dark and in a dry atmosphere
for 24 h at ca. 60 °C, poured into a mixture of 30 g of ice and 50 mL
concentrated HCl, and extracted with ca. 300 mL of chloroform. The
organic layer was washed sequentially with water, aqueous NaHCO3,
and water. After being dried (anhydrous Na2SO4), solvent was removed
(rotary evaporator) to afford a dark yellow-brown solid which was
eluted by column chromatography (silica gel, 60-200 mesh). The
resulting light brown-yellow solids were recrystallized to obtain white
or light yellow powders. Details of specific solvent mixtures used for
column chromatography and recrystallization of different homologues
are available as supporting information.
Syntheses. A general outline of the synthetic procedures for the
GP is shown in Scheme 2.
3,4-Dialkyloxybenzaldehydes (1a-e). A mixture of 3,4-dihydroxy-
benzaldehyde (6.9 g, 50 mmol), 1-bromoalkane (140 mmol), and
anhydrous K2CO3 (34.5 g, 250 mmol) in 200 mL of 2-butanone was
heated under reflux with stirring for 24 h in a dry atm. Most of the
solvent was removed on a rotary evaporator, and ca. 200 mL water
and ca. 250 mL chloroform were added. The organic layer was
separated from the aqueous layer, washed with more water (3 × 150
mL), and dried (anhydrous Na2SO4). Evaporation of the solvent
afforded the crude product. The hexyloxy homologue (1a) was used
without further purification, 1b-d were purified by column chroma-
tography (silica gel, hexane followed by 1/1 hexane/chloroform), and
1e was recrystallized from acetone. Product yields were typically
70-80%: 1b, mp 47-49 °C (lit.14 mp 48 °C); 1c, mp 52-54 °C (lit.14
mp 55 °C); 1d, mp 62 °C (lit.14 mp 65 °C); 1e, mp 76-78 °C (lit.14
1,2,3,4,6-Pentakis-O-(trans-3,4-dihexyloxycinnamoyl)-â(D)-glu-
copyranose (6âGP): 45% yield of a pale yellow, amorphous solid (99%
pure by HPLC), mp ca. 72 °C (solid to mesophase) and 85.2-86.1 °C
(mesophase to isotropic). IR (KBr) 2930, 2870, 1718 (ester CdO),
1
mp 78 °C). IR and H NMR spectra are consistent with the assigned
structures.
3,4-Dialkoxycinnamic acids (2) were prepared by the procedure
described for 2a. A mixture of 3,4-dihexyloxybenzaldehyde (crude
material from the previous reaction, ca. 40 mmol), malonic acid (10.4
g, 100 mmol), 120 mL of pyridine, and 35 mL of dipropyl amine was
heated with stirring at ca. 110 °C for 1.5 h in a dry atm. The reaction
mixture was cooled to ca. 40 °C and then poured into a mixture of 300
mL of concentrated HCl and ca. 300 g of ice. The crude product was
collected by filtration, washed with ca. 600 mL water, suction-dried,
and recrystallized (chloroform/hexane and ethanol) to yield 4.1 g (23%
from 3,4-dihydroxybenzaldehyde) of light brown solid, mp 124-127
°C. IR (KBr) 3300-2340 (broad, acid O-H), 1662 (CdO), 1595 cm-1
1
1633 (CdC) cm-1; H NMR (270 MHz, CDCl3/TMS) δ 7.7-7.5 (m,
5H, olefinic), 7.1-6.75 (two m, 15H, aromatic), 6.4-6.0 (m, 6H,
olefinic and -COO-CH-), 5.7-5.4 (m, 3H, -COO-CH-), 4.4 (broad m,
2H, -COO-CH-), 4.2-3.9 (m, 21H, -COO-CH- and -O-CH2-), 1.9-
0.8 (m, 110H, aliphatic). Anal. Calcd for C111H162O21: C, 72.79; H,
8.85. Found: C, 73.03; H, 9.02.
1,2,3,4,6-Pentakis-O-(trans-3,4-dihexyloxycinnamoyl)-r(D)-glu-
copyranose (6rGP): 9% yield of a pale yellow solid (>99% pure by
HPLC), mp ca. 50 °C (solid to mesophase) and 60.4-62.0 °C
(mesophase to isotropic). IR (KBr) 2932, 2872, 1718 (ester CdO),
1
1
1631 (CdC) cm-1; H NMR (270 MHz, CDCl3/TMS) δ 7.8-7.5 (m,
(aromatic); H NMR (270 MHz, CDCl3/TMS) δ 7.7 (d, J ) 15.8 Hz,
1H, olefinic), 7.1 (m, 2H, aromatic), 6.9 (d, J ) 7.9 Hz, 1H, aromatic),
6.3 (d, J ) 16.0 Hz, 1H, olefinic), 4.0 (m, 4H, -O-CH2-), 1.9-0.8 (m,
22H, aliphatic).
5H, olefinic), 7.2-5.9 (several m, 22H, aromatic, olefinic and -COO-
CH-), 5.6-5.4 (m, 2H, -COO-CH-), 4.4 (broad m, 3H, -COO-CH-),
4.1-3.9 (m, 20H, -O-CH2-) and 1.9-0.8 (m, 110H, aliphatic). Anal.
Calcd for C111H162O21: C, 72.79; H, 8.85. Found: C, 72.77; H, 8.66.
1,2,3,4,6-Pentakis-O-(trans-3,4-diheptyloxycinnamoyl)-â(D)-glu-
copyranose (7âGP): 28% yield of a pale yellow solid (>97% pure by
HPLC), mp ca. 78 °C (solid to mesophase) and 104.8-105.6 °C
(mesophase to isotropic). IR (KBr) 2930, 2856, 1718 (ester CdO),
3,4-Diheptyloxycinnamic acid (2b) was obtained in 35% yield as
a light brown solid (recrystallized from ethanol), mp 124.6-128.8 °C.
IR (KBr) 3184-2500 (broad, acid O-H), 1664 (CdO), 1622 (olefinic
CdC) 1595 (aromatic) cm-1; 1H NMR (270 MHz, CDCl3/TMS) δ 7.7
(d, J ) 15.8 Hz, 1H, olefinic), 7.1 (m, 2H, aromatic), 6.85 (d, J ) 8.3
Hz, 1H, aromatic), 6.3 (d, J ) 15.8 Hz, 1H, olefinic), 4.0 (m, 4H,
-O-CH2-), 1.9-0.8 (m, 26H, aliphatic).
1
1633 (CdC) cm-1; H NMR (270 MHz, CDCl3) δ 7.7-7.5 (m, 5H,
olefinic), 7.1-6.7 (two m, 15H, aromatic), 6.4-6.0 (m, 6H, olefinic
and -COO-CH-), 5.7-5.4 (m, 3H, -COO-CH-), 4.4 (broad m, 2H,
-COO-CH-), 4.2-3.9 (m, 21H, -COO-CH- and -O-CH2-), 1.9-0.8 (m,
130H, aliphatic). Anal. Calcd for C121H182O21: C, 73.71; H, 9.24.
Found: C, 73.81; H, 9.25.
3,4-Dioctyloxycinnamic acid (2c) was obtained in 38% yield as a
light brown solid (recrystallized from acetic acid), mp 125.1-129.2
°C. IR (KBr) 3280-2380 (broad, acid O-H), 1664 (CdO), 1662
1
(olefinic CdC), 1595 (aromatic) cm-1; H NMR (270 MHz, CDCl3/
TMS) δ 7.7 (d, J ) 15.8 Hz, 1H, olefinic), 7.1 (m, 2H, aromatic), 6.85
(d, J ) 8.3 Hz, 1H, aromatic), 6.3 (d, J ) 15.8 Hz, 1H, olefinic), 4.0
(m, 4H, -O-CH2-), 1.9-0.8 (m, 30H, aliphatic).
1,2,3,4,6-Pentakis-O-(trans-3,4-diheptyloxycinnamoyl)-r(D)-glu-
copyranose (7rGP): 17% yield as a light yellow solid (>99% pure
by HPLC), mp ca. 50 °C (solid to mesophase) and 80.1-81.2 °C
(mesophase to isotropic). IR (KBr) 2926, 2856, 1718 (ester CdO),
3,4-Didecyloxycinnamic acid (2d) was obtained in 42% yield as
white solid (recrystallized from acetic acid), mp 123.8-128.3 °C. IR
(KBr) 3200-2400 (broad, acid O-H), 1664 (CdO), 1622 (olefinic
1
1631 (CdC) cm-1; H NMR (270 MHz, CDCl3/TMS) δ 7.8-7.5 (m,
5H, olefinic), 7.2-5.9 (several m, 22H, aromatic, olefinic and -COO-
CH-), 5.6-5.4 (m, 2H, -COO-CH-), 4.4 (broad m, 3H, -COO-CH-),
4.1-3.9 (m, 20H, -O-CH2-) and 1.9-0.8 (m, 130H, aliphatic). Anal.
Calcd for C121H182O21: C, 73.71; H, 9.24. Found: C, 73.91; H, 9.28.
1,2,3,4,6-Pentakis-O-(trans-3,4-dioctyloxycinnamoyl)-â(D)-glu-
copyranose (8âGP). Following the general procedure, a mixture of
the cinnamoyl chloride, â-D-glucopyranose, pyridine, and benzene was
stirred at room temperature for 96 h under a dry atmosphere. Ethanol
(200 mL) was added, and the solid that formed was collected by
filtration and recrystallized from dichloromethane/ethanol to afford 22%
1
CdC), 1595 (aromatic) cm-1; H NMR (270 MHz, CDCl3/TMS) δ
7.7 (d, J ) 15.8 Hz, 1H, olefinic), 7.1 (m, 2H, aromatic), 6.85 (d, J )
8.1 Hz, 1H, aromatic), 6.3 (d, J ) 15.8 Hz, 1H, olefinic), 4.0 (m, 4H,
-O-CH2-), 1.9-0.8 (m, 38H, aliphatic).
3,4-Ditetradecyloxycinnamic acid (2e) was eluted by column
chromatography (silica gel, 7/3 chloroform/hexane) and recrystallized
(13) By Hyper Cube Inc.
(14) Nguyen, H. T.; Destrade, C.; Malthete, J. Liq. Crystals 1990, 8,
797.

Photopolymerization of Discotic Mesogens
J. Am. Chem. Soc., Vol. 118, No. 40, 1996 9501
yield of a white solid (99% pure by HPLC), mp ca. 60 °C (solid to
mesophase) and 110.6-111.7 °C (mesophase to isotropic). IR (thin
film) 2926, 2854, 1717 (ester CdO), 1634 (CdC) cm-1; 1H NMR (270
MHz, CDCl3/TMS) δ 7.65-7.4 (m, 5H, olefinic), 7.0-6.65 (two m,
15H, aromatic), 6.3-5.95 (m, 6H, olefinic and -COO-CH-), 5.65-
5.35 (m, 3H, -COO-CH-), 4.35 (broad m, 2H, -COO-CH-), 4.1-3.8
(m, 21H, -COO-CH- and -O-CH2-), 1.9-0.8 (m, 150H, aliphatic). Anal.
Calcd for C131H202O21: C, 74.50; H, 9.57. Found: C, 74.73; H, 9.69.
1,2,3,4,6-Pentakis-O-(trans-3,4-dioctyloxycinnamoyl)-r(D)-glu-
copyranose (8rGP): 10% yield of a pale yellow solid (>98% pure
by HPLC), mp ca. 60 °C (solid to mesophase) and 90.1-91.2 °C
(mesophase to isotropic). IR (thin film) 2926, 2855, 1719 (ester CdO),
n ) 5-14) were attached at the para position of the cinnamoyl
rings. However, neither the neat homologues nor their mixtures
in various proportions15 were liquid-crystalline.16 Since one
chain per cinnamoyl group is probably insufficient to fill the
interstices near the glucopyranose cores, alkoxy chains were
then placed at both the meta and para positions. The resultant
disk-shaped molecules, nRGP and nâGP, form the desired
discotic columnar liquid-crystalline phases.17 Optical micro-
scopic (OM) and differential scanning calorimetric (DSC)
analyses demonstrate that the liquid-crystalline phases of all
of the nRGP and nâGP are enantiotropic. Designations of the
mesophases as discotic columnar liquid-crystals and of the
molecular packing arrangements in solid phases are based upon
powder X-ray diffraction (XRD) and optical microscopy data
at appropriate temperatures.
To predict the shapes and structures of oligomeric products
obtained from irradiation of the neat phases of the GP, their
molecular packing and its topological consequences on inter-
molecular reactivity must be known. Prior studies employing
[2 + 2] cycloaddition reactions of cinnamate double bonds have
demonstrated clearly the need for proximity and orientational
specificity if reaction selectivity is to be attained.18
The design of GP molecules places the reactive double bonds
near the core and in conformations that make GP appear disk-
shaped; the alkyl chains lie along the molecular periphery. In
this way, intercolumnar [2 + 2] cycloaddition reactions between
cinnamate double bonds are blocked when the molecules are
stacked in columns. Such packing does not preclude intramo-
lecular [2 + 2] reactions which compete with the intracolum-
nar-intermolecular reactions leading to oligomerization. Ad-
ditionally, there are features of the GP that enhance the
possibility of intermolecular linkages and oligomerizations in
their columns: (1) the reactive cinnamoyl groups are projected
both above and below the rough plane defined by the glucopy-
ranose ring and (2) the GP contain an odd number of reactive
groups. Thus, even if two pairs of cinnamate double bonds
react intramolecularly, the fifth cannot and will be available
for intermolecular reactions exclusively. However, at least two
intermolecular reactions per GP, involving two different
neighbors is the minimum requirement for columnar polymer-
ization along a column. The dissymmetry of the projections
of the five cinnamate groups of each GP has other desirable
consequences with respect to columnar oligomerization. In
more symmetrical and (especially) planar discotic molecules,
the initial [2 + 2] cycloaddition favors subsequent reactions of
other double bonds within the same molecular pair.5d As such,
oligomerization is intrinsically disfavored.
1
1631 (CdC) cm-1; H NMR (270 MHz, CDCl3/TMS) δ 7.8-7.5 (m,
5H, olefinic), 7.2-5.9 (several m, 22H, aromatic, olefinic and -COO-
CH-), 5.6-5.4 (m, 2H, -COO-CH-), 4.4 (broad m, 3H, -COO-CH-),
4.1-3.9 (m, 20H, -O-CH2-) and 1.9-0.8 (m, 130H, aliphatic). Anal.
Calcd for C131H202O21: C, 74.50; H, 9.57. Found: C, 74.26; H, 9.53.
1,2,3,4,6-Pentakis-O-(trans-3,4-didecyloxycinnamoyl)-â(D)-glu-
copyranose (10âGP): 17% yield of a pale yellow solid (>98% pure
by HPLC), mp ca. 50 °C (solid to mesophase) and 121.9-123.1 °C
(mesophase to isotropic). IR (thin film) 2923, 2853, 1717 (ester CdO),
1
1633 (CdC) cm-1; H NMR (270 MHz, CDCl3/TMS) δ 7.7-7.5 (m,
5H, olefinic), 7.1-6.7 (two m, 15H, aromatic), 6.35-6.0 (m, 6H,
olefinic and -COO-CH-), 5.7-5.4 (m, 3H, -COO-CH-), 4.4-3.8 (three
m, 23H, -COO-CH- and -O-CH2-), 1.9-0.8 (m, 190H, aliphatic). Anal.
Calcd for C151H242O21: C, 75.82; H, 10.13. Found: C, 75.76; H, 10.27.
1,2,3,4,6-Pentakis-O-(trans-3,4-didecyloxycinnamoyl)-r(D)-glu-
copyranose (10rGP): 8% yield of a pale yellow solid (98% pure by
HPLC), mp ca. 50 °C (solid to mesophase) and 102.7-103.9 °C
(mesophase to isotropic). IR (KBr) 2924, 2854, 1718 (ester CdO),
1
1633 (CdC) cm-1; H NMR (270 MHz, CDCl3/TMS) δ 7.8-7.5 (m,
5H, olefinic), 7.2-5.9 (several m, 22H, aromatic, olefinic and -COO-
CH-), 5.6-5.4 (m, 2H, -COO-CH-), 4.4 (broad m, 3H, -COO-CH-),
4.1-3.9 (m, 20H, -O-CH2-) and 1.9-0.8 (m, 190H, aliphatic). Anal.
Calcd for C151H242O21: C, 75.82; H, 10.13. Found: C, 75.81; H, 10.29.
1,2,3,4,6-Pentakis-O-(trans-3,4-ditetradecyloxycinnamoyl)-â(D)-
glucopyranose (14âGP): 37% yield of a white solid (99% pure by
HPLC), mp ca. 75 °C (solid to mesophase) and 117.4-118.5 °C
(mesophase to isotropic). IR (KBr) 2924, 2854, 1718 (ester CdO),
1
1633 (CdC) cm-1; H NMR (270 MHz, CDCl3/TMS) δ 7.7-7.5 (m,
5H, olefinic), 7.1-6.7 (two m, 15H, aromatic), 6.4-6.0 (m, 6H, olefinic
and -COO-CH-), 5.7-5.4 (m, 3H, -COO-CH-), 4.4-3.9 (three m, 23H,
-COO-CH- and -O-CH2-), 1.9-0.8 (m, 270H, aliphatic). Anal. Calcd
for C191H322O21: C, 77.69; H, 10.92. Found: C, 77.56; H, 10.95.
1,2,3,4,6-Pentakis-O-(trans-3,4-ditetradecyloxycinnamoyl)-r(D)-
glucopyranose (14rGP): 17% yield of a white amorphous solid (>97%
pure by HPLC), mp ca. 70 °C (solid to mesophase) and 104.6-106.5
°C (mesophase to isotropic). IR (KBr) 2920, 2851, 1718 (ester CdO),
1
1633 (CdC) cm-1; H NMR (270 MHz, CDCl3/TMS) δ 7.8-7.5 (m,
5H, olefinic), 7.2-5.9 (several m, 22H, aromatic, olefinic and -COO-
CH-), 5.6-5.4 (m, 2H, -COO-CH-), 4.5-3.9 (two broad m, 23H,
-COO-CH- and -O-CH2-) and 1.9-0.8 (m, 270H, aliphatic). Anal.
Calcd for C191H322O21: C, 77.69; H, 10.92. Found: C, 77.72; H, 10.69.
Ethyl trans-3,4-Ditetradecyloxycinnamate (3). A mixture of 3,4-
ditetradecyloxycinnamic acid (0.55 g, 0.96 mmol), 40 mL of absolute
ethanol, 15 mL of dry benzene, and 1 mL of concentrated sulfuric acid
was heated under reflux with stirring for 20 h in the dark. Chloroform
(150 mL) was added to the cooled reaction mixture, and the organic
layer was washed sequentially with water, aqueous NaHCO3, and water.
After being dried (anhydrous Na2SO4), solvent was removed (rotary
evaporator) to afford a light yellow solid which was eluted by column
chromatography (silica gel, 60-200 mesh, 1/99 ethyl acetate/hexane)
to obtain 0.2 g (35%) of a white solid (>99% pure by HPLC), mp
66.5-67.6 °C. IR (thin film) 2915, 2847, 1706 (CdO), 1631 (CdC)
Structures of GP Phases. Optical Microscopy. Upon
being heated, samples sandwiched between cover slips under-
went solid-to-liquid crystal transitions (K-D) that were difficult
to measure precisely over ranges less than 1-2°, but liquid
crystal-to-isotropic transitions (D-I) were more easily detected.
The patterns became increasingly birefringent and deformable
at higher temperatures. Optical textures between crossed polars
(15) It is possible, in some cases, to introduce mesomorphism in
homologous mixtures of non-mesomorphic materials. See: Sheikh-Ali, B.
M.; Weiss, R. G. Liq. Crystals 1994, 17, 605.
(16) Burns, C. L., Jr.; Weiss, R. G. Unpublished results.
(17) Mukkamala, R.; Burns, C. L., Jr.; Weiss, R. G. 29th ACS Middle
Atlantic Regional Meeting, May 1995, Washington, DC, Abstract no. 255.
(18) For details on cinnamate photochemistry in anisotropic phases,
see: (a) Schmidt, G. M. J., et al. Solid State Photochemistry; Ginsburg, D.,
Ed.; Verlag-Chemie: Weinheim, 1976. (b) Venkatesan, K.; Ramamurthy,
V. In Photochemistry in Organized and Constrained Media; Ramamurthy,
V., Ed.; VCH: Weinheim, 1991; pp 133-184. (c) Weiss, R. G. In
Photochemistry in Organized and Constrained Media; Ramamurthy, V.,
Ed.; VCH: Weinheim, 1991; pp 603-690. (d) Enkelman, V.; Wegner, G.;
Novak, K.; Wagener, K. B. J. Am. Chem. Soc. 1993, 115, 203. (e) Greiving,
H.; Hopf, H.; Jones, P. G.; Bubenitschek, P.; Desvergne, J. P.; Bouas-
Laurent, H. J. Chem. Soc., Chem. Commun. 1994, 1075.
1
cm-1; H NMR (270 MHz, CDCl3/TMS) δ 7.6 (d, J ) 15.8 Hz, 1H,
olefinic), 7.1-7.0 (m, 2H, aromatic), 6.85 (d, J ) 8.8 Hz, 1H, aromatic),
6.3 (d, J ) 15.8 Hz, 1H, olefinic), 4.25 (q, J ) 7.0 Hz, 2H, -COOCH2-
), 4.0 (m, 4H, -O-CH2-), 1.9-0.8 (m, 57H, aliphatic).
Results and Discussion
In an initial attempt to induce liquid-crystallinity in discotic
molecules like GC, alkoxy chains of various lengths (-OCnH2n+1
,

9502 J. Am. Chem. Soc., Vol. 118, No. 40, 1996
Mukkamala et al.
Figure 1. Optical mesophase textures of (a, top left) 7RGP at 63 °C (× 560), (b, top right) 8âGP at 81 °C, after shearing (× 340), (c, bottom left)
14âGP at 102 °C (× 340) upon cooling the isotropic phase, and (d, bottom right) 14RGP at 89 °C (× 700) upon cooling from the isotropic phase.
of the mesophases of 6RGP and 7RGP from heating or cooling
were very similar (Figure 1a) and did not change with gentle
pressing/shearing. For the other R homologues and the â
anomers, gentle pressing/shearing motions resulted in textures
from aligned mesophases (Figure 1b) that were distinctly
different from those obtained upon slow cooling of the
undisturbed isotropic phases (Figures 1c,d); textures similar to
those in Figure 1b-d have been observed for the hexagonal
discotic (Dh) mesophases of cellobiose octa(n-alkanoates).10b
The liquid crystal-to-solid transitions (D-K) of cooled samples
were difficult to detect optically as well. The mesophases
became increasingly viscous and gradually lost most of their
birefringence while the optical textures were frozen in a glassy
state. When reheated, the samples regained their highly
birefringent appearance and fluidity.
observed to give nonsuperimposable fan-shaped textures (similar
to Figure 1c) with only one type of fan pairs with opposite
points.19 Although the GP are chiral and optically pure, well-
defined focal-conic (fan-shaped) textures with “off-centered
meeting points” are observed only for 7-, 8-, 10-, and 14âGP.
Even in these cases, the direction of the off-centered points
appears to be somewhat random (points indicated by arrows in
Figure 1c).
Differential Scanning Calorimetry. Mesophases of many
GP have a tendency to supercool significantly before crystal-
lizing. For that reason, samples were incubated at room
temperature for 24 h and 2-3 weeks before recording the second
and third heating-cooling cycles, respectively. Except for
6RGP and 7RGP, the D-K transition in cooling thermograms
was not observed above room temperature. However, the I-D
The enantiomers of a triphenylene-based columnar discotic
liquid crystal with optically active side chains have been
(19) Malthete, J.; Jacques, J.; Tinh, N. H.; Destrade, C. Nature 1982,
298, 46.

Photopolymerization of Discotic Mesogens
J. Am. Chem. Soc., Vol. 118, No. 40, 1996 9503
Table 1. Solid-Mesophase (K-D) and Mesophase-Isotropic (D-I)
Transition Temperatures (T, °C, Endotherm Peak Maxima) and
Enthalpies (∆H, Jg-1) and Entropies (∆S, Jg-1 K-1) of GP from
First and Second DSC Heating Scans
first heating
second heatinga
compd
T
∆H ∆S × 103
T
∆H ∆S × 103
6RGP K-D 58.1 4.15
D-I 62.4 2.93
7RGP K-D 57.4 4.99
D-I 81.0 5.24
8RGP K-D 48.7 6.12
D-I 90.8 5.66
57.8 6.04
62.0 3.27
58.7 7.20
81.4 5.21
47.2 4.94
91.1 5.86
42.6 6.10
8.7
14.8
15.6
9.8
14.7
16.0
15.4
15.4
5.3
10RGP K-D 45.2 4.51
D-I 102.9 5.56
14.8 102.8 5.79
41.4 22.68
16.5 106.4 5.86
c
14RGP K-D 54.1 42.0b
D-I 106.6 6.22
6âGP K-D 53.4 2.54
D-I
84.2 1.93
5.4
84.7 1.88
7âGP K-D 56.1 3.25
c
D-I 105.5 3.05
8.0 105.5 2.95
c
8.3 113.8 2.99
c
10.9 120.6 4.09
41.9 38.75
11.0 120.0 4.18
7.7
8âGP K-D 64.3 14.15
D-I 113.6 3.20
7.7
10âGP K-D
c,d
D-I 120.8 4.29
14âGP K-D 55.4 46.27b
D-I 119.9 4.34
10.3
Figure 2. Powder X-ray diffractograms of solid I (a), liquid-crystalline
(b), and solid II (c) phases of 14RGP. × marks the scattering peak
from the beam-stop cutoff.
10.6
a Twenty-four h at ca. 25 °C after first heating/cooling scan.
b Combined heat of multiple transitions. c Transition not observed.
d Appearance is a soft solid at room temperature.
alkanoates) are considerably higher for the R-anomers!10d The
entropy changes (∆S ) ∆H/To) were calculated for D-I
transitions using the onset temperatures (To) from both heating
(endotherm) and cooling (exotherm) peaks. The heats per mass
unit for D-I transitions, also, tend to increase with increasing
chain length for lower homologues and do not vary significantly
when n > 8. This indicates that phase packing is dominated
by the alkyl chains in the longer homologues. Studies on
benzenehexayl hexa-n-alkanoates have shown that the disorder
created by alkyl chains affects K-D the enthalpies and temper-
atures much more than those of the D-I transitions.20 Enthalpies
for the D-I transitions of the GP are comparable to those of
benzenehexayl hexa-n-alkanoates11a and triphenylenehexayl
hexa-n-alkanoates.21 The higher values of ∆H and ∆S for the
R anomers indicate that their mesophases are more closely
packed and more ordered than those of the â anomers.
transitions showed no significant supercooling, as expected for
enantiotropic phases.
For reasons that remain to be determined, the â anomers have
a greater tendency to form glass phases than the R anomers.
No K-D transition was detected in the second or third heating
scans of 7âGP, 8âGP, and 10âGP, and no K-D transition was
observed for 10âGP even in the first heating scan, although it
appears to be a soft solid at room temperature (Table 1 and
supporting information).
From Table 1, the temperatures of the K-D transitions (TK-D
)
from the second heating scans change very little with increasing
chain length in both the R- and â-anomeric series, while the
enthalpies remain fairly constant, also (except for 14RGP and
14âGP). This behavior was reproducible in the third heating
scans. Both 14RGP and 14âGP exhibit rather anomalous
thermal behavior when compared to the shorter homologues.
Besides the enthalpies of their K-D transitions being much
higher than for the other homologues, the 14GP also have some
low enthalpy (presumably solid-to-solid) transitions before
melting into mesophases during first heating scans; no analogous
transitions were detected for any of the other GP. Also, the
solids obtained by cooling mesophases (solid II) of 14RGP and
14âGP exhibit very different thermal properties from the solids
obtained from solvent recrystallization (solid I).
The anomalous phase behavior of the two solid morphs of
each 14GP anomer may be a consequence of their molecular
packing arrangements (Vide infra). Another possibility, that
solvent molecules are incorporated in the crystal lattices of the
solvent crystallized 14GP in solid I, is very unlikely since
thermal gravimetric analyses of 14âGP showed no discernible
weight loss between room temperature and 130 °C (above the
clearing temperature). Differences in temperatures, heats, and
entropies for the D-I transitions during the first, second, and
third (data not presented) heating scans of all the GP are
negligibly small. This and other evidence indicate very little
thermal decomposition even above the clearing temperatures.
For both anomeric series of GP, TD-I increases with increasing
chain length and reaches a plateau near n ) 9 that is ca. 20°
higher for the âGP. By contrast, both the clearing temperatures
and enthalpies of the Dho-I transitions of cellobiose octa(n-
Powder X-ray Diffraction. Diffraction data were collected
on nonoriented samples of several GP in solid I and solid II
phases at room temperature and in the liquid-crystalline phase
at 52-55 °C. Figure 2 contains the diffraction patterns of
14RGP as an example. The d-spacings in Table 2 are calculated
from 2θ values using Bragg’s law (nλ ) 2d sin θ).22
A single, high-intensity reflection at low Bragg angles, weak,
secondary reflections (with d-spacings in the multiples of
x
x
x
1/ 3, 1/ 4, 1/ 7, etc. of the principal low-angle reflection) in
the midangle region, and a weak, broad hump at higher angles
are the common features of diffractograms obtained from
mesophases of all the GP. They indicate a hexagonal packing
arrangement of columns of molecules with disordered stacking
of core groups within the columns (i.e., a Dhd phase).22,23 The
d-spacing corresponding to the principal (100) reflection
increases with increasing alkyl chain length (Table 2). At a
(20) Collard, D. M.; Lillya, C. P. J. Am. Chem. Soc. 1991, 113, 8577.
(21) Destrade, C.; Mondon, M. C.; Malthete, J. J. Phys. (Les Ulis, Fr.)
1979, 40, C3.
(22) (a) Glusker, J. P.; Lewis, M.; Rossi, M. Crystal Structure Analysis
for Chemists and Biologists; VCH: New York, 1994. (b) Milburn, G. H.
W. X-Ray Crystallography; Butterworth: London, 1973.
(23) (a) Lai, C. K.; Serrette, A. G.; Swager, T. M. J. Am. Chem. Soc.
1992, 114, 7948. (b) Baehr, C.; Frick, E. G.; Wendorff, J. H. Liq. Crystals
1990, 7, 601. (c) Sauer, T.; Wegner, G. Mol. Cryst. Liq. Cryst. 1988, 162,
97 (d) Goltner, C.; Pressner, D.; Mullen. K. Angew. Chem., Int. Ed. Engl.
1993, 32, 1660.

9504 J. Am. Chem. Soc., Vol. 118, No. 40, 1996
Mukkamala et al.
Table 2. 2θ (deg) and Derived d-Spacing (Å) Values and Miller
Indices (hkl) of Selected GP in Various Phases from Powder X-ray
Diffraction Measurements
solid Ia
liquid crystalb
solid IIa
compd 2θ
d(hkl)
2θ d(hkl)
2θ d(hkl)
14âGP 1.73 50.93 (100) 2.54 34.71 (100) 2.50 35.29 (100)
3.04 29.03 (110) 5.05 17.47 (200) 4.97 17.75 (200)
3.42 25.81 (200) 20.11 4.41 (001)c 7.53 11.74 (300)
6.83 12.92 (400)
21.38 4.15 (001)
21.25 4.18 (001)
14rGP 1.77 49.87 (100) 2.68 32.91 (100) 2.78 31.74 (100)
6.89 12.83 (400) 5.34 16.54 (200) 5.45 16.21 (200)
21.31 4.17 (001) 19.94 4.45 (001)c 8.23 10.73 (300)
21.29 4.17 (001)
8âGP
3.05 28.93 (100) 3.17 27.83 (100) 3.01 29.37 (100)
6.07 14.55 (200) 5.55 15.90 (110) 6.03 14.63 (200)
8.64 10.22 (210) 6.26 14.10 (200) 7.75 11.40 (210)
18.84 4.71 (001) 8.32 10.62 (210) 19.92 4.45 (001)c
19.55 4.54 (001)c
Figure 3. Intercolumnar distances from powder X-ray diffraction data
of the mesophases (R series: 9; â series: 0) and calculated molecular
diameters (Å, from MM+, X) versus alkyl chain length (n).
8rGP
7âGP
d
3.20 27.58 (100)
6.48 13.62 (200)
19.75 4.49 (001)c
e
3.37 26.17 (100) 3.26 27.12 (100) 3.20 27.59 (100)
5.98 14.78 (110) 6.54 13.49 (200) 6.41 13.77 (200)
6.31 13.99 (200) 9.83 8.99 (300) 9.67 9.14 (300)
20.97 4.23 (001)c 19.17 4.63 (001)c 12.37 7.15 (400)
20.00 4.44(001)c
7rGP
6âGP
3.24 27.28 (200) 3.34 26.39 (100) 3.03 29.09 (200)
4.02 21.98 (110) 6.83 12.94 (200) 3.85 22.93 (110)
6.07 14.55 (310) 10.34 8.54 (300) 5.78 15.29 (310)
19.31 4.59 (001) 19.72 4.49 (001)c 19.07 4.65 (001)
3.74 23.59 (100) 3.52 25.06 (100) 3.43 25.71 (100)
6.57 13.45 (110) 6.00 14.72 (110) 6.83 12.94 (200)
9.34 9.46 (210) 6.98 12.67 (200) 20.54 4.32(001)c
21.14 4.19 (001) 9.95 9.66 (210)
20.09 4.42 (001)c
6rGP
3.43 25.76 (200) 3.55 24.89 (100)
4.14 21.30 (110) 19.17 4.62 (001)c
6.34 13.93 (310)
e
19.54 4.54 (001)
a Ca. 25 °C. b 52-54 °C. c Broad. d Not measured. e Very weak
peaks.
common chain length, the d-spacings for the R anomers are
somewhat smaller than for the â anomers. The significantly
smaller d-values of the 14RGP may be related to the anoma-
lously higher ∆H measured for its D-I transition. If 14RGP is
more interdigitated in its mesophase than 14âGP and, as a result,
experiences greater dispersive interactions, more energy would
be required to separate the molecules into a phase with random
orientations.
To probe this possibility further, intercolumnar distances (a)
were calculated for a hexagonal lattice using the formula a )
1.154 × d100 (where d100 is the d-spacing corresponding to the
(100) reflection)22a and compared with molecular diameters (2R)
calculated from MM+ modeling studies13 of energy-minimized
GP conformations with fully extended alkyl chains (Figure 3).
The calculated diameters of the R and â anomers of one chain
length are essentially the same. Although 2R is markedly
greater than a, indicating that the 14GP are much more
interdigitated than their shorter homologues, intracolumnar and
intercolumnar chain entanglements cannot be distinguished.
The 14GP structures from MM+ calculations predict an
overall disc-like shape, although the 10 alkyl chains are projected
somewhat above and below the rough plane of the core. The
glucopyranose ring of the R anomer is calculated to assume a
half-chair shape that places the cinnamoyl group on the anomeric
carbon in an equatorial position (Figure 4a).24 However, the
Figure 4. Energy minimized structures of 14RGP (a) and 14âGP (b)
from MM+ calculations and glucopyranose ring portions with other
groups omitted for clarity; oxygen atoms are shown to indicate the
ether group and where cinnamate groups are attached. The orientations
of the two representations for each molecule are not correlated.
actual conformation in the neat phases may not correspond to
that from the “gas phase” calculations, and the energy difference
between it and a full chair conformation is probably small. Both
the R and â anomers of GC are calculated to have lowest energy
(24) This may be due to an intrinsic preference for C-O bonds,
specifically, to be in equatorial positions in six-membered rings. Paquette,
L. A.; Stepanian, M.; Branan, B. M.; Edmondson, S. D.; Bauer, C. B.;
Rogers, R. D. J. Am. Chem. Soc. 1996, 118, 4504.

Photopolymerization of Discotic Mesogens
J. Am. Chem. Soc., Vol. 118, No. 40, 1996 9505
full-chair conformations, requiring that the anomeric cinnamate
group of the R-GC be in axial orientations. Also, X-ray
diffraction studies with R-GA have led us to conclude that the
anomeric alkanoyl groups of some of the homologues examined
are projected axially, allowing them to occupy the space in a
column near the ether group of a neighboring GA molecule.8
The â anomer can place all of its cinnamoyl substituents in
equatorial positions when the glucopyranose ring is chair-like,
and that is the predicted lowest energy conformation (Figure
4b).
Solid II phases of 14âGP and 14RGP consist of hexagonally
arranged molecular columns, also, with almost the same
d-spacings as in the mesophases (Table 2). In the solid II
diffractograms, the broad hump in the wide angle region of the
mesophase is replaced by a narrower and more intense reflection
with smaller d-spacing (ca. 4.2 Å), indicative of regular packing
and significantly reduced motion of the core groups within a
column. To be well-ordered, a GP column must have specific
rotational orientations and specific chiral facial arrangements
between neighboring molecules;8 the broadness of the high angle
peak, when compared to the sharpness of the low angle
reflection, is indicative of continued orientational disorder.
Since planar, achiral disc-like mesogens typically have 6-fold
(or greater) rotational symmetry and a σ-plane of symmetry
defined by the core ring atoms, molecular orientations within a
column do not usually impede crystallization from their liquid-
crystalline phases. Crystallization may be suppressed by either
chain entanglements, hindered rotational isomerism,25 or other
factors leading to rapid increases of viscosity at lowered
temperatures that prohibit a decrease in entropy content.
Conditions for crystallization of chiral molecules like GP that
lack rotational and facial symmetry are much more severe.8
Diffraction patterns of the solid II phases of 6âGP, 7âGP,
and 8âGP retain the general appearance of the d-spacings (ca.
4.5 Å) and diffuse, weak high-angle diffraction of the me-
sophase, indicating a glassy phase with mesophase-like ordering
of the core groups. The lack of an observable D-K transition
by DSC in the initial and subsequent cooling scans and of a
K-D transition in the second and third heating scans of 6âGP,
7âGP, and 8âGP, even when samples were incubated at room
temperature, is consistent with glassy phases.
Extremely weak peaks from which no meaningful information
could be extracted were observed in diffractograms of solid II
6RGP and 8RGP. Very faint birefringent patterns were detected
by optical microscopy. On these bases, the solid II phases of
6RGP and 8RGP appear to be amorphous glasses. Although
the diffraction peaks from the solid II phases of 7RGP were
also broad and of low intensity, the two peaks in the low angle
region could be indexed to an orthorhombic packing of columns.
Reflections from the solid I phases were generally broader
and of lower intensity than those from the corresponding
mesophases. Apparently, disorder among molecules or the
distribution of their packing arrangements in a column and
among columns in the (frozen) solid I phases is greater than in
the (mobile) mesophases. Diffraction patterns of the solid I
phases of 14RGP and 14âGP are consistent with hexagonal
packing of columns. Although the d-spacings in the low- and
mid-angle regions of the mesophase or solid II and of solid I
are quite different, those from the meridional reflections remain
at 4.2-4.5 Å. In fact, the solid I intercolumnar distances (a),
57.5 Å (R) and 58.7 Å (â), are ca. 20 Å longer than the a values
for the liquid-crystalline or solid II phases and almost the same
as the calculated molecular diameters (ca. 58 Å).
Figure 5. Cartoon representation of the proposed dimer-type aggregates
of 14RGP and 14âGP in their solid I phases.
the molar mass, Z is the number of molecules present in a
column slice of thickness t, A is the area of a column, and N
is Avogadro's number), are reasonable (i.e., ca. 0.9-1.0 g/cm3)
if Z ) 19c,10c when t is equal to the d-spacing. However,
densities near 1 g/cm3 for the solid I phases are calculated only
when Z ) 2; dimeric aggregation, like that in Figure 5, is the
likely cause. Similar dimeric, disk-like structures, but stabilized
by hydrogen-bonding interactions, have been invoked for some
tetraalkyl ether derivatives of inositols.9c In the absence of
strong binding interactions, dimers like those in Figure 5 can
dissociate easily. Such packing changes may be involved in
the low enthalpy solid-to-solid transitions detected by DSC in
first heating scans of 14âGP and 14RGP initially in their solid
I phases.26
Diffractograms of solid I phases of 6âGP, 7âGP, and 8âGP
indicate hexagonal columnar packing with d-spacings like those
of the solid II and liquid-crystalline phases. However, columns
in solid I 6RGP and 7RGP appear to be orthorhombically
packed. Except for 6âGP and 7âGP, the intermolecular
distances within a column are >4.5 Å, indicating intracolumnar
disorder.
Photo-oligomerization of GP in Various Phases and the
Nature of the Oligomers. General Considerations. Samples
of the R and â anomers of 7GP and 14GP have been irradiated
in dilute chloroform and benzene solutions, neat discotic
mesophases, solid I and solid II phases, and isotropic melt phases
at ca. 320-360 nm to effect [2 + 2] photodimerizations of
cinnamoyl double bonds.18
The absorption spectra of the GP in chloroform are nearly
the same: for 7âGP, λmax 333 nm (ꢀ 107 000 L mol-1 cm-1
)
with a shoulder at 305 nm (ꢀ 73 000 L mol-1 cm-1) and λmax
247 nm (ꢀ 58 000 L mol-1 cm-1). Thin films of neat melt and
solid II phases were prepared by sandwiching the molten
compounds between Pyrex discs and thermostatting them at the
desired temperature. Solid I samples were irradiated as powders
between Pyrex discs at room temperature. Conversions ((5%)
were calculated from the loss in optical density at 333 nm (after
dissolving the solid in chloroform) by assuming that [2 + 2]
cycloaddition and trans-cis isomerization of cinnamoyl double
bonds are the dominant photoprocesses. Previous studies
demonstrate that some trans-cis isomerization occurs during
even the solid state irradiations of GC molecules.5c The
cyclobutane products of the [2 + 2] process do not absorb at
333 nm or at the irradiation wavelengths. Gross product
1
structures were identified by H NMR and IR spectroscopies,
and molecular mass averages were determined by gel permeation
chromatography.
Calculated densities (F) of the liquid-crystalline and solid
II phases of the 14GP, using F ) [(MZ)/(AtN)] (where M is
(26) Diffraction patterns of the higher temperature crystalline phases were
not recorded due to our inability to thermostat the samples with adequate
precision.
(25) Kim, S.-J.; Karis, T. E. J. Mater. Res. 1995, 10, 2128.

9506 J. Am. Chem. Soc., Vol. 118, No. 40, 1996
Mukkamala et al.
Table 3. Molecular Mass Averages (Mw) and Polydispersities
(Mw/Mn) of Irradiated 14RGP and 14âGP in Various Phases
molecular
mass averages
% OD loss irradiation
at 333 nm period (h)
sample
phase
compd
14RGP
(MW ) 2950)
14âGP
Mw
Mw/Mn
87a
93b
87a
94b
91b
82b
92b
83a
0a
2
4
2
4
4
4
2
2
0
0
mesophasec 33401
mesophasec 123747
mesophased 28426
mesophased 50925
2.4
2.0
1.5
1.7
2.1
3.8
1.3
1.0
1.0
1.0
(MW ) 2950)
solid IIe
solid Ie
meltf
58994
31419
18707
3662
solutiong
3721
3870
0b
a Data from GPC instrument 1. b Data from GPC instrument 2. c At
95 °C. d At 110 °C. e At 25-35 °C. f At 130 °C. g 10-5 M in N2-
saturated benzene at ca. 30 °C.
Figure 6. Change in optical density of a 10-5 M N2-saturated benzene
solutions of 3 (2) at 327 nm, and 14âGP (0) and 7âGP (×) at 333
nm as a function of irradiation time. The OD values are normalized to
time ) 0.
achieved. Apparently, some double bonds are in optimal
orientations for [2 + 2] cycloaddition reactions during the early
stages of irradiation. The attenuated movement of molecules
caused by the initial intracolumnar-intermolecular reactions
may retard cycloadditions between the remaining double bonds
during the latter stages.
In spite of the large conversions, the optical textures of the
liquid-crystalline phases of 7RGP, 7âGP,30 14RGP, and
14âGP were not Visibly changed by extended irradiation. They
are unaffected by cooling to room temperature, where the
monomers are solids or by heating to where the monomers are
isotropic liquids. This observation and knowledge of the initial
packing arrangement of the monomers requires that the oligomer
chains be unbranched.
For example, the optical texture of the material obtained after
irradiation of 14âGP in its mesophase at 110 °C to ca. 85%
reduction in optical density faded gradually and disappeared
only near 230 °C; it did not reappear upon cooling. The optical
texture was also lost when oligomerized 14âGP was dissolved
in chloroform and reprecipitated. Another sample of oligomer-
ized 14âGP was cooled to room temperature, and its DSC
thermograms were recorded; no transitions were detected upon
initial heating to 250 °C or upon cooling the heated sample to
room temperature. Similarly, after a sample of 7RGP (TD-I
81 °C), had been irradiated in its liquid-crystalline phase at
65 °C to ca. 80% conversion, its optical clearing temperature
was near 260-280 °C and birefringence did not reappear upon
cooling.
Another sample of 14âGP that had been irradiated at 110
°C (to ca. 85% conversion) was cooled to room temperature,
and its X-ray diffraction pattern was recorded (see supporting
information). As expected, an intense low-angle peak and a
diffuse high-angle peak like those of the unirradiated liquid-
crystalline phase were present. The d-spacing of the (100)
reflection, 35.6 Å, is 0.9 Å larger than in the unirradiated liquid-
crystalline phase. The small increase may be due to an ancillary
consequence of bringing the facial portions of linked GP cores
closer together. Unfortunately, the high angle peak was too
weak to determine with reasonable precision the magnitude of
the decrease in interfacial distances. When the oligomerized
sample was heated above its melting temperature and then
recooled, the lower angle peak lost most of its intensity and
became broader, as expected from the optical microscopic
observations.
In thicker neat samples, penetration of radiation is limited
initially to regions near the surfaces; molecules farther from
the surface react only after those near the surface are depleted.
High conversions (ca. 90%) could be realized in relatively short
times (4 h) for samples of thicknesses less than 50 µm (prepared
by sandwiching an isotropic-melt between Pyrex disks).
To explore the influence of cis isomers on optical densities
of irradiated GP samples,27 a N2-saturated benzene solution of
10-5 M ethyl trans-3,4-ditetradecyloxycinnamate (3, Scheme
2)28 was irradiated to its photostationary state (pss) (Figure 6).
Under these conditions, intermolecular cycloadditions are
inconsequential since the lifetimes of the photoreactive excited
singlet states18 are in the picosecond range.29 From integration
of the olefinic protons in the 1H NMR spectrum of the mixture,
the trans/cis ratio was 3/2. Using this information and the ca.
25% reduction in optical density at 327 nm, the ꢀ of cis-3 at
this wavelength is calculated to be 9000 L mol-1 cm-1
.
Solution Phase Irradiations. Irradiation of 10-5 M 7âGP,
14âGP, or 14RGP in air-saturated chloroform or N2-saturated
benzene (under conditions where intermolecular cycloaddition
reactions are not possible,27 but intramolecular ones are) resulted
in indistinguishable results: rapid loss of absorption at 333 nm
to ca. 15% of its initial value followed by no further decreases
(Figure 6). Assuming two cycloadditions per molecule of GP
(i.e., reactions of two pairs of cinnamoyl groups), the same ꢀ
value for individual cis-chromophores of GP at 333 nm as that
of cis-3 at 327 nm and the same pss mixture of geometric
isomers for the remaining GP double bond, the final optical
density is predicted (and found) to be ca. 15% of the initial
value. The absence of intermolecular reaction is supported by
GPC analyses of the GP products (Table 3) that show a
monodisperse peak corresponding to the normalized molecular
mass (Vide infra) of the initial species. Previously, we
determined that irradiation of 1,2,3,4,6-penta-O-(trans-cin-
namoyl)-(D)-glucopyranose in dilute solutions yields predomi-
nantly truxinic (head-to-head) type cyclobutanes,5c and a similar
result is expected here.
Discotic Liquid-Crystalline Phase Irradiations. Decreases
in optical density with time during irradiation of the liquid-
crystalline GP were rapid initially and slower at later times (as
in Figure 6) due to a combination of some trans-cis isomer-
ization of cinnamate double bonds and [2 + 2] cycloadditions.5
However, >90% decreases in optical density, corresponding to
>90% conversions of double bonds to cyclobutanes, could be
Infrared spectra of 14âGP samples irradiated at 110 °C
showed the appearance of a saturated ester carbonyl stretch
(1745 cm-1) and disappearance of bands from the unsaturated
ester (1720 cm-1) and double bond (1630 cm-1) (see supporting
information). 1H NMR spectra of irradiated samples lacked
(27) Lewis, F. D.; Oxman, J. D.; Gibson, L. L.; Hampsch, H. L.; Quillen,
S. L. J. Am. Chem. Soc. 1986, 108, 3005.

Photopolymerization of Discotic Mesogens
J. Am. Chem. Soc., Vol. 118, No. 40, 1996 9507
most of the olefinic proton resonances of the GP and included
new aliphatic proton resonances around 3-4 ppm from cy-
clobutane protons. The severe broadening of proton resonances
from groups near the molecular core (cyclobutyl moieties,
aromatic rings, and -OCH2- groups of the alkoxy chains) indicate
very long relaxation times (and impeded torsional motions) due
to the formation of large oligomers. Although 1H NMR spectra
of the monomeric products from solution irradiation show more
resolved and narrower resonances for cyclobutyl and other
protons near the core, some of the aromatic protons are also
broadened (see supporting information). Mobility of the alkyl
chain protons is not expected to be affected by the [2 + 2]
cycloaddition reactions and their resonances remain narrow for
the irradiated samples.
From molecular mass averages (Mw) determined by gel
permeation chromatography (Table 3), 5-8 monomers31 were
linked during irradiation of liquid-crystalline 14âGP to ca. 85%
conversion. At >90% conversion (by increasing the irradiation
time), oligomers whose molecular mass averages correspond
to 8-14 GP units (Table 3) are formed. Curiously, irradiation
of 14RGP in its Dhd liquid-crystalline phase at 95 °C produced
oligomers of much higher average molecular masses, corre-
sponding to the linking of 16 to 33 monomers at similar
photoconversions. The polydispersities of these oligomers, near
2, are surprisingly low considering the nature of the linking
process.
As mentioned, MM+ based computations indicate that a half-
chair conformation for the glucopyranose rings of the RGP is
lowest in energy, and there is precedent for axial projections of
less bulky anomeric groups in related discotic phases.8 Al-
though the much larger size of the dialkoxycinnamoyl groups
of RGP makes a packing arrangement like that stabilizing the
full-chair conformations of RGA molecules less likely, axial
projection of the anomeric groups of the RGP would promote
intermolecular cycloadditions, and we consider this a possible
explanation for the greater degree of oligomerization in the R
series.
Solution CD spectra of 14âGP and the oligomers obtained
from its irradiation (>80% conversion) in the liquid-crystalline
phase display very little dichroism (ca. 1 mdeg for either sample
at 0.08 mg/mL concentrations) where the corresponding absorp-
tion spectra have high optical densities. Several glucopyranose
and other saccharide derivatives containing p-methoxycinnamoyl
chromophores are known to have very high dichroic absorptions
with exciton-coupled bands, and intensity of the dichroism
increases in an additive fashion as the number of chromophores
on the saccharide core is increased.32 The GP molecules and
their oligomers may favor a conformation that essentially cancels
the dichroic absorption of individual cinnamoyl chromophores
rather than leading to their additivity.33
Solid II Phase Irradiations. Irradiation of 14âGP in its solid
II phase (obtained by cooling the liquid-crystalline phase) at
25-35 °C and mesophase at 110 °C under otherwise comparable
conditions led to similar conversions. Initial optical textures
of the solid II samples were maintained after irradiation as well,
but heating and cooling did not convert them to a texture like
that in Figure 1c: the texture of a sample irradiated to >90%
conversion was intact up to ca. 220 °C and became completely
isotropic only at ca. 250 °C; like samples oligomerized in their
liquid-crystalline phases, the solid II oligomer lost its bire-
fringence permanently once heated above its clearing temper-
ature was increasingly deformable by pressure as the temperature
was increased. Completely analogous results were obtained
upon irradiation of 7RGP and 7âGP in their solid II phases.30
Even at elevated temperatures, the molecular motions necessary
to form a liquid-crystalline phase are blocked by the covalent
linkages between GP moieties.
Although powder diffraction patterns indicate almost identical
hexagonal columnar packing for the liquid-crystalline and solid
II phases (Table 2), their comparable reaction rates are surprising
since the more rigid solid should be more demanding to-
pochemically than the mesophase.18 Either a large fraction of
the cinnamates must be frozen at near bonding distances and
orientations, or significant motion must be possible.
Molecular mass averages of oligomers from the solid II phase
of 14âGP irradiated for 4 h at 25-35 °C (>90% conversion)
correspond to 7-15 linked GP units, slightly more than in the
mesophase (Table 3). On the basis of the optical micrographs,
the oligomers must be cylindrical in shape and packed in parallel
columns (like the monomeric phase) as formed. However, it
and the other solid II oligomers cannot be converted by heating
to liquid-crystalline phases.
These observations indicate that cylindrical oligomers formed
initially from the columnar phases of GP are neither rigid nor
in their preferred conformations. Preassembly of the columns
forces the oligomers to be columnar. When provided with the
opportunity to explore other shapes, the cylinders “unwind”.
In order for such shape changes to occur, at least some of the
GP units in each cylinder must be linked through no more than
one [2 + 2] cycloaddition (Scheme 3a); if two (or more)
cyclobutane rings are shared by a pair of GP units, escape from
a face-on orientation is essentially blocked (Scheme 3b).
Solid I Phase Irradiations. Irradiation of 14âGP in its solid
I phase (obtained by recrystallization from solvent) at 25-35
°C was slower than in its liquid-crystalline or solid II phase;
4 h were needed for ca. 80% conversion. GPC analysis revealed
a low degree of oligomerization with higher polydispersity
(Table 3). A large sharp peak with elution time corresponding
to the monomer and a smaller one that may be from dimers,
along with broad and less prominent peaks for oligomers (Figure
7) are present. Clearly, intramolecular [2 + 2] processes are
more favored in the solid I phase than in the solid II and liquid-
crystalline phases. Additionally, the molecular pairing nature
of the solid I columns of the 14GP (Figure 5) suggests that the
oligomers derived from them are branched.
(28) Except for a small blue shift of the lowest energy band and an
expected 5-fold reduction in ꢀ, the absorption spectrum of 3 in chloroform
or benzene is like those of the GP: λmax 327 nm; ꢀ 20 300 L mol-1 cm-1
.
(29) Lewis, F. D.; Quillen, S. L.; Elbert, J. E. J. Photochem. Photobiol.,
Part A 1989, 47, 173.
(30) The 7RGP and 7âGP exhibit photochemical behavior that is similar
to their tetradecyl homologues. Irradiations of mesophases near the clearing
temperatures or of solid II phases at 25-35 °C gave high photoconversions
with no detectable changes in optical textures. Proton NMR and IR spectra
of the photoproducts were comparable to those of similarly irradiated
tetradecyl homologues. However, GPC analyses of oligomers from irradia-
tion of the various phases of the 7GP led only to very small peaks from
which no usable information could be extracted. The oligomers were not
completely soluble in the initial eluent, THF, and do not have sufficiently
different indices of refraction from a second eluent, toluene, to allow
reasonable signals from the RI detector. For that reason, we have emphasized
data from the longer homologues.
(31) Since the two GPC instruments employed gave slightly different
Mw values for unirradiated 14âGP, the degrees of oligomerization have
been calculated assuming the monomeric molecular masses from the
appropriate data set (rather than using 2950, the correct molar mass).
(32) Nakanishi, K.; Berova, N. In Circular Dichroism; Nakanishi, K.,
Berova, N., Woody, R. W., Eds.; VCH: New York, 1994; pp 361-398,
and references cited therein.
Irradiation in the Melt Phases. Among the neat phases of
14âGP, the “isotropic” melt phase at 130 °C underwent the
fastest conversion (ca. 90% in 2 h) to photoproducts. However,
GPC analysis of this sample showed that only 4-5 monomers
(33) (a) Zhao, N.; Lo, L.-C.; Berova, N.; Nakanishi, K.; Tymiak, A. A.;
Ludens, J. H.; Haupert, G. T., Jr. Biochemistry 1995, 34, 9893. (b) Wiesler,
W. T.; Berova, N.; Ojika, M.; Meyers, H. V.; Chang, M.; Zhou, P.; Lo,
L.-C.; Niwa, M.; Takeda, R.; Nakanishi, K. HelV. Chim. Acta 1990, 73,
509.

9508 J. Am. Chem. Soc., Vol. 118, No. 40, 1996
Mukkamala et al.
Conclusions
The nRGP and nâGP (n ) 6-8, 10, and 14) investigated
here form enantiotropic discotic columnar liquid-crystalline
phases that persist over wide temperature ranges; clearing
temperatures are intrinsically higher in the â anomeric series.
They increase with increasing chain lengths and plateau at n g
10. Columnar packing has been identified in some of the solid
phases, also. Columns are arranged hexagonally or orthorhom-
bically.
Irradiation of dilute solutions of GP homologues leads only
to intramolecular [2 + 2] dimerizations of no more than four
cinnamoyl double bonds per molecule. Although short (4-5
GP units), probably branched oligomers are produced by
irradiation of the neat melt (nonbirefringent) phases, reaction
continues to be primarily intramolecular. The oligomers are
nonbirefringent and glassy when cooled; more than four double
bonds per molecule can be induced to react.
Figure 7. Gel permeation chromatograms for 14âGP (a) before and
(b) after 4 h irradiation in the solid I phase at 25-35 °C, and (c) after
2 h irradiation in the melt (“isotropic”) phase at 130 °C. The shape of
the peak for the 14âGP product after irradiation of a 10-5
M
After photoinduced oligomerization, the columnar arrange-
ments of GP molecules in their neat discotic liquid-crystalline
and solid II phases are retained to temperatures ca. 100° above
the monomer clearing transitions. Under these conditions,
reaction is exclusively intracolumnar, primarily intermolecular,
and (except for the solid I phase of 14GP, where two molecules
are along a columnar cross-section) unbranched. In the most
favorable cases, more than 30 monomer units of GP have been
linked in unbranched chains while retaining, for the first time,
the columnar organization of the monomers. The oligomers,
as produced initially from these phases, are nanoscale cylinders
of one diameter and varying lengths. Once the initial oligomer
conformation is lost (via heating to much higher temperatures
than the monomer clearing points or via dissolution), it is not
reestablished (upon cooling or precipitation) due to the occur-
rence of some [2 + 2] cycloadditions that lead consequently to
increased potential mobility within the polymer chains.
N2-saturated benzene solution is virtually the same as in (a).
Scheme 3
a
b
Results from these studies pose a challenge for construction
of cylindrical oligomers that are shape-persistent at high
temperatures and in solutions.
were covalently linked (Table 3 and Figure 7c). When cooled,
the irradiated sample was glassy, and its optical texture lacked
any discernible birefringence. The disorganization of the melt
phase makes the topology for intermolecular reaction more
difficult to attain. Also, the photoproducts are unable to pack
in columns when cooled.
Acknowledgment. We are indebted to Mr. Ashabir Fiseha
and Dr. Steven Pollack of the Chemistry Department, Howard
University, and Dr. Tai Ho of the Naval Research Laboratory,
Washington, DC, for use of their gel permeation chromato-
graphs. Financial support to RGW by the National Science
Foundation (Grants CHE-9213622 and CHE-9422560) is grate-
fully acknowledged. In memory of Professor Ernesto Giesbre-
cht, an excellent scientist, a wonderful person, and a visionary
whose pioneering efforts helped make the Instituto de Quimica
of the Universidade de Sao Paulo a premiere scientific institu-
tion.
The mode of intermolecular reaction in the melt phase should
be very different from that in the solid and liquid-crystalline
phases. Individual molecules in the melt are much more mobile
in all dimensions and lack orientational and positional order.
Most importantly, one GP may be linked to as many as fiVe
other GP molecules; highly branched oligomers are possible.5a
In all of the columnar phases except those from solid I of the
14GP, each GP can be linked to no more than two other GP
molecules by virtue of packing constraints. The observation
of low-molecular mass oligomers (ca. four linked monomers)
from irradiation of 14âGP in its melt phase indicates that the
proximity among the five cinnamate groups on each GP leads
to preferred intramolecular reactions and that columnar order
among GP molecules is a key entropic factor for effective [2
+ 2] intermolecular cycloadditions.
The behavior of melt phase 14âGP is in sharp contrast to
irradiation results from isotropic hexakis-N-(4-tetradecyloxy-
cinnamoyl)hexacyclen from which very high molecular weight,
highly-branched polymers were obtained.5a In that case,
intermolecular reactivity is clearly favored over the intramo-
lecular since cinnamoyl groups are kept farther apart in a
hexacyclen molecule than in a GP.
Supporting Information Available: One table of solvents
for recrystallizations and silica gel chromatographic purifica-
tions, seven figures (DSC thermograms of 8âGP, 14RGP, and
14âGP, powder X-ray diffractograms of 14âGP in various
phases, infrared and 1H NMR spectra of 14âGP before and after
irradiation of a neat, thin, liquid-crystalline film, and gel
permeation chromatograms before and after irradiation of
1
14âGP in various phases), and ten H NMR and ten FT-IR
spectra of the GP (30 pages). See any current masthead page
for ordering and Internet access instructions.
JA961665X