Synthesis and electronic properties of bis-styryl substituted trimeric aryleneethynylenes. Comparison of cruciforms with iso-cruciforms

Article abstract of DOI:10.1016/j.tet.2004.06.040

Four 1,2-distyryl-4,5-bis(phenylethynyl)benzenes are described. A two step synthesis, a combination of Horner and Sonogashira methods is utilized. The targets and their para isomers were examined by UV-vis and fluorescence spectroscopies as well as by cyc

Full text of DOI:10.1016/j.tet.2004.06.040

Tetrahedron 60 (2004) 7157–7167
Synthesis and electronic properties of bis-styryl
substituted trimeric aryleneethynylenes.
Comparison of cruciforms with iso-cruciforms
James N. Wilson,a Kenneth I. Hardcastle,b Mira Josowicza and Uwe H. F. Bunza
,
*
aSchool of Chemistry and Biochemistry, Georgia Institute of Technology, 770 State Street, Atlanta, GA 30332, USA
bDepartment of Chemistry, Emory University, 1515 Pierce Drive, Atlanta, GA 30322, USA
Received 4 May 2004; revised 27 May 2004; accepted 4 June 2004
Dedicated to Professor Robert H. Grubbs on the occasion of the Tetrahedron prize
Abstract—Four 1,2-distyryl-4,5-bis(phenylethynyl)benzenes are described. A two step synthesis, a combination of Horner and Sonogashira
methods is utilized. The targets and their para isomers were examined by UV–vis and fluorescence spectroscopies as well as by cyclic
voltammetry. They show solvatochromic behavior and are easily oxidized if the styryl units carry dialkylamino substitutents. Single crystal
structures of three derivatives have been obtained.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
There has been considerable interest in poly(paraphenyl-
eneethynylene)s (PPE)3 as active layers in light emitting
devices.4 And while several encouraging reports have
appeared in the literature, the hole injecting properties of
PPEs present a problem in their use as OLED materials. The
difficulty in hole injection is due to the electron withdrawing
quality of the alkyne groups. It would be of interest, while
preserving the PPE skeleton, to improve the hole injection
capabilities of these polymers.5 As a consequence, a series
of PPEs with styryl side chains was synthesized.6 These
materials are more electron rich and showed elctrooptical
properties that were different from those displayed by the
PPEs. To understand the properties of these polymers
better, cruciform models 1-5 were prepared and their
emission and absorption were recorded both in solution
and in the solid state.7,8
Conjugated organic materials are—generally speaking—
wide bandgap semiconductors.1 Their band gap and their
emissive properties are primarily manipulated by choice of
the covalently assembled chemical structure. This approach
leads to an inexhaustible variability of chromophores. Upon
combination of two or more organic modules this variability
increases if their connection takes place in topologically
different ways; absorption and emission will change as a
result.
Contrary to inorganic semiconductors that only exist in the
solid state, as covalent assemblies,2 most organic semi-
conductors are soluble and processible from solution into
amorphous or crystalline films.1 The processing aspect adds
a further layer of complexity to the behavior of most organic
semiconductors, with the optical properties being dependent
upon solid-state ordering and concomitant aggregation
phenomena.3 As a consequence, organic semiconductors
might be compared to proteins where primary, secondary
and tertiary structures are present and critically important
for the correct function. While organic semiconductors are
much less complex than proteins, higher order structures—
as in the case of nature’s polymers—often define the organic
semiconductor’s electronic properties.
We describe the electronic effects of different topological
arrangements of two phenylethynyl and two styryl groups
around a central benzene ring. We compare two series of
emissive oligomers, the cruciforms A (1–5) and the iso-
cruciforms B (6–9) with respect to their optical, electronic,
and electroactive properties in solution and in the solid state;
we present single crystal X-ray structures of 6–8.
2. Results
2.1. Synthesis
Keywords: Organic semiconductors; Distyrylbenzenes; Alkynes;
Cruciforms; Solvatochromicity; Bandgap.
*
Corresponding author. Tel./fax: þ1-404-385-1795;
e-mail address: uwe.bunz@chemistry.gatech.edu
The synthesis of the cruciforms 1–3 and 5 has been
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.06.040

7158
J. N. Wilson et al. / Tetrahedron 60 (2004) 7157–7167
cruciform 4 (Scheme 1) in high yield and purity after
chromatography and crystallization from hexanes.9 For the
isocruciforms 6–9 (Schemes 2 and 3) the central inter-
mediates 17 and 18 were prepared. Starting from the
Scheme 2. (a) N-Bromo succinic imide, sun lamp, CHCl3, 6 h. (b) Triethyl-
phosphite (solvent), reflux 8 h.
described elsewhere.7 The cruciform 4 and the isocruci-
forms 6–9 were prepared utilizing a similar route. Starting
from the known diiodide 10 a Heck–Cassar–Sonogashira–
Hagihara coupling with the alkyne 11 furnished the
Scheme 3. (a) NaH, heat; slow addition of either 15 or 16. (b) (Ph3P)2PdCl2,
CuI, piperidine.
Scheme 1. (a) (Ph3P)2PdCl2, CuI, piperidine, THF.

J. N. Wilson et al. / Tetrahedron 60 (2004) 7157–7167
7159
compound 12 photochemical bromination furnished the
tetrahalide 13 after multiple recrystallizations in 19% yield.
furnished the isocruciforms 7–9 in yields between 80 and
94% after crystallization and chromatography. All of the
isolated materials are yellow to yellow–orange solids,
freely soluble in non-polar organic solvents and stable
crystalline materials in the solid state.
An Arbuzov reaction transformed 13 into the phosphonate
14, which in the presence of sodium hydride was subjected
to the conditions of a Horner10 olefination utilizing the
aldehydes 15 and 16 to give the intermediates 17 and 18.
The yields in these reactions were considerably lower than
those obtained in an analogous coupling for molecules
of the type 10. This is probably due (Scheme 4) to
intramolecular ring closure of the sodium salt of 14. The
ring closed species may then decompose into further
unidentified products. And indeed, this Horner reaction
was always accompanied by the formation of polar, but hard
to identify, spectra side products that gave rise to a forest of
signals in their proton NMR spectra. In the bisphosphonate
precursor to 10 and similar para-distyrylbenzenes this
intramolecular ring closure is sterically not possible and the
yields of those double Horner reactions are considerably
higher.7
2.2. Optical and photochemical properties; comparison
of cruciforms with the isocruciforms
The optical properties of 1–9 are listed in Table 1. The
absorption spectra of the conjugated crosses are unremark-
able and their bandgap decreases with increasing donor–
acceptor quality of the styryl and phenylethynyl side chains.
As a conesquence, 1 and 6 show the highest band gaps in
solution and in the solid state, while 5 and 9 show the smallest
ones. The isocruciforms show a somewhat larger band gap
than the cruciforms, suggesting that conjugation is attenuated.
In the isocruciforms all of the conjugative interactions of
one type of substituent (either styryl or phenylethynyl) are
of ortho- instead of the para-directionality.
Scheme 4. (a) NaH in mineral oil.
The intermediate 17 (Scheme 3) is coupled to 4-methyl-
(phenylacetylene), and gave 6 in 46% yield. The inter-
mediate 18 was subjected to a similar Heck–Cassar–
Sonogashira–Hagihara9 coupling utilizing phenylacetylene,
4-trifluoromethylphenylacetylene and 3,5-bis(trifluoro-
methyl)(phenylacetylene) (Scheme 5). This reaction
Figure 1. Emission spectra of compound 5 in different solvents and in the
solid state. The emission maximum of 5 varies from 521 to 608 nm
depending upon the environment.
The tetrasubstituted benzenes 1–9 are emissive in the solid
state and in solution. Figure 1 shows the emission spectra of
5 in differrent solvents. The emission in 5 ranges from 521
to 608 nm. It is instructive to note that the emission of
the pure hydrocarbon crosses 1 and 6 do not show any
appreciable dependence on solvent (Fig. 2, Table 1).
In Figure 2 a colored bar representation of all of the
fluorescence data of 1–9 is displayed to give an overview of
their emissive properties in different solvents and in the
solid state.
Some general trends can be gleaned. (1) The position of the
Scheme 5. (a) (Ph3P)2PdCl2, CuI, piperidine.
Table 1. Absorption and emission data for the cruciforms and isocruciforms 1–9
Substituents compound
Absorption (nm)
CHCl3
Emission (nm)
Acetone
Film
Hex.
411
426 sh 433
471
455
501
492
472
521
CHCl3
THF
EtOAc
MeOH
Film
Crystal
tert-Butyl, H; 1
Me, Me; 6
H, Nbu2; 2
369
366
442
410
405 sh
397 sh
474
421
433
510
518
542
539
530
558
539
418
430
515
521
548
549
545
573
562
418
431
535
540
562
564
564
591
591
415
429
510
514
546
546
541
568
556
416
430
532
536
554
555
555
583
580
491
479
560
539
573
584
543
585
566
487
497
603
535
595
594
549
608
581
H, Net2; 7
451
o-CF3, NBu2; 3
p-CF3, NBu2; 4
p-CF3, Et2; 8
M, m-(CF3)2, Nbu2; 5
M, m-(CF3)2, NEt2; 9
440
441
424 sh
470 sh
437 sh
486 sh
480 sh
462 sh
518 sh
484 sh
484

7160
J. N. Wilson et al. / Tetrahedron 60 (2004) 7157–7167
Figure 4. Emission spectra of 8 under irradiation in hexanes after
1,3,5….30 min. After 30 min the spectrum of 8 has disappeared, instead a
broad emission centered at 401 nm has developed.
Figure 2. Bar graph of the emission spectra of 1–9. The compounds are
grouped according to their substituent patterns. y-axis is the emission
maximum in nm.
prolonged exposure to UV light in a fluorometer at 315 nm.
In the case of the isocruciforms 6–9 fast bleaching in dilute
solution and in thin films is observed via an isosbestic point
(Fig. 4). Attempts to perform the irradiation in synthetically
useful concentrations led to the isolation of an unidentified
brownish polymeric-insoluble residue that resisted attempts
of a meaningful spectroscopic characterization. Mu¨llen et al.
had observed similar behavior when irradiating simple
1,2-distyrylbenzenes.11 A mass spectrum taken from a very
dilute sample of 8, irradiated in chloroform, showed the
presence of a molecular ion at 794 amu suggesting the
uptake of a chlorine atom under the reaction conditions.
We speculate that this is a photo-induced radical process,
possibly involving a Bergman-type reaction of the enediyne
moieties of the isocruciforms. Further investigations of this
interesting process are ongoing.
CF3-group on the phenylethynyl substituent (ortho or para
with respect to the alkyne group) in 3 and 4 does not have a
significant influence on their emission spectra. That is not
unexpected, because the CF3-groups have an inductive
effect. Their position should not be critical. (2) The emission
of the isocruciforms is blue shifted with respect to that
of their cruciform isomers. (3) In thin film and in the
crystalline state the emission of 1-9 is bathochromically
shifted, probably due to the formation of excimers. This red
shift, when going from solution into the crystalline state is
significantly larger in the cruciforms when compared to
their isocruciforms. Similar trends are observed in thin
films, but the emission is generally less red-shifted. (4) The
more polar the solvent, the more bathochromically shifted
the emission for the donor–acceptor substituted oligomers.
The distinct solvatochromicity of the emission is easily
understood in terms of the stabilization of the excited state.
The more donor–acceptor substituted the oligomer is, the
more its excited state is charge-separated, and the more red-
shifted the observed emission is. In the compounds 3–5, 8
and 9 these effects are most strongly developed, due to the
presence of both a donor (dialkylamino) and one or two
acceptor groups (CF3).
In the solid state the bleaching process can be utilized to
irreversibly etch patterns into thin films of 8. Figure 5 shows
such a pattern ‘RHG’ etched into a thin film of 8.
The quantum yields of 1, 2 and 4–9 were determined in
hexanes and in chloroform (Fig. 3). Overall the quantum
yields tend to be higher in hexanes than in chloroform, but
there is no obvious correlation of the quantum efficiency
with the structure or with the solvent utilized. The
cruciforms 1–5 are photostable and do not bleach after
Figure 5. Thin film of 8 irradiated through an RHG mask at 315 nm.
Irradiated areas are non-fluorescent.
2.3. Electrochemistry of the isocruciforms 6–9
The electrochemistry of the isocruciforms 6–9 was
examined by cyclic voltammetry to correlate 6–9’s redox
properties to their structure. Table 2 shows the electro-
chemical potentials of 6–9 for oxidation and for reduction.
Comparison of the oxidation potentials of 6–9 reveals that 6
is oxidized at 1.27 V but 7–9 are oxidized at a considerably
lower potential, that is, at 0.52–0.57 V. The CF3-groups in 8
or 9 do not seem to play a role in the determination of the
oxidation potential; the diethylamino groups have the
determining influence on the oxidation potential. The
positive charge of the oxidized species—we speculate—is
Figure 3. Quantum yield (y-axis) of 1, 2 and 4–9 in hexanes and in
chloroform.

J. N. Wilson et al. / Tetrahedron 60 (2004) 7157–7167
Table 2. Reduction and oxidation potentials of 6–9 versus standard
7161
hydrogen electrode, HOMO and LUMO positions of 6–9 calculated from
oxidation and reduction potentials as well as by SPARTAN with B3LYP
utilizing the 6-31Gp p basis set
6
7
8
9
Oxidation (V)
1.27
1.35
E1/2
Peak
0.52
0.56
0.57
0.62
0.55
0.63
HOMO
25.77
25.02
25.07
25.05
Reduction (V)
21.77
21.86
E1/2
Peak
LUMO
21.83
21.94
22.67
21.61
21.71
22.89
21.62
21.70
22.88
22.73
Band gap
3.04
2.35
2.18
2.17
Quantum Chemical Calculations (eV)
HOMO
LUMO
25.16
22.04
3.12
24.74
21.68
3.06
24.96
22.05
2.91
25.04
22.20
2.84
Band gap
located on the distyrylbenzene branch of 7–9. That is
reasonable, because the presence of the diethylamino group
pumps electron density into the already electron rich
p-system. The phenyleneethynylene branch does not seem
to be involved in the reversible electrochemical oxidation
event.
Figure 7. HOMO (top) and LUMO (bottom) of 9. The diethylamino groups
are substituted by simple NH2 groups to decrease the time to calculate the
structures.
The isocruciforms 6 and 7 are reduced between 21.63 and
21.70 V while 8 and 9 are reduced at around 21.47 V, at
significantly lower negative potential than 6 and 7. This is
easily understood, because the LUMO of 6–9 should be
located on the electron deficient bis(phenylethynyl)benzene
branch of the molecules. The electron accepting trifluoro-
methyl substituents help to accommodate the negative
charge on this branch leading to a lower reduction potential.
2.4. Quantum chemical calculations
To understand the optical (i.e., band gap, solvatochromicity)
and the redox properties of 6–9, quantum chemical
calculations (B3LYP utilizing 6-31Gp p basis set;
SPARTAN) were performed. Table 2 contains the results
of the calculations for 6–9. In Figures 6 and 7 the calculated
HOMOs and LUMOs of 6 and 9 are shown. The values in
Table 2 demonstrate the decreasing band gap when going
from the hydrocarbon 6 to the donor–acceptor substituted
arene 9. The localization of the HOMO and the LUMO of
6 and of 9 are critical to understand the solvatochromic
behavior of the isocruciforms. In 6 the HOMO as well as
the LUMO are spread out over the whole p-system. Both
orbitals are decidedly delocalized (Fig. 6). In 9 that is not
the case (Fig. 7). Instead the HOMO is mostly located on the
distyrylbenzene branch of the molecule, while the LUMO
resides mostly on the aryleneethynylene branch of the
isocruciform 9. The central ring is an integral part of both
orbitals. Upon photonic excitation, an electron from the
HOMO will be advanced into the LUMO to give a highly
polarized state. This polarized excited state should be
significantly stabilized by polar solvents, as observed
(Table 1). Increasing solvent polarity leads to a significantly
red shifted emission but only to a small change in absorption
as expected for an excited state effect. As a means of
comparison we have calculated the structure and the
energetics of 5 (B3LYP; 6-31Gp p basis set). In Figure 8
the HOMO and the LUMO of 5 are shown. The calculated
band gap of 5 is 2.7 eV.
The band gap of 5 is lower than that of 9, which is 2.91 eV.
The HOMO and the LUMO of 5 (Fig. 8) are even more
Figure 6. HOMO (top) and LUMO (bottom) of 6. The methyl groups are
omitted to minimize the time to calculate the structures.

7162
J. N. Wilson et al. / Tetrahedron 60 (2004) 7157–7167
Figure 8. Calculated (B3LYP, 6-31Gp p , SPARTAN 04) HOMO (top,
24.98 eV) and LUMO (bottom, 22.28 eV) of 5. The calculated band gap is
2.70 eV.
Figure 9. Comparison of the calculated and experimental band gaps of the
compounds 1, 2 and 4–9. The values are taken from Table 3.
Table 3. Band gap of 1,2 and 4–9 calculated from oxidation and reduction
potentials, from the optical band gap as well as by SPARTAN with B3LYP
utilizing the 6-31Gp p basis set. Electrochemical band gaps of 1–5 are from
Ref. 7
Calculated bandgap Optical bandgap Electro-chemical bandgap
(eV)
(eV)
(V)
1
2
4
5
6
7
8
9
3.06
3.08
2.72
2.70
3.12
3.06
2.89
2.84
3.08
2.56
2.47
2.39
3.08
2.75
2.63
2.52
2.35
2.33
2.33
2.35
3.04
2.35
2.18
2.17
Figure 10. Top: ADP plot representation of 6. Notable is the disorder, that
renders alkyne and alkene units identical in the solid state. Middle, bottom:
tetragonal packing of 6. The molecules of 6 are packed in tilted stacks.

J. N. Wilson et al. / Tetrahedron 60 (2004) 7157–7167
7163
localized on their relative branches than the HOMO and the
LUMO of 9 (Fig. 9, Table 3).
The quantum chemical calculations support a self-consistent
picture that explains the decreased band gap when going
from 6 to 9 as well the increased solvatochromicity. Both
effects are due to the increased localization of the HOMO
and the LUMO respectively on the distyrylbenzene and the
aryleneethynylene branches in 9. When the HOMO and the
LUMO of 9 are compared to the frontier orbitals of 57 one
can see that the geometrical separation of HOMO and
LUMO (Figs. 7 and 8) is even more succinct in 5. So is 5’s
solvatochomicity. The herein presented calculations are in
good qualitative agreement with the experimental band
gap data obtained from fluorescence spectroscopy and
from cyclic voltammetry and explain the observed
solvatochromicity.
2.5. X-ray crystal structures of the isocruciforms 6, 7,
and 8
To get a better feel for the structures and the conformations
of the isocruciforms in the solid state, single crystalline
specimen of 6–8 were grown and their structures solved
(Figs. 10–12). Compound 9 did not form single crystals but
a microcrystalline powder. However, 6–8 are quite
disordered in the solid state. The compound 6 crystallizes
in an unusual tetragonal space group. That is due to the
structural similarity of the styryl and the phenylethynyl
substituents that are completely disordered in the solid state.
In Figure 10 the ADP plot of 6 and two views of its packing
Figure 12. Top: ADP plot representation of a representative molecule of 8.
The trifluoromethyl groups are rotationally completely disordered. Middle,
˚
bottom: packing diagram of 8. Distance between two layers is 3.51 A.
in the solid state are shown. The alkyne/alkyne connectors
are indistinguishable in the solid state. The arene rings are
somewhat twisted with respect to each other. The twist
angles between the central benzene and the outer rings vary
from 12 to 328. Interestingly, pairs of oppositely located
‘arms’ show similar torsion angles. The molecules are
packed in tilted stacks with an interstack distance of approx.
˚
3.6 A, the van der Waals radius of carbon.
An ADP plot representation and views of the packing of 7
are shown in Figure 11. The ethyl groups of the amine
substituents in 7 are disordered, and the outer rings are
twisted with respect to the central one. One of the
Figure 11. Top: ADP plot representation of 7. Bottom: packing diagram of
˚
7. Distance between two layers is 3.596 A.

7164
J. N. Wilson et al. / Tetrahedron 60 (2004) 7157–7167
Table 4. X-ray data of 6–8
6
7
8
Empirical formula
Formula weight
Temperature
C42 H30
534.66
173(2) K
0.71073 A
Tetragonal
P4/n
C46 H44 N2
624.83
173(2) K
1.54178 A
Monoclinic
P2(1)/c
C48 H42 F6 N2
760.84
173(2) K
1.54178 A
Triclinic
P-1
˚
˚
˚
Wavelength
Crystal system
Space group
Unit cell dimensions
˚
˚
˚
˚
˚
˚
˚
a¼23.809(2) A, a¼908.
a¼8.3889(9) A a¼908.
a¼10.0454(7) A a¼91.787(1)8.
˚
b¼23.809(2) A b¼908.
b¼31.632(3) A b¼92.992(5)8.
b¼13.88826(7) A b¼101.471(4)8.
˚
c¼5.8229(8) A g¼908.
c¼13.489(1) A g¼908.
c¼15.3(10) A g¼107.723(4)8.
3
3
3
˚
3300.8(6) A
˚
3574.4(6) A
˚
1981.7(2) A
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F 2
Final R indices (I.2s(I))
R indices (all data)
Extinction coefficient
Largest diff. peak and hole
4
4
2
1.076 Mg/m3
1.161 Mg/m3
1.275 Mg/m3
0.061 mm21
1128
0.505 mm21
1336
0.775 mm21
794
0.75£0.12£0.12 mm3
0.40£0.20£0.17 mm3
0.57£0.12£0.10 mm3
1.71–21.248
224#h#24, 224#k#24, 25#l#5
23096
2.79–66.698
29#h#7, 237#k#25, 213#l#16
12497
2.96–38.078
28#h#, 211#k#11, 212#l#12
18747
1825 (R(int)¼0.0457)
4226 (R(int)¼0.0644)
8695 (R(int)¼0.0000)
99.9%
67.0%
99.9%
Semi-empirical from equivalents
1.000 and 0.757805
Full-matrix least-squares on F 2
1825/0/194
Nd
1.000 and 0.425485
4226/0/438
1.069
R1¼0.0980, wR2¼0.2759
R1¼0.1692, wR2¼0.3198
0.0045(8)
8695/0/252
1.180
R1¼0.139, wR2¼0.3246
R1¼0.1815, wR2¼0.3481
Nd
2.227
R1¼0.1489, wR2¼0.4577
R1¼0.1567, wR2¼0.4654
Nd
23
23
23
˚
˚
˚
0.786, 20.323 e A
0.371, 20.311 e A
0.710 and 20.573 e A
phenyleneethynylene units is twisted around 138, while the
other one is twisted around 378. The degree of twisting with
respect to the central core ranges from 11 to 588. In 7 two of
the substituents are almost coplanar with the central ring,
while the other two show a significant twist. There are four
independent molecules in the unit cell and each of them has
a set of somewhat different twist angles of the benzene rings
with respect to the center ring.
pathways in the ortho connected isocruciforms. The
electrochemical oxidation and reduction of the isocruci-
forms combined with the quantum chemical calculations
and the emission data in a series of different solvents shows
that HOMO and LUMO of the isocruciforms are located on
the distyrylbenzene and the bisphenylethynyl branch of the
molecules respectively, similar to those observed for 1–5.
This trend is pronounced in the donor–acceptor substituted
isocruciforms 8 and 9 where an electronic decoupling of
HOMO and LUMO is more prevalent. The cruciforms and
the isocruciforms are cross conjugated molecules, even
though in a non-classical sense.12,13 The large bathochromic
shift in emission of 1–9 in the crystalline state can be
explained by the packing of the p-systems that leads to
strong electronic interactions in the excited state.14 The
presence of the 1,2-distyrylbenzene unit in 6–9 makes them
potentially attractive as materials for photopatterning. In
future we will report single crystal reactivity of 6–9 and
their incorporation into polymers. These materials are
interesting for third order NLO-phores and similar
applications.
In the crystals of the compound 8 (Fig. 12) the situation is
similar to that of 7. Here as well two out of the four
substituents are more twisted than the other two. An
additional feature is the rotational disorder of the CF3
groups that is not unexpected in this type of structures.
In all of the cases, the p-systems are packed closely and in a
herringbone or a modified herringbone pattern. The
distances between the p-systems are in the range of the
van der Waals distance of carbon, that is, between 3.5 and
˚
3.8 A. The close proximity of the p-systems leads to their
interaction in the solid state, and we interpret the observed
bathochromic emissions of crystalline or microcrystalline
species of 6–9 as a sign of excimer formation of the
isocruciforms in the solid state (Table 4).
4. Experimental
4.1. General
3. Conclusions
4.1.1. 1,2-Bis-bromomethyl-4,5-diiodo-benzene (13). A
round bottom flask with a stirbar was charged with 50.0 g
(0.140 mol) of 12, 120 g (0.667 mol) of N-bromosuccini-
mide, 1.3 L of chloroform and 1.00 g (6.85 mmol) of t-butyl
peroxide. A reflux condenser was placed atop the flask and
the reaction mixture was irradiated with a sun lamp (120 W,
General Electric 120R40/P1) while refluxing for 6 h. The
In conclusion we have prepared the isocruciforms 6–9 by a
combination of Horner and Heck–Cassar–Sonogashira–
Hagihara reactions. The optical and electrochemical proper-
ties of the isocruciforms are similar to those of the
cruciforms 1–5, however, their emission spectra are
somewhat blue shifted, due to the decreased conjugative

J. N. Wilson et al. / Tetrahedron 60 (2004) 7157–7167
7165
reaction mixture was allowed to cool to room temperature
and crystals of succinimide formed which were filtered off.
The chloroform solution was washed three times with
500 mL of 5% sodium sulfite solution followed by two
washes with 500 mL of water. The chloroform was removed
by roto-vap and the resulting solid was crystallized from
hexane. Yield: 14.0 g, 19.4%. Mp: 98 8C. 1H NMR (CDCl3)
d 7.79 (s, 2H), 4.47 (s, 4H). 13C NMR (CDCl3) 141.1, 137.6,
108.7, 28.0. IR (cm21): 3014.5, 2896.9, 2849.6, 2735.4,
2660.1, 2408.9, 2291.3, 1992.3, 1972.1, 1777.8, 1729.6,
156.5, 1451.8, 1432.1, 1341.9, 1268.1, 1212.2, 1168.3,
1142.3, 1098.4, 898.8, 877.1.
capped with a balloon. The schlenk cock was closed and the
stem was capped with a septum. The reaction mixture was
heated to reflux for 30 min at which point it had turned
yellow. Diethylaminobenzaldehyde 16 (2.30 g, 12.9 mmol)
was dissolved in the minimum volume of dry THF (,8 mL)
and was added via a syringe in 8 aliquots every 5 min. The
reaction was allowed to cool to room temperature. The
crude reaction mixture was poured slowly into a 2 L
Erlenmeyer flask was filled with 1 kg of ice. This mixture
was separated with the addition of NaCl and the THF
portion was extracted then reduced. The crude product was
redissolved in 50 mL of hexane with 10 mL of dichloro-
methane. This solution was run over a fritted funnel with a
silica plug in the same solvent mixture. The product
proceeded as a yellow band, was collected and the solution
was evaporated and further purified by crystallization from
hexane/dichloromethane mixture. The product was obtained
as large yellow crystals (1.55 g, 39% yield). Mp: 175 8C. 1H
NMR (CDCl3) d 7.94 (s, 2H), 7.36–7.35 (d, 4H, 8.24 Hz),
7.01–6.96 (d, 2H, J3H,H¼15.9 Hz), 6.89–6.86 (d, 2H,
J3H,H¼15.9 Hz), 3.41–3.34 (q, 8H, J3H,H¼7.1 Hz), 1.52–
1.14 (t, 12H, J3H,H¼7.1 Hz). 13C NMR (CDCl3) d 147.9,
138.0, 136.5, 132.5, 128.4, 124.5, 119.4, 111.8, 104.9, 44.7,
13.0. IR (cm21): 3039.1, 3006.3, 2959.6, 2923.4, 1601.8,
1596.0, 1576.2, 1557.9, 1512.6, 1495.7, 1490.4, 1479.3,
1464.8, 1441.2, 1429.6, 1373.7, 1357.8, 1355.4, 1265.2,
1200.1, 1236.6, 1138.4, 1092.1, 1078.1, 1070.4, 1025.1,
947.0, 930.1.
4.1.2. [2-(Diethoxy-phosphorylmethyl)-4,5-diiodo-ben-
zyl]-phosphonic acid diethyl ester (14). A 250 mL round
bottom flask was charged with a stirbar, 23.2 g (45.0 mmol)
of 13, and 100 mL of triethylphosphite. A condensor
column was placed atop the reaction vessel and the reaction
refluxed for 8 h. The reaction was allowed to cool briefly
before the excess triethylphosphite was distilled off. The
product was crystallized from hexane to offer clear crystal-
line cubes. Yield: 27.5 g, 97%. Mp: 115 8C. 1H NMR
(CDCl3) d 7.70 (d, 2H, J4H,P¼1.9 Hz), 4.06–3.97 (m, 8H),
3.31–3.24 (d, 4H, J2H,P¼20.33 Hz), 1.28–1.23 (t, 12H,
J3H,H¼7.1 Hz). 13C NMR (CDCl3) d 141.6, 133.1, 105.7,
62.3, 31.5–29.6 (d, J1P,C¼137.4 Hz) , 16.5. IR (cm21):
2978.4, 2909.9, 2863.1, 2828.9, 2810.1, 1748.5, 1733.4,
1569.5, 1476.4, 1452.3, 1408.9, 1394.0, 1366.5, 1341.9,
1249.8, 1221.8, 1202.5, 1160.1, 1099.4, 1047.3, 1019.3,
893.0, 864.5.
4.1.5. 1,2-Bis-p-tolylethynyl-4,5-bis-(2-p-tolyl-vinyl)-
benzene (6). 1.00 g (1.78 mmol) of 17 were placed into a
Schlenk tube with a stirbar under nitrogen purge. To this
was added 5.0 mL of THF and 3.0 mL of piperidine. This
solution was allowed to stir vigorously for 10 min. 10 mg
(14 mmol) of Pd(PPh3)2Cl2 and 5.0 mg (26 mmol) of CuI
were added and the tube was fitted with a septum. With a
syringe, 516 mg (4.45 mmol) of 4-methyl-1-ethynylben-
zene were added. The reaction was allowed to proceed at
room temperature for 48 h. The crude reaction mixture was
diluted with hexane and run through a silica plug on a fritted
funnel with hexane (100 mL) as an eluent. The hexane
solution was collected and discarded. A 50:50 dichloro-
methane:hexane mixture (250 mL) was then passed through
the silica plug and collected. This solution was reduced to
yield 437 mg (46% yield) of a green crystalline solid. To
obtain crystals for XRD analysis 100 mg were dissolved in
approximately 10 mL of dichloromethane. Aliquots of this
solution (2–3 mL) were placed in the bottom of a test tube
filled with hexane and capped with a septum. Mp: 233 8C.
4.1.3. 1,2-Diiodo-4,5-bis-(2-p-tolyl-vinyl)-benzene (17).
A 500 mL Schlenk flask was charged with a stirbar and
5.40 g (8.57 mmol) of 14 while under nitrogen purge. Dry
THF (250 mL) was added followed by 1.2 g (50 mmol) of
NaH. A reflux condenser was placed atop the reaction vessel
and capped with a balloon. The Schlenk cock closed and the
stem was capped with a septum. The reaction mixture was
heated to reflux for 30 min at which point it had turned
yellow. Tolualdehyde 15 (2.57 g, 21.4 mmol) was added via
a syringe in approximately 10 0.25 mL aliquots every 5 min.
The reaction was allowed to cool to room temperature. The
crude reaction mixture was poured slowly into a 2 L
Erlenmeyer flask filled with 1 kg of ice. The product was
extracted with hexane (2£200 mL), reduced and run over a
fritted funnel with a silica plug. The product was obtained as
1
a pale yellow solid (1.72 g, 36% yield). Mp: 150 8C. H
NMR (CDCl3) d 8.01 (s, 2H), 7.41–7.39 (d, 4H,
J3H,H¼7.69 Hz), 7.21–7.14 (m, 6H), 6.97–6.92 (d, 2H,
J3H,H¼15.9 Hz), 1.53 (s, 6H). 13C NMR (CDCl3) d 138.2,
137.2, 136.7, 132.5, 129.4, 126.6, 123.1, 106.0, 99.3, 21.4.
IR (cm21): 3045.9, 3040.1, 3017.9, 3006.8, 2965.8, 2917.1,
2853.0, 1628.3, 1624.0, 1607.6, 1568.0, 1508.7, 1456.6,
1448.0, 1436.9, 1409.4, 1376.6, 1354.9, 1325.5, 1300.4,
1281.6, 1279.2, 1262.3, 1229.5, 111.81.3, 1118.2, 1108.5,
1040.0, 1038.3, 1016.9, 949.9, 847.2.
1H NMR (CDCl3) d 7.78, 7.51–7.48 (d, 4H, J3H,H
¼
7.97 Hz), 7.46–7.43 (d, 4H, J3H,H¼7.97 Hz), 7.38–7.33
(d, 2H, 15.9 Hz), 7.21–7.16 (m, 8H), 7.09–7.03 (d, 2H,
J3H,H¼15.9 Hz), 2.37 (s, 12H). 13C NMR (CDCl3) d 138.4,
137.9, 135.6, 134.3, 132.1, 131.5, 129.5, 129.4, 129.1,
126.6, 124.5, 124.1, 120.2, 94.0, 87.9, 21.7, 21.4. IR (cm21):
3024.2, 2918.1, 2855.4, 2731.0, 2203.0, 1905.1, 1792.2,
1656.7, 1629.3, 1605.6, 1512.6, 1481.7, 1447.5, 1412.3,
1377.1, 1302.3, 1281.1, 1264.7, 1209.8, 1181.8, 1118.2,
1104.7, 1039.6, 1033.3, 1016.9, 970.1, 962.4, 816.8, 806.7.
4.1.4. 1,2-Bis-(3,5-bis-trifluoromethyl-phenylethynyl)-
4,5-bis(4-diethylaminostyryl)benzene (18). A 500 mL
Schlenk flask was charged with a stirbar and 3.70 g
(5.87 mmol) of 14 while under nitrogen purge. Dry THF
(250 mL) was added followed by 1.2 g (50 mmol) of NaH.
A reflux condenser was placed atop the reaction vessel and
4.1.6. 1,2-Bis-phenylethynyl-4,5-bis(4-diethylamino)-
styryl-benze (7). 520 mg (0.769 mmol) of 18 were placed
into a Schlenk tube with a stirbar under nitrogen purge. To

7166
J. N. Wilson et al. / Tetrahedron 60 (2004) 7157–7167
this was added 5.0 mL of THF and 3.0 mL of piperidine.
This solution was allowed to stir vigorously for 10 min.
10 mg (14 mmol) of Pd(PPh3)2Cl2 and 5.0 mg (26 mmol) of
CuI were added and the tube was fitted with a septum. With
a syringe, 500 mg (4.9 mmol) of phenylacetylene were
added. The reaction was allowed to proceed at room
temperature for 48 h. The crude reaction mixture was
diluted with hexane and run through a silica plug on a fritted
funnel with hexane (100 mL) as an eluent. The hexane
solution was collected and discarded. A 50:50 dichloro-
methane:hexane mixture (250 mL) was then passed through
the silica plug and collected. This solution was reduced to
25 mL and poured into 200 mL of methanol. The precipitate
was collected to yield 384 mg (80%) of a yellow crystalline
solid. To obtain crystals for XRD analysis 100 mg were
dissolved in approximately 10 mL of dichloromethane.
Aliquots of this solution (2–3 mL) were placed in the
bottom of a test tube filled with methanol and capped with a
septum. Mp: 191–193 8C. 1H: d 7.88 (s, 2H), 7.64–7.61 (m,
4H), 7.46–7.43 (d, 4H, J3H,H¼8.5 Hz), 7.41–7.35 (m, 6H),
7.24–7.19 (d, 2H, J3H,H¼15.9 Hz), 7.06–7.00 (d, 2H,
16.2 Hz), 6.71–6.68 (d, 4H, 8.5 Hz), 3.44–3.37 (q, 8H,
J3H,H¼6.9 Hz), 1.23–1.18 (t, 12H, J3H,H¼6.9 Hz) 13C: d
147.8, 136.5, 132.3, 131.8, 129.5, 128.5, 128.4, 124.9,
123.8, 123.6, 120.4, 111.8, 93.5, 89.3, 44.8, 13.1. IR
(cm21): 3061.8, 3017.9, 2977.4, 2965.8, 2001.5, 1951.4,
1876.6, 1597.4, 1583.9, 1552.1, 1511.1, 1462.9, 1441.2,
1426.7, 1396.4, 1393.0, 1371.3, 1293.2, 1250.8, 1219.4,
1176.5, 1156.7, 1139.9, 1092.1, 1070.9, 1064.2
1154.3, 1120.1, 1104.2, 1064.6, 1013.5, 1001.5, 966.8,
955.2, 931.1.
4.1.8. 1,2-Bis-(3,5-bis-trifluoromethylphenylethynyl)-
4,5-bis(4-diethylaminostyryl)benzene
(9).
520 mg
(0.769 mmol) of 18 were placed into a Schlenk tube with
a stirbar under nitrogen purge. To this was added 5.0 mL of
THF and 3.0 mL of triethylamine. This solution was
allowed to stir vigorously for 10 min. 10 mg (14 mmol) of
Pd(PPh3)2Cl2 and 5.0 mg (26 mmol) of CuI were added and
the tube was fitted with a septum. With a syringe, 3.00 g
(12.6 mmol) of 3,5-bistrifluoromethylethynylbenzene were
added. The reaction was allowed to proceed at room
temperature for 48 h. The crude reaction mixture was
diluted with hexane and run through a silica plug on a fritted
funnel with hexane (100 mL) as an eluent. The hexane
solution was collected and discarded. A 50:50 dichloro-
methane:hexane mixture (250 mL) was then passed through
the silica plug and collected. This solution was reduced to
25 mL and poured into 200 mL of methanol. The precipitate
was collected to yield 646 mg (94% yield) of a yellow
crystalline solid. To obtain single crystalline specimen
100 mg were dissolved in approximately 10 mL of dichlor-
omethane. Aliquots of this solution (2–3 mL) were placed
in the bottom of a test tube filled with methanol and capped
1
with a septum. Mp: 207 8C. H NMR (CDCl3) d 7.97 (s,
4H,), 7.82 (s, 2H), 7.80 (s, 2H), 7.45 (d, 4H, J3H,H
¼
8.79 Hz), 7.21 (d, 2H, J3H,H¼15.9 Hz), 7.06 (d, 2H,
J3H,H¼15.9 Hz), 6.70 (d, 4H, J3H,H¼8.80 Hz), 3.44–3.37
(q, 8H, J3H,H¼7.12 Hz), 1.22–1.18 (t, 12H, J3H,H
¼
4.1.7. 1,2-Bis(4-diethylaminostyryl)-4,5-bis-(4-trifluoro-
methyl-phenylethynyl)benzene (8). 510 mg (0.754 mmol)
of 18 were placed into a Schlenk tube with a stirbar under
nitrogen purge. To this was added 5.0 mL of THF and
3.0 mL of piperidine. This solution was allowed to stir
vigorously for 10 min. 10 mg (14 mmol) of Pd(PPh3)2Cl2
and 5.0 mg (26 mmol) of CuI were added and the tube was
fitted with a septum. With a syringe, 3.00 g (17.6 mmol)
of 4-trimethylfluoromethyethynylbenznene were added.
The reaction was allowed to proceed at room temperature
for 48 h. The crude reaction mixture was diluted with
hexane and run through a silica plug on a fritted funnel with
hexane (100 mL) as an eluent. The hexane solution was
collected and discarded. A 50:50 dichloromethane:hexane
mixture (250 mL) was then passed through the silica plug
and collected. This solution was reduced to 25 mL and
poured into 200 mL of methanol. The precipitate was
collected to yield 457 mg (80%) of a yellow crystalline
solid. To obtain crystals for XRD analysis 100 mg were
dissolved in approximately 10 mL of dichloromethane.
Aliquots of this solution (2–3 mL) were placed in the
bottom of a test tube filled with methanol and capped with a
7.13 Hz). 13C NMR (CDCl3) d 148.0, 137.6, 133.2,
133.0–131.6 (q, J2C,F¼33.2 Hz), 131.4, 129.9, 128.5,
128.5–117.6 (q, JC,F¼272.6 Hz), 125.8, 124.5, 122.2,
121.2, 119.7, 111.8, 92.7, 90.5, 44.8, 13.0. IR (cm21):
2967.8, 2207.4, 1605.2, 1576.2, 1521.3, 1517.4, 1487.0,
1466.3, 1429.6, 1398.8, 1390.1, 1349.1, 1296.6, 1284.5,
1279.7, 1267.6, 1245.9, 1176.5, 1169.3, 1105.6, 1097.9,
1077.2, 1074.3, 959.5, 937.8.
4.2. Cyclic voltametry
Electrochemical experiments were carried out with CH
Instruments model 660 electrochemical workstation.
Cyclic voltammograms (CV) were obtained by using a
conventional three-electrode system. A platinum foil was
used as the counter electrode. A platinum disk electrode
(f¼ 1.2 mm) from Bioanalytical Systems serves as a
working electrode. Reference electrode A, Ag/0.1 M
AgNO3 in acetonitrile, was separated from the test by a
fritted bridge containing the background electrolyte
(0.1 M Bu4NPF6 in acetonitrile). The reference electrode
was calibrated before each experiment with the ferrocene/
ferrocenium (Fc/Fcþ) redox system. The E1/2 of 5 mM of
Fc/Fcþ in 0.1 M Bu4NPF6 in in acetonitrile was
0.37 V. The standard redox potential of the Fc/Fcþ
system has been determined to be 0.190 V (Bard, A. J.;
Faulkner, L. R. Electrochemical Methods; Wiley: New
York, 1980, p 701). Therefore, the potential of our
reference electrode A was 0.227 V versus S.H.E. All
solutions were purged prior to electrochemical measure-
ments using nitrogen gas. All solvents were dried with
1
septum. Mp: 220 8C. H NMR (CDCl3) d 7.73 (s, 2H),
7.64–7.57 (m, 8H), 7.41 (d, 4H, J3H,H¼8.24 Hz), 7.18 (d,
2H, J3H,H¼15.9 Hz), 7.01 (d, 2H, J3H,H¼15.9 Hz), 6.66 (d,
4H, J3H,H¼8.24 Hz), 3.40–3.37 (q, 8H, J3H,H¼6.87 Hz),
1.21–1.17 (t, 12H, J3H,H¼6.59 Hz). 13C NMR (CDCl3) d
137.05, 132.67, 131.90, 130.67–129.37 (q, J2C,F¼33.2 Hz),
129.68, 129.56–118.76 (q, J1C,F¼271.4 Hz), 128.47,
127.53, 125.54, 124.7, 122.82, 120.09, 111.76, 92.03,
91.51, 44.76, 13.03. IR (cm21): 2967.3, 2866.0, 2207.9,
1873.7, 1616.9, 1601.8, 1576.2, 1475.9, 1456.6, 1448.0,
1428.2, 1398.8, 1358.3, 1349.6, 1310.1, 1297.0, 1267.6,
˚
molecular sieves (3 A). All the salts were used as
received from Aldrich.

J. N. Wilson et al. / Tetrahedron 60 (2004) 7157–7167
7167
652–654. (d) Scherf, U. Top. Curr. Chem. 1999, 201,
163–222.
4.3. Quantum chemical calculations
4. (a) Schmitz, C.; Posch, P.; Thelakkat, M.; Schmidt, H. W.;
Montali, A.; Feldman, K.; Smith, P.; Weder, C. Adv. Funct.
Mater. 2001, 11, 41–46. (b) Kokil, A.; Shiyanovskaya, I.;
Singer, K. D.; Weder, C. Synth. Met. 2003, 138, 513–517.
5. Pschirer, N. G.; Miteva, T.; Evans, U.; Roberts, R. S.;
Marshall, A. R.; Neher, D.; Myrick, M. L.; Bunz, U. H. F.
Chem. Mater. 2001, 13, 2691–2696.
The program SPARTAN 04 was implemented on a
Windows XP platform (Dell, 3.0 GHz processor speed).
The geometries were first determined utilizing an AM1
calculation. The obtained geometries were utilized in a
second set of minimizations to determine the equilibrium
structure utilizing B3LYP with a 6-31Gp p basis set.
4.4. Single crystal structure determination15–20
6. Wilson, J. N.; Windscheif, P. M.; Evans, U.; Myrick, M. L.;
Bunz, U. H. F. Macromolecules 2002, 35, 8681–8683.
7. Wilson, J. N.; Josowicz, M.; Wang, Y. Q.; Bunz, U. H. F.
Chem. Commun. 2003, 2962–2963.
Suitable crystals of 6, 7 and 8 were coated with Paratone N
oil, suspended in a small fiber loop and placed in a cooled
nitrogen gas stream at 173 K on a Bruker D8 SMART
8. Wilson, J. N.; Smith, M. D.; Enkelmann, V.; Bunz, U. H. F.
Chem. Commun., in press.
˚
APEX CCD graphite monochromated Mo Ka (0.71073 A)
diffractometer for 6 and a D8 SMART 1000 CCD graphite
9. (a) Yamamoto, T. Synlett 2003, 425–450. (b) Yamamoto, T.
Bull. Chem. Soc. Jpn 1999, 72, 621–638. (c) Sonogashira, K.
J. Organomet. Chem. 2002, 653, 46–49. (d) Sonogashira, K.;
Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 4467–4470.
(e) Negishi, E.; Anastasia, L. Chem. Rev. 2003, 103,
1979–2017.
˚
monochromated Cu Ka (1.5418 A) difractometer for 7 and
8. Data were measured using a series of combinations of phi
and omega scans with 20 s frame exposures and 0.38 frame
widths. Data collection, indexing and initial cell refinements
were all carried out using SMART software. Frame
integration and final cell refinements were done using
SAINT software. The final cell parameters were determined
from least-squares refinement on 2911, 3793 and 3786
reflections for 6, 7 and 8, respectively. The SADABS
program was used to carry out absorption corrections. The
structure was solved using Direct methods and difference
Fourier techniques (SHELXTL, V6.12). Hydrogen atoms
were placed their expected chemical positions using the
HFIX command and were included in the final cycles of
least squares with isotropic Uij’s related to the atom’s ridden
10. (a) Horner, L.; Hoffmann, H.; Wippel, H. G. Chem. Ber. 1958,
91, 61. (b) Horner, L.; Klink, W. Tetrahedron Lett. 1964, 36,
2467.
¨
¨
11. (a) Bohm, A.; Adam, M.; Mauermann, H.; Stein, S.; Mullen,
¨
K. Tetrahedron Lett. 1992, 33, 2795–2798. (b) Bohm, A.;
Mu¨llen, K. Tetrahedron Lett. 1992, 33, 611–614. (c) Mu¨ller,
M.; Mauernann-Du¨ll, H.; Wagner, M.; Enkelmann, V.;
Mu¨llen, K. Angew. Chem., Int. Ed. 1995, 34, 1583–1586.
12. (a) Tykwinski, R. R.; Zhao, Y. M. Synlett 2002, 1939–1953.
(b) Zhao, Y. M.; McDonald, R.; Tykwinski, R. R. J. Org.
Chem. 2002, 67, 2805–2812. (c) Tykwinski, R. R.; Schreiber,
M.; Carlon, R. P.; Diederich, F.; Gramlich, V. Helv. Chem.
Acta 1996, 79, 2249–2281. (d) Tykwinski, R. R.; Diederich, F.
Liebigs Ann.-Recueil 1997, 649–661.
˚
upon. The C–H distances were fixed at 0.93 A (aromatic
˚
and amide), 0.98 A (methine), 0.97 A (methylene), or
˚
˚
0.96 A (methyl). All non-hydrogen atoms were refined
anisotropically. Scattering factors and anomalous dispersion
corrections are taken from the International Tables for X-ray
Crystallography.19 Structure solution, refinement, graphics
and generation of publication materials were performed by
using SHELXTL, V6.12 software. Additional details of data
collection and structure refinement are given in Table 4.
13. (a) Hagen, S.; Hopf, H. Top. Curr. Chem. 1998, 196, 45–89.
(b) Hopf, H.; Kreutzer, M.; Jones, P. G. Chem. Ber. 1991, 124,
1471–1475.
14. (a) Cao, J.; Curtis, M. D. Chem. Mater. 2003, 15, 4424–4430.
(b) Koren, A. B.; Curtis, M. D.; Francis, A. H.; Kampf, J. W.
J. Am. Chem. Soc. 2003, 125, 5040–5050. (c) Koren, A. B.;
Curtis, M. D.; Kampf, J. W. Chem. Mater. 2000, 12,
1519–1523.
Acknowledgements
15. SMART Version 5.628, Bruker AXS, Inc Analytical X-ray
Systems, 5465 East Cheryl Parkway, Madison WI 53711,
55373, 2003.
U. Bunz and J. Wilson thank the National Science
Foundation (DMR 0138948) for generous support.
16. SAINT Version 6.36A, Bruker AXS, Inc, Analytical X-ray
Systems, 5465 East Cheryl Parkway, Madison, WI 53711,
55373, 2002.
References and notes
17. SADABS Version 2.08, George Sheldrick, University of
¨
Gottingen, 2003.
1. Electronic Materials, the Oligomers Approach; Mu¨llen, K.,
Wegner, G., Eds.; Wiley-VCH: Weinheim, 1998.
18. SHELXTL V6.12, Bruker AXS, Inc, Analytical X-ray
Systems, 5465 East Cheryl Parkway, Madison, WI 53711,
55373, 2002.
2. Ball, P. Made to Measure; Princeton: Paperback, Princeton,
1999.
3. (a) Bunz, U. H. F. Chem Rev. 2000, 100, 1603–1644.
(b) Halkyard, C. E.; Rampey, M. E.; Kloppenburg, L.; Studer-
Martinez, S. L.; Bunz, U. H. F. Macromolecules 1998, 31,
8655–8659. (c) Miteva, T.; Palmer, L.; Kloppenburg, L.;
Neher, D.; Bunz, U. H. F. Macromolecules 2000, 33,
19. International Tables for X-ray Crystallography, Volume C;
Wilson, A. J. C., Ed.; Kynoch, Academic: Dordrecht, 1992;
p 500 Tables 6.1.1.4 (pp 500–502) and 4.2.6.8 (pp 219–222).
20. CCDC reference numbers for 6–8 are 238654-6.