Steric communication of chiral information observed in dendronized polyacetylenes

Article abstract of DOI:10.1021/ja0665848

Structural and retrostructural analysis of helical dendronized polyacetylenes (i.e., self-organizable polyacetylenes containing first generation dendrons or minidendrons as side groups) synthesized by the polymerization of minidendritic acetylenes with [R

Full text of DOI:10.1021/ja0665848

Published on Web 11/30/2006
Steric Communication of Chiral Information Observed in
Dendronized Polyacetylenes
Virgil Percec,*,† Emad Aqad,† Mihai Peterca,‡ Jonathan G. Rudick,† Lance Lemon,†
Juan C. Ronda,† Binod B. De,† Paul A. Heiney,‡ and E. W. Meijer§
Contribution from the Roy & Diana Vagelos Laboratories, Department of Chemistry, UniVersity
of PennsylVania, Philadelphia, PennsylVania 19104-6323, Department of Physics and
Astronomy, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6396, and Laboratory
of Macromolecular and Organic Chemistry, EindhoVen UniVersity of Technology, P.O. Box 513,
5600 MB EindhoVen, The Netherlands
Received September 12, 2006; E-mail: percec@sas.upenn.edu
Abstract: Structural and retrostructural analysis of helical dendronized polyacetylenes (i.e., self-organizable
polyacetylenes containing first generation dendrons or minidendrons as side groups) synthesized by the
polymerization of minidendritic acetylenes with [Rh(nbd)Cl]2 (nbd ) 2,5-norbornadiene) reveals an ∼10%
change in the average column stratum thickness (l) of the cylindrical macromolecules with a chiral periphery,
through which a strong preference for a single-handed screw-sense is communicated. The cylindrical
macromolecules reversibly interconvert between a three-dimensional (3D) centered rectangular lattice (Φr-c,k
)
exhibiting long-range intracolumnar helical order at lower temperatures and a two-dimensional (2D)
hexagonal columnar lattice (Φh) with short-range helical order at higher temperatures. A polymer containing
chiral, nonracemic peripheral alkyl tails is found to have a larger l as compared to the achiral polymers. In
methyl cyclohexane solution, the same polymer exhibits an intense signal in circular dichroism (CD) spectra,
whose intensity decreases upon heating. The observed change in l indicates that the chiral tails alter the
polymer conformation from that of the corresponding polymer with achiral side chains. This change in
conformation results in a relatively large free energy difference (∆Gh) favoring one helix-sense over the
other (per monomer residue). The capacity to distort the polymer conformation and corresponding free
energy is related to the population of branches in the chiral tails and their distance from the polymer backbone
by comparison to recently reported first and second generation dendronized polyphenylacetylenes.
Introduction
exhibit helical conformations are valued for molecular recogni-
tion and have been employed as stationary phase materials in
Helical chirality in macromolecules and supramolecular
assemblies offers a wellspring of challenges and inspiration at
the interface of scientific disciplines.1-3 We, for example, have
demonstrated the unique ability of dendritic dipeptide assemblies
to form helical porous nanostructures inspired by transmembrane
proteins.4 We and others have identified helical arrangements
of electroactive components as a critical opportunity to improve
self-assembled organic electronics.3,5,6 Chiral polymers that
HPLC columns and as sensory elements.2,7
Dynamic helical polymers exhibit an equilibrium between
left- and right-handed screw-senses. The equilibrium can be
shifted in the presence of chiral, nonracemic information either
by appending the aromatic ring with chiral, nonracemic sub-
stituents or noncovalent association of chiral substrates.1-3 The
former approach is valuable in the chromatography2a and
electronics examples above,3,5,6 which require a static excess
of single-handed chirality. The latter approach has been explored
for sensor applications.7 Principles for transfer of chiral informa-
tion from side chains to the polymer backbone stress the
importance of the chiral element-to-backbone distance and the
need for steric or secondary bonding interactions to impede
helix-sense reversal. Often, steric effects or hydrogen bonding
are described in a fashion similar to the i to i + 3 connections
† Department of Chemistry, University of Pennsylvania.
‡ Department of Physics and Astronomy, University of Pennsylvania.
§ Eindhoven University of Technology.
(1) (a) Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem.
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9
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J. AM. CHEM. SOC. 2006, 128, 16365-16372
16365

A R T I C L E S
Scheme 1
Percec et al.
Scheme 2
found in R-helical polypeptides.8 cis-Polyarylacetylenes are
dynamic helical polymers for which these rules have proven to
be reliable.2
We have investigated helical chirality in dendronized poly-
mers as a tool to program hierarchical ordering within self-
organizable cylindrical and spherical macromolecules9,10 and
supramolecular assemblies.11 2-Ethynyl- and 3-ethynylcarbazole
polymers carrying first generation self-assembling achiral and
chiral dendrons (i.e., minidendrons) self-organize into nematic
phases.12 Interestingly, the copolymers of the chiral and achiral
2-ethynylcarbazole monomers exhibited a weaker sergeants-and-
soldiers effect13 than the corresponding 3-ethynylcarbazole
copolymers even though the chiral, nonracemic 2-ethynylcar-
bazole homopolymer exhibits a larger circular dichroism (CD)
signal associated with the helical polyene backbone than the
3-ethynylcarbazole homopolymer.12 We have also reported a
library of dendronized polyphenylacetylenes (PPAs) that exhibit
hexagonal columnar lattices (Φh).14 It was shown that for
specific dendron substitution patterns the Φh exhibited intra-
columnar helical order whose screw sense can be selected by
chiral, nonracemic alkyl tails at the periphery. CD spectra of a
second generation dendronized PPA confirmed helix-sense
selection over a remarkable distance.14a
Meijer and co-workers have reported15 structural analysis of
poly[(3,4,5)12G1-A] (P[(3,4,5)12G1-A]) and poly[(3,4,5)-
dm8G1-A] (P[(3,4,5)dm8G1-A]) (Scheme 1) and chiroptical
properties of the latter in ortho-dichlorobenzene. These are
polyacetylenes carrying self-assembling minidendrons. The
XRD pattern from an unoriented P[(3,4,5)12G1-A] sample at
50 °C revealed a single low-angle diffraction peak, which was
ascribed to an interdigitated lamellar liquid crystalline phase.
(9) (a) Percec, V.; Anh, C.-H.; Ungar, G.; Yeardley, D. J. P.; Mo¨ller, M.;
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S. Z. D.; Zhang, A.; Ungar, G.; Yeardley, D. J. P. J. Am. Chem. Soc. 1998,
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G.; Liu, Y.; Zeng, X.; Percec, V.; Cho, W.-D. Science 2003, 299, 1208-
1211. (c) Zeng, X.; Ungar, G.; Liu, Y.; Percec, V.; Dulcey, A. E.; Hobbs,
J. K. Nature 2004, 428, 157-160. (d) Percec, V.; Cho, W.-D.; Ungar, G.;
Yeardley, D. J. P. J. Am. Chem. Soc. 2001, 123, 1302-1315. (e) Percec,
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128, 3324-3334. (g) Percec, V.; Holerca, M. N.; Nummelin, S.; Morrison,
J. J.; Glodde, M.; Smidrkal, J.; Peterca, M.; Rosen, B. M.; Uchida, S.;
Balagurusamy, V. S. K.; Sienkowska, M. J.; Heiney, P. A. Chem.-Eur.
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Dendronized Polyacetylenes
A R T I C L E S
Table 1. Polymerization Conditions and Structural Characteristics of the Polymersa
[M]0
(M)
reaction
time (h)
yield
(%)
cis-content
(%)c
b
polymer
[M]0/[Rh]0
Mn
×
10-3 b
Mw/Mn
P[(3,4,5)12G-A]
P[(3,4,5)dm8G1-A]
P[Cl-(3,4,5)12G1-A]
0.072
0.081
0.066
179
87
50
14
12
12
84
80
65
188
71
29
1.21
1.42
1.57
91
89
85
a Polymerizations were carried out with [Rh(nbd)Cl]2 in 1,2-dichloroethane/NEt3 (10:1 v/v) at 23 °C. b Determined by GPC (THF, 1 mL × min-1
)
calibrated with polystyrene standards. c Determined as described in ref 21a.
Figure 1. 1H NMR Spectrum of P[(3,4,5)12G1-A] in CDCl3.
Variable-temperature CD spectra of P[(3,4,5)dm8G1-A] dem-
onstrated that the polymer backbone exhibits a helical confor-
mation whose handedness reversibly inverts upon heating (i.e.,
stereomutation). Percec and co-workers have previously inves-
tigated bulk self-assembly and self-organization of P[(3,4,5)-
12G1-A] and its analogue poly[Cl-(3,4,5)12G1-A] (P[(3,4,5)-
12G1-A]) (Scheme 2). However, the results of these experiments
were not published.16 We have re-synthesized and re-investi-
gated these structures15,16 to elucidate structural parameters for
communicating chiral information over increasingly larger
distances.
Herein we report the synthesis of three minidendritic acetylene
monomers and their polymerization using [Rh(nbd)Cl]2 (nbd
) 2,5-norbornadiene).17 First generation self-assembling den-
drons, or minidendrons, serve as models for the elaboration and
elucidation of novel self-assembly processes found in larger
dendrons.18,10,19 This is analogous to simple peptide sequences
used to understand molecular engineering principles for as-
sembling more complex protein structures.20 In methyl cyclo-
hexane solution, we confirm using CD spectroscopy that
P[(3,4,5)dm8G1-A] adopts a preferred helix screw-sense and
that this preference decreases with increasing temperature. The
corresponding UV-vis spectra indicate stretching of the polyene
backbone as evidenced by a red shift of the long wavelength
absorption. XRD studies of the polymers in bulk and in oriented
fibers revealed additional low-angle diffraction peaks. The
Figure 2. Variable-temperature CD (left side) and UV-vis (right side)
spectra of P[(3,4,5)dm8G1-A] (2.4 × 10-4 M) in methyl cyclohexane.
Table 2. Thermal Transitions and Corresponding Enthalpy
Changes of the Self-Organized Periodic Arrays of Dendronized
Polyacetylenes
thermal transitions (
°C) and
corresponding enthalpy changes (kcal/mol)a
first and second heating first cooling scan
scans ( C, kcal/mol) C, kcal/mol)
polymer
°
(°
c
P [(3,4,5)12G1-A]
Φr-c, kb 46 (4.20) Φh
Φh 21 (3.60) Φr-c, k
Φh-15 (0.27) Φr-c, k
Φh 24 (3.40) Φr-c, k
Φ
Φ
Φ
Φ
Φ
r-c, k 24 (2.85) Φh
r-c, k 14 (0.38) Φh
r-c, k 15 (0.41) Φh
r-c, k 36 (3.41) Φh
r-c, k 35 (3.74) Φh
P [(3,4,5)dm8G1-A]
P[Cl-(3,4,5)12G1-A]
a Thermal transitions (°C) and enthalpy changes in between parenthesis
(kcal/mol) were determined by DSC (10 °C/min). Data from the first heating
and cooling scans are on the first line, and data from the second heating
b
scans are on the second line. Φr-c,k ) c2mm 3D centered rectangular
columnar lattice. c Φh ) p6mm 2D columnar hexagonal lattice.
cylindrical macromolecules reversibly interconvert between a
three-dimensional (3D) centered rectangular lattice (Φr-c,k) at
lower temperatures and a two-dimensional (2D) Φh lattice at
higher temperatures. Intracolumnar helical order is found in both
phases for all polymers. P[(3,4,5)dm8G1-A] has a larger
average column stratum thickness (l) as compared to P[(3,4,5)-
12G1-A] and P[Cl-(3,4,5)12G1-A]. For the first time, the steric
effect involved in communication of chiral information to the
polymer backbone is shown to result in distortion of the helical
conformation relative to an achiral congener. The magnitude
of the distortion relates to the magnitude of the free energy
difference (∆Gh) favoring one helix-sense over the other.
(16) Lemon, L. Synthesis and polymerization of arylacetylene monomers. M.S.
Thesis, Case Western Reserve University, Cleveland, OH, 1996.
(17) Sedla´cek, J.; Vohl´ıdal, J. Collect. Czech. Chem. Commun. 2003, 68, 1745-
1790.
(18) Percec, V.; Anh, C.-H.; Bera, T. K.; Ungar, G.; Yeardley, D. J. P. Chem.-
Eur. J. 1999, 5, 1070-1083.
(19) (a) Percec, V.; Holerca, M. N.; Magonov, S. N.; Yeardley, D. J. P.; Ungar,
G.; Duan, H.; Hudson, S. D. Biomacromolecules 2001, 2, 706-728. (b)
Percec, V.; Holerca, M. N.; Uchida, S.; Yeardley, D. J. P.; Ungar, G.
Biomacromolecules 2001, 2, 729-740. (c) Duan, H.; Hudson, S. D.; Ungar,
G.; Holerca, M. N.; Percec, V. Chem.-Eur. J. 2001, 7, 4134-4141. (d)
Percec, V.; Holerca, M. N.; Uchida, S.; Cho, W.-D.; Ungar, G.; Lee, Y.;
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Results and Discussion
Synthesis of Dendritic Acetylenes. Dendritic monomers
were prepared as described in Schemes 1 and 2 by a new
synthetic method.15 This method allows the synthesis of the
previously unknown ortho-substituted compounds from Scheme
2. Details are described in Supporting Information.
9
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A R T I C L E S
Percec et al.
Figure 3. DSC traces of P[(3,4,5)12G1-A], P[Cl-(3,4,5)12G1-A], and P[(3,4,5)dm8G1-A]: (a) first heating; (b) first cooling; (c) second heating.
Table 3. Measured d-Spacings and Structural Analysis of the Self-Organized Periodic Arrays of Dendronized Polyacetylenes
polymer
T (°
C)
lattice
d-Spacings (Å)
a or (a, b) (Å)
Dcol (Å)
P[(3,4,5)12G1-A]
80
40
80
24
-40
60
Φh
d
d
d
d
d
d
d
10 (23.3) d11 (13.4)
20 (24.6) d11 (20.2)
10 (21.3)
26.90a
26.90c
Φr-c, k
Φh
49.27, 22.09b
24.32
31.4,d 22.09e
24.32
P[(3,4,5)dm8G1-A]
P[Cl-(3,4,5)12G1-A]
Φh
10 (21.1)
24.6
43.83, 21.13
27.09
24.6
27.9, 21.13
27.09
Φr-c, k
Φh
20 (21.9) d11 (19.0)
10 (23.5) d11 (13.5)
20 (25.0) d11 (20.3)
24
Φr-c, k
49.90, 22.20
31.8, 22.2
a p6mm ) 2D hexagonal columnar (Φh); a ) 2 d100 3, d100 ) (d100
+
3d110)/2. b c2mm ) 3D centered rectangular (Φr-c,k) lattice parameters a and
x
x
b; a ) h0, b ) k0; (h0) and (k0) from diffractions. c Column diameter for the Φh phase, Dcol ) a. d Column diameter along the a-direction of the Φr-c,k
phase, Dcol ) 2a/π. e Column diameter along the b-direction of the Φr-c,k phase, Dcol ) b.
Polymerization of Monomers and Characterization of the
Polymers in Solution. All monomers were polymerized using
[Rh(nbd)Cl]2 at 23 °C in 1,2-dichloroethane/NEt3 (10:1 v/v).
Table 1 reports the reaction conditions, the number (Mn) and
weight (Mw) average molecular weights, and the stereochemistry
of the polymers. Figure 1 depicts the 1H NMR spectrum of P-
[(3,4,5)12G1-A] in CDCl3. The cis-polyene proton is visible at
∼5.7 ppm.21 The cis-content of all polymers was determined
from 1H NMR spectra using a method developed in our group.21
observed.2,7,8,12-14a,15,23,26 In contrast to our observations with
poly[4-((+)-l-mentholoxycarbonyl)phenylacetylene],23 subse-
quent cooling and heating show no hysteresis. Therefore, for
the time scale of the experiment, intramolecular electrocycliza-
tion of the PPA backbone is not significant.
Structural and Retrostructural Analysis of the Den-
dronized Polymers. Thermal optical polarized microscopy
(TOPM) and differential scanning calorimetry (DSC) were used
to identify transition temperatures. Figure 3 depicts DSC traces
of all polymers. Reversible heating and cooling cycles are
observed. Samples from DSC experiments were analyzed by
GPC to confirm the thermal stability of the polymers. Table 2
enumerates the transition temperatures and corresponding en-
thalpy changes. The large enthalpies and supercooling are
consistent with the 3D order of the low-temperature phase.
Branching in the peripheral alkyl chains results in a depression
of the melting temperature and lower transition enthalpy.
Furthermore, during the second DSC heating scans P[Cl-(3,4,5)-
12G1-A] has slightly higher temperature transitions than P-
[(3,4,5)12G1-A]. This might be a consequence of the dipole
introduced by the chlorine.
Typically we would also be concerned with the formation of
intramolecular 1,3-cyclohexadiene sequences along the polymer
backbone via electrocyclization.14,21-25 However, the alkoxy
methylene overlaps the methine resonance of the cyclohexadiene
units so that we cannot assess these defects.14 Nevertheless, the
high cis-content indicates that 1,3-cyclohexadiene defects cannot
be significant in these dendronized polymers. Additionally, we
have shown that their occurrence is usually negligible even for
PPA prepared with Rh(I)-based catalysts.22
Figure 2 presents variable-temperature CD and UV-vis
spectra of P[(3,4,5)dm8G1-A] in methyl cyclohexane. The long
wavelength absorption in the UV-vis spectrum is associated
with conjugation along the polyene backbone; therefore, the
signal in this region of the CD spectrum indicates that the
polyene backbone adopts a preferred helical screw sense. Upon
heating from 10 to 70 °C, the intensity of the CD signal
decreases, and a red shift in the UV-vis spectrum is
The columnar phases identified by DSC and TOPM were
assigned by XRD. Table 3 reports the lattice symmetry, observed
d-spacings, lattice parameter (a and b for the Φr-c,k phase and
a for the Φh phase), and column diameter (Dcol) of each phase.
Notably, we have indexed both d20 and d11 reflections in the
Φr-c,k phase for all three polymers. For P[(3,4,5)12G1-A] and
P[Cl-(3,4,5)12G1-A], we have indexed both the d10 and d11
reflections in the Φh phase. All three polymers exhibit a Φr-c,k
phase at low temperature and a Φh phase at higher temperature.
This clearly eliminates the previous lamellar phase assignment
for P[(3,4,5)12G1-A] at 50 °C.15 Despite the differences in
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Dendronized Polyacetylenes
A R T I C L E S
Figure 4. Wide-angle X-ray diffraction patterns of aligned fibers collected at the indicated temperatures; the fiber axis is vertical. l, average column stratum
thickness; i, diffuse short-range helical feature; t, dendron tilt feature; s, 4.3 Å stacking along the column feature; (10), (11), (hk0) reflections of the 2D Φh
phase; (hkl) and (hk0), (hkl) reflections of the 3D Φr-c,k phase.
respectively, whereas l for the achiral polymers is 4.1 and 4.4
Å in the Φr-c,k and Φh phases, respectively (Figure 6). A larger
(i.e., 7.5 Å vs 7.2 Å) first-order (hk) reflection position (i.e.,
7.5 Å vs 7.2 Å) for P[(3,4,5)dm8G1-A] is shown in the
meridional q-plots (Figure 6a).
P[(3,4,5)12G1-A], P[(3,4,5)dm8G1-A], and P[Cl-(3,4,5)-
12G1-A] are high cis-content stereoregular cis-transoidal den-
dronized polyacetylenes that in bulk behave as cylindrical
objects that self-organize into 3D Φr-c,k and 2D Φh phases.
From the structural information presented above, we have
prepared a model that describes the intracolumnar organization
of P[(3,4,5)12G1-A] (Figure 7). The helical arrangement
enforced by the polyene backbone and the dendritic side groups
is highlighted. The dihedral angle (θ) is set to account for the
observed structural features.
The model in Figure 7 is general for the three dendronized
polyacetylenes. In the low temperature Φr-c,k phase common
to the three polymers, we do not find significant differences in
Figure 5. Azimuthal angle ø plots for the fiber patterns shown in
the dendron tilt feature (t in Figures 4 and 5) or the first-order
(hk) reflection position. Both observations suggest that the
polymers adopt similar 3D arrangements in the Φr-c,k phase.
Comparing the column diameter of P[(3,4,5)12G1-A], P[(3,4,5)-
dm8G1-A], and P[Cl-(3,4,5)12G1-A] in the 2D Φh phase
(Table 3) also points to structures that are very similar to each
other.
Both the DSC and XRD results indicate that the transition
from the 3D Φr-c,k to 2D Φh phase is largely related to packing
of the peripheral alkyl tails. Dramatic decreases in the melt
transition temperature and transition enthalpy are found for P-
[(3,4,5)dm8G1-A] as compared to P[(3,4,5)12G1-A] and P-
[Cl-(3,4,5)12G1-A] (Table 2, Figure 3). Branching in the chiral
tails is the likely source of these differences. Furthermore, the
small variations of Dcol and l with temperature observed by XRD
Figure 4.
molecular structure, there is very little difference in Dcol found
in the Φh phase for each polymer. In the Φh phase the lattice
parameter, a, is equivalent to the diameter of the cylindrical
macromolecule (Dcol).
XRD patterns from oriented fibers of each polymer in its
Φr-c,k and Φh phase are presented in Figure 4. In the Φr-c,k
phase, no significant differences of the dendron tilt feature (t
in Figures 4 and 5) or of the position of the first-order (hk)
reflections between polymers were observed. The only major
difference is the larger value of the average column thickness
(l in Figures 4 and 5) of the chiral polymer. This trend is
observed in both the Φh and the Φr-c,k phases. For P[(3,4,5)-
dm8G1-A] l is 4.5 and 4.8 Å in the Φr-c,k and Φh phases,
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Percec et al.
Figure 6. Meridional q plots for the fiber patterns shown in Figure 4 for the 3D Φr-c,k phase (a) and for the 2D Φh phase (b).
Figure 7. Structure of the cylindrical P[(3,4,5)12G1-A]. Side-view renderings as (a) stick and (b) space filling models, (c) top view of the column and of
a column stratum, and (d) detail of the polymer backbone. An increase of the indicated dihedral angle (θ) of approximately 3-5° is enough to modify the
average column stratum thickness from 4.1 Å observed in the 3D Φr-c,k phase to 4.4 Å in the 2D Φh phase.
Scheme 3a
indicate that also small changes in the backbone conformation
take place during the Φr-c,k to Φh phase transition. It is most
likely that the major changes that the columns undergo during
this transition are in the alkyl tails. In the low temperature Φr-c,k
phase, the rigidity of the tails distorts space-filling at the
periphery of the column. At higher temperatures, in the Φh
phase, increased flexibility allows the formation of the Φh
organization.
Similar to other polyarylacetylenes these dendronized poly-
mers are dynamic helical polymers. Helical order has been
identified in both the Φr-c,k and Φh phases of P[(3,4,5)dm8G1-
A]. From CD spectra in methyl cyclohexane, we recognize that
the chiral alkyl chains in P[(3,4,5)dm8G1-A] are able to select
a preferred screw-sense for the helical backbone. The screw-
sense bias is temperature-dependent.26 Helical order has also
been identified in both the Φr-c,k and Φh phases of the two
polymers derived from achiral monomers. On the basis of the
close structural analogy between the three polymers, we can
gain insight to the mechanism by which helix-sense selection
occurs.
a For the achiral monomer scenario, P and M refer to the respective helix
screw sense. For the chiral monomer scenarios, the matched (m) and
unmatched (u) energy levels correspond to the monomer in its preferred
and non-preferred helical screw senses, respectively.
degree of polymerization is greater than the persistence length
of a helical segment. This is because there is no free energy
difference to favor one helix sense or the other per monomer
residue (∆Gh ) 0, Scheme 3). Separating the oppositely handed
helical segments is a helix reversal with which there is an
associated excess energy (∆Gr). On the other hand, in a polymer
with chiral, nonracemic monomers there is preferred helix-sense
because there is a free energy penalty for putting the monomer
in its non-preferred helical conformation. Scheme 3 illustrates
two scenarios for a helical polymer with chiral, nonracemic side
According to the model for macromolecular helix-sense
selection developed by Green and co-workers,13 both helical-
senses will be found in polymers with achiral side chains if the
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A R T I C L E S
chains. In one case, the polymer with chiral side chains adopts
a conformation very similar to the achiral polymer. The chiral
information in the side chain creates a modest ∆Gh, which
translates into weak helix-sense selection. In the second scenario,
the polymer with chiral side chains adopts a conformation more
different from the achiral polymer. A larger ∆Gh manifests
greater bias of a single preferred helix-sense.
while the latter carries four. When the dodecyl chains are
replaced with chiral, nonracemic alkyl tails the result is
dramatically different. Poly[(4-3,4,5)AmylG1-4EBn] has
chiral tails derived from (S)-amyl alcohol27 but no signal is
observed in the CD spectrum in methyl cyclohexane even at 2
°C. The distance of the chiral information from the polymer
backbone is most clearly noticed by comparing the diameter of
the cylindrical macromolecules in the Φh phase. P[(3,4,5)-
dm8G1-A] has a column diameter of 24.6 Å (24 °C) while poly-
[(3,4-3,5)dm8G2-4EBn] has a column diameter of 43 Å (23
°C).14a The lower melt temperature and transition enthalpy for
P[(3,4,5)dm8G1-A] as compared to P[(3,4,5)12G1-A] dissuade
us from the rationale that the dependence on the distance and
population of chiral, nonracemic braches is to foster i to i + n
contacts that reinforce the preferred helical screw-sense.
From XRD of oriented fiber samples we have seen that the
average column stratum thickness (l) is significantly larger for
P[(3,4,5)dm8G1-A] than P[(3,4,5)12G1-A] or P[Cl-(3,4,5)-
12G1-A] in both the Φr-c,k and Φh phases. Clearly, this larger
layer separation is caused by the presence of methyl branches
in the peripheral alkyl chains, including that of the chiral C(3).
Evidence that this increase is transmitted to the core region can
be seen in the larger d-spacings of the first order (hk) reflection
in the Φr-c,k phase. Our model shows that a small change in
dihedral angle (θ) can accommodate the necessary structural
changes (Figure 7d).
Recently we have reported a library of dendronized PPAs.14
Some of the polymers contain chiral alkyl tails at the periphery.
For the second generation dendronized PPA, poly[(3,4-
3,5)mG2-4EBn], with achiral (m ) 12) and chiral (m ) dm8)
alkyl chains there is no significant change in l (4.1 Å). In methyl
cyclohexane solution, poly[(3,4-3,5)dm8G2-4EBn] exhibits
a signal in the polyene backbone region of the CD spectrum at
2 °C that decreases upon warming to 22 °C. We associate this
behavior with the chiral information in the peripheral alkyl tails
being less efficiently conveyed to the polymer backbone in the
second generation PPAs as compared to dendronized polyacetyl-
enes herein.
We understand these structural effects by considering space-
filling lateral to the dendron wedge (i.e., perpendicular to the
column). The chiral carbons are relatively confined to fit along
the circumference of a circle of given diameter, which is
determined by the size of the dendron or distance to the cylinder
core. As the population of chiral carbons increases without
changing the diameter, the chiral centers begin to get crowded.
A critical level of crowding appears necessary to achieve even
small degrees of helix-sense selection. When overcrowding
occurs, the vertical dimension of the dendron (i.e., parallel to
the cylinder axis) must compensate. The increase of the average
column stratum thickness (l), as observed here experimentally,
is the product of overcrowding.
Conclusions
Greater bias in the helix-sense selection by P[(3,4,5)dm8G1-
A] than poly[(3,4-3,5)dm8G2-4EBn] is a consequence of a
larger energy penalty (∆Gh) for placing the chiral monomer into
its non-preferred helix-sense. The behavior observed for P-
[(3,4,5)12G1-A] and poly[(3,4-3,5)12G2-4EBn] is illustrated
by the “achiral monomer” scenario in Scheme 3. We liken the
observed helix-sense selection by poly[(3,4-3,5)dm8G2-
4EBn] to the middle energy diagram in Scheme 3. There is a
negligible energy difference between the preferred helical
handedness and the helix adopted by the polymer composed of
achiral monomers. The penalty for putting the chiral, nonracemic
monomer in its non-preferred helix-sense is correspondingly
small. The final energy diagram in Scheme 3 is used to
understand P[(3,4,5)dm8G1-A], where we can find structural
evidence for a conformational difference imposed by the chiral
alkyl tails. The helical conformation adopted by P[(3,4,5)-
dm8G1-A] has a distinctly different energy than that adopted
by P[(3,4,5)12G1-A]. The greater difference between the achiral
and chiral polymers translates into a correspondingly larger
energy penalty for placing the chiral monomer into its non-
preferred helix-sense.
Structural and retrostructural analysis of three dendronized
polyacetylenes has shed light on the molecular mechanism by
which chiral information is conveyed over large distances in
dynamically helical polyacetylenes. Steric effects due to branch-
ing in the side chain can distort the polymer backbone
conformation compared to one without branching. When this
steric bulk is associated with a chiral center, the change in
conformation distorts the free energy difference (∆Gh) favoring
one helix-sense over the other (per monomer residue). The
magnitude of such distortions is related to the population of
chiral branches and their distance from the polymer backbone.
Peripheral chiral, nonracemic alkyl tails in dendronized polymers
are able to impose a conformational change that stretches the
polymer backbone. We have observed this directly as the ∼10%
increase of the average column stratum thickness (l) in XRD
patterns of oriented fibers of P[(3,4,5)dm8G1-A] as compared
to P[(3,4,5)12G1-A] and P[Cl-(3,4,5)12G1-A]. CD spectra of
P[(3,4,5)dm8G1-A] in methyl cyclohexane confirm that the
polymer backbone adopts a preferred helix-sense. This is the
first example where the steric impact of chiral branching has
been shown to directly relate to communication of chiral
information. The model described above contrasts with other
explanations2,7,8,26 for steric communication of chiral information
in that it does not require chiral groups to associate like teeth
Drawing comparisons between the polymers herein and those
of the previous library14 indicate that the number of chiral,
nonracemic centers should increase as their distance from the
polymerbackboneincreases.Firstgenerationpoly[(4-3,4,5)mG1-
4EBn] is of comparable size to poly[(3,4-3,5)mG2-4EBn]
for the same m. For example, when m ) 12, the column
diameter is 46.2 Å (115 °C) for poly[(4-3,4,5)12G1-4EBn],
and it is 44.9 Å (105 °C) for poly[(3,4-3,5)12G2-4EBn] in
the Φh phase.14 The former dendron carries three alkyl tails
(27) Helix sense-selection was possible for dendronized poly(ethynylcarbazole)s
with (S)-amyl tails; see ref 9.
(28) For recent selected reviews and highlights on dendronized polymers, see:
(a) Percec, V. Phil. Trans. R. Soc. A 2006, 364, 2709-2719. (b) Schlu¨ter,
A. D. Top. Curr. Chem. 2005, 245, 151-191. (c) Frauenrath, H. Prog.
Polym. Sci. 2005, 30, 325-384.
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in a set of gears, nor does it invoke atropisomeric conformations
between the conjugated polymer backbone and the aromatic side
chain. The potential generality of this mechanism in other related
macromolecular and supramolecular helical structures is under
investigation. The experiments reported here also support the
hypothesis that the higher dynamics of minidendrons allows
them to be used as models or maquettes18,20 that can assess the
molecular mechanism of structure formation19 in higher genera-
tions of self-organizable dendronized polymers.14,28
Acknowledgment. Financial support by the National Science
Foundation (DMR-0548559 and DMR-0520020) and P. Roy
Vagelos Chair at Penn are gratefully acknowledged.
Supporting Information Available: Experimental section
containing materials, techniques, synthesis, and analytical data;
a discussion of the monomer synthesis; a discussion of the chiral,
nonracemic monomers; the complete ref 14b. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA0665848
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