Self-assembling phenylpropyl ether dendronized helical polyphenylacetylenes

Article abstract of DOI:10.1002/chem.200701008

The first example of a self-assembling phenylpropyl ether based dendronized polymer has been reported and its preferred helical handedness has been determined. Dendronized polymer poly(10) and its nondendritic analogue poly(8) are high-cis-content polyphenylacetylenes (PPAs) prepared by using [Rh(nbd)Cl]₂/NEt₃ (nbd: 2,5-nor-bornadiene). Both polymers possess a Stereocenter in their side chain, which selects a preferred helical handedness.Based on negative exciton chirality observed in the CD spectra of poly(10), we have designated this molecule as a right-handed helical polymer, which persists over a wide temperature range. Poly(10) self-organizes into both Φ_h^iO and Φ_h lattices in bulk. The Φ_h^iO-to-Φ_h transition is associated with thermore-versible cis-cisoidal to cis-transoidal isomerization of the helical PPA, accompanied by a dramatic decrease in the column diameter and a decrease in the π-stacking correlation length along the column. A model for the right-handed helical dendronized PPA has been proposed wherein dendrons from adjacent column strata interdigitate to effectively fill space.

Full text of DOI:10.1002/chem.200701008

DOI: 10.1002/chem.200701008
Self-Assembling Phenylpropyl Ether Dendronized Helical
Polyphenylacetylenes
Virgil Percec,*[a] Mihai Peterca,[a, b] Jonathan G. Rudick,[a] Emad Aqad,[a]
Mohammad R. Imam,[a] and Paul A. Heiney[b]
Abstract: The first example of a self-
assembling phenylpropyl ether based
dendronized polymer has been report-
ed and its preferred helical handedness
has been determined. Dendronized po-
lymer poly(10) and its nondendritic an-
alogue poly(8) are high-cis-content pol-
yphenylacetylenes (PPAs) prepared by
Based on negative exciton chirality ob-
served in the CD spectra of poly(10),
we have designated this molecule as a
right-handed helical polymer, which
persists over a wide temperature range.
Poly(10) self-organizes into both Fiho
and Fh lattices in bulk. The Fiho-to-Fh
transition is associated with thermore-
versible cis-cisoidal to cis-transoidal
isomerization of the helical PPA, ac-
companied by a dramatic decrease in
the column diameter and a decrease in
the p-stacking correlation length along
the column. A model for the right-
handed helical dendronized PPA has
been proposed wherein dendrons from
adjacent column strata interdigitate to
effectively fill space.
using [Rh(nbd)Cl]2/NEt3 (nbd: 2,5-nor-
A
Keywords: dendrons · helical struc-
tures · polymers · self-assembly ·
supramolecular chemistry
bornadiene). Both polymers possess a
stereocenter in their side chain, which
selects a preferred helical handedness.
Introduction
mechanism through retrostructural analysis.[2,11] We[2,4a,b,5–7,9]
and others[4,8] have developed a robust strategy to prepare
self-organizing dendronized polymers by using benzyl ether
based self-assembling dendrons.
Dendronized polymers approach the architectural complexi-
ty of biomacromolecules and provide some of the most pow-
erful building blocks currently explored at the interface be-
tween chemistry, biology, physics, medicine, and nano-
science.[1,2] Divergent,[3] attach-to,[4,5] macromonomer,[6–8]
and supramolecular[9,10] strategies to prepare these polymers
facilitate tremendous diversity of structural composition. A
subset of these polymers has been demonstrated to self-or-
ganize into periodic[2,4,6,7,8d,9] and quasiperiodic[11] arrays,
which enable detailed understanding of the self-assembly
We have recently expanded the palette of self-assembling
dendrons from which to prepare supramolecular dendrimers
and self-organizing dendronized polymers by using (AB)y–
AB3 and constitutional isomeric (AB)y–AB2 building block-
s,[11i] biphenyl methyl ether dendrons,[11o] hybrid dendrons,[11r]
and self-assembling phenylpropyl ether dendrons.[11p] Self-as-
sembling phenylpropyl ether based dendrons are more flexi-
ble and stable toward heat and acid than their benzyl ether
based analogues. Faster dynamics of self-assembly yield
larger supramolecular structures with higher order and
lower transition temperatures than those observed with the
corresponding benzyl ether based self-assembling dendrons.
Furthermore, self-assembling phenylpropyl ether dendrons
provide more examples of flat-tapered dendrons than had
been encountered in previous libraries.[2,11p]
Motivated in part by the desire to improve charge-carrier
mobility in complex electronic materials derived from supra-
molecular dendronized polymers,[9] we have investigated
strategies to program the helical handedness of dendronized
polymers.[2,7e] A helical arrangement of the polymer back-
bone that penetrates cylindrical dendronized polymers had
been demonstrated previously by our group.[6f–p] We have
[a]Prof. Dr. V. Percec, Dr. M. Peterca, Dr. J. G. Rudick, Dr. E. Aqad,
M. R. Imam
Roy & Diana Vagelos Laboratories
Department of Chemistry
University of Pennsylvania
Philadelphia, PA 19104-6323 (USA)
Fax : (+1)215-573-7888
E-mail: percec@sas.upenn.edu
[b]Dr. M. Peterca, Prof. Dr. P. A. Heiney
Department of Physics and Astronomy
University of Pennsylvania
Philadelphia, PA 19104-6396 (USA)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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FULL PAPER
elucidated design principles for
expressing the chirality of the
peripheral tails in the back-
bone
of
self-organizing
dendronized polyphenylacety-
lenes (PPAs) and other polyar-
ylacetylenes.[2,7f] Additionally,
we have demonstrated control
over the helicity of supra-
molecular polymers generated
from self-assembling dendrons
appended with chiral apex
groups.[11] These are currently
used to investigate the struc-
tural origin of functions and
thus create new functions
through 3D tertiary and qua-
ternary structures in accord-
ance with biological design
principles.
Scheme 1. Synthesis of poly(8): i) LiAlH4, THF, 238C, 2 h; ii) TrCl, DMAP, DCM, NEt3, 238C, 14 h; iii) MsCl,
dichloromethane, NEt3, 238C, 2 h; iv) p-iodophenol, K2CO3, DMF, 758C, 6 h; v) TsOH, dichloromethane,
238C, 8 h; vi) trimethylsilylacetylene, [PdCl2
A
10 h; vii) [Rh(nbd)Cl]2, NEt3, 1,2-dichloroethane, 238C. TrCl: triphenylmethylchloride; DMAP: 4-dimethyl-
AHCTREUNG
aminopyridine; MsCl: methanesulfonyl chloride; DMF: N,N-dimethylformamide; TsOH: toluenesulfonic acid;
TMS: trimethylsilyl; nbd: 2,5-norbornadiene.
Herein, we report the syn-
thesis and structural and retro-
structural analysis of the first self-assembling phenylpropyl
ether based dendronized polymer as well as the correspond-
group of 5 with pTsOH. Sonogashira cross-coupling of 6
with trimethylsilylacetylene by using Pd0/CuI/PPh3 in NEt3
proceeded with high yields. Base-induced elimination of the
trimethylsilyl moiety provided the desired chiral monomer
ing
poly{(R)-4-[(2-hydroxy)propyloxy]phenylacetylene}
(poly(8)), which lacks any dendrons. That both polymers
adopt a preferred helical handedness in solution serves to
distinguish helix sense selection from helix induction. Varia-
ble temperature CD and UV/Vis spectra of the dendronized
polymer (poly(10)) demonstrate it to be a relatively stable
dynamic helical polymer. Furthermore, these spectra indi-
cate that the CD signal is due to negative exciton chirality
that is consistent with a right-handed helical conforma-
tion.[12] Poly(10) self-organizes into a hexagonal columnar
lattice with internal order (Fiho) and a hexagonal columnar
liquid crystalline lattice (Fh). X-ray diffraction (XRD) ex-
periments reveal that the helical polymer undergoes reversi-
ble stretching and compression during thermal cycling.
Structural and retrostructural analysis of self-organized
poly(10) culminates in a model for the internal right-handed
helical order.
1
8. The H NMR spectrum of 8 is shown in Figure 1. Charac-
terization of 8 and its preceding intermediates confirms
their molecular structure, but we have not verified their en-
antiomeric purity and therefore must presume that some of
the racemate is present.[14]
Scheme 2 illustrates the synthesis of the dendritic mono-
mer 10. The dendritic carboxylic acid 9 was prepared as de-
scribed previously.[11m] Esterification of 9 with 8 was accom-
Results and Discussion
Synthesis of the monomers: Scheme 1 outlines the synthesis
of the chiral monomer (R)-2-(4-ethynylphenoxy)propane-1-
ol (8). Compound 4 is prepared in a three-step sequence
from ethyl l-lactate (1) without purification of the inter-
mediates.[13] Ethyl l-lactate is reduced with LiAlH4 and the
resulting diol 2 is selectively etherified with triphenylmethyl
chloride at the less sterically hindered hydroxy group. Sub-
sequently, the remaining hydroxy group of 3 is transformed
into a leaving group by using methanesulfonyl chloride. Wil-
liamson etherification of p-iodophenol with 4 takes place
with inversion of stereochemistry to provide 5. The aryl
iodide 6 was obtained by elimination of the triphenylmethyl
Figure 1. 1H NMR spectrum of monomer 8 in [D6]DMSO. The chemical
structure with proton assignments is also shown.
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V. Percec et al.
Polymerization of the mono-
mers and characterization of
the polymers in solution: As
shown in Schemes 1 and 2, the
polymerizations of 8 and 10
were effected by using [Rh-
(nbd)Cl]2/NEt3. This catalyst
yields high-cis-content polyary-
lacetylenes,[15]
dendronized
including
polyarylacety-
lenes.[2,7] We have previously
noted that polymerizations of
benzyl ether based dendritic
monomers with RhI-based cat-
alysts can be complicated by
side reactions, a fact that we
have attributed to cleavage of
the benzyl ether linkages.[2,7c]
We envision that the acid tol-
erance of the phenylpropyl
ether based dendrons will
permit access to larger and
more structurally diverse libra-
Scheme 2. Synthesis of poly(10): i) EDAC, DPTS, 8, THF; ii) [Rh(nbd)Cl]2, NEt3, 1,2-dichloroethane, 238C.
EDAC: 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride; DPTS: 4-(N,N-dimethylamino)pyridi-
nium toluene sulfonate; nbd: 2,5-norbornadiene.
A
plished by using EDAC and DPTS. The dendritic monomer
10 was isolated in only 50% yield. We have previously dis-
cussed the challenges associated with using carbodiimide
coupling agents with dendritic carboxylic acids.[7b,c] The in-
creased solubility associated with the self-assembling phe-
nylpropyl ether based dendrons makes purification even
more difficult. Nonetheless, pure 10 was obtained, as dem-
onstrated by thorough characterization including 1H and
13C NMR spectroscopy, HPLC, and MALDI-TOF mass
ries of dendronized polyarylacetylenes.
Table 1 reports polymerization conditions, molecular
weight, molecular-weight distribution, and cis content for
the polymers. Under dilute conditions, an extremely high-
Table 1. Polymerization conditions and structural characteristics of the
polymers.[a]
Polymer [M]0 [M]0/
[m] [Rh]0
t
Yield
Mn ”10À3[b] Mw/
cis content
[b]
[h] [%]
Mn
[%][c]
1
spectrometry. Figure 2 shows the H NMR spectrum of 10.
poly(8) 0.182 40
poly(10) 0.041 58
14 50
14 69
3.4
231.7
1.36
2.14
91
88
[a]Polymerizations were carried out with [Rh (nbd)Cl]2 in 1,2-dicloro-
G
ethane/NEt3 (10:1 v/v) at 238C. [M]0: initial concentration of the mono-
mer; [Rh]0: initial concentration of the catalyst; Mn: number-average mo-
lecular weight; Mw: weight-average molecular weight. [b]Determined by
GPC (THF, 1 mLminÀ1) calibrated with polystyrene standards. [c] cis
content determined by 1H NMR spectroscopy by using the methods de-
veloped in our laboratory (see reference [16a]).
molecular-weight dendronized PPA is obtained. In the cases
of dendronized polystyrenes and polymethacrylates, we have
demonstrated anomalous elution during GPC of similarly
high-molecular-weight polymers.[6o] In such cases, some of
the extremely high-molecular-weight fraction is retarded
during elution from the GPC column and thus coelutes with
small polymers; the lower-molecular-weight polymers elute
normally. Consequently, we expect that the Mn and Mw/Mn
values reported in Table 1 for poly(10) are underestimated
by this method[6o] by more than that due to hydrodynamic
volume discrepancies between dendronized polymers and
polystyrene calibration standards.[3c,6o,8c]
The cis content of both polymers was found to be high by
using methods previously elaborated by our group (Ta-
Figure 2. 1H NMR spectrum of dendritic monomer 10 in CDCl3. The
chemical structure with proton assignments is also shown.
1
ble 1).[7b,c,e,16] The H NMR spectra of poly(8) and poly(10)
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Dendronized Helical Polyphenylacetylenes
FULL PAPER
are shown in Figures 3 and 4, respectively. The proton reso-
nance of the cis-polyene backbone is labeled in each as
“Ha” for reference. Integration of this resonance versus the
rest of the aromatic region of the spectrum permits quanti-
tative determination of the cis content.[7b,c,e,16] The trans-
polyene backbone resonance is overlapped by the aromatic
protons of the side chain (for example, “Hb” and “Hc”); this
prevents direct quantification of the trans-polyene content.
The methylene resonances of poly(8) (labeled “Hf” in
Figure 3) prevent qualitative and quantitative identification
of the methine resonance that is indicative of 1,3-cyclohexa-
diene defect moieties in the polyene backbone.[2,7,16] Similar-
ly, the alkyl ether methylene resonances of poly(10) prevent
such analysis. This has been observed for previously report-
ed examples of dendronized polyarylacetylenes.[2,7] It is
worth stressing that the integration method employed for
dendronized polyarylacetylenes only determines the pres-
ence of cis-polyene repeat units and not their conformation-
al disposition. In PPA, the conformation (that is, cisoid
versus transoid) can be determined because cis-cisoidal PPA
is crystalline and therefore insoluble.
In DMSO, poly(8) exhibits a strong Cotton effect associ-
ated with a long-wavelength absorption due to the conjugat-
ed polyene backbone (Figure 5). Like other examples of
Figure 3. 1H NMR spectrum of poly(8) in [D6]DMSO. The chemical
structure with proton assignments is also shown.
Figure 5. a) CD and b) UV/Vis spectra of poly(8) at 238C in DMSO.
polyarylacetylenes with chiral, nonracemic side chains,
poly(8) adopts
a
preferred helical handedness.[16g,17,18]
Poly(10) also adopts a preferred helical handedness in meth-
ylcyclohexane (Figure 6). We have demonstrated for other
self-organizing dendronized polyarylacetylenes that this
occurs due to helix sense selection rather than induction of
a helical conformation.[2b,7b,e] Limited comparisons of the
CD spectra for poly(8) and poly(10) can be made since the
spectra are recorded in such different solvents. The second
Cotton effect (ꢀ380 nm) in both polymers is comparable in
magnitude and sign, a fact that leads us to speculate that
these polymers adopt rather similar conformations in solu-
tion. That is to say that the dendron structure appears to
have little impact on the communication of chiral informa-
tion in this instance.
Polyarylacetylenes are dynamic helical polymers,[17] and
variable-temperature CD and UV/Vis spectroscopy can be
used to gauge the stability of such synthetic helical poly-
mers.[17] Longer wavelength absorption and the correspond-
ing CD signal are due to more flat, extended conformations
of the conjugate polyene backbone. Thus, we can probe the
stretching/unwinding of the helical conformation. We have
Figure 4. 1H NMR spectrum of poly(10) in CDCl3. The chemical structure
of the linear polymer backbone with proton assignments is also shown.
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V. Percec et al.
Figure 6. Variable-temperature CD (top) and UV/Vis (bottom) spectra of poly(10) in methylcyclohexane ([rpu]=8.4”10À4 m). a) First heating, b) first
cooling, and c) second heating cycles are shown.
shown previously that structural transformations associated
with the polyene backbone can interfere with the interpreta-
tion of such experiments.[16g] cis–trans isomerization, 6p elec-
trocyclization to form 1,3-cyclohexadiene moieties, chain
cleavage due to rearomatization of these moieties accompa-
nied by extrusion of triarylbenzene, and photooxidative
cleavage of the backbone are all possible transformations.[16]
Nonetheless, we get a qualitative sense of the stability of the
helix even when we cannot directly measure the molecular
weight, molecular-weight distribution, cis content, and
amount of 1,3-cyclohexadiene defect structures. Both the
overall magnitude of the Cotton effects and the temperature
range over which they can be observed decrease as the ste-
reocenter is moved farther away from the backbone in
dendronized polyarylacetylenes.[7a,b,e]
The variable-temperature CD and UV/Vis spectra for
poly(10) in methylcyclohexane are shown in Figure 6. We
show the first heating and cooling scans as well as a second
heating scan. Immediately visible is the hysteresis of the
Cotton effects measured at 58C in each scan. The initial
magnitude is never recovered. We have noted this effect in
menthol-derived PPAs and have shown that this is due to
structural transformations of the polyene backbone.[16g] Fur-
thermore, close inspection of the UV/Vis spectra at 708C re-
veals a gradual blue shift through the experiment. This is
consistent with a reduction of the effective conjugation
length of the polyene backbone.
from the cooling scans at 108C and 408C. The identification
of a negative exciton chirality[12] is demonstrated here by the
second and third Cotton effects, which are both observed in
the vicinity the first UV/Vis absorption maximum at
ꢀ336 nm (Figure 7a). At the higher temperature, the
second and third Cotton effects, as well the corresponding
UV/Vis absorption maximum, exhibit a slight blue shift,
thus making it easier to separate these from the first nega-
tive Cotton effect. The higher wavelength UV/Vis absorp-
tion maximum shifts slightly toward longer wavelengths
with the temperature increase. We can assign this UV/Vis
absorption maximum to a negative Cotton effect that over-
laps with the one localized at 381.4–376.0 nm (Figure 7).
The presence of this overlapped Cotton effect explains the
asymmetry of the second peak from the CD spectra, as well
the slight mismatch in the crossover of the CD spectra local-
ized at 348 nm with the first UV/Vis maximum localized at
336.5 nm. Consequently, the negative Cotton effect consid-
ered in the exciton coupling has its true maximum at a
lower wavelength than that shown in Figure 7. The negative
exciton chirality identified here is commensurate with a
right-handed helical structure.[12]
Evolution of helical structure in macromolecules and supra-
molecules: Phenomena by which an excess of one helical
handedness is observed in polyarylacetylenes are frequently
categorized as helix induction.[9,10,17b,d,f,18] This suggests that
in the absence of a chiral stimulus, no helix would be found.
Helix induction occurs through increased rotational barriers
that prevent the polymer from accessing nonhelical confor-
The CD spectra of poly(10) show three Cotton effects of
opposite sign at ꢀ440, ꢀ380, and ꢀ325 nm (Figure 6).
Figure 7 shows overlay plots of the CD and UV/Vis spectra
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Dendronized Helical Polyphenylacetylenes
FULL PAPER
to their steric bulk.[20] The same mechanism responsible for
the preferred helical handedness in poly(8) is likely respon-
sible for that in poly(10).
Structural and retrostructural analysis of the dendronized
polymers: Thermal optical polarized microscopy and differ-
ential scanning calorimetry (DSC) were used to identify
phase transitions for poly(10). The scans are shown in
Figure 8 and the phase-transition temperatures are reported
Figure 7. Overlays of CD (left axis) and UV/Vis (right axis) spectra at
a) 108C and b) 408C from the first cooling of poly(10) in methylcyclohex-
ane ([rpu]=8.4”10À4 m). The overlays highlight the negative exciton chir-
ality.
mations,[19] such as for cylindrical, self-organizing dendron-
ized polystyrenes and polymethacrylates.[6f–p] We proposed
that polyarylacetylenes adopt a helical conformation[16a] in
the absence of a chiral field (for example, a side chain, sol-
vent, or ligand) and have shown this to be the case for
dendronized polyarylacetylenes.[2b,7a,b,e] Observations of an
excess of one helical handedness are due to helix sense se-
lection by a chiral bias rather than helix induction. This is
also the case for polyisocyanates[20] and polyguanidines.[21]
Covalently[8,16d,17f] and noncovalently[18] attached chiral side
chains program the preferred helical handedness due, in
part, to infrequent helix reversals in substituted PPAs.[17]
The spectra in Figures 3 and 5 are critical to the distinc-
tion between helix sense selection[2b,7a,b,e,20,21] and helix in-
Figure 8. DSC traces (108CminÀ1) of poly(10).
with their corresponding enthalpies in Table 2. Similar to
previously reported dendronized PPAs,[7b,c] poly(10) exhibits
a transition between Fiho lattices that is only found on the
first heating. The highest temperature phase is Fh. The poly-
Table 2. Thermal transitions [8C]and corresponding enthalpy changes
[kcalmolÀ1]of the self-organized periodic arrays of poly( 10).[a]
1
duction.[9,10d,17b,d,f,18] In [D6]DMSO, the H NMR peak shape
and area are consistent with facile rotation of the backbone
and side chains in poly(8) (Figure 3). Steric, hydrogen-bond-
ing, and solvophobic effects are not meaningful for poly(8)
in [D6]DMSO or DMSO. The preferred helical handedness,
which gives rise to the observed Cotton effects for poly(8)
(Figure 5), is therefore due to the chiral environment associ-
ated with the side chains, rather than restricted motion due
First and second heating scans
First cooling scan
1
2
Fiho 12 (2.55) Fiho 50 (0.12) Fiho 81 (0.13)
Fh 72 (0.27) Fiho 11 (4.40)
Fhc
Fiho
Fiho 18 (4.57) Fiho 81 (0.28) Fh
[a]Thermal transitions and enthalpy changes (in parenthesis) were deter-
mined by DSC (108CminÀ1). Fiho: hexagonal columnar lattice with inter-
nal order; Fh: p6mm hexagonal columnar lattice.
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V. Percec et al.
mer decomposes before isotropization. Below the decompo-
sition temperature, transitions in subsequent heating and
cooling cycles are reversible and reproducible.
All phase assignments were confirmed by XRD. The
structural and retrostructural analysis of poly(10) is reported
in Table 3. In addition to the data in Table 3, Figure 9 re-
Table 3. Structural and retrostructural analysis of the self-organized peri-
odic arrays of poly(10).
T [8C]Phase
d Spacings [Š]
d11 d20
t [Š][a] a=Dcol [Š][b] m[c]
d10
d21
À20
30
80
p6mm 46.2 26.9 23.1 17.5 4.3
p6mm 47.3 27.2 23.6 17.8 4.46
p6mm 46.1 26.6 22.9 17.4 4.56
53.5
54.5
53.1
48.0
4.7
5.1
4.9
4.0
110
p6mm 41.6 24.0 20.8
4.58
[a] t: Average column stratum thickness. [b]Lattice parameter for the
p6mm columnar hexagonal lattice; a=2<d100 >3À0.5, in which <d100 >=
0.25(d10+30.5d11+2d20+70.5d21) and is equal to the diameter of a column
(Dcol). [c] m: Number of dendrons in
a
column stratum=
(30.5atNA1)(2M)À1, in which Avogadroꢁs number NA =6.0220455”1023, 1
is the density of the polymer (assumed to be 0.93 gcmÀ3), and M is the
molecular weight of the monomer.
Figure 10. a) Fiber XRD patterns for poly(10) and b) meridional plots for
poly(10) showing the gradual increase of the average column strata thick-
ness, noted with the s value, with the increase of temperature. Collection
temperatures and correlation lengths are indicated and the plots are y-
axis offset for clarity.
All of the above observations correlate to give a molecu-
lar model that partially (high temperatures, above 808C) or
completely (low temperatures, below 808C) includes in each
column stratum dendrons from the next adjacent layer, as
shown in Figure 11. A plausible mechanism for this involves
the relatively large empty space around the PPA backbone
and dendron linkage created by the flexible self-assembling
phenylpropyl ether dendrons. Thus, in order to maximize
van der Waals interactions both at the nearby backbone
region and at the peripheral region, the layers are thermally
dynamically interdigitated, thereby causing the number of
dendrons per column strata average thickness to vary from
m=4.7 at 208C to m=4.0 at 1108C (both numbers are calcu-
lated by assuming a constant density of 1=0.93 gcmÀ3).
Figure 9. Plot of the average column diameter determined from XRD as
a function of temperature.
ports the average column diameter measured by XRD for
poly(10) at various temperatures above and below the Fiho-
to-Fh transition. We have previously associated this transi-
tion with the unprecedented conversion of the PPA back-
bone from a cis-cisoidal conformation into a cis-transoidal
conformation.[7b] The large variation of the column diameter
indicates an unwinding of the helical backbone with the
temperature increase, as confirmed also by the small gradual
increase in the average column strata thickness observed in
the wide-angle fiber experiments (Figure 10). Furthermore,
wide-angle XRD from oriented fiber samples suggests long
correlation lengths within the cylindrical PPAs (Figure 10).
Conclusion
The first example of a self-assembling phenylpropyl ether
based dendronized polymer has been reported and its pre-
ferred helical handedness has been determined. Poly(10)
and its nondendritic analogue poly(8) are high cis-content
PPAs prepared by using [Rh(nbd)Cl]2/NEt3. Both polymers
A
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Dendronized Helical Polyphenylacetylenes
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Figure 11. Structure of poly(10): a) Top and b) side views of the core region. The dotted circles mark the dendrons from the adjacent layer that are inter-
digitated to facilitate a better packing at the region close to the PPA backbone. c) Top view of two layers, d) top space-filling view of the supramolecular
column, and e)–g) detailed side views of the supramolecular columns. The surface marks the right-handed helix array of aromatic rings templated by the
right-handed helical polyene backbone, which is selected by the R chiral center.
possess a stereocenter in their side chain, which selects a
preferred helical handedness. Based on negative exciton
chirality observed in the CD spectra of poly(10), we have
designated it as a right-handed helical polymer. Likely, the
same helical handedness is found in poly(8). Due the close
proximity of the stereocenter to the polyene backbone,
poly(10) exhibits a CD signal over a wide temperature
range.
self-assembling phenylpropyl ether dendrons appended with
chiral apex groups make it possible to create complex elec-
tronic materials comprising larger supramolecular dendron-
ized polymers whose helical handedness can be defined.
New concepts such as these will be the subject of future
publications.
Poly(10) self-organizes into both Fiho and Fh lattices in
bulk. The Fiho-to-Fh transition corresponds with a dramatic
decrease in the column diameter and decrease in the p-
stacking correlation length along the column. We attribute
this to thermoreversible cis-cisoidal to cis-transoidal isomeri-
zation of the helical PPA. A model for the dendronized
PPA has been proposed wherein dendrons from adjacent
column strata interdigitate to effectively fill space. This is
likely a consequence of the smaller projected molecular
solid angle occupied by self-assembling phenylpropyl ether
based self-assembling dendrons compared to that of benzyl
ether based self-assembling dendrons.[11p]
The introduction of self-assembling phenylpropyl ether
based self-assembling dendrons offers previously unobtaina-
ble possibilities for dendronized polyarylacetylenes. We
have previously discussed the limitations of RhI-based poly-
merization catalysts with increasingly larger benzyl ether
based dendritic monomers.[2b,7c] Side reactions due to cleav-
age of the benzyl ether linkages are obviated by the acid tol-
erance of self-assembling phenylpropyl ether based self-as-
sembling dendrons.[11p] We have shown that compact helical
structures more efficiently communicate chiral information
from the periphery of dendronized polymers to the polymer
backbone.[2b,7a,b,e] Larger dendrons will allow us to probe
whether the more extended helix is fundamentally more dif-
ficult to program. Reversible, temperature-responsive exten-
sion and compression of the polyene backbone offers rudi-
mentary sensing in both solution and in bulk. We envision
further possibilities for multianalyte sensing based on the
helical chirality of the polyarylacetylene.[17f,18] Furthermore,
Acknowledgements
Financial support by the National Science Foundation (grant nos.: DMR-
05-48559 and DMR-05-20020) and the P. Roy Vagelos Chair at the Uni-
versity of Pennsylvania is gratefully acknowledged.
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FULL PAPER
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Received: July 2, 2007
Published online: September 24, 2007
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9581