Communications
DOI: 10.1002/anie.200905222
Helicity
Control of the Helicity of Poly(phenylacetylene)s: From the
Conformation of the Pendant to the Chirality of the Backbone**
Iria Louzao, Josꢀ M. Seco,* Emilio Quiꢁoꢂ, and Ricardo Riguera*
Dedicated to Professor Pelayo Camps on the occasion of his 65th birthday
Since the seminal work of Percec and co-workers,[1a] the
design, synthesis, and applications of helical polymers with a
controlled helix sense has become a field of major interest in
recent years.[1b,c,2] The possibility of controlling and switching
the helicity of these polymers by an external agent[2,3] (e.g.
temperature,[3a,b] solvent,[3c–e] light[3f,g]) makes them suitable[4]
for several applications.[1b,c,2]
polarity effects[2,5] and is based on the characteristics of the
conformational equilibrium of the pendants. We performed
variable-temperature circular dichroism (CD) experiments in
a variety of solvents, atomic force microscopy (AFM) on
highly oriented pyrolytic graphite (HOPG), NMR, IR, and
Raman spectroscopy, and theoretical calculations (MM
(MMFF94), DFT (B3LYP), PCM).
We now present a novel reversible way to control the
helicity of poly(phenylacetylene)s with phenylglycine methyl
ester pendant groups (poly-(R)-1 and poly-(S)-1; Figure 1).
We show herein that the manipulation of the conformational
equilibrium of the pendant allows one to choose the right- or
left-handed sense of the helix. This phenomenon is achieved
by complexation with appropriate metal cations or by solvent
(R)- and (S)-Phenylglycine methyl esters were chosen as
suitable pendants for the planned studies. Accordingly, poly-
(R)-1 and poly-(S)-1 (Figure 1) were prepared by following
known procedures[5a] with [Rh(nbd)Cl]2 (nbd = 2,5-norborna-
diene) as catalyst from monomer 2 and obtained with
stereoregular cis-transoid[5] backbones as shown by the
chemical shifts of the vinyl protons (d = 5.7–5.8 ppm) and
Raman resonances (1553, 1343, 1003 cmÀ1) (see the Support-
ing Information for experimental details and spectroscopic
data). Poly-(R)-1 adopts a right-handed helical conformation
and poly-(S)-1 a left-handed one[5a] in CHCl3 (positive and
negative Cotton effects, respectively, at 375 nm; Figure 1),
and the polymers have positive and negative dihedral angles,
respectively (1808 < w1 < 08 and 1808 > w1 > 08, Figure 2c),
between vicinal double bonds.
CD spectra of the two polymers after addition of a series
of perchlorates of mono- and divalent metal cations (Li+, Na+,
Ag+, Mg2+, and Ba2+) showed, in all cases, that inversion of
the helicity had taken place (opposite CD signs); Ba2+ gave
the strongest response. The addition of acetylacetone (acac)
reversed the helicity, causing the recovery of the original CD
spectra in all cases.[6]
Figure 1. a) Structure of poly-(S)-1 and monomer (S)-2. b) CD spectra
of poly-(S)-1 taken before and after the addition of Ba(ClO4)2 and
recovery of the original helicity after the addition of acac (CHCl3).
To reveal the mechanism beyond this inversion of helicity,
a series of studies were performed:
1) AFM (HOPG)[7] gave important insights into the
helicity and morphology of poly-(R)-1 (see the Supporting
Information for details). The images show two types of
structures (Figure 2): individual and associated chains.
The single chains, packed parallel one after another,
display a left-handed (counterclockwise) pendant disposi-
tion[3b] (Figure 2a,d) with the periodic oblique strips forming
angles close to 458 (i.e. w1 ꢀ + 1488, Figure 2c). This value
justifies the right-handedness of the backbone (Figure 2d)
and allows intrachain hydrogen bond formation between the
nth and (n + 2)th amide groups (essential to stabilize the
helical structure).[8] AFM also shows multistranded left-
handed helices, in which interchain hydrogen bonds are
likely to play a main role[9] (Figure 2b). The AFM images
show, after the partial addition of Ba(ClO4)2 (1.0 equiv), the
coexistence of both senses of handedness (see the Supporting
Information).
[*] Dr. I. Louzao, Dr. J. M. Seco, Prof. Dr. E. Quiꢀoꢁ, Prof. Dr. R. Riguera
Departamento de Quꢂmica Orgꢁnica, Facultad de Quꢂmica
Universidad de Santiago de Compostela
15782 Santiago de Compostela (Spain)
Fax: (+34)981-59-1091
E-mail: josemanuel.seco@usc.es
[**] We thank the Ministerio de Ciencia e Innovaciꢃn (CTQ2008-01110/
BQU and CTQ2009-08632/BQU) and Xunta de Galicia (PGI-
DIT09CSA029209PR; PPIAI 2007/000028-0) for financial support
and the Centro de Supercomputaciꢃn de Galicia (CESGA) for their
assistance with the computational work. We also thank the Servicio
de Nanotecnologꢂa y Anꢁlisis de Superficies (CACTI, Universidad de
Vigo) for recording AFM experiments and Grupo de Magnetismo y
Nanotecnologꢂa (Universidad de Santiago de Compostela) for
experimental time on its spin-coater.
Supporting information for this article is available on the WWW
1430
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 1430 –1433