Light-orchestrated macromolecular "accordions": Reversible photoinduced shrinking of rigid-rod polymers

Article abstract of DOI:10.1002/anie.201106879

Light can play: Irradiation causes dramatic changes in the shape of rigid-rod polymers incorporating azobenzene photochromes in the main chain. The embedded photoswitches act as hinges, which upon light-induced isomerization lead to reversible shrinking a

Full text of DOI:10.1002/anie.201106879

DOI: 10.1002/anie.201106879
Smart Polymers
Light-Orchestrated Macromolecular “Accordions”: Reversible
Photoinduced Shrinking of Rigid-Rod Polymers**
David Blꢀger,* Tobias Liebig, Raphael Thiermann, Michael Maskos,* Jꢁrgen P. Rabe,* and
Stefan Hecht*
Synthetic molecular systems undergoing structure and prop-
erty changes as a response to external stimuli offer a series of
relevant functions. Their direct use in solution, immobilized at
surfaces, or integrated into adequate matrices, allow for the
elaboration of nano-carriers,[1] molecular electronic memory
circuits,[2] adaptive systems,[3] artificial muscles,[4] or healable
materials,[5] among others. Reversible systems are commonly
generated by utilizing molecular switches[6] as responsive bi-
stable molecules. Of particular interest are switches con-
trolled by non-invasive triggers, especially light, which can
display high spatial and temporal resolution. Photoswitches[7]
were used to change properties, such as conductance[8] and
basicity,[9] based on light-induced alteration of the molecular
geometry, that is, their dimensions and shape. In nature, light-
induced geometrical modifications of molecular systems[10]
lead to primary biological functions, such as the visual
perception in vertebrates,[11] or proton pumping in some
bacteria,[12] two examples that nicely illustrate the potential of
photo-inducing structural modifications in organic systems.
Changing the geometry of macromolecules can be
exploited for the elaboration of guest-encapsulation struc-
tures,[13] and opto-mechanical materials,[14] which enable
direct conversion of photon energy into mechanical work.
To prepare efficient structures and materials however, it is
desirable to design macromolecules apt to dramatically
change shape and dimensions upon exposure to light.[15]
Azobenzenes[16] are ideal candidates, since upon UV and
visible light irradiation, they reversibly undergo a large
geometrical modification, isomerizing from their linear and
flat E isomer to a more compact and kinked Z form. The
groups of Aida,[17] and in particular McGrath,[18] later on also
Mꢀllen and co-workers,[19] introduced azobenzene photo-
chromes into the core of rather spherical dendrimers, result-
ing in (quasi)isotropic nano-objects that display significant
changes in hydrodynamic volume upon photoirradiation, with
contractions as high as DVh = 37%,[19] as evaluated by gel
permeation chromatography (GPC). It is reasonable to
expect that volume contractions of larger magnitude can be
realized in anisotropic architectures, for example, cylindrical
objects with high aspect ratio (defined as the ratio of a
cylinderꢁs length to its diameter). Herein we report on the
synthesis of such rigid-rod polymers consisting of azobenzene
photochromes in the main chain and the dramatic change of
their shape induced upon irradiation. The embedded photo-
switches act as hinges, which upon light-induced isomeriza-
tion lead to reversible shrinking and stretching of the polymer
backbone.
Rigid-rod polymer P1 (Figure 1) incorporates azobenzene
photoswitches in a poly(para-phenylene) (PPP) backbone.
The critical step to achieve large shrinkage of the rods is to
[*] Dr. D. Blꢀger, Prof. Dr. S. Hecht
Department of Chemistry, Humboldt-Universitꢁt zu Berlin
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
E-mail: david.bleger@staff.hu-berlin.de
sh@chemie.hu-berlin.de
Homepage: http://www.hechtlab.de
T. Liebig, Prof. Dr. J. P. Rabe
Department of Physics, Humboldt-Universitꢁt zu Berlin
Newtonstrasse 15, 12489 Berlin (Germany)
E-mail: rabe@physik.hu-berlin.de
R. Thiermann, Prof. Dr. M. Maskos
BAM Bundesanstalt fꢂr Materialforschung und -prꢂfung
Unter den Eichen 87, 12205 Berlin (Germany)
E-mail: michael.maskos@bam.de
[**] D.B. and S.H. conceived the idea and designed the experiments.
D.B. synthesized the molecules and carried out optical spectros-
copy; R.T. and M.M. designed and performed LS measurements,
T.L. and J.P.R. designed and performed the SFM measurements.
D.B. and S.H. wrote the paper. D.B., S.H., J.P.R., and M.M.
discussed the results and commented on the manuscript. Generous
support by the German Research Foundation (DFG) through SFB
658, subprojects A3 and B8 is gratefully acknowledged. Wacker
Chemie AG, BASF AG, Bayer Industry Services, and Sasol Germany
are thanked for generous donations of chemicals.
Figure 1. Chemical structure and schematic representation of P1
(R=C12H25) in the extended (100%-E) and compressed (86%-Z) state.
Molecularly dissolved P1 can reversibly shrink and stretch upon
exposure to UV (l=357 nm) and visible (l>400 nm) light, respec-
tively.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106879.
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12559

Communications
attain a Z-rich photostationary state (PSS), in which a large
amount of azobenzenes are in their Z-configuration. This
point is not straightforward since azobenzenes directly
connected at their para-positions display rather Z-poor
PSS.[20,21] However, we recently developed an electronic
decoupling approach allowing for quantitative photoswitch-
ing in linear bisazobenzenes.[21] It was shown that introducing
large twist angles between the chromophores and therefore
breaking the conjugation, lead to systems with restored
photoswitching efficiency. Rigid rod P1 displays large twist
angles (F)[22] along its polyaromatic backbone leading to a
86% overall Z content[23] in the PSS. To our knowledge, this is
the first time that such a high amount of Z forms has been
observed in rigid multi-azobenzene architectures.[24]
Polymer P1 was synthesized by Suzuki polycondensa-
tion[25] using [Pd2dba3] (dba = dibenzylideneacetone) in com-
bination with Buchwaldꢁs SPhos ligand,[26] a catalytic system
developed to couple sterically hindered substrates (for details
see the Supporting Information). The use of the SPhos ligand
was essential for reaching high molecular weight polymers
because P1 is sterically crowded at its coupling positions by
methyl groups and dodecyl chains, introduced in the back-
bone to confer large twist angles and good solubility,
respectively. The sample studied herein has a molecular
weight of Mn ꢀ 46000 gmolꢁ1 [27] (theoretically corresponding
to 72 repeat units), with a polydispersity index of 1.3.
Irradiating solutions of all-E P1 (extended state) with UV
light[28] induces modifications in the UV/Vis absorption
spectra (Figure 2a), typically observed for single azoben-
zenes. As the embedded photochromes isomerize, the band
corresponding to the p–p* transition (at 357 nm) decreases
and the band attributed to the n–p* transition (centered at
450 nm) increases, in agreement with the large amount of
Z-configured azobenzenes in the backbone. In addition,
shrinkage of the P1 rods is readily determined from the
GPC curves (Figure 2b). By virtue of the large twist angles
(F) UV irradiation leads to a Z-rich PSS with a significantly
increased retention volume, corresponding to a change of
hydrodynamic volume DVh = ꢁ75%, (based on the GPC
data).[29]
Static and dynamic light-scattering (SLS/DLS) measure-
ments were performed to measure the accurate dimensions
and conformations of P1 in the two states.[28] At concentra-
tions of approximately 1 gLꢁ1 in THF, both the radius of
gyration (Rg) and the hydrodynamic radius (Rh) of P1
decrease upon UV light irradiation, from Rh = 10 nm for
100% E-P1 to Rh = 5.3 nm for 86% Z-P1. This change
corresponds to a significant drop of the hydrodynamic volume
(DVh = ꢁ72%, in good agreement with the change deter-
mined by GPC), as illustrated by the increase of the apparent
diffusion coefficient (Dapp, see Figure 2c). Light-scattering
data additionally allow the change in shape to be determined,
by calculating the so-called 1 ratio.[30] The 1 ratio decreases
from 2.3–2.9 (for a rod-like molecule of this size—length l >
40 nm, diameter D = 2 nm—the 1-ratio should be > 2) to 1.5–
1.9 (for random coil structures, 1 = 1.505).[31] Therefore,
shrinkage of P1 is accompanied by a conformational rod!
coil transition, as a consequence of the E!Z isomerization of
the embedded azobenzenes. Moreover, the process was found
Figure 2. Monitoring P1 shrinking upon UV irradiation (l=357 nm).
a) Time evolution of the UV/Vis absorption spectra of a solution of P1
in CH2Cl2 until the PSS was reached (in ca. 45 s), b) GPC curves of P1
in the extended all-E (before irradiation, solid line) and compressed
Z-rich state (after irradiation, broken line) in THF, and c) angular
dependence of the apparent diffusion coefficient (Dapp) in THF
(0.7 gLꢁ1) before (filled squares/solid line) and after (open circles/
dashed line) irradiation. Arrows indicate changes on UV irradiation.
to be reversible (as shown by UV/Vis absorption spectrosco-
1
py, H NMR spectroscopy, GPC, and light scattering meas-
urements), that is, irradiation with visible light (l > 400 nm)[28]
triggers reverse Z!E isomerizations leading to the recovery
of the rod-like geometry.[32]
The possibility to exploit the rod!coil transition in an
ensemble of molecules was further examined by synthesizing
a polymer homologue of P1. It is anticipated that Z-rich
polymers will display a diminished ability to stack, when
compared to all-E rigid rods, owing to the kinked config-
uration of Z-azobenzenes. A variation in the stacking
propensity should enable the elaboration of photoswitchable
assemblies. Similar to polymer P1, PPP derivative P2 (see
12560 www.angewandte.org
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Angew. Chem. Int. Ed. 2011, 50, 12559 –12563

spheres were reformed within a
few minutes upon irradiation with
visible light (> 400 nm). Analo-
gous observations were made by
light-scattering
measurements:
Upon UV-light irradiation, the Rh
decreases from 45 nm for the
aggregates to 25 nm for the pre-
sumably molecularly dissolved
polymer and the 1-ratio increases
from 0.9–1 (1 = 0.775 for monodis-
perse homogeneous spheres) to
1.2–1.5 (for random coil structures,
1 = 1.505).[31] The disruption was
additionally observed by monitor-
1
ing the H NMR spectra, where a
very broad peak, centered around
6.2 ppm and attributed to the
aggregated areas, completely van-
ishes upon UV-light irradiation
(see Figure S4). In the literature,
triggered disassembly of sphere-
like nano-objects is almost exclu-
sively realized in water using
switchable amphiphiles.[35] From
this perspective, our P2 system,
Figure 3. Light induced reversible disruption of molecular aggregates of P2 (R=C12H25). a) Selected
snapshots of the disruption process (SFM), scale bar: 1 mm. The inset (t=40 min) shows a
z-expansion (z-scale: 3.3 nm) of the surface, emphasizing the absence of aggregates after UV-light
irradiation. The aggregates were spin-coated on mica from a 1 gLꢁ1 solution of P2 in THF. b) Polymer
P2 contains a few solubilizing groups and therefore aggregates in the extended (100%-E) state. In the
compressed (81%-Z) state, the intermolecular aggregates are disrupted.
Figure 3) features large twist angles (F,F’)[33] leading to a
Z-rich PSS with a 81% overall Z content, as determined by
1H NMR spectroscopy (see Supporting Information, Fig-
ure S3). However, P2 is functionalized with only two dodecyl
chains per 2.4 nm long repeat unit (vs. two dodecyl chains per
1.4 nm long repeat unit in P1). As a result, all-E P2 has a
slightly reduced solubility and is consequently able to form
aggregates in apolar solvents.[34] The sample studied here (for
synthesis and characterization, see the supporting informa-
tion) displays a molecular weight of Mn ꢀ 56000 gmolꢁ1 [27]
(theoretically corresponding to 66 repeat units), with a
polydispersity index of 1.7. Light scattering measurements
indicate that clear solutions of P2 in THF (ca. 1 gLꢁ1) contain
nano-sized spherical (1 ꢀ 1) aggregates, with hydrodynamic
radii of Rh ꢀ 45 nm.[28] A visualization of the spherical
aggregates on mica surfaces by scanning force microscopy
(SFM) is shown in Figure 3a. The distribution of the
aggregates is rather narrow, with an average height of 80 nm
(see Figure S12 in the Supporting Information).
which relies on the ability of the polyaromatic rods to stack in
one switching state but not the other, is rather unique and
should generally be operating in most common non-polar
solvents.
In summary, this study shows a unique approach for
maximizing the photodeformation of individual molecules,
using azobenzene-containing anistropic molecular architec-
tures. Reversible and efficient shrinkage of rod-like polymers,
such as P1, demonstrated and quantified by SLS/DLS
measurements, is accompanied with an unprecedented
decrease in hydrodynamic volume. Furthermore, designing
multi-azobenzene rods with reduced solubility, such as P2,
leads to the formation of spherical aggregates in organic
solvents, which could reversibly be disrupted by alternating
irradiation with UV and visible light. Based on this work, we
aim to use the optimized shrinkable polymers for the bottom-
up construction of efficient opto-mechanical materials.[14]
Furthermore, the transfer of this concept to aqueous solution
promises possible applications as “smart” carriers.
When solutions of P2 are exposed to UV light, the rod !
coil transition takes place, as indicated by the GPC curves of
molecularly dissolved P2 in THF (see Supporting Informa-
tion, Figure S1). Remarkably, this conformational transition
induces a disruption of the sphere-like aggregates, according
to SFM as well as SLS/DLS measurements. Selected SFM
snapshots of samples precipitated during a disruption–reag-
gregation cycle are shown in Figure 3a. Upon ex situ photo-
irradiation over 40 min, the dimensions of the sphere-like
aggregates continuously decrease from almost 80 nm height
(corresponding roughly to the diameter) to a residual
approximately 3 nm height (see inset Figure 3a). We did not
observe any large deformations of the aggregates which
remain rather spherical until complete disruption. The
Received: September 28, 2011
Published online: November 24, 2011
Keywords: aggregation · azo compounds · nanostructures ·
.
photochromism · polymers
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