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Intermolecular Coupling of Motion under Photochemical Control.

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Angewandte
Chemie
DOI: 10.1002/anie.200602516
Molecular Machines
Intermolecular Coupling of Motion under
Photochemical Control
Franisco M. Raymo*
Keywords:
chirality и molecular devices и photochromism и
supramolecular chemistry
Biology is replete with examples of
sophisticated processes that rely on the
precise control of motion at the molecular level.[1, 2] In these systems, appropriate stimulations induce cascades of
programmed molecular motions and
regulate crucial biological functions as
a result. The cytoplasmic proteins dyneins, kinesins, and myosins, for example, shuttle substrates along linear filaments in response to adenosine triphosphate (ATP).[3] The hydrolysis and synthesis of this particular molecule are in
turn correlated to the rotary motion of
ATP synthase in response to transmembrane fluxes of ions.[4] Similarly, the
hydrolysis of ATP can open and close
the cylindrical cavity of chaperonin
proteins, controlling the release of entrapped guests.[5]
The complexity of biological processes, based on controlled molecular
motions, together with their relevance to
cellular functions have stimulated a
wealth of investigations aimed at understanding the basic factors that regulate
these fascinating systems.[1?5] These fundamental studies have in turn encouraged the identification of viable strategies to control the relative movements
of artificial molecular assemblies by
external stimuli.[6?9] Clever operating
principles and structural designs have
emerged as a result in the form of
functional nanostructures?molecular
[*] Prof. F. M. Raymo
Center for Supramolecular Science
Department of Chemistry
University of Miami
1301 Memorial Drive, Coral Gables, FL
33146-0431 (USA)
Fax: (+ 1) 305-284-4571
E-mail: fraymo@miami.edu
Angew. Chem. Int. Ed. 2006, 45, 5249 ? 5251
machines.[6] Generally, these systems
are composed of at least two moving
parts, which are covalently, mechanically, or noncovalently connected to each
other. The position of one component
relative to the other can be controlled
reversibly with chemical, electrical, or
optical inputs.
Chemically controllable molecular
machines are in principle reversible.[6?9]
However, the regeneration of their original state often requires the addition of a
second chemical input to neutralize the
influence of the first. Thus, multiple
switching cycles lead to the accumulation of side products. Electrically controllable molecular machines are instead based on interfacial electron transfer.[6?9] Electrochemical oxidation and
reduction steps are responsible for their
reversible operation. Nonetheless, both
processes require the physical contact of
the molecular machine with a macroscopic electrode. Optically controllable
systems instead do not suffer these
limitations.[6?9] In fact, many photochemical processes can simply be reversed by changing the irradiation wavelength. Furthermore, photons can travel
through transparent media and reach a
molecular target even if this species is
not in contact with the irradiation
source. Indeed, optical control is emerging as a convenient strategy to operate
molecular machines,[10] and numerous
photoresponsive systems with diverse
structures and functions have recently
been developed.[6?10] For example, the
relative movements of the interlocked
components of photoactive catenanes[11, 12] and rotaxanes[13?19] can be
controlled with light. Similarly, the unidirectional rotation of one molecular
fragment relative to another can be
implemented on the basis of the photoinduced isomerizations of overcrowded
alkenes.[20]
The controlled motions associated
with the photoresponsive molecular machines developed so far involve either
the relative movements of two molecular portions integrated within the same
structure or the association/dissociation
of supramolecular assemblies.[6?10] A
recent investigation[21] has taken the
design of these functional nanostructures one step further. This clever study
demonstrates that the motion of one
molecule can be exploited to regulate
the movement of another under photochemical control. Indeed, the assistance
of supramolecular interactions can be
invoked to couple intermolecularly the
dynamics of a receptor with those of a
complementary substrate. Specifically,
compound 1 (Figure 1) combines an
azobenzene switch, a ferrocene pivot,
and two porphyrin chelators. Irradiation
of this molecular assembly with ultraviolet light induces the trans!cis isomerization of the azobenzene photochrome.[22] This transformation shortens
the distance between the two 1,3-phenylene rings of the photochromic fragment, causing a rotation of the two
cyclopentadienyl rings of the ferrocene
pivot about the main axis of the metallocene.[23] The photoinduced motion
alters the separation and relative orientation of the two porphyrin appendages.
The overall process is fully reversible,
and the original state is regenerated
after irradiation with visible light.[24]
The photoinduced and reversible
transformations of the receptor 1 can
be exploited to regulate the conformation of a complementary substrate.[21] In
particular, the bidentate 4,4?-biisoquino-
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5249
Highlights
Figure 1. The azobenzene switch of 1 interconverts
between trans and cis forms upon irradiation with UV
and visible light. As a result, the two cyclopentadienyl
rings of the ferrocene pivot adjust their relative orientation, altering the dihedral angle about the bond that
connects the two isoquinoline fragments of the substrate 2 bound in the bis(porphyrin) cleft of 1.
line 2 (Figure 1) binds the two porphyrin
chelators of 1 as a result of NиииZn
coordination. In the resulting supramolecular assembly, the photoinduced
regulation of the distance between the
two porphyrin units controls the dihedral angle about the CC bond that
connects the two isoquinoline fragments. Thus, the geometrical changes
associated with the photoisomerization
of the azobenzene switch in 1 are
coupled intermolecularly to the relative
rotation of the two main fragments of 2.
The photoinduced geometrical
modifications of the receptor 1 and the
substrate 2 in the corresponding supramolecular assembly translate into profound changes in their chiroptical response.[21] Indeed, the receptor itself is
chiral and the trans isomer can adopt the
two configurations (+)-trans-1 and ()trans-1, as illustrated in Figure 2. The
photoisomerization of the azobenzene
5250
www.angewandte.org
switch in (+)-trans-1 bound to 2
modulates the intensity of a
band at 421 nm in the circular
dichroism (CD) spectrum. This
band corresponds to an electronic transition centered on
the porphyrin chromophores,
and increases with the trans!
cis isomerization of the azobenzene switch and decreases
with the opposite transformation (Figure 3 a). Similarly, the
substrate 2 is locked into a
chiral conformation upon binding to (+)-trans-1 with the con- Figure 2. The two enantiomers associated with the
comitant appearance of a pos- trans isomer of the receptor 1.
itive band at 307 nm in the CD
spectrum. This band decreases
with the trans!cis isomerization of the azobenzene component and increases with the
opposite transformation (Figure 3 b).[25] The spectra show
that the changes in the chiroptical response of the receptor
(Figure 3 a) parallel those of
the substrate (Figure 3 b), confirming that the photoinduced
motion of the former is coupled
to the rotation of the latter.
The analysis of the receptor
1 and substrate 2 demonstrates Figure 3. Changes at l = 421 (a) and 307 nm (b) in the
that their supramolecular asso- CD spectrum of the complex between (+)-trans-1 and 2
ciation mediates effectively the upon irradiation with UV and visible light (partially
reproduced from Ref. [21] with permission).
transfer of dynamic information from one component to
the other.[21] This study proves
unequivocally that the intermolecular motions, it is essential to learn how their
coupling of motion can be designed into functions can be reproduced. Only then
artificial molecular machines. In princi- will artificial molecular machines be
ple, similar mechanisms can further be able to evolve into nanostructured dedeveloped to concatenate the dynamics vices with moving molecular compoof more than two moving components. nents and unique functions and properUltimately, the design of programmed ties.
cascades of molecular motions in multicomponent assemblies can certainly be Published online: July 26, 2006
envisaged. It is important to stress,
however, that this unique example of
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The ratio between the trans and cis
isomers at the photostationary state was
22:78 (l = 350 10 nm).[21]
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T. Aida, J. Am. Chem. Soc. 2003, 125,
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The ratio between the trans and cis
isomers at the photostationary state was
63:37 (l > 420 nm).[21]
The CD spectrum of the supramolecular
assembly was deconvoluted with the aid
of model compounds to determine the
contribution of the substrate 2, as the
receptor 1 also absorbs at 307 nm.[21]
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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