Funding information
European Research Council grants (ERC Advanced Grant No. 74092 to
R.J.M.N. and ERC Advanced Grant No. 227897 to B.L.F.); Dutch Ministry of
Education, Culture, and Science grant (Gravitation program 024.001.035).
Abstract: Molecular motors change conformation under the
influence of light and when attached to host molecules they may find
applications as sensors and switchable catalysts. Here we present a
porphyrin macrocyclic host functionalized with two motor appendages for
future catalytic applications. The compound is formed as a mixture of
six stereoisomers (three sets of enantiomers), which have been separated
by (chiral) chromatography. 1H NMR and chiral
spectroscopy revealed that in one set of diastereomers the two motors
interact with the cavity of the host (bound-bound ), whereas in a
second set one interacts and the other one does not
(bound-loose ). In the third set both motors do not interact with
the host compound (loose-loose ). The motorized hosts bind guest
molecules in the order: (loose-loose ) >
(bound-loose ) > (bound-bound ). They can be
switched with light to pseudo-identical diastereomers, leading to
orthogonal behavior in the light-gated binding of guest molecules.
Whereas the photo-isomerization of the diastereomer setloose-loose significantly lowers the binding affinity for
viologen guests, the opposite is true for the diastereomer setbound-bound , i.e. the binding affinity increases. For the
diastereomer set bound-loose no influence on guest binding is
observed as the effect of photoisomerization on the motors is cancelled
out.
Stimuli-responsive host-guest systems mimic the dynamic and regulatory
behavior of enzyme-substrate complexes and various examples of such
artificial systems have been published in the
past.1–6 A frequently used non-invasive stimulus is
light,2 which can activate a chromophore present in
the host or the guest leading to a change in the electronic or
three-dimensional structure of these molecules and hence a change in
their affinity. Photo-responsive macrocyclic hosts based on various
chromophores have been described in the literature, including
azobenzenes, stilbenes, and dithienylethenes. While
azobenzenes7 and stilbenes8 undergo
photochemical E -Z isomerizations,
dithienylethenes9 are involved in photochemical
cyclizations and ring-openings. Various host molecules, e.g.
cyclodextrins, have been equipped with photo-switches in order to tune
their binding properties.10,11 In addition, an
artificial muscle was developed employing cyclodextrins in combination
with azobenzene-functionalized polymeric guest
molecules.12 Other extensively studied hosts are crown
ethers, which have been connected to azobenzene13 and
stilbene photoswitches,14,15 for photo-switchable
guest binding and on/off-catalysis. Azobenzene-functionalized cationic
guests have been used for light-powered threading through crown
ethers.16 More recently, a molecular
motor-functionalized crown ether was developed,17 in
which host-guest interactions were used to tune specific properties of
the molecular motor, i.e. the rotation rate and the
photostationary state. In addition, molecular motors capable of tuning
the chiral environment of the crown ether host, allowing
enantio-divergent binding and recognition of chiral guests have been
reported.18,19 Molecular motors offer the unique
property of photochemically induced helical
inversion,20,21 which allowed for the construction of
photo-switchable asymmetric catalysts.22,23
In our group, we have developed a porphyrin macrocyclic host
(i.e. cage compound Zn1 , Figure 1),24consisting of a glycoluril framework that is connected to a porphyrin
roof via xylylene side walls and crown ether-like linkages. Zn1is an excellent host for viologen guests, which are bound in the cavity
of Zn1 by a combination of electrostatic and π-stacking
effects.24 As part of a program aimed at the
development of a supramolecular catalytic system that can write
information on a polymeric thread we recently reported on the synthesis
and properties of molecular motor-functionalized porphyrin cageZn2 ,25 which combines the rotational
properties of a free molecular motor with the host-guest binding
properties of the parent porphyrin cage Zn1 . Since Zn2contains only one motor substituent the two faces of the cavity of this
porphyrin host are non-identical, causing ambiguity in the binding and
threading process of guest molecules. Here we report the synthesis and
physical properties of a porphyrin cage (Zn3 ), of which both
faces of the molecule are equipped with a motor function, eliminating
this ambivalence. Host molecule Zn3 possesses three distinct
stereochemical elements, i.e. point chirality at the four chiral
centers, planar chirality resulting from the presence of substituents on
the xylylene sidewalls,26 and axial chirality on both
molecular motor substituents. The combination of fixed planar chirality
and photo-switchable axial chirality (helicity) is exploited in the
orthogonal dynamic binding of viologen guests.