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.