Macrocyclic photoisomerizations

The chemical structures of [2]catenane 19 and the related [3]catenane 20 (Fig. 8) were conceived as an extension of their work on molecular shuttles. The larger macrocycle in 19 comprises two fumaramide stations with differing macro cycle binding affinities. In station B (red) the methyl groups on the fumaramide motif cause it to have lower affinity than the standard fumaramide station. The non-methylated fumaramide station (station A, green) is located next to a benzophenone unit. This allows selective, photosensitized isomerization of station A by irradiation at 350 nm. Station B (red) can be photoisomerized by direct irradiation at 254 nm. The third station, a succinic amide ester (station C, orange), is not photoactive and is intermediate in macrocycle binding affinity between the two fu-... 

Illuminated alternately with UV and visible light, the porphyrinic dithienylethene underwent a reversible photoisomerization reaction (Scheme 1). Meanwhile, the fluorescence intensity of the porphyrin macrocycles could be regulated by alternate irradiation with UV (313nm) and visible light (longer than 480 nm). Moreover, the selective excitation band of the fluorophore was beyond the spectral band which could induce the photoisomerization reaction. Therefore, the authors asserted that this compound could act as a system for reversible data processing using fluorescence as the non-destructive detection method. 

Figure 25. Light-controlled reversible shuttling of the macrocyclic component of 27 + along its axle, based on the cis/trans photoisomerization of the azobenzene unit [111].

There are several examples of catenanes where ring movements can be induced by external stimulations like simple chemical reactions or homogeneous or heterogeneous electron transfer processes [91-93], but only very few cases are reported in which the stimulus employed is light. It has been shown that in azobenzene-containing [2]catenanes like 31 + (Fig. 29) it is possible to control the rate of thermally activated rotation of the macrocyclic components by photoisomerization of the azobenzene moiety [119, 120]. Such systems can be viewed as molecular-level brakes operated by light. 

Figure 29. Photoconlrollable molecular-level brake. The thermally activated circumrotation of the macrocyclic polyether component of 2 catenane 31 can be modulated reversibly by cisitrans photoisomerization of the azobenzene unit incorporated into the tetracationic macrocycle [119, 120].

The triplet-sensitized photoreaction of stilbenophanes that caused trans-ds photoisomerization of stilbenophanes trans,trans-2, Z = SiMe2CH2-l,3-C6H4CH2SiMe2 trans,trans-, Z = CH2SiMe2CH2-l,4-CH2 and cis,cis-3, Z = CH2) was reported [98], A series of macrocyclic and medium-size stilbenophanes tethered by silyl chains... 

Leigh et al. also reported a chiroptical molecular switch (Figure 63) with a large amplitude displacement of a tetraamide macrocycle driven by light. The tetraamide macrocycle resides on the fumaramide unit of the dumbbell component initially. Upon photoisomerization of the... 

Incorporation of a triplet sensitizer benzophenone into the macrocycle 33c has little influence on the self-assembly nor the dimension of the resulting nanotube, but the reaction environment of the cavity is totally changed. Photodimerization of the above enones could not happen within 33c-tube. Instead, the nanotube was able to act as a triplet sensitizer for the cis-trans photoisomerization of trans- >-mQ hy -styrene and selective oxidation of 2-methyl-2-butene to a primary allylic alcohol (Scheme 3.14) [70, 109]. As well, an enlarged nanotube 33d-tube was shown to facilitate the selective photodimerization of coumarin to the awr/-head-to-head product [71]. 

Recently, we synthesized a complicated switchable CD-based [3]rotaxane (Fig. 5) to simulate the abacus system [28]. The E- Z photoisomerization of azobenzene results in the shuttling of the two macrocycles close to NS stopper accompanied by... 

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