Synthesis and Dimerization Studies of a Lipophilic Photoresponsive Aryl‐Extended Tetraurea‐Calix[4]pyrrole

Abstract

We describe the syntheses of the lipophilic aryl-extended α,α,α,α-tetraurea-phenyl-calix[4]pyrrole 1, featuring four appended azo-phenyl groups with two tert-butoxy carbonyl meta-substituents and its photo-inactive counterpart 2. In CD Cl solutions, both tetraurea-calix[4]pyrroles self-assemble into dimeric capsules by encapsulating one molecule of a suitable bis-N-oxide or two molecules of a mono-N-oxide. The dimeric capsules are mainly stabilized by a cyclic array of sixteen hydrogen bonds established between the eight unidirectionally oriented urea groups. Photoirradiation experiments demonstrated the trans-to-cis isomerization of the azo-phenyl groups and the formation of a plethora of stereo isomeric cis-azo-enriched capsular assemblies. The highly cis-azo enriched capsular assemblies seem to show a reduced stability and their involvement in equilibria with non-capsular counterparts that also bind the N-oxides. The thermally induced cis-to-trans interconversion processes demonstrated the reversibility of the photoisomerization and the photostability of most binding partners. An equimolar mixture of the two tetraureas produced two homodimeric capsules and the heterodimeric counterpart in a ratio close to statistical distribution.

A Regulable Internal Cavity inside a Resorcinarene‐Based Hemicarcerand

“…[6][7][8][9] Carcerands and hemicarcerands, [10,11] which are composed of covalently linked cavitands, are interesting types of single-molecular capsules. In contrast to self-assembled capsules, in which the components are held together by noncovalent interactions such as hydrogen, [12][13][14][15][16][17][18][19][20][21] chalcogen, [22] halogen, [23] and coordination bonds, for example, palladium-nitrogen coordination bonds, [24][25][26][27] covalently linked molecular containers are thermally and chemically stable,t hought heir production is synthetically more difficult.…”

Recent Advances of Photoresponsive Supramolecular Switches

“…It was noteworthy that the isomerization of azo did not lead to the dissociation of dimer, but result in the change of the dimer cavity. In the follow‐up study, they investigated the light‐controlled behavior of different kinds of host‐guest complexes based on calix[4]arenes and calix[4]pyrroles [13] . In 2018, the azo moieties were utilized to regulate the depth of the aryl‐extended cavity in a tetra‐α‐meso substituted calix[4]pyrrole by the same group (Figure 1b) [14] .…”

“…Incorporating photoresponsive moieties into a known host molecule is an effective method to make the host become photoresponsive. [12][13][14][15][16][17][18][19][20] The switching of host molecules via light stimuli leads to the reversible binding affinity changes for guest molecules, finally realizing release and capture of guests in a controlled manner. The group of Ballester conducted numerous studies on photoresponsive calix [4]arene and calix [4]pyrrole in recent years.…”

“…kinds of host-guest complexes based on calix [4]arenes and calix [4]pyrroles. [13] In 2018, the azo moieties were utilized to regulate the depth of the aryl-extended cavity in a tetra-α-meso substituted calix [4]pyrrole by the same group (Figure 1b). [14] In comparison with all-trans isomer of the calix [4]pyrrole, the allcis-isomer had a shallower aryl-extended cavity, which results in the decreased binding affinity for 4-phenyl pyridine N-oxide by one order of magnitude in DCM: from (7.0 � 2.0) × 10 6 M À 1 in alltrans form to (5.0 � 1.0) × 10 5 M À 1 in all-cis form.…”

Hydrogen‐Bonded Dimeric Capsules with Appended Spiropyran Units: Towards Controlled Cargo Release

“…Recently, we described several photo‐responsive homo‐ and hetero‐dimeric tetra‐urea capsules equipped with four azobenzene units at their upper rims. [ 11 , 38 , 39 , 40 ] Depending on the substitution of the terminal phenyl ring of the azobenzene units, the trans ‐to‐ cis photo‐isomerization reaction of the switches induced the dimer's disassembly at different extents. We discovered that the release of the cargo to the bulk solution was not directly related to the isomerization level.…”