Photocontrolled molecular assembler based on cucurbit[8]uril: [2+2]autophotocycloaddition of styryl dyes in the solid state and in water

Three styryl dyes of the 4‐pyridine series that form syn‐head‐to‐tail dimeric pairs in polycrystalline films were synthesised. NMR and UV/Vis spectroscopic studies showed that this promotes stereospecific [2+2]‐autophotocycloaddition (PCA) in the dimeric pairs to give rctt‐isomers of cyclobutane derivatives. For the dye with an N‐ammoniopropyl substituent, this transformation was accomplished according to the single‐crystal‐to‐single‐crystal pattern. In aqueous solutions, the dyes and cucurbit[n]urils (CB[n], n = 7, 8) form complexes with a pseudorotaxane structure. CB[7] tends to form 1:1 complexes, and CB[8] forms 1:1 and 2:1 complexes. The structure of the termolecular complex formed by the betaine of the N‐sulfonatopropyl styryl dye and CB[8] was determined by X‐ray diffraction. The stability of the complexes was measured by ¹H NMR titration in a D₂O/[D₃]MeCN (10:1) mixture (log K_1:1 ≥ 3.2, log K_2:1 ≥ 2.6). Free dyes and their complexes with CB[7] in water undergo only (E)/(Z) photoisomerisation. Termolecular complexes of all dyes with CB[8] were found to undergo effective stereospecific PCA reactions to give rctt isomers of cyclobutane derivatives. The 1:1 complexes of the obtained cyclobutanes with CB[8] (log K_cb = 3.2–4.8) are less stable than those of the starting dyes with the same stoichiometry. Thus, CB[8] can play the role of a photocontrolled molecular assembler, i.e., even in the presence of 5 mol‐% of CB[8], complete conversion of styryl dyes into the corresponding cyclobutane derivatives takes place upon irradiation in solution.