Dansylated resorcinarenes

The synthesis, X-ray crystal structures and spectroscopic characterization (UV-Vis absorption and fluorescence emission) of regioselective tetradansylated resorcinarene 2 and octadansylated resorcinarene 3 are described. In solution, the resorcinarene backbone of 2 shows a boat conformation with C-2v symmetry, while the octasubstituted 3 shows at room temperature a time-averaged C-4v symmetry, which turns into stable boat conformation at low temperatures. The four hydroxyl groups of 2, not present in 3, form hydrogen bonds to the sulfoxide oxygens of the dansyl moieties and stabilize the C-2v conformation in solution. The X-ray crystal structures of 2 and 3 confirm the C-2v symmetry of both compounds in the crystalline state and revealed that the tetradansyl resorcinarene has two rather strong intramolecular O - H center dot center dot center dot O=S hydrogen bonds. In addition 2 forms a directly hydrogen-bonded dimeric assembly via four intermolecular O - H center dot center dot center dot O and O - H center dot center dot center dot O=S hydrogen bonds. In the solid state both 2 and 3 exhibit a multitude of intra- and intermolecular pi - pi interactions between the adjacent dansyl moieties and also with the aromatic parts of the macrocyclic skeleton. The absorption and fluorescence spectra of 2 and 3, together with a reference compound, dansylphenolate 4, were recorded. In chloroform, with the same dansyl concentration and absorption at excitation wavelength, fluorescence from 4 was clearly stronger than that from either 2 or 3, an indication of excited-state dansyl - dansyl interactions in the dansyl substituted resorcinarenes. The absorption and emission bands of the dansyl units were gradually substituted by the absorption and emission bands of the protonated dansyl units when 2 and 3 were titrated with trifluoroacetic acid.