Clathrate formation by and self-assembled supramolecular structures of a “molecular spring”

Abstract

The inclusion properties and self-assembly of racemic and optically active helicenediols have been thoroughly investigated. The racemic helicenediol (PM-1) crystallizes with ethanol or 1,2-dichloroethane in different host–guest stoichiometric ratios to form (PM-1)·(ethanol) or (PM-1)₂·(1,2-dichloroethane), respectively. Single crystal X-ray analyses of the clathrates show that the helicenediol 1 has greater flexibility to accommodate guest molecules than might have been anticipated. The helical pitch of the helicenediol, which controls the interplanar angle between the terminal thiophene rings, ranges from 38.0, for the ethanol clathrate, to 54.5° for the 1,2-dichloroethane clathrate. This represents an increase of 16.5° or 44%. Testosterone is selectively incorporated into the left-handed helicenediol (M-1) to afford a 1∶1 inclusion complex, (M-1)·(testosterone), in which the interplanar angle decreases from 54.5 to 46.2°. Without guest molecules, racemic helicenediols self-assemble through a unique supramolecular network of hydrogen bonds to form an alternate-leaf motif, while right-handed helicenediols form a four-leaf clover motif in projection. In the self-assembled structures, the interplanar angle of the helicenediol 1 changes from 44.7, for the racemic case, to 33.8° for the right-handed helical case. All of the above evidence points to the surprising conclusion that helicenediol 1 can expand and contract as a “molecular spring”. The maximum elongation of the spring is about 61%.