When a 1:1 complex of cis-3,3′-bis(diphenylhydroxymethyl)stilbene (1) with acetone was irradiated in the solid state at room temperature using a high-pressure mercury lamp with a Pyrex filter for 6 h, a 1:1 complex of the trans-isomer (2) with acetone was obtained without losing the guest acetone molecules. Similar results were obtained without a guest or in the presence of other guests. Internal rotation around the double bond bearing the very large triphenylhydroxymethyl substituent is hardly imaginable in the confinement of the crystal. Therefore, the geometrically easier 'hula-twist' (HT) mechanism with its movements primarily in the molecular plane provides a viable mechanistic alternative for the cis-trans isomerization. The solid-state mechanism of the reaction was studied by x-ray and atomic force microscopy (AFM) analyses. Face-selective efflorescence formed a protective cover on (100) and can be related to the crystal structure. Further molecular migration was not detected with the sensitivity of AFM and the crystal of 1·acetone did not lose its clarity and microscopic shape. The cis-trans conversion profits from cages that are present in the crystal lattice. X-ray structural analysis confirmed a strong loss of crystallinity upon the photochemical conversion of 1·acetone which precludes a definitive conformational proof of HT by x-ray diffraction. The formation of the amorphous phase excludes a topotactic reaction.
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