Cis-Trans Photoisomerization in a Tri-o-thymotide Clathrate. Crystal Structure of Tri-o-thymotide Clathrates of cis-and trans-Silibene

New clathrate inclusion complexes of tri-o-thymotide (TOT) which contain cis-stilbene or rrani-stilbene have been prepared and characterized; both crystallize in the triclinic space group P1 and have closely similar cell constants. Each unit cell of the trans-stilbene-TOT complex contains four TOT molecules and two stilbene molecules; the stilbenes lie on crystallographic centers of symmetry within two crystallographically independent sausage-like channels. The cis-stilbene-TOT complex contains partially empty channels, the TOT : stilbene ratio being approximately 2.5:1, and the disordered guest molecules could not be located. On irradiation through Pyrex, the m-stilbene-TOT clathrate is transformed to trans-stilbene-TOT clathrate; phenanthrene is also formed. The experimental results are consistent with a pathway involving photoisomerization of the cis-stilbene within the clathrate inclusion complex. The photoreactivity of the cis-stilbene complex, and stability of the trans-stilbene clathrate, may be ascribed to the disorder, larger volume, and higher energy of the cis species, but we ascribe particular importance to the coincidence or noncoincidence of molecular symmetry and cavity symmetry. The centrosymmetric cavity appears to stabilize centrosymmetric molecules and favor reaction pathways from noncentrosymmetric reactants to centrosymmetric products. The cis and trans isomers of methyl cinnamate also form triclinic TOT clathrate crystals, but here, where neither reactant nor product can achieve the symmetry of the cavity, irradiation yields approximately equal amounts of cis and trans isomer starting from either pure cis-or pure trans-cinnamate clathrate.