Supramolecular solids as a medium for single-crystal-to-single-crystal E/Z photoisomerization: kinetic study of the photoreactions of two Zn-coordinated tiglic acid molecules.

[Zn(TA)2(H2O)2] (H-TA=tiglic acid) has been embedded in a framework composed of CECR (CECR=C-ethylcalix[4]resorcinarene) molecules to examine its E-->Z photoisomerization in a periodic framework. The photoisomerization of tiglic acid in CECR-[Zn(TA)2(H2O)2]4 H2O proceeds without the [2+2]-dimerization reaction that often occurs in crystals of uncomplexed analogues, and without breakdown of the crystal lattice that frequently occurs in neat crystals. The two Zn-coordinated TA molecules are located in different size cavities. The rate constants of the isomerization reaction are strongly affected by the size of the reaction cavity. Analysis of the temperature dependence of the reaction rates and the occupancies in the final photostationary state shows that the activation energies and the standard enthalpies of activation are dependent on the difference between the reaction cavities. This is the first quantitative diffraction study of solid-state E/Z isomerization of a metal-coordinated ligand in a periodic host environment.