Invited Paper Ligand-tuning of the Excitation Wavelength of a Solid State E/Z Isomerization: [Zn(TA) 2 (2,2¢-bipyridyl)] in a Supramolecular Framework Shao-Liang Zheng,* Milan Gembicky, Marc Messerschmidt and Philip Coppens* Department of Chemistry, State University of New York at Buffalo, Buffalo, New York, 14260-3000, USA. The single-crystal-to-single-crystal (SCSC) E®Z photoisomerization of TA in the supramolecular solid CECR-[Zn(TA) 2 (bpy)]·H 2 O (CECR = C-ethylcalixresorcinarene, HTA = tiglic acid, and bpy = 2,2¢- bipyridine, is induced by 458 nm light, indicating a red-shift of the photo-active wavelength on introduc- tion of the aromatic bpy ligand compared with the previously studied reaction of CECR-[Zn(TA) 2 (H 2 O) 2 ]· 4H 2 O. Theoretical calculations show that the initial excitation involves the bipyridyl ligand, which acts as an intramolecular photosensitizer for the isomerization process. The reaction is topotactic and illustrated by photodifference maps. Keywords: Photocrystallography; Ligand-tuning; Supramolecular framework; E/Z isomerization reaction. INTRODUCTION Photochemical E/Z photoisomerizations have re- ceived considerable attention as they trigger important physiological reactions. In photoactive yellow protein (PYP) for example, the E/Z isomerization of the p-cou- maric acid chromophore triggers the conformational change of the protein which leads to the physiological response of marine organism to exposure to blue light. 1,2,3 Although the process has been the subject of extensive time-resolved dif- fraction studies, the limited resolution of protein diffrac- tion, prevents atomic-resolution observation of the trigger- ing reaction, thus providing an impetus for study of the re- action in smaller systems. But unlike intramolecular cy- clizations 4,5 and many [2+2], 6 [4+4] 7 photodimerizations, especially when initiated by illumination in the long wave- length tail of an absorption band, 8 E/Z-photoisomerizations in the solid state do usually not occur as topotactic, sin- gle-crystal-to-single-crystal (SCSC) reactions. However, when it is possible to embed photoactive molecules within cavities in framework supramolecular crystals, the crystal lattice is much less affected by the photoinduced molecular change and SCSC reaction become common. 9,10,11,12 We have reported a number of E/Z-photoisomerization of framework-embedded photo-reactive molecules. 13,14 They include tiglic acid (HTA, Scheme I), and tiglic acid ligated to Zn(II) as [Zn(TA) 2 (H 2 O) 2 ], both within a supramolecular framework composed of CECR (CECR = C-ethylcalix- [4]resorcinarene). Both photo-isomerize without loss of crystallinity, allowing a detailed kinetic study of the prog- ress of the SCSC reaction. As part of our investigations of SCSC E/Z-photoisomerizations in supramolecular envi- ronments we have now isolated [Zn(TA) 2 (bpy)] (bpy = 2,2¢ -bipyridine) in the same framework, yielding crystals of composition CECR-[Zn(TA) 2 (bpy)]·H 2 O(1). The re- sults show that the excitation wavelength of E/Z-photo- isomerizations can be tuned by judicious variation of a co- ligand which remains unaltered in the final reaction prod- 16 Journal of the Chinese Chemical Society, 2009, 56, 16-21 * Corresponding author. E-mail: coppens@buffalo.edu Scheme I The structure of (a) CECR, and (b) HTA