Tetrathiafulvalenenaphthalenophanes: Planar chirality and cis/trans photoisomerization

A cyclophane incorporating one 1,5-dioxynaphthalene ring system and one tet rathiafulvalene (TTF) unit bridged by [SCH2CH2O] linkages has been synthesi zed. In this cyclophane, the TTF unit can adopt either cis or traits config urations. In addition, the 1,5-dioxynaphthalene ring system imposes one ele ment of planar chirality on this cyclophane. A second element of planar chi rality is introduced by the trans form of the TTF unit. Thus, the cyclophan e exists in diastereoisomeric forms as three pairs of enantiomers. The enan tiomeric pairs associated with the cis form of the TTF unit, as well as one of those associated with the trans form, have been isolated by crystalliza tion, and their structures assigned in the solid state by single-crystal X- ray analyses. In solution, cis/trans isomerization occurs when either the c is or the trans form of the cyclophane is exposed to light. The photoisomer ization reaction can be followed by H-1 NMR and UV-vis spectroscopies, as w ell as by HPLC. The photoisomerization quantum yield has been measured at t wo different excitation wavelengths (406 and 313 nm). In both cases, the tr ans --> cis process (Phi = 0.20 at 406 nm) is much more efficient than the reverse cis --> traits process (Phi = 0.030 at 406 nm). Since the absorptio n spectra of the trans and cis isomers are different and the quantum yield of the trans --> cis photoisomerization reaction depends on the excitation wavelength, the mole fraction of the two diastereoisomers present at the ph otostationary state depends on the wavelength of the exciting light. No iso merization occurs when the solutions, regardless of the mole fraction of th e two diastereoisomers, are stored in the dark.