The indirect coupling of external energy sources (e.g. light, redox and proton gradients, and exergonic reactions) to molecular switches [1] to drive a function is one of the most interesting problems in supramolecular chemistry. To our knowledge, light-driven, carrier-mediated, active transport through a membrane against a concentration gradient has not yet been achieved. To this end, we developed a photoswitchable ligand [2] based on the known norbornadieneQquadricyclane isomerization.[3] Quadricyclanes with electrondonating substitutents on the cyclobutane ring isomerize to the norbornadienes with a concerted "grapplelike" motion, which can be used to bind or release guests (cations). The main advantage of these compounds is the large reisomerization energy. More than 20 kcal mol À1 are released in forming the norbornadiene, which makes the switching process almost independent of the presence of a guest ion. Additional prerequisites for the application of this photosystem as an active ion carrier are the high lipophilicity of the ligand, a considerable difference in the complexation energy of the two isomers, and the high yields of the switching process. Amide substituents are promising for this photoswitchable system because they can act as chromophores for effective photoisomerization and are also efficient donor ligands for alkalimetal cations. [4] In this context calixarene derivatives [5] (some of them even conformationally switchable [6] ) have been studied.Norbornadiene tetramide 1 was prepared from the corresponding tetracarboxylic acid 2 (Scheme 1). After generation of the acid chloride in situ by chlorination with oxalyl dichloride, the reaction mixture was treated directly with diisopropylamine. Quadricyclane tetramide 3 was synthesized photochemically from 1. According to the 1 H NMR spectra, this conversion-a photochemical [2 + 2] cycloaddition-was almost quantitative (> 95 %, 85 % yield after purification by chromatography); side products were not detected. The cycloaddition proceeds with the same efficiency in the presence of Na + ions under the same conditions. The reverse reaction (3!1, Scheme 2) was achieved with UV light of shorter wavelength (mercury low-pressure lamp, quartz filter) with a yield of 50 % (photostationary equilibrium). Upon prolonged irradiation the elimination of propene (Norrish Type-II reaction) was observed as a minor side reaction.In view of the complexation of metal cations, the spatial arrangement of the substituents and the size of the cavity they form is particularly interesting. The structures of tetramides 1 and 3 were elucidated by X-ray analysis. The symmetry of norbornadiene tetramide 1 is lowered to C 2 , and quadricyclane tetramide 3 exhibits a similar, approximately C 2 -symmetrical structure. For steric reasons two carbonyl oxygen atoms diagonal to each other point toward the center of the cavity and the other two point outward. Consequently the bulky diisopropyl groups assume a sterically favorable alternating orientation (Figure 1, top). The distance be...
