Photoresponsive Supramolecular Systems: Synthesis and Photophysical and Photochemical Study of Bis-(9,10-anthracenediyl)coronands AAOnOn

Article abstract of DOI:10.1021/jo00129a045

Monocyclic bis-(9,10-anthracenediyl)coronands denoted AAOnOn (A = (9,10-anthracenediyl, n indicating the number of ethyleneoxy repeating units between two aromatic rings) were designed as photoresponsive systems.Their synthesis is described; their spectroscopic properties were found to be in agreement with the X-ray structures.Special attention was devoted to AAO5O5, which gives coronates with Na(1+) and K(1+).It was observed that one molecule of AAO5O5 displays a positive cooperative effect in binding two sodium cations, in contrast with previous reports on bicyclic coronands.Moreover, in acetonitrile, this effect, clear in the ground state (K₁₁ = 180 M^-1, K_1,2 = 240 M^-1) was found to be stronger in the excited state (K₁₁ = 200 M^-1, K_1,2 = 500 M^-1).With K(1+), AAO5O5 generates a 1:1 complex (K ca. 9 M^-1 in methanol).Spectroscopic properties were shown to be triggered by cation concentration, specifically the fluorescence emission which undergoes an important intensity and wavelength redistribution in favor of the excimer: the maximum wavelength is shifted from 530 nm (free ligand) to 570 nm (sodium biscoronate), i.e., Δ<*> ca. 1350 cm^-1 and the excimer intensity is multiplied by ca. 3.The photocyclomerization (intramolecular dimerization of the anthracene nucleus) was demonstrated to be regiospecifically directed by Na(1+); in methanol, the free receptor exclusively generates the 9,10:1',4' photoadduct (Φ_R ca. 2E-4) whereas the sodium biscoronate leads to the classical 9,10:9',10' photoadduct (Φ_R ca. 8E-3).Finally, a transient kinetic analysis versus temperature allowed the determination of the conformational mobility of the free receptor within the nanosecond range; in the singlet excited state it is best described by the sequence M₁^* -> M₂^* -> E where M^* stands for the locally excited state and E the excimer state species.At room temperature, in methanol, excimer lifetimes were found to be as follows: free ligand, 23 ns; sodium biscoronate, 165 ns; and potassium monocoronate, 215 ns.