Synthesis, X-ray structure, spectroscopic and cation complexation studies of macrocyclic ligands incorporating the 9,9'-(ethane-1,2-diyl)bis(anthracene) photoactive subunit

Two 9,9'-(ethane-1,2-diyl)bis(anthracene) ligands. incorporating an ethano bridge, namely the bis-anthracene coronand 1 and the bis-anthracene cryptand 2. respectively, have been studied in parallel to compare their binding abilities, spectroscopic and photochemical properties in the presence and absence of cations as a function of their X-ray structure. The ethano bridge imposes a twisted shape (torsion angles between the vicinal C-H being 47 degrees for 1 and 62 degrees for 2a and 47 degrees for 2b, the two conformers of compound 2), which inhibits the cation-binding ability of 1 and attenuates that of cryptand 2 (log K-s approximate to 1.8 for Na+ or K-double dagger, approximate to 4 for Tl+ and approximate to 8 for Ag+); in CH3OH, both 1 and 2 emit essentially excimer-type fluorescence at lambda(max) 520-535 nm, which is red-shifted compared with that of the single crystals (approximate to 465-480 nm) where the rotation about the CH2-CH2 bond is hindered. The presence of cations, which has no influence on the photocycloisomerization quantum yield of 1 (as expected), clearly affects that of 2. Cryptand 2 was shown to display photochromic properties.