Synthesis, structural and photophysical evaluations of urea based fluorescent PET sensors for anions

The design, synthesis and photophysical evaluation of four anthracene based photoinduced electron transfer (PET) sensors (3a-d) for anions is described. The [4+4] photodimerization product, 4, was also obtained from 3b, by slow evaporation from DMSO solution and its X-ray crystal structure determined. The structure of 4 showed the classical dimerization at the 9,10-positions of anthracene. Sensors 3a-d are all based on the use of charge neutral aryl urea receptors, where the recognition of anions such as acetate, phosphate and iodide, was due to the formation of strong hydrogen bonding interactions in DMSO. This anion recognition resulted in enhanced quenching of the anthracene excited state via electron transfer from the receptor; hence the emission was 'switched on-off'. The sensing of fluoride was, however, found to be a two-step process, which involved initial hydrogen bonding interactions with the receptor, followed by deprotonation and the formation of bifluoride ([image omitted]). The changes in the emission spectra upon sensing of chloride and bromide were, however, minor. The photophysical properties of these sensors in the presence of various anions were further studied, including investigation of their excited state lifetimes and quantum yields, as well as detailed Stern-Volmer kinetic analysis with the aim of determining the dynamic and static quenching constants, k(q) and k(s) for the above anion recognition. These measurements indicated that while the anion dependant quenching was mostly by a dynamic process, some contribution from static quenching was also observed at lower anion concentrations.