Detection of organic anions in water through complexation enhanced fluorescence of a macrobicyclic tris-acridine cryptand

Abstract

The water-soluble macrobicyclic cryptand 1 containing three fluorogenic acridine units has been synthesized and its photophysical and substrate binding properties have been examined. The free hexaprotonated compound 1-6H⁺ displays dual (monomer/excimer) fluorescence with a particularly low quantum yield (Φ_F= 10^–3) in comparison with that of the related monochromophoric compound 2 (Φ_F= 6.8 × 10^–2). Its fluorescence is strongly affected on complexation with various organic anions (carboxylates, sulfonates, phosphates), i.e. a strong increase of the monomer band is observed with concomitant disappearance of the excimer contribution. The fluorescence revival of 1, which arises from both conformational changes and specific electronic interactions with the substrate, allowed the determination of high stability constants for the 1 : 1 complexes (K_s ranging from 10³ to 10⁷ dm³ mol^–1). The magnitude of this effect has been correlated in terms of relative fluorescence quantum yields (Φ_R) to the selectivities observed (polyanion/monoanion; aromatic/aliphatic; chain length selectivity) and to the factors involved in the strength of the association (electrostatic and hydrophobic forces; structural complementarity). In the case of the naphthalene derivatives, the occurrence of a donor–acceptor energy transfer process demonstrates the formation of an association where the substrate and the receptor are in close proximity. Finally 1-6H⁺ binds tightly to mono- and oligo-nucleotides and moreover appears able to discriminate between homopyrimidine and homopurine sequences by its light-sensitive response.