Study of interaction between fluorescent dyes and Cucurbituril host in aqueous solution

Abstract

The photophysics of PRODAN (6-propionyl-2-dimethyaminonaphthalene), BP(OH)2 (2,2′-bipyridine-3,3′-diol), and HPTS (8-hydroxypyrene-1,3,6-trisulfonate, pyranine) guest molecules have been studied in cucurbiturils (CBn, where n=6-8) having different cavity sizes. Cucurbituril has interior hydrophobic cavity and hydrophilic carbonyl groups present at its portal. It has been observed that fluorescence intensity of PRODAN is quenched upon encapsulation by CBn cavity. The restriction of intramolecular charge transfer (ICT) within PRODAN inside the CBn cavity causes the quenching of fluorescence. In CB cavity due to higher shift of pKa value, dimethylamino group is getting protonated which causes the restraint of ICT. BP(OH)2 exists in two forms, one di-enol (DE) form in the ground state and other di-zwitterion (DZ) form in the excited state after intramolecular double-proton transfer. DZ form exists in the ground state exclusively in water. Binding of BP(OH)2 to CBs was studied by following the changes in its absorption and fluorescence spectra after addition of CBs. It was found that inclusion of BP(OH)2 inside CB7 and CB8 causes enhancement in fluorescence intensity. The spectral behavior of HPTS is studied in CB8 using absorption, steady state emission and time-resolved emission techniques. It has been observed that the absorption and fluorescence intensity of HPTS is getting reduced due to hydrogen bonding between –OH group of HPTS and carbonyl groups of CBs. The lifetime measurement values also decreases as CB8 concentration increases.