EFFECTS OF CHARGE AND STRUCTURE OF HYALURONIC-ACID ON THE LUMINESCENCE QUENCHING IN AQUEOUS-SOLUTION

The quenching of electronically excited Ru(bpy)(3)(2+) (bpy = Tris-2,2 '-bipyridine) by methylviologen (MV) and ferricyanide (FC) in aqueous solutions of hyaluronic acid (HA) was studied. The structural and visc osity changes occuring with increasing HA concentration were found to influence the photophysical and photochemical properties of the sensit izer. Different kinetic models had to be used for the quenchers studie d. The kinetics of the quenching of Ru(bpy)(3)(2+) by MV can be descr ibed by the pseudophase model, which indicates that the rate for the e xchange of the quencher between the microdroplets is higher than that for the excited state decay of the Ruthenium complex. In contrast, the quenching by the negatively charged quencher, FC, can be described by the Infelta-Tachiya equation, which indicates that the distribution o f this quencher on the aqueous microdroplets is of the Poisson type an d there is no exchange of quencher molecules during the lifetime of th e sensitizer. The lifetimes of the excited Ruthenium complex, the unim olecular constants for its quenching by FC and the average concentrati on of the aqueous microdroplets increase with increasing HA concentrat ion, reflecting the change in the solution structure during the transi tion from semidilute to concentrated regions. For MV no significant de pendence of the quenching constant on the HA content of the solution w as found. The reaction behavior of charged reactants in HA solution de pends strongly on the sign of the charge.