Luminescence quenching of Ru(bpy)32+ by nitrophenols in silicate thin films

The luminescence quenching of ruthenium(II)–tris-1,2-bipyridine ion (Ru(bpy)32+) included in methyltriethoxysilane-derived films by 2-, 3- and 4-nitrophenol was investigated. Sol–gel technology was used to prepare the “sol”. Coating thin films were obtained from the “sol” by spin-coating on glass slide. A blue shift in the Ru(bpy)32+ emission spectrum included in the films with respect to the aqueous solution was observed. Ru(bpy)32+ luminescence quenching experiments were carried out with the quencher in aerated aqueous solution at pH 2 in contact with the film. The quenching plots obtained from either steady-state or time-resolved luminescence intensity measurements showed a downward curvature. Both plots were fitted satisfactorily by a sum of two terms of Stern–Volmer with quenching constants KSV1 and KSV2 associated to two different binding sites of the ruthenium complex, which indicates the presence of a matrix microheterogeneity in the films. The KSV1 and KSV2 values and the corresponding fractions of the total emission f10 and f20 for both sites suggest that only a low percentage of the probe is accessible to the quencher and that the probe is efficiently quenched in one of the sites, assigned to site 1. The value of its respective constant KSV1 was higher than the value of the constant KSV in homogeneous aqueous solution. A dynamic process of electron transfer was suggested for the luminescence quenching of Ru(bpy)32+ in the film by nitrophenols in aqueous solution.