Excited-state behavior of Nile Blue H-aggregates bound to SiO2 and SnO2 colloids

The spectroscopic characterization of a cationic dye, Nile Blue A (NB), in negatively charged SnO2 and SiO2 colloidal suspensions has been carried out . The results show that NB, in the presence of these colloidal suspensions, forms aggregates of H-type which are nonfluorescent and have blue-shifted absorption compared to the NB monomer in aqueous solution. Laser photolysis experiments have been carried out to study the excited-state dynamics of t he NB aggregates. The transient absorption measurements of NB adsorbed on S iO2 colloids (inert support) show that the H-aggregates of NB have a relati vely short-lived excitonic singlet state (tau = 40 ps); the lifetime of the excited singlet of the monomer in aqueous solution is similar to 390 ps. O n a reactive semiconductor support like SnO2, the NB aggregates also inject electrons into SnO2. This is characterized by the formation of the cation radical, (NB)(2)(.+), of the NB aggregates and by the trapping of electrons the in SnO2 nanocrystallites. The monophotonic dependence of the formation of (NB)(2)(.+) on SnO2 surface supports the charge transfer from NB aggreg ates to SnO2. The rate constant for this heterogeneous electron-transfer pr ocess is similar to 3.3 x 10(8) s(-1).