Parsing The Aggregation- and Photodegradation-Induced Effects of Rhodamine-Sensitized Tio2 and Zro2 Films

The impact of rhodamine aggregates on the photophysical properties of rhodamine dyes adsorbed to TiO2 were investigated using diffuse reflectance spectroscopy, steady-state fluorescence, and time-correlated single photon counting (TCSPC). Photocatalyzed de-alkylation of rhodamine dyes containing tertiary amine groups (5-ROX, R101, and RB) was observed on TiO2, which resulted in a ~50 nm hypsochromic shift. Therefore, concentration dependent diffuse reflectance spectra of R560/TiO2 samples, which contained primary amines, demonstrated the formation of H- and J-aggregates at the expense of the monomers. The formation of J-aggregates resulted in FRET between monomers and J-aggregates which yielded a bathochromic shifted fluorescence spectra as a function of increased dye concentration. Additionally, the J-aggregates were believed to be less fluorescent than the corresponding monomer, which, in addition to the presence of non-fluorescent H-aggregates, led to the quenched fluorescence. Time dependent fluorescent measurements established that electron injection to TiO2 from R560 occurred from both the monomer and J-aggregates, as lifetimes of each species on TiO2 were quenched relative to lifetimes on ZrO2.