Probing the Aggregation and Photodegradation of Rhodamine Dyes on TiO2

The aggregation and photodegradation of rhodamine derivatives adsorbed to TiO2 are investigated using diffuse reflectance spectroscopy, steady-state fluorescence, and time-correlated single-photon-counting (TCSPC) measurements. Rhodamine dyes containing substituted amines (i.e., 5-ROX, R101, RB) exhibit an ∼50 nm hypsochromic shift in λmax upon adsorption to TiO2 relative to solution. By examining a rhodamine derivative with primary amines (i.e., R560) as well as control experiments on insulating ZrO2 substrates, we demonstrate that photocatalyzed N-dealkylation is largely responsible for the spectral changes observed upon surface adsorption to TiO2. For R560, which does not undergo N-dealkylation, diffuse reflectance spectra show that mainly monomers and J-aggregates are present on TiO2. Comparative lifetime measurements for R560 on TiO2 and ZrO2 show that the injection yield for R560/TiO2 is increased with dye-loading concentration (i.e., from 0.63 for monomers to ∼0.80 for heavily doped films), indicating that the presence of aggregates enhances electron injection. The residual fluorescence of R560/TiO2 is attributed to subpopulations of monomers and weakly fluorescent J-aggregates of R560 that do not undergo efficient electron injection to TiO2. The fluorescence intensity, energy, and lifetime of R560 on TiO2 and insulating ZrO2 films are dependent on dye concentration, consistent with a resonance energy transfer quenching process. This study shows that contributions due to molecular photodegradation and energy transfer interactions must be considered when pursuing the development of a controlled aggregation strategy for solar energy conversion materials and devices.