Probing the Aggregation and Photodegradation of Rhodamine Dyes on TiO2

5 years ago

Probing the Aggregation and Photodegradation of Rhodamine Dyes on TiO2

Jenna A. Tan, Kristin L. Wustholz, James P. Cassidy

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 λ_max 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 TiO_2. 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.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b04604

DOI: 10.1021/acs.jpcc.7b04604