Chemometric modeling of spectroscopic data for characterizing the visible-light-driven photocatalytic N-dealkylation of rhodamine B on a TiO2 film.

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The green-light driven photocatalytic N-deethylation reaction of Rhodamine B (RhB) on a TiO2 film was investigated by UV-vis absorption and fluorescence emission spectroscopies, in addition to HPLC and HR-MS, to ascertain the nature of the reaction products. The evolution of the photocatalytic reaction was chemometrically analyzed using Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) of the spectroscopic data to obtain the kinetic and spectral decomposition of the RhB derivatives involved in the reaction. This was then compared with the results obtained by standard HPLC analysis. The MCR-ALS analysis yielded satisfactory spectral and kinetic profiles for RhB and the fully deethylated product, Rhodamine 110. However, the spectral profiles for the N-triethyl (3EtRh) and the mixture of the two isomeric N-diethyl (2EtRh) derivatives exhibited some spectral distortions due to significant spectral overlap between these compounds. In contrast to the HPLC analysis, the MCR-ALS could not resolve the N-ethylrhodamine (EtRh) derivative. The deethylation reactions occurred via independent zero-order steps at the surface of TiO2, indicating that the RhB degradation reaction is governed by the adsorption-desorption equilibrium of the dye and derivatives on the photocatalyst surface, thereby enhancing the diffusion of compounds on the surface.