Electronic Excited States of Carotenoid Dyes Adsorbed on TiO₂

The fast development of new organic sensitizers for the purposes of dye-sensitized solar cells and photocatalysis requires a better understanding of the complexity of their electronic states on TiO₂ surface. This paper describes a study of carboxylated derivatives of all-trans carotenoids retinoic acid (ATRA) and bixin by Stark effect (electroabsorption) spectroscopy in glassy ethanol at low temperature, both as free monomeric species and when adsorbed to nanoparticles of titanium dioxide in the presence of a variable concentration of acetic acid. Both pigments adsorbed strongly at the lowest acid contents (<0.2%), but become partially desorbed at the higher acid concentration (2%). Adsorption of pigments on TiO₂ was accompanied by an increase of difference dipole moment Δμ between the ground and excited states from 14.5 to 22.5 D for ATRA. A smaller change in Δμ was estimated for bixin. The remarkable spectral inhomogeneity exhibited by significant fractions of both pigments with larger values of Δμ is ascribed to the differences in bonding to TiO₂. The gradual spectral shift due to the charging of nanoparticles induced by the increasing acetic acid concentration was used to estimate the change in surface electric field of charged nanoparticles.