Monitoring N719 Dye Configurations on (1 × n)-Reconstructed Anatase (100) by Means of STM: Reversible Configurational Changes upon Illumination

We report experimental results concerning the STM imaging of cis-bis (isothiocyanate)-bis-(2,2′-bipyridyl-4,4′dicarboxylate)ruthenium(II)bis(tetrabutylammonium) dye (known as N719) adsorbed on a single crystal of anatase TiO₂(100). The cleaning pretreatment, by sputtering and annealing, of TiO₂(100) yields a reproducible (1 × n) surface reconstruction. Previous to dye deposition, TiO₂ was covered with one monolayer of 4-tert-butylpyridine (4-TBP) in ultrahigh vacuum (UHV) in order to protect the surface against air contamination. N719 was subsequently deposited by dipping the crystal into the dye solution. 4-TBP was removed partially in the solution and totally by heating the sample to around 285−300 °C in UHV. The images of the deposited 4-TBP on TiO₂(100) revealed a complete surface coverage showing three modes of adsorption on TiO₂. The relatively uncomplicated desorption of 4-TBP enables the accommodation and chemisorption of most N719 molecules directly onto the TiO₂ surface. The STM imaging of N719 was affected, in a reversible way, by illumination, because the quality of the image changed after a few hours in the dark or under illumination conditions. The results presented herein are discussed in terms of changes in molecular configurations and in open circuit potentials.