Flipping Molecules over on TiO₂ Surfaces with Light and Electric Fields

Light excitation of the sensitizer [Ru(NH₃)₅(eina)](PF₆)₂, where eina is ethyl isonicotinate, anchored to anatase TiO₂ nanocrystallites interconnected in a mesoporous thin film and immersed in CH₃CN resulted in spectroscopic changes consistent with both excited-state injection and sensitizer reorientation, termed flipping. When the light irradiation was removed, the sensitizers flipped back over. Such flipping was absent when the carboxylic acid derivative of the sensitizer was utilized or when SnO₂/TiO₂ core/shell materials were employed in place of TiO₂. The flipping was attributed to the torque on the sensitizer in the electric field generated by the injected electrons. Pulsed light excitation was utilized to time-resolve flipping and charge recombination with this and the per-deuterated complex (ND₃)₅Ru^II(eina)|TiO₂. In all cases, charge recombination was more rapid when the oxidized sensitizer was flipped over, behavior consistent with stronger electronic coupling. Kinetic isotope effects of 26.7 and 0.12 were determined for charge recombination and for flipping, respectively. Spectro-electrochemical measurements showed that thermal reduction of TiO₂ with an applied potential also initiated flipping yet required much larger field strengths. The data show that the electric fields created at illuminated semiconductor interfaces are sufficient to reorientate molecules anchored to its surface.