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 CH3 CN 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),Ru-II(eina)vertical bar 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.