Continuous Surface Electric Field Contraction Accompanying Electron Transfer from TiO₂ to Oxidized Sensitizers

The electric field present while electrons injected into TiO 2 recombine with oxidized sensitizers has been quantified for the first time. This advance was enabled by transient study of [Ru(NH 3 ) 5 (ina)] 2+, where ina is isonicotinic acid, anchored to the mesoporous TiO 2 thin films used in dye-sensitized solar cells. Light excitation of the characteristic metal-to-ligand charge transfer (MLCT) resulted in a significant change in the molecular dipole, Δμ = 9.1 D, that enabled the surface electric field to be transiently quantified after pulsed light excitation. The field present 70 ns after excited-state injection was E = 0.35 MV/cm, and this value decreased continuously with charge recombination. The observed behavior is most consistent with these surface-anchored sensitizers experiencing a continuous contraction of the electric fields due to delocalized electrons, rather than a discrete number of sensitizers experiencing a localized field, as recombination proceeds.