Optical modulation of molecular conductance.

A novel scanning probe microscope stage permits break junction measurements of single molecule conductance while the molecules are illuminated with visible light. We studied a porphyrin-fullerene dyad molecule designed to form a charge separated state on illumination. A significant fraction of illuminated molecules become more conductive, returning to a lower conductance in the dark, suggesting the formation of a long-lived charge separated state on the indium-tin oxide surface. Transient absorption spectra of these molecular layers are consistent with formation of a long-lived charge separated state, a finding with implications for the design of molecular photovoltaic devices.