Switching ability of nitro-spiropyran on Au(111): electronic structure changes as a sensitiveprobe during a ring-opening reaction

Abstract

Spiropyran is a prototype molecular switch which undergoes a reversible ring-opening reactionby photoinduced cleavage of a C–O bond in the spiropyran (SP) to the merocyanine (MC)isomer. While the electronic states and switching behavior are well characterized insolution, adsorption on metal surfaces crucially affects these properties. Using two-photonphotoemission and scanning tunneling spectroscopy, we resolve the molecular energy levelson a Au(111) surface of both isomeric forms. Illumination at various wavelengths does notyield any observable switching rate, thus evidencing a very small upper limit of thequantum efficiency. Electron-induced switching from the SP to the MC isomer viageneration of a negative ion resonance can be detected with a quantum yield of(2.2 ± 0.2) × 10−10 events/electron in tunneling spectroscopy. In contrast, the back reaction could not be observed. This studyreveals that the switching properties of surface-bound molecular switches can be verydifferent compared with free molecules, reflecting the strong influence of the interactionwith the metal substrate.