Photochromic molecular switches immobilized by direct contact with surfaces typically show only weak response to optical excitation, which often is not reversible. In contrast, here, it is shown that a complete and reversible ringopening and ring-closing reaction of submonolayers of spironaphthopyran on the Bi(111) surface is possible. The ring opening to the merocyanine isomer is initiated by ultraviolet light. Switching occurs in a two-step process, in which after optical excitation, an energy barrier needs to be overcome to convert to the merocyanine form. This leads to a strong temperature dependence of the conversion efficiency. Switching of the merocyanine isomer back to the closed form is achieved by a temperature increase. Thus, the process can be repeated in a fully reversible manner, in contrast to previously studied nitrospiropyran molecules on surfaces. This is attributed to the destabilization of the merocyanine isomer by the electron-donating nature of the naphtho group and the reduced van der Waals interaction of the Bi(111) surface. The result shows that molecules designed for switching in solutions need to be modified to function in direct contact with a surface.

Diarylethenes are molecular switches, the state of which can efficiently be controlled by illumination with ultraviolet or visible light. To use the change in the molecular properties when switching between the two states for a specific function, direct contact with solid surfaces is advantageous as it provides immobilization. Here we present a study of a diarylethene derivate (T-DAE, 1,2-bis(5-methyl-2-phenylthiazol-4-yl)cyclopent-1-ene) in direct contact with highly ordered graphite as well as with semimetallic Bi(1 1 1) surfaces by x-ray photoelectron spectroscopy, x-ray absorption spectroscopy and simulated spectra based on density functional theory. On both surfaces, the molecule can be switched from its open to its closed form by 325-475 nm broadband or ultraviolet illumination. On the other hand, back isomerization to the ring-open T-DAE was not possible.