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
ring-opening 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
