  Many natural and artificial reactions including photosynthesis or
photopolymerization are initiated by stimulating organic molecules into an
excited state, which enables new reaction paths. Controlling light-matter
interaction can influence this key concept of photochemistry, however, it
remained a challenge to apply this strategy to control photochemical reactions
at the atomic scale. Here, we profit from the extreme confinement of the
electromagnetic field at the apex of a scanning tunneling microscope (STM) tip
to drive and control the rate of a free-base phthalocyanine
phototautomerization with submolecular precision. By tuning the laser
excitation wavelength and choosing the STM tip position, we control the
phototautomerization rate and the relative tautomer population. This
sub-molecular optical control can be used to study any other photochemical
processes.
