Photoexcitation of molecules on metal surfaces can drive chemical reactions within the excited state along otherwise not accessible reaction pathways. Fast relaxation to substrate induced dissipation channels, however, significantly reduces the reaction yield. The concentration of visible light in the near-field of metal nanostructures has been demonstrated before to enhance the excitation rate of adsorbed molecules and compensate increased losses. Quantitative insight into the underlying mechanism at the single-molecule level within a well-defined nanostructure geometry is nevertheless missing and usually inaccessible in ensemble averaging techniques. Low-temperature scanning tunneling microscopy (STM) is ideally suited to study single molecules, while the junction, formed between a noble metal tip and and substrate, provides an individual, precisely tunable nano-gap, in which the electric field of the incident light is dramatically enhanced. In this thesis, a combination of STM with wavelength tunable laser excitation is used to investigate single-molecule photochemistry within a tip-sample junction. The molecular photochemical reactivity is monitored as function of photon energy which provides insight into the underlying excitation mechanism while comparing optical far- and near-field excitation. Porphycene molecules on copper surfaces act as multiresponsive, bi-stable tautomeric switches. Within this work, the optical stimulus is explored through far-field excitation. We find that photogenerated substrate charge carriers drive the tautomerization via non-adiabatic coupling of vibrational energy to the reaction coordinate. By narrowing the tip-sample gap into the range of optical near-field enhancement, the photochemical reactivity is enhanced by 10^2-10^3. In the tunneling regime, photogenerated charge carriers mediate inelastic tunneling between tip and sample and can attach to molecule electronic states before thermalization. As optically induced porphycene tautomerization is found highly responsive to the near-field magnitude at the apex, it is further employed as a sensitive sensor to study the plasmonic response of scanning tunneling tips. Microstructured tips are found to exhibit a spectrally modulated near-field at the apex due to the formation of surface plasmon cavity resonances which provides a facile tool for customizing the near-field response. The here presented findings contribute to the understanding of plasmon enhanced chemistry at the level of a single metallic nano-gap and molecule which is unattainable in ensemble averaging techniques and demonstrates a possibility to fabricate tailored near-field optical probes.
