Control of optical properties of single molecules by plasmonic nanostructures is an important issue in nanoplasmonics and nanophotonics, particularly valuable for the development of molecular plasmonic devices and ultrasensitive high-resolution microscopic techniques. The nanocavity defined by the coinage-metal tip and substrate in a scanning tunneling microscope (STM) can provide highly localized and dramatically enhanced electrical fields upon appropriate plasmonic resonant tuning, which can modify the excitation and emission of a single molecule inside and produce interesting new optoelectronic phenomena. In this talk, I shall demonstrate two STM-based phenomena related to single-molecule optical spectroscopy. The first is single-molecule Raman scattering. The spatial resolution of tip enhanced Raman spectromicroscopy has been driven down to sub-nanometer scale for a single porphyrin molecule. I shall demonstrate further applications of this technique to chemically distinguish different adjacent molecules on a surface, from relatively large porphyrin molecules to small DNA-base molecules. The second phenomenon is single-molecule electroluminescence. I shall first demonstrate the realization of electrically driven single-photon emission from a well-defined isolated single molecule. Then, by using STM manipulation to construct a molecular dimer, I shall demonstrate the visualization of coherent intermolecular dipole-dipole coupling in real space through sub-nanometer resolved electroluminescence imaging, together with a demonstration of single-photon superradiance in artificially constructed oligomers. These findings provide unprecedented spatial details about the coherent dipole-dipole coupling in molecular systems, which may open up new research avenues to study molecular interactions and enable rational engineering of light harvesting structures and quantum light sources.