A scanning tunneling microscope (STM) can do more than atomic imaging and manipulation, its tunneling current can also be used for the excitation of light, converting electron energy to photon energy. In this talk, I shall first demonstrate the realization of STM based single-molecule electroluminescence by adopting a strategy of both efficient electronic decoupling and nanocavity plasmonic enhancement. The emission intensity, achieved through optimized material combination for molecule, spacer, tip, and substrate, is strong and stable enough for us to carry out second-order photon correlation measurements. The observation of an evident photon antibunching effect demonstrates clearly the nature of single-photon emission from an isolated single molecule that is electrically excited by tunneling electrons. Strikingly, the spectral peak in an isolated monomer is found to split when a molecular dimer is artificially constructed through STM manipulation, which suggests that the excitation energy from tunneling electrons is likely to rapidly delocalize over the whole molecular dimer and form a delocalized exciton. The spatial distribution of the excitonic coupling for different energy states in a dimer can be visualized in real space through sub-nanometer resolved electroluminescence imaging technique, which correlates very well with the local optical responses predicted in terms of coherent intermolecular dipole-dipole coupling. Finally, I shall also demonstrate electrically driven single-photon superradiance for constructed molecular oligomers, which strongly suggests the formation of multi-molecule quantum entangled states. These findings open up new avenues to fabricate electrically driven quantum light sources and to study intermolecular energy transfer at the single-molecule level.