Positioning of a scanning tunneling microscope (STM) tip above Cu-tetra- (3,5-di-tertiary-butyl- phenyl)-porphyrin (Cu-TBPP) molecules on Cu(100) is found to induce plasmon-mediated emission and molecular luminescence when bias voltages are above ` 2.3 V. Optical spectra acquired at a low current of 0.2 nA suggest not only the enhancement effect of the molecules on light emission but also new features associated with the molecules. The quantum efficiency of such light emission excited by inelastic tunneling is on the order of 10-6 photons per electron.