In a rigid matrix at low temperature, a single molecule presents one intense and very narrow absorption line, called zero-phonon line. Its spectral position is shifted randomly by defects in the environment of the molecule. Such sharp spectral features open many experiments in quantum optics, molecular spectroscopy, and nanoscale probing. In this project, we wish to probe charge injection and conduction on nanometer scales in an organic material. After charge injection, new local electric fields are created in the material and can dynamically shift the spectral positions of individual molecules by linear and quadratic Stark effect. The spectral traces of single molecules show a rich variety of complex behaviors. The non-exponential relaxation of the lines indicates a slow relaxation of the charge distribution. The noisy apparence of the spectral diffusion in some cases indicates possible instabilities in the current flow, felt differently by different molecules, depending on their positions. The spectral selection of up to thousands of molecules in each focal spot makes it possible to obtain sub-wavelength resolution in the study of these effects, for example in the interaction of molecules with metallic electrodes or nanostructures. Our favorite system for these studies is a thin anthracene crystal doped with fluorescent dibenzoterrylene molecules which present two main absorption sites at 785 and 795 nm.