Two dye molecules, rhodamine B (RB) and sulforhodamine (SR), were adsorbed on doped silicon. The effect of photoexcitation of the adsorbed species on the electronic properties of the surface was investigated applying wavelength- and time-dependent two-photon photoemission spectroscopy (WD-TPPE). Despite the very similar electronic properties of the two molecules, their adsorption affects very differently the electronic properties of the substrate. It was found that while the adsorbed RE interacts weakly with the substrate and hence preserves its spectral properties, SR interacts strongly with the substrate, causes a decrease in the work function, and looses its "molecular" spectral features. It is shown that photoexcitation of a small fraction (0.01%) of the weakly bound RE species causes a significant change in the electronic properties of the substrate.