Two-dimensional dye crystals formed after electrostatic adsorption onto monolayers of a saturated chain lipid and of an unpolymerized and a polymerized diacetylene lipid are studied by electron diffraction and optical microscopy and spectroscopy. We demonstrate the mutual influence of the epitaxially related structures in the unpolymerized system. Photopolymerization can be sensitized by the dye crystals, indicating efficient electron transfer. This leads to just one modification of the polydiacetylenic monolayer which is insensitive to radiation damage by electron beam or UV light. It also influences the dye lattice and its orientation relative to the polymer backbone. Structural changes yield changes in the position and splitting of optical absorption bands. These are well understood by means of recent quantum-chemical calculations.