Systems of monomolecular layers can form functional units having properties not exhibited by the individual layers. On UV irradiation, for example, an energy transfer takes place from a sensitizer (S) layer to an acceptor (A) layer and results in fluorescence of A. S and A may be cyanine dyes. Monomolecular layers can be manipulated in many ways and combined to construct novel systems; the energy transfer method can be used to establish whether the desired structure has in fact been produced. The monolayer assembling technique can also be used for the investigation of the deactivation of excited molecules; an interesting example is the spectral sensitization of the photographic process.