System of the investigated dye is exceptional in terms of general observations of Langmuir monolayers. Lipophilic luminophores extremely rarely possess bright emission while situated on a water subphase. For BODIPY-C18, we could observe orange emission of a monomolecular layer with a naked eye (Figure ). It's possible due to equilibrium between two kinds of excited state species. Once molecules are pushed towards each other below certain critical distance, dye molecules do not interact to water molecules in excited state anymore.