Compounds of the formula I ##STR1## in which R is a C.sub.1 - to C.sub.22 -alkyl radical, A.sup.1 and A.sup.2 are hydrogen or methyl or, A.sup.1 is hydrogen and A.sup.2 is the nitro group, and B is the cyano group, the nitro group or an electron-attracting organic group having a double bond which is in conjugation with the thieno�3,2-b!thiophene ring system, have a high dipole moment and a high polarizability and are suitable for purposes of nonlinear optics. If R in the formula I is a C.sub.12 - to C.sub.22 -alkyl radical, monomolecular layers can be applied to a substrate by the Langmuir-Blodgett method. The resulting layer elements are suitable, inter alia, as waveguides.