Langmuir–Blodgett (LB) films of isomeric donor–(π-bridge)–acceptor molecules with either a hydrophobically substituted acceptor [D–π–A–C16H33 (1)] or donor [A–π–D–C16H33 (2)] exhibit second-harmonic generation and asymmetric current–voltage characteristics: a reversed polarity for rectification arises from an inversion of the dipole orientations in the two types of monolayer.