We found unimolecular rectification in a single molecule, (N-hexadecylquinolin-4-ium-1-yl)methylidenetricyanoquinomethanide, 1, in which the ground state is zwitterionic: D+–π–A−, and has a large dipole moment (43 D) while the first excited state is undissociated: D0–π–A0 (and has a smaller moment, between 3 and 9 D). The intervalence optical absorption band connecting these two states is strongly hypsochromic. Spectroscopic measurements (FTIR, NMR, VIS–UV, XPS) all confirm this assignment. This significant ‘first step' to unimolecular electronics, i.e. a rectifier only 2.3 nm thick, confirms a 1974 proposal by Aviram and Ratner. However, the ‘real proof' that the large current under forward bias actually travels through the molecule, has not yet been obtained.