Self-assembled monolayers of cationic donor-(pi-bridge)-acceptor dyes coupled with anionic donors exhibit asymmetric current-voltage (I-V) characteristics when contacted by Au or PtIr probes. Rectification ratios of 3000 at +/- 1 V are obtained from Au-S-C10H20-A+-pi-D|D-|Au structures in which the cationic moiety is 5-(4-dimethylaminobenzylidene)-5,6,7,8-tetrahydro-isoquinolinium and the counterion is copper phthalocyanine-3,4',4'',4'''-tetrasulfonate (SAM ). Similar behaviour, with a high rectification ratio of 700-900 at +/- 1 V, is also obtained for the CuPc(SO3-)4 salt of 4-[2-(4-dimethylaminonaphthalen-1-yl)-vinyl]-quinolinium (SAM ). The properties are dependent upon the D-pi-A+ moieties which, for these highly rectifying salts, have sterically locked non-planar structures causing the conjugation to be effectively broken. Its effect on the electrical asymmetry is less spectacular when the cationic species is sterically unhindered: the rectification ratio decreases to 15-70 at +/- 1 V for films of the 4-[2-(4-dimethylaminophenyl)-vinyl]-pyridinium salt (SAM ), which has single-ring substituents on opposite sides of the -CH=CH- bridge and an almost planar D-pi-A+ structure. Rectification ratios from the sterically hindered structures are on a par with electrical asymmetries from metal-insulator-metal (MIM) devices where oxide-induced Schottky barriers dominate the behaviour.