Unimolecular rectifiers and what lies ahead

Six unimolecular rectifiers are discussed: γ-hexadecylquinolinium tricyanoquinodimethanide, (1); 2,6-di[dibutylamino-phenylvinyl]-1-butylpyridinium iodide, (2); dimethylanilino-aza[C60]-fullerene, (3); fullerene-bis-[4-diphenylamino-4″-(N-ethyl-N-2‴-ethyl)amino-1,4-diphenyl-1,3-butadiene] malo-nate, (4); N-(10-nonadecyl)-N-(2-ferrocenyl-ethyl)-pyrenylmethyl)perylene-3,4,9,10-bis(dicarboxy-imide), (5); and 4,5-dipentyl-5′-methyltetrathiafulvalen-4′-methyloxy 2,4,5-trinitro-9-dicyanomethylene-fluorene-7-(3-sulfonylpropionate), (6). Langmuir–Blodgett (LB) or Langmuir–Schaefer (LS) monolayers of these molecules show asymmetric electrical conductivity between Au or Al electrodes. When the films are compact (LS of 4, LB of 5), and if there is finite intramolecular charge transfer (ICT, or intervalence transfer, IVT), then the electrical behavior persists for many cycles of measurement.