Molecular rectification: experimental and theoretical confirmation of anion-induced dipole reversal in cationic dyes

E-4-[(N-alkyl-5,6,7,8-tetrahydro-5-isoquinolylidene)methyl]-N,N-dibutylaniline octadecylsulfate is a molecular rectifier. Its Langmuir–Blodgett (LB) films exhibit asymmetric current–voltage characteristics but with rectification occurring in opposite quadrants of the I/V plot for the dodecyl and octadecyl analogues. This is attributed to an anion-induced reversal of the molecular dipole, the aromatic form of the dye being stabilized when the counterion is adjacent to the heterocyclic group and the quinonoid form when close to the anilino end. Such changes have been modeled theoretically using MNDO, AM1 and PM3 calculations, which show a dependence of the bond alternation on the location of the probe charge.