Molecular rectification: dipole reversal in a cationic donor–(π-bridge)–acceptor dye

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Abstract
The cationic donor–(π-bridge)–acceptor dye, E-4-[(N-alkyl-5,6,7,8-tetrahydroisoquinolinium-5-ylidene)methyl]-N,N-dibutylaniline octadecyl sulfate is a molecular rectifier. Its Langmuir–Blodgett films exhibit asymmetric current–voltage characteristics but the dodecyl and octadecyl analogues behave differently. Rectification occurs in opposite quadrants of the I–V plot as a consequence of anion-induced dipole reversal: the aromatic form (D–π–A+–CnH2n + 1) exists when the negatively charged sulfate is adjacent to the heterocycle and the quinonoid form (D+ πA′–CnH2n + 1 where A′ is the donor) when it is adjacent to the amino group. This is supported by the fact that mixed monolayers of the two dyes do not exhibit second-harmonic generation whereas films of the individual dyes have high second-order susceptibilities (e.g.χ(2) zzz = 100 pm V−1 at 1.064 µm for the dodecyl analogue). The monolayer structure is non-centrosymmetric and, therefore, the nonlinear optical behaviour can only be interpreted as above. Theoretical modelling has also verified the transition as the counterion is relocated. MNDO, AM1 and PM3 calculations indicate changes of ca. 0.08 Å in the C–C bonds of the central bridging unit as well as the exocyclic C–N bond. It is the first time that anion-induced dipole reversal has been observed.

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