Semiconducting Langmuir–Blodgett films of ethylenedithiotetrathiafulvalene (EDT–TTF) derivatives bearing charged and uncharged aromatic substituents

Abstract

Ethylenedithiotetrathiafulvalene (EDT–TTF) derivatives1–4 functionalised with a single aromatic ring have been synthesised and their Langmuir–Blodgett (LB) films have been assembled utilising only 25% molar ratio of fatty acid. For compounds 1, 3 and 4, predominantly Y-type deposition onto solid supports was observed with a transfer ratio close to unity. After doping with iodine vapour, the maximum in-plane conductivity values obtained were σ _rt =10 ^-3 S cm ^-1 for 1 and 3, and 10 ^-5 S cm ^-1 for 4. LB deposition of 2 was not uniform and the conductivity value after doping was low. UV–VIS spectra of the LB films reveal the appearance of a charge-transfer (CT) band at λ _max =ca. 900 nm for 1 and 4 after iodine doping. A solution of compound 3 exhibited a weak absorption band at ca. 665 nm which is assigned to an intramolecular CT band; the intensity of this band increases on exposure of the solution to light. This band is not observed in LB films of 3, neither as-deposited nor after doping. Molecular orbital calculations indicate that in the minimum energy conformation of 3, the pyridinium moiety is practically orthogonal to the TTF unit and this conformation may be obtained in solution, enabling charge-transfer to occur. A more linear conformation of 3 in the LB films may prevent intramolecular charge-transfer from occurring. Monolayers of 1, 3 and 4 were characterised by cyclic voltammetry which revealed two redox steps consistent with the formation of the EDT–TTF cation radical and dication, respectively.