4-Ethoxycarbonyl-4′,5,5′-trimethyltetrathiafulvalene and its radical cation: Langmuir–Blodgett film studies, EPR spectra and the X-ray crystal structure of (Me3TTF-CO2Et)2·TCNQ complex

Abstract

Langmuir–Blodgett films of the title trimethyltetrathiafulvalene derivative 10 have been assembled without the need for added fatty acid: room temperature, in-plane, dc conductivity values of the films before and after doping with iodine vapour were σ_rt = 10^–5 S cm^–1 and 10^–1 S cm^–1, respectively. Over the temperature range 300–100 K the films exhibited a linear current versus voltage relationship, with an activation energy for conductivity E_a = 0.11 eV. Cyclic voltammetric data were obtained for LB films of 10 and the best response was recorded for a 2-layer film; electroactivity gradually disappeared after a few cycles, which is consistent with film desorption from the electrode. UV–vis and IR spectra of the LB films are consistent with a conjugative intramolecular charge-transfer interaction between the triMe-TTF (donor) and ester (acceptor) moieties of 10 (see structure 10′). This increases the amphiphilic nature of 10 and is believed to play an important role in stabilising the LB film structure. Iodine doping of the LB films leads to the formation of radical cations 10·⁺, and polarised spectra show that the molecules are aligned at a high angle to the substrate surface. Simultaneous electrochemistry and EPR (SEEPR) studies provide insight into the electronic structure of the radical cation species 10·⁺ in solution; the ethoxycarbonyl group strongly polarises the spin density, and these data are supported by ab initio UHF and B3LYP calculations. The single crystal X-ray structure of the complex (10)₂·TCNQ (TCNQ = tetracyano-p-quinodimethane) reveals a mixed ⋯ADDADD⋯ stacking motif with a low degree of charge transfer between the D and A moieties. Analysis of the bond lengths in the donor moiety confirms a contribution from the zwitterionic structure 10′.