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)center dot TCNQ complex

Langmuir-Blodgett films of the title trimethyltetrathiafulvalene derivative 10 have been assembled without the need for added fatty acid: room tempera ture, in-plane, dc conductivity values of the films before and after doping with iodine vapour were sigma(rt) = 10(-5) cm(-1) and 10(-1) cm(-1), respe ctively. Over the temperature range 300-100 K the films exhibited a linear current versus voltage relationship, with an activation energy for conducti vity 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 g radually disappeared after a few cycles, which is consistent with film deso rption from the electrode. UV-vis and IR spectra of the LB films are consis tent 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 L B films leads to the formation of radical cations 10(.+), and polarised spe ctra show that the molecules are aligned at a high angle to the substrate s urface. Simultaneous electrochemistry and EPR (SEEPR) studies provide insig ht into the electronic structure of the radical cation species 10(.+) in so lution; the ethoxycarbonyl group strongly polarises the spin density, and t hese 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-q uinodimethane) reveals a mixed ...ADDADD... stacking motif with a low degre e of charge transfer between the D and A moieties. Analysis of the bond len gths in the donor moiety confirms a contribution from the zwitterionic stru cture 10'.