Valence electronic structures of tetrakis(alkylthio)tetrathiafulvalenes

Abstract

Tetrakis(alkylthio)tetrathiafulvalenes constitute a new class of organic semiconductors offering unusually high conductivities; ultraviolet photoelectron spectroscopic experiments have been performed to investigate the valence electronic structures of these molecular solids. The results have been analysed with help from extended-Hückel molecular-orbital calculations. In this series of compounds the central tetrathiotetrathiafulvalene portion is non-planar, but the degree of departure from planarity has little effect on the electronic structure. On the other hand, members of the series with many (≳8) carbon atoms in each alkyl chain possess extraordinarily low photoionisation thresholds (4.6₅ eV at minimum). For such solids large polarisation energies have been deduced, implying a relatively strong intermolecular interaction. Organic semiconductors in which, between adjacent molecules, the close packing of side chains enhances the interaction between the conjugated part of the molecule, are candidates for which high conductivity is desired.