Alkyl-substituted oligothiophenes: crystallographic and spectroscopic studies of neutral and doped forms

The alkyl-substituted oligothiophenes with varying degree of polymerization have been characterized both in their neutral and in their doped forms. Large blade-shaped crystals of these compounds are readily accessible through recrystallization. Thin films of the oligothiophenes are also obtained from casting or vacuum evaporation. The oligothiophene compounds can be doped (oxidized) with an acceptor such as iodine and TCNQF4(7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane). The doping makes the yellow transparent oligothiophene materials dark green or purple and highly conducting. The oligothiophenes can also be doped with an acceptor in solution.Crystal structure analysis has been carried out for a neutral species of DMQtT (α,α′-dimethylquaterthiophene). The crystal structure is characterized by a regular stack of DMQtT molecules with a monolayer thickness of ca. 18.0 Å. A similar layered structure has also been confirmed from the X-ray θ-2θ measurements for the oligothiophenes doped with an acceptor. Anisotropic conductivities were measured for the blade-shaped crystals doped with iodine. The X-ray and conductivity results show that the conductivity in the direction vertical to the molecular long axis is about a thousand times as high as that along that axis. This implies that electronic motion through the face-to-face molecular array is the dominant carrier transport mechanism. The doped materials were further examined by IR and electronic spectroscopy. The effects of the alkyl substitution upon the doped species are discussed on the basis of these results.