CONDUCTIVITY AND ELECTRON-DENSITY OF UNDOPED MODEL COMPOUNDS OF POLY(PHENYLENE VINYLENE)

Fourteen derivatives of trans,trans-1,4-bis[2-phenylethenyl]benzene were synthesized by Wittig reactions as model compounds of poly(paraphenylene vinylene). Structure, configurational homogeneity and absence of ionic impurities were controlled by mass spectrometry, infrared and neutron activation analysis, respectively. Crystallographic unit cell parameters were obtained from X-ray powder patterns and measurements of electrical conductivities were performed on undoped samples. The data of four more compounds containing one or more thiophene rings instead of phenyl rings were added from the literature. If NO2 and Cl groups are excluded from the electron count a good linear correlation is found between the logarithm of the conductivity and the non-sigma electron density (e angstrom-3). The position of the substituents, on the central or on the terminal ring, also plays a role in as much as it affects the molecular volume of the compound but not the non-sigma electron density. The correlation between the logarithm of the conductivity and the absorption coefficient of the longest wavelength of UV absorption identifies the pi electrons in the chromophore as the principal charge carriers.