Electronic properties of model compounds simulating carbons prepared at low temperatures

The effect of the separation distance between the aromatic ring systems on the electronic properties of carbons were simulated by model compounds. The model compounds included perylene nuclei separated by aliphatic chains with varying numbers of methylene groups. They were prepared by polycondensation of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and ω-diamines with various lengths of methylene chains. The optical absorption spectra consisted of two components. The first was a peak characteristic to PCTDA and the other was a broad absorption decreasing towards the near infra-red. The latter developed at the expense of the former with the decrease in the number of methylene units (n). Electrical conductivity and photocurrent were found to increase exponentially with decreasing n. Analyses of these trends in terms of a parameter proportional to the separation distance revealed that both the electrical conductivity and the photocurrent obeyed the relation expected for hopping conduction. The analyses were also applied to polycondensates doped with iodine or ammonia. The dopants were found to facilitate the hopping by lowering the potential barrier for tunneling or by extending the overlap of the wave functions of the π-electrons. The importance of the control of the separation distance in designing electronically functional carbons is emphasized.