Electrical conductivity of trhe complexes obtained from 1,4 - Diaminounthraguinone and of the pryolitic cabon films

Abstract

Electrically conducting metal complexes of 1,4?diaminoanthraquinone are synthesised and characterised. The conductivity of the complexes varies from 1×10?? to 1×10?? S/cm depending on the metal atom coordinated in the complex. There is a good correlation between the number of d?electrons and the conductivity of the complex. Electrical conductivity in these complexes is attributed to the delocalisation of electrons in the d?orbitals of the metal through the ligand framework. Copper, which favours square?planar coordination, exhibits relatively high conductivity. The amount of copper present in the system contributes to the electrical conductivity of copper complexes. The copper complex with 22.2% copper exhibits relatively high conductivity (0.198 S/cm). Interestingly, 1,4?diaminoanthraquinone forms conducting stacks when it is oxidised with Ce??. Cerium initiates the complex formation, but in the final complex formed, cerium is absent. The conductivity of this complex is in the same range as that obtained by oxidising 1,4?diaminoanthraquinone with copper, where the copper content is 22.2%. The absence of cerium was confirmed by several spot tests and Inductively Coupled Plasma Atomic Emission Spectroscopy. In this complex, conductivity is attributed to the formation of donor–acceptor stacks. Low?temperature electrical?conducting properties of 1,4?diaminoanthraquinone complexes reveal that the conduction is governed by hopping of carriers excited to localised states near the mobility edge. This shows that the conductivity mechanism is of the Variable Range Hopping (VRH) type. The resistivity varies as (ln? ? T¹??). In the complex obtained by oxidising 1,4?diaminoanthraquinone with Ce??, the resistivity varies as (ln? ? T¹?²). The deviation from T?¹?? behaviour is explained on the basis of Coulomb interaction between the electrons of the conducting stacks. The high?pressure effect on these complexes is explained on the basis of molecular?orbital overlap. The behaviour observed is similar to that seen in many organic semiconductors. The electrical properties of the film obtained from 3,4,9,10?perylenetetracarboxylic dianhydride through pyrolysis are very interesting. The resistivity of the film depends on the pyrolysis temperature. Film formed during pyrolysis at 700°C exhibits typical Variable Range Hopping conduction. However, the film formed during pyrolysis at 900°C exhibits temperature?independent electrical conductivity over a wide temperature range from 600 K to 1.4 K. This behaviour is explained in terms of the conjugation length of the polymer film. In conclusion, this thesis presents several new results on metal–anthraquinone complexes and pyrolysed carbon films, opening up many new avenues for research. The following two lines of work seem promising:

Study of a new class of synthetic conductors—particularly the complex of 1,4?diaminoanthraquinone oxidised with ceric ammonium nitrate, where cerium is absent—opens a new branch of conducting complexes. Many other aminoanthraquinones and anthracene derivatives may be suitable substitutes for 1,4?diaminoanthraquinone. Among these, hydroxyanthraquinones, 2,6?diaminoanthraquinone, 1?amino?4?hydroxyanthraquinone, and 1,4,5,8?tetraaminoanthraquinone are promising.

The film obtained through pyrolysis of 3,4,9,10?perylenetetracarboxylic dianhydride is highly interesting. Doping this film with alkali metals or halides may further increase its conductivity. Another way of enhancing conductivity could be through sulphur bridging. If such bridges form between aromatic molecules of the conjugated chains, the conductivity behaviour could become even more interesting. Using derivatives of perylenetetracarboxylic dianhydride (where hydrogen atoms are replaced by methyl, ethyl, alkali metals, or polarisable dye groups) could result in even higher?conducting carbon films.