Synthesis of polyaromatic molecules for applications in organic electronics

Abstract

The synthesis of a number of novel polyaromatic compounds are reported, starting with novel polyheterocyclic systems containing sulphur and oxygen, otherwise known as thienoacenes and furanoacenes. These were generally produced via a four-step synthetic strategy utilising palladium-catalysed cross-coupling reactions such as Sonogashira and Suzuki cross-couplings to build the precursors, which were then subjected to a base-catalysed cycloisomerisation to give the final angular systems of several fused aromatic rings. Eight thienoacenes and seven furanoacenes, all consisting of between five and eight fused rings, were synthesised, as well as a mixed system featuring a thiophene and furan ring at either end of the molecule. Use of a five-ring thienoacene as a building block allowed for extension of the aromatic framework, and a stable fifteen-ring thienoacene was synthesised in a total of nine steps.

The same synthetic method was applied in producing a series of polycyclic aromatic hydrocarbons (PAHs) with phenyl and naphthyl substituents replacing thiophene and furan. Within this chapter, significant work was carried out in investigating the final cyclisation mechanism. Six highly crystalline PAHs are reported, consisting of five to eight fused benzene rings, and the addition of extra steps enabled longer polyaromatics of nine, eleven and twelve rings to be created.

The final challenge was the design and synthesis of a series of polyaromatic compounds for use as molecular wires. Similar approaches were used to build systems containing terminal pyridine and nitrile substituents which can act as anchoring groups to secure the molecule to a metal surface. Nine compounds containing five to eight fused aromatic rings with a suitable anchoring group were brought through, and the development of strategies for significantly lengthening these wires gave five systems of either nine or twelve rings.

Optical and electrochemical characterisation of the compounds was carried out at the University of Caen Lower Normandy, France, in collaboration with Prof. Bernhard Witulski. The results of these measurements generally gave good correlation with computationally calculated values.

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ORCID for Richard Whitby: orcid.org/0000-0002-9891-5502

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