ESR studies of radical cations produced on doping discotic liquid crystals with Lewis acids

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Abstract
We have shown previously that doping the Dhd phase of 2,3,6,7,10,11-hexahexyloxytriphenylene (HAT6) with 1–5 mole per cent of a Lewis acid converts it from an insulator to a p-type semiconductor. Herein, ESR spectroscopic studies establish that this and related oxidation reactions produce radical cations, but that these radical cations can be of several types. For instance, oxidation of the parent hydrocarbon triphenylene (TP) with aluminium trichloride in carbon disulphide solution gives a non-covalently bonded dimer (TP)2 + in which the charge is delocalized over two aromatic nuclei. Oxidation of HAT6 in solution gives either charged aggregates (HAT6). n + or the monomeric radical cation (HAT6). +. Formation of (TP).2+, (HAT6). n +, or (HAT6).+ involves removal of an electron from a π-orbital. An alternative reaction is revealed by studies of 2,3,6,7,10,11-hexahexyl-thiotriphenylene (HTT6): removal of an electron from a [sgrave]-orbital. In this case a cation with a novel two-centre, three-electron, sulphur-sulphur bond is formed. Electronic conduction in doped neat HAT6 involves a mechanism in which the charge carrier hops between localized states. These localized states may be discrete (HAT6).+ ions but are more probably (HAT6). n + aggregates or polarons.