We have made electrical measurements on a system using carbon nanotubes as the dopant material. A semiconjugated, organic polymer was mixed with carbon nanotubes to form a wholly organic composite. Composite formation from low to high nanotube concentration increases the conductivity dramatically by ten orders of magnitude, indicative of percolative behavior. Effective mobilities were calculated from the spacecharge regions of the current-voltage characteristics for the 0-8 % mass fractions. After an initial rise these were seen to fall from 1-8 % doping levels as predicted by theory. From the values for conductivity and mobility, an effective carrier density was calculated. This was seen to decrease between 0% and 1%, before rising steadily. ͓S0163-1829͑98͒51536-6͔

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ring overnight at room temperature, monomer and initiator were readily absorbed in the vesicle bilayer. Photoinitiated polymerizations were performed in a thermostatted quartz reactor using either an UV-lamp (HPR 125 W, Philips) or a pulsed excimer laser (Lambda Physics XeF, 351 nm, 2 Hz pulse frequency, 30 mJ energy per pulse) as irradiation source. Conversions were determined by HPLC analysis of the residual monomers.Details concerning the use of cryo-electron microscopy have been described earlier [19].

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The effect on optical emission of the presence of carbon-carbon triple bond linkages in conjugated aromatic copolymers is studied and leads to fluorescence quantum yields of up to 50%, substantially higher than most conventional conjugated polymers.Since the first demonstration of organic polymer light emitting diodes (polymer LEDs),l materials research in this area has intensified. The main issues to be addressed are device efficiency and material stability. Since the light emitting state in devices appears to be an excited neutral singlet,2 the issue of efficiency is very much dependent on the quantum yield of emission of the fluorescent polymers employed. To this end, conjugated polymers with high quantum yields of fluorescence are required.One approach commonly employed is to limit the delocalisation length of the polymer by interrupting x-conjugation or synthesising short chain oligomers.3 This effectively increases the 'rigidity' of such systems and reduces relaxation from excited states through non-radiative (e.g. thermal) processes.The principal problem with this approach is that limited conjugation leads to lower charge carrier mobility. In general, the lower the carrier mobility, the greater the bias required to drive the device. The ideal material therefore possesses uninterrupted n-conjugation as well as a high quantum yield of fluorescence.To date, work has focused primarily on the paraphenylene: paraphenylene vinylene and thiophene5 families of conjugated polymers. Fluorescence quantum yields for these polymers are typically between 0.1 %6 and 30%.7 There has been little study of arylene ethynylene polymers, in which aromatic moieties are connected by carbon-carbon triple bonds.8.9We have synthesised a series of soluble paraphenylene ethynylene copolymers and measured their quantum yields in solution by comparison with values for previously characterised laser dyes. The general synthetic route to these polymers is illustrated in Scheme 1. The polymerisation method is adapted from that of Giesa.9 Polymers 2 and 4 were previously synthesised by this author; however, no comprehensive structural or optical analysis was performed. The yields for this condensation polymerisation are reasonable (50-100%)t and give molecular masses ranging from 10s to 100s of repeat units.The visible absorption and emission spectra of 3 are shown in Fig. 1.1 The spectra of the samples are all sharp in comparison to other conjugated polymers and are inhomogeneously broadened. The position of the absorption maximum depends on the nature of the co-unit. For small units such as thiophene or pyridine, the maxima are around 450 nm. If a more electron rich unit (such as anthracene) is introduced (e.g. in 4), a significant red-shift of the absorption maximum is observed. Table 1 lists the position of the absorption maxima for the copolymers studied. It is tentatively suggested that the position of the absorption maximum is related to the degree of destabilisation of the HOMO and stabilisation of the LUMO. The presence of the elect...