Stephen C. Moratti

Bis(dithienothiophene) organic field-effect transistors with a high ON/OFF ratio

A conjugated, fused-thiophene oligomer, bis(dithienothiophene) (BDT), has been synthesized and deposited by vacuum sublimation as the active layer in organic thin film transistors (TFTs). The TFTs show exceptionally high ON/OFF ratios up to 108 between accumulation and depletion with sharp turn-on characteristics comparable to that of amorphous silicon TFTs (subthreshold slope S=0.6 V/decade). Field-effect mobilities are 0.02–0.05 cm2/Vu2009s. The good performance is explained by the relatively high π-π* gap of the short-chain BDT molecule and the favorable coplanar π-π stacking in BDT, differing from the herringbone stacking in the oligothiophenes.

A blue light emitting copolymer with charge transporting and photo-crosslinkable functional units

The synthesis of a new poly(methacrylate) copolymer bearing an efficient blue light emitting chromophore, a charge transporting aromatic oxadiazole and a UV-sensitive crosslinkable unit is reported. The statistical copolymer was synthesized via a multi-step route and has been characterised by NMR, FT-IR, and DSC. The copolymer has excellent solution processibility and exhibits strong luminescence. Upon cross-linking via UV-treatment, the copolymer becomes insoluble, while gaining both mechanical strength and thermal stability without loss of luminescence. The structure and properties of this novel copolymer and its application in light emitting devices are discussed.

The synthesis, optical and charge transport properties of poly(aromatic oxadiazole)s

In this work we report the synthesis and characterization of processable poly(aromatic oxadiazole)s. The polymers are rendered processable by either dissolving the polymer in strongly acidic organic solvent or by the thermal conversion of a soluble precursor polyhydrazide. The optical and charge transporting properties of the polymers have been investigated by UV-Vis absorption, photoluminescence spectroscopy and cyclic voltammetry. All the polymers have been found to exhibit strong purple-blue fluorescence and good hole-blocking and electron-injecting properties.

Novel poly(arylene vinylene)s carrying donor and acceptor substituents

New electroluminescent poly(arylene vinylene)s 1 containing electron-withdrawing/donating substituents have been synthesized by the precursor route outlined below. The synthesis of two novel polymers and the evaluation of their photoluminescent and electroluminescent properties in polymer light-emitting diodes (LEDs) are reported.

Förster energy transfer and control of the luminescence in blends of an orange-emitting poly(p-phenylenevinylene) and a red-emitting tetraphenylporphyrin

We report on the luminescence of a tetraphenylporphyrin, TPP-d, blended into poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene], MEH-PPV. We find significant energy transfer from MEH-PPV to the porphyrin, in spite of the low absorption of the porphyrin at the emission wavelength of MEH-PPV, reflected in a Forster transfer radius (2.5xa0nm) smaller than for materials with larger spectral overlap. The overall photoluminescence, PL, efficiency decreases monotonically with increasing porphyrin content, whereas the porphyrin contribution to the total efficiency, referred as an “apparent” PL efficiency, exhibits a maximum at 1.4% porphyrin content (by weight). We attribute this non-monotonic behaviour to the interplay of the exciton transfer probability and PL quenching, both of which increase with concentration. We also observed the energy transfer under electrical excitation, but noticed that, at low concentrations, the porphyrin contribution to the electroluminescence is higher than that observed in PL. This indicates significant emission from excitons formed directly at the porphyrin sites, which are likely to act as charge trapping sites. We also compare the luminescence properties of the blends with those of copolymers based on the same host–guest pair.

New luminescent polymers for LEDs

Abstract Recent results are reported on the synthesis of statistical copolymers 8 and 9 derived from the highly luminescent silyl PPV derivative (DMOS-PPV) with various alternative building blocks carrying side chains capable of supporting polyelectrolytes. These polymers are prepared by Gilch dehydrohalogenation polymerization. It is of interest to study the effect on charge transport in standard electroluminescent (EL) devices using these PPV copolymers. Moderately efficient conventional single-layer devices are obtained for copolymer 9 . When fabricated in a light-emitting electrochemical cell (LEC) configuration (lithium triflate) without any extra supporting polymer electrolyte (e.g., poly(ethylene oxide), PEO), devices based on both 8 and 9 show lower turn-on voltages and improved efficiency.

Substituted polythiophenes : effect of substituent and regiochemistry on physical properties

Summary form only given. A range of substituted poly(2,5-thienytene)s (polythiophenes) has been synthesised, which contain straight chain alkyl and/or functionalised substituents. Polythiophenes have been chemically synthesised, via known methodology, to consist almost exclusively of head-to-tail coupling (regioregular) and are compared against those synthesised by standard chemical means. The effect of the substituent and the polymer structure on electroluminescence, order, material properties and other physical properties is described.

Synthesis and optoelectronic properties of aromatic oxadiazole polymers

A one-pot polycondensation of 4,4′-(hexafluoroisopropylidene)bis(benzoic acid)1 with terephthalic dihydrazide 2 and/or hydrazine sulfate 3 affords aromatic oxadiazole polymers 4, 5 and 6, respectively, the optical and electrochemical properties of which have been investigated for applications as charge transporting layers in electroluminescent devices.

Synthesis of new conjugated thiophene polymers

Results are presented which explore the synthesis, characterization and photoluminescence of new types of conjugated copolymers which incorporate fused thiophene units. Such materials possess intense purple-red colours and some absorb in the near-infrared. The redox processes showed that the polymers were both p- and n-dopable.

The synthesis and crosslinking of substituted regioregular polythiophenes

A range of substituted head-to-tail polythiophene random copolymers containing alkyl and ω-hydroxyalkyl side chains has been synthesized by protection of the alcohol as a tetrahydropyranyl acetal during polymerization. The ω-hydroxyl group can be transformed into an azide which, on heating, decomposes and leads to crosslinking which renders the solid polythiophene film insoluble and modifies its optical properties. An ω-hydroxyalkyl homopolymer has also been synthesized by this route and affords polymers with unexpected optical properties.