D.A. Halliday

Optical spectroscopy of highly ordered poly(p-phenylene vinylene)

The authors report a study of the photophysical properties of poly(p-phenylene vinylene), PPV, prepared in a way that gives an especially high degree of intrachain order. Optical absorption, photoluminescence, photoinduced absorption, and photoconductivity excitation spectra are presented and compared to data reported for less well ordered PPV. Spectral red shifts, sharpening of spectral lines, and a transfer of oscillator strength into the vibronic ground states of the electronic transitions are observed. Photoinduced absorption due to long-lived charged excitations, previously reported for less ordered PPV, could not be detected in this material. Photoconductivity excitation spectra show a steep rise at the absorption edge with no appreciable offset between the onsets for photoconduction and absorption. A very slow photocurrent component is observed, which the authors associated with the trapping and subsequent thermal release of photocarriers.

Conformational effects in poly(p-phenylene vinylene)s revealed by low-temperature site-selective fluorescence

Low-temperature site-selective fluorescence (SSF) spectroscopy is employed to study morphological effects on the conformation of poly(p-phenylene vinylene) (PPV) and its phenyl-substituted, soluble derivative poly(phenylphenylenevinylene) (PPPV). Samples of PPV prepared as spin-coated thin films and stretch-aligned free-standing films, and samples of PPPV prepared as cast films and as blends with poly(methylmethacrylate) and polycarbonate have been studied. The results that the authors present are considered with the notion that each polymer sample consists of an array of ordered chain segments whose average length reflects the perfection of the local structure. The statistical distribution of the segment lengths is responsible for inhomogeneous broadening of the optical spectra (absorption and emission). The dominant electronic excitation created by photoexcitation across the pi - pi * energy gap is a singlet exciton that can execute a random walk among the chain segments. SSF spectroscopy allows the authors to distinguish the contributions to the apparent fluorescence Stokes shift that arise from energy relaxation through excitation migration (spectral diffusion) and from structural relaxation of the polymer chain (self-localization). The structural contribution to the Stokes shift approaches zero in well aligned PPV and reaches values of up to 500 cm-1 in highly disordered PPPV films. The SSF method also provides a means of assessing the extent of phase separation that occurs in PPPV blends.

PHOTOLUMINESCENCE AND ELECTROLUMINESCENCE IN CONJUGATED POLYMERIC SYSTEMS

The basic working principles and construction of electroluminescent polymer devices are described. The opportunities for combining creative synthetic chemistry and imaginative device physics to address issues of colour, efficiency, and control of processing of semiconducting polymeric materials are reviewed. New advances in the construction of multilayer devices and controlled syntheses of poly(p-phenylenevinylene) are reported.

Determination of the average molecular weight of poly(p-phenylenevinylene)

Abstract For any polymer the determination of the average molecular weight and its distribution is an important part of characterising its physical properties. We report values for the M n, M w and dispersity, determined using gel permeation chromatography, of a neutral precursor polymer to poly(p-phenylenevinylene) in relation to polymerisation conditions. We have observed a maximum M n value of approximately 100,000 with a dispersity of 5.7 and compare this value to that calculated for the “effective conjugation length”, that is the number of repeat units over which uninterrupted π-electron delocalisation actually takes place, in fully converted poly(p-phenylenevinylene).

Precursor route chemistry and electronic properties of poly(p-phenylenevinylene), poly[(2,5-dimethyl-p-phenylene)vinylene] and poly[(2,5-dimethoxy-p-phenylene)vinylene]

Several precursor polymer routes to poly(p-phenylenevinylene)4, poly[(2,5-dimethyl-p-phenylene)vinylene]10 and poly[(2,5-dimethoxy-p-phenylene)vinylene]15 are described. Base induced polymerisation of bis-sulfonium salt monomers afforded the polyelectrolytes 3 and 9 which were converted respectively directly into either polymer 4 or 10. Optimised conditions for the complete displacement of sulfonium groups by methanol to produce the corresponding methoxy precursor polymers 5, 11 and 17 are reported. The fully conjugated polymers obtained after heat and acid treatment of these precursors have been characterised and compared with those obtained via different precursor routes. The 1H NMR spectra of the precursor polymers, as well as IR and UV–VIS data of both the precursor polymers and the materials obtained after conversion into the poly(arylenevinylene)s are discussed.

Extended π-conjugation in poly(p-phenylenevinylene) from a chemically modified precursor polymer

We have developed a synthetic method that allows a controlled increase in the one-dimensional character of poly(p-phenylenevinylene) by subtle chemical modification of the precursor polymer. Examples of the unoriented fully conjugated material exhibit room temperature optical absorption spectra (uncorrected for reflectivity) which have a peak in absorption at 2.45 eV with subsidiary shoulders at higher photon energies. The absorption maximum is red-shifted with respect to previously observed values. Luminescence spectra of these samples recorded at 10K are complementary with a peak in emission at 2.38 eV and accompanying subsidiary maxima at lower energies. The peaks in absorption and emission are identified as the transitions between the vibrational ground states of the electronic and the first excited singlet states which we term the (0,0) transitions.

Electro-Absorption Spectroscopy on Poly(Arylene Vinylene)s

Abstract We report spectrally resolved electroabsorption (EA) measurements on poly(arylene vinylene)s. The materials poly(thienylene vinylene), PTV, and poly(p-phenylene vinylene), PPV, are particularly attactive as they can be prepared from soluble precursors which allow ready fabrication of films. The subsequent thermal conversion of the precursor yields samples with good optical and mechanical properties and environmental stability. Both materials give a strong electro-optic response, an induced change in the optical absorption leading to a refractive index change of order 10or applied electric fields between 25 and 100 kV/cm. The EA signal scales quadratically with applied field and shows an anisotropy of 2. 3 with respect to the polarisation of the incident light at the main peak.

Photoinduced absorption of structurally improved poly(p-phenylene vinylene) - no evidence for bipolarons

We have recently developed a synthetic route to poly(p-phenylene vinylene), PPV, which gives improved intrachain order. This has a large effect on the optical properties. We present data on photoinduced absorption for this form of PPV. The changes as compared to PPV prepared from the standard sulphonium precursor polymer are significant. Firstly, on the time scale of our experiments there is no evidence for photogenerated metastable bipolarons, usually observed in standard PPV. Secondly, the strong induced absorption due to triplet-triplet transitions is red-shifted to about 1.36 eV and is slightly narrower. This triplet absorption has a much stronger temperature dependence than observed in ordinary PPV, and is not seen above ca. 70 K. We consider that in this new form of PPV there are fewer defects which are responsible for the stabilisation of long-lived charged excitations.

Synthesis and characterisation of doped and undoped poly(2,5-dimethoxy phenylene vinylene)

Abstract Conjugated polymers exhibit large changes in their optical and electronic properties upon either n- or p- type doping. We report details of the synthesis of poly(2,5-dimethoxyphenylene vinylene)s (DMPPV)s, prepared from sulphonium precursor polymers with a range of counter-anions, and optical measurements of their electronic transitions and infrared active vibrations. We have observed that the DMPPV produced when the sulphonium polyelectrolyte precursor polymer contained SbF 6 − as the counterion shows sub band-gap optical absorption bands. This was observed to take place during thermal conversion from the precursor to the conjugated polymer under acidic conditions.

Structural order in poly(p-phenylene vinylene)

Poly(p-phenylene vinylene) [PPV] is fast becoming one of the most widely studied of the conjugated polymers. Its preparation via a precursor polymer synthesis allows ready processing and presents many opportunities for subtle control of the physical and electronic properties of the final polymer. A number of precursor routes have now been developed and we report an X-ray and electron diffraction study of the effects of different synthesis routes on the structure of the PPV polymer films thus prepared.