Hugh D. Burrows

Reaction pathways and mechanisms of photodegradation of pesticides

The photodegradation of pesticides is reviewed, with particular reference to the studies that describe the mechanisms of the processes involved, the nature of reactive intermediates and final products. Potential use of photochemical processes in advanced oxidation methods for water treatment is also discussed. Processes considered include direct photolysis leading to homolysis or heterolysis of the pesticide, photosensitized photodegradation by singlet oxygen and a variety of metal complexes, photolysis in heterogeneous media and degradation by reaction with intermediates generated by photolytic or radiolytic means.

S1∼>T1 intersystem crossing in π-conjugated organic polymers

Quantum yields for triplet formation have been determined for seven common π-conjugated polymers in benzene solution using time-resolved photoacoustic calorimetry (PAC) in conjunction with fluorescence quantum yields, singlet and triplet energies. The polymers studied include three poly(thiophenes), poly(2-methoxy,5-(2′-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV), a cyano derivative of MEH-PPV, a ladder type poly(p-phenylene) (MeLPPP), and a poly(fluorene). Yields of singlet oxygen formation have also been determined for these polymers in benzene by time-resolved phosphorimetry, and are in reasonable agreement with triplet yields obtained by PAC. Polythiophenes show the highest intersystem crossing yields, which are suggested to result from extensive spin-orbit coupling. Where singlet oxygen yields are less than triplet yields, it is suggested that interaction of molecular oxygen with the ground state of the polymers may be involved.

Photophysics of thiophene based polymers in solution: The role of nonradiative decay processes

An investigation has been undertaken of the photophysics of six thiophene-based polymers. This includes the measurement of fluorescence quantum yields, quantum yields for triplet formation, and determination of fluorescence and triplet lifetimes in benzene solution at room temperature. From the above-mentioned data, the overall set of rates for the deactivation processes (fluorescence, intersystem crossing, and internal conversion), has been evaluated. Mechanisms of nonradiative decay are discussed, and it is shown that both intersystem crossing and internal conversion are important in the decay of the lowest singlet excited state of isolated polythiophenes molecules in solution. Comparison of spectroscopic and photophysical properties of the polymers with analogous oligothiophenes shows that with the polymer, the S1⇝S0 internal conversion deactivation pathway plays a major role, in contrast to the behavior found with the oligomers where this internal conversion deactivation channel is essentially negligible.

The thermal behaviour of divalent and higher valent metal soaps: a review

The thermal behaviour of di-, tri- and tetravalent metal soaps (carboxylates) is reviewed, with reference to both the phase behaviour and thermal decomposition. General methods of preparation and purification are considered, and results on the structures of the solid phase obtained by techniques such as X-ray diffraction, NMR and vibrational spectroscopy are discussed. The general phase behaviour of these systems is reviewed, and the effects of metal ion, unsaturation and chain branching on this is considered. Transport and other physical properties of molten metal carboxylates, both as pure systems and mixtures, are discussed. Both the products of thermal decomposition and the proposed reaction mechanisms for the degradation of these compounds are considered, and the literature on the kinetics of these reactions is reviewed. Although this literature survey demonstrates that a considerable amount is already known about the thermal behaviour of these systems, it is clear that there is still much to be learnt.

Measurement of the S0–T1 energy gap in poly(2-methoxy,5-(2′-ethyl-hexoxy)–p-phenylenevinylene) by triplet–triplet energy transfer

Abstract We have elucidated the lowest triplet state in the soluble luminescent conjugated polymer poly(2-methoxy,5-(2′-ethyl-hexoxy)– p -phenylenevinylene) (MEH–PPV) in benzene. Using a range of triplet sensitisers we determine that the S 0 –T 1 energy separation is 1.27±0.07 eV. Triplet–triplet absorption is observed at 1.5 eV. In solution, the triplet state has a lifetime ⩾100 μs. The molar absorption coefficient at 1.5 eV is concentration dependent; at 200 mg/l, e =1.19×10 5 M −1 cm −1 . Up to 50 mg/l, the triplet decay rate showed no change, indicating self-quenching of triplets is not significant. Oxygen quenches the MEH–PPV triplet, leading to singlet oxygen formation with a quantum yield of 2.5±1%.

Triplet state dynamics on isolated conjugated polymer chains

Triplet state behaviour has been studied with several conjugated polymers in dilute benzene solutions by flash photolysis, photoacoustic calorimetry (PAC) and pulse radiolysis/energy transfer. With polythiophenes and the ladder poly(p-phenylene) MeLPPP, singlet–triplet intersystem crossing (ISC) is relatively efficient. In contrast, it is inefficient with poly(p-phenylenevinylene)s (PPVs) and polyfluorene, while with cyano-substituted PPV, there is no evidence for any long-lived triplet state. Energy transfer from triplet biphenyl to MEH-PPV is diffusion controlled and triplet state lifetimes are typically tens or hundreds of μs. All the triplet states are quenched by molecular oxygen, leading to formation of singlet oxygen with yields which are generally close to those for triplet formation. With pulse radiolysis at high doses, it is possible to have more than one triplet state per polymer chain. This can lead to delayed fluorescence via intrachain triplet–triplet annihilation. Kinetic analysis of this shows slow movement of triplets by hopping along the chain.

All-conjugated polyelectrolyte block copolymers

Novel, all-conjugated polyelectrolyte block copolymers of the rod-rod type can be generated in a “grafting from” scheme and exhibit a preferred tendency to self-assemble into layered aggregates both in solution and the solid state. Here, the rigid-rod structure of the individual, complex macromolecules favours the formation of low-curvature vesicular and lamellar aggregates. Our poly(9,9-dialkylfluorene)-b-poly[3-(6-ammoniumhexyl)thiophene] (PF2/6-b-P3TMAHT and PFO-b-P3TMAHT, where PF2/6 and PFO denote 2-(ethyl)hexyl and linear octyl alkyl pendant groups, respectively), and poly(9,9-dialkylfluorene)-b-poly[3-(6-pyridylhexyl)thiophene] (PF2/6-b-P3PyHT and PFO-b-P3PyHT) polyelectrolyte diblock copolymers allow for simple and reliable control of the occurring self-organisation process and the resulting nano-scaled architectures. They are, therefore, promising candidates for application as the active layer in electronic devices or as functional membranes (e.g. for sensor applications). Moreover, the electronic properties of the materials (especially the excitation energy transfer between both blocks) strongly depend on the aggregation state present. Aggregation can be further controlled via addition of oppositely charged surfactants resulting in the formation of ordered polyelectrolyte/surfactant complexes.

Cationic polythiophene-surfactant self-assembly complexes: phase transitions, optical response, and sensing.

The absorption and photoluminescence spectra of the cationic conjugated polyelectrolyte poly[3-(6-trimethylammoniumhexyl)thiophene] (P3TMAHT) were observed to be dramatically altered in the presence of anionic surfactants due to self-assembly through ionic complex formation. Small-angle neutron scattering (SANS), UV/vis, and photoluminescence spectroscopy were used to probe the relationship between the supramolecular complex organization and the photophysical response of P3TMAHT in the presence of industrially important anionic surfactants. Subtle differences in the surfactant mole fraction and chemical structure (e.g., chain length, headgroup charge density, perfluorination) result in marked variations in the range and type of complexes formed, which can be directly correlated to a unique colorimetric and fluorimetric fingerprint. Our results show that P3TMAHT has potential as an optical sensor for anionic surfactants capable of selectively identifying distinct structural subgroups through dual mode detection.

Structure and "surfactochromic" properties of conjugated polyelectrolyte (CPE): Surfactant complexes between a cationic polythiophene and SDS in water

We report on the phase transitions, solution structure, and consequent effect on the photophysical properties of poly[3-(6-trimethylammoniumhexyl)thiophene] bromide (P3TMAHT) in aqueous sodium dodecylsulfate (SDS). Polythiophene was mixed with SDS or deuterated SDS to form P3TMAHT(SDS)(x) complex (x = the molar ratio of surfactant over monomer units) in D(2)O and studied by small-angle neutron and X-ray scattering (SANS/SAXS) and optical spectroscopy. At room temperature, P3TMAHT forms charged aggregates with interparticle order. The addition of SDS eliminates the interparticle order and leads to rod-like (x = 1/5) or sheet-like polymer-SDS aggregates (x = 1/2 to 1) containing rod-like (x = 1/5 to 1/2) or sheet-like (x = 1/2 to 1) polymer associations. Partial precipitation occurs at the charge compensation point (x = 1). Ellipsoidal particles without interparticle order, reminiscent of SDS micelles modified by separated polymer chains, occur for x = 2 to 5. Free SDS micelles dominate for x = 20. Structural transitions lead to a concomitant variation in the solution color from red (P3TMAHT) to violet (x = 1/5 to 1) to yellow (x > 2). The photoluminescence fingerprint changes progressively from a broad featureless band (x = 0) through the band narrowing and appearance of vibronic structure (x = 1/5 to 1) to the return to a blue-shifted broad emission band (x = 5). The polymer stiffness reaches a maximum for x = 1, which leads to minimization of the Stokes shift (0.08 eV). This work gives fundamental information upon how surfactant complexation can influence both the solution structure and photophysical properties of a water-soluble polythiophene.

The structure and thermal behaviour of some long chain cerium(III) carboxylates

The even chain length cerium(III) carboxylates from the octanoate to the octadecanoate have been synthesised by metathesis. Thermogravimetry shows the presence of coordinated water for the short chain homologues, whereas the longer chain ones only contain adsorbed water. X-Ray diffraction and IR spectral measurements show that the solid phase has a lamellar, bilayer structure with planes of the cerium(III) ions coordinated to the carboxylate groups. The phase behaviour of the carboxylates has been studied by DSC and polarized-light microscopy. One or more mesophases are observed over the temperature range 70–120°C and melting occurs between 130 and 150°C. The textures observed on the polarizing microscope clearly show the anisotropic nature of the mesophases. Although the overall enthalpy and entropy of melting of these compounds increase with increasing chain length, the values are considerably lower than expected for complete fusion of the alkyl chains. Competition between melting of the chains and changes in the metal–carboxylate coordination region is the major factor responsible for the differences observed in the phase behaviour between the short and long chain derivatives.