M. da Graça Miguel

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.

Synthesis, Spectra and Photophysics of some Free Base Tetrafluoroalkyl and Tetrafluoroaryl Porphyrins with Potential Applications in Imaging¶

Abstract The synthesis, characterization and photophysical properties of two perfluoroalkyl (5,10,15,20-tetrakis-[trifluoromethyl]- and [heptafluoropropyl]-porphyrin) and two perfluoroaryl (5,10,15,20-tetrakis-[2,6-difluorophenyl]- and [pentafluorophenyl]-porphyrin) are described, with reference to their potential in both photodynamic therapy (PDT) and in vivo imaging by fluorescence and 19F nuclear magnetic resonance spectroscopy. Absorption and fluorescence spectra, fluorescence lifetimes and triplet–singlet difference spectra are reported. Triplet yields have been obtained by flash photolysis and pulse radiolysis, whereas yields of sensitized singlet oxygen formation have been determined by time-resolved phosphorimetry. All four compounds show high yields of triplet formation and singlet oxygen sensitization. The spectral properties, stability and attractive solubility characteristics of the perfluoroalkyl derivatives make them particularly suitable candidates for future study for applications in PDT.

Fluorescence studies of polymer–surfactant association

Abstract Fluorescence spectroscopy has been successfully used for the study of central issues of solutions of surfactants and associating polymers. Different fluorescence techniques and methods are uniquely adapted to investigate problems in this field and can, by using extrinsic or intrinsic probes, provide information on molecular association, microstructure and molecular dynamics. This constitutes an important contribution to the understanding and control of macroscopic properties, as well as to their biological functions and technical applications. Important aspects of these mixed systems, related to their self-assembly, are: formation of micelles and hydrophobic microdomains in general; size and shape of surfactant molecular aggregates; formation and stability of vesicles; intra- vs. intermolecular association in polymers; conformational changes in polymers as affected by polymer–surfactant association; surfactant organization in adsorbed layers; kinetic aspects of the formation and disintegration of self-assembly structures; residence times of molecules in microdomains and migration of active molecules. Some of these issues will be addressed in this paper.

Solvatochromic fluorescent probes in bicontinuous microemulsions

Abstract The fluorescence probes nile red and prodan have been used to study the structure of the bicontinuous phase of C 12 E 5 /alkane/water microemulsions. For nile red the steady state fluorescence spectrum show contributions from probe located in alkane and surfactant rich regions, together with indications of surfactant hydration. Fluorescence lifetime measurements show multiexponential decay kinetics consistent with this idea. The steady state fluorescence spectrum of prodan in the bicontinuous region also suggests different environments for the probe. Fluorescence anisotropy measurements using rhodamine 6G show that this bicontinuous phase is more ordered than either oil in water or water in oil microemulsions. The fluorescence technique provides a valuable tool with which to elucidate the structure of these complex systems.

Diffusion coefficients of sodium dodecylsulfate in aqueous solutions and in aqueous solutions of β-cyclodextrin

Abstract Differential diffusion coefficients have been measured of sodium dodecylsulfate (SDS) in aqueous solutions of β-cyclodextrin (β-CD) at 298.15 K over the concentration range 0.001 M to 0.0817 M using a conductimetric cell and an automatic apparatus to follow the diffusion. The cell uses an open ended capillary method, while a conductimetric technique is used to follow the diffusion process by measuring the resistance of a solution inside the capillaries, at various recorded times. The β-CD is known to for strong 1:1 complexes with SDS, and the effect of this on the diffusion of this electrolyte was investigated. The presence of β-cyclodextrin can influence the diffusion coefficients of sodium dodecylsulfate both above and below the critical micelle concentration (cmc) of this surfactant. For concentrations of β-cyclodextrin of 0.001 mol dm −3 the behaviour of the diffusion of SDS in aqueous solutions is the same in the absence or the presence of β-cyclodextrin. In contrast, when the β-CD concentration is 0.016 mol dm −3 we obtain diffusion coefficients higher than those obtained in aqueous solutions. Further, we do not observe the dramatic decrease in diffusion normally found at the cmc of the surfactant. These results are interpreted in terms of the effect of incorporation of dodecylsulfate chains inside the cyclodextrin cavities.

Triplet state spectroscopy of conjugated polymers studied by pulse radiolysis

Using the technique of pulse radiolysis we have elucidated the energies and kinetics of triplet states in soluble luminescent conjugated polymers. Using poly(2-methoxy,5-(2 0 -ethyl-hexoxy)-p-phenylenevinylene) MEH-PPV as an example we explain this technique and show how it can be used to study the triplet states in conjugated polymers. Triplet energy transfer is used to determine 1 1 Agˇ1 3 Bu energy gaps and the kinetics of triplet‐triplet absorption yields triplet lifetimes. In the case of MEH-PPV, at concentrations up to 50 mg/l, the triplet decay rate shows no change, indicating self-quenching of triplets is not significant. However, if very high electron beam doses are used, high intra chain triplet concentrations can be generated. In this high concentration regime triplet‐triplet annihilation becomes effective, as determined by the onset of delayed fluorescence. # 2001 Elsevier Science B.V. All rights reserved.

Diffusion coefficients of sodium dodecylsulfate in aqueous solutions of sucrose and in aqueous solutions

Abstract Differential diffusion coefficients of sodium dodecylsulfate (SDS) in aqueous solutions of sucrose and in aqueous solutions at 298.15 K, over the concentration range 0.0018 M to 0.0817 M, have been measured using a conductimetric cell and an automatic apparatus to follow the diffusion. The results are discussed on the basis of the Onsager-Fuoss model. The cell uses an open ended capillary method, while a conductimetric technique is used to follow the diffusion process by measuring the resistance of a solution inside the capillaries, at recorded times. The influence of molecules of sucrose on the diffusion of this electrolyte was investigated. The presence of sucrose increases the diffusion coefficients of sodium dodecylsulfate both above and below the critical micelle concentration (cmc) of the surfactant.

Quenching of the luminescent state of the uranyl ion (UO2+2) by metal ions. Evidence for an electron transfer mechanism

The quenching of the luminescence of the uranyl ion by other metal ions has been studied in aqueous solution. The quenching is shown to be a dynamic process, and the correlation of the logarithm of the quenching rate with the metal ion ionization potential suggests that intermolecular electron transfer is the predominant mechanism. Evidence that this involves complete electron transfer comes from flash photolysis of solutions of UO2+2 and manganese(II), where a broad absorption (λmax= 505 nm) is observed which is assigned to Mn3+. Consideration of the energetics of the quenching process suggests that in the quenching of uranyl by silver(I), the products (UO+2 and Ag2+) are produced in their electronic ground states. Studies of the effect of temperature on the quenching suggest that if an intermediate complex (exciplex) is involved in the quenching then this must involve only very weak binding. With silver(I), the quenching is sensitive to the ionic strength of the solution. Further studies suggest that the lifetime of the luminescent state of the uranyl ion in aqueous solution varies with both temperature and uranyl ion concentration.

Transient absorption spectra of triplet states and charge carriers of conjugated polymers

Abstract The use of pulse radiolysis to study transient species in conjugated polymers in solutions is discussed, with particular reference to poly(2-methoxy-5-(2′-ethylhexoxy)- p -phenylenevinylene) (MEH-PPV). The excited triplet state is specifically generated by energy transfer on radiolysis of benzene solutions in the presence of suitable sensitisers, and its spectrum is reported. The presence of vibronic structure suggests a localised triplet state. By varying the energy of the sensitiser, it is possible to determine the energy of the lowest triplet state. Results are also presented for the triplet–triplet absorptions and energies of five other common π-conjugated polymers. The MEH-PPV triplet absorption, produced on radiolysis, does not increase linearly with either polymer concentration or radiation dose. This is suggested to be due to the presence of multiple triplet excitations on the isolated polymer chains. Intrachain triplet–triplet annihilation is observed, and is seen to lead to delayed fluorescence. Spectra are also reported for the positive and negative charge carriers of MEH-PPV and the other polymers specifically produced by pulse radiolysis of the polymers in chloroform or tetrahydrofuran solutions.