The magnetic field effect in the presence of the electric field on fluorescence of a methylene-linked compound of pyrene and n,n-dimethylaniline doped in a polymer film

Recent experimental data by Mizoguchi and Ohta are analyzed under the assumption of fast equilibrium between the locally excited (LE), radical-ion-pair (RIP), and exciplex states. In the absence of the equilibrium, no magnetic field effect on the LE fluorescence would be observed. Owing to the equilibrium, the relative electric-field-induced changes of the quantum yields of the LE and exciplex emissions as functions of the magnetic field are linearly connected to each other. The electric field shifts the equilibrium from the LE state toward the RIP state. Predictions are made for the magnetic field effect on the fluorescence kinetics.     

External electric field effects on absorption, fluorescence, and phosphorescence spectra of diphenylpolyynes in a polymer film

External electric field effects on absorption, fluorescence, and phosphorescence spectra of a series of unsubstituted diphenylpolyynes have been examined in a PMMA film. The analysis of the electroabsorption spectra indicates that the shorter diphenylpolyynes exhibit only the change in molecular polarizability, whereas the longer ones exhibit the change both in dipole moment and in molecular polarizability following absorption. The finding of the change in dipole moment following absorption of centrosymmetric diphenylpolyynes is interpreted in terms of the symmetry distortion upon doping a polymer film. When the external electric field is applied, the fluorescence yield is reduced and enhanced, respectively, in diphenylacetylene and diphenyloctatetrayne, indicating that the rate of the nonradiative process from the fluorescence state is accelerated in diphenylacetylene and decelerated in diphenyloctatetrayne by an external electric field. All of the diphenylpolyynes used in the present study exhibit the change in molecular polarizability following the phosphorescence process.     

Effect of a d.c. electric field on the fluorescence spectra of cationic polymethine dyes in a polymer film

We have observed that in an external d.c. electric field, in the differential fluorescence spectra of both symmetric and asymmetric polymethine dyes in a polymer film, the intensity of the emission in the long-wavelength region of the band increases while the intensity in the short-wavelength region decreases. In symmetric polymethines, these effects are opposite to those in the differential absorption spectra, while in asymmetric polymethines they are symbatic. The spectral effects were interpreted by a change in the probability of the vibronic transitions due to redistribution of electron density in the chromophore, in the ground state and the excited state of the dye, in an external electric field. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 44, No. 6, pp. 343–347, November–December, 2008.     

External electric field effects on fluorescence of pyrene butyric acid in a polymer film: concentration dependence and temperature dependence

Fluorescence spectra and electrofluorescence spectra (plots of the electric field-induced change in fluorescence intensity as a function of wavelength) have been measured at different temperatures for pyrene butyric acid (PBA) in a PMMA film at different concentrations. At a low concentration of 0.5 mol % where fluorescence emitted from the locally excited state of PBA (LE fluorescence) is dominant, LE fluorescence spectra show only the Stark shift in the presence of an electric field (F), which results from the difference in molecular polarizability between the ground and emitting states. At a high concentration of 10 mol % where the so-called sandwich-type excimer fluorescence (EX(1)) is dominant, both EX(1) and LE fluorescence are quenched by F. Another fluorescence assigned to a partially overlapped excimer (EX(2)) also exists at room temperature, and this emission is enhanced by F. As the temperature decreases, three fluorescence emissions whose electric field effects are different from each other become clear besides EX(1) and LE fluorescence, indicating that at least five fluorescence components exist at high concentrations at low temperatures. At a medium concentration of 5 mol % where EX(1) is comparable in intensity to the LE fluorescence, the intensity of EX(1) is not affected by F at any temperature, but LE fluorescence and EX(2) are markedly influenced by F at room temperature, and four fluorescence emissions are confirmed at low temperatures.     

Magnetic field effect on the fluorescence of glyoxal

Fluorescence from the ¹A_u state of glyoxal is quenched by a magnetic field and the pre-exponential factor of phosphorescence from the ³A_u state increases with increasing magnetic field. These findings are explained by the enhancement of intramolecular intersystem crossing by the magnetic field.     

External electric field effects on absorption and fluorescence spectra of a fullerene derivative and its mixture with zinc-tetraphenylporphyrin doped in a PMMA film

Electroabsorption and electrofluorescence spectra of a fullerene derivative, C60(C18)2, and its mixture with zinc-tetraphenylporphyrin (ZnTPP) have been measured by using electric field modulation spectroscopy. The change in dipole moment is significant in the electroabsorption spectra both of C60(C18)2 and of a complex composed of C60(C18)2 and ZnTPP, indicating that the excited states both of C60(C18)2 and of a complex between C60(C18)2 and ZnTPP have a large charge-transfer character. The fluorescence quantum yield of C60(C18)2 decreases in the presence of an electric field, which probably arises from the field-induced acceleration of the intramolecular nonradiative process of C60(C18)2 in the fluorescent state. In a mixture between ZnTPP and C60(C18)2, electrofluorescence spectra show the field-induced enhancement for the fluorescence of ZnTPP and the field-induced de-enhancement for the fluorescence both of C60(C18)2 and of the complex between ZnTPP and C60(C18)2. A theoretical analysis clearly shows that the field-induced enhancement of the ZnTPP fluorescence in a mixture results from the field-induced deceleration of the rate of the electron transfer from the excited ZnTPP to C60(C18)2. The standard free energy gap for the photoinduced electron-transfer process is estimated based on the theoretical simulation of the field-dependent fluorescence intensity.     

Influence of the electric field on a bio-mimetic film supported on a gold electrode

A model biological membrane was formed by fusion of mixed cholesterol and DMPC (dimyristoylphosphatidylcholine) phospholipid vesicles onto a gold-coated quartz support. The gold surface was charged and the influence of the charge at the solid support on the structure and integrity of the phospholipid bilayer was investigated using the specular reflection of neutrons and electrochemical measurements. When the surface charge density is close to zero, the lipid vesicles fuse directly on the surface to form a bilayer with a small number of defects and hence low water content. When the support's surface is negatively charged the film swells and incorporates water due to the field driven poration of the membrane. When the charge density is more negative then -8 microC cm(-2) the bilayer is detached from the metal surface. However, it remains in close proximity to the metal electrode, suspended on a thin cushion of water. The film thicknesses, calculated from neutron reflectivity, have allowed us to determine the tilt angle of the lipid molecules as a function of the support's charge density. The lipid molecules are tilted 55 degrees from the surface normal at zero charge density but become significantly more perpendicular (30 degrees tilt angle) at charge densities more negative than -8 microC cm(-2). The tilt angle measurements are in very good agreement with previous IR studies. This paper describes the highlights of a more in-depth study which is fully described in [1].     

Measuring chirality in NMR in the presence of a static electric field

The scalar Hamiltonian of nuclear spins in the presence of a static electric field supports chirality. However, the eigenvalues of the Hamiltonian are not chiral; hence, chirality is not manifested in the usual NMR experiment. In this work, we show that the magnetization response to certain radio frequency pulse sequences exhibits chirality as well as handedness.     

Dynamics of a conducting polymer jet in an electric field

The dynamics of a non-Newtonian liquid jet emanating from the Taylor cone during electrospinning of a polymer solution has been studied. The viscoelastic properties of the solution are described in terms of the finitely extensible nonlinear elastic FENE-P model. The shape of the jet has been determined, and the current and tension in the jet have been calculated. Calculations have shown the existence of a critical tension at which the jet stops stretching such that it can exhibit time-dependent behavior.     

Host matrix dependent fluorescence intensity modulation by an electric field in single conjugated polymer chains

An electric field oscillating at a frequency approximately 1 Hz is found to induce strong modulation of the fluorescence intensity of single poly[2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) molecules (MW approximately 10(6)) embedded in a poly(methyl methacrylate) (PMMA) matrix. The MEH-PPV polymer chains are carefully isolated from the electrodes to avoid effects of injection. In a polystyrene matrix, fluorescence intensity modulations are on average much less pronounced. The difference in average modulation depth can be explained in terms of lower field-induced exciton dissociation rates in the MEH-PPV/polystyrene system compared to MEH-PPV/PMMA because of a lack of suitable acceptor sites. The observed electric field dependence of single-molecule fluorescence strongly suggests that energy transfer from singlet or even triplet excitons to long-living on-chain hole polarons contributes to the observed modulations. The observed large qualitative differences between the responses of different molecules probably reflect differences in chain topology and strongly anisotropic distributions of acceptor sites, while the hysteretic response of some molecules indicates conformational switching.     

Magnetic field intensity effect on plane electric capacitor characteristics and viscoelasticity of magnetorheological elastomer

Magnetorheological (MR) elastomer was prepared using silicone rubber and soft magnetic carbonyl iron microspheres, and then examined as dielectric materials for manufacturing electric capacitors. As a specific element, capacity of the capacitors located in a magnetic field was found to be sensitive to both the MR suspension proportion to the silicone rubber and the intensity of the applied magnetic field. Viscoelastic characteristics of the MR elastomer, represented by storage modulus and creep behavior, were also studied.     

Behaviour of biaxial nematics in the presence of electric and magnetic fields

The behaviour of a biaxial nematic liquid crystal in the presence of electric and magnetic fields is discussed. In terms of the values of the magnetic susceptibilities and the dielectric permittivities, each biaxial nematic compound can be classified to belong to one of thirty-six different states. These states can be grouped together into three different classes, denoted by us as type A, B and C. The states belonging to each class exhibit a different qualitative behaviour in the presence of perpendicular electric and magnetic fields. While type A biaxial nematics always exhibit the same stable configuration in the presence of the fields, type B and C biaxial nematics exhibit two possible stable equilibrium configurations. Which of these is stable is determined by the magnitudes of the applied fields. The exchange of stability for type B systems can be modelled as a second order transition, while the exchange of stability for type C systems is of first order. In addition, the latter type can develop a bistable behaviour if certain conditions for the magnitudes of the electric and magnetic fields are fulfilled.     

Peculiarities of electric field alignment of nonlinear optical chromophores incorporated into thin film polymer matrix

The electric field poling process of free-standing films of poly(methyl methacrylate) (PMMA) matrix doped with the nonlinear optical compound 4-(dimethylamino)-4'-nitrostilbene (DMANS) was investigated by molecular simulation methods. The influence of the vacuum/bulk interfacial regions on static and dynamic properties, including the glass transition temperature T_g and the field-induced chromophore reorientation, was studied by employing films of three different thicknesses and by comparison with previous work on the bulk system. The interfacial region, defined as the region, where the local density increases from zero to the bulk density, is about 2 nm wide, independent of the film thickness. T_g decreases with decreasing film thickness, in accord with previous experimental work and theoretical predictions. The resistance against field-induced chromophore reorientation in the liquid state is found to increase strongly with the film thickness.     

Phase diagram of a two-frequency chiral nematic mixture in an electric field

The behaviour of a chiral nematic mixture with low frequency dielectric relaxtion was studied between two electrodes, with homeotropic anchoring, in the presence of an AC electric field E. The phase diagram in the parameter plane (d/p, E) was determined for this mixture which could be dielectrically negative or positive in different frequency ranges. The determination was carried out by changing the sample thickness d and both the amplitude and frequency of the electric field at a constant cholesteric pitch p.     

Magnetic field effects on the fluorescence of intramolecular electron-donor-acceptor systems

The external magnetic field effects on the exciplex fluorescence of α-(4-dimethylaminophenyl)-ω-(9-phenanthryl)alkanes have been studied by photostationary, time-resolved, and magnetic field modulation fluorescence spectroscopy. The singlet-triplet degeneracy of the radical ion pair is suggested to occur at a methylene chain length of about ten.     

Forces at the nuclei of a molecule in the presence of non-uniform electric field

The interaction Hamiltonian within the Bloch gauge for the potentials of the electric field has been used to define electric multipole moment operators. Perturbation theory has been applied to evaluate the induced electronic moments and electric field at the nuclei in the presence of spatially non-uniform electric fields of high intensity. Multipole nuclear electric shielding tensors have been defined to describe the contributions arising from a non-homogeneous electric field. These quantities are useful to rationalize linear and non-linear responses of a molecule in the presence of intense external electric perturbations.     

Preparation and characterization of nonfouling polymer brushes on poly(ethylene terephthalate) film surfaces

In this study, a surface grafting of nonfouling poly(ethylene glycol) methyl ether acrylate (PEGMA) on poly(ethylene terephthalate) (PET) was carried out via surface-initiated atom-transfer radical polymerization (SI-ATRP) to improve hemocompatibility of polymer based biomaterials. To do this, the coupling agent with hydroxyl groups for the ATRP initiator was first anchored on the surface of PET films using photochemical method, and then these hydroxyl groups were esterified by bromoisobutyryl bromide, from which PET with various main chain lengths of PEGMA was prepared. The structures and properties of modified PET surfaces were investigated using water contact angle (WAC), ATR-FTIR, X-ray photoelectron spectroscopy (XPS) and Atomic force microscopy (AFM). The molecular weights of the free polymer from solution were determined by gel permeation chromatography (GPC). These results indicated that grafting of PEGMA on PET film is a simple way to change its surface properties. The protein adsorption resistance on the surfaces of PET was primarily evaluated by an enzyme-linked immunosorbent assay (ELISA). The result demonstrated that the protein adsorption could be well suppressed by poly(PEGMA) brush structure on the surface of PET. This work provides a new approach for polymers to enhance their biocompatibility.