Synergy Effects of Electric and Magnetic Fields on Locally Excited-State Fluorescence of Photoinduced Electron Transfer Systems in a Polymer Film

Photoluminescence of electron donor−acceptor pairs that show photoinduced electron transfer (PIET) has been measured in a polymer film under simultaneous application of electric field and magnetic field. Fluorescence emitted from the locally excited state (LE fluorescence) of 9-methylanthracene (MAnt) and pyrene (Py) is quenched by an electric field in a mixture of 1,3-dicyanobenzene (DCB) with MAnt or Py, indicating that PIET from the excited state of MAnt or Py to DCB is enhanced by an electric field. Simultaneous application of electric and magnetic fields enhances the reverse process from the radical-ion pair produced by PIET to the LE fluorescent state of MAnt or Py. As a result, the electric-field-induced quenching of the LE fluorescence is reduced by application of the magnetic fields. Thus, the synergy effect of electric and magnetic fields is observed on the LE fluorescence of MAnt or Py. Exciplex fluorescence spectra resulting from PIET can be obtained by analyzing the field effects on photoluminescence spectra, even when the exciplex fluorescence is too weak to be determined from the steady-state or time-resolved photoluminescence spectra at zero field

Electroabsorption Spectra of Quantum Dots of PbS and Analysis by the Integral Method

Electroabsorption (E-A) spectra of semiconductor quantum dots (QDs) of PbS have been analyzed by the integral method, which is powerful not only to determine the change in electric dipole moment and/or polarizability following absorption precisely but also to confirm the weak absorption bands buried under other strong absorption bands. In the results, two weak absorption bands which induce blue-shift and red-shift, respectively, with application of electric fields have been newly confirmed in PbS QDs to be located just above the intense first exciton band. The energy separation between these two bands becomes smaller, as the applied field strength becomes stronger, suggesting that the interaction between the states excited by these transitions becomes weaker by application of electric fields. The QD size dependence has been reported both for the transition energy and for the magnitude of the change in electric dipole moment and molecular polarizability following the absorption of each band. The assignment of the absorption bands has also been discussed

Effects of Nanosecond Pulsed Electric Fields on the Intracellular Function of HeLa Cells As Revealed by NADH Autofluorescence Microscopy

The fluorescence lifetime of the endogenous fluorophore of reduced nicotinamide adenine dinucleotide (NADH) in HeLa cells is affected by the application of nanosecond pulsed electric fields (nsPEFs). In this study, we found that after nsPEF application, the fluorescence lifetime became longer and then decreased in a stepwise manner upon further application, irrespective of the pulse width in the range of 10–50 ns. This application time dependence of the NADH fluorescence lifetime is very similar to the time-lapse dependence of the NADH fluorescence lifetime following the addition of an apoptosis inducer, staurosporine. These results, as well as the membrane swelling and blebbing after the application of nsPEFs, indicate that apoptosis is also induced by the application of nsPEFs in HeLa cells. In contrast to the lifetime, the fluorescence intensity remarkably depended on the pulse width of the applied nsPEF. When the pulse width was as large as 50 ns, the intensity monotonically increased and was distributed over the entire cell as the application duration became longer. As the pulse width of the applied electric field became smaller, the magnitude of the field-induced increase in NADH fluorescence intensity decreased; the intensity was reduced by the electric field when the pulse width was as small as 10 ns. These results suggest that the mechanism of electric-field-induced apoptosis depends on the pulse width of the applied nsPEF

Integral Method Analysis of Electroabsorption Spectra and Its Application to Quantum Dots of PbSe

The integral method is proposed to analyze the electroabsorption (E-A) spectra, since the change in the electric dipole moment and/or polarizability following absorption can be determined precisely and the bands buried under strong absorption bands can be confirmed. This method, where not only the observed E-A spectra but also each of their first and second integral spectra are fitted using the absorption and their derivative and integral spectra, has been successfully applied to the E-A spectra of semiconductor quantum dots of PbSe. In the results, one absorption band, which is not identified in the absorption spectrum because of the extremely weak intensity and also showing a remarkable blue shift in the presence of an electric field because of the large difference in polarizability between the ground state and the excited state, has been confirmed to be located between the first and second strong exciton bands. The size dependence of PbSe QDs of the peak position of the newly confirmed band as well as the magnitude of the change in electric dipole moment and polarizability following the absorption of each absorption band is also reported, based on the analysis by the integral method

Interface- and Temperature-Sensitive Linear Electric Field Effects on Exciton Absorption of CH₃NH₃PbI₃ Perovskite Films

The influence of applied electric field (FA) on the absorption spectra of a methylammonium lead tri-iodide (MAPbI3) crystalline film sandwiched between a fluorine-doped tin oxide (FTO) layer and a polymer film of poly(methyl methacrylate) (PMMA) (Sample I) and sandwiched between a compact layer of titanium oxide (cp-TiO2) and a PMMA film (Sample II) has been investigated by first harmonic modulation spectroscopy on various temperatures in the range of 290–60 K. The linear electroabsorption spectra, EA(1f), of the MAPbI3 film in Sample I showed very different behaviors in the temperature range above and below 200 K. EA(1f) spectra show a shape similar to the second derivative of the exciton absorption band having a Gaussian profile, and switching between positive and negative signs is generated at 290 K on alternating polarity of FA. With decreasing temperature, the same tendency of FA polarity-dependent EA(1f) switching was maintained until ∼200 K. At temperatures below 200 K, the inversion of the EA(1f) signal was found with the same field direction of FA, and the signal intensity increased with decreasing temperature. In sample II, the polarity-dependent EA(1f) signal was negligibly small at room temperature but became prominent with decreasing temperature and followed the same trend as that in Sample I observed at temperatures below 200 K, indicating that the substrate on which the MAPbI3 film was coated played a conclusive role as the origin of the polarity-dependent EA(1f) spectra. The second-derivative-like shape of the interface- and temperature-sensitive EA(1f) spectra is interpreted in terms of the polarity-dependent linear Stark shift of the exciton absorption band whose peak position depends on the temperature and substrate on which the MAPbI3 film is coated

Application of Nanosecond Pulsed Electric Field and Autofluorescence Lifetime Microscopy of FAD in Lung Cells

Exposure of nanosecond pulsed electric fields (nsPEFs) to live cells is an increasing research interest in biology and medicine. Despite extensive studies, a question still remains as to how effects of application of nsPEF on intracellular functions are different between cancerous cells and normal cells and how the difference can be detected. Herein, we have presented an approach of autofluorescence lifetime (AFL) microscopy of flavin adenine dinucleotide (FAD) to detect effects of application of nsPEF having 50 ns of a pulse width, nsPEF(50), on intracellular function in lung cancerous cells, A549 and H661, which show nsPEF(50)-induced apoptosis, and normal cells, MRC-5, in which the field effect is less or not induced. Then, the application of nsPEF(50) is shown to increase the lifetime of FAD autofluorescence in lung cancerous cells, whereas the electric field effects on the autofluorescence of FAD was not significant in normal healthy cells, which indicates that the lifetime measurements of FAD autofluorescence are applicable to detect the field-induced change in intracellular functions. Lifetime and intensity microscopic images of FAD autofluorescence in these lung cells were also acquired after exposure to the apoptosis-inducer staurosporine (STS). Then, it was found that the AFL of FAD became longer after exposure not only in the cancerous cells but also in the normal cells. These results indicate that nsPEF(50) applied to lung cells induced apoptotic cell death only in lung cancerous cells (H661 and A549) but not in lung normal cells (MRC-5), whereas STS induced apoptotic cell death both in lung cancerous cells and in lung normal cells. The lifetime microscopy of FAD autofluorescence is suggested to be very useful as a sensitive detection method of nsPEF-induced apoptotic cell death

Illumination Power-Dependent Electroabsorption of Excitons in a CH₃NH₃PbI₃ Perovskite Film

Electroabsorption (E-A) spectra of methylammonium lead triiodide (MAPbI3) are found to depend on the power density of the illumination light (IL-D) in both tetragonal and orthorhombic phases when the modulation frequency (M-F) of the applied electric field (F) is low. As IL-D increased, the E-A intensity decreased and the shape of the E-A spectra altered from that similar to the second derivative of the exciton absorption band having a Gaussian profile to the one similar to the first derivative, when the M-F of F is low. When the M-F of F is high, E-A spectra were independent of the power density of illumination. Orientational polarization of excitons is considered to be induced by F having a low M-F with strong photoirradiation, following ion migration of MA+ and I– along the applied field direction enhanced by photoirradiation

Temperature-Dependent Electroabsorption and Electrophotoluminescence and Exciton Binding Energy in MAPbBr₃ Perovskite Quantum Dots

Organic–inorganic lead halide perovskite nanocrystals have attracted much attention as promising materials for the development of solid-state light-emitting devices, but the existence of free or bound excitons or the formation of trap states remains under debate. We recorded the temperature-dependent electroabsorption (E-A) and electrophotoluminescence (E-PL) spectra, that is, electric-field-induced change in absorption and photoluminescence spectra, for methylammonium lead tribromide (MAPbBr3) colloidal perovskite nanocrystals, that is, quantum dots (QD), doped in a poly­(methyl methacrylate) film in the temperature range of 40–290 K. Based on the results, the binding energy of the exciton (electron–hole pair) was estimated. The exciton binding energy of QD of MAPbBr3 estimated from the absorption and E-A spectra (∼17 meV) is nearly the same as that of a MAPbBr3 polycrystalline thin solid film, while the exciton binding energy estimated from the temperature-dependent PL spectra (∼70 meV) is much greater than that estimated from the absorption profile. The frequency dependence of the E-A intensity observed at 40 and 290 K for the modulated applied electric field indicates a slow ion migration in nanocrystals, which follows the modulation of the applied electric field at a frequency less than 500 Hz. The observed E-A spectra were analyzed with an integral method on assuming the Stark effect; the magnitudes of the changes in electric dipole moment and polarizability following photoexcitation were determined at each temperature from 40 to 290 K. E-PL spectra show that the PL of QD of MAPbBr3 is quenched on the application of an external electric field; the extent of quenching is much greater for trap emission than for exciton emission. Exciton–phonon scattering, which is responsible for the line broadening of the PL spectra, is also discussed based on the temperature-dependent PL spectra