32 research outputs found

    External Electric Field Effect on Fluorescence Spectra of Pyrene in Solution

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    Electrophotoluminescence (E-PL) spectra, i.e., plots of the electric-field-induced change in photoluminescence intensity as a function of wavenumber, have been measured for pyrene solution. At high concentrations of pyrene where excimer fluorescence is observed along with the monomer fluorescence emitted from the locally excited state, both excimer fluorescence and monomer fluorescence are enhanced by application of electric fields. The results show that the nonradiative decay process at the excimer emitting state is decelerated by application of electric fields. It is also found that molecular polarizability of pyrene excimer is larger than that of pyrene monomer in the ground state by ∼270 ± 90 Å<sup>3</sup>, based on the analysis of the Stark shift of the excimer fluorescence

    Insulator–Metal Transitions Induced by Electric Field and Photoirradiation in Organic Mott Insulator Deuterated κ-(BEDT-TTF)<sub>2</sub>Cu[N(CN)<sub>2</sub>]Br

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    The Mott insulator–metal transition induced by an external stimulus such as electric field, pressure, chemical doping, or photoirradiation has received considerable attention because of the potential use in new optoelectronic functional devices. Here we report an abrupt Mott insulator–metal transition observed as a current jump in a molecular-based Mott insulator, namely, deuterated κ-(BEDT-TTF)2Cu­[N­(CN)2]­Br, where BEDT-TTF = bis­(ethylenedithio)­tetrathiafulvalene, upon application of a pulsed voltage of certain magnitude (threshold voltage). Furthermore, the threshold voltage needed for the transition is shown to be reduced by photoirradiation. Thus, the Mott insulator–metal transition can be controlled by a combination of an external electric field and photoirradiation

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

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    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

    A Memory Effect Controlled by a Pulsed Voltage in Photoinduced Conductivity Switching in an Organic Charge-Transfer Salt

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    We report that the photoinduced high conductivity (HC) state in the organic charge-transfer salt α-(BEDT-TTF)2I3, which is initially triggered by a pulsed voltage and synchronous photoirradiation, can be repeatedly recovered without further photoirradiation by application of a pulsed voltage even after the current has been reduced to zero. The recovery to the HC state can be controlled by adjusting the pulse width and/or amplitude. By proper selection of pulsed voltage parameters, the switching can be made reversible or irreversible. Hysteresis loops in the current−voltage and current−irradiation intensity curves of the circuit demonstrate a memory effect with potential application in unconventional photoswitching operations. The characteristic of the hysteresis loop and current bistability can be flexibly varied by changing the width of the applied pulsed voltage

    Insulator–Metal Transitions Induced by Electric Field and Photoirradiation in Organic Mott Insulator Deuterated κ-(BEDT-TTF)<sub>2</sub>Cu[N(CN)<sub>2</sub>]Br

    No full text
    The Mott insulator–metal transition induced by an external stimulus such as electric field, pressure, chemical doping, or photoirradiation has received considerable attention because of the potential use in new optoelectronic functional devices. Here we report an abrupt Mott insulator–metal transition observed as a current jump in a molecular-based Mott insulator, namely, deuterated κ-(BEDT-TTF)<sub>2</sub>Cu­[N­(CN)<sub>2</sub>]­Br, where BEDT-TTF = bis­(ethylenedithio)­tetrathiafulvalene, upon application of a pulsed voltage of certain magnitude (threshold voltage). Furthermore, the threshold voltage needed for the transition is shown to be reduced by photoirradiation. Thus, the Mott insulator–metal transition can be controlled by a combination of an external electric field and photoirradiation

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

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    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

    Stark Spectroscopy of Rubrene. II. Stark Fluorescence Spectroscopy and Fluorescence Quenching Induced by an External Electric Field

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    We report Stark fluorescence spectroscopy investigation of rubrene dispersed in a poly­(methyl methacrylate) film. The features of the fluorescence spectrum are analogous to those in solutions. In the Stark fluorescence spectrum, the decrease of the fluorescence quantum yield in the presence of an external electric field is observed. This result shows that the yield of nonradiative decay processes is increased by the application of an external electric field. It is known that the fluorescence quantum yield for rubrene, which is nearly unity at room temperature, depends on temperature, and a major nonradiative decay process in photoexcited rubrene is ascribed to a thermally activated intersystem crossing (ISC). Equations that express the field-induced fluorescence quenching in terms of the molecular parameters are derived from the ensemble average of electric field effects on the activation energy of the reaction rate constant in random orientation systems. The molecular parameters are then extracted from the observed data. It is inferred that the field-induced increase in the yield of other intramolecular and intermolecular photophysical processes in addition to the ISC should be taken into account

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

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    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

    Insulator–Metal Transitions Induced by Electric Field and Photoirradiation in Organic Mott Insulator Deuterated κ-(BEDT-TTF)<sub>2</sub>Cu[N(CN)<sub>2</sub>]Br

    No full text
    The Mott insulator–metal transition induced by an external stimulus such as electric field, pressure, chemical doping, or photoirradiation has received considerable attention because of the potential use in new optoelectronic functional devices. Here we report an abrupt Mott insulator–metal transition observed as a current jump in a molecular-based Mott insulator, namely, deuterated κ-(BEDT-TTF)<sub>2</sub>Cu­[N­(CN)<sub>2</sub>]­Br, where BEDT-TTF = bis­(ethylenedithio)­tetrathiafulvalene, upon application of a pulsed voltage of certain magnitude (threshold voltage). Furthermore, the threshold voltage needed for the transition is shown to be reduced by photoirradiation. Thus, the Mott insulator–metal transition can be controlled by a combination of an external electric field and photoirradiation

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

    No full text
    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
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