Toshifumi Iimori

Insulator-metal transitions induced by electric field and photoirradiation in organic Mott insulator deuterated κ-(BEDT-TTF)2Cu[N(CN)2]Br.

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)(2)Cu[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.

Molecular-based light-activated thyristor

A photoinduced electrical conductivity switching is observed in the molecular conductor of α-[bis(ethylenedithio)tetrathiafulvalene]2I3 at different temperatures with different irradiation light intensities. The threshold voltage for the differential-negative-resistance effect appearing in the current-voltage characteristic curve decreases and increases, respectively, as the light intensity is increased and as the temperature decreases below the insulator-metal phase transition. The potential application of molecular conductor in bidirectional light-activated thyristor devices is demonstrated.

Comparative study of electroabsorption spectra of polar and nonpolar organic molecules in solution and in a polymer film

Electroabsorption (EA) spectra of polar and nonpolar molecules of coumarin 153 (C153) and pyrene in solution and in a polymer film of polymethylmethacrylate (PMMA) have been measured in the UV-visible region at room temperature. The shape of the EA spectra of C153 in benzene, 1,4-dioxane, or monochlorobenzene remarkably depends on the angle between the polarization direction of the absorption light and the applied electric field, whereas the EA spectra of C153 doped in PMMA show only the Stark shift and the field-induced change in spectral shape is negligible. These results demonstrate that C153 is reoriented by application of electric fields in solution, but the molecules are immobilized in a PMMA film. Based on the EA spectra, electric dipole moments both in the ground state and in the excited state have been evaluated for C153 in different solvents. In the EA spectra of pyrene, only the Stark shift is observed both in solution and in PMMA, indicating that the field-induced molecular reorientation does not occur both in solution and in PMMA. The change in dipole moment of C153 as well as the change in molecular polarizability of pyrene following absorption is much larger in solution than that in PMMA.

Electroabsorption spectra of PbSe nanocrystal quantum dots

The quantum-confined Stark effect of PbSe nanocrystal quantum dots (QDs) in a polymer film has been studied for the near-infrared absorption in the region of 1300–1600 nm (corresponding to diameters of 4.0–7.0 nm) by using electric field modulation spectroscopy. Electroabsorption spectra of QDs show the electric-field-induced energy shift in the exciton state, resulting in the spectral broadening given by the second derivative of the absorption spectra. It is shown that the magnitude of the change in electric dipole moment following excitation into the first exciton state increases with an increase in the size of QDs.

Reversible photoswitching behavior in bulk resistance and in color of polycrystalline AgI at room temperature

A photoinduced reversible change in bulk resistance of polycrystalline AgI is observed at room temperature. The original yellow color of the sample changes to dark brown with UV (308 nm) photoirradiation, associated with the small decrease in the bulk resistance. A reversible switching of color between dark brown and yellow is observed by alternative UV-visible photoirradiation, associated with a switching between high and low resistance states. The observed reversible photoswitching is interpreted in terms of the photoinduced reversible change in the β-γ-polytype stacking structure of the polycrystalline AgI.

Magnetic Field Effect on Fluorescence in a Mixture of N-Ethylcarbazole and Dimethyl Terephthalate in a Polymer Film in the Presence of Electric Fields

Magnetic field effects on the fluorescence spectrum and on the electrofluorescence spectrum (plots of the electric field-induced change in fluorescence intensity as a function of wavelength) have been examined in electron donor and acceptor pairs of N-ethylcarbazole (ECZ) and dimethyl terephthalate (DMTP) in polymer films at different ratios of donor/acceptor concentration. In the mixture having a high concentration of ECZ, electric field-induced quenching of the exciplex fluorescence originating from the photoinduced electron transfer becomes less efficient in the presence of a magnetic field. In the mixture having a low concentration of ECZ, on the other hand, no magnetic field effect was observed in the electrofluorescence spectrum, indicating that the hole carrier plays an important role in synergy effects of magnetic and electric field effects on exciplex fluorescence. In the absence of the applied electric field, the magnetic field does not affect either exciplex fluorescence with a peak at 450 nm or LE fluorescence emitted from the locally excited state of ECZ but enhances the broad emission with a peak at approximately 380 nm, probably assigned to the fluorescence of another type of exciplex between ECZ and DMTP. Thus, two kinds of magnetic field effects on fluorescence have been observed in a mixture of ECZ and DMTP in a polymer film.

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.

Stark Spectroscopy of Rubrene. I. Electroabsorption Spectroscopy and Molecular Parameters

Electroabsorption spectroscopy investigation and the determination of molecular parameters for rubrene dispersed in a poly(methyl methacrylate) (PMMA) matrix are reported. The features of the band system in the absorption spectrum in PMMA are analogous to those in solutions. The changes in the electric dipole moment and the polarizability between the excited and ground states are determined from analysis of the Stark effect in the absorption band. The change in the transition dipole moment in the presence of an external electric field is also observed. Although rubrene is predicted to be classified as a nonpolar molecule, there is a contribution of the difference in the electric dipole moment between the excited and ground states to the electroabsorption spectrum. The origin of the nonzero difference in the electric dipole moment is argued. Stark fluorescence spectroscopy investigation is reported in Part II of this series.

Photoirradiation Effect on Magnetic Susceptibility in Organic Superconductors

Photoirradiation effects on magnetic susceptibility are studied in organic superconductors of κ-(BEDT-TTF)2Cu[N(CN)2]Br and β-(BEDT-TTF)2I3 (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene), for which we have reported photoresponses of the electrical conductivity elsewhere. With photoirradiation by cw visible laser light, the diamagnetic susceptibility originating from Meissner effect is reduced, indicating that the percentage of superconducting volume decreases with photoirradiation. These results agree with the ones of the time-resolved photoresponse measurements of the electrical conductivity using pulsed laser light.

Excitation wavelength dependence of photoinduced phase transition in polycrystalline AgI at room temperature

Excitation wavelength dependence of the photocurrent and the photoinduced change in bulk resistance has been investigated at room temperature for the polycrystalline AgI using steady state photocurrent measurement method and impedance spectroscopy. A small decrease of the bulk resistance is induced by 308 nm excitation, and the bulk resistance further decreases with 450 nm excitation following the 308 nm excitation. The photoinduced reversible switching in bulk resistance between 308 nm excitation and 450 nm excitation has been interpreted in terms of the photoinduced phase transition in the polycrystalline AgI, and these properties are obtained only in the polycrystalline β-AgI.