14 research outputs found
External Electric Field Effect on Fluorescence Spectra of Pyrene in Solution
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 Ã…<sup>3</sup>, based on the analysis of the Stark shift of
the excimer fluorescence
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
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
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>CuÂ[NÂ(CN)<sub>2</sub>]ÂBr, where BEDT-TTF = bisÂ(ethylenedithio)Âtetrathiafulvalene,
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
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
Stark Spectroscopy of Rubrene. II. Stark Fluorescence Spectroscopy and Fluorescence Quenching Induced by an External Electric Field
We
report Stark fluorescence spectroscopy investigation of rubrene
dispersed in a polyÂ(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
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
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>CuÂ[NÂ(CN)<sub>2</sub>]ÂBr, where BEDT-TTF = bisÂ(ethylenedithio)Âtetrathiafulvalene,
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
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
Anisotropic Electric Field Effect on the Photoluminescence of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> Perovskite Sandwiched between Conducting and Insulating Films
Photoluminescence
(PL) of a nanocrystalline film of methylammonium
lead iodide perovskite (MAPbI<sub>3</sub>) sandwiched between an electrode
of a fluorine-doped tin oxide (FTO) layer and an insulating film of
polyÂ(methyl methacrylate) is found to increase and decrease significantly
with the application of an external electric field (<i><b>F</b></i><sub>ext</sub>), depending on the direction of the
applied field, based on the measurements of electrophotoluminescence
(E-PL) spectra, i.e., field-induced change in PL spectra. The field-induced
change in PL intensity is confirmed to originate from the field-induced
change in the number of free carriers which induce radiative recombination,
based on temporally resolved E-PL measurements. We propose that an
internal field (<i><b>F</b></i><sub>int</sub>) exists
even without application of <i><b>F</b></i><sub>ext</sub>. The anisotropic behavior of the effect of <i><b>F</b></i><sub>ext</sub> on PL is interpreted in terms of a synergy
effect of <i><b>F</b></i><sub>int</sub> and <i><b>F</b></i><sub>ext</sub>; both fields are additive with
the applied field direction from Ag to FTO electrode (positive direction)
or subtractive with the opposite applied field direction (negative
direction), where FTO is the positive electrode, resulting in an increased
or decreased total electric field as well as quenching or enhancement
of PL, respectively. The PL lifetime in the nanosecond region increased
and decreased with an application of an electric field in the positive
and negative directions, respectively, which is attributed to a field-induced
change in the concentration of free carriers
External Electric Field Effects on Excited-State Intramolecular Proton Transfer in 4′‑<i>N</i>,<i>N</i>‑Dimethylamino-3-hydroxyflavone in Poly(methyl methacrylate) Films
The
external electric field effects on the steady-state electronic
spectra and excited-state dynamics were investigated for 4′-<i>N</i>,<i>N</i>-(dimethylamino)-3-hydroxyflavone (DMHF)
in a polyÂ(methyl methacrylate) (PMMA) film. In the steady-state spectrum,
dual emission was observed from the excited states of the normal (N*)
and tautomer (T*) forms. Application of an external electric field
of 1.0 MV·cm<sup>–1</sup> enhanced the N* emission and
reduced the T* emission, indicating that the external electric field
suppressed the excited-state intramolecular proton transfer (ESIPT).
The fluorescence decay profiles were measured for the N* and T* forms.
The change in the emission intensity ratio N*/T* induced by the external
electric field is dominated by ESIPT from the Franck–Condon
excited state of the N* form and vibrational cooling in potential
wells of the N* and T* forms occurring within tens of picoseconds.
Three manifolds of fluorescent states were identified for both the
N* and T* forms. The excited-state dynamics of DMHF in PMMA films
has been found to be very different from that in solution due to intermolecular
interactions in a rigid environment
Inhomogeneous Photoluminescence Characteristic in Carbon Nanodots and Electrophotoluminescence Measurements
Photoluminescence
(PL) spectra, time-resolved PL spectra, and PL
decay profiles have been observed for carbon nanodots (CDs) with different
excitation wavelengths in an embedded solid film and in solution.
PL excitation spectra have been also observed with different monitoring
wavelengths. Then, it is found in both solid film and solution that
not only the location of the PL spectra but also the peak of the excitation
spectra show a significant red shift, as the excitation and monitoring
wavelengths become longer, respectively, indicating that the emitting
states of the excitation-dependent PL are the real state to which
direct absorption occurs from the ground state, not the transient
trapped states produced by photoexcitation. It is shown that not only
the excitation-dependent PL but also the excitation-independent PL
with a peak at ∼375 nm exist. The lifetimes of both PL emissions
are very sensitive to the surroundings. Multiple emitting states that
give excitation-dependent PL are ascribed to the inhomogeneous properties
in prepared carbon nanodots, which is supported by the fluorescence
lifetime image measurements. Electrophotoluminescence spectrum, that
is, the electric-field-induced change in PL spectrum, has also been
observed for the excitation-independent PL of CDs embedded in a polyÂ(methyl
methacrylate) film, and the magnitude of the change in electric dipole
moment and molecular polarizability following emission has been determined