12 research outputs found
Coherent Nuclear Wave Packets in Q States by Ultrafast Internal Conversions in Free Base Tetraphenylporphyrin
Persistence
of vibrational coherence in electronic transition has
been noted especially in biochemical systems. Here, we report the
dynamics between electronic excited states in free base tetraphenylporphyrin
(H<sub>2</sub>TPP) by time-resolved fluorescence with high time resolution.
Following the photoexcitation of the B state, ultrafast internal conversion
occurs to the Q<sub><i>x</i></sub> state directly as well
as via the Q<sub><i>y</i></sub> state. Unique and distinct
coherent nuclear wave packet motions in the Q<sub><i>x</i></sub> and Q<sub><i>y</i></sub> states are observed through
the modulation of the fluorescence intensity in time. The instant,
serial internal conversions from the B to the Q<sub><i>y</i></sub> and Q<sub><i>x</i></sub> states generate the coherent
wave packets. Theory and experiment show that the observed vibrational
modes involve the out-of-plane vibrations of the porphyrin ring that
are strongly coupled to the internal conversion of H<sub>2</sub>TPP
Excited State Intramolecular Proton Transfer Dynamics of 1‑Hydroxy-2-acetonaphthone
Excited
state intramolecular proton transfer (ESIPT) of 1-hydroxy-2-acetonaphthone
(HAN) has been in controversy, mainly because its Stokes shift is
small compared to those of typical ESIPT molecules. We have investigated
excited state dynamics of HAN by time-resolved fluorescence with a
resolution high enough to record the nuclear wave packet motions in
the excited state. Population dynamics of both the normal and tautomer
forms were recorded together with the wave packet motions of the tautomer
in the excited state, which confirm the ESIPT of HAN. The population
dynamics of the normal and tautomer forms imply that the ESIPT dynamics
is biphasic with two time constants <25 and 80 fs. Theoretical
analysis of the vibrational modes of the tautomer excited impulsively
reveals that major part of the change for the ESIPT reaction is on
the naphthalene ring
Active Role of Proton in Excited State Intramolecular Proton Transfer Reaction
Proton transfer is one of the most important elementary
reactions
in chemistry and biology. The role of proton in the course of proton
transfer, whether it is active or passive, has been the subject of
intense investigations. Here we demonstrate the active role of proton
in the excited state intramolecular proton transfer (ESIPT) of 10-hydroxybenzoÂ[<i>h</i>]Âquinoline (HBQ). The ESIPT of HBQ proceeds in 12 ±
6 fs, and the rate is slowed down to 25 ± 5 fs for DBQ where
the reactive hydrogen is replaced by deuterium. The results are consistent
with the ballistic proton wave packet transfer within the experimental
uncertainty. This ultrafast proton transfer leads to the coherent
excitation of the vibrational modes of the product state. In contrast,
ESIPT of 2-(2′-hydroxyphenyl)Âbenzothiazole (HBT) is much slower
at 62 fs and shows no isotope dependence implying complete passive
role of the proton
Multifaceted Ultrafast Intramolecular Charge Transfer Dynamics of 4‑(Dimethylamino)benzonitrile (DMABN)
Intramolecular charge transfer (ICT) of DMABN has been
the subject
of extensive investigations. Through the measurements of highly time-resolved
fluorescence spectra (TRFS) over the whole emission region, we have
examined the ICT dynamics of DMABN in acetonitrile free from the solvation
dynamics and vibronic relaxation. The ICT dynamics was found to be
characterized by a broad range of time scales; nearly instantaneous
(<30 fs), 160 fs, and 3.3 ps. TRFS revealed that an ICT state with
partially twisted geometry, ICTÂ(P), is formed within a few hundred
femtoseconds either directly from the initial photoexcited state or
via the locally excited (LE) state. The ICTÂ(P) state undergoes further
relaxation along the intramolecular nuclear coordinate to reach the
twisted ICT (TICT) state with the time constant of 4.8 ps. A conformational
diversity along the rotation of the dimethylamino group was speculated
to account for the observed diffusive dynamics
Coherent and Homogeneous Intramolecular Charge-Transfer Dynamics of 1-<i>tert</i>-Butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6), a Rigid Analogue of DMABN
We
report the intramolecular charge-transfer (ICT) dynamics of
1-<i>tert</i>-butyl-6-cyano-1,2,3,4-tetrahydroquinoline
(NTC6), a planar analogue of 4-(dimethylamino)Âbenzonitrile (DMABN),
by using time-resolved fluorescence (TRF) and TRF spectra (TRFS).
TRFS allow accurate determination of the ICT dynamics free from the
spectral relaxation caused by the solvation and vibronic relaxation.
For NTC6 in tetrahydrofuran (THF), the locally excited (LE) state
is populated exclusively presumably via a conical intersection from
the initial photoexcited S<sub>2</sub> (L<sub>a</sub>) state, and
the LE state undergoes ICT single exponentially with a time constant
of 1.8 ± 0.2 ps. In acetonitrile, however, both LE (22%) and
ICT (78%) states are populated from the S<sub>2</sub> state, and the
population in the LE state undergoes ICT in 800 ± 100 fs. The
ICT state undergoes further relaxation in 1.2 ps along the solvation
and the intramolecular nuclear coordinates involving the rotation
of the amino group to form a twisted ICT state. Coherent nuclear wave
packet motions of 130 cm<sup>–1</sup>, which can be assigned
to the −CN group bending mode, were observed in the
TRF of the reactant (LE) and product (ICT) states, indicating that
the ICT reaction is partially coherent. Compared with DMABN, the ICT
dynamics of NTC6 are quite homogeneous, and we speculated on the narrow
conformational distribution of NTC6 in the ground state along the
rotation of the amino group due to its rigid structure
Exciton Scattering Mechanism in a Single Semiconducting MgZnO Nanorod
Excitonic phenomena, such as excitonic absorption and
emission,
have been used in many photonic and optoelectronic semiconductor device
applications. As the sizes of these nanoscale materials have approached
to exciton diffusion lengths in semiconductors, a fundamental understanding
of exciton transport in semiconductors has become imperative. We present
exciton transport in a single MgZnO nanorod in the spatiotemporal
regime with several nanometer-scale spatial resolution and several
tens of picosecond temporal resolution. This study was performed using
temperature-dependent cathodoluminescence and time-resolved photoluminescence
spectroscopies. The exciton diffusion length in the MgZnO nanorod
decreased from 100 to 70 nm with increasing temperature in the range
of 5 and 80 K. The results obtained for the temperature dependence
of exciton diffusion length and luminescence lifetime revealed that
the dominant exciton scattering mechanism in MgZnO nanorod is exciton–phonon
assisted piezoelectric field scattering
Coherent Nuclear Wave Packets Generated by Ultrafast Intramolecular Charge-Transfer Reaction
Intramolecular charge-transfer (ICT) dynamics, including
reaction
coordinates, structural changes, and reaction rate, has been noted
experimentally and theoretically. Here we report the ICT dynamics
of laurdan investigated by time-resolved fluorescence at extreme time
resolution of 30 fs. A single high-frequency coherent nuclear wave-packet
motion on the product potential surface is observed through the modulation
of the fluorescence intensity in time. Theory and experiment show
that this vibrational mode involves large displacement of the carbon
atoms in the naphthalene backbone, which indicates that the naphthalene
backbone coordinates are strongly coupled to the ICT reaction of laurdan,
not the twisting or planarization of the dimethylamino group
Effects of Gold-Nanoparticle Surface and Vertical Coverage by Conducting Polymer between Indium Tin Oxide and the Hole Transport Layer on Organic Light-Emitting Diodes
The effect of varying
degrees of surface and vertical coverage
of gold nanoparticles (Au-NPs) by polyÂ(styrenesulfonate)-doped polyÂ(3,4-ethylenedioxythiophene)
(PEDOT:PSS), which was used as a capping layer between indium tin
oxide (ITO) and a hole transport layer (HTL) on small-molecule fluorescent
organic light-emitting diodes (OLEDs), was systemically investigated.
With respect to the Au-NP loading amount and size, the resultant current
densities influenced the charge balance and, therefore, the OLED device
performance. When the capping layer consisted of ITO/Au-NPs/PEDOT:PSS+Au-NPs,
superior device performance was obtained with 10-nm Au-NPs through
increased surface coverage in comparison to other Au-NP PEDOT:PSS
coverage conditions. Furthermore, the Au-NP size determined the vertical
coverage of the capping layer. The current densities of OLEDs containing
small Au-NPs (less than 30 nm, small vertical coverage) covered by
PEDOT:PSS decreased because of the suppression of the hole carriers
by the Au-NP trapping sites. However, the current densities of the
devices with large Au-NPs (over 30 nm, large vertical coverage) increased.
The increased electromagnetic fields observed around relatively large
Au-NPs under electrical bias were attributed to increased current
densities in the OLEDs, as confirmed by the finite-difference time-domain
simulation. These results show that the coverage conditions of the
Au-NPs by the PEDOT:PSS clearly influenced the OLED current density
and efficiency
Fluorescence from Multiple Chromophore Hydrogen-Bonding States in the Far-Red Protein TagRFP675
Far-red fluorescent proteins are critical for in vivo imaging applications,
but the relative importance of structure versus dynamics in generating
large Stokes-shifted emission is unclear. The unusually red-shifted
emission of TagRFP675, a derivative of mKate, has been attributed
to the multiple hydrogen bonds with the chromophore <i>N</i>-acylimine carbonyl. We characterized TagRFP675 and point mutants
designed to perturb these hydrogen bonds with spectrally resolved
transient grating and time-resolved fluorescence (TRF) spectroscopies
supported by molecular dynamics simulations. TRF results for TagRFP675
and the mKate/M41Q variant show picosecond time scale red-shifts followed
by nanosecond time blue-shifts. Global analysis of the TRF spectra
reveals spectrally distinct emitting states that do not interconvert
during the S<sub>1</sub> lifetime. These dynamics originate from photoexcitation
of a mixed ground-state population of acylimine hydrogen bond conformers.
Strategically tuning the chromophore environment in TagRFP675 might
stabilize the most red-shifted conformation and result in a variant
with a larger Stokes shift
Fluorescence from Multiple Chromophore Hydrogen-Bonding States in the Far-Red Protein TagRFP675
Far-red fluorescent proteins are critical for in vivo imaging applications,
but the relative importance of structure versus dynamics in generating
large Stokes-shifted emission is unclear. The unusually red-shifted
emission of TagRFP675, a derivative of mKate, has been attributed
to the multiple hydrogen bonds with the chromophore <i>N</i>-acylimine carbonyl. We characterized TagRFP675 and point mutants
designed to perturb these hydrogen bonds with spectrally resolved
transient grating and time-resolved fluorescence (TRF) spectroscopies
supported by molecular dynamics simulations. TRF results for TagRFP675
and the mKate/M41Q variant show picosecond time scale red-shifts followed
by nanosecond time blue-shifts. Global analysis of the TRF spectra
reveals spectrally distinct emitting states that do not interconvert
during the S<sub>1</sub> lifetime. These dynamics originate from photoexcitation
of a mixed ground-state population of acylimine hydrogen bond conformers.
Strategically tuning the chromophore environment in TagRFP675 might
stabilize the most red-shifted conformation and result in a variant
with a larger Stokes shift