We hereby report the results of our direct investigation into the excited-state dynamics of thermally activated delayed fluorescence (TADF) molecules in solution using pump-probe transient absorption spectroscopy (TAS). We found that the charge-transfer (CT) state commonly stated for TADF molecules encompasses two forms: localized and delocalized CT states. A highly efficient TADF molecule, 4CzIPN [Uoyama et al., Nature, 492, 234-238 (2012)], showed both the localized and delocalized CT states, while an inefficient TADF molecule, 2CzPN, exhibited only a localized CT state. By analyzing the time profile of triplet species observed in TAS, we propose that the reverse intersystem crossing (RISC) of 4CzIPN occurs via a mutual interaction in multiple energy levels of localized neutral and CT states, and delocalized CT states.
Transient absorption of visible light responsive powder photocatalysts, solid solution of GaN and ZnO (denoted as
GaN:ZnO), and other related materials were measured by using femtosecond diffuse reflectance spectroscopy in order to
evaluate the nature of photogenerated electrons and holes through spectral information in visible and near-infrared region
as well as kinetics from 100 fs to 500 ps. The GaN:ZnO is known to be one of successful photocatalysts which are able
to split water into oxygen and hydrogen molecules under visible-light irradiation. Since photoexcitation generates
electrons and holes in the conduction and valence bands, respectively, it is important to understand their trapping and
recombination processes in details. Generally efficient and quick trapping and slow recombination of them are required
to increase the chance of charge transfer of them to protons and water molecules. We have elucidated that the charge
trapping was within time resolution (< 1 ps) and recombination time was about 100 ps for 26% of carriers and much
longer for 74%, clearly indicating that most of photogenerated carriers have long lifetime. Other photocatalysts with
lower photocatalytic activity showed shorter lifetimes. These results indicated that the long carrier lifetime in GaN:ZnO
is one of the reasons for the efficient reactivity.
studied primary photocatalytic reactions in TiO2 nanoparticle films by using time-resolved laser spectroscopy. In
this report, we assign absorption spectra due to active species in TiO2. We also observed the reaction of holes in TiO2
with methanol. Finally, we detected mobile electrons in a strong light-scattering sample by means of time-resolved
microwave conductivity.k
Study of plasmon-induced charge transfer mechanism in gold-TiO2 system is crucial and promising in the solar cell
application. To investigate charge separation and recombination dynamics in gold/TiO2 nanoparticle systems, we used
ultrafast visible-pump/IR-probe femtosecond transient absorption spectroscopy method. In our experimental study,
anatase TiO2 with different particle size 9 nm and 20 nm were chosen as electron acceptors. Plasmon-induced electron
transfer from the gold nanoparticle to the conduction band of TiO2 was studied by optical excitation of the surface
plasmon band of gold nanoparticle at 550 nm. The transient absorption kinetics were studied by probing at 3440 nm to
observe intraband free electron adsorption in TiO2. In our experimental results, electron injection was found to be
completed within the apparatus time resolution (240 fs), the charge recombination decay within 1.5 ns was
nonexponential. And when laser power changed from 0.5 μJ to 1.9 μJ, the recombination decay didn't depend on the
excitation intensity. It is interesting that we found the measured back electron transfer kinetics up to 1.5 ns were strongly
dependent on the particle size of TiO2. The plasmon-induced charge transfer mechanisms will be discussed.
The transient absorption of nanocrystalline TiO2 films in the visible-to-IR wavelength region was measured under UV
excitation conditions at different wavelengths of 266 nm and 355 nm. Under weak 355 nm excitation, the generated
charge carrier density could be reduced as low as the second-order electron-hole recombination process could be ignored
as we reported previously (Y. Tamaki et al. Phys. Chem. Chem. Phys. 9, 1453-1460 (2007)). The result was compared
with data obtained under 266 nm excitation, where the band-gap exaction was strong and efficient electron-hole
recombination occurred due to the high charge carrier density. Taking into account the dynamics of the electrons and
holes in the femtosecond to picosecond time range, such as ultrafast charge carrier trapping and slow deep trapping of
electrons, intra-band relaxation in the conduction and the valence bands and intra-particle diffusion of electrons in the
shallow trap levels were revealed.
Dye sensitized nanocrystalline semiconductor films are used as a photoactive part in dye-sensitized solar cells, which are recently attracting much interest both in basic and applied studies. Electron transfer reaction from a photoexcited dye molecule, which is chemically adsorbed on the surface of semiconductor, into the semiconductor conduction band is the primary step to generate photocurrent. Ultrafast pump-probe spectroscopy with a <100 fs time resolution and in a visible-to-IR wavelength range was used to elucidate the interfacial electron transfer mechanism in dye-sensitized nanocrystalline metal oxide films of ZnO, TiO2, and others. We found two types of reaction paths; one is direct electron transfer from the excited molecule to the conduction band and the other is stepwise transfer through an intermediate, which was assigned to a charge transfer complex formed by the excited molecule and a surface state on the semiconductor. The order of the observed electron transfer rates for different semiconductors was qualitatively explained by the idea of the density of electron acceptor states; that is, the larger the density of states near the energy level of the excited molecules was, the faster the electron transfer took place.
Molecular photonics, information processing based on strong interactions of photons and molecules, has attracted much attention, since it can fully utilize many superior properties of both photons and molecules. We have been developing materials showing ultrafast absorption changes in the visible to near infrared regions by photoinduced electron transfer and reverse reactions or by excited electronic state formation. Guided wave mode (GWM) geometry composed of a prism, a metal thin film, and a photoresponsive polymer film successfully achieved sensitive, all optical, and very fast control of reflectance by transient or persistent changes of GWM conditions by means of photoinduced complex refractive index changes upon nanosecond to femtosecond laser excitation. All-optical parallel control of reflected probe light by pump (writing) light was demonstrated by photochromic spiropyran or fulgide, and various metallophthalocyanines dispersed in appropriate polymers.
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