Tsuyoshi Asahi

The bell-shaped energy gap dependence of the charge recombination reaction of geminate radical ion pairs produced by fluorescence quenching reaction in acetonitrile solution

Abstract In view of the theoretical prediction that the observation of the inverted region in the photoinduced charge separation will be difficult, as it is actually the case in the fluorescence quenching reaction, but the charge recombination of the produced geminate ion pair will show a clear-cut bell-shaped energy gap dependence, we have made systematic studies on the charge recombination processes of geminate ion pairs by directly observing their dynamics with ultrafast laser spectroscopy. We have obtained not only the results for the inverted region, but also the results for the top region as well as normal region, confirming the bell-shaped energy gap dependence of the charge recombination. On the basis of this result, some discussions on the nature of the inter- and intra-molecular ion pairs with respect to the charge recombination processes have been given in the case of some typical exciplexes and porphyrin-quinone systems.

Direct observation of a picosecond charge separation process in photoexcited platinum-loaded TiO2 particles by femtosecond diffuse reflectance spectroscopy

Transient absorption spectra of Pt-loaded TiO2 powder photocatalysts were measured by femtosecond diffuse reflectance spectroscopy and the dynamics of photogenerated electrons in TiO2 were investigated. The Pt-loaded TiO2 showed a new decay component of a few ps under 390 nm excitation in addition of normal charge recombination kinetics. This fast decay process can be interpreted as migration of electrons in TiO2 to Pt on the basis of detailed measurements changing experimental conditions such as Pt-loading, excitation wavelength and excitation fluence. These results clearly indicate that charge separation is responsible for the well-known enhancement of the catalytic reactivity of Pt-loaded TiO2.

Femtosecond light scattering spectroscopy of single gold nanoparticles

We have developed an ultrafast, light-scattering spectroscopic system combining a conventional microscope with a pump–probe setup. We successfully measured the surface plasmon resonance band for the individual gold particles with a mean radius of 40 nm. The results on the pump–probe experiment demonstrate that both the electron–phonon and the phonon–phonon coupling processes in the individual gold particles take place with the lifetimes of 4 ps and >25 ps, respectively.

Absorption and scattering of near-infrared light by dispersed lanthanum hexaboride nanoparticles for solar control filters

Optical absorption and scattering behaviors of lanthanum hexaboride (LaB 6 ) nanoparticulate dispersion coatings with various particle sizes have been investigated in the ultraviolet to near-infrared (NIR) wavelengths for application to solar control filters. Large characteristic near-infrared absorption is introduced as the decreased particle size falls into nanoscale, and its origin is discussed in terms of a localized surface plasmon resonance (LSPR) of conduction electrons. Optical constants of LaB 6 have been measured and Mie scattering theory analysis was conducted. The theory was found to generally replicate the optical characteristics, and variations of absorbed and scattered wave fractions as well as the LSPR wavelength have been discussed with varying particle size. The absorption peaks are found as shaped narrower and located at shorter wavelength in theory than in experiment, which is suggested as ascribed mainly to the nonspherical distorted shape of LaB 6 nanoparticles.

Determination of the bimolecular rate constant of photoinduced charge separation in the energy gap region where the reaction is diffusion controlled — analysis of the transient effect in the course of fluorescence quenching reaction

Abstract The photoinduced intermolecular charge separation (CS) rate constant ( k CS ) at the encounter in acetonitrile solution has been evaluated for various donor—acceptor pairs by analyzing the transient effect in the fluorescence decay curves, in the energy gap region −Δ G 0 =0.37–2.21 eV, where the reaction is diffusion controlled. Although the obtained k CS values are larger than the diffusion rate constant, they do not show the typical bell-shaped −Δ G 0 dependence as observed in the charge recombination reaction of the geminate ion pairs produced by fluorescence quenching reaction, but show a rather flat shape.

Development of a femtosecond diffuse reflectance spectroscopic system, evaluation of its temporal resolution, and applications to organic powder systems

An ultrafast time-resolved diffuse reflectance spectroscopic system has been developed as a potential instrument for analyzing photophysical and photochemical dynamics of light scattering materials such as powder and opaque suspension, using an amplified femtosecond Ti:sapphire laser as a light source. Transient absorption spectra of organic powder, covering the wavelength region from 400 to 760 nm, were precisely obtained by using a stable and bright femtosecond white-light continuum as a probe light. An ultrashort light pulse is temporally broadened owing to numerous times of refraction, reflection, and diffraction in scattering materials. This affects the temporal resolution of transient absorption measurements, which was examined and discussed in detail by using a simple numerical model with time-dependent light propagation of a short pulse. From the simulation and experimental results, it was shown that the temporal resolution of transient absorption measurement is less than a few ps under a certain measurement condition, although it strongly depends on the optical properties of the sample; scattering and absorption coefficients. Some applications to the analysis of excited dynamics of organic molecules in polymer latex and molecular microcrystalline powder are also demonstrated.

Mechanisms of the strongly exothermic charge separation reaction in the excited singlet state. Picosecond laser photolysis studies on aromatic hydrocarbon-tetracyanoethylene and aromatic hydrocarbon-pyromellitic dianhydride systems in polar solutions

Abstract In relation to the fact that there are no clear-cut experimental results indicating the “inverted region” in the strongly exothermic charge separation (CS) in the fluorescence quenching reaction, the possibilities of (a) formation of a nonfluorescent charge transfer complex in the course of quenching and (b) participation of excited electronic states of the ion pair in the course of CS at the encounter of fluorescer and quencher are examined for aromatic hydrocarbon-tetracyanoethylene and aromatic hydrocarbon-pyromellitic dianhydride systems in acetonitrile with the picosecond laser photolysis method. Both (a) and (b) are shown improbable as mechanisms for the lack of an inverted region in the photoinduced CS reaction. Discussions on these results are given on the basis of a new theoretical treatment.

Direct observation of interfacial hole transfer from a photoexcited TiO2 particle to an adsorbed molecule SCN- by femtosecond diffuse reflectance spectroscopy

On the basis of transient absorption spectral measurements in the whole visible region using femtosecond diffuse reflectance spectroscopy, a hole transfer process from photoexcited TiO2 to an adsorbed molecule SCN- was elucidated. In a TiO2 aqueous suspension with KSCN, absorption rise of dimer radical anion (SCN)•2- was observed indicating hole transfer. Interestingly, there were two rise components with < 1 ps and several 100 ps, which were shorter in lower pH of the solution. Ultrafast hole transfer and subsequent intermolecular geometrical change of a dimmer radical anion are also discussed.

Photochromic reactions of crystalline spiropyrans and spirooxazines induced by intense femtosecond laser excitation

Photocoloration of microcrystalline powder of spirooxazine and spiropyran compounds induced by femtosecond laser excitation was investigated using time-resolved and steady state diffuse reflectance spectroscopy. The intermediate products were observed in a ns time region, and assigned to non-planar open forms. On the other hand, photocoloration can occur even in the crystalline phase by intense femtosecond pulse excitation, and the yield of colored forms increased nonlinearly with laser fluence. The mechanism of photochromic reaction induced by high-density excitation is discussed on the basis of excitation fluence and shot number dependences of photocoloration, thermal bleaching of colored forms, and transient absorption spectra. It is considered that free volume around molecules to change the molecular conformation is generated due to the local deformed crystal lattice following the densely formed excited states and intermediate products.

Intersystem crossing of benzophenone by femtosecond transient grating spectroscopy

Abstract Transient grating spectroscopy using a femtosecond white-light continuum was developed and applied to the spectral analysis of intersystem crossing of benzophenone in solution. The diffraction spectra corresponding to Sn←S1 and Tn←T1 transient absorption were observed for the first time. The rate constants of intersystem crossing in acetonitrile and CCl4 were estimated to be (9.6 ± 0.9 ps)−1 and (11.6 ± 0.9 ps)−1, respectively.