Hiroshi Miyasaka

Picosecond Absorption Spectra and Relaxation Processes of the Excited Singlet State of Pyrene in Solution

The method of analysis for the ps transient absorption spectra obtained by a ps Nd3+: YAG laser photolysis system is described in detail, showing the results of pyrene in solution. The S n ← S 1 spectrum observed immediately after excitation is rather broad and its intensity in the short wavelength region is relatively enhanced, compared to the spectrum observed at later stages. For elucidating this fast spectral change, the spectrum at 0 ps was simulated from that at 100 ps, evaluating numerically the inner filter effect, depletion of the ground state molecules and temporal characteristics of the monitoring ps continuum. The difference between the simulated and experimentally observed spectra may be ascribed to the nonlinear refractive index change of the solvent, the thermal lensing effect, or to an intrinsic relaxation of the excited molecule. Examining absorption spectra obtained under various conditions, the former two possibilities were denied for the excited pyrene system. Vibrational relaxation as well as the S2 → S1 internal conversion with a time constant of less than 10 ps have been suggested to be responsible for this difference between the simulated and observed spectra.

Picosecond laser photolysis studies on a photochromic dithienylethene in solution and in crystalline phases

Abstract Picosecond transient absorption spectroscopy and fluorescence measurement were applied to the direct observation of photochromic reaction of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (DMTF) in solutions as well as in crystalline phase. In both phases, very rapid (⩽ ≈ 10 ps) reaction channel of the ring-closure was found. the mechanism of ultrafast reaction process was discussed by comparing the present result with those of other diarylethene derivatives.

Picosecond laser photolysis studies on photochromic reactions of 1,2-bis(2,4,5-trimethyl-3-thienyl)maleic anhydride in solutions

Abstract Picosecond laser photolysis and transient absorption spectroscopy were applied to the direct observation of the photochromic reactions, such as ring-closure and ring-opening processes, of 1,2-bis(2,4,5-trimethyl-3-thienyl)maleic anhydride in solutions. Both in the ring-closure and the ring-opening processes, very rapid (

One-color reversible control of photochromic reactions in a diarylethene derivative: three-photon cyclization and two-photon cycloreversion by a near-infrared femtosecond laser pulse at 1.28 μm.

One-color control of colorization/decolorization reactions of diarylethene molecules was attained by using nonresonant high-order multiphoton absorption processes with a near-infrared (NIR) femtosecond laser pulse at 1.28 μm with 35 fs full width at half-maximum (fwhm). The intensity of a rather weak laser pulse (<1 nJ/pulse) can induce the simultaneous three-photon absorption leading to the colorization, while much weaker intensity induces two-photon absorption resulting in the decolorization. The spatial patterning concomitant with higher-order multiphoton absorption processes was also demonstrated.

Picosecond dynamics of photoinduced electron transfer processes in poly(N-vinylcarbazole) solid film doped with electron acceptors as revealed by transient absorption spectroscopy and dichroism measurements

Abstract Combining the measurements of the picosecond transient absorption spectra and the transient dichroism sensitive to the charge shift reaction processes, photoinduced charge separation (CS), recombination (CR) and hole transfer (HT) processes in poly(N-vinylcarbazole) solid films doped with electron acceptors were investigated. It was observed that the HT process was in competition with the CR in the initial charge-separated state produced via the excitation of the ground state CT complex. The time constant for the HT from the cation state of carbazole moiety (Cz) to neighboring Czs was obtained to be ≈ 2 ns. Primary processes in these photoinduced electron transfer phenomena were discussed by comparing the present results with those in solutions.

Picosecond-nanosecond laser photolysis studies on the photochemical reaction of excited benzophenone with 1,4-diazabicyclo[2.2.2]octane in acetonitrile solution: proton abstraction of the free benzophenone anion radical from the ground state amine

Abstract Picosecond and nanosecond dynamics of the ion pair produced by the electron transfer reaction between the triplet state benzophenone ( 3 BP*) and 1,4-diazabicyclo[2.2.2]octane (DABCO) was investigated by means of transient absorption spectroscopy and laser-induced photoconductivity measurement. It has been revealed that the solvated free anion radical of BP, produced by the rapid ionic dissociation of the ion pair within 2 ns, abstracts proton from the neutral DABCO giving benzophenone ketyl radical, competing with the charge recombination reaction at encounter with DABCO + , decomposition and/or impurity scavenging processes.

Nondestructive micropatterning of living animal cells using focused femtosecond laser-induced impulsive force

Micropatterning of mouse NIH3T3 fibroblast cells was performed using focused femtosecond laser-induced impulsive force in a culture medium. The cells were detached from an upper substrate by the force and transferred to an underlying substrate with less than spatial resolution of 80μm full width at half maximum. About 80% of the cells were confirmed to be alive at 3h after the patterning. The force exerted to the cell was investigated by high-speed imaging and estimated to be an order of micronewtons. The force origin was not only due to cavitation bubble but also due to shockwave and jet flow.

Picosecond and femtosecond laser photolysis studies of a photochromic diarylethene derivative: multiphoton gated reaction

Cycloreversion (ring-opening) process of one of the photochromic diarylethene derivatives, 1,2-bis(2-methyl-3-benzothienyl)perfluorocyclopentene (BPFC), was investigated by means of picosecond and femtosecond laser photolysis methods. The drastic enhancement of the reaction yield was observed by the picosecond laser exposure. On the other hand, the cycloreversion reaction yield under femtosecond laser exposure was consistent with the steady-state light irradiation. The excitation intensity effect of the reaction profiles revealed that the successive multiphoton absorption process leading to higher excited states opened the efficient cycloreversion process.

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.

Photochemistry of fac-[Re(bpy)(CO)3Cl]

The photochemistry of fac-[Re(bpy)(CO)3Cl] (1 a; bpy=2,2′-bipyridine) initiated by irradiation using <330 nm light has been investigated. Isomerization proceeded in THF to give the corresponding mer-isomer 1 b. However, in the presence of a small amount of MeCN, the main product was the CO-ligand-substituted complex (OC-6-24)-[Re(bpy)(CO)2Cl(MeCN)] (2 c; bpy=2,2′-bipyridine). In MeCN, two isomers, 2 c and its (OC-6-34) form (2 a), were produced. Only 2 c thermally isomerized to produce the (OC-6-44) form 2 b. A detailed investigation led to the conclusion that both 1 b and 2 c are produced by a dissociative mechanism, whereas 2 a forms by an associative mechanism. A comparison of the ultrafast transient UV-visible absorption, emission, and IR spectra of 1 a acquired by excitation using higher-energy light (e.g., 270 nm) and lower-energy light (e.g., 400 nm) gave detailed information about the excited states, intermediates, and kinetics of the photochemical reactions and photophysical processes of 1 a. Irradiation of 1 a using the higher-energy light resulted in the generation of the higher singlet excited state with τ≤25 fs, from which intersystem crossing proceeded to give the higher triplet state (3HES(1)). In THF, 3HES(1) was competitively converted to both the triplet ligand field (3LF) and metal-to-ligand charge transfer (3mLCT) with lifetimes of 200 fs, in which the former is a reactive state that converts to [Re(bpy)(CO)2Cl(thf)]+ (1 c) within 10 ps by means of a dissociative mechanism. Re-coordination of CO to 1 c gives both 1 a and 1 b. In MeCN, irradiation of 1 a by using high-energy light gives the coordinatively unsaturated complex, which rapidly converted to 2 c. A seven-coordinate complex is also produced within several hundred femtoseconds, which is converted to 2 a within several hundred picoseconds.