Akira Itaya

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 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 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.

Fluorescence dynamics of poly(N-vinylcarbazole) in solution as revealed by multicomponent analysis of picosecond time-resolved fluorescence spectra: dependence on tacticity and molecular weight

The steady-state fluorescence spectra of poly(N-vinylcarbazole) (PVCz) with various molecular weights in tetrahydrofuran solution were examined. The spectra of PVCz(r) prepared by radical polymerization were almost independent of the molecular weight, while those of PVCz(c) prepared by cationic polymerization were dependent on it. The difference in the temperature dependence of the spectra between PVCz(r) and PVCz(c) with high molecular weights was found. The time-resolved fluorescence spectra of PVCz(r) and PVCz(c) with high and low molecular weights were analysed by using a principal multicomponent spectral estimation method, whose results indicate clearly that the spectra were composed of only three component fluorescences (monomer, partial overlap excimer and sandwich excimer). The spectra were analysed by means of a least-squares fitting method using the three component spectral shapes, giving rise and decay curves of these component fluorescences. The curves were non-exponential and depended upon tacticity and the molecular weight. On the basis of these results, the excimer dynamics of PVCz in solution was discussed.

AFM observation of silk fibroin on mica substrates: morphologies reflecting the secondary structures

Bombyx mori silk fibroin was fixed on mica substrates by cast of aqueous fibroin solutions, and the microscopic morphologies of the samples were revealed by means of atomic force microscopy. By adjusting the method used to prepare the solution, we succeeded in forming quasi-2-dimensional thin films in which a network of fibroin molecules developed over the substrate. The film network consisted of fibroin in a random coil structure. The morphology of the network changed after thermal or methanol treatments, which are known to convert the secondary structure of fibroin from the random coil to the β-sheet type. In both of these cases, the network morphology disappeared and characteristic island-like morphologies appeared. On the other hand, temporally evolving gelation occurred in a fibroin solution due to the formation of β-sheet crystals. Such islands were also observable in a specimen prepared by the cast of the gel-containing solution. Based on these results, it was concluded that the islands consist of β-sheet crystals. Of particular interest is the observation that all of the islands had a common thickness value of 1.3 nm. These morphologies are discussed in terms of the secondary structure of fibroin.

Photon statistics in enhanced fluorescence from a single CdSe/ZnS quantum dot in the vicinity of silver nanoparticles

Single-photon emission behavior in the enhanced fluorescence from single colloidal quantum dots (QDs) near silver nanoparticles (AgNPs) was investigated using a single molecule fluorescence spectroscopy technique. It was found that the degree of fluorescence enhancement from single QDs with AgNPs increased with decrease in the lifetime and the probability of single-photon emission, that is, highly enhanced fluorescence with a shortened lifetime exhibited a low probability of single-photon emission. The present results yield new insights into fundamentals of QD-metal nanostructure interactions, and are also important to understand the mechanism of the fluorescence enhancement by localized surface plasmon of metal nanostructures.

Singlet Excitation Energy Transfer in the Vinyl Polymers with Pendant Carbazolyl Groups

The triplet energy transfer in poly(N-vinylcarbazole)(PVCz) prepared by radical and cationic polymerization (PVCz(r) and PVCz(c) respectively), poly[2-(N-carbazolyl)ethyl vinyl ether](PCzEVE), and brominated PVCz (BPVCz) has been investigated by measuring the delayed emission spectra in solid films and rigid solutions at 77 K. The prompt and delayed fluorescence spectra both in the solid film and in the rigid solution at 77 K were different for PVCz(r) and PVCz(c). This seems to be due to the difference in tacticity between them. PCzEVE, in which the Cz groups are connected from afar to the skeletal chains by means of –O–CH2–CH2–bonds, showed the delayed fluorescence and the phosphorescence both in the rigid solutions and in the solid films at 77 K. The BPVCz film showed only the excimer phosphorescence. In the solid films of PVCz(r), PVCz(c), and PCzEVE, the Cz chromophore-delayed fluorescence increased with an increase in the concentration of triplet quenchers, such as naphthalene and fluorene. This phe...

Excimer dynamics of poly(n-vinylcarbazole) films revealed by time-correlated single photon counting measurements

Abstract Precise rise and decay curves of excimer emissions of poly (N-vinylcarbazole) films in the picosecond time region have been measured by using a time-correlated single photon counting technique. The partial overlap excimer fluorescence was observed immediately after excitation, while the sandwich excimer fluorescence showed delayed formation processes. These results were quite different from those reported before, discussed in terms of high intensity excitation and photo-oxidation effects.

Pulsed laser deposition of collagen and keratin

Pulsed laser deposition of collagen and keratin, which are representative fibrous proteins, were investigated in terms of laser wavelength and fluence. Chemical structure and surface morphology of thin deposited films were analyzed by means of infrared (IR) spectroscopy and atomic force microscopy (AFM), respectively. For the couple of proteins, IR spectra of the films deposited under some irradiation conditions were almost similar to those of starting targets. This means that a large part of primary structure of the proteins was maintained in the thin deposited films. Also the secondary structures in the thin films were discussed briefly. The random-coil domains increased both for collagen and keratin. Thus we have demonstrated that laser lights are capable of preparing thin films of proteins.

Pulsed laser deposition of silk protein: Effect of photosensitized-ablation on the secondary structure in thin deposited films

Silk fibroin is a simple protein expected to have functional applications in medicine and bioelectronics. The primary structure of this protein is quite simple, and the main secondary structures are β-sheet crystals and amorphous random coils. In the present study, we investigated pulsed laser deposition (PLD) of fibroin with the β-sheet structures as targets. The primary and secondary structures in films deposited were analyzed using infrared spectroscopy. Normal laser deposition at 351 nm using neat fibroin targets produced thin films of fibroin with a random coiled structure. Ablation was triggered by two-photonic excitation of the peptide chains, which resulted in the destruction of β-sheet structure in PLD. In order to avoid the two-photonic excitation, we adopted a PLD method utilizing anthracene (5–0.1 wt %) in a photosensitized reaction involving doped fibroin targets. Laser light (351 or 355 nm) was absorbed only by anthracene, which plays an important role converting photon energy to thermal ene...