Koichi Iwata

Conjugated polymers incorporated with cycloplatinated complexes (P1-Pt and P2-Pt) were used as dispersants for single-walled carbon nanotubes (SWCNTs). Significant changes in the UV-vis absorption spectra were observed after the formation of the polymer/SWCNT hybrids. Molecular dynamics (MD) simulations revealed the presence of a strong interaction between the cycloplatinated complex moieties and the SWCNT surface. The photoinduced electron transfer processes in these hybrids were strongly dependent on the type of the comonomer unit. Upon photoexcitation, the excited P1-Pt donates electrons to the SWCNT, while P2-Pt accepts electrons from the photoexcited SWCNT. These observations were supported by results from Raman and femtosecond time-resolved transient absorption spectroscopy experiments. The strong electronic interaction between the Pt complexes and the SWCNT gives rise to a new hybrid system that has a controllable photoinduced electron transfer flow, which are important in regulating the charge transport processes in SWCNT-based optoelectronic devices.

The excited-state dynamics of pyrene incorporated into poly(substituted methylene)s is investigated by picosecond time-resolved fluorescence spectroscopy and femtosecond time-resolved near-IR absorption spectroscopy in the 900-1400 nm region. The pyrene rings in poly (substituted methylene)s are photoexcited to the monomer excited state immediately after UV irradiation, followed by prompt excimer formation with time constants of a few picoseconds to a few hundred picoseconds. The excimer formation in poly(substituted methylene)s proceeds with much shorter time constants than that in pyrene-incorporated polyacrylates, vinyl polymer counterparts with the same side-chain structures, indicating the presence of stronger electronic interaction between the pyrene rings in poly(substituted methylene)s. The effects of every methylene substitution hold when each pyrene ring is connected to the polymer backbone with a monomethylene linker, while the effects are observed only weakly when a tetramethylene linker is employed. The results demonstrate the effectiveness of every methylene substitution in the prompt excimer formation of pyrene connected to the polymer backbone either directly or with the monomethylene linker.

Novel regioisomeric alkylated-naphthalene liquids were designed and synthesized. In the solvent-free liquid state, 1-alkyloxy regioisomers showed excimeric luminescence, whereas 2-alkyloxy analogues exhibited monomer-rich luminescence features. Correlations among the molecular structures and the photophysical, calorimetric, and rheological properties are presented, demonstrating the impact of regioisomerism on the alkylated-chromophore liquid systems.

Novel methodologies utilizing pulsed or intense CW irradiation obtained from lasers have a major impact on biological sciences. In this article, recent development in biophysical researches fully utilizing the laser irradiation is described for three topics, time-resolved fluorescence spectroscopy, time-resolved thermodynamics, and manipulation of the biological assemblies by intense laser irradiation. First, experimental techniques for time-resolved fluorescence spectroscopy are concisely explained in Section 2. As an example of the recent application of time-resolved fluorescence spectroscopy to biological systems, evaluation of the viscosity of lipid bilayer membranes is described. The results of the spectroscopic experiments strongly suggest the presence of heterogeneous membrane structure with two different viscosity values in liposomes formed by a single phospholipid. Section 3 covers the time-resolved thermodynamics. Thermodynamical properties are important to characterize biomolecules. However, measurement of these quantities for short-lived intermediate species has been impossible by traditional thermodynamical techniques. Recently, development of a spectroscopic method based on the transient grating method enables us to measure these quantities and also to elucidate reaction kinetics which cannot be detected by other spectroscopic methods. The principle of the measurements and applications to some protein reactions are reviewed. Manipulation and fabrication of supramolecues, amino acids, proteins, and living cells by intense laser irradiation are described in Section 4. Unconventional assembly, crystallization and growth, amyloid fibril formation, and living cell manipulation are achieved by CW laser trapping and femtosecond laser-induced cavitation bubbling. Their spatio-temporal controllability is opening a new avenue in the relevant molecular and bioscience research fields. This article is part of a Special Issue entitled “Biophysical Exploration of Dynamical Ordering of Biomolecular Systems” edited by Dr. Koichi Kato.

The electronic and vibrational relaxation of carotenoids is one of the key processes in the protection of living cells as well as in the functions of proteins involved in photosynthesis. In this study, the electronic and vibrational relaxation dynamics of b-carotene and its derivatives with substituents on the terminal rings is investigated using femtosecond time-resolved absorption and stimulated Raman spectroscopy in the near-IR region. The carbonyl substituent induces low-frequency shifts of the steady-state and transient absorption bands, decreases of the excited-state lifetimes and the acceleration of vibrational relaxation of the conjugated main chain, whereas the hydroxyl substituent only slightly affects them. The effects of the carbonyl group in the electronic relaxation dynamics are explained well by the lengthening of effective conjugation by the carbonyl group through a partial conjugation between the main chain and the terminal ring. Time-resolved near-IR stimulated Raman spectroscopy demonstrates the significance of the peripheral substitution with the carbonyl group for the vibrational energy relaxation of b-carotene derivatives in the lowest excited singlet state.

We investigated the fluorescence properties of dialkoxyphenyl-pyrene molecules experimentally as well as theoretically. Our experiments confirmed fluorescence solvatochromism in 2,5-dimethoxyphenylpyrene and, in contrast there was no significant solvent-effect on the emission properties of the isomers, 3,5-and 2,6-dimethoxyphenyl-pyrene. This clear difference in the solvent-dependence would reflect the difference in character of the excited-state between the isomers, which differ only in the substitution positions of the two methoxy groups. The positional effects of the di-substituted molecules are successfully explained theoretically by the topologies of the highest occupied molecular orbital of the phenyl group that are governed by the relative positions of the two substituents, though it is somewhat contradictory to the meta-effect for the mono-substituted molecules. Theoretical calculations were also used to analyze the character of the excited states; 2,5-dimethoxyphenyl-pyrene alone exhibited an intramolecular charge transfer character for the excited state, which was responsible for the solvatochromism effect. The dynamics of the excited states were analyzed using time-resolved fluorescence measurements, in which a characteristic increase of the fluorescence intensity was observed for 2,5-dialkoxyphenyl-pyrene; this observation was supported by the theoretical calculations as well.

A morphology transformation of hybrid liposomes was shown to occur from spherical vesicles to tubular micelles when increasing the ratio of the metal complex lipid present. Phase transition temperatures increased while viscosities decreased, indicating that the hybrids exhibit stronger interaction between heads but weaker interaction between alkyl chains than occurs in pristine liposomes.

Two series of new 3d-4f heterodinuclear [Zn(II)Ln(III)] Schiff base complexes of general formula [Zn(mu-L1)(mu-CH3COO)Ln(NO3)(2)(S)] [Ln = Pr (1), Nd (2), Sm (3), Gd (4); S = MeOH; H(2)L1 = N,N-bis(3-methoxysalicylidene)-1,4-diaminobutane] and [Zn(mu-L-2)(-CH3COO)Ln(NO3)(2)(S)] [Ln = Pr (5), Nd (6), Sm (7), Gd (8), S= MeOH; H(2)L2 = N,N-bis(3-ethoxysalicylidene)-1,4-diaminobutane] are synthesized and fully characterized. Complexes 1, 2, 3, 4, 5 &7 are structurally characterized by single crystal X-ray crystallography. The crystallographic investigation indicates that the complexes contain 9 and 10-coordinated Ln(III) ions, while the Zn-II ions always display a distorted square-pyramidal geometry. The crystal structure of the complexes are stabilized by inter molecular extended hydrogen bonding and C-H....pi interactions resulting in supramolecular frameworks. Luminescence studies for the heterodinuclear compounds containing Nd-III & Sm-III reveal that the Zn-complex moiety acts as antenna for the emission from Ln(III) ions. In addition to the ligand-centered emission in the UV-Vis region, complex 2 (having Nd-III) exhibits emission in NIR region also, thus resulting a new NIR emitting material.

Solvent-free, nonvolatile, room-temperature alkylated-pi functional molecular liquids (FMLs) are rapidly emerging as a new generation of fluid matter. However, precision design to tune their physicochemical properties remains a serious challenge because the properties are governed by subtle pi-pi interactions among functional pi-units, which are very hard to control and characterize. Herein, we address the issue by probing pi-pi interactions with highly sensitive pyrene-fluorescence. A series of alkylated pyrene FMLs were synthesized. The photophysical properties were artfully engineered with rational modulation of the number, length, and substituent motif of alkyl chains attached to the pyrene unit. The different emission from the excimer to uncommon intermediate to the monomer scaled the pyrene-pyrene interactions in a clear trend, from stronger to weaker to negligible. Synchronously, the physical nature of these FMLs was regulated from inhomogeneous to isotropic. The inhomogeneity, unexplored before, was thoroughly investigated by ultrafast time-resolved spectroscopy techniques. The result provides a clearer image of liquid matter. Our methodology demonstrates a potential to unambiguously determine local molecular organizations of amorphous materials, which cannot be achieved by conventional structural analysis. Therefore this study provides a guide to design alkylated-pi FMLs with tailorable physicochemical properties.

Charge transfer and charge delocalisation processes play key roles in the functions of large biomolecular systems and organic/inorganic devices. Many of the short-lived transients involved in these processes can be sensitively detected by monitoring their low-energy electronic transitions in the near-IR region. Ultrafast time-resolved near-IR Raman spectroscopy is a promising tool for investigating the structural dynamics of the short-lived transients as well as their electronic dynamics. In this study, we have developed a femtosecond time-resolved near-IR multiplex stimulated Raman spectrometer using the Raman pump pulse at 1190 nm and a broadband probe pulse covering the 900-1550 nm region. Spectral and temporal instrument responses of the spectrometer are estimated to be 5 cm(-1) and 120 fs, respectively. Time-resolved near-IR stimulated Raman spectra of poly(3-dodecylthiophene) (P3DDT) are recorded in toluene solution for investigating its structural changes following the photoexcitation. The spectra strongly indicate conformational changes of P3DDT in excited states associated with the elongation of its effective conjugation length. The results on P3DDT fully demonstrate the effectiveness of the newly developed femtosecond time-resolved near-IR stimulated Raman spectrometer.

The dynamic motions of solvent molecules in the solvation shell of phenols in binary water-ethanol solutions have been examined experimentally by measuring the solvent relaxation times using phase-fluorometry. The results show that the mono-exponential character of the solvent relaxation changes to bi-exponential just after the ethanol content of the bulk solution exceeds a critical value. This value depends on the electron density of the aromatic rings of the phenolic molecules. Above the critical values much higher ethanol content is observed in the solvation shell compared to the bulk solutions. This observation is applicable in wide scale of chemical processes in condensed phase and indicates that further research is needed towards development of a picture at the atomic scale.

Nanosecond time-resolved infrared spectra of photoexcited 9,9'-bianthryl (BA) and its deuterated derivative (BA-d(18)) were recorded in acetonitrile-d(3) and cyclohexane-d(12). The charge-transfer (CT) bands observed at the 1200-1400 cm(-1) region are contributed mostly by the intra- and inter-ring CC stretch vibrations. The intrinsic frequency of the inter-ring CC stretch falls within the 1200-1400 cm(-1) region, which is consistent more with a twisted structure than with a fully conjugated planar structure of the CT state.

Vibrational cooling rate of the first excited singlet (S1) state of trans-stilbene and bulk thermal diffusivity are measured for seven room temperature ionic liquids, C2mimTf2N, C4mimTf2N, C4mimPF6, C5mimTf2N, C6mimTf2N, C8mimTf2N, and bmpyTf2N. Vibrational cooling rate measured with picosecond time-resolved Raman spectroscopy reflects solute-solvent and solvent-solvent energy transfer in a microscopic solvent environment. Thermal diffusivity measured with the transient grating method indicates macroscopic heat conduction capability. Vibrational cooling rate of S1 transstilbene is known to have a good correlation with bulk thermal diffusivity in ordinary molecular liquids. In the seven ionic liquids studied, however, vibrational cooling rate shows no correlation with thermal diffusivity; the observed rates are similar (0.082 to 0.12 ps-1 in the seven ionic liquids and 0.08 to 0.14 ps-1 in molecular liquids) despite large differences in thermal diffusivity (5.4-7.5 x 10-8 m2 s-1 in ionic liquids and 8.0-10 x 10-8 m2 s-1 in molecular liquids). This finding is consistent with our working hypothesis that there are local structures characteristically formed in ionic liquids. Vibrational cooling rate is determined by energy transfer among solvent ions in a local structure, while macroscopic thermal diffusion is controlled by heat transfer over boundaries of local structures. By using " local" thermal diffusivity, we are able to simulate the vibrational cooling kinetics observed in ionic liquids with a model assuming thermal diffusion in continuous media. The lower limit of the size of local structure is estimated with vibrational cooling process observed with and without the excess energy. A quantitative discussion with a numerical simulation shows that the diameter of local structure is larger than 10 nm. If we combine this lower limit, 10 nm, with the upper limit, 100 nm, which is estimated from the transparency (no light scattering) of ionic liquids, an order of magnitude estimate of local structure is obtained as 10 nm < L < 100 nm, where L is the length or the diameter of the domain of local structure. c 2012 American Institute of Physics. [ http:// dx. doi. org/ 10.1063/ 1.3691839]

Picosecond time-resolved visible absorption and Raman spectra of trans-stilbene in acetonitrile have been measured. A new transient absorption band is observed at 440 nm, whose decay time constant coincides with that of the rise of the radical cation. In addition to the known Raman bands of the radical cation, a new Raman feature showing a fast decay is found at 1599 cm(-1). We assign the 440 nm absorption band and the 1599 cm(-1) Raman band to a precursor of the radical cation. The precursor may well be an ion pair that holds an ejected electron still around the cation. (C) 2012 Elsevier B. V. All rights reserved.

2-Methylresorcinarene and its methylene-bridged cavitand derivative as host compounds were investigated in selective complexation of alkali metal ions as guests in methanol media by photoluminescence measurements. These host molecules possess either flexible (2-methylresorcinarene) or rigid (cavitand) molecular skeleton. The Benesi-Hildebrand method and the van't Hoff theory have been applied to determine the stability constants and the thermodynamic parameters, respectively. Considerable interactions between 2-methylresorcinarene and Li+ or Na+ ions have been observed while the rigid cavitand derivative can interact only with K+ or Cs+ ions. Neither the complexes of 2-methylresorcinarene with K+ or Cs+ nor those of the cavitand derivative with Li+ or Na+ ions are stable at room temperature in methanol media. Quantum-chemical investigations justified that only solvated Li+ and Na+ ions can form stable complexes with 2-methylresorcinarene while unsolvated K+ and Cs+ ions form stable complexes with the methylene-bridged cavitand. These results highlight that the stability of the guest solvation shell and its size could play a key role in the selectivity behaviour of host molecules.

Viscosity inside the lipid bilayer of egg-PC (egg yolk phosphatidylcholine) liposome with a diameter of 100 nm is examined with picosecond time-resolved fluorescence spectroscopy. The viscosity is estimated independently from the photoisomerization rate and from the rotational relaxation time of the first excited singlet state of trans-stilbene solubilized within the lipid bilayer. The presence of two solvation environments within the bilayer is suggested from both of the methods. One environment is 50 to 100 times more viscous than the other. The use of trans-stilbene as a probe provides valuable information on the solvation environments inside the lipid bilayer of liposome, which serves as a field for a number of biochemical reactions.

The interaction of phenol guest molecules with 2-methylresorcinarene and its methylene-bridged cavitand derivative has been investigated in methanol. The host molecules were selected according to the flexibility of their cavities by varying the conformational freedom of the molecular skeleton prior to molecular association. The results show stronger host-phenol interactions when the host molecule possesses a rigid molecular skeleton (i.e., cavitand) compared to that of the flexible resorcinarene with phenol. Although the enthalpy change associated with the molecular interactions was found to be the same in both cases, higher negative entropy change was obtained when the resorcinarene interacted with the phenol molecules at room temperature. As a result, stronger host-guest complexes are formed at room temperature when the host molecules, possessing a rigid molecular skeleton, participated in the complex formation. Furthermore, since the higher entropy change results in higher temperature-dependence of the interactions, the stability of the complexes formed with the flexible resorcinarene is smaller at higher temperature. These results highlight that the decreasing flexibility of the host molecular skeleton itself can determine the entropy change during the complexation process; therefore, the temperature dependence of the complex stabilities highly depends on the flexibility of the host's molecular skeleton. This information might contribute to the development of selective and sensitive sensor molecules toward phenol derivatives.

Transient absorption spectra of 9,9'-bianthryl (BA) in heptane, in acetonitrile, and in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (bmimTf(2)N) are observed with a nanosecond time-resolved near-IR absorption spectrometer for the wavenumber range of 4500-10 500 cm(-1) (2200-950 nm). In nonpolar heptane solution, a broad absorption band is observed at 6700 cm(-1)(1500 urn), in addition to a strong absorption band of the locally excited (LE) state centered at 9800 cm(-1) (1020 nm). The broad band is assigned to a partial charge transfer (PCT) band. The decay time constants of the PCT band and the LE band are both (13 +/- 1) ns. The agreement of the two decay constants strongly suggests that the PCT state is in equilibrium with the LE state in heptane. In acetonitrile, an absorption band of the charge transfer (CT) state is observed at 8000 cm(-1) (1250 nm). This band decays in (41 +/- 2) ns. In bmimTf(2)N, the CT band appears at 8500 cm(-1) (1180 nm) and decays in (34 +/- 1) ns. The difference in peak position for the CT bands in acetonitrile and in bmimTf(2)N, and the PCT bands in heptane, is explained well by the model based on the charge resonance between the two equivalent electronic structures of the CT state.

The formation of the solvatation shell of anthracene molecule was investigated in binary mixtures of protic alcoholic solvents by solvent-relaxation measurements, anisotropy decay studies and quantum-chemical modeling. Results show significant change of the solvatation shell of anthracene dissolved in binary solutions of methanol, ethanol or n-propanol with n-butanol when the molar fraction of n-butanol in the bulk solution exceeds a critical value. Both the solvent relaxation and anisotropy decay data suggest clusterization process in the solvatation shell of the anthracene probe. Quantum chemical calculation supports formation of energetically preferred heteromolecular (X(2)(BuOH)(2), X = MeOH, EtOH or PrOH) clusters around the solute. (c) 2009 Elsevier B.V. All rights reserved.

The rate of vibrational energy relaxation (VER) is a valuable probe of microscopic intermolecular interaction. In the present article, the VER rate of the T-1u CO stretch of W(CO)(6) is measured in binary solutions composed of ail alkane (hexane, decane, tetradecane) and cyclohexane or CCl4, with seven mixing volume ratios. The volume ratio dependence indicates that, among the three alkane-cyclohexane solutions, only decane shows ail exceptionally strong contribution to the VER process. It is strongly suggested that decane forms a specific solute-solvent complex with W(CO)(6). The "V-shaped" dependence of the VER rate on the number of carbons in alkane solvents (Banno, M.; Sato, S.; Iwata, K.; Hamaguchi, H. Client. Phys. Lett. 2005, 412, 464.) is explained by the solute-solvent complex model. There are no experimental results that suggest the formation of the specific solute-solvent complex in alkane-CCl4 solutions.

Femtosecond time-resolved absorption anisotropy spectroscopy by multichannel detection has been developed. The charge transfer (CT) character and dynamics of the UV-photoexcited 9,9(')-bianthryl (BA) in heptane, acetonitrile, and ethanol are revealed with this method. The transient absorption spectra are decomposed into two absorption components with different anisotropy values by the absorption anisotropy spectra. The decomposition results show two absorption bands having different anisotropy values or different directions of the transition dipole moment. One band that has the transition dipole perpendicular to the central C-C bond has almost an identical spectral shape with transient absorption of anthracene in the singlet excited state. This band is assigned to a transition in a locally excited anthracene ring. The other band is broad and structureless. This band is assigned to partial charge transfer (PCT) absorption because its transition dipole moment is parallel to the central C-C bond. Because the PCT band is observed in a nonpolar solvent heptane as well as in polar solvents, the PCT occurs in both nonpolar and polar solvents. The PCT band rises within the instrumental response, indicating that the PCT takes place immediately after the photoexcitation. In acetonitrile, the CT component shows a significant blueshift, indicating the formation of the stabilized CT state from the PCT state. In ethanol, the CT band does not show a spectral shift, suggesting that the stabilization is smaller than in acetonitrile. From these results, a new kinetic model on the intramolecular CT in BA is discussed.