Alignment of fibrous J-aggregates and the resulting macroscopic optical anisotropy observed in static solution.

J-aggregates, which are supramolecular assemblies that exhibit unique optical properties owing to their excitonic interactions, have potential applications in artificial light-harvesting systems and fluorescence biosensing. Although J-aggregates are formed in solution, in situ observations of their structures and behaviors in solution remain scarce. In this study, we investigated the J-aggregates of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate [DiIC18(3)] in methanol/water (M/W) binary solvents using fluorescence imaging as well as polarized absorption and fluorescence measurements to explore the relationship between their structure and macroscopic optical properties under static conditions. Fluorescence images revealed that the DiIC18(3) J-aggregates have fibrous structures in the M/W = 44/56 (v/v) binary solvent. We measured the polarization-angle dependence of the fluorescence intensity of the fibrous J-aggregates to determine the direction of their transition dipole moment. Furthermore, the J-band absorbance was dependent on the polarization angle of the linearly polarized incident light, even in the absence of an external force such as that generated by a flow or stirring, indicating that the J-aggregates "spontaneously" aligned in solution. We also monitored the time evolution of the degree of alignment of the fibrous J-aggregates, which revealed that the formation and elongation of the fibers induced their alignment, resulting in the observed macroscopic optical anisotropy in solution.

Charge-Dependent Metastable Dissociations of Multiply Charged Decafluorobiphenyl Formed by Femtosecond Laser Pulses.

Femtosecond laser ionization is a unique means to produce multiply charged organic molecules in the gas phase. The charge-dependent chemical reactions of such electron-deficient molecules are interesting from both fundamental and applied scientific perspectives. We have reported the production of quadruply charged perfluoroaromatics; however, they were so stable that we cannot obtain information about their chemical reactions. In general, it might be difficult to realize the conflicting objectives of observing multiply charged molecular ion themselves and their metastable dissociations. In this study, we report the first example showing metastable dissociations of several charge states within the measurable time range of a time-of-flight mass spectrometer. Metastable dissociations were analyzed by selecting a precursor ion with a Bradbury-Nielsen ion gate followed by time-of-flight analysis using a reflectron. We obtained qualitative information that triply and quadruply charged decafluorobiphenyl survived at least in the acceleration region but completely decomposed before entering a reflectron. In contrast, three dissociation channels for singly and one for doubly charged molecular ions were discriminated by a reflectron and determined with the help of ion trajectory simulations.

Smallest Organic Tetracation in the Gas Phase: Stability of Multiply Charged Diiodoacetylene Produced in Intense Femtosecond Laser Fields.

Coulomb explosion imaging, which is the reconstruction of a molecular structure by measuring the three-dimensional momenta of atomic ions formed by a Coulomb explosion of multiply charged molecular cations (MMCs), has been utilized widely. In contrast, intact MMCs, whose properties and reactions are interesting from both fundamental and applied scientific perspectives, themselves have been little explored to date. This study demonstrates that the four-atom molecule diiodoacetylene (DIA) can survive as a long-lived species in the gas phase after the removal of four electrons in intense femtosecond laser fields. The electron configurations of the equilibrium structures of the electronic ground states calculated by the complete active space self-consistent field (CASSCF) method reveal the stability of multiply charged DIA. The dissociation energies are estimated to be 3.01, 3.59, 2.57, 1.82, and 1.61 eV for neutral, cation radical, dication, trication radical, and tetracation, respectively. A fairly deep potential well suggests that a DIA tetracation is metastable toward dissociation, whereas the repulsive potential of a pentacation radical confirms its absence in the mass spectrum. With their sufficiently long lifetimes, minimum number of atoms, and simple dissociation paths, DIA MMCs are promising candidates for further experimental and theoretical investigations of multiply charged ion chemistry.

Charge Transfer and Metastable Ion Dissociation of Multiply Charged Molecular Cations Observed by Using Reflectron Time-of-Flight Mass Spectrometry.

The front cover artwork is provided by the group of Prof. Tomoyuki Yatsuhashi (Osaka City University) as well as Dr. Akimasa Fujihara (Osaka Prefecture University). The image shows that the potential energy of a product ion formed by metastable ion dissociation can be larger than that of a multiply charged precursor ion. Read the full text of the Article at 10.1002/cphc.202000021.

Charge Transfer and Metastable Ion Dissociation of Multiply Charged Molecular Cations Observed by Using Reflectron Time-of-Flight Mass Spectrometry.

A multiply charged molecule expands the range of a mass window and is utilized as a precursor to provide rich sequence coverage; however, reflectron time-of-flight mass spectrometer has not been well applied to the product ion analysis of multiply charged precursor ions. Here, we demonstrate that the range of the mass-to-charge ratio of measurable product ions is limited in the cases of multiply charged precursor ions. We choose C6 F6 as a model molecule to investigate the reactions of multiply charged molecular cations formed in intense femtosecond laser fields. Measurements of the time-of-flight spectrum of C6 F6 by changing the potential applied to the reflectron, combined with simulation of the ion trajectory, can identify the species detected behind the reflectron as the neutral species and/or ions formed by the collisional charge transfer. Moreover, the metastable ion dissociations of doubly and triply charged C6 F6 are identified. The detection of product ions in this manner can diminish interference by the precursor ion. Moreover, it does not need precursor ion separation before product ion analysis. These advantages would expand the capability of mass spectrometry to obtain information about metastable ion dissociation of multiply charged species.

Synthesis of Single-Nanometer-Sized Gold Nanoparticles in Liquid-Liquid Dispersion System by Femtosecond Laser Irradiation.

Gold nanoparticles (AuNPs) show unique optical properties and catalytic activities, and their synthesis from gold ions has been widely studied. One of the additive-reagent-free and noncontact production procedures is the reduction of gold ions in solution by femtosecond laser pulses; however, the aggregation of AuNPs is unavoidable in homogeneous solution. Here, we report the synthesis of single-nanometer-sized AuNPs in a mixture of aqueous HAuCl4 solution and n-hexane (the mixture) and in aqueous HAuCl4 solution (the aqueous solution) by femtosecond laser irradiation in the absence of any additive reagents. Transmission electron microscopy revealed that circlelike colonies consisting of well-separated AuNPs were obtained from the mixture, while highly stacked and agglomerated AuNPs were obtained from the aqueous solution. The mean size of AuNPs in the mixture was nearly independent of the laser irradiation time, whereas that obtained in aqueous solution was gradually shifted to smaller size by laser irradiation period. We propose that the adsorption of primary AuNPs on the surface of hexane microdroplets and the fragmentation of large AuNPs in water by successive laser pulses retain single-nanometer-sized AuNPs in the mixture. The use of liquid-liquid interface on hexane microdroplets in aqueous solution provides a simple and useful environment to synthesize small AuNPs without the aid of surfactants or capping agents.

Synthesis of Bare Iron Nanoparticles from Ferrocene Hexane Solution by Femtosecond Laser Pulses.

Iron-based nanoparticles (FeNPs) have unique and attractive properties such as superparamagnetism, biocompatibility, and catalytic activity. Although the synthesis of precious metal NPs from a metal in liquid and/or metal salt solution by a pulsed laser has been investigated, comparably little effort has been devoted to examine the production of FeNPs. Here we report the synthesis of carbon-shell free spherical NPs of iron oxide (magnetite) from ferrocene hexane solution by femtosecond near infrared laser pulses. Nanosecond UV laser pulses are used to compare the evolution of the particle size distribution as a function of laser irradiation time. The size of NPs remains constant even for extended exposure to femtosecond laser pulses, whereas it grows with exposure to nanosecond laser pulses. The primary particles are generated by photochemical reactions regardless of pulse duration; however, the fragmentation of NPs by successive femtosecond laser pulses regulates the particle size.

Selection of a Single Isotope of Multiply Charged Xenon (A Xez+ , A=128-136, z=1-6) by Using a Bradbury-Nielsen Ion Gate.

The inclusion of an ion gate in a tandem mass spectrometer allows a specific precursor ion to be selected, and the fragment ions are then used for structure analysis and to investigate chemical reactions. However, the performance of an ion gate has been judged simply by whether or not the target ion was selected. In this study, we designed, manufactured, constructed, and characterized a Bradbury-Nielsen ion gate (BNG). The actual ion selection ability, i.e. the gate function, of the BNG was measured for isotopes of Xez+ (z=1-6). The gate function of the BNG was 36.5±0.5 ns in width and 3-13 ns in rise and fall times. The BNG provides a simple way to select multiply charged molecular cations of small organic molecules as well as large molecules such as proteins and peptides.

Synthesis of Fluorine-Doped Hydrophilic Carbon Nanoparticles from Hexafluorobenzene by Femtosecond Laser Pulses.

We report on the preparation and characterization of fluorine-doped hydrophilic carbon nanoparticles by the exposure of hexafluorobenzene or a water/hexafluorobenzene bilayer solution to femtosecond laser pulses. Uniform atom distributions are achieved not only on the particle surface but also inside the particles. The semi-ionic character of C-F bonds and the non-aggregating feature of the nanoparticles play key roles in the water-dispersible character of fluorine-doped carbon nanoparticles. We suggest the following building-up process of carbon nanoparticles: the fragmentation of hexafluorobenzene initiated by the electrons generated in laser-induced plasma followed by the reconstruction of a carbon framework of nanoparticles.

Anisotropic Coulomb Explosion of CO Ligands in Group 6 Metal Hexacarbonyls: Cr(CO)6, Mo(CO)6, W(CO)6.

Multiple ionization and subsequent Coulomb explosion have been studied for many organic molecules and their clusters; however, the metal complexes, particularly the large Coulombic interactions expected between a metal and its ligands, have not yet been explored. In this study, the angular distribution of CO(+), oxygen, and carbon ions ejected from metal hexacarbonyls (M(CO)6, M: Cr, Mo, W) having Oh symmetry by Coulomb explosion in femtosecond laser fields (>1 × 10(14) W cm(-2)) is investigated. The emissions of oxygen ions are well-explained in terms of the geometric alignment along a line inclined 45° relative to the CO-M-CO axis in a M(CO)4 plane. Unlike the explosion behavior of the oxygen ions located on the outer part of the molecule, the explosion behavior of the carbon ions was affected by the laser intensity, kinetic energy, and metal. This finding that the emission trends of carbon sandwiched between oxygen and metal atoms were the opposite of those for oxygen was explained by the obstruction by oxygen, the deformation of structure in bending coordinates, and the strong interaction with charged metal. The anisotropic Coulomb explosion of metal complexes reflecting their structural symmetry and central metal charge is a promising candidate for use in the investigation of large Coulombic interactions at the molecular level.

Intact Four-atom Organic Tetracation Stabilized by Charge Localization in the Gas Phase.

Several features distinguish intact multiply charged molecular cations (MMCs) from other species such as monocations and polycations: high potential energy, high electron affinity, a high density of electronic states with various spin multiplicities, and charge-dependent reactions. However, repulsive Coulombic interactions make MMCs quite unstable, and hence small organic MMCs are currently not readily available. Herein, we report that the isolated four-atom molecule diiodoacetylene survives after the removal of four electrons via tunneling. We show that the tetracation remains metastable towards dissociation because of the localization (91-95 %) of the positive charges on the terminal iodine atoms, ensuring minimum Coulomb repulsion between adjacent atoms as well as maximum charge-induced attractive dipole interactions between iodine and carbon. Our approach making use of iodines as the positively charged sites enables small organic MMCs to remain intact.

Reduction of Yb(III) to Yb(II) by two-color two-photon excitation.

Ytterbium 3+ ions in alcohol were found to be reduced to the corresponding 2+ ions upon laser irradiation with a stepwise two-color two-photon excitation. The infrared (975-nm) pulse with a duration of 4 ns pumps the ground state to the 4f excited state with the transition of (2)F(5/2) ← (2)F(7/2), and the second photon (355-nm) generates the charge transfer (CT) state of Cl 3p to Yb 4f; the reduction then occurs. Laser energy and excitation wavelength dependencies well-explain the above mechanism. The product Yb(2+) was detected by its absorption spectrum peak at 367 nm. The absorption spectrum of the intermediate in the two-photon chemistry was measured from the 4f excited state ((2)F(5/2)) to the CT state by nanosecond laser photolysis. The intermediate spectrum appears in the wavelengths shorter than 400 nm with the molar extinction coefficient on the order of (10(2) M(-1) cm(-1)) at 340 nm and can be explained in terms of the CT absorption shifted by IR photon energy. A UV nanosecond laser pulse (266 nm from a YAG laser with a duration of 6 ns) can generate the reactive CT state by one-photon absorption and leads to Yb(2+) formation. The reaction yields for single-photon UV excitation and the second photon in the two-photon excitation are on the order of 0.1, suggesting that the reactive states are a common CT state.

Coulomb explosion of dichloroethene geometric isomers at 1 PW cm(-2).

Strikingly different Coulomb explosion behavior under intense laser fields is shown between the cis and trans geometric isomers of dichloroethene using 40-fs pulses at 0.8 µm. Although the fragment-ion distributions in the mass spectra did not aid in the identification of the geometric and positional isomers of the dichloroethenes, we found that the angular distributions of atomic ions were strongly dependent on the geometric structures. The angular distributions of chlorine ions, carbon ions, and protons were similar between 1,1- and cis-1,2-dichloroethene, whereas trans-1,2-dichloroethene showed a very sharp distribution of chlorine ions and quite different distributions of carbon ions and protons. The origin of the anisotropic ion angular distributions is the geometric selection of molecules in the tunnel-ionization process followed by a Coulomb explosion, although molecules are randomly oriented in the gas phase. The highly charged molecular ions exploded into pieces, and the direction of atomic-ion ejection was strongly correlated with the relative configuration of atoms with respect to the electron-extraction axis, the repulsion with adjacent atomic ions within the molecule, and the degree of the persistence of a molecular frame. We propose herein that the most probable electron-extraction axis by tunneling, which is governed by the configuration of molecular orbitals, is different among three dichloroethene isomers. Because multiple ionization under intense laser fields occurs by sequential tunneling processes, the first ionization step at the leading edge of the laser pulse dominates the further ionization steps. Therefore, the shapes of the highest occupied molecular orbitals and probably the underlying orbitals determine the anisotropic emission of atomic ions that can be used to identify isomers.

Fe(z+) (z = 1-6) generation from ferrocene.

Multiply charged iron atoms up to Fe(6+) with few carbon ions were produced from ferrocene under intense femtosecond laser fields. The production of Fe(4+) and Fe(5+) from ferrocene requires much less laser intensity than theoretically expected for iron atoms. The dissociation of ferrocene and the generation mechanism of multiply charged iron atoms are discussed.

Dissociation and multiply charged silicon ejection in high abundance from hexamethyldisilane.

Quadruply charged, neon-like silicon and helium-like carbon were generated by the exposure of hexamethyldisilane to intense femtosecond laser pulses. Dissociation of the silicon-silicon bond, the formation of highly charged silicons, as well as the saturation intensity of their formation were studied by mass spectroscopy. The production of these ions in high abundance, but also with lower laser intensity than theoretically expected for the element, was accomplished by using organosilicon compounds. Multiply charged silicon was generated at low laser intensity because stripping electrons from organosilicon compounds is much easier than from pure silicon due to the loose binding of electrons belonging to molecular orbitals. Femtosecond laser ionization is a valuable methodology for producing highly charged ions in high abundance and is useful in many fields of interest.

Linear response of multiphoton reaction: three-photon cycloreversion of anthracene biplanemer in solution by intense femtosecond laser pulses.

The photocycloreversion of anthracene photodimers and biplanemer in solution was investigated by nonresonant intense femtosecond laser pulses. Cycloreversion of biplanemer showed a pseudolinear response to laser intensity whereas the formation of anthracene from photodimer was proportional to the cubic of laser intensity. The unusual intensity dependence of biplanemer was explained in terms of the sum of three-photon intramolecular cycloreversion and the recovery of reactant by a two-photon intramolecular cyclodimerization. The coexistence of high- and low-order multiphoton processes within the same laser pulse originated in the spatial distribution of the laser intensity. We observed white light emerging from the sample solution; however, the effect of solvated electrons was not observed in the present system. The saturation of both the photoreaction and white light due to a volume effect was observed at high intensity.

Formation and fragmentation of quadruply charged molecular ions by intense femtosecond laser pulses.

We investigated the formation and fragmentation of multiply charged molecular ions of several aromatic molecules by intense nonresonant femtosecond laser pulses of 1.4 mum with a 130 fs pulse duration (up to 2 x 10(14) W cm(-2)). Quadruply charged states were produced for 2,3-benzofluorene and triphenylene molecular ion in large abundance, whereas naphthalene and 1,1'-binaphthyl resulted only in up to triply charged molecular ions. The laser wavelength was nonresonant with regard to the electronic transitions of the neutral molecules, and the degree of fragmentation was strongly correlated with the absorption of the singly charged cation radical. Little fragmentation was observed for naphthalene (off-resonant with cation), whereas heavy fragmentation was observed in the case of 1,1'-binaphthyl (resonant with cation). The degree of H(2) (2H) and 2H(2) (4H) elimination from molecular ions increased as the charge states increased in all the molecules examined. A striking difference was found between triply and quadruply charged 2,3-benzofluorene: significant suppression of molecular ions with loss of odd number of hydrogen was observed in the quadruply charged ions. The Coulomb explosion of protons in the quadruply charged state and succeeding fragmentation resulted in the formation of triply charged molecular ions with an odd number of hydrogens. The hydrogen elimination mechanism in the highly charged state is discussed.

Reduction of Sm(3+) to Sm(2+) by an intense femtosecond laser pulse in solution.

Samarium 3+ ions in methanol were found to be reduced to the corresponding 2+ ions upon irradiation with intense femtosecond laser pulses. The reduction was observed at both pulses with central wavelengths of 403 nm converted from an 800 fs fundamental pulse and 800 nm with a duration of 43 fs. When the laser wavelength was tuned to the 4f-4f absorption at 403 nm corresponding to the (6)P(3/2) <-- (6)H(5/2) transition, the reduction occurred by multiphoton absorption, presumably due to reaching the deep charge transfer state. In the case of excitation by 800 nm pulses of the fundamental wavelength of the Ti:sapphire laser, the reduction is considered to occur via solvent ionization followed by electron capture by Sm(3+). The product Sm(2+) was detected by its fluorescence, which was observed for the first time in solution and showed a broad spectrum peak around 750 nm with a quantum yield of 0.050 in methanol in the presence of 15-crown-5-ether.

Ionization and fragmentation of alkylphenols by 0.8-1.5 microm femtosecond laser pulses.

Ionization and fragmentation were studied on alkylphenols with long alkyl chains (p-(C6H4)(OH)(C(n)H(2n+1)), n = 1,3,5,8,9) and, for reference, on alkylbenzenes ((C6H5)(C(n)H(2n+1)), n = 1,3,5,7,9) by intense femtosecond laser pulses, typically with 43 fs duration at 0.8 microm and 140 fs at 1.3 microm in an intensity range of 10(14) W cm(-2). The major products were the corresponding molecular and C7 fragment ions from the alkylphenols and alkylbenzenes. The molecular ion yields decreased from nearly 1 (n = 1) to 0.3-0.5 (n = 9) when the carbon number in the alkyl chain increased for both excitation wavelengths. Higher yields of the molecular ions were observed at a longer wavelength of 1.3 microm. The long wavelengths in the range of 1.3-1.5 microm were used to determine whether or not -OH absorption had any increase in fragment ions. No effect was observed by vibrational overtone excitation of the -OH group in this wavelength range. Direct dissociation by cation absorption is the most plausible explanation of the present fragmentation results. Other possible mechanisms were discussed, including a statistical model, an effect of electron rescattering, a multiactive electron model, and dissociation from the superexcited state. In the case of cyclohexane, nonresonant wavelength excitation with a pulse of 1.3 microm (150 fs) effectively suppressed fragmentation more than excitation by a resonant but short-duration pulse (0.8 microm, 15 fs).