Anomalous spin relaxation in graphene nanostructures on the high temperature annealed surface of hydrogenated diamond nanoparticles.

The electronic and magnetic structures of diamond nanoparticles with a hydrogenated surface are investigated as a function of annealing temperature under vacuum annealing up to 800-1000 °C. Near edge X-ray absorption fine structure (NEXAFS) spectra together with elemental analysis show successive creation of defect-induced nonbonding surface states at the expense of surface-hydrogen atoms as the annealing temperature is increased above 800 °C. Magnetization and ESR spectra confirm the increase in the concentration of localized spins assigned to the nonbonding surface states upon the increase of the annealing temperature. Around 800 °C, surface defects collectively created upon the annealing result in the formation of graphene nano-islands which possess magnetic nonbonding edge states of π-electron origin. Interestingly, extremely slow spin relaxation is observed in the magnetization of the edge state spins at low temperatures. The relaxation time is well explained in terms of a lognormal distribution of magnetic anisotropy energies instead of the classical Néel relaxation mechanism with a unique magnetic anisotropy energy, in addition to the contribution of the quantum mechanical tunnelling mechanism. The spin-orbit interaction enhanced by the electrostatic potential gradient created at the interface between the core diamond particle and surface graphene nano-islands is responsible for the slow spin relaxation.

Synthesis, optical and electrochemical properties of 4,4'-bibenzo[c]thiophene derivatives.

We designed and synthesized unsubstituted 4,4'-bibenzo[c]thiophene 4,4'-BBT and its silyl-substituted derivatives 1,1'-Si-4,4'-BBT and 1,1',3,3'-Si-4,4'-BBT with one or two tert-butyldimethylsilyl groups on each thiophene ring, as new π-building blocks in emitters, photosensitizers and semiconductors for organic optoelectronic devices. The characterization of 4,4'-BBT, 1,1'-Si-4,4'-BBT and 1,1',3,3'-Si-4,4'-BBT was successfully determined by FTIR, 1H and 13C NMR measurements, high-resolution mass spectrometry (HRMS) analysis, photoabsorption and fluorescence spectroscopy, cyclic voltammetry (CV) and density functional theory (DFT) calculations. Moreover, a single-crystal X-ray structural analysis was successfully made for 1,1'-Si-4,4'-BBT and 1,1',3,3'-Si-4,4'-BBT. The photoabsorption and fluorescence maxima (λ abs max and λ fl max) of the three 4,4'-bibenzo[c]thiophene derivatives in toluene exhibit bathochromic shifts in the order of 4,4'-BBT (359 nm and 410 nm) < 1,1'-Si-4,4'-BBT (366 nm and 420 nm) < 1,1',3,3'-Si-4,4'-BBT (371 nm and 451 nm). The HOMO and LUMO energy levels rise in the order of 4,4'-BBT (-5.55 eV and -2.39 eV) < 1,1'-Si-4,4'-BBT (-5.45 eV and -2.34 eV) < 1,1',3,3'-Si-4,4'-BBT (-5.34 eV and -2.30 eV), but the rise of the HOMO energy level is larger than that of the LUMO energy level, resulting in the bathochromic shift of the photoabsorption band from 4,4'-BBT to 1,1',3,3'-Si-4,4'-BBT. The fluorescence quantum yields (Φ fl) of 4,4'-BBT, 1,1'-Si-4,4'-BBT and 1,1',3,3'-Si-4,4'-BBT in toluene are 0.41, 0.41 and 0.36, respectively. It is worth mentioning that in the solid state 1,1'-Si-4,4'-BBT and 1,1',3,3'-Si-4,4'-BBT show relatively high Φ fl-solid values of 0.22 and 0.25, respectively, whereas 4,4'-BBT exhibits poor solid-state fluorescence properties (Φ fl-solid < 0.02). This work provides an efficient synthetic method for the 4,4'-bibenzo[c]thiophene derivatives and their photophysical properties in the solution and solid state, electrochemical properties and X-ray crystal structures.

Synthesis, photophysical and electrochemical properties of pyridine, pyrazine and triazine-based (D-π-)2A fluorescent dyes.

The donor-acceptor-π-conjugated (D-π-)2A fluorescent dyes OUY-2, OUK-2 and OUJ-2 with two (diphenylamino)carbazole thiophene units as D (electron-donating group)-π (π-conjugated bridge) moiety and a pyridine, pyrazine or triazine ring as electron-withdrawing group (electron-accepting group, A) have been designed and synthesized. The photophysical and electrochemical properties of the three dyes were investigated by photoabsorption and fluorescence spectroscopy, Lippert-Mataga plots, cyclic voltammetry and density functional theory calculations. The photoabsorption maximum (λmax,abs) and the fluorescence maximum (λmax,fl) for the intramolecular charge-transfer characteristic band of the (D-π-)2A fluorescent dyes show bathochromic shifts in the order of OUY-2 < OUK-2 < OUJ-2. Moreover, the photoabsorption bands of the (D-π-)2A fluorescent dyes are nearly independent of solvent polarity, while the fluorescence bands showed bathochromic shifts with increasing solvent polarity (i.e., positive fluorescence solvatochromism). The Lippert-Mataga plots for OUY-2, OUK-2 and OUJ-2 indicate that the Δµ (= µe - µg) value, which is the difference in the dipole moment of the dye between the excited (µe) and the ground (µg) states, increases in the order of OUY-2 < OUK-2 < OUJ-2. Therefore, the fact explains our findings that OUJ-2 shows large bathochromic shifts of the fluorescence maxima in polar solvents, as well as the Stokes shift values of OUJ-2 in polar solvents are much larger than those in nonpolar solvents. The cyclic voltammetry of OUY-2, OUK-2 and OUJ-2 demonstrated that there is little difference in the HOMO energy level among the three dyes, but the LUMO energy levels decrease in the order of OUY-2 > OUK-2 > OUJ-2. Consequently, this work reveals that for the (D-π-)2A fluorescent dyes OUY-2, OUK-2 and OUJ-2 the bathochromic shifts of λmax,abs and λmax,fl and the lowering of the LUMO energy level are dependent on the electron-withdrawing ability of the azine ring, which increases in the order of OUY-2 < OUK-2 < OUJ-2.

Colorimetric and ratiometric fluorescence sensing of water based on 9-methyl pyrido[3,4-b]indole-boron trifluoride complex.

In this work, 9-methyl pyrido[3,4-b]indole-boron trifluoride complex, 9-MP-BF3, was designed and developed as a colorimetric and ratiometric fluorescent sensor for the detection of water in the low- and high-water-content regions in solvents. In the low-water-content region, a new photoabsorption band at around 360 nm and a fluorescence band at around 370 nm gradually appeared due to the dissociation of 9-MP-BF3 into 9-methyl pyrido[3,4-b]indole (9-MP) by water molecules with a simultaneous decrease in the photoabsorption band at around 390 nm and the fluorescence band at around 460 nm originating from 9-MP-BF3. In the moderate-water-content region, the photoabsorption band at around 360 nm and the fluorescence band at around 370 nm gradually shifted to a longer wavelength region with an increase in the fluorescence intensity, which could be ascribed to the formation of a hydrogen-bonded complex (9-MP-H2O) with water molecules. Furthermore, in the high-water-content region, two photoabsorption bands at around 305 nm and 390 nm and one fluorescence band at around 460 nm gradually reappeared with simultaneous decrease in the photoabsorption band at around 290 nm and the fluorescence band at around 370 nm, which was attributed to the formation of a hydrogen-bonded proton transfer complex (9-MP-H+) with water molecules. Thus, this work revealed the mechanism of a colorimetric and ratiometric fluorescent sensor based on pyrido[3,4-b]indole-boron trifluoride complex for the detection of water over a wide range from low water content to high water content in solvents.

Development of an intramolecular charge transfer-type colorimetric and fluorescence sensor for water by fusion with a juloidine structure and complexation with boron trifluoride.

An optical sensor with the ability to detect and determine water over a wide concentration range is highly desirable in the laboratory and industry. Here the sensitivity and spectral responses of an intramolecular charge transfer-type colorimetric and fluorescence sensor with ß-carboline structure are tuned and improved significantly over various water contents in the organic solvent by fusion with an electron-donating juloidine structure and complexation with boron trifluoride (BF3). The sensors, ET-1 and ET-1-BF3, developed in this study can respond differently depending on water content. ET-1-BF3 releases BF3 to generate ET-1 by addition of a trace amount of water, and ET-1 forms hydrogen bonds with one water molecule in low water contents and a hydrogen-bonded proton transfer complex with several water molecules in high water contents, accompanying gradual color and fluorescence changes. This work shows a promising approach to the sensitive detection and precise determination of water over the whole concentration range using a simple and practical method with optical sensors.

A colorimetric and fluorescent sensor for water in acetonitrile based on intramolecular charge transfer: D-(π-A)2-type pyridine-boron trifluoride complex.

A D-(π-A)2-type pyridine-boron trifluoride complex composed of a carbazole skeleton and two pyridine-boron trifluoride units was designed and developed as a colorimetric and fluorescent sensor based on intramolecular charge transfer (ICT) for the detection of water over a wide range from low water content to high water content in solvents.

Emerging chemical strategies for imprinting magnetism in graphene and related 2D materials for spintronic and biomedical applications.

Graphene, a single two-dimensional sheet of carbon atoms with an arrangement mimicking the honeycomb hexagonal architecture, has captured immense interest of the scientific community since its isolation in 2004. Besides its extraordinarily high electrical conductivity and surface area, graphene shows a long spin lifetime and limited hyperfine interactions, which favors its potential exploitation in spintronic and biomedical applications, provided it can be made magnetic. However, pristine graphene is diamagnetic in nature due to solely sp2 hybridization. Thus, various attempts have been proposed to imprint magnetic features into graphene. The present review focuses on a systematic classification and physicochemical description of approaches leading to equip graphene with magnetic properties. These include introduction of point and line defects into graphene lattices, spatial confinement and edge engineering, doping of graphene lattice with foreign atoms, and sp3 functionalization. Each magnetism-imprinting strategy is discussed in detail including identification of roles of various internal and external parameters in the induced magnetic regimes, with assessment of their robustness. Moreover, emergence of magnetism in graphene analogues and related 2D materials such as transition metal dichalcogenides, metal halides, metal dinitrides, MXenes, hexagonal boron nitride, and other organic compounds is also reviewed. Since the magnetic features of graphene can be readily masked by the presence of magnetic residues from synthesis itself or sample handling, the issue of magnetic impurities and correct data interpretations is also addressed. Finally, current problems and challenges in magnetism of graphene and related 2D materials and future potential applications are also highlighted.

Mitochondria-Targeting Polyamine-Protoporphyrin Conjugates for Photodynamic Therapy.

Two polyamine derivatives of protoporphyrin IX (PPIX) were tested as photodynamic therapy (PDT) agents in HT29 colorectal cancer and HEP3B liver cancer cell lines. These compounds exhibit excellent singlet oxygen quantum yields and show strong in vitro PDT efficacy after 660 nm laser irradiation, whereas exogenous PPIX itself exhibits much weaker PDT effects. Confocal microscopy imaging studies reveal that a protoporphyrin derivative with eight amine moieties has excellent water solubility, and localizes mainly in the mitochondria of both HT29 and HEP3B cells, whereas the cellular distribution of a protoporphyrin derivative with four amine moieties is not as specific. This work demonstrates that polyamine moieties on macrocycles can enhance PDT efficacy by targeting mitochondria.

Synthesis and optical and electrochemical properties of a phenanthrodithiophene (fused-bibenzo[c]thiophene) derivative.

We designed and developed a fused-bibenzo[c]thiophene, namely, 2,9-bis(tert-butyldimethylsilyl)phenanthro[9,8-bc:10,1-b'c']dithiophene (PHDT-Si), as a new π-building block in the emitters, photosensitizers and semiconductors for organic optoelectronic devices. Based on photophysical (photoabsorption, fluorescence and time-resolved fluorescence spectroscopy) and electrochemical measurements (cyclic voltammetry), and density functional theory (DFT) calculations, this work reveals that the fused-bibenzo[c]thiophene PHDT-Si, which is prepared by an efficient synthesis method, has a rigid, high planar and expanded π-conjugation structure, and possesses intense photoabsorption and fluorescence properties (λ = 598 nm (εmax = 41 000 M-1 cm-1) and λ = 613 nm (Φf = 0.74) in toluene) in the long-wavelength region and undergoes an electrochemically reversible oxidation process, compared to non-fused 1,1'-bis(tert-butyldimethylsilyl)-4,4'-bibenzo[c]thiophene (BBT-Si).

Synthesis and optical and electrochemical properties of julolidine-structured pyrido[3,4-b]indole dye.

The julolidine-structured pyrido[3,4-b]indole dye ET-1 has been newly designed and developed as a small D-A fluorescent dye. ET-1 showed bathochromic shifts of the fluorescence band upon changing from aprotic solvents to protic solvents, as well as positive fluorescence solvatochromism. Moreover, it was found that ET-1 can form a 1 : 1 Py(N)-B complex with boron trifluoride and a hydrogen-bonded proton transfer (Py(N)-H) complex with trifluoroacetic acid, which exhibit photoabsorption and fluorescence bands at a longer wavelength region than the pristine ET-1. Based on optical (photoabsorption and fluorescence spectroscopy) and electrochemical (cyclic voltammetry) measurements, Lippert-Mataga plots, 1H NMR spectral measurement and density functional theory (DFT) calculation, this work indicated that the Py(N)-B complex or the Py(N)-H complex is effectively formed and stable in solution. This is due to the strong Py(N)-B interaction or Py(N)-hydrogen-bond, which can be attributed to the enhanced basicity or the accumulated electron density on the nitrogen atom of the pyridine ring caused by the introduction of a julolidine (quinolizidine) moiety as a strong electron-donating group. We propose that the D-A-type dye ET-1 based on the julolidine-structured pyrido[3,4-b]indole possesses the ability to act as a calorimetric and fluorescent sensor for Brønsted and Lewis acids.

Data mining graphene: correlative analysis of structure and electronic degrees of freedom in graphenic monolayers with defects.

The link between changes in the material crystal structure and its mechanical, electronic, magnetic and optical functionalities-known as the structure-property relationship-is the cornerstone of modern materials science research. The recent advances in scanning transmission electron and scanning probe microscopies (STEM and SPM) have opened an unprecedented path towards examining the structure-property relationships of materials at the single-impurity and atomic-configuration levels. However, there are no statistics-based approaches for cross-correlation of structure and property variables obtained from the different information channels of STEM and SPM experiments. Here we have designed an approach based on a combination of sliding window fast Fourier transform, Pearson correlation matrix and linear and kernel canonical correlation methods to study the relationship between lattice distortions and electron scattering from SPM data on graphene with defects. Our analysis revealed that the strength of coupling to strain is altered between different scattering channels, which can explain the coexistence of several quasiparticle interference patterns in nanoscale regions of interest. In addition, the application of kernel functions allowed us to extract a non-linear component of the relationship between the lattice strain and scattering intensity in graphene. The outlined approach can be further used to analyze correlations in various multi-modal imaging techniques where the information of interest is spatially distributed and generally has a complex multi-dimensional nature.

Single oxygen generation sensitized by spiro(dipyridinogermole)(dithienogermole)s.

Three spiro(dipyridinogermole)(dithienogermole) derivatives (1-3), including newly prepared spiro(dipyridinogermole)[di(2-pyridyl)dithienogermole] (3), were examined as photosensitizers for singlet oxygen (1O2) generation in dichloromethane-methanol. Irradiation of their air-saturated solutions led to the generation of 1O2, which was readily trapped by well-known scavengers, dihydronaphthoquinone (DHN) and diphenylisobenzofuran (DPBF). Spiro(dipyridinogermole)[bis(n-hexylbithiophenyl)dithienogermole] (2) showed the best performance with a first-order rate constant that was higher than that of tetraphenylporphyrin (TPP), an efficient photosensitizer for 1O2 generation. This is ascribable to the efficient intersystem crossing characteristic of the dipyridinogermole unit. The quantum yield of 1O2 generation was φΔ = 0.72 for 2, relative to that for rose bengal (RB) in methanol as reference (φΔ = 0.8).

Development of type-I/type-II hybrid dye sensitizer with both pyridyl group and catechol unit as anchoring group for type-I/type-II dye-sensitized solar cell.

A type-I/type-II hybrid dye sensitizer with a pyridyl group and a catechol unit as the anchoring group has been developed and its photovoltaic performance in dye-sensitized solar cells (DSSCs) is investigated. The sensitizer has the ability to adsorb on a TiO2 electrode through both the coordination bond at Lewis acid sites and the bidentate binuclear bridging linkage at Brønsted acid sites on the TiO2 surface, which makes it possible to inject an electron into the conduction band of the TiO2 electrode by the intramolecular charge-transfer (ICT) excitation (type-I pathway) and by the photoexcitation of the dye-to-TiO2 charge transfer (DTCT) band (type-II pathway). It was found that the type-I/type-II hybrid dye sensitizer adsorbed on TiO2 film exhibits a broad photoabsorption band originating from ICT and DTCT characteristics. Here we reveal the photophysical and electrochemical properties of the type-I/type-II hybrid dye sensitizer bearing a pyridyl group and a catechol unit, along with its adsorption modes onto TiO2 film, and its photovoltaic performance in type-I/type-II DSSC, based on optical (photoabsorption and fluorescence spectroscopy) and electrochemical measurements (cyclic voltammetry), density functional theory (DFT) calculation, FT-IR spectroscopy of the dyes adsorbed on TiO2 film, photocurrent-voltage (I-V) curves, incident photon-to-current conversion efficiency (IPCE) spectra, and electrochemical impedance spectroscopy (EIS) for DSSC.

Role of edge geometry and chemistry in the electronic properties of graphene nanostructures.

The geometry and chemistry of graphene nanostructures significantly affects their electronic properties. Despite a large number of experimental and theoretical studies dealing with the geometrical shape-dependent electronic properties of graphene nanostructures, experimental characterisation of their chemistry is clearly lacking. This is mostly due to the difficulties in preparing chemically-modified graphene nanostructures in a controlled manner and in identifying the exact chemistry of the graphene nanostructure on the atomic scale. Herein, we present scanning probe microscopic and first-principles characterisation of graphene nanostructures with different edge geometries and chemistry. Using the results of atomic scale electronic characterisation and theoretical simulation, we discuss the role of the edge geometry and chemistry on the electronic properties of graphene nanostructures with hydrogenated and oxidised linear edges at graphene boundaries and the internal edges of graphene vacancy defects. Atomic-scale details of the chemical composition have a strong impact on the electronic properties of graphene nanostructures, i.e., the presence or absence of non-bonding π states and the degree of resonance stability.

Potential-dependent hydration structures at aqueous solution/graphite interfaces by electrochemical frequency modulation atomic force microscopy.

Potential-dependent solvation structures of aqueous electrolyte-graphite interfaces were studied using electrochemical frequency modulation atomic force microscopy. Oscillatory modulations on the force curves reversibly changed with the applied potential on the graphite electrode, and also strongly depended on the anion species in electrolyte solutions.

Transfer hydrogenation of alkenes using Ni/Ru/Pt/Au heteroquatermetallic nanoparticle catalysts: sequential cooperation of multiple nano-metal species.

Quatermetallic alloy nanoparticles of Ni/Ru/Pt/Au were prepared and found to promote the catalytic transfer hydrogenation of non-activated alkenes bearing conjugating units (e.g., 4-phenyl-1-butene) with 2-propanol, where the composition metals, Ni, Ru, Pt, and Au, act cooperatively to provide significant catalytic ability.