Solid-state excimer emission of o-carborane derivatives and applications for stimuli-responsive luminescent materials.

o-Carborane is an icosahedral cluster containing boron and has been paid attention as a versatile platform for constructing stimuli-responsive materials with solid-state luminescence properties by suppressing aggregation-caused quenching (ACQ), which is the emission annihilation process observed in condensed states, such as concentrated solutions, solids and films. Non-specific intermolecular interactions in the condensed states resulting in ACQ are disturbed by the spherical boron cluster. More recently, it has been demonstrated from several reports that o-carborane is capable of assisting solid-state excimer emission in aryl-modified o-carboranes. Moreover, some of these materials provide unique stimuli-responsiveness. In the commodity design for solid-state luminescent materials, o-carboranes are responsible for disturbing intermolecular interactions and subsequently presenting solid-state luminescence. On the other hand, excimer emission, which is inducible from the intermolecular photophysical process, has been discovered in the solid state based on o-carborane materials. In this manuscript, recent progress in stimuli-responsive luminescent materials via excimer formation will be described. Moreover, as the further potential of o-carborane materials, recent findings on the photo-salient effect which can be induced through excimer formation followed by photoreaction will also be explained.

Highly Efficient and Thermally Durable Luminescence of 1D Eu3+ Coordination Polymers with Arsenic Bridging Ligands.

In this work, bisarsine oxides were evaluated as novel bridging ligands, aiming to develop practical and efficient luminescent lanthanide coordination polymers. We have synthesized one-dimensional (1D) Eu3+ coordination polymers that incorporate bisarsine oxide bridging ligands and hexafluoroacetylacetonate anions. These polymers exhibited a denser packing of chains compared to analogous polymers bridged with bisphosphine oxides. The coordination polymers demonstrated exceptional thermal stability and substantial emission quantum yields. Additionally, the bisarsine oxides induced a pronounced polarization effect, facilitating a sensitive electric dipole transition that yields considerably narrow band red emission. Remarkably, the Eu3+ coordination polymers with bisarsine oxides maintained intense emission even at 550 K. A distinctive feature of these polymers is their heating-induced emission enhancement observed when the temperature was increased from 300 K to 400 K.

Solvent-free synthesis and chiroptical properties of a C-N axially chiral cruciform dimer of benzo[b]phenoxazine.

A novel C-N axially chiral molecule composed of two tert-butyl-substituted benzo[b]phenoxazine (BPO) was synthesized via solvent-free reactions. The absolute configurations of the enantiomers were determined by X-ray single-crystal analysis. The enantiomers had a sufficiently high racemization barrier to ignore racemization at room temperature (149 ± 20 kJ mol-1), and the solutions exhibited dual circularly polarized emissions stemming from fluorescence and phosphorescence of |gCPL| = ca. 1 × 10-3.

NIR-II Absorption/Fluorescence of D-A π-Conjugated Polymers Composed of Strong Electron Acceptors Based on Boron-Fused Azobenzene Complexes.

Luminescence in the second near-infrared (NIR-II, 1,000-1,700 nm) window is beneficial especially for deep tissue imaging and optical sensors because of intrinsic high permeability through various media. Strong electron-acceptors with low-lying lowest unoccupied molecular orbital (LUMO) energy levels are a crucial unit for donor-acceptor (D-A) π-conjugated polymers (CPs) with the NIR-II emission property, however, limited kinds of molecular skeletons are still available. Herein, D-A CPs involving fluorinated boron-fused azobenzene complexes (BAz) with enhanced electron-accepting properties are reported. Combination of fluorination at the azobenzene ligand and trifluoromethylation at the boron can effectively lower the LUMO energy level down to -4.42 eV, which is much lower than those of conventional strong electron-acceptors. The synthesized series of CPs showed excellent absorption/fluorescence property in solution over a wide NIR range including NIR-II. Furthermore, owing to the inherent solid-state emissive property of the BAz skeleton, obvious NIR-II fluorescence from the film (up to λFL=1213 nm) and the nanoparticle in water (λFL=1036 nm, brightness=up to 29 cm-1 M-1) were observed, proposing that our materials are applicable for developing next-generation of NIR-II luminescent materials.

The Photosalient Effect and Thermochromic Luminescence Based on o-Carborane-Assisted π-Stacking in the Crystalline State.

Herein, we report the unique multiple-stimuli responsiveness of anthracene-tethered o-carborane derivatives. We designed and synthesized anthracene derivatives with different substitution positions and numbers of the o-carborane units. Two compounds had characteristic crystal structures involving the columnar π-stacking structures of the anthracene units. From the analysis of crystalline-state structure-property relationships, it was revealed that the crystals exhibited the photosalient effect accompanied by photochemical [4+4] cycloaddition reactions and temperature-dependent photophysical dual-emission properties including excimer emission of anthracene. Those properties were considered as non-radiative and radiative deactivation pathways through the excimer formation in the excited state and the formation of excimer species was facilitated by the π-stacking structure of anthracene units. Moreover, we found unusual temperature dependency on the occurrence of the photosalient effect. According to the data from variable temperature X-ray crystallography, a strong correlation between lattice shrinkage and strain accumulation is suggested.

Synthesis of Regioregular and Random Boron-Fused Azomethine Conjugated Polymers for Film Morphology Control.

Regioregular and random conjugated polymers based on a boron-fused azomethine unit were synthesized by Sonogashira-Hagihara cross coupling reaction. Although these polymers exhibited similar optical properties in the solution states, a distinct difference was observed in the aggregation forming ability in the film states; scanning electron microscope (SEM) observation indicated the existence of fiber-like aggregates in the spin-coated film of the regioregular polymer, while regiorandom polymer showed no aggregate in the film state. Accordingly, the UV-vis absorption spectrum of the regioregular polymer showed an increased shoulder peak due to the aggregate formation, whereas the random one showed no change. Furthermore, an absolute fluorescence quantum efficiency of the regioregular polymer was enhanced in response to the aggregate disassembly via thermal annealing treatment. In this study, we demonstrate that controlling regioregularity of the conjugated polymers can induce the different morphological structures and thermal-responsive behaviors. These findings could be beneficial for the design strategy and potential applications of thin-film optoelectronic devices with stimuli-responsive properties.

Benzannulated Double Aza[9]helicenes: Synthesis, Structures, and (Chir)optical Properties.

A benzannulated double aza[9]helicene 1 was successfully synthesized via a one-pot oxidative fusion reaction. 1 was derivatized to N-alkylated double aza[9]helicene 1-Et and 1-Bu, whose structures were determined by X-ray diffraction analysis. 1-Et and 1-Bu exhibited red-shifted absorption and fluorescence spectra compared to single aza[9]helicene. The double aza[9]helicenes were expected to have two different conformers. Consistent with solid-state structure, the chiral-isomer was estimated to be more stable by 16 kcal/mol relative to meso-isomer. Indeed, enantiomers of 1-Et and 1-Bu were optically resolved by HPLC and showed mirror-imaged CD and CPL spectra with the CPL brightness up to 19.2 M-1cm-1 for 1-Bu.

A triple-imaging-modality system for simultaneous measurements of prompt gamma photons, prompt x-rays, and induced positrons during proton beam irradiation.

Objective. Prompt gamma photon, prompt x-ray, and induced positron imaging are possible methods for observing a proton beam's shape from outside the subject. However, since these three types of images have not been measured simultaneously nor compared using the same subject, their advantages and disadvantages remain unknown for imaging beam shapes in therapy. To clarify these points, we developed a triple-imaging-modality system to simultaneously measure prompt gamma photons, prompt x-rays, and induced positrons during proton beam irradiation to a phantom.Approach. The developed triple-imaging-modality system consists of a gamma camera, an x-ray camera, and a dual-head positron emission tomography (PET) system. During 80 MeV proton beam irradiation to a polymethyl methacrylate (PMMA) phantom, imaging of prompt gamma photons was conducted by the developed gamma camera from one side of the phantom. Imaging of prompt x-rays was conducted by the developed x-ray camera from the other side. Induced positrons were measured by the developed dual-head PET system set on the upper and lower sides of the phantom.Main results. With the proposed triple-imaging-modality system, we could simultaneously image the prompt gamma photons and prompt x-rays during proton beam irradiation. Induced positron distributions could be measured after the irradiation by the PET system and the gamma camera. Among these imaging modalities, image quality was the best for the induced positrons measured by PET. The estimated ranges were actually similar to those imaged with prompt gamma photons, prompt x-rays and induced positrons measured by PET.Significance. The developed triple-imaging-modality system made possible to simultaneously measure the three different beam images. The system will contribute to increasing the data available for imaging in therapy and will contribute to better estimating the shapes or ranges of proton beam.

Polynomial Fuzzy Observer-Based Feedback Control for Nonlinear Hyperbolic PDEs Systems.

This article explores the observer-based feedback control problem for a nonlinear hyperbolic partial differential equations (PDEs) system. Initially, the polynomial fuzzy hyperbolic PDEs (PFHPDEs) model is established through the utilization of the fuzzy identification approach, derived from the nonlinear hyperbolic PDEs model. Various types of state estimation and controller design problems for the polynomial fuzzy PDEs system are discussed concerning the state estimation problem. To investigate the relaxed stability problem, Euler's homogeneous theorem, Lyapunov-Krasovskii functional with polynomial matrices (LKFPM), and the sum-of-squares (SOSs) approach are adopted. The exponential stabilization condition is formulated in terms of the spatial-derivative-SOSs (SD-SOSs). Additionally, a segmental algorithm is developed to find the feasible solution for the SD-SOS condition. Finally, a hyperbolic PDEs system and several numerical examples are provided to illustrate the validity and effectiveness of the proposed results.

X-ray free electron laser observation of ultrafast lattice behaviour under femtosecond laser-driven shock compression in iron.

Over the past century, understanding the nature of shock compression of condensed matter has been a major topic. About 20 years ago, a femtosecond laser emerged as a new shock-driver. Unlike conventional shock waves, a femtosecond laser-driven shock wave creates unique microstructures in materials. Therefore, the properties of this shock wave may be different from those of conventional shock waves. However, the lattice behaviour under femtosecond laser-driven shock compression has never been elucidated. Here we report the ultrafast lattice behaviour in iron shocked by direct irradiation of a femtosecond laser pulse, diagnosed using X-ray free electron laser diffraction. We found that the initial compression state caused by the femtosecond laser-driven shock wave is the same as that caused by conventional shock waves. We also found, for the first time experimentally, the temporal deviation of peaks of stress and strain waves predicted theoretically. Furthermore, the existence of a plastic wave peak between the stress and strain wave peaks is a new finding that has not been predicted even theoretically. Our findings will open up new avenues for designing novel materials that combine strength and toughness in a trade-off relationship.

Effects of Central Elements on the Properties of Group 13 Dialdiminate Complexes.

Invited for the cover of this issue is the group of Kazuo Tanaka at Kyoto University. The image depicts the element-dependent properties of group 13 dialdiminate complexes Read the full text of the article at 10.1002/chem.202300654.

Design and Synthesis of Far-Red to Near-Infrared Chromophores with Pyrazine-Based Boron Complexes.

We synthesized new binuclear boron complexes based on pyrazine with ortho and para substitution patterns. It was demonstrated that the para-linked complexes possess a significantly narrow energy gap between highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), leading to their far-red to near-infrared emission properties. Meanwhile, the ortho-substituted complex showed orange emission. Considering the HOMO and LUMO distributions of pyrazine, the boron complexation to the nitrogen atoms would stabilize its LUMO more efficiently than its HOMO because a nodal plane in the HOMO passes through the two nitrogen atoms. The theoretical study suggests that the para-substitution would not significantly perturb such a characteristic HOMO distribution originating from pyrazine in stark contrast to the ortho-substituted one. As a result, the HOMO-LUMO gap of the para-linked complex is dramatically narrower than that of the ortho-linked one.

Stimuli-Responsive π-Conjugated Polymers Showing Solid-State Emission Based on Boron-Fused Azomethine Complexes with NNO-Tridentate Ligands.

We report stimuli-responsive luminescent π-conjugated polymers involving the boron-fused azomethine (BAm) structure with the NNO-tridentate ligands and demonstrate their application to film-type sensors. The acid-responsive properties of the polymers are provided by the free lone-pair electrons of the nitrogen atom on the BAm unit. The polymer films showed red shifts in emission wavelengths under acid vapor, and the responses were reversible and corresponded to deprotonation. Furthermore, owing to environmental sensitivity in the film state, unique luminescent color changes were observed. The bilayer films of the BAm polymer coated by poly(n-butyl methacrylate) exhibited vapochromic luminescence with gradual blue shifts (orange → yellow → green) by vapor annealing with chloroform depending on the exposure time. We demonstrated that these changes were applicable for evaluating solvent-vapor permeability of target membranes. This study shows that nitrogen substitution into existing organic compounds is effective for creating stimuli-responsive materials.

Hybrid imaging of prompt x-rays and induced positrons using a pinhole gamma camera during and after irradiation of protons.

Objective. Prompt x-ray imaging using a low-energy x-ray camera is a promising method for observing a proton beam's shape from outside the subject. Furthermore, imaging of positrons produced by nuclear reactions with protons is a possible method for observing the beam shape. However, it has not been possible to measure these two types of images with a single imaging system due to the limited imaging capability of existing systems. Imaging of both prompt x-rays and the distribution of positrons may compensate for the shortcomings of each method.Approach. We conducted imaging of the prompt x-ray using a pinhole x-ray camera during irradiation with protons in list mode. Then, after irradiation with protons, imaging of annihilation radiations from the produced positrons was conducted using the same pinhole x-ray camera in list mode. After this imaging, list-mode data were sorted to obtain prompt x-ray images and positron images.Main results. With the proposed procedure, we could measure both prompt x-ray images and induced positron images with a single irradiation by a proton beam. From the prompt x-ray images, ranges and widths of the proton beams could be estimated. The distributions of positrons were slightly wider than those of the prompt x-rays. From the time sequential positron images, we could derive the time activity curves of the produced positrons.Significance. Hybrid imaging of prompt x-rays and induced positrons using a pinhole x-ray camera was achieved. The proposed procedure would be useful for measuring prompt x-ray images during irradiation to estimate the beam structures as well as for measuring the induced positron images after irradiation to estimate the distributions and time activity curves of the induced positrons.

Direction Switching and Self-Recovering Mechanochromic Luminescence of Anthracene-Modified o-Carboranes.

Luminochromic behaviors regarding mechanochromic luminescence (MCL) of o-carborane-modified anthracene derivatives are reported. We have previously synthesized bis-o-carborane-substituted anthracene and found that its crystal polymorphs show dual-emission properties composed of excimer and charge transfer (CT) emission bands in the solid state. Initially, we observed the bathochromic MCL behavior from 1 a originating from emission mechanism alteration from dual emission to CT emission. By inserting the ethynylene spacers between anthracene and o-carboranes, compound 2 was obtained. Interestingly, 2 showed hypsochromic MCL originating from the emission mechanism alteration from CT to excimer emission. Furthermore, luminescent color of ground 1 a can be recovered to the initial state by allowing to stand at room temperature, meaning that self-recovery proceeds. Detailed analyses are described in this study.

Efficient synthesis and unit-selective π-extension of π-fused [4.3.3]propellane as a chiral building block.

A three-dimensional skeleton, π-fused [4.3.3]propellane, was constructed and derivatized by selective π-extension at the two naphthalene units. The obtained propellanes existed as stereoisomers different in spatial arrangement, one of which displayed a chiroptical response originating from through-space interactions between 5-azachrysenes in a skew position.

Effects of Central Elements on the Properties of Group 13 Dialdiminate Complexes.

Novel luminescent dialdiminate complexes of the Group 13 elements were prepared to evaluate the effects of the central element on their properties. We demonstrate that their absorption wavelength and the response to Lewis bases apparently depend on the central atom. The aluminum complex exhibited the absorption band in the higher-energy region than the gallium and indium congeners. Theoretical calculations suggest that the aluminum complex has a lower-lying highest-occupied molecular orbital than the other complexes. Additionally, the emission intensity of the aluminum complex clearly changed in response to a Lewis base. Quantum chemical calculations suggest that these element-dependent optical properties could originate from the difference in the electric charges on the central elements. Interestingly, the ligand exchange reactions were observed in the indium complexes together with the changes in the optical properties and controlled by the addition of InCl3 and InMe3 . Furthermore, all the complexes showed aggregation-induced emission enhancement (AIEE) and crystallization-induced emission enhancement (CIEE) properties. These results lead to proposing a practical strategy for manipulating the optoelectronic properties coupled with the reactivities of complexes by choosing the central elements in the same group.

Tuning of the height of energy barrier between locally-excited and charge transfer states by altering the fusing position of o-carborane in phenylnaphthalene.

We synthesized two types of the regioisomers fused by a phenylnaphthalene ring with variable connection points to the o-carborane scaffold. In this paper, we describe their photoluminescence (PL) properties and detailed photochemical mechanisms. According to the series of optical measurements, interestingly, they showed different PL characters in terms of wavelength and the dual-emission character despite that they have the common aromatic unit. Variable-temperature PL measurements and quantum chemical calculations suggested that the substitution position of aryl groups to o-carborane plays an important role in determining the energy barrier to the intramolecular charge-transfer (ICT) state at the S1 state. Finally, it is revealed that the relative position of the C-C bond of o-carborane and the aryl center should be responsible for the photophysical events of aryl-o-carboranes.

Effects of Hypervalent Bismuth on Electronic Properties of the Azobenzene Tridentate Ligand and Roles of Lewis Acidity in Controlling Optical Properties.

Organobismuth compounds have been studied in various fields, including electronic states, pnictogen bonds, and catalysis. Among them, one of the unique electronic states of the element is the hypervalent state. So far, many issues regarding the electronic structures of bismuth in hypervalent states have been revealed; meanwhile, the influence of hypervalent bismuth on the electronic properties of π-conjugated scaffolds is still vailed. Here, we synthesized the hypervalent bismuth compound, BiAz, by introducing hypervalent bismuth into the azobenzene tridentate ligand as a π-conjugated scaffold. The influence of hypervalent bismuth on the electronic properties of the ligand was evaluated from optical measurements and quantum chemical calculations. The introduction of hypervalent bismuth revealed three significant electronic effects: first, hypervalent bismuth shows position-dependent electron-donating and electron-accepting effects. Second, BiAz can have a larger effective Lewis acidity than the hypervalent tin compound derivatives reported in our previous research. Finally, the coordination of dimethyl sulfoxide transformed the electronic properties of BiAz, similar to the hypervalent tin compounds. The data from quantum chemical calculations showed that the optical properties of the π-conjugated scaffold were able to be changed by introducing hypervalent bismuth. To the best of our knowledge, we first demonstrate that the introduction of hypervalent bismuth should be a new methodology for controlling the electronic properties of π-conjugated molecules and developing sensing materials.

Near-Infrared Emissive Hypervalent Compounds with Germanium(IV)-Fused Azobenzene π-Conjugated Systems.

A novel molecular design for showing near-infrared (NIR) emission is still required for satisfying growing demands for NIR-light technology. In this research, hypervalent compounds with germanium (Ge)-fused azobenzene (GAz) scaffolds were discovered that can exhibit NIR emission (λPL =690∼721 nm, ΦPL =0.03∼0.04) despite compact π-conjugated systems. The unique optical properties are derived from the trigonal bipyramidal geometry of the hypervalent compounds constructed by combination of Ge and azobenzene-based tridentate ligands. Experimental and theoretical calculation results disclosed that the germanium-nitrogen (Ge-N) coordination at the equatorial position strongly reduces the energy level of the LUMO (lowest unoccupied molecular orbital), and the three-center four-electron (3 c-4 e) bond in the apical position effectively rises the energy level of the HOMO (highest occupied molecular orbital). It is emphasized that large narrowing of the HOMO-LUMO energy gap is achieved just by forming the hypervalent bond. In addition, the narrow-energy-gap property can be enhanced by extension of π-conjugation. The obtained π-conjugated polymer shows efficient NIR emission both in solution (λPL =770 nm and ΦPL =0.10) and film (λPL =807 nm and ΦPL =0.04). These results suggest that collaboration of a hypervalent bond and a π-conjugated system is a novel and effective strategy for tuning electronic properties even in the NIR region.