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.

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.

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.

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.

Alternately π-Stacked Systems Assisted by o-Carborane: Dual Excimer Emission and Color Modulation by Bcage -Methylation.

Herein, we report the unique solid-state excimer emission of three types of acridine-tethered o-carboranes with variable degrees of methylation at the o-carborane unit. They all showed columnar packing structures based on dimer formation, and two types of π-overlapping motifs were alternately stacked. From the photoluminescence (PL) measurements on the crystalline samples, it was found that three types of luminescence bands can simultaneously appear: monomer emission, excimer emission from the moderately π-stacked intra-dimer unit, and excimer emission from the widely π-stacked inter-dimer unit. Consequently, the PL colors were drastically changed by the steric effect of the methyl groups, with a strong correlation found between the π-overlapping and excimer character. In addition, variable-temperature PL measurements revealed that these PL species should be in thermal equilibrium at room temperature, with the intensity ratios sensitive toward temperature changes.

Development of a fluoride-anion sensor based on aggregation of a dye-modified polyhedral oligomeric silsesquioxane.

We report a new concept for the turn-on fluoride sensor based on the aggregation of dye-modified polyhedral oligomeric silsesquioxane (POSS). The dye-modified POSS aggregation initially shows weak fluorescence, while intense fluorescence can be obtained when fluoride breaks POSS cores following dye release.

Modulation of Properties by Ion Changing Based on Luminescent Ionic Salts Consisting of Spirobi(boron ketoiminate).

We report development of luminescent ionic salts consisting of the boron ketoiminate structure, which is one of the robust skeletons for expressing aggregation-induced emission (AIE) properties. From the formation of the boron-centered spiro structure with the ketoiminate ligands, we obtained stable ionic salts with variable anions. Since the ionic salts show Tms below 100 °C, it was shown that these salts can be classified as an ionic liquid. By using PF6 anion, the single crystal-which is applicable for X-ray crystallography-was obtained. According to the optical measurements, it was proposed that electronic interaction should occur through the boron center. Moreover, intense emission was observed both in solution and solid. Finally, we demonstrated that the emission color of the PF6 salt was altered from crystal to amorphous by adding mechanical forces. Based on boron complexation and intrinsic solid-state luminescent characters, we achieved obtainment of emissive ionic materials with environmental responsivity.

Controlling the Dual-Emission Character of Aryl-Modified o-Carboranes by Intramolecular CH⋠⋠⋠O Interaction Sites.

Invited for the cover of this issue is the group of Kazuo Tanaka at Kyoto University. The image depicts the control of solid-state dual-emissive properties by modulating the intramolecular hydrogen bonding in boron clusters. Read the full text of the article at 10.1002/chem.202200155.

Recent Progress on Designable Hybrids with Stimuli-Responsive Optical Properties Originating from Molecular Assembly Concerning Polyhedral Oligomeric Silsesquioxane.

In this review, we describe recent progress on stimuli-responsive hybrid materials based on polyhedral oligomeric silsesquioxane (POSS) and their applications as a chemical sensor. In particular, we explain the unique functions originating from molecular assembly concerning POSS-containing soft materials mainly from our studies. POSS has an inorganic cubic core composed of silicon-oxygen (Si-O) bonds and organic substituents at each vertex. Owing to intrinsic properties of POSS, such as high thermal stability, rigidity, and low chemical reactivity, various robust hybrid materials have been developed. From the numerous numbers of POSS hybrids, we herein focus on the environment-sensitive optical materials in which molecular assembly of POSS itself and functional units connected to POSS should be a key factor for expressing material properties. We also explain the mechanisms of chemical sensors originating from these stimuli-responsive optical properties. Stimuli-responsive excimer emission and pollutant detectors, nanoplastic sensors with the water-dispersive POSS networks, trans fatty acid sensors, turn-on luminescent sensors for aerobic condition and fluoride anion sensors are described. We also mention the mechanochromic polyurethane hybrids and the thermally-durable mechanochromic luminescent materials. The roles of the unique optical properties from soft materials composed of rigid POSS, which doesn't have significant light-absorption and emission properties in the visible region, are surveyed.

Controlling the Dual-Emission Character of Aryl-Modified o-Carboranes by Intramolecular CH⋠⋠⋠O Interaction Sites.

It is still challenging to realize a dual-emission system, in which two luminescent bands simultaneously appear by photoexcitation, in solid with organic dyes due to the difficulty in regulation of electronic properties in the excited state and concentration quenching. o-Carborane is known to be a versatile platform for constructing solid-state emitters since the sphere boron cluster is favorable for suppressing intermolecular interactions and subsequently concentration quenching. Here, we show solid-state dual-emissive o-carborane derivatives. We prepared 4 types of o-carborane derivatives and found dual-emission behaviors both in solution and solid states. By regulating the rotation at the o-carborane unit with the intramolecular Ccage H⋅⋅⋅O interaction, the dual-emission intensity ratios were changed. Finally, it was demonstrated that the overall photoluminescence spectra can be estimated using the binding energy of intramolecular interactions.

Development of NIR emissive fully-fused bisboron complexes with π-conjugated systems including multiple azo groups.

Development of novel near-infrared (NIR) emitters is essential for satisfying the growing demands of advancing optical telecommunication and medical technology. We synthesized elemental skeletons composed of robust π-conjugated systems including two boron-fused azo groups, which showed an intense emission in the red or near-infrared (NIR) region both in solution and solid states. Two types of bisboron complexes with different aromatic linkers showed emission properties with larger bathochromic shifts and emission efficiencies in solution than the corresponding monoboron complex. Transient absorption spectroscopy disclosed that the inferior optical properties of the monoboron complex can be attributed to fast nonradiative deactivation accompanied by a large structural relaxation after photoexcitation. The expanded π-conjugated system through multiple boron-fused azo groups can contribute to rigid molecular skeletons followed by improved emission properties. Moreover, the anti-form of the bisboron complex with fluorine groups in the opposite directions to the π-plane exhibited crystallization-induced emission enhancement in the NIR region. The molecular design by using multiple boron-fused azo groups is expected to be a critical strategy for creating novel NIR emitters.

Development of Long Wavelength Light-Absorptive Homopolymers Based on Pentaazaphenalene by Regioselective Oxidative Polymerization.

We report the synthesis and absorption properties of homopolymers consisting of 1,3,4,6,9b-pentaazaphenalene (5AP). Oxidative polymerization in the Scholl reaction was accomplished, and various lengths of homopolymers can be isolated. It should be noted that we scarcely observed the generation of structural isomers at the connecting points, which is often observed in this type of reaction. Therefore, we were able to evaluate electronic structures of the synthesized homopolymers. In addition, it was observed that absorption bands were obtained in the longer wavelength region than the monomer. The computer calculation suggests that the highest occupied molecular orbital (HOMO) energy levels could be lowered by electronic interaction through spatially-separated HOMOs of 5AP. Moreover, we can evaluate the extension of the conjugated system through the meta-substituted skeleton and distance dependency of the main-chain conjugation.

Molecular design and application of luminescent materials composed of group 13 elements with an aggregation-induced emission property.

Complexation of π-conjugated ligands by metal or semimetal ions leads to the enhancement of the planarity and rigidity of π-conjugated systems. Boron, especially, has played a central role in the design of luminescent main-group complexes. However, these complexes still suffer the disadvantage of aggregation-caused quenching as well as typical organic fluorophores. It has recently been reported that some types of boron complexes exhibit the aggregation-induced emission (AIE) property. Moreover, AIE behavior from complexes and organometallic compounds composed of the other group 13 elements, such as aluminum and gallium, has emerged in this decade. These observations greatly encourage us to develop advanced functional materials based on the group 13 elements. Indeed, recent research has demonstrated that these classes of materials are potentially versatile scaffolds for constructing chromic luminophores, efficiently emissive π-conjugated polymers and so on. This review mainly describes AIE-active group 13 complexes with four-coordinate structures and their application as photo-functional materials. Proposed mechanisms of the origins of AIE behavior are briefly discussed.

Stimuli-Responsive Self-Assembly of π-Conjugated Liquids Triggers Circularly Polarized Luminescence.

We developed novel room-temperature stimuli-responsive N-heteroacene-based liquid materials bearing a chiral alkyl chain. When these liquid materials were exposed to HCl vapor as an external stimulus, a disordered-ordered state change occurred immediately to yield self-assembled solid states from fluidic liquids. The self-assembly mechanism during this state change was evaluated by experimental in situ observations and molecular dynamics simulations over various spatiotemporal scales. These self-assembled structures led to supramolecular chirality through the influence of the chiral alkyl chain. As a result, circularly polarized luminescence (CPL) was triggered in the solid state, which was absent in the precursor liquid, thereby rendering this the first report on a stimuli-responsive CPL on/off liquid material. In addition, the initial state was recovered by exposure to air or upon heating. Moreover, the synergy between the experimental and the theoretical studies opens a new avenue to develop a novel class of stimuli-responsive materials and to discover novel phenomena in such materials.

Dimerization-Induced Solid-State Excimer Emission Showing Consecutive Thermochromic Luminescence Based on Acridine-Modified o-Carboranes.

Although excimer emission is a useful luminescent phenomenon for fabricating optical sensors and probes, it is difficult to apply excimer emission for film sensors due to critical concentration quenching in the solid state. Therefore, robust molecular designs for solid-state excimer emission are still being explored. One of the key examples is the previously reported acridine-ethynyl-o-carborane AcE1, which showed a bright solid-state excimer emission assisted by characteristic CcageH···N interactions. In this paper, we report the newly synthesized acridine-diehynyl-o-carborane AcE2 and comprehensively compare it to AcE1. Both compounds had the same crystalline packing mode based on dimer formation, resulting in an efficient π-overlapping area and solid-state excimer emission. Variable-temperature photoluminescence (VT-PL) measurements revealed the consecutive thermochromic luminescence of these compounds. Finally, on the basis of the easily accessible spray-coating method, we constructed the thermochromic luminescent sensors on quartz substrates. According to the mechanistic studies, it is demonstrated that the design strategy based on a dimer-induced solid-state excimer should have great potential for applications as a molecular thermometer.

Reversible Vapochromic Luminescence Accompanied by Planar Half-Chair Conformational Change of a Propeller-Shaped Boron ß-Diketiminate Complex.

Leakage of volatile organic compounds (VOCs) is one of the most severe industrial problems, because it can cause environmental pollution, global warming, fire, and explosion. Hence, the visualization of leakage is an essential technology to detect it at an early stage. Molecular crystals, fluorescence color of which can be changed by the exposure to VOCs could potentially serve as the sensing materials for realizing rapid and facile VOC detection. However, these materials usually require harsh conditions, such as heating or a vacuum, to recover their initial phases for reuse. Therefore, it remains a challenge to obtain completely reversible sensing systems without such energy-consuming recycling processes. Herein, the reversible color change of fluorescence from the crystals of a propeller-shaped boron ß-diketiminate complex is reported. The complex was crystallized in distinct crystalline phases having different luminescent colors. Importantly, these phases were interconverted very rapidly (time constant <60 s) and repeatedly upon exposure to the vapors of the appropriate VOCs. The small energy differences between conformers of the complex could lead to this pseudopolymorphic behavior. This finding could be applied for the development of further eco-friendly reversible sensing materials based on four-coordinated boron complexes.

Paintable Hybrids with Thermally Stable Dual Emission Composed of Tetraphenylethene-Integrated POSS and MEH-PPV for Heat-Resistant White-Light Luminophores.

Thermally stable dual emission followed by white-light luminescence from hybrid materials is reported. Hybrid films were prepared with a spin-coating method with the mixture solution containing tetraphenylethene (TPE)-integrated polyhedral oligomeric silsesquioxane (POSS) and poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV). TPE-tethered POSS (TPE-POSS) showed high compatibility with MEH-PPV. Therefore, homogeneous films with variable concentrations of TPE-POSS were obtained. Owing to good dispersion of rigid silica cubes into matrices, POSS-containing films demonstrated high thermal stability toward molecular rearrangement by annealing as well as pyrolysis, similar to conventional polymer hybrids. Furthermore, it was found that TPE-POSS was able to enhance emission efficiencies, probably by suppressing chain aggregation. By modulating introduction ratios of TPE-POSS, dual-emission properties followed by white-light luminescence composed of cyan and orange emissions from TPE-POSS and MEH-PPV, respectively, were accomplished. It should be noted that these color balances can be preserved even in the high-temperature region (425 K). Finally, white-light luminescent materials with thermal durability were obtained.

Vapochromic Luminescent π-Conjugated Systems with Reversible Coordination-Number Control of Hypervalent Tin(IV)-Fused Azobenzene Complexes.

The dynamic and reversible changes of coordination numbers between five and six in solution and solid states, based on hypervalent tin(IV)-fused azobenzene (TAz) complexes, are reported. It was found that the TAz complexes showed deep-red emission owing to the hypervalent bond composed of an electron-donating three-center four-electron (3c-4e) bond and an electron-accepting nitrogen-tin (N-Sn) coordination. Furthermore, hypsochromic shifts in optical spectra were observed in Lewis basic solvents because of alteration of the coordination number from five to six. In particular, vapochromic luminescence was induced by attachment of dimethyl sulfoxide (DMSO) vapor to the coordination point at the tin atom accompanied with a crystal-crystal phase transition. Additionally, the color-change mechanism and degree of binding constants were well explained by theoretical calculation. To the best of our knowledge, this is the first example of vapochromic luminescence by using stable and variable coordination numbers of hypervalent bonds.

Controlling Energy Gaps of π-Conjugated Polymers by Multi-Fluorinated Boron-Fused Azobenzene Acceptors for Highly Efficient Near-Infrared Emission.

We demonstrate that multi-fluorinated boron-fused azobenzene (BAz) complexes can work as a strong electron acceptor in electron donor-acceptor (D-A) type π-conjugated polymers. Position-dependent substitution effects were revealed, and the energy level of the lowest unoccupied molecular orbital (LUMO) was critically decreased by fluorination. As a result, the obtained polymers showed near-infrared (NIR) emission (λPL =758-847 nm) with high absolute photoluminescence quantum yield (ΦPL =7-23%) originating from low-lying LUMO energy levels of the BAz moieties (-3.94 to -4.25 eV). Owing to inherent solid-state emissive properties of the BAz units, deeper NIR emission (λPL =852980 nm) was detected in film state. Clear solvent effects prove that the NIR emission is from a charge transfer state originating from a strong D-A interaction. The effects of fluorination on the frontier orbitals are well understandable and predictable by theoretical calculation with density functional theory. This study demonstrates the effectiveness of fluorination to the BAz units for producing a strong electron-accepting unit through fine-tuning of energy gaps, which can be the promising strategy for designing NIR absorptive and emissive materials.

Discovery of Functional Luminescence Properties Based on Flexible and Bendable Boron-Fused Azomethine/Azobenzene Complexes with O,N,O-Type Tridentate Ligands.

Azomethine (C=N) and azo (N=N) scaffolds are a part of structural units in poly(p-phenylene azomethine) (PAM) and poly(p-phenylene azo) (PAZ), respectively. Poly(p-phenylene vinylene) (PPV) is known to be one of luminescent π-conjugated polymers, meanwhile PAM and PAZ, which are the aza-substituted PPV analogues, are regarded as weak or no emissive materials. However, by the boron complexation, intense emission can be induced. Furthermore, environment-sensitivity and stimuli-responsivity were also observed. In this review, we demonstrate unique and versatile luminescent properties based on "flexible and bendable" π-conjugated systems composed of the boron-fused azomethine and azobenzene complexes (BAm and BAz) with the O,N,O-type tridentate ligands. The "flexible and bendable" luminophores showed intriguing optical behaviors, such as thermosalient effect, aggregation-induced emission (AIE) and crystallized-induced emission (CIE). Moreover, highly efficient emissions both in solution and film states were observed from the polymers. We illustrate the results and mechanisms on these luminescent properties from the series of our recent studies with BAm and BAz complexes and polymers.