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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Preparation of Near-Infrared Emissive π-Conjugated Polymer Films Based on Boron-Fused Azobenzene Complexes with Perpendicularly Protruded Aryl Substituents.

Most organic luminescent dyes usually show poor emission in solid due to aggregation-caused quenching due to nonspecific intermolecular interaction, such as π-π stacking. Furthermore, since commodity molecules having near-infrared (NIR) emission properties tend to have extended π-conjugated systems, development of luminescent organic materials with solid-state NIR emission has been still challenging. Herein, the series of the azobenzene complexes with the perpendicularly-protruded aryl derivative at the boron atom toward π-conjugated system is synthesized. From the optical measurements, it is shown that these complexes can show crystallization-induced emission enhancement behaviors. The donor-acceptor type π-conjugated polymers composed of the azobenzene complexes are also synthesized. Highly-efficient NIR emission from the phenyl-substituted polymers both in solution (λPL  = 742 nm, ΦPL  = 15%) and film states (λPL  = 793 nm, ΦPL  = 9%) is obtained. Furthermore, emission wavelengths can be tuned by changing the substituent at the boron atom to the modified aryl groups. From mechanistic studies including theoretical calculations, it is shown that electronic interaction is allowable between the aryl substituent to the π-conjugated system through the tetradentate boron.

Enantioselective Synthesis of Triple Helicenes by Cross-Cyclotrimerization of a Helicenyl Aryne and Alkynes via Dynamic Kinetic Resolution.

Optically active triple helicenes (TH-1) were prepared via a palladium-catalyzed enantioselective cross-cyclotrimerization of two helicenyl arynes 5, which are generated in situ from 3, with dialkyl acetylenedicarboxylate 4. Enantiomeric ratios of up to 98:2 were obtained when using 4a and (S)-QUINAP as the alkyne and chiral ligand, respectively. The absolute stereochemistry of TH-1a was revealed to be (M,P,M) by a single-crystal X-ray diffraction analysis. Kinetic studies of the racemization of enantiomerically pure TH-1a at elevated temperatures were conducted based on a high-performance liquid chromatography analysis. The activation energy for the racemization was found to be 29.1 kcal mol-1. Density functional theory calculations revealed that the palladium-catalyzed enantioselective cross-cyclotrimerization reactions proceed via the dynamic kinetic resolution of a five-membered palladacycle 6a with two [5]helicenes. Several initially formed stereoisomers of 6a eventually isomerize into the most thermodynamically stable palladacycle intermediate (M,P,M)-6a by inversion of the [5]helicenyl moiety. Then, the insertion of 4 into 6a to form (M,P,M)-12a, followed by a reductive elimination, leads to the formation of (M,P,M)-TH-1a in a stereoselective manner. The optical properties of TH-1a were studied by circular dichroism and circularly polarized luminescence.

Recent Progress in the Development of Solid-State Luminescent o-Carboranes with Stimuli Responsivity.

o-Carborane, a cluster compound containing boron and adjacent carbon atoms, displays intriguing luminescent properties. Recently, compounds containing o-carborane units were found to show suppressed aggregation-induced quenching and intense solid-state emission; they also show potential for the development of stimuli-responsive luminochromic materials. In this Minireview, we introduce three kinds of fundamental photochemical properties: aggregation-induced emission, twisted intramolecular charge transfer in crystals, and environment-sensitive excimer formation in solids. Based on these properties, several types of luminochromism, such as thermos-, vapo-, and mechanochromism, have been discovered. Based mainly on results from recent studies, we illustrate these mechanisms as well as unique luminescent behaviors of o-carborane derivatives.

Near-Infrared Circularly Polarized Luminescence through Intramolecular Excimer Formation of Oligo(p-phenyleneethynylene)-Based Double Helicates.

Carbon-based double helicates consisting of two anthracene-containing oligo(p-phenyleneethynylene) units and two flexible chiral 1,1'-binaphthyl units or two rigid chiral 9,9'-spirobifluorene units were developed. The curved oligo(p-phenyleneethynylene) fragments in the double helicates were successfully constructed by tin-mediated reductive aromatization. Helical oligo(p-phenyleneethynylene) double strands fixed by two rigid spirobifluorene units showed little structural change under photoirradiation, thereby emitting circularly polarized luminescence (CPL) in the visible region with a high quantum yield (ΦPL =0.93). In contrast, flexible binaphthyl units induced dynamic structural change of the oligo(p-phenyleneethynylene) luminophores under photoirradiation, leading to strong CPL (|glum |=1.1×10-2 ) in the near-infrared (NIR) region. UV/Vis, circular dichroism (CD), CPL and NMR spectroscopic analyses of the binaphthyl-hinged double helicate suggested excimer formation between two π-conjugated strands in the excited state. Theoretical calculations highlight the importance of the tightly interlocked excimer structure of the carbon-based double helicate in controlling the angle between the electric and magnetic transition dipole moments for strong NIR CPL generation.

Independently Tuned Frontier Orbital Energy Levels of 1,3,4,6,9b-Pentaazaphenalene Derivatives by the Conjugation Effect.

Herein reported are syntheses and the unique substituent effect in 1,3,4,6,9b-pentaazaphenalene (5AP) derivatives. We developed the new synthetic route via ( N, N-pyridine-2,6-diyl)bisamides, which enabled us to prepare a variety of substituted 5APs and to systematically investigate the substituent effects on the electronic structures of the multi- N-heterocyclic 5APs using UV-vis absorption spectroscopy, cyclic voltammetry, and density functional theory calculations. From the analyses, we found that the substituent effects regarding the electronic states of 5APs drastically varied depending on the substitution position. It was revealed that the conjugation effects from the substituents were specifically expressed either highest occupied molecular orbitals (HOMOs) or lowest unoccupied molecular orbitals (LUMOs) by altering the substitution position at the 5AP ring. As a result, small HOMO-LUMO gaps were accomplished only by elevating HOMOs or by lowering LUMOs. The intrinsically separated HOMO and LUMO of 5AP are the origin of this unique substituent effect. These results suggested that HOMOs and LUMOs can be independently tuned by the conjugation effects in 5APs. This selective tunability of either frontier orbital is a desired character for application of organic molecules as modern optoelectronic devices and advanced organic electronic materials.

Hash-Mark-Shaped Azaacene Tetramers with Axial Chirality.

The tetramers of azapentacene derivatives with unique hash mark (#)-shaped structures were prepared in a quite facile manner. The #-shaped tetramers are optically active due to possessing extended biaryl skeletons, and the structure of the tetramer composed of four dihydrodiazapentacene (DHDAP) units (1) was investigated as the first example of this kind of molecule. The tetramer 1 showed characteristic chiroptical properties reflecting its orthogonally arranged quadruple DHDAP moieties, as well as redox activity. The solution of enantiopure 1 exhibited intense circularly polarized luminescence (CPL) with a dissymmetry factor of 2.5 × 10-3. The absolute configuration of the enantiomers of 1 was experimentally determined by X-ray crystal analysis for the dication salt of the enantiomer of 1 with SbCl6- counterions. The solutions of enantiopure 12+·2[SbCl6-] also showed NIR circular dichroism (CD) spectra over the entire range from visible to 1100 nm, enabling the modulation of the chiroptical properties by redox stimuli.

High Surface Area, Thermally Stable, Hydrophobic, Microporous, Rigid Gels Generated at Ambient from MeSi(OEt)3 /(EtO)3 SiCH2 CH2 Si(OEt)3 Mixtures by F- -Catalyzed Hydrolysis.

High surface area materials are of considerable interest for gas storage/capture, molecular sieving, catalyst supports, as well as for slow-release drug-delivery systems. We report here a very simple and fast route to very high surface area, mechanically robust, hydrophobic polymer gels prepared by fluoride-catalyzed hydrolysis of mixtures of MeSi(OEt)3 and bis-triethoxysilylethane (BTSE) at room temperature. These materials offer specific surface areas up to 1300 m2 g-1 , peak pore sizes of 0.8 nm and thermal stabilities above 200 °C. The gelation times and surface areas can be controlled by adjusting the solvent volume (dichloromethane), percent fluoride (as nBu4 NF or TBAF) and the BTSE contents. Polymers with other corners and linkers were also explored. These materials will further expand the materials databank for use in vacuum insulation panels and as thermally stable release and capture media.

Synthesis of enantiopure planar chiral bis-(para)-pseudo-meta-type [2.2]paracyclophanes.

A new type of planar chiral (Rp )- and (Sp )-4,7,12,15-tetrasubstituted [2.2]paracyclophanes was prepared from racemic 4,7,12,15-tetrabromo[2.2]paracyclophane as the starting substrate. Regioselective lithiation and transformations afforded racemic bis-(para)-pseudo-meta-type [2.2]paracyclophane (4,15-dibromo-7,12-dihydroxy[2.2]paracyclophane). Its optical resolution was performed by the diastereomer method using a chiral camphanoyl group as the chiral auxiliary. The diastereoisomers were readily isolated by simple silica gel column chromatography, and the successive hydrolysis afforded (Rp )- and (Sp )-bis-(para)-pseudo-meta-type [2.2]paracyclophanes ((Rp )- and (Sp )-4,15-dibromo-7,12-dihydroxy[2.2]paracyclophanes). They can be used as pseudo-meta-substituted chiral building blocks.

A Highly Efficient Near-Infrared-Emissive Copolymer with a N=N Double-Bond π-Conjugated System Based on a Fused Azobenzene-Boron Complex.

Fused azobenzene-boron complexes (BAzs) show highly efficient near-infrared (NIR) emission from the nitrogen-nitrogen double bond (N=N) containing π-conjugated copolymer. Optical measurements showed that BAz worked as a strong electron acceptor because of the intrinsic electron deficiency of the N=N double bond and the boron-nitrogen (B-N) coordination which dramatically lowered the energy of the lowest unoccupied molecular orbital (LUMO) of the azobenzene ligand. The simple donor-acceptor (D-A) type copolymer of bithiophene (BT) and BAz exhibited intense photoluminescence (PL) in the NIR region both in the dilute solution (λPL =751 nm, ΦPL =0.25) and in the film (λPL =821 nm, ΦPL =0.038). The BAz monomer showed slight PL in the dilute solution, and aggregation-induced emission (AIE) was detected. We proposed that N=N double bonds should be attractive and functional building blocks for designing π-conjugated materials.

Heat-Resistant Mechanoluminescent Chromism of the Hybrid Molecule Based on Boron Ketoiminate Modified Octasubstituted Polyhedral Oligomeric Silsesquioxane.

The development of a heat-resistant mechanoluminescent chromic (MLC) dye based on an organic-inorganic hybrid molecular scaffold is reported. Luminescent organoboron complexes were introduced into polyhedral oligomeric silsesquioxane (POSS) at each vertex through the rigid linker structure in the Suzuki-Miyaura cross-coupling reaction. From optical measurements, it was found that the crystalline state of the modified POSS showed orange emission. By adding mechanical forces to the sample, regular structures collapsed. Correspondingly, the emission color turned to yellow. Interestingly, the yellow-emissive amorphous state can be preserved during heating. By radially distributing luminescent chromophores on the rigid cube, both optical and thermal properties can be modulated to realize heat- resistant MLC behavior. POSS can contribute not only to expressing the MLC behavior, but also to improving thermal stability in the amorphous state. This is the first example, to the best of our knowledge, to offer the rational design of a heat-resistant MLC material.