Enhancement of Near-Infrared Emission Based on Hypervalent Germanium(IV)-Fused Azobenzene Compounds with Electron-Donating Groups

NIR-light emitting materials are significant as a source for NIR-light technology. Previously, we have found that hypervalent germanium-fused azobenzene (GAz) compounds showed NIR emissions despite the small π-conjugated systems owing to trigonal bipyramidal geometry with three-center four-electron (3c-4e) bonds. Herein, we succeed in enhancing their luminescence properties by introduction of electron-donating groups. It is revealed that the frontier molecular orbitals of the π-conjugated systems with the hypervalent bonds can be varied depending on the type of substituents. As a result, a GAz compound with diphenylamine groups shows efficient NIR emission at room temperature (λᴘʟ = 761 nm and Φᴘʟ = 0.07) and at 77 K (λᴘʟ = 749 nm and Φᴘʟ = 0.20). Furthermore, data from theoretical calculations support the experimental results and provide additional information on the producibility of the hypervalent compounds. Our findings afford strategies to expand the library of the fine-tunable NIR-luminescent materials involving heteroatoms

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

We report stimuli-responsive luminescent -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

Acceleration of Chemiluminescence Reactions with Coumarin-modified Polyhedral Oligomeric Silsesquioxane

Chemiluminescence has attracted much attention as a light source and an excitation mediator without electric power sources. To develop advanced optical materials, it is necessary to control the chemiluminescence behavior more precisely by molecular design. Herein, we describe influence of the connection to the inorganic scaffold on the chemiluminescence properties with coumarin-modified polyhedral oligomeric silsesquioxane (D1421-POSS). Accordingly, when D1421-POSS was chemically excited using bis(2-carbopentyloxy-3, 5, 6-trichlorophenyl) oxalate (CPPO), it was shown that the initiation of chemical emission reactions was accelerated by the POSS connection, comparing to the model compound D1421-arm. From cyclic voltammetry measurements, it was revealed that the oxidation potential of D1421-POSS was significantly lower than that of D1421-arm. From these data, we propose the mechanism that aggregation assisted by the POSS core induces low oxidation potential, resulting in the acceleration of chemiluminescence reactions

Vapochromic films of pi-conjugated polymers based on coordination and desorption at hypervalent tin(iv)-fused azobenzene compounds

We report the synthesis and vapochromic behaviors of film materials consisting of hypervalent tin-containing π-conjugated polymers. We prepared copolymers with brominated tin-fused azobenzenes and modified fluorene having tetraethylene glycol as a side chain. The synthesized polymers showed good film-formability and high affinity with coordinating solvent molecules such as dimethyl sulfoxide (DMSO). In particular, we discovered distinct color changes from blue to purple when exposed to DMSO vapor. It was revealed that color changes should originate from reversible alteration of the coordination-number between five and six of hypervalent tin(IV) in the azobenzene compounds involved in the main-chain conjugation. Moreover, we also observed that binding constants between tin and coordinating solvents could be influenced by two substitutions on the tin atom and subsequently modulated responsivity of vapochromism in films by altering the type of substituent. Furthermore, the color-change behaviors can be estimated by quantum calculations with density functional theory. We demonstrate not only that hypervalent tin can work as a switching unit for modulating the electronic structures of π-conjugated polymers triggered by solvent coordination but also that vapochromic behaviors in films can be predicted by estimating the affinity between hypervalent tin and solvent molecules with theoretical calculations

Facile strategy for obtaining luminescent polymorphs based on the chirality of a boron-fused azomethine complex

A chloro-substituted boron-fused azomethine complex (BAmCl) having a stereogenic boron center was synthesized for obtaining a luminescent chiral crystal. We succeeded in isolating the (R)- and (S)-enantiomers of BAmCl and preparing the homochiral polymorphic crystal, while we obtained the racemic crystal with rac-BAmCl. Single crystal X-ray diffraction analyses suggest that a variety of intermolecular interaction patterns and intrinsic flexibility of the molecular framework should play a significant role in stabilizing the homochiral crystal. We found the difference in molecular arrangements between the racemic and the homochiral crystals, and we observed distinctly different emission colors. In particular, we observed heat-initiated homogeneous racemization without the need for a solvent or catalyst in the molten state of the homochiral crystal (R)-BAmCl. Our results mean that chiral resolution of a flexible fused-skeleton having a stereogenic boron center can be a platform for creating luminescent polymorphic materials

Cyclic arrays of five pyrenes on one rim of a planar chiral pillar[5]arene

Spatial arrangement of multiple planar chromophores is an emerging strategy for molecule-based chiroptical materials via easy and systematic synthesis. We attached five pyrene planes to a chiral macrocycle, pillar[5]arene, producing a set of chiroptical molecules in which pyrene-derived absorption and emission were endowed with dissymmetry by effective transfer of chiral information. The chiroptical response was dependent on linker structures and substituted patterns because of variable interactions between pyrene units. One of these hybrids showed larger dissymmetry factor and response wavelength (g[lum] = 7.0 × 10⁻³ at ca. 547 nm) than reported pillar[5]arene-based molecules using the pillar[5]arene cores as parts of photo-responsive π-conjugated units

Controlling Energy Gaps of pi-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

PPV-type π-conjugated polymers based on hypervalent tin(IV)-fused azobenzene complexes showing near-infrared absorption and emission

We demonstrate near-infrared (NIR) absorptive and emissive poly(p-phenylene vinylene) (PPV)-type π-conjugated polymers based on hypervalent tin-fused azobenzene (TAz) complexes. Taking advantage of the inherent narrow-energy gap of TAz complexes originating from the three-center four-electron (3c-4e) bond and nitrogen–tin (N–Sn) coordination, the synthesized polymers, TAz-PPVs, showed absorption, and emission in wavelength regions of >750 and 810 nm in diluted solution, respectively. From the experimental and theoretical investigations, the elevation of the highest occupied molecular orbital (HOMO) and the reduction of the lowest unoccupied molecular orbital (LUMO) were simultaneously shown to be caused by the extension of π-conjugation. The effective conjugation length was calculated to be n > 10 (n: degree of polymerization), and the value was comparable to conventional PPV systems. Through this research, we revealed that π-conjugated systems including hypervalent bonds were able to expand π-conjugation. According to the concept of “element blocks”, the development of heteroatom-containing narrow-energy-gap monomers should be a novel approach for the construction of new NIR-absorptive and emissive bland materials

Facile Preparation of Near Infrared-Luminescent Protein Complexes with Conjugated Polymers Consisting of Boron Azobenzene Units

We report herein facile preparation of near infrared (NIR)-luminescent protein complexes with conjugated polymers. We have discovered that solid-state luminescence in the NIR region can be obtained from the series of conjugated polymers consisting of boron azobenzene complexes. We demonstrate in this paper that protein molecules can be modified through adsorption with the boron azobenzene-containing conjugated polymers simply by mixing in an aqueous buffer and subsequently purification with filtration followed by freeze drying. The modified protein complexes can exhibit NIR emission in the buffer and high dispersibility. In particular, comparing to the complex with indocyanine green (ICG), which is a conventional NIR-luminescent dye, polymer-modified protein complexes showed higher resistance to photobleaching. Finally, by using lipase as a scaffold, we confirmed that enzymatic activity can be detected after polymer modification

CPL on/off control of an assembled system by water soluble macrocyclic chiral sources with planar chirality

Herein, we report the synthesis and planar chiral properties of a pair of water-soluble cationic pillar[5]arenes with stereogenic carbons. Interestingly, although units of the molecules were rotatable, only one planar chiral diastereomer existed in water in both cases. As a new type of chiral source, these molecules transmitted chiral information from the planar chiral cavities to the assembly of a water-soluble extended π-conjugated compound, affording circularly polarized luminescence (CPL). The chirality transfer process and resulting CPL were extremely sensitive to the feed ratio of the chiral pillar[5]arenes owing to the combined action of their planar chirality, bulkiness, and strong binding properties. When a limited amount of chiral source was added, further assembly of the extended π-conjugated compound into helical fibers with CPL was triggered. Unexpectedly, larger amounts of chiral source destroyed the helical fiber assemblies, resulting in elimination of the chirality and CPL properties from the assembled structures