Acceptor-acceptor (A-A) copolymerization is an effective strategy to develop high-performance n-type conjugated polymers. However, the development of A-A type conjugated polymers is challenging due to the synthetic difficulty. Herein, a distannylated monomer of strong electron-deficient double B←N bridged bipyridine (BNBP) unit is readily synthesized and used to develop A-A type conjugated polymers by Stille polycondensation. The resulting polymers show ultralow LUMO energy levels of -4.4 eV, which is among the lowest value reported for organoboron polymers. After n-doping, the resulting polymers exhibit electric conductivity of 7.8 S cm-1 and power factor of 24.8 μW m-1  K-2 . This performance is among the best for n-type polymer thermoelectric materials. These results demonstrate the great potential of A-A type organoboron polymers for high-performance n-type thermoelectrics.

A Distannylated Monomer of a Strong Electron-Accepting Organoboron Building Block: Enabling Acceptor-Acceptor-Type Conjugated Polymers for n-Type Thermoelectric Applications.

Near-infrared-emitting polymers were prepared using four boron-difluoride-curcuminoid-based monomers using ring-opening metathesis polymerization (ROMP). Well-defined polymers with molecular weights of ≈20 kDa and dispersities <1.07 were produced and exhibited near-infrared (NIR) emission in solution and in the solid state with photoluminescence quantum yields (ΦPL ) as high as 0.72 and 0.18, respectively. Time-resolved emission spectroscopy revealed thermally activated delayed fluorescence (TADF) in polymers containing highly planar dopants, whereas room-temperature phosphorescence dominated with twisted species. Density functional theory demonstrated that rotation about the donor-acceptor linker can give rise to TADF, even where none would be expected based on calculations using ground-state geometries. Incorporation of TADF-active materials into water-soluble polymer dots (Pdots) gave NIR-emissive nanoparticles, and conjugation of these Pdots with antibodies enabled immunofluorescent labeling of SK-BR3 human breast-cancer cells.

Near-Infrared-Emitting Boron-Difluoride-Curcuminoid-Based Polymers Exhibiting Thermally Activated Delayed Fluorescence as Biological Imaging Probes.

Flexible molecules are unfavorable for designing luminescent dyes because their excitation states rapidly decay through molecular motions. We recently found that some flexible boron complexes, which potentially show a larger degree of structural relaxation in the excited state, and their polymers exhibit unique optical properties with high environmental sensitivity, such as aggregation-induced emission and luminochromism triggered by external stimuli, upon the addition of structural restrictions. Moreover, these optical properties were drastically changed by modulating the connecting points in the polymers. In this review, recent progress in the development of luminescent polymer films with stimuli responsiveness is illustrated. In particular, the influence of the alteration of connecting points on luminescent behaviors is explained. Polymerization is a versatile strategy not only for transforming a class of nonemissive molecules into luminescent dyes but also for precisely regulating the optical properties of film materials; the resulting materials are promising for application as scaffolds for advanced chemical sensors. Recent progresses in the developments of luminescent polymer films with stimuli responsiveness are illustrated. In particular, the influence of the alteration of connecting points in polymers on luminescent behaviors is explained. It is demonstrated that polymerization is a versatile strategy not only for transforming a class of nonemissive flexible boron complexes to luminescent dyes but also for precisely regulating optical properties of film materials that are promised to be applied as a scaffold for advanced chemical sensors.

/pdf/modulation-of-the-solid-state-luminescent-properties-of-4jsf3sin3j.pdf

Modulation of the solid-state luminescent properties of conjugated polymers by changing the connecting points of flexible boron element blocks

Poly(p-phenylene vinylene) (PPV) is known to be a typical π-conjugated polymer and used as a commodity platform for constructing optoelectronic organic devices because of superior optical and material properties. PPV derivatives generally show a large degree of light absorption and intense emission in the visible region; meanwhile, very few examples have been reported to offer near-infrared (NIR) absorptive and emissive PPV derivatives. In this study, we designed and synthesized a novel PPV derivative, named BAz-PPV, containing boron-fused N═N double bond units in the main chain. BAz-PPV showed intense NIR absorption and emission (λₐbₛ = 702 nm, λPL = 760 nm, and ΦPL = 2.0% in diluted toluene). This polymer had a narrow energy band gap because of not only extension of main chain π-conjugation over 50 monomer units but also stabilization of the energy level of the lowest unoccupied molecular orbital. Moreover, the polymer shows high stability toward photodegradation and sufficient carrier-transport ability for the applications in organic semiconducting devices. Advantages of the introduction of the N═N double bond to main-chain conjugation followed by boron coordination for the development of NIR materials are demonstrated in this manuscript.

Near-Infrared Absorptive and Emissive Poly(p-phenylene vinylene) Derivative Containing Azobenzene–Boron Complexes

Stimuli-responsive luminochromic polymers consisting of multi-state emissive fused boron ketoiminate

A recent report illustrated superior optical properties, such as near-infrared emission, of polymers connected at the 4,4'-positions to a fused azobenzene-boron complex (BAz). In this study, it is initially demonstrated that further narrowing of the band gap can be realized through the substituent effect with bromine groups at the 5,5'-positions of BAz compared with those at the 4,4'-positions. From a series of mechanistic studies, perturbation of the energy levels was rationally explained by the difference in contributions of the inductive effect and the variable resonance effect, which was correlated with the degree of electron distribution of molecular orbitals at the substituent positions. Moreover, it was found that unique electronic states, such as delocalized HOMOs and LUMOs, should appear on the main chains of the BAz-containing copolymers with fluorene and bithiophene units, according to the optical and electrochemical data and theoretical calculations. By taking advantage of property tunability and the dramatically low LUMO energy level (near -4.0 eV) of the BAz unit, it can be said that BAz should be a conjugated building block favorable for building advanced optoelectronic devices.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/asia.201801659

Unique Substitution Effect at 5,5'-Positions of Fused Azobenzene-Boron Complexes with a N=N π-Conjugated System.

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.

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

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 (-394 to -425 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

Junko Wakabayashi

Papers

Near-Infrared Absorptive and Emissive Poly(p-phenylene vinylene) Derivative Containing Azobenzene–Boron Complexes

Unique Substitution Effect at 5,5'-Positions of Fused Azobenzene-Boron Complexes with a N=N π-Conjugated System.

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

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