Kenta Kokado

Multicolor Tuning of Aggregation-Induced Emission through Substituent Variation of Diphenyl-o-carborane

We demonstrate multicolor tuning of aggregation-induced emission (AIE) derived from o-carborane. Both electron-donating and electron-accepting arylacetylenes underwent efficient palladium coupling reaction with bis(4-bromophenyl)-o-carborane, resulting in moderate yields. The emission spectra of these compounds span almost the entire visible spectrum (λ(max) = 452-662 nm). Study on AIE mechanism indicated that CT-based emission of o-carborane derivatives was enhanced by the restriction of molecular motions. The computational study also suggests the possibility of precise color-control of AIE through substituent variation.

Highly luminescent BODIPY-based organoboron polymer exhibiting supramolecular self-assemble structure.

A novel class of rod-coil type-organoboron polymers with p-phenylene-ethynylene as the rod segment and long alkyl chain (decyl group) as coil segment has been prepared from a Sonogashira-Hagihara coupling reaction of a BODIPY-based monomer having bis-iodophenyl and decyl groups with diyne monomers, 1,4-diethynylbenzene (a), 1,4-diethynyl-2,5-bis(trifluoromethyl)benzene (b), and 2,7-diethynyl-9,9-dihexyl-9H-fluorene (c). The characterization by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed the strong tendency of the obtained polymers, 2a, 2b, and 2c, to self-assemble into particles in solution and as-casted films on a micron-nm scale. Especially, 2a showed the presence of nm-sized particles and micron-sized fiber-like structures formed by aggregation of each particle. Further, in CHCl3, the gelation of 2a by three-dimensional aggregation of each fiber was observed at room temperature after 24 h. Their luminescent properties showed high energy transfer efficiency from pi-conjugated polymer linkers to BODIPY moieties (PhiF > 71%).

Highly Intense Fluorescent Diarylboron Diketonate

Diarylboron diketonates were successfully prepared by the reaction of 1,3-diketone derivatives and arylboron compounds such as triphenylborane [BPh3] and fluorobis(pentafluorophenyl)borane diethyl etherate [(C6F5)2BF x OEt2]. The fluorescent emission of their complexes took place depending on the substituent of the arylboron moiety. In particular, a boron 1,3-bis(4-methoxyphenyl)-1,3-diketonate chelated by a strong electron-withdrawing C6F5 group exhibited the most intense fluorescence.

Polymer Phase‐Transition Behavior Driven by a Charge‐Transfer Interaction

Charge-transfer (CT) interactions are intermolecular interactions between p-electron-rich (donor molecules) and pelectron-deficient species (acceptor molecules). The high specificity of CT interactions enables the arrangement between a donor and an acceptor to be controlled, and the resulting CT complex shows a characteristic absorption band in the visible region; this absorption band provides information on the association of supramolecular complexes. Therefore, CT interactions are among the most powerful tools for the design of supramolecular complexes, such as organic crystalline materials with superconductivity or conductivity, low-molecular-weight organic gelators, preorganized building blocks for rotaxane, and supramolecular polymers. In many cases, the supramolecular CT complexes can be collapsed readily by heating, since a CT interaction provides a bond of modest strength unless a cavity, such as that of cucurbit[8]uril, is used to stabilize the CT complex. Herein, we demonstrate the precise and facile control of the lower critical solution temperature (LCST) of a polymer solution, that is, its phase-transition behavior, by the use of a CT interaction. The well-known phase-transition behavior associated with the LCST of a polymer solution occurs as a result of primitive molecular recognition between the polymer chains and solvent molecules (e.g., H2O). It has attracted broad interest with respect to the development of stimuli-sensitive materials because the solubility of the polymer chains changes drastically upon heating. However, one of the major drawbacks of this phenomenon is the restriction imposed by the required conditions (temperature, solvent, or pressure); as result, stimuli-sensitive materials are limited to those based on intrinsic LCST polymers, such as poly(N-isopropylamide) (PNIPAM) and poly(ethylene glycol) (PEG). Sophisticated supramolecular control of the LCST has been paid much attention, because such studies can uncover not only a fundamental perspective of the phenomenon but also potential applications in smart materials. As an outstanding example, Ritter and co-workers reported that a polymer bearing adamantyl moieties showed LCST behavior in the presence of methylated b-cyclodextrins (b-CD) in water owing to the dissociation of b-CD from the adamantyl group in the polymer chain upon heating and the subsequent association of adamantyl groups through a hydrophobic interaction. We also demonstrated readily adjustable LCST behavior based on hydrogen bonding between a polymer bearing urea units and an alcohol as an “effector” for polymer solubility. In both cases, the dissociation of supramolecular complexes at an increased temperature triggered a drastic change in solubility of the polymers. These findings inspired us to explore the applicability of other weak intermolecular interactions between polymers and effectors to clarify the molecular design of LCST polymer chains and external effectors. Herein, we demonstrate the first example of LCST behavior based on a CT interaction as the driving force (Figure 1). As the result of the appearance of a CT band in the UV/Vis absorption spectrum, a quantitative evaluation of the relationship between LCST behavior and the formation of supramolecular complexes was possible. Previously, the distinction between polymer–polymer interactions and polymer–effector (solvent) interactions was considered problematic in the common case of LCST behavior driven by hydrogen bonding.

BODIPY-Based Chain Transfer Agent: Reversibly Thermoswitchable Luminescent Gold Nanoparticle Stabilized by BODIPY-Terminated Water-Soluble Polymer

Well-defined poly(N-isopropylacrylamide)s (PNIPAMs) with boron dipyrromethene (BODIPY) in the terminal end were prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization of NIPAM with BODIPY-chain transfer agent (CTA), which was synthesized by the reaction of pyrrole and BODIPY derivative having benzyl chloride in the meso position. The controlled character of the polymerization of NIPAM was confirmed by the formation of the narrow molecular weight distribution products and linear increase of molecular weight with the feed ratios of [NIPAM]/[CTA]. The RAFT-prepared PNIPAM exhibited strong green luminescence with emission maxima at 558 nm upon excitation with the absorption maxima at 526 nm and was used for the aqueous synthesis of stabilized gold nanoparticles (AuNPs). The in situ reduction of the RAFT-prepared polymers and HAuCl(4) resulted in the formation of stable modified AuNPs with size distributions of ∼10 nm in water solution. Temperature-dependent change in photoluminescence (PL) spectra (excited at 526 nm) of the RAFT-prepared polymers measured in water showed an increase in emission intensity with the rise at temperature (20 → 40 °C). On the contrary, AuNPs exhibited a decrease in emission intensity with increasing of temperature, resulting that shrinking of the distance between BODIPY units and Au core in the AuNP across border LCST gives rise to both quenching of emission of BODIPY unit by Förster resonance energy transfer (FRET) and dye quenching of aggregated BODIPY units. The emission/quenching of AuNP occurs reversibly and efficiently, regardless of the heating and cooling cycle.

Luminescent alternating boron quinolate–fluorene copolymers exhibiting high electron mobility

π-Conjugated copolymers composed of 9,9-didodecylfluorene and boron quinolate were synthesized by an efficient chelating reaction of quinolinol-based polyfluorene derivative, as a polymeric ligand, with arylborane compounds such as triphenylborane (BPh3) or fluorobis(pentafluorophenyl)borane diethyl etherate ((C6F5)BF·OEt2). The polymeric ligand was prepared from the deprotection reaction of boron tribromide and benzyloxyquinoline-based polyfluorene, which was produced via the efficient Suzuki cross-coupling reaction of 5,7-dibromo-8-benzylquinoline and 9,9-didodecylfluorene-2,7-diboronic acid in the presence of 2-(2′,6′-dimethoxybiphenyl)dicyclohexylphosphine (S-Phos) and palladium(II) acetate. Their optical properties were studied by UV-vis absorption and photoluminescence spectroscopies. The absorption and PL spectra of the obtained polymers were red-shifted in comparison with those of the polymeric ligand due to the decrease of the LUMO level by the formation of boron complexes, explained by theoretical calculations of the model compounds using the density-functional theory (DFT) method. The molar extinction coefficients and emission maxima of boron-chelating polymers were analogous values to those of (κ2-(N,O)-5,7-bis(9,9-dihexylfluoren-2-yl)-8-quinolinolate)diphenylborane (BFluorene2q). Furthermore, the electron mobilities of the polymers were determined from the space-charge limited current (SCLC) with electron-only device structure of ITO/Ca/polymer/LiF/Al. As a result, the mobilities for the polymers (3.9 or 2.0 × 10−5 cm2 V−1 s−1) were reasonably close to the value (7.9 × 10−5 cm2 V−1 s−1) calculated from the Alq3-fabricated device.

Conversion of azide to primary amine via Staudinger reaction in metal–organic frameworks

Facile conversion of azide to primary amine in metal–organic frameworks (MOFs) was accomplished by Staudinger reduction. After the reaction, MOFs retained high crystallinity confirmed by X-ray diffraction patterns, meaning a high usability of this method for post-synthetic modification of MOFs. Bulky phosphine groups provided surface-selective modification of MOFs via a reaction between resulting amine groups of MOF and activated ester with fluorescein, illustrated by confocal laser scanning microscope (CLSM) observation. This method opens up new possibilities for the preparation of MOFs having core-shell structures.

Rigidity-induced emission enhancement of network polymers crosslinked by tetraphenylethene derivatives

In this paper, we prepared various network polymers with crosslinkers having two or four acryloyl groups derived from an AIE luminogen, and investigated the relationship between the rigidity of network polymers and emission behaviour. The crosslinkers showed representative AIE properties. A conventional radical polymerization with various vinyl monomers produced corresponding network polymers. The variation of vinyl monomers or the crosslinkers allowed us to obtain network polymers with different rigidities. Fluorescence spectroscopy of the network polymers revealed rigidity dependent emission enhancement accompanying the hypsochromic shift. In addition, the network polymer showed temperature and solvent dependent emission behaviour. The crosslinker with less crosslinkable substituents showed higher susceptibility toward the environmental conditions such as rigidity, temperature, and solvent.

Anisotropically Swelling Gels Attained through Axis-Dependent Crosslinking of MOF Crystals.

Anisotropically deforming objects have attracted considerable interest for use in molecular machines and artificial muscles. Herein, we focus on a new approach based on the crystal crosslinking of organic ligands in a pillared-layer metal-organic framework (PLMOF). The approach involves the transformation from crosslinked PLMOF to polymer gels through hydrolysis of the coordination bonds between the organic ligands and metal ions, giving a network polymer that exhibits anisotropic swelling. The anisotropic monomer arrangement in the PLMOF underwent axis-dependent crosslinking to yield anisotropically swelling gels. Therefore, the crystal crosslinking of MOFs should be a useful method for creating actuators with designable deformation properties.

Visualization of the complexation between chloride and anion receptors using volume change of ionomer gels in organic solvents

Stimuli-responsive gels that can change their volumes drastically in response to various external physical and chemical stimuli have been of much interest due to their various applications. Herein, novel stimuli-responsive gels were achieved through the complexation between anion receptors and the chloride anion of polystyrene ionomer gels in aprotic organic solvents. The volume expansions induced by the addition of the receptors originated from breaking the aggregation of ionic groups and enhancing the dissociation of the ion-pairs, accompanied by their colour changes.