Experimental and Theoretical Studies on Cobalt-Catalyzed Regioselective Hydrosilylation of Tetrafluorinated Cyclohexa-1,3-dienes.

Cyclohexa-1,3-dienes bearing a tetrafluoroethylene group underwent highly regioselective hydrosilylation in the presence of 1-10 mol % Co2(CO)8 in 1,2-dichloroethane under mild conditions (reflux, 3 h), which led to an abundant yield of homoallylsilanes. Mechanistic studies proved that the reaction proceeds as per the modified Chalk-Harrod mechanism; via DFT calculation, the reason for homoallylsilanes being exclusively obtained was demonstrated. The formal synthesis of a tetrafluorinated negative-type liquid crystal demonstrated the synthetic utility of such hydrosilylation.

Diverse Synthetic Transformations Using 4-Bromo-3,3,4,4-tetrafluorobut-1-ene and Its Applications in the Preparation of CF2 CF2 -Containing Sugars, Liquid Crystals, and Light-Emitting Materials.

Organic molecules with fluoroalkylene scaffolds, especially a tetrafluoroethylene (CF2 CF2 ) moiety, in their molecular structures exhibit unique biological activities, or can be applied to functional materials such as liquid crystals and light-emitting materials. Although several methods for the syntheses of CF2 CF2 -containing organic molecules have been reported to date, they have been limited to methods using explosives and fluorinating agents. Therefore, there is an urgent need to develop simple and efficient approaches to synthesize CF2 CF2 -containing organic molecules from readily available fluorinated substrates using carbon-carbon bond formation reactions. This personal account summarizes the simple and efficient transformation of functional groups at both ends of 4-bromo-3,3,4,4-tetrafluorobut-1-ene and discusses its synthetic applications to biologically active fluorinated sugars and functional materials, such as liquid crystals and light-emitting molecules.

Development of Donor-π-Acceptor-Type Fluorinated Tolanes as Compact Condensed Phase Luminophores and Applications in Photoluminescent Liquid-Crystalline Molecules.

Fluorinated tolanes, produced by introducing fluorine atoms into one of the aromatic rings of tolane, emitted almost no fluorescence in a solution state, but the fluorescence intensity increased dramatically in the crystalline state because of intermolecular H⋅⋅⋅F hydrogen bonds. The photoluminescent (PL) colors depend on the molecular orbitals, dipole moments, and molecular aggregated structures can be varied by controlling terminal substituents along the major molecular axis. The introduction of a long alkoxy or semifluoroalkoxy unit as a flexible chain into the terminal positions along the major molecular axis induced the formation of a liquid-crystalline (LC) phase; fluorinated tolanes act both as luminophores and as mesogens, leading to the molecular design of new photoluminescent LC molecules (PLLCs). The results also indicated that a fluorinated tolane dimer, which consists of two fluorinated tolanes linked by a flexible alkylene spacer, also becomes a novel PLLC.

Donor-π-Acceptor-Type Fluorinated Tolane Containing a Semifluoroalkoxy Chain as a Condensed-Phase Luminophore.

Photoluminescent liquid-crystalline (PLLC) molecules, which can easily tune the PL behavior through the crystal (Cry)-LC phase transition, have attracted significant attention. Previously, we have demonstrated that the incorporation of a semifluoroalkoxy chain into π-conjugated mesogen is a promising approach for developing PLLC molecules with PL and SmA LC characteristics. We focused on the LC and PL characteristics of the molecules induced by the semifluoroalkoxy chain and fluorinated tolanes in the condensed phase. In this study, we developed cyano- or ethoxycarbonyl-terminated donor-π-acceptor-type fluorinated tolanes containing a semifluoroalkoxy flexible chain. The cyano-terminated fluorinated tolanes exhibited intense light-blue photoluminescence in the crystalline phase and did not exhibit any LC phase. In contrast, blue photoluminescence in the ethoxycarbonyl-terminated analogs was slightly weak; however, they exhibited Cry-SmA phase transition during the heating and cooling processes. The PL intensity of the ethoxycarbonyl-terminated fluorinated tolanes significantly decreased in the SmA phase; however, their PL colors changed during the Cry-SmA phase transition. This indicates that the developed tolanes are promising temperature-dependent PL materials, such as PL thermosensors or PL thermometers.

Development of Yellow-to-Orange Photoluminescence Molecules Based on Alterations in the Donor Units of Fluorinated Tolanes.

Since the aggregation-induced emission (AIE) phenomenon was first reported by Tang et al., much effort has been devoted to the development of solid-state luminescent molecules by chemists worldwide. Our group successfully developed fluorinated tolanes as novel compact π-conjugated luminophores with blue photoluminescence (PL) in the crystalline state. Moreover, we reported the yellow-green PL molecules based on their electron-density distributions. In the present study, we designed and synthesized fluorinated tolanes with various amine-based donors and evaluated their photophysical properties. The carbazole-substituted fluorinated tolane exhibited strong PL in the solution state, whereas piperidine- or phenothiazine-substituted fluorinated tolanes showed a dramatic decrease in PL efficiency. Notably, fluorinated tolanes with piperidine or phenothiazine substituents displayed yellow-to-orange PL in the crystalline state; this may have occurred because these tolanes exhibited tightly packed structures formed by intermolecular interactions, such as H···F hydrogen bonds, which suppressed the non-radiative deactivation process. Moreover, fluorinated tolanes with amine-based donors exhibited AIE characteristics. We believe that these yellow-to-orange solid PL molecules can contribute to the development of new solid luminescent materials.

Practical Synthesis of α-Trifluoromethylated Pyridines Based on Regioselective Cobalt-Catalyzed [2+2+2] Cycloaddition using Trifluoromethylated Diynes with Nitriles.

Regioselective cobalt-catalyzed [2+2+2] cycloaddition using fluorine-containing diynes with nitriles was described. Cycloaddition of fluorinated diynes with nitriles under the influence of CoCl2(phen), zinc bromide, and zinc dust in dichloroethane at 80°C for 3 h took place smoothly, exclusively affording the corresponding α-fluoroalkylated pyridines in excellent yields. In addition, dinitriles as substrate were also found to be suitable for this reaction, giving the corresponding fluoroalkylated bipyridine derivatives in excellent yields.

Systematic Studies on the Effect of Fluorine Atoms in Fluorinated Tolanes on Their Photophysical Properties.

In this study, we synthesized a series of fluorinated and non-fluorinated tolanes, in which one or more fluorine atoms were systematically introduced into one aromatic ring of a tolane scaffold, and systematically evaluated their photophysical properties. All the tolanes with or without fluorine substituents were found to have poor photoluminescence (PL) in tetrahydrofuran (THF) solutions. On the other hand, in the crystalline state, non-fluorinated and fluorinated tolanes with one or four fluorine atoms were less emissive, whereas fluorinated tolanes with three or five fluorine atoms exhibited high PL efficiencies (ФPL) up to 0.51. X-ray crystallographic analyses of the emissive fluorinated tolanes revealed that the position of the fluorine substituent played a key role in achieving a high ФPL. Fluorine substituents at the ortho (2/6) and para (4) positions led to tight and rigid packing due to plural π-π stacking and/or hydrogen bonding interactions, resulting in enhanced ФPL caused by the suppression of non-radiative deactivation. Additionally, fluorinated tolanes with three fluorine atoms exhibited notable aggregation-induced PL emission enhancement in THF/water mixed solvents. This demonstrates that the PL characteristics of small PL materials can be tuned depending on the usage requirements.

Cobalt-Catalyzed C-H Activation/Annulation of Benzamides with Fluorine-Containing Alkynes: A Route to 3- and 4-Fluoroalkylated Isoquinolinones.

The C-H activation/annulation reaction of various benzamides with fluoroalkylated alkynes in the presence of a Co(acac)2·2H2O catalyst proceeded very smoothly to give the corresponding 3- and 4-fluoroalkylated isoquinolinones in excellent yields with approximately 70% regioselectivities. These regioisomers could be successfully separated and obtained in pure form. Major or minor regioisomers were determined as 4- or 3-fluoroalkylated isoquinolinones, respectively, based on X-ray crystallographic analyses.

Excited-State Dynamics of Room-Temperature Phosphorescent Organic Materials Based on Monobenzil and Bisbenzil Frameworks.

Room-temperature phosphorescent (RTP) materials have been attracting tremendous interest, owing to their unique material characteristics and potential applications for state-of-the-art optoelectronic devices. Recently, we reported the synthesis and fundamental photophysical properties of new RTP materials based on benzil, i.e., fluorinated monobenzil derivative and fluorinated and non-fluorinated bisbenzil derivative analogues [Yamada, S. et al., Beilstein J. Org. Chem. 2020, 16, 1154-1162.]. To deeply understand their RTP properties, we investigated the excited-state dynamics and photostability of the derivatives by means of time-resolved and steady-state photoluminescence spectroscopies. For these derivatives, clear RTP emissions with lifetimes on the microsecond timescale were identified. Among them, the monobenzil derivative was found to be the most efficient RTP material, showing both the longest lifetime and highest amplitude RTP emission. Time-resolved photoluminescence spectra, measured at 77 K, and density functional theory calculations revealed the existence of a second excited triplet state in the vicinity of the first excited singlet state for the monobenzil derivative, indicative of the presence of a fast intersystem crossing pathway. The correlation between the excited state dynamics, emission properties, and conformational flexibility of the three derivatives is discussed.

Regioselective cobalt(II)-catalyzed [2 + 3] cycloaddition reaction of fluoroalkylated alkynes with 2-formylphenylboronic acids: easy access to 2-fluoroalkylated indenols.

[2 + 3] cycloaddition reactions of fluorinated alkynes with 2-formylphenylboronic acids under the influence of Co(acac)2·2H2O in two-component solvents of acetonitrile/2-propanol at reflux temperature for 18 h took place smoothly, affording the corresponding fluoroalkylated indenol derivatives in good yields. This reaction shows excellent regioselectivity, giving 2-fluoroalkylated indenols, together with a very small amount of 3-fluoroalkylated indanones as side products.

Development of fluorinated benzils and bisbenzils as room-temperature phosphorescent molecules.

Pure organic phosphorescent molecules are attractive alternatives to transition-metal-complex-based phosphores for biomedical and technological applications owing to their abundance and nontoxicity. This article discloses the design, synthesis, and photophysical properties of fluorinated benzil and bisbenzil derivatives as potential pure organic room-temperature phosphorescent molecules. These compounds were separately converted from the corresponding fluorinated bistolanes via PdCl2-catalyzed oxidation by dimethyl sulfoxide, while nonfluorinated bistolane provided the corresponding bisbenzil derivatives exclusively in a similar manner. Intensive investigations of the photophysical properties of the benzil and bisbenzil derivatives in toluene at 25 °C showed both fluorescence with a photoluminescence (PL) band at a maximum wavelength (λPL) of around 400 nm and phosphorescence with a PL band at a λPL of around 560 nm. Interestingly, intersystem crossing effectively caused fluorinated benzils to emit phosphorescence, which may arise from immediate spin-orbit coupling involving the 1(n, π)→3(π, π) transition, unlike the case of fluorinated or nonfluorinated bisbenzil analogues. These findings offer a useful guide for developing novel pure organic room-temperature phosphorescent materials.

Novel V- and Y-Shaped Light-Emitting Liquid Crystals with Pentafluorinated Bistolane-Based Luminophores.

Herein, we describe the synthesis of novel light-emitting liquid-crystalline (LC) compounds bearing pentafluorinated bistolane-based luminophores with a V- or a Y-shaped molecular geometry and the evaluation of their LC and photophysical characteristics. The V- or Y-shaped compounds exhibited a unique LC phase and showed photoluminescence (PL) behavior under various circumstances, such as in dilute solution or in the solid state. Notably, PL characteristics were observed even under high-temperature conditions with a crystal (Cr) to LC phase transition, although the PL efficiency (Φ PL) was gradually reduced because of thermal molecular motion. Interestingly, Φ PL was found to be completely recovered through the LC → Cr phase transition during the cooling process; the PL characteristics of the V- or Y-shaped compounds were sensitively changed by external thermal stress, giving these compounds the ability to act as thermoresponsive PL sensing materials.

Synthesis and characterization of bent fluorine-containing donor-π-acceptor molecules as intense luminophores with large Stokes shifts.

Herein, we prepared novel bent fluorine-containing donor-π-acceptor (D-π-A) molecules from commercially available octafluorocyclopentene using a facile two-step procedure, revealing that the above molecules absorb UV-light and exhibit yellow photoluminescence (PL) with high PL efficiencies (ΦPL) in solution. The corresponding Stokes shifts exceeded 10 000 cm-1, and the maximum PL wavelength (λPL) strongly depended on solvent polarity or intermolecular interactions in the solid state. On the basis of a Lippert-Mataga plot, PL was confidently assigned to radiative relaxation from an intramolecular charge-transfer excited state. Moreover, the synthesized luminophores showed intense PL even in the crystalline state and exhibited alkoxy chain length-dependent PL behavior (e.g., high ΦPL, λPL = 486-540 nm).

Stabilization of SF5- with Glyme-Coordinated Alkali Metal Cations.

The stabilization of complex fluoroanions derived from weakly acidic parent fluorides is a significant and ongoing challenge. The [SF5]- anion is recognized as one such case, and only a limited number of [SF5]- salts are known to be stable at room temperature. In the present study, glyme-coordinated alkali metal cations (K+, Rb+, and Cs+) are employed to stabilize [SF5]-, which provides a simple synthetic route to a [SF5]- salt. The reactivities of KF and RbF with SF4 are significantly enhanced by complexation with G4, based on Raman spectroscopic analyses. A new room-temperature stable salt, [Cs(G4)2][SF5] (G4 = tetraglyme), was synthesized by stoichiometric reaction of CsF, G4, and SF4. The vibrational frequencies of [SF5]- were assigned based on quantum chemical calculations, and the shift of the G4 breathing mode accompanying coordination to metal cations was confirmed by Raman spectroscopy. Single-crystal X-ray diffraction revealed that Cs+ is completely isolated from [SF5]- by two G4 ligands and [SF5]- is disordered along the crystallographic two-fold axis. Hirshfeld surface analysis reveals that the H···H interaction between two neighboring [Cs(G4)2]+ moieties is more dominant on the Hirshfeld surface than the interaction between the H atom in glyme molecules and the F atom in [SF5]-, providing a CsCl-type structural model where the large and spherical [Cs(G4)2]+ cations contact each other and the [SF5]- anions occupy interstitial spaces in the crystal lattice. The [SF5]- anion, combined with [Cs(G4)2]+, exhibits a very limited deoxofluorinating ability toward hydroxyl groups in both neat conditions and THF solutions.

Practical tetrafluoroethylene fragment installation through a coupling reaction of (1,1,2,2-tetrafluorobut-3-en-1-yl)zinc bromide with various electrophiles.

(1,1,2,2-Tetrafluorobut-3-en-1-yl)zinc bromide was prepared by insertion of the zinc-silver couple into the CF2-Br bond of commercially available 4-bromo-3,3,4,4-tetrafluorobut-1-ene in DMF at 0 °C for 0.5 h, The resultant polyfluorinated zinc reagent was found to be thermally stable at ambient temperature and storable for at least 1.5 years in the refrigerator. This CF2CF2-containing organozinc reagent could be easily transmetallated to copper species, which underwent cross-coupling reactions with various aromatic iodides or acyl chlorides to produce a broad range of CF2CF2-containing organic molecules in good-to-excellent yields. Therefore, the zinc reagent could become a new and practical synthetic tool for producing functional molecules with a CF2CF2 fragment.

Thermoresponsive luminescence properties of polyfluorinated bistolane-type light-emitting liquid crystals.

We developed and characterized four polyfluorinated bistolane derivatives. These compounds, which possess either two alkoxy substituents or an alkoxy group and a bromine atom in their two molecular terminals, were synthesized from readily available 4-alkoxy-1-ethynylbenzene with a facile three-step procedure. Their thermodynamic and photophysical properties were evaluated in detail, and they were found to display both liquid-crystalline (LC) and photoluminescence properties. Remarkably, the photoluminescence behaviors dramatically changed during the thermal phase transition between the crystal and LC phases. Thus, these polyfluorinated bistolanes may be promising candidates for thermoresponsive luminous molecules.

Gram-scale preparation of negative-type liquid crystals with a CF2CF2-carbocycle unit via an improved short-step synthetic protocol.

Herein, we demonstrate an improved short-step protocol for the synthesis of multicyclic molecules having a CF2CF2-containing cyclohexadiene or cyclohexane framework in a mesogenic structure. These molecules are promising candidates for vertical alignment (VA)-mode liquid crystal (LC) display devices owing to their large negative dielectric constant. The tetrafluorinated multicyclic molecules were successfully obtained in only five or six reaction steps without the need for special handling techniques, as is generally required for thermally unstable organometallic species, representing a reduction of three reaction steps. The improved short-step synthetic protocol was also amenable to the multigram preparation of these promising molecules, which may contribute significantly to the development of novel negative-type LC molecules containing CF2CF2 carbocycles.

Development of Novel Solid-State Light-Emitting Materials Based on Pentafluorinated Tolane Fluorophores.

We herein describe the synthesis of novel pentafluorinated tolane fluorophores, which possess an extended π-conjugated structure with a large molecular dipole moment along the longitudinal axis. We also report a detailed evaluation of both the photophysical and thermal behaviors of these fluorophores. All molecules displayed photoluminescence (PL) characteristics in both the crystalline state and in dilute solutions. The large longitudinal dipole moment induced solvatochromic PL behavior, which switched sensitively with changes in the solvent polarity. In addition, incorporation of the fluorinated tolane-based solid-state light-emitting moiety into the polymer side chain was found to be responsible for the PL characteristics observed in the solid state. It was also noteworthy that the polymerization protocol led to a significant enhancement in the thermal stability, with the thermal decomposition temperature increased by 90 °C. Accordingly, novel solid-state light-emitting materials with high thermal stabilities were successfully developed as promising candidates for use in light-emitting and optoelectronic applications.

Rational design and synthesis of a novel laterally-tetrafluorinated tricyclic mesogen with large negative dielectric anisotropy.

Guided by theoretical calculations, we predicted that 5,5,6,6-tetrafluoro-1-methoxycyclohex-1-ene, a new structural motif, exhibits a large negative dielectric anisotropy, and then developed a synthetic strategy for the preparation of the novel tetrafluorinated tricyclic mesogen in twelve reaction steps starting from a commercially available fluorinated starting material. During the evaluation of its liquid-crystalline characteristics, the tetrafluorinated mesogen was found to display low birefringence and a large negative dielectric anisotropy, confirming that its molecular structure is a reasonable design for the further development of novel mesogenic molecules with large negative dielectric anisotropies.

Fluorine-containing bistolanes as light-emitting liquid crystalline molecules.

We synthesised a series of dissymmetric bistolane derivatives and evaluated their liquid-crystalline (LC) and photoluminescence properties in detail. In measuring LC behaviours, rational structural design based on the dissymmetric molecular structure and electron-density distribution facilitated the production of the LC phase with a wide temperature range (up to 97 °C). In addition, dissymmetric bistolane derivatives were shown to strongly emit blue-photoluminescence in dilute solution and in crystalline states. It was found that dissymmetric bistolanes possess emissive features in even the LC phase and photoluminescence behaviours such as emission intensity and colour were sensitively switched depending on the molecular aggregate structure caused by applying a thermal stimulus.