Theoretical and Experimental Investigations of Stable Arylfluorene-Based Radical-Type Mechanophores.

Invited for the cover of this issue are the groups of Hideyuki Otsuka at the Tokyo Institute of Technology and Koichiro Mikami at the Sagami Chemical Research Institute. The image depicts theoretical and experimental investigations of stable arylfluorene-based radical-type mechanophores. Read the full text of the article at 10.1002/chem.202203249.

Theoretical and Experimental Investigations of Stable Arylfluorene-Based Radical-Type Mechanophores.

Radical-type mechanophores (RMs) can undergo homolytic cleavage of their central C-C bonds upon exposure to mechanical forces, which affords radical species. Understanding the characteristics of these radical species allows bespoke mechanoresponsive materials to be designed and developed. The thermal stability of the central C-C bonds and the oxygen tolerance of the generated radical species are crucial characteristics that determine the functions and applicability of such RM-containing mechanoresponsive materials. In this paper, we report the synthesis and characterization of two series of arylfluorene-based RM derivatives, that is, 9,9'-bis(5-methyl-2-pyridyl)-9,9'-bifluorene (BPyF) and 9,9'-bis(4,6-diphenyl-2-triazyl)-9,9'-bifluorene (BTAF). BPyF and BTAF derivatives were synthesized without generating any peroxides initially, albeit that BPyF slowly converted to the corresponding peroxide in solution. DFT calculations revealed the importance of the thermodynamic stability and the values of the α-SOMO levels of the corresponding radical species for their thermal stability and oxygen tolerance. Furthermore, the mechanochromism of BTAF was demonstrated by ball-milling a BTAF-centered polymer, which was synthesized by atom-transfer radical polymerization (ATRP).

Mechanochemical Reactions of Bis(9-methylphenyl-9-fluorenyl) Peroxides and Their Applications in Cross-Linked Polymers.

The exploration of mechanochemical reactions has brought new opportunities for the design of functional materials. We synthesized the novel organic peroxide mechanophore bis(9-methylphenyl-9-fluorenyl) peroxide (BMPF) and examined its mechanochromic properties. The mechanism behind its mechanofluorescence was clarified and harnessed in polymer networks that can release the small fluorescent molecule 9-fluorenone upon exposure to a mechanical stimulus. Additionally, polymer networks cross-linked with BMPF units are able to tolerate temperatures up to 110 °C without any change in optical properties or mechanical strength. As mechanophores based on organic peroxide have rarely been documented so far, these fascinating results suggest excellent potential for applications of BMPF in stress-responsive materials. The mechanochemical protocol demonstrated here may provide guiding principles to expand the field of mechanochromic peroxides.

Segmented Polyurethane Elastomers with Mechanochromic and Self-Strengthening Functions.

Mechanochromic elastomers that exhibit force-induced cross-linking reactions in the bulk state are introduced. The synthesis of segmented polyurethanes (SPUs) that contain difluorenylsuccinonitrile (DFSN) moieties in the main chain and methacryloyl groups in the side chains was carried out. DFSN was selected as the mechanophore because it dissociates under mechanical stimuli to form pink cyanofluorene (CF) radicals, which can also initiate the radical polymerization of methacrylate monomers. The obtained elastomers generated CF radicals and changed color by compression or extension; they also became insoluble due to the mechanically induced cross-linking reactions. Additionally, an SPU containing diphenylmethane units also exhibited highly sensitive mechanofluorescence. To the best of our knowledge, this is the first report to demonstrate damage detection ability and changes in the mechanical properties of bulk elastomers induced by simple compression or extension.

Aryne polymerization enabling straightforward synthesis of elusive poly(ortho-arylene)s.

Herein we present the first straightforward aryne polymerization that successively connects aromatic rings at their ortho-positions, directly giving poly(ortho-phenylene)s of up to ca. 100-mer size. The polymerization proceeds smoothly in a chain-growth fashion in the presence of a monovalent copper reagent. Direct synthesis of poly(ortho-arylene)s has been a "missing piece" of aromatic chemistry for a long time (although the discovery of arynes dates back to 1902), in contrast to the well-investigated para- and meta-linked polyarylenes. Our achievement reported here is expected to open up new areas of nanocarbon and materials science.

Polycarbonates derived from glucose via an organocatalytic approach.

An organocatalyzed ring-opening polymerization methodology was developed for the preparation of polycarbonates derived from glucose as a natural product starting material. The cyclic 4,6-carbonate monomer of glucose having the 1, 2, and 3 positions methyl-protected was prepared in three steps from a commercially available glucose derivative, and the structure was confirmed by means of NMR and IR spectroscopies, electrospray ionization mass spectrometry (MS), and single-crystal X-ray analysis. Polymerization of the monomer, initiated by 4-methylbenzyl alcohol in the presence of 1,5,7-triazabicyclo[4.4.0]dec-5-ene as the organocatalyst, proceeded effectively in a controlled fashion to afford the polycarbonate with a tunable degree of polymerization, narrow molecular weight distribution, and well-defined end groups, as confirmed by a combination of NMR spectroscopy, gel-permeation chromatography, and MALDI-TOF MS. A distribution of head-to-head, head-to-tail, and tail-to-tail regiochemistries was determined by NMR spectroscopy and tandem MS analysis by electron transfer dissociation. These polycarbonates are of interest as engineering materials because of their origination from renewable resources combined with their amorphous character and relatively high glass transition temperatures as determined by X-ray diffraction and differential scanning calorimetry studies.

Helicity Induction in Three π-Conjugated Chromophores by Planar Chirality of Calixamide.

Shall we twist? Three-dimensional arrangement of π-conjugated chromophores with triple-stranded helicity was achieved by using the planar chirality of m-calix[3]amide. Based on spectroscopic data and theoretical calculations, the dynamic and preferred helical characters of bithiophene units embedded in the tubular molecule were elucidated, and the absolute configuration was determined.

A rational design strategy of radical-type mechanophores with thermal tolerance.

Radical-type mechanophores (RMs) are attractive molecules that undergo homolytic scission of their central C-C bond to afford radical species upon exposure to heat or mechanical stimuli. However, the lack of a rational design concept limits the development of RMs with pre-determined properties. Herein, we report a rational design strategy of RMs with high thermal tolerance while maintaining mechanoresponsiveness. A combined experimental and theoretical analysis revealed that the high thermal tolerance of these RMs is related to the radical-stabilization energy (RSE) as well as the Hammett and modified Swain-Lupton constants at the para-position (σp). The trend of the RSE values is in good agreement with the experimentally evaluated thermal tolerance of a series of mechanoresponsive RMs based on the bisarylcyanoacetate motif. Furthermore, the singly occupied molecular orbital (SOMO) levels clearly exhibit a negative correlation with σp within a series of RMs that are based on the same skeleton, paving the way toward the development of RMs that can be handled under ambient conditions without peroxidation.

Assessment of electroencephalography modification by antipsychotic drugs in patients with schizophrenia spectrum disorders using frontier orbital theory: A preliminary study.

AIM: Schizophrenia is characterized by an abnormality in electroencephalography (EEG), which can be affected by antipsychotic drugs. Recently, the mechanism underlying these EEG alterations in schizophrenia patients was reframed from the perspective of redox abnormalities. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) can be calculated using a computational method and may be useful for evaluating the antioxidant/prooxidant effect of antipsychotic drugs. Thus, we examined the association between the effects of antipsychotic monotherapy on quantitative EEG and HOMO/LUMO energy. METHODS: We used medical report data including EEG results of psychiatric patients admitted to Hokkaido University Hospital. We extracted the EEG records of patients diagnosed with a schizophrenia spectrum disorder undergoing antipsychotic monotherapy during the natural course of treatment (n = 37). We evaluated the HOMO/LUMO energy of all antipsychotic drugs using computational methods. Multiple regression analyses were used to examine the relationship between the HOMO/LUMO energy of all antipsychotic drugs and spectral band power in all patients. Statistical significance was set at p < 6.25 × 10-4 adjusted with Bonferroni correction. RESULTS: We showed that the HOMO energy of all antipsychotic drugs had weak positive correlations with delta- and gamma-band power (e.g., standardized ß = 0.617 for delta in the F3 channel, p = 6.6 × 10-5 ; standardized ß = 0.563 for gamma in the O1 channel, p = 5.0 × 10-4 ). CONCLUSION: Although there may be unexpected bias and confounding factors, our findings suggest that the effect of antipsychotic drugs on EEG may be related to their antioxidant actions.

Non-symmetric mechanophores prepared from radical-type symmetric mechanophores: bespoke mechanofunctional polymers.

A non-symmetric radical-type mechanophore (CF/ABF) was synthesized by molecular crossing between two radical-type mechanophores. The thermal stability and mechanoresponsiveness of CF/ABF were found to be tunable by altering the properties of the parent RMs. The CF/ABF-centred polymers showed mixed mechanochromism derived from the simultaneous generation of two radical species.

A two-coordinate boron cation featuring C-B+-C bonding.

Two-coordinate boron cations (R2B(+)), referred to as borinium ions, are chemical species in which the boron bears only four valence electrons, and that are isoelectronic with hypothetical carbon dications (R2C(2+)). Although lone-pair-donating substituents such as amino groups have enabled the isolation of several borinium ions, diarylated and dialkylated borinium derivatives remain entirely unexplored. Here, we present the synthesis, structure and reactivity of the dimesitylborinium ion, which displays unexpectedly high thermal stability. X-ray crystallography and (11)B NMR spectroscopy, supported by density functional theory calculations, reveal that the borinium ion adopts a linear two-coordinate structure in both the solid state and in solution. The boron centre is stabilized by pπ bonding from the mesityl groups and is free from coordination by the counterion or solvent molecules. This diarylborinium ion possesses exceptional Lewis acidity, accepting a pair of electrons from CO2 to cause an unusual deoxygenation reaction.