Nobuhumi Nakamura

Spontaneous orientation polarization (SOP) of polar molecules is formed in vacuum‐deposited films by tilting their permanent dipole moment against the substrate surface direction. In this study, we developed SOP molecules with high structural asymmetry by introducing multiple fluoroalkyl groups into polar molecules because SOP is driven by asymmetric intermolecular interactions on the film surface during the vacuum deposition. The developed polar molecules exhibited high dipole orientation degrees in vacuum‐deposited films and achieved a high surface potential growth rate relative to the film thickness, over −350 mV nm ⁻¹ , which is a record high for the reported compounds. Furthermore, the dipolar layers introduced at organic thin‐film interfaces in hole‐only devices and organic photovoltaics to study the impact of dipole interlayers on device performance. The characteristics of the device were observed to be significantly influenced by the SOP polarity, suggesting that the SOP at the organic thin‐film interface plays a crucial role in charge transfer and energy level alignment. The findings of this study provide methodologies for the formation of highly anisotropic glassy films, leading to improved performance of organic devices.

Upon heating, the LCST-type phase transition is induced in both the monomeric and gel states of [N4][C6] by the dehydration of water molecules in the secondary hydration shell near the ammonium cation.

LCST phase transition behaviour of some phosphonium-based ionic liquids in water has been widely tuned by the host-guest interaction between these ionic liquids and α-cyclodextrin.

Organic polar small molecules exhibit spontaneous orientation polarization (SOP) in vacuum-deposited amorphous films. This phenomenon arises because of the asymmetric molecular interactions at the interface between the film surface and vacuum during the deposition process. Here, we investigate the impact of functional groups on the SOP of bent-shaped polar molecules. The results demonstrate that the differences in polarizability between the head and tail functional groups in polar molecules drive the orientation polarization in the films. Enhancing the SOP involves the introduction of two functional groups with distinct polarizabilities in the head and tail groups to promote asymmetric interactions. The developed polar molecule, incorporating bulky alkyl and sulfonyl groups, exhibits a high surface potential growth rate of +321 mV nm⁻¹ in the SOP film. These findings enable the precise control of the molecular orientation in amorphous films and the development of highly polarized electret materials without charging processes.

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Redox enzyme catalysis coupled with electrode reaction is called bioelectrocatalysis and has become a key technology applicable to bioelectrochemical devices such as biosensors, biofuel cells and bioreactors. Since electron transfer between the enzyme and the electrode is the most important phenomenon, discussion of it has dominated the study of bioelectrocatalysis. Direct electron transfer (DET) between oxidoreductases and electrodes is important not only for understanding the fundamental properties of redox proteins, but also for developing mediator-free bioelectronic devices. Among enzymes with different coenzymes, pyrroloquinoline quinone (PQQ)-dependent dehydrogenases are promising biocatalysts for both biosensors and biofuel cells. However, only a limited number of studies have reported DET between PQQ in the enzyme and the electrode. Fungal PQQ-dependent pyranose dehydrogenase from Coprinopsis cinerea (CcPDH)is a multifactor-containing enzyme with superior DET ability [1].The enzyme has a three-domain structure, an N-terminal heme b-binding cytochrome domain, a central catalytic domain with PQQ as a cofactor, and a C-terminal cellulose-binding domain. The substrate undergoes oxidation in the PQQ domain followed by interdomain electron transfer (IET) from the reduced PQQ cofactor to heme b in the cytochrome domain. CcPDHis the attractive quinohemoprotein with a PQQ domain and a cytochrome domain, both of which are possible domains for DET. Previous work has shown that the PQQ domain can engage in direct bioelectrocatalysis without the cytochrome domain. In addition, the DET of the PQQ domain was investigated using self-assembled monolayer (SAM)-coated electrodes. Importantly, a high catalytic current density of 1.6 mA/cm² was achieved for the oxidation of L-fucose under optimised conditions [2]. These results indicate a highly efficient DET to PQQ in the active site of the fungal PQQ-dependent dehydrogenase. On the other hand, it is unclear how DET proceeds at the electrode for the full-length enzyme. The study of DET and IET by an electron transfer protein linked via a linker to a catalytic protein, such as CcPDH, is useful for incorporating these processes into biosensors and biofuel cells based on direct bioelectrocatalysis, as well as for creating fusion proteins that enable DET capability. In the present study, we demonstrated that this process distinguishes direct bioelectrocatalysis via the cytochrome domain and direct bioelectrocatalysis from the PQQ domain by regulating the distance from the electrode to CcPDH using various alkyl chains of SAMs [3]. Catalytic currents by the full-length enzyme were obtained on SAM with chain alkyl lengths of C6 or more, whereas no catalytic currents were obtained by the isolated PQQ domain. The results indicated that direct bioelectrocatalysis occurred through the cytochrome domain of CcPDH for chain lengths of 6 or more. The heme-to-electrode distance is shorter than the PQQ-to-electrode distance when the redox center of each domain is closest to the SAM layer. The PQQ-to-electrode distance exceeds 15 Å for C6-OH-SAM (16.9 Å), but the heme-to-electrode distance is less than 15 Å even for C11-OH-SAM (14.2 Å). Thus, in full-length CcPDH, DET of the PQQ domain does not occur at distances greater than 15 Å, and direct bioelectrocatalysis proceeds through the heme in the cytochrome domain when chain lengths are 6 or greater. Although the optimum pH in the PQQ domain is pH 6.0, the optimal pH is approximately 8.5 for electron transfer of the full-length enzyme through the cytochrome domain. It has been suggested that IET is the rate-limiting step in the pH range 6.0 to 8.5 for the enzyme activity in aqueous solution. The pH dependence of the catalytic current for L-fucose oxidation was examined on the enzyme electrode with C6-OH-SAM, in which DET proceeds only from the cytochrome domain. An enzymatic turnover rate (k _cat) at a limiting catalytic current was obtained using the electroactive coverage of the enzyme on the electrode. In direct bioelectrocatalysis through interdomain electron transfer of the cytochrome domain, k _cat was found to be pH dependent with an optimal pH of 8.5; therefore, the rate-limiting step that governs pH dependence is likely the IET process. [1] K. Takeda, et al., Curr Opin Chem Biol, 49 (2019) 113-121, [2] K. Takeda, et al., Electrochim Acta, 359 (2020) 136982, [3] K. Takeda, et al., Electrochemistry, 92 (2024) 022011.

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