Strategy for Stimuli-Induced Spin Control Using a Liquescent Radical Cation

Stimuli-Induced Controls of Magnetic and Photophysical Properties Using Liquescent Open-Shell Ionic Molecular Systems

This study explores the remarkable properties of liquescent open-shell ionic molecular systems, emphasizing the magnetic and photophysical characteristics arising from their associated structures in the condensed state under various conditions. Well-investigated open-shell molecules, namely, phenothiazine, dihydrophenazine, and tetrathiafulvalene radical cations, and bis(malononitriledithiolato)nickel(III) anionic complexes were examined, and the concept of liquescent open-shell ionic molecular systems was devised. Transformations in their associated structures are induced by external stimuli, resulting in significant variations in their physical properties. These experimental findings open new avenues for exploring and applying stimuli-responsive molecule-based materials.

Temperature-Dependent Modulation of Short-Wave-Infrared Light Transparency Based on Associated Structures of a Liquescent Nickel(III) Complex

A liquescent bis(malononitriledithiolato)nickel(III) complex with a bis(methoxyethyl)imidazolium cation, 1[Ni(mnt)2 ], exhibits three-stage thermochromic modulation of transparency/absorption in the short-wave-infrared (SWIR) region (1000-2500 nm), driven by associated structural changes. Upon heating, the electronic spectra of 1[Ni(mnt)2 ] in the SWIR region shift to shorter wavelengths accompanying with the solid-liquid phase transition at 76 °C. Further heating to over 109 °C induces a second transition of the electronic spectra, characterized by a blue-shift of the SWIR absorption in the liquid phase. The results of temperature-dependent electronic spectra and magnetic susceptibility indicated that the thermochromic changes can be attributed to the two-step dissociation of the associated structures of [Ni(mnt)2 ]- , occurring during the solid-liquid phase transition and the shift of dimer-monomer equilibrium in the liquid state. These changes can be visualized using an SWIR imaging camera under appropriate SWIR lights.

On-Demand Control of Short-Wave Infrared Light Transparency Based on Stimuli-Responsive Association of Tetrathiafulvalene Radical Cations

The light-transmissive properties of a solid-state tetrathiafulvalene radical cation-bis(trifluoromethanesulfonyl)imide, 1-C5 ⋅+  ⋅ NTf2 - , underwent instantaneous changes in the short-wave infrared (SWIR) region (1000-2500 nm) upon exposure to solvent vapor or the application of mechanostress at room temperature. The initial solid state of 1-C5 ⋅+  ⋅ NTf2 - exhibited strong absorption in the near-infrared (NIR; 700-1000 nm) and SWIR regions, whereas the absorption in the SWIR region was significantly diminished in the stimulated state induced by dichloromethane vapor. Upon cessation of vapor stimulation, the solid state spontaneously and promptly reverted to its original state, characterized by absorption bands in the NIR/SWIR region. Moreover, the SWIR absorption was absent upon the application of mechanical stress using a steel spatula. The reversal was fast and occurred within 10 s. These changes were visualized using a SWIR imaging camera under 1450-nm light irradiation. Experimental investigations demonstrated that the transparency to the SWIR light in the solid states was modulated through significant structural transformations of the associated radical cations, with transitions between columnar and isolated π-dimer structures under ambient and stimulated conditions, respectively.

Sterically Frustrated Aromatic Enes with Various Colors Originating from Multiple Folded and Twisted Conformations in Crystal Polymorphs

Overcrowded ethylenes composed of 10-methyleneanthrone and two bulky aromatic rings contain a twisted carbon-carbon double (C=C) bond as well as a folded anthrone unit. As such, they are unique frustrated aromatic enes (FAEs). Various colored crystals of these FAEs, obtained in different solvents, correspond to multiple metastable conformations of the FAEs with various twist and fold angles of the C=C bond, as well as various dihedral angles of attached aryl units with respect to the C=C bond. The relationships between color and these parameters associated with conformational features around the C=C bond were elucidated in experimental and computational studies. Owing to the fact that they are separated by small energy barriers, the variously colored conformations in the FAE crystal change in response to various external stimuli, such as mechanical grinding, hydrostatic pressure and thermal heating.

Rapid Luminescent Enhancement Triggered by One-shot Needlestick-stimulus Using a Liquescent Gold(I) Salt

Luminescence from a gold(I) complex with an N-heterocycliccarbene-based ligand, 1⁺ ⋅NTf₂ ^- , increased rapidly upon the application of one-shot needlestick-stimulus. The weakly orange-emitting solid-state of 1⁺ ⋅NTf₂ ^- was prepared by cooling its melted liquid to 90 °C. Upon applying a weak pinpoint stimulus with a needle, this weakly orange-emitting solid state transformed into an intensively violet-blue-emitting state on a timescale of seconds. The emission after applying the stimulus could be visualized upon UV excitation even under ambient room light. This sequential phase transition from a stable solid to a liquid and then to a metastable solid could occur repeatedly without any measurable degradation of the complex. Various shapes could be prepared by casting the liquid-state complex into molds of different designs. This rapid response is thought to be triggered by the flexible intermolecular interactions in the kinetically generated aggregates formed upon cooling the liquid state, and by the strong Au-Au interactions in the thermodynamically stable crystals after applying the needlestick-stimulus.

Redox-Active Polymeric Ionic Liquids with Pendant N-Substituted Phenothiazine

Polymers that are elastic while supporting charge transport are desirable for flexible and soft electronics. Many polymers with bulky and conjugated redox-active pendant units have high glass transition temperatures (T_g) in their neutral form that will not lead to elasticity at room temperature. Their behavior in charged form in the solid state without an electrolyte has not been extensively studied. Here, the design strategy of polymeric ionic liquid where two weakly interacting ionic groups are used to maintain a low T_g is shown to lead to flexible redox active polymers. The use of a flexible ethylene backbone and redox-active phenothiazine (PTZ)-based pendant group resulted in polymers with relatively low T_g that are electrically conductive. PTZ that was N-substituted with 2-(2-ethoxyethoxy)ethoxy)ethyl was found to promote solubility of the polymer and lower the T_g of the neutral polymer by ∼150 °C relative to that of the T_g of a variant without the N-substituent. Doping with trifluoromethanesulfonimide leads to an electrically conductive polymer without significantly increasing the T_g. Physical characterization by UV-vis-NIR spectroscopy, electron spin resonance spectroscopy, and impedance spectroscopy verified that the molecular design leads to an efficient charge hopping between the PTZ groups.

Condensed Phenoxazine Dimer and Its Radical Cation

A condensed phenoxazine dimer was synthesized and characterized. X-ray crystallographic analysis of the dimer shows a double-butterfly structure, in which the nitrogen atoms are located above and below the molecular plane. A radical cation salt of the dimer was obtained using tris(4-bromophenyl)aminium hexafluoroantimonate as the oxidant. The salt is air-stable in solid and solution states. The cation structure was evaluated by X-ray crystallographic analysis, showing that the phenoxazine units were converted to a planar structure upon oxidation.

Emission Control by Molecular Manipulation of Double-Paddled Binuclear PtII Complexes at the Air-Water Interface

Molecular functions depend on conformations and motions of the corresponding molecular species. An air-water interface is a suitable asymmetric field for the control of molecular conformations and motions under a small applied force. In this work, double-paddled binuclear Pt^II complexes containing pyrazole rings linked by alkyl spacers were synthesized and their orientations and emission properties dynamically manipulated at the air-water interface. The complexes emerge from water with concurrent variation of interface orientation of the planes of the Pt^II complexes from perpendicular to parallel during mechanical compression suggesting a unique 'submarine emission'. Phosphorescence of the complexes is quenched at the air-water interface prior to monolayer formation with intensities subsequently rapidly increasing during monolayer compression. These results indicate that asymmetric reactions and motions might be controlled by applying mechanical force at the air-water interface.