Reversible Transition between Discrete and 1D Infinite Architectures: A Temperature-Responsive Cu(I) Complex with a Flexible Disilane-Bridged Bis(pyridine) Ligand.

A thermoresponsive structural change based on a disilane-bridged bis(pyridine) ligand and CuI is reported. Single-crystal X-ray analysis revealed that there are two polymorphs in the Cu(I) complex: octanuclear copper(I) complex at 20 °C and 1D staircase copper(I) polymer complex at -173 °C. The formation of these polymorphs is due to the flexibility of the ligand. Cu-I bond formation is observed upon cooling the sample from -10 °C to -170 °C. The temperature-induced phase transition progression was clarified by DSC, VT-PXRD, and VT-photoluminescence measurements and indicated a reversible temperature-controlled crystal-to-crystal phase transition. Observation on a VT-stage using a high-speed camera showed crystal cracking during single-crystal to single-crystal transitions between these polymorphic forms.

A Series of D-A-D Structured Disilane-Bridged Triads: Structure and Stimuli-Responsive Luminescence Studies.

A series of σ-π extended octamethyltetrasilanes, which have phenothiazine, 9,9-dimethyl-9,10-dihydroacridine, or phenoxazine (1, 2, and 3) groups as donor moieties and thienopyrazine or benzothiadiazole (a and b) groups as acceptor fragments, has been prepared, and their optical properties have been studied as an extension of our work. All six compounds exhibited fluorescence in the solid state with maximum wavelengths centered in the range of 400 and 650 nm upon excitation by a UV lamp. Compound 2b showed apparent dual emission behavior in solution, which depends on solvent polarity, and a reversible photoluminescent change under mechanical and thermal stimuli in the solid state. Quantum chemical calculations suggest the contribution of a quasi-axial conformer of the 9,9-dimethyl-9,10-dihydroacridine moiety in 2b to the dual emission in solution and the mechanofluoroluminescence in the solid state, similarly to 1a. These studies provide new insight into the preparation of disilane-bridged triads capable of responding to multiple stimuli.

Synthesis, Structure, and Photophysical Properties of Yellow-Green and Blue Photoluminescent Dinuclear and Octanuclear Copper(I) Iodide Complexes with a Disilanylene-Bridged Bispyridine Ligand.

The synthesis, structural, and photophysical investigations of CuI complexes with a disilanylene-bridged bispyridine ligand 1 are herein presented. Dinuclear (2) and ladder-like (3) octanuclear copper(I) complexes were straightforwardly prepared by exactly controlling the ratio of CuI/ligand 1. Single-crystal X-ray analysis confirmed that dinuclear complex 2 had no apparent π…π stacking whereas octanuclear complex 3 had π…π stacking in the crystal packing. In the solid state, the complexes display yellow-green (λem = 519 nm, Φ = 0.60, τ = 11 µs, 2) and blue (λem = 478 nm, Φ = 0.04, τ = 2.6 µs, 3) phosphorescence, respectively. The density functional theory calculations validate the differences in their optical properties. The difference in the luminescence efficiency between 2 and 3 is attributed to the presence of π…π stacking and the different luminescence processes.

Luminescent Behavior Elucidation of a Disilane-Bridged D-A-D Triad Composed of Phenothiazine and Thienopyrazine.

A σ-π extended aryldisilane, comprising a thienopyrazine group as an acceptor fragment and phenothiazine groups as the donor moiety, has been prepared through the introduction of two Si-Si bridges (compound 1). X-ray diffraction analysis determined the crystal structure of 1, and experimental and theoretical approaches investigated its optical properties. Solvatochromic studies revealed the dual emission of 1 in all solvents tested. Compound 1 also exhibited fluorescence in the solid state upon excitation with a hand-held UV lamp, as well as mechanochromic luminescent properties. The packing mode in the crystal structure, variation of phenothiazine conformation, morphological changes between crystalline and amorphous phases are the major factors showing reversible fluorescence under external stimuli. A theoretical conformer study found that 1 exists in distinct conformational groups differing in Gibbs free energy by less than 3 kcal mol-1 . The conformer in the crystalline state of 1 can promote the complete separation of the HOMO and LUMO between the phenothiazine donor and the thienopyrazine acceptor, linked by the disilane linker. HOMO-LUMO energy transition in the crystalline state is forbidden due to the lack of frontier orbital overlap. Crystalline state emission showed LUMO → HOMO-1 transition (locally excited (LE) state). In the amorphous state, the partial presence of quasi-axial conformers allows intramolecular charge-transfer type emission via energy transfer from dominant quasi-equatorial conformers. The strategy proposed in this work provides important guidance for developing stimuli-responsive materials with controlled excited states.

Thermosalience in Macrocycle-Based Soft Crystals via Anisotropic Deformation of Disilanyl Architecture.

We describe here the preparation of soft crystals using disilanyl macrocycle C4 possessing four p-phenylenes circularly connected by four flexible disilane bonds. Single crystals of C4 exhibited a reversible thermal single-crystal-to-single-crystal (SCSC) phase transition behavior between two crystal phases accompanied by remarkable mechanical motion (thermosalient effect), as revealed by thermal analyses and X-ray diffraction measurements. Detailed structural analyses implied that flexibility of the parallelogram disilanyl architecture and molecular packing mode via weak intermolecular interactions facilitated a concerted structural transformation (parallel crank motion) of macrocycles in the crystal, thus resulting in the SCSC phase transition accompanied by anisotropic shrinking/elongation of the cells to induce the thermosalient effect. This work explores a new area of organosilicon chemistry and presents the potential utility of disilanyl macrocycles as soft crystals.

CO Release from N,C,S-Pincer Iron(III) Carbonyl Complexes Induced by Visible-to-NIR Light Irradiation: Mechanistic Insight into Effects of Axial Phosphorus Ligands.

Light-induced CO release from newly synthesized N,C,S-pincer iron(III) carbonyl complexes with two phosphorus ligands- trans-[Fe(L-κ3 N,C,S)(CO)(PR2R')2]PF6 ([1]PF6, R = Me, R' = Ph; [2]PF6, R = R' = Me; [3]PF6, R = R' = OEt)-were investigated. All the iron(III) carbonyl complexes were stable in solution and showed light-inducible CO release under ambient conditions. Studies on the wavelength dependence of photoreaction revealed that the phosphite complex [3]PF6 exhibited the most extended photosensitivity including all visible and a part of near-IR light (390-800 nm wavelengths). The phosphine complexes [1]PF6 and [2]PF6 showed sensitivity to only the higher-energy region of visible light (390-450 nm). Quantum-chemical calculations and spectroscopic data suggested that all complexes [1]PF6-[3]PF6 have dπ-dπ excitation modes to depopulate Fe-C(carbonyl) bonding and potentially induce the CO release by irradiation of light in the near-IR region, although moderately weakened Fe-C(carbonyl) bonding due to stronger π-backbonding by the phosphite ligand rendered the excitation effective on the CO release exclusively in [3]PF6.

15N CP/MAS NMR as a Tool for the Mechanistic Study of Mechanical Stimuli-Responsive Materials: Evidence for the Conformational Change of an Emissive Dimethylacridane Derivative.

Mechanochromic luminescent molecules are currently attracting considerable attention because of their promising technological applications, but understanding their mechanism of action is challenging and is thus hindering our deeper understanding of these materials. The conformational change of 9,9'-dimethyl-9,10-dihydroacridane derivative 1 was examined using solid-state 15N nuclear magnetic resonance (NMR) spectroscopic techniques without using a specifically 15N-labeled compound. A difference between the two conformers was clearly observed in the measurements and was assigned to the ⟨pl⟩ and ⟨bf⟩ spatial structures. The results were supported by quantum chemical calculations on 15N NMR chemical shifts of each isomer. The technique presented here can clearly identify the structural changes caused by crushing a powder sample. Such structural changes are difficult to determine using X-ray diffraction (XRD) measurements.

Crystal structures and electrochemical properties of nickel(II) complexes with N,N',N'',S-tetra-dentate Schiff base ligands.

The thiol-ate nickel complexes {2-[({2-[(2-amino-ethyl-κN)(meth-yl)amino-κN]eth-yl}imino-κN)meth-yl]benzene-thiol-ato-κS}nickel(II) chloride, [Ni(C12H18N3S)]Cl (1), and [2-({[2-(piperazin-1-yl-κ2 N 1,N 4)eth-yl]imino-κN}meth-yl)benzene-thiol-ato-κS]nickel(II) hexa-fluoro-phosphate di-chloro-methane monosolvate, [Ni(C13H18N3S)]PF6·CH2Cl2 (2), were synthesized by the reactions of 2-(tert-butyl-thio)-benzaldehyde, tri-amines, and nickel(II) salts. Both complexes have a nickel ion surrounded by an N,N',N'',S-tetra-dentate ligand, forming a square-planar geometry. The terminal N,N-chelating moiety is N,N-di-alkyl-ethane-1,2-di-amine for 1 and 1-alkyl-piperazine for 2. The N-Ni-N bite angle in the terminal N,N-chelate ring in 2 [76.05 (10)°] is much smaller than that in 1 [86.16 (6)°]. Cyclic voltammograms of 1 and 2 in aqueous media indicated that the reduction and oxidation potentials of 2 are more positive than those of 1. The smaller bite angle of the terminal piperazine chelate in 2 reduces the electron-donating ability of the tetra-dentate ligand, resulting in a positive shift of the redox potentials. Both complexes exhibit catalytic activity for proton reduction, and the piperazine moiety in 2 is effective in reducing the overpotential.

Visible-light-induced release of CO by thiolate iron(iii) carbonyl complexes bearing N,C,S-pincer ligands.

Iron(iii) carbonyl complexes are stabilized by a pincer ligand containing pyridine-N, phenyl-C and thiolate-S donors and two axial phosphine ligands. The N,C,S-pincer iron(iii) carbonyl complexes show CO-releasing properties induced by visible light.