Structuring of Interfacial Water on Silica Surface in Cyclohexane Studied by Surface Forces Measurement and Sum Frequency Generation Vibrational Spectroscopy

We investigated interfacial water, formed by adsorption or phase separation (prewetting transition), on a silica surface in water–cyclohexane binary liquids using a combination of colloidal probe atomic force microscopy (AFM) and sum frequency generation (SFG) vibrational spectroscopy. At 33 ± 9 ppm water, the long-range attraction extending to 19.4 ± 2.9 nm appeared, which was caused by the contact of water layers formed on silica surfaces. The attraction range increased with increasing water concentration and reached 97 ± 17 nm at the saturation concentration of water in cyclohexane (<i>C</i>*), indicating that the thickness of the water layer formed on silica was ca. 50 nm. The interfacial energy between the water adsorption layer and bulk solution (γ = 79.3 ± 2.0 mN/m) was estimated from the pull-off force, and was significantly larger than the value for the bulk water/cyclohexane interface (γ = 50.1 mN/m). SFG spectroscopy demonstrated that the interfacial water formed an icelike structure at <i>C</i>*. These results indicated that the interfacial water molecules formed an icelike ordered structure induced by the hydrogen bonding with surface silanol groups, resulting in the free OH groups being more exposed to the bulk solution. On the other hand, the water adsorption layer induced by phase separation at water concentrations above <i>C</i>* was found to be less ordered and its structure at the adsorption layer/bulk interface was almost the same as that of bulk water, although its thickness was almost the same as that formed at <i>C</i>*. To our knowledge, this is the first report of the observation of liquid adsorption layers formed by chemical interaction up to saturation and by the wetting transition above saturation, and their differences in the structure and properties at the molecular level

Crystal Engineering of Vapochromic Porous Crystals Composed of Pt(II)-Diimine Luminophores for Vapor-History Sensors

A novel Pt­(II) diimine complex, [Pt­(CN)₂­(H₂d<i>p</i>cpbpy)] (<b>1</b>, H₂d<i>p</i>cpbpy = 4,4′-di­(<i>p</i>-carboxyphenyl)-2,2′-bipyridine), was synthesized, and its vapochromic behavior was investigated. The <b><u>y</u></b>ellow <b><u>a</u></b>morphous form of <b>1</b>, <b>1-Ya</b>, transformed into the porous <b><u>o</u></b>range <b><u>c</u></b>rystalline form, <b>1-Oc</b>, upon exposure to ethanol vapor. This behavior is similar to that of the previously reported complex, [Pt­(CN)₂(H₂dcphen)] (<b>2</b>, H₂dcphen = 4,7-dicarboxy-1,10-phenanthroline). X-ray diffraction study showed that <b>1-Oc</b> possessed similar but larger porous channels (14.3 × 8.6 Å) compared to the <b><u>r</u></b>ed <b><u>c</u></b>rystalline form of <b>2</b>, <b>2-Rc</b> (6.4 × 6.8 Å). Although the porous structure of <b>2-Rc</b> was retained after vapor desorption, that of <b>1-Oc</b> collapsed to form the <b><u>o</u></b>range <b><u>a</u></b>morphous solid, <b>1-Oa</b>. However, the orange color was unchanged in this process. The initial color was recovered by grinding <b>1-Oa</b> and <b>2-Rc</b>. These <i>vapor-writing</i> and <i>grinding-erasing</i> functions can be applied to both in situ vapor sensing and vapor-history sensing, i.e., sensors that can memorize the existence of previous vapors. A notable difference was observed for humid air sensitivity; the orange emission of <b>1-Oa</b> was largely unaffected upon exposure to humid air, whereas the red emission of <b>2-Rc</b> was significantly affected. The lesser sensitivity of <b>1-Oa</b> toward humidity is important for stable vapor-history sensor applications

Importance of the Molecular Orientation of an Iridium(III)-Heteroleptic Photosensitizer Immobilized on TiO₂ Nanoparticles

To elucidate the effect of the molecular orientation of a photosensitizing (PS) dye molecule on photoinduced interfacial electron transfer to a semiconductor substrate, we have synthesized two new Ir­(III) heteroleptic complexes each comprising two phosphonic acid groups: [Ir­(ppy)₂(CPbpy)]⁺ and [Ir­(CPppy)₂(bpy)]⁺ (<b>1B</b> and <b>1P</b>, respectively; Hppy = 2-phenylpyridine, bpy = 2,2′-bipyridine, CPbpy = 4,4′-bis­(methylphosphonic acid)-2,2′-bipyridine, and CPppy = 4-(methylphosphonic acid)-2-phenylpyridine). Both Ir­(III) complexes exhibit similar UV–vis absorption spectra and quasi-reversible Ir­(IV)/Ir­(III) redox behavior at a potential of 1.67 V vs NHE. On the other hand, the triplet metal-to-ligand charge-transfer (³MLCT) phosphorescence energy of <b>1B</b> was ∼0.12 eV higher than that of <b>1P</b>. This difference was attributed to the electron-donating methyl phosphonate groups attached to the bpy ligand that destabilize the ³MLCT excited state in which the photoexcited electron is localized in the bpy moiety. Both Ir­(III) PS dyes were immobilized onto the surface of the Pt-co-catalyst-loaded TiO₂ nanoparticles (<b>1B@Pt-TiO</b>_<b>2</b> and <b>1P@Pt-TiO</b>_<b>2</b>). Immobilization was comparable, suggesting that the effect of the positions of the methyl phosphonate groups on the immobilization behavior was negligible. On the other hand, the photocatalytic H₂ evolution activity of <b>1B@Pt-TiO</b>_<b>2</b> was about 6-fold higher than that of <b>1P@Pt-TiO</b>_<b>2</b>, indicating the importance of the methyl phosphonate anchoring group position in regulating not only the redox potentials but also the orientation of the molecular photosensitizer on the semiconductor substrate

Reduction in Crystal Size of Flexible Porous Coordination Polymers Built from Luminescent Ru(II)-Metalloligands

In this study, we examined the reduction in crystal size of the porous coordination polymers (PCPs) {Sr₄(H₂O)₉]<b>­[4Ru]</b>₂­·9H₂O]} (<b>Sr</b>_<b>2</b><b>[4Ru]</b>) and [Mg­(H₂O)₆]­{[Mg₂(H₂O)₃<b>­[4Ru]</b>­·4H₂O} (<b>Mg</b>_<b>2</b><b>[4Ru]</b>) composed of a luminescent metalloligand [Ru­(4,4′-dcbpy)]^4–­(<b>[4Ru]</b>; 4,4′-dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) using a coordination modulation method. Scanning electron microscopy measurements clearly show that the sizes of crystals of <b>Sr</b>_<b>2</b><b>[4Ru]</b> and <b>Mg</b>_<b>2</b><b>[4Ru]</b> were successfully reduced to the mesoscale (about 500 nm width and 10 nm thickness for <b>Sr</b>_<b>2</b><b>[4Ru]</b> (abbreviated as <i>m</i>-<b>Sr</b>_<b>2</b><b>[4Ru]</b>) and about 1 μm width and 30 nm thickness for <b>Mg</b>_<b>2</b><b>[4Ru]</b> (abbreviated as <i>m</i>-<b>Mg</b>_<b>2</b><b>[4Ru]</b>)) using lauric acid as a coordination modulator. Interestingly, the nanocrystals of <i>m</i>-<b>Sr</b>_<b>2</b><b>[4Ru]</b> formed flower-like aggregates with diameters of 1 μm, whereas flower-like aggregates were not formed in <i>m</i>-<b>Mg</b>_<b>2</b><b>[4Ru]</b>. Water vapor adsorption isotherms of these nanocrystals suggest that the water adsorption behavior of <i>m</i>-<b>Sr</b>_<b>2</b><b>[4Ru]</b>, which has a three-dimensional lattice structure containing small pores, is significantly different from that of the bulk <b>Sr</b>_<b>2</b><b>[4Ru]</b> crystal, as shown by the vapor adsorption isotherm. In contrast, <i>m</i>-<b>Mg</b>_<b>2</b><b>[4Ru]</b>, which has a two-dimensional sheet structure, had an adsorption isotherm very similar to that of the bulk sample. These contrasting results suggest that the dimensionality of the coordination framework is an important factor for the guest adsorption behavior of nanocrystalline PCPs

Environmentally Friendly Mechanochemical Syntheses and Conversions of Highly Luminescent Cu(I) Dinuclear Complexes

Luminescent dinuclear Cu­(I) complexes, [Cu₂X₂(dpypp)₂] [<b>Cu-X</b>; X = Cl, Br, I; dpypp = 2,2′-(phenylphosphinediyl)­dipyridine], were successfully synthesized by a solvent-assisted mechanochemical method. A trace amount of the assisting solvent plays a key role in the mechanochemical synthesis; only two solvents possessing the nitrile group, CH₃CN and PhCN, were effective for promoting the formation of dinuclear <b>Cu-X</b>. X-ray analysis revealed that the dinuclear structure with no Cu···Cu interactions, bridged by two dpypp ligands, was commonly formed in all <b>Cu-X</b> species. These complexes exhibited bright green emission in the solid state at room temperature (Φ = 0.23, 0.50, and 0.74; λ_em = 528, 518, and 530 nm for <b>Cu-Cl</b>, <b>Cu-Br</b>, and <b>Cu-I</b>, respectively). Emission decay measurement and TD-DFT calculation suggested that the luminescence of <b>Cu-X</b> could be assigned to phosphorescence from the triplet metal-to-ligand charge-transfer (³MLCT) excited state, effectively mixed with the halide-to-ligand charge-transfer (³XLCT) excited state, at 77 K. The source of emission changed to thermally activated delayed fluorescence (TADF) with the same electronic transition nature at room temperature. In addition, the CH₃CN-bound analogue, [Cu₂(CH₃CN)₂­(dpypp)₂]­(BF₄)₂, was successfully mechanochemically converted to <b>Cu-X</b> by grinding with solid KX in the presence of a trace amount of assisting water

Importance of the Molecular Orientation of an Iridium(III)-Heteroleptic Photosensitizer Immobilized on TiO₂ Nanoparticles

To elucidate the effect of the molecular orientation of a photosensitizing (PS) dye molecule on photoinduced interfacial electron transfer to a semiconductor substrate, we have synthesized two new Ir­(III) heteroleptic complexes each comprising two phosphonic acid groups: [Ir­(ppy)₂(CPbpy)]⁺ and [Ir­(CPppy)₂(bpy)]⁺ (<b>1B</b> and <b>1P</b>, respectively; Hppy = 2-phenylpyridine, bpy = 2,2′-bipyridine, CPbpy = 4,4′-bis­(methylphosphonic acid)-2,2′-bipyridine, and CPppy = 4-(methylphosphonic acid)-2-phenylpyridine). Both Ir­(III) complexes exhibit similar UV–vis absorption spectra and quasi-reversible Ir­(IV)/Ir­(III) redox behavior at a potential of 1.67 V vs NHE. On the other hand, the triplet metal-to-ligand charge-transfer (³MLCT) phosphorescence energy of <b>1B</b> was ∼0.12 eV higher than that of <b>1P</b>. This difference was attributed to the electron-donating methyl phosphonate groups attached to the bpy ligand that destabilize the ³MLCT excited state in which the photoexcited electron is localized in the bpy moiety. Both Ir­(III) PS dyes were immobilized onto the surface of the Pt-co-catalyst-loaded TiO₂ nanoparticles (<b>1B@Pt-TiO</b>_<b>2</b> and <b>1P@Pt-TiO</b>_<b>2</b>). Immobilization was comparable, suggesting that the effect of the positions of the methyl phosphonate groups on the immobilization behavior was negligible. On the other hand, the photocatalytic H₂ evolution activity of <b>1B@Pt-TiO</b>_<b>2</b> was about 6-fold higher than that of <b>1P@Pt-TiO</b>_<b>2</b>, indicating the importance of the methyl phosphonate anchoring group position in regulating not only the redox potentials but also the orientation of the molecular photosensitizer on the semiconductor substrate

Vapor-Controlled Linkage Isomerization of a Vapochromic Bis(thiocyanato)platinum(II) Complex: New External Stimuli To Control Isomerization Behavior

We synthesized a novel Pt­(II)–diimine complex with a typical ambidentate thiocyanato ligand, [Pt­(<i>thiocyanato</i>)₂(H₂dcbpy)] (<b>1</b>; H₂dcbpy =4,4′-dicarboxy-2,2′-bipyridine), and found that the complex <b>1</b> exhibits unique linkage isomerizations with drastic color and luminescence changes driven by exposure to volatile organic chemical (VOC) vapors in the solid state. Reaction between [PtCl₂(H₂dcbpy)] and KSCN in aqueous solution at 0 °C enabled successful isolation of an isomer with the S-coordinated thiocyanato ligand, [Pt­(<u>S</u>CN)₂(H₂dcbpy)] (<b>1SS·H</b>_<b>2</b><b>O</b>), as a nonluminescent orange solid. Interestingly, <b>1SS·H</b>_<b>2</b><b>O</b> was isomerized completely to one isomer with the N-coordinated isothiocyanato ligand, [Pt­(<u>N</u>CS)₂(H₂dcbpy)] (<b>1NN·3DMF</b>) by exposure to DMF vapor, and this isomerization was accompanied by significant color and luminescence changes from nonluminescent orange to luminescent red. IR spectroscopy and thermogravimetric analysis revealed that adsorption of the DMF vapor and transformation of the hydrogen-bonded structure both played important roles in this vapor-induced linkage isomerization. Another isomer containing both S- and N-coordinated thiocyanato ligands, [Pt­(<u>S</u>CN)­(<u>N</u>CS)­(H₂dcbpy)] (<b>1SN</b>), was obtained as a nonluminescent yellow solid simply by exposure of <b>1SS·H</b>_<b>2</b><b>O</b> to acetone vapor at room temperature, and about 80% of <b>1SS·H</b>_<b>2</b><b>O</b> was found to be converted to <b>1SN</b>. In the solution state, each isomer changed gradually to an isomeric mixture, but pure <b>1SS</b> was regenerated by UV light irradiation (λ_irr. = 300 nm) of an MeOH solution of the mixture. In the crystal structure of <b>1SN</b>, the complex molecules were hydrogen-bonded to each other through the carboxyl groups of the H₂dcbpy ligand and the N site of the thiocyanato ligand, whereas the <b>1NN</b> molecules in the <b>1NN·4DMF</b> crystal were hydrogen-bonded to the solvated DMF molecules. Competition of the hydrogen-bonding ability among the carboxyl groups of the H₂dcbpy ligand, N and S atoms of the thiocyanato ligand, and the vapor molecule was found to be one of the most important factors controlling linkage isomerization behavior in the solid state. This unique linkage isomerization controlled by vapor can provide an outstanding vapochromic system as well as a new molecular switching function driven by vapor molecules

Vapor-Controlled Linkage Isomerization of a Vapochromic Bis(thiocyanato)platinum(II) Complex: New External Stimuli To Control Isomerization Behavior

We synthesized a novel Pt­(II)–diimine complex with a typical ambidentate thiocyanato ligand, [Pt­(<i>thiocyanato</i>)₂(H₂dcbpy)] (<b>1</b>; H₂dcbpy =4,4′-dicarboxy-2,2′-bipyridine), and found that the complex <b>1</b> exhibits unique linkage isomerizations with drastic color and luminescence changes driven by exposure to volatile organic chemical (VOC) vapors in the solid state. Reaction between [PtCl₂(H₂dcbpy)] and KSCN in aqueous solution at 0 °C enabled successful isolation of an isomer with the S-coordinated thiocyanato ligand, [Pt­(<u>S</u>CN)₂(H₂dcbpy)] (<b>1SS·H</b>_<b>2</b><b>O</b>), as a nonluminescent orange solid. Interestingly, <b>1SS·H</b>_<b>2</b><b>O</b> was isomerized completely to one isomer with the N-coordinated isothiocyanato ligand, [Pt­(<u>N</u>CS)₂(H₂dcbpy)] (<b>1NN·3DMF</b>) by exposure to DMF vapor, and this isomerization was accompanied by significant color and luminescence changes from nonluminescent orange to luminescent red. IR spectroscopy and thermogravimetric analysis revealed that adsorption of the DMF vapor and transformation of the hydrogen-bonded structure both played important roles in this vapor-induced linkage isomerization. Another isomer containing both S- and N-coordinated thiocyanato ligands, [Pt­(<u>S</u>CN)­(<u>N</u>CS)­(H₂dcbpy)] (<b>1SN</b>), was obtained as a nonluminescent yellow solid simply by exposure of <b>1SS·H</b>_<b>2</b><b>O</b> to acetone vapor at room temperature, and about 80% of <b>1SS·H</b>_<b>2</b><b>O</b> was found to be converted to <b>1SN</b>. In the solution state, each isomer changed gradually to an isomeric mixture, but pure <b>1SS</b> was regenerated by UV light irradiation (λ_irr. = 300 nm) of an MeOH solution of the mixture. In the crystal structure of <b>1SN</b>, the complex molecules were hydrogen-bonded to each other through the carboxyl groups of the H₂dcbpy ligand and the N site of the thiocyanato ligand, whereas the <b>1NN</b> molecules in the <b>1NN·4DMF</b> crystal were hydrogen-bonded to the solvated DMF molecules. Competition of the hydrogen-bonding ability among the carboxyl groups of the H₂dcbpy ligand, N and S atoms of the thiocyanato ligand, and the vapor molecule was found to be one of the most important factors controlling linkage isomerization behavior in the solid state. This unique linkage isomerization controlled by vapor can provide an outstanding vapochromic system as well as a new molecular switching function driven by vapor molecules

Effect of Water Coordination on Luminescent Properties of Pyrazine-Bridged Dinuclear Cu(I) Complexes

Two luminescent pyrazine-bridged dinuclear Cu­(I) complexes, namely, [{Cu­(PPh₃)₂(H₂O)}­(μ-MeOpyz)­{Cu­(PPh₃)₂(CH₃CN)}]­(BF₄)₂ and [{Cu­(PPh₃)₂(H₂O)}­(μ-MeOpyz)­{Cu­(PPh₃)₂(H₂O)}]­(BF₄)₂ (<b>H</b>_<b>2</b><b>O–Cu</b>_<b>2</b><b>–AN</b> and <b>H</b>_<b>2</b><b>O–Cu</b>_<b>2</b><b>–H</b>_<b>2</b><b>O</b>; PPh₃ = triphenylphosphine, MeOpyz = 2-methoxypyrazine), were successfully synthesized and characterized by single-crystal X-ray diffraction and luminescence measurements. X-ray analysis revealed that the water molecules are coordinated to both Cu­(I) ions to form almost the same P₂N₁O₁ coordination structure in <b>H</b>_<b>2</b><b>O–Cu</b>_<b>2</b><b>–H</b>_<b>2</b><b>O</b>, whereas one of the two Cu ions in <b>H</b>_<b>2</b><b>O–Cu</b>_<b>2</b><b>–AN</b> was coordinated by acetonitrile instead of water to form a different P₂N₂ coordination environment. The asymmetric <b>H</b>_<b>2</b><b>O–Cu</b>_<b>2</b><b>–AN</b> exhibits very bright yellow-green emission with a high emission quantum yield (λ_em = 550 nm, Φ = 0.70) at room temperature in the solid state in spite of the coordination of water molecule, which usually tends to deactivate the emissive state through O–H vibration. The intense emission at room temperature is a result of thermally activated delayed fluorescence, and the remarkable temperature dependence of emission lifetimes indicates the existence of unique multiple emission states for the asymmetric dinuclear complex. In contrast, the emission of <b>H</b>_<b>2</b><b>O–Cu</b>_<b>2</b><b>–H</b>_<b>2</b><b>O</b> was observed at longer wavelengths with remarkably a lower quantum yield (λ_em = 580 nm, Φ = 0.05). Time-dependent density functional theory calculations suggested that the emission could result from the metal-to-ligand charge-transfer transition state. However, it could be rapidly deactivated by the structural distortion around the Cu ion with a less-bulky coordination environment in <b>H</b>_<b>2</b><b>O–Cu</b>_<b>2</b><b>–H</b>_<b>2</b><b>O</b>

Emission Tuning of Luminescent Copper(I) Complexes by Vapor-Induced Ligand Exchange Reactions

We have synthesized two luminescent mononuclear Cu­(I) complexes, [Cu­(PPh₂Tol)­(THF)­(4Mepy)₂]­(BF₄) (<b>1</b>) and [Cu­(PPh₂Tol)­(4Mepy)₃]­(BF₄) (<b>2</b>) (PPh₂Tol = diphenyl­(<i>o</i>-tolyl)­phosphine, 4Mepy = 4-methylpyridine, THF = tetrahydrofuran), and investigated their crystal structures, luminescence properties, and vapor-induced ligand exchange reactions in the solid state. Both coordination complexes are tetrahedral, but one of the three 4Mepy ligands of complex <b>2</b> is replaced by a THF solvent molecule in complex <b>1</b>. In contrast to the very weak blue emission of the THF-bound complex <b>1</b> (wavelength of emission maximum (λ_em) = 457 nm, emission quantum yield (Φ_em) = 0.02) in the solid state at room temperature, a very bright blue-green emission was observed for <b>2</b> (λ_em = 484 nm, Φ_em = 0.63), suggesting a contribution of the THF ligand to nonradiative deactivation. Time-dependent density functional theory calculations and emission lifetime measurements suggest that the room-temperature emissions of the complexes are due to thermally activated delayed fluorescence from the metal-to-ligand charge transfer excited state. Interestingly, by exposing the solid sample of THF-bound <b>1</b> to 4Mepy vapor, the emission intensity drastically increased and the emission color changed from blue to blue-green. Powder X-ray diffraction measurements revealed that the emission change of <b>1</b> is due to the vapor-induced ligand exchange of THF for 4Mepy, forming the strongly emissive complex <b>2</b>. Further emission tuning was achieved by exposing <b>1</b> to pyrimidine or pyrazine vapors, forming green (λ_em = 510 nm) or orange (λ_em = 618 nm) emissive complexes, respectively. These results suggest that the vapor-induced ligand exchange is a promising method to control the emission color of luminescent Cu­(I) complexes