Low Temperature Metal-Enhanced Fluorescence

In this short letter, we describe the effects of low temperature on the Metal-Enhanced Fluorescence (MEF) phenomenon. Fluorophores close to Silver Island Films (SiFs) show on average two- to ten-fold enhancements in their fluorescence signatures at room temperature. However, at 77 K, we have observed that MEF is even more pronounced as compared to an identical glass control sample. We also demonstrate that the further enhancements in MEF occur at low temperature over a range of visible wavelengths for different fluorophores, for both SiFs and 20 nm surface deposited gold colloids.     

A Molecular Analogue of the C−H Activation Intermediate of the Silica-Supported Ga(III) Single-Site Propane Dehydrogenation Catalyst: Structure and XANES Signature

Propane dehydrogenation is an important field of research due to an increasing world-wide demand of propene while classical production routes through naphtha cracking are in decline. In that context, silica-supported Ga(III) sites, synthesized from surface organometallic chemistry principles, show high selectivity and stability in the propane dehydrogenation reaction. This performance is in significant contrast to the reported fast deactivation and lower selectivity of most Ga₂O₃ and CrO₃ based materials. The Ga-catalyzed propane dehydrogenation reaction is proposed to proceed through the formation of Ga alkyl intermediates for which it would be desirable to have detailed structural and spectroscopic information. Here, we prepare a consistent series of Ga(III) molecular complexes with varying numbers of alkyl and siloxide ligands; they are characterized by single crystal X-Ray diffraction and X-Ray Absorption Near Edge Structure analysis, which is known to be highly sensitive to the Ga coordination environment. We report in particular the structure and the spectroscopic signatures of [Ga(ⁱPr)(OSi(O^tBu)₃)₂(HOSi(O^tBu)₃)], a molecular mimic of the key proposed reaction intermediates in the Ga-catalyzed PDH reaction.     

Triruthenium clusters containing mono and bidentate phosphines: Synthesis,structure, thermal reactivity and fluxional behavior

The syntheses, structures and thermal reactions of [Ru₃(CO)₉{P(C₄H₃E)₃}(μ-dppe)] (2, E = S; 3, E = O; dppe = 1,2-bis(diphenylphosphino)ethane) are described. These triphosphine-substituted clusters can be easily obtained in high yield from the Me₃NO initiated room temperature reaction between [Ru₃(CO)₁₀(μ-dppe)] (1) and P(C₄H₃E)₃. Both clusters have been structurally characterized which reveals that the functionalized phosphine P(C₄H₃E)₃ is coordinated to the remote ruthenium atom using the phosphorus atom, while the NMR spectroscopic data indicate that both clusters are fluxional in solution mainly due to the ring-flipping process involving the dppe ligand which has been probed by VT NMR spectroscopy. Thermolysis of 2 at 66 °C affords 1 via P(C₄H₃S)₃ dissociation, whilst that of 3 under similar experimental conditions also furnishes the diruthenium σ,π-furyl complex [Ru₂(CO)₆(μ,η²-C₄H₃O){μ-P(C₄H₃O)₂] (4) in addition to 1.     

低功率超声增强碘帕醇氯氧化的效果和路径

研究了低功率超声(US, <38 W)对NaClO氧化非离子型碘代X射线造影剂—碘帕醇(IPM)的增强作用及机理, 考察了NaClO添加浓度和超声功率的影响, 分析并计算了体系中的主要活性物种及其贡献. 采用高效液相色谱/串联质谱(HPLC/MS/MS)对降解产物进行分析, 推测IPM的降解路径. 结果表明, 低功率US显著增强了NaClO对IPM的氧化效果, 在25 ℃, pH=5.8, NaClO浓度为0.12 mmol/L条件下, 10 mg/L IPM在60 min的降解率达到85.8%. 其中NaClO氧化、 HO·和活性氯自由基(RCSs)是US/NaClO增强IPM降解的主要原因, 自由基分析计算它们的贡献率分别为15.82%, 4.65%和79.53%. NaClO浓度在0~0.24 mmol/L范围内, IPM的降解率随NaClO浓度升高而增加, 60 min后降解率由4.75%增加到91.12%; 超声功率为28.5 W, 降解率达到最高. 在 15~45 ℃温度范围内, IPM的降解过程符合表观一级反应动力学, 反应活化能(E_a)为59.03 kJ/mol. HPLC/MS/MS共检测出5种中间产物, 结合密度泛函理论(DFT)计算结果, 初步推测了IPM在US/NaClO体系中的降解途径和机理.     

理论研究晶体场效应和电荷转移效应对Co2+的2p电子X射线L2,3吸收边光谱的影响

运用多重态计算方法研究了在正八面体对称性的晶体场中Co²⁺离子的2p电子X射线L_2,3吸收边光谱, 研究了Co²⁺离子和周围的配位离子之间的正八面体(O_h)晶体场效应和相应的电荷转移效应对于吸收光谱的影响. 系统讨论了在多重态计算中起作用的所有物理参数对CoO和CoCl₂的X射线吸收光谱特性的特定影响及其物理机制. 将计算得出的光谱数据和同样具有O_h对称性结构Co²⁺离子的CoO和CoCl₂实验光谱数据进行了对比, 在实验光谱数据中发现的特征被确定为来自不同自旋态, 并且光谱强度的变化与晶体场的强度相关, 揭示了其中包含的电荷转移效应. 本文为低对称性复杂系统的多重态计算提供了一个基础的参考标准, 可以适用于含有钴元素或其它过渡金属的复杂体系的X射线吸收光谱的理论计算.     

Click Chemistry in Ultra-high Vacuum – Tetrazine Coupling with Methyl Enol Ether Covalently Linked to Si(001)

The additive-free tetrazine/enol ether click reaction was performed in ultra-high vacuum (UHV) with an enol ether group covalently linked to a silicon surface: Dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate molecules were coupled to the enol ether group of a functionalized cyclooctyne which was adsorbed on the silicon (001) surface via the strained triple bond of cyclooctyne. The reaction was observed at a substrate temperature of 380 K by means of X-ray photoelectron spectroscopy (XPS). A moderate energy barrier was deduced for this click reaction in vacuum by means of density functional theory based calculations, in good agreement with the experimental results. This UHV-compatible click reaction thus opens a new, flexible route for synthesizing covalently bound organic architectures.     

Molecular Cage Assembly by Sn−O−Sn Bridging of Di-, Tri- and Tetranuclear Organotin Tectons: Extending the Spacing in Double Ladder Structures

Hydrolysis reactions of di- and trinuclear organotin halides yielded large novel cage compounds containing Sn−O−Sn bridges. The molecular structures of two octanuclear tetraorganodistannoxanes showing double-ladder motifs, viz., [{Me₃SiCH₂(Cl)SnCH₂YCH₂Sn(OH)CH₂SiMe₃}₂(μ-O)₂]₂ [ 1 , Y=p-(Me)₂SiC₆H₄-C₆H₄Si(Me)₂] and [{Me₃SiCH₂(I)SnCH₂YCH₂Sn(OH)CH₂SiMe₃}₂(μ-O)₂]₂ ⋅ 0.48 I₂ [ 2⋅ 0.48 I₂, Y=p-(Me)₂SiC₆H₄-C₆H₄Si(Me)₂], and the hexanuclear cage-compound 1,3,6-C₆H₃(p-C₆H₄Si(Me)₂CH₂Sn(R)₂OSn(R)₂CH₂Si(Me)₂C₆H₄-p)₃C₆H₃-1,3,6 ( 3 , R=CH₂SiMe₃) are reported. Of these, the co-crystal 2⋅ 0.48 I₂ exhibits the largest spacing of 16.7 Å reported to date for distannoxane-based double ladders. DFT calculations for the hexanuclear cage and a related octanuclear congener accompany the experimental work.     

Soluble Congeners of Prior Insoluble Shape-Persistent Imine Cages

One of the most applied reaction types to synthesize shape-persistent organic cage compounds is the imine condensation reaction and it is assumed that the formed cages are thermodynamically controlled products due to the reversibility of the imine condensation. However, most of the synthesized imine cages reported are formed as precipitate from the reaction mixture and therefore rather may be kinetically controlled products. There are even examples in literature, where resulting cages are not soluble at all in common organic solvents to characterize or study their formation by NMR spectroscopy in solution. Here, a triptycene triamine containing three solubilizing n-hexyloxy chains has been used to synthesize soluble congeners of prior insoluble cages. This allowed us to study the formation as well as the reversibility of cage formation in solution by investigating exchange of building blocks between the cages and deuterated derivatives thereof.     

Polyoxotungstate Archetype {P4W27} and its 3d Derivatives

The propensity of the new, phenylphosphonate-stabilized polyoxotungstate [(C₆H₅P^VO)₂P₄W₂₄O₉₂]¹⁶⁻ to act as a precursor for new 3d metal-functionalized polyanions has been investigated. Reactions with Mn^II and Cu^II induce the formation of the previously unknown polyoxotungstate archetype {P₄W₂₇}, isolated as salts of the polyanions [Na⊂{Mn^II(H₂O)}{WO(H₂O)}P₄W₂₆O₉₈]¹³⁻ ( 1 ) and [K⊂{Cu^II(H₂O)}{W(OH)(H₂O)}P₄W₂₇O₉₉]¹⁴⁻ ( 2 ), which were characterized in the solid state (single-crystal X-ray diffraction, elemental and TG analyses, IR spectroscopy, SQUID magnetometry) and in aqueous solution (UV/Vis spectroscopy, cyclic voltammetry).     

Photophysics of Azobenzene Constrained in a UiO Metal–Organic Framework: Effects of Pressure,Solvation and Dynamic Disorder

Photophysical studies of chromophoric linkers in metal–organic frameworks (MOFs) are undertaken commonly in the context of sensing applications, in search of readily observable changes of optical properties in response to external stimuli. The advantages of the MOF construct as a platform for investigating fundamental photophysical behaviour have been somewhat overlooked. The linker framework offers a unique environment in which the chromophore is geometrically constrained and its structure can be determined crystallographically, but it exists in spatial isolation, unperturbed by inter-chromophore interactions. Furthermore, high-pressure studies enable the photophysical consequences of controlled, incremental changes in local environment or conformation to be observed and correlated with structural data. This approach is demonstrated in the present study of the trans-azobenzene chromophore, constrained in the form of the 4,4’-azobenzenedicarboxylate (abdc) linker, in a UiO topology framework. Previously unobserved effects of pressure-induced solvation and conformational distortion on the lowest energy, nπ* transition are reported, and interpreted the light of crystallographic data. It was found that trans-azobenzene remains non-fluorescent (with a quantum yield less than 10⁻⁴) despite the prevention of trans-cis isomerization by the constraining MOF structure. We propose that efficient non-radiative decay is mediated by the local, pedal-like twisting of the azo group that is evident as dynamic disorder in the crystal structure.