Unexpected Nonresponsive Behavior of a Flexible Metal-Organic Framework under Conformational Changes of a Photoresponsive Guest Molecule

In this article, we describe the synthesis, characterization, and optical properties of a photochromic-guest-incorporated metal-organic framework (MOF). The photochromic guest molecule, 2-phenylazopyridine (PAP), was introduced into a pre-synthesized porous crystalline host MOF, [Zn₂(1,4-bdc)₂(dabco)]<i>_n</i> (<b>1</b>). The successful embedment of PAP has been confirmed by elemental analysis, powder X-ray diffraction measurements, IR spectroscopy, etc. The number of PAP molecules per unit cell of host was 1.0, as evidenced by elemental and thermogravimetric analyses of the host–guest composite, <b>1⊃PAP</b>. The <b>1⊃PAP</b> composite did not adsorb N₂, revealed by the adsorption isotherm of <b>1⊃PAP</b>, which indicates the pore blockage by the close contact of the host framework with the guest PAP in the trans form. The light-induced trans/cis isomerization with partial reversibility of the guest molecule (PAP) in this hybrid host–guest compound (<b>1⊃PAP</b>) has been investigated by detailed IR spectroscopy and UV–vis spectroscopy. The structural transformation from tetragonal in <b>1</b> to orthorhombic in <b>1⊃PAP</b> exhibits dynamic nature of the framework upon inclusion of guest in the framework, which remarkably becomes nonresponsive with the photoirradiation of guest PAP, retaining its orthorhombic structure in the photoirradiated complex, <b>1⊃PAP­(UV)</b>

Conformational Control of Ultrafast Molecular Rotor Property: Tuning Viscosity Sensing Efficiency by Twist Angle Variation

Fluorescent molecular rotors find widespread application in sensing and imaging of microscopic viscosity in complex chemical and biological media. Development of viscosity-sensitive ultrafast molecular rotor (UMR) relies upon the understanding of the excited-state dynamics and their implications for viscosity-dependent fluorescence signaling. Unraveling the structure–property relationship of UMR behavior is of significance toward development of an ultrasensitive fluorescence microviscosity sensor. Herein we show that the ground-state equilibrium conformation has an important role in the ultrafast twisting dynamics of UMRs and consequent viscosity sensing efficiency. Synthesis, photophysics, and ultrafast spectroscopic experiments in conjunction with quantum chemical calculation of a series of UMRs based on dimethylaniline donor and benzimidazolium acceptor with predefined ground-state torsion angle led us to unravel that the ultrafast torsional dynamics around the bond connecting donor and acceptor groups profoundly influences the molecular rotor efficiency. This is the first experimental demonstration of conformational control of small-molecule-based UMR efficiencies which can have wider implication toward development of fluorescence sensors based on the UMR principle. Conformation-controlled UMR efficiency has been shown to exhibit commensurate fluorescence enhancement upon DNA binding

Solvent- and DNA-Controlled Phototriggered Linkage Isomerization in a Ruthenium Sulfoxide Complex Incorporating Dipyrido[3,2‑<i>a</i>:2′,3′‑<i>c</i>]phenazine (dppz)

A new <i>tris</i>-heteroleptic complex [Ru­(bpy)­(dppz)­(OSO)]­(ClO₄), [<b>1</b>]­(ClO₄) (bpy = 2,2′-bipyridine, dppz = dipyrido­[3,2-<i>a</i>:2′,3′-<i>c</i>]­phenazine and OSO = 2-methylsulfinylbenzoate), was synthesized and characterized to control the photochromic Ru–S → Ru–O linkage isomerization. Details isomerization kinetics studied by UV–visible absorption spectroscopy and cyclic voltammetry revealed that efficient photochromic S → O isomerization and thermal O → S reversal take place in solvents like propylene carbonate (PC), methanol, and dichloromethane. Strikingly, photoisomerization of [<b>1</b>]­(ClO₄) is arrested in water although is active in the analogous compound [Ru­(bpy)₂(OSO)]­(ClO₄). Effective excited state deactivation through dark ³MLCT state involving dppz ligand of [<b>1</b>]­(ClO₄) switches off photochromism in aqueous medium. Interestingly, the photochromism is activated in aqueous solution in the presence of DNA which shields the dppz localized dark state through intercalation. Ultrafast transient absorption spectroscopic measurement sheds light on the differential behavior of photochromism in aqueous and nonaqueous solvents

Temperature-Independent Catalytic Two-Electron Reduction of Dioxygen by Ferrocenes with a Copper(II) Tris[2-(2-pyridyl)ethyl]amine Catalyst in the Presence of Perchloric Acid

Selective two-electron plus two-proton (2e^–/2H⁺) reduction of O₂ to hydrogen peroxide by ferrocene (Fc) or 1,1′-dimethylferrocene (Me₂Fc) in the presence of perchloric acid is catalyzed efficiently by a mononuclear copper­(II) complex, [Cu^II(tepa)]²⁺ (<b>1</b>; tepa = tris­[2-(2-pyridyl)­ethyl]­amine) in acetone. The <i>E</i>_1/2 value for [Cu^II(tepa)]²⁺ as measured by cyclic voltammetry is 0.07 V vs Fc/Fc⁺ in acetone, being significantly positive, which makes it possible to use relatively weak one-electron reductants such as Fc and Me₂Fc for the overall two-electron reduction of O₂. Fast electron transfer from Fc or Me₂Fc to <b>1</b> affords the corresponding Cu^I complex [Cu^I(tepa)]⁺ (<b>2</b>), which reacts at low temperature (193 K) with O₂, however only in the presence of HClO₄, to afford the hydroperoxo complex [Cu^II(tepa)­(OOH)]⁺ (<b>3</b>). A detailed kinetic study on the homogeneous catalytic system reveals the rate-determining step to be the O₂-binding process in the presence of HClO₄ at lower temperature as well as at room temperature. The O₂-binding kinetics in the presence of HClO₄ were studied, demonstrating that the rate of formation of the hydroperoxo complex <b>3</b> as well as the overall catalytic reaction remained virtually the same with changing temperature. The apparent lack of activation energy for the catalytic two-electron reduction of O₂ is shown to result from the existence of a pre-equilibrium between <b>2</b> and O₂ prior to the formation of the hydroperoxo complex <b>3</b>. No further reduction of [Cu^II(tepa)­(OOH)]⁺ (<b>3</b>) by Fc or Me₂Fc occurred, and instead <b>3</b> is protonated by HClO₄ to yield H₂O₂ accompanied by regeneration of <b>1</b>, thus completing the catalytic cycle for the two-electron reduction of O₂ by Fc or Me₂Fc

Acid-Induced Mechanism Change and Overpotential Decrease in Dioxygen Reduction Catalysis with a Dinuclear Copper Complex

Catalytic four-electron reduction of O₂ by ferrocene (Fc) and 1,1′-dimethylferrocene (Me₂Fc) occurs efficiently with a dinuclear copper­(II) complex [Cu^II₂(XYLO)­(OH)]²⁺ (<b>1</b>), where XYLO is a <i>m</i>-xylene-linked bis­[(2-(2-pyridyl)­ethyl)­amine] dinucleating ligand with copper-bridging phenolate moiety], in the presence of perchloric acid (HClO₄) in acetone at 298 K. The hydroxide and phenoxo group in [Cu^II₂(XYLO)­(OH)]²⁺ (<b>1</b>) undergo protonation with HClO₄ to produce [Cu^II₂(XYLOH)]⁴⁺ (<b>2</b>) where the two copper centers become independent and the reduction potential shifts from −0.68 V vs SCE in the absence of HClO₄ to 0.47 V; this makes possible the use of relatively weak one-electron reductants such as Fc and Me₂Fc, significantly reducing the effective overpotential in the catalytic O₂-reduction reaction. The mechanism of the reaction has been clarified on the basis of kinetic studies on the overall catalytic reaction as well as each step in the catalytic cycle and also by low-temperature detection of intermediates. The O₂-binding to the fully reduced complex [Cu^I₂(XYLOH)]²⁺ (<b>3</b>) results in the reversible formation of the hydroperoxo complex ([Cu^II₂(XYLO)­(OOH)]²⁺) (<b>4</b>), followed by proton-coupled electron-transfer (PCET) reduction to complete the overall O₂-to-2H₂O catalytic conversion

Baseline clinical and demographic characteristics.

<p>Baseline clinical and demographic characteristics.</p

Mutational analysis of the HBV polymerase and surface ORFs for the occult HBV subjects.

<p>Mutational analysis of the HBV polymerase and surface ORFs for the occult HBV subjects.</p

Identification of occult HBV infection among HIV treatment naïve patients.

<p>HBsAg negative/ HIV positive individuals were screened for the presence of HBV DNA to identify occult HBV infection. The HBsAg negative group was stratified into anti-HBc positive and anti-HBc negative groups.</p

Analysis of the association of HBV sub-genotypes with biochemical parameters.

<p>Median (A) ALT (IU/liter) and (B) bilirubin (IU/liter) levels in patients vary with circulating HBV sub-genotypes. Only significant differences (p<0.05) have been marked in the figure.</p

Phylogenetic analysis of HBV isolates from Eastern India with acute infection created using the neighbour-joining method.

<p>Phylogenetic tree was constructed from surface gene sequences of this study (denoted by AEI) along with reference sequences derived from GenBank (denoted by accession numbers).</p