26 research outputs found
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, [Zn<sub>2</sub>(1,4-bdc)<sub>2</sub>(dabco)]<i><sub>n</sub></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 N<sub>2</sub>, 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⊃PAPÂ(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 [RuÂ(bpy)Â(dppz)Â(OSO)]Â(ClO<sub>4</sub>), [<b>1</b>]Â(ClO<sub>4</sub>) (bpy = 2,2′-bipyridine, dppz =
dipyridoÂ[3,2-<i>a</i>:2′,3′-<i>c</i>]Âphenazine 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>]Â(ClO<sub>4</sub>) is arrested in water although is active in the
analogous compound [RuÂ(bpy)<sub>2</sub>(OSO)]Â(ClO<sub>4</sub>). Effective
excited state deactivation through dark <sup>3</sup>MLCT state involving
dppz ligand of [<b>1</b>]Â(ClO<sub>4</sub>) 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<sup>–</sup>/2H<sup>+</sup>) reduction of O<sub>2</sub> to hydrogen peroxide
by ferrocene
(Fc) or 1,1′-dimethylferrocene (Me<sub>2</sub>Fc) in the presence
of perchloric acid is catalyzed efficiently by a mononuclear copperÂ(II)
complex, [Cu<sup>II</sup>(tepa)]<sup>2+</sup> (<b>1</b>; tepa
= trisÂ[2-(2-pyridyl)Âethyl]Âamine) in acetone. The <i>E</i><sub>1/2</sub> value for [Cu<sup>II</sup>(tepa)]<sup>2+</sup> as
measured by cyclic voltammetry is 0.07 V vs Fc/Fc<sup>+</sup> in acetone,
being significantly positive, which makes it possible to use relatively
weak one-electron reductants such as Fc and Me<sub>2</sub>Fc for the
overall two-electron reduction of O<sub>2</sub>. Fast electron transfer
from Fc or Me<sub>2</sub>Fc to <b>1</b> affords the corresponding
Cu<sup>I</sup> complex [Cu<sup>I</sup>(tepa)]<sup>+</sup> (<b>2</b>), which reacts at low temperature (193 K) with O<sub>2</sub>, however
only in the presence of HClO<sub>4</sub>, to afford the hydroperoxo
complex [Cu<sup>II</sup>(tepa)Â(OOH)]<sup>+</sup> (<b>3</b>).
A detailed kinetic study on the homogeneous catalytic system reveals
the rate-determining step to be the O<sub>2</sub>-binding process
in the presence of HClO<sub>4</sub> at lower temperature as well as
at room temperature. The O<sub>2</sub>-binding kinetics in the presence
of HClO<sub>4</sub> 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 O<sub>2</sub> is shown to result from the existence of
a pre-equilibrium between <b>2</b> and O<sub>2</sub> prior to
the formation of the hydroperoxo complex <b>3</b>. No further
reduction of [Cu<sup>II</sup>(tepa)Â(OOH)]<sup>+</sup> (<b>3</b>) by Fc or Me<sub>2</sub>Fc occurred, and instead <b>3</b> is
protonated by HClO<sub>4</sub> to yield H<sub>2</sub>O<sub>2</sub> accompanied by regeneration of <b>1</b>, thus completing the
catalytic cycle for the two-electron reduction of O<sub>2</sub> by
Fc or Me<sub>2</sub>Fc
Acid-Induced Mechanism Change and Overpotential Decrease in Dioxygen Reduction Catalysis with a Dinuclear Copper Complex
Catalytic
four-electron reduction of O<sub>2</sub> by ferrocene
(Fc) and 1,1′-dimethylferrocene (Me<sub>2</sub>Fc) occurs efficiently
with a dinuclear copperÂ(II) complex [Cu<sup>II</sup><sub>2</sub>(XYLO)Â(OH)]<sup>2+</sup> (<b>1</b>), where XYLO is a <i>m</i>-xylene-linked
bisÂ[(2-(2-pyridyl)Âethyl)Âamine] dinucleating ligand with copper-bridging
phenolate moiety], in the presence of perchloric acid (HClO<sub>4</sub>) in acetone at 298 K. The hydroxide and phenoxo group in [Cu<sup>II</sup><sub>2</sub>(XYLO)Â(OH)]<sup>2+</sup> (<b>1</b>) undergo
protonation with HClO<sub>4</sub> to produce [Cu<sup>II</sup><sub>2</sub>(XYLOH)]<sup>4+</sup> (<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 HClO<sub>4</sub> to 0.47 V;
this makes possible the use of relatively weak one-electron reductants
such as Fc and Me<sub>2</sub>Fc, significantly reducing the effective
overpotential in the catalytic O<sub>2</sub>-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 O<sub>2</sub>-binding to the fully reduced complex [Cu<sup>I</sup><sub>2</sub>(XYLOH)]<sup>2+</sup> (<b>3</b>) results in the
reversible formation of the hydroperoxo complex ([Cu<sup>II</sup><sub>2</sub>(XYLO)Â(OOH)]<sup>2+</sup>) (<b>4</b>), followed by proton-coupled
electron-transfer (PCET) reduction to complete the overall O<sub>2</sub>-to-2H<sub>2</sub>O 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