Clear Isosbestic Points Research Articles

An ESIPT-ICT steered naphthylthioic-based ionic probe with dual emissive channels exhibiting CHEF and CHEQ effects.

A naphthylthioic-based emissive probe (M) bearing a hydroxyl and amine group was designed and synthesized via a one-step Schiff base reaction process. The probe was characterized spectroscopically using 1H NMR, UV-Vis and fluorescence spectrophotometers. The probe turned out to be spectroscopically and colorimetrically selective and sensitive to multiple cations and anions. Interestingly, the probe displayed characteristics of excited-state intramolecular proton transfer (ESIPT)-driven dual emissive channels; experiencing fluorescence enhancement upon the molar additions of Al3+ as well as the anions used, events presumably ascribed to chelation fluorescence enhancement (CHEF), hydrogen bonding and deprotonation effects. Moreover, the fluorometric titration with Hg2+ resulted in ratiometric spectral behaviors of M, with the disappearance of the peak at 450 nm, concomitant with the appearance of a new peak at 520 nm, distinguished by a clear isosbestic point, the same behaviors exhibited by Sn2+ and Ag+ analytes towards M. The introduction of all other cations used, resulted in fluorescence quenching, attributable to chelation enhanced fluorescence quenching (CHEQ), thereby inhibiting the ESIPT process. The experiments were all carried out in the aqueous environment medium of DMSO-H2O (9 : 1) at ambient temperature. Theoretical density functional theory calculations were carried out to gain insight into the interaction of M with cations and anions, and their influence on the HOMO-LUMO energy gaps.

Mechanistic insight into copper cation exchange in cadmium selenide semiconductor nanocrystals using X-ray absorption spectroscopy

In terms of producing new advances in sustainable nanomaterials, cation exchange (CE) of post-processed colloidal nanocrystals (NCs) has opened new avenues towards producing non-toxic energy materials via simple chemical techniques. The main processes governing CE can be explained by considering hard/soft acid/base theory, but the detailed mechanism of CE, however, has been debated and has been attributed to both diffusion and vacancy processes. In this work, we have performed in situ x-ray absorption spectroscopy to further understand the mechanism of the CE of copper in solution phase CdSe NCs. The x-ray data indicates clear isosbestic points, suggestive of cooperative behavior as previously observed via optical spectroscopy. Examination of the extended x-ray absorption fine structure data points to the observation of interstitial impurities during the initial stages of CE, suggesting the diffusion process is the fundamental mechanism of CE in this system.

Radical Cyclic [3]Daisy Chains

Summary Mechanically interlocked molecules (MIMs) that undergo controllable internal motions of their component parts in more than one dimension are rare entities in the molecular world. Cyclic [2]daisy chains ([c2]DCs) are a class of MIMs that have been identified as prototypes for molecular muscles. It remains, however, a challenge to synthesize [cn]DCs with n > 2 selectively and efficiently. Herein, we report the design and synthesis of [c3]DCs employing radical and anionic templates. Two mechanically interlocked [c3]DCs with 18 positive charges were obtained in >90% yields. One [c3]DC displayed good air stability in its radical cationic form, while the other underwent reversible “co-conformational” switching between open macrocyclic and closed trisarm-shaped forms under electrochemical control. These findings provide not only two-dimensional MIMs with attractive electronic and switchable properties, but also a starting point for the design of extended molecular arrays, which could become the forerunners of adjustable molecular nets and breathable molecular membranes.

β-carboline-based turn-on fluorescence chemosensor for quantitative detection of fluoride at PPB level.

A novel β-carboline-based chemosensor, having an acidic NH proton that leads to fluoride-induced deprotonation involving a vivid color change from colorless to yellow is described. The absorption spectrum of the chemosensor in acetonitrile has a peak at 375 nm, which changes to 428 nm with the gradual addition of only fluoride in the solution with a clear isosbestic points at 357 nm and 392 nm. More interestingly, the chemosensor gives a turn-on type of fluorescence at 554 nm in the presence of fluoride. Further it was found that the sensor is highly selective towards fluoride over other anions including chloride, bromide, iodide, nitrate, borate, perchlorate and can quantitatively detect fluoride at ppb level with a limit of detection of 0.02 mg/ L or 20 ppb. The chemosensor was successfully demonstrated to assess the fluoride concentration in the tap water.

Extraordinary Redox Activities in Ladder-Type Conjugated Molecules Enabled by B ← N Coordination-Promoted Delocalization and Hyperconjugation.

The introduction of B ← N coordinate bond-isoelectronic to C-C single bond-into π-systems represents a promising strategy to impart exotic redox and electrochromic properties into conjugated organic molecules and macromolecules. To achieve both reductive and oxidative activities using this strategy, a cruciform ladder-type molecular constitution was designed to accommodate oxidation-active, reduction-active, and B ← N coordination units into a compact structure. Two such compounds (BN-F and BN-Ph) were synthesized via highly efficient N-directed borylation. These molecules demonstrated well-separated, tworeductive and twooxidative electron-transfer processes, corresponding to five distinct yet stable oxidation states, including a rarely observed boron-containing radical cation. Spectroelectrochemical measurements revealed unique optical characteristics for each of these reduced/oxidized species, demonstrating multicolor electrochromism with excellent recyclability. Distinct color changes were observed between each redox state with clear isosbestic points on the absorption spectra. The underlying redox mechanism was elucidated by a combination of computational and experimental investigations. Single-crystal X-ray diffraction analysis on the neutral state, the oxidized radical cation, and the reduced dianion of BN-Ph revealed structural transformations into two distinct quinonoid constitutions during the oxidation and reduction processes, respectively. B ← N coordination played an important role in rendering the robust and reversible multistage redox properties, by extending the charge and spin delocalization, by modulating the π-electron density, and by a newly established hyperconjugation mechanism.

Binding mode of proflavine to DNA probed by polarized light spectroscopy

It is well known that proflavine binds to DNA. Here we investigate the binding mode of proflavine to native DNA using absorption, circular dichroism (CD), and linear dichroism (LD) spectroscopy as well as by fluorescence techniques. The observed changes of proflavine upon complexation with DNA can be summarized as a red shift and hypochromism in the absorption spectrum. The negative LDr in the proflavine absorption region has a magnitude comparable to or larger than that of the DNA absorption region, confirming the intercalative binding mode of proflavine to DNA. Saturation of the LD spectrum in the proflavine absorption region at R = 0.25 and a decrease in the fluorescence intensity provide further evidence of intercalation. Furthermore, the coupling of electric transition of intercalated proflavine is observed. Although proflavine has been reported to position itself along the DNA stem at high [proflavine]/[DNA base] ratios, the spectral characteristics, which include a clear isosbestic point in the absorption spectrum and proportionality in the LD magnitude in the proflavine absorption region, do not show any possibility of exterior binding.

A simple two-output near-infrared fluorescent probe for hydrazine detection in living cells and mice

Abstract Hydrazine is an important reactive base in chemistry, pharmacy and agriculture and an effective propellant in aeronautics and astronautics. However, tiny leakage might induce serious environmental contamination and fatal biological damage. Thus, how to rapidly detect hydrazine with high selectivity and sensitivity is of significance and challenge. In this report, we demonstrate a hydrazine-specific fluorescent probe on the modulation of intramolecular charge transfer process. This probe contains a 4-bromobutyl group as a recognition site for hydrazine anchored on the fluorescent dye. Leaving of 4-bromotutylate moiety leads to a marked NIR fluorescent emission at 715 nm induced by hydrazine. Additionally, presence of hydrazine leads to a color change from green to blue-purple and a ratiometric UV–vis absorption change with a clear isosbestic point at 635 nm. The good linear relationship between fluorescence intensity and concentration of hydrazine results in a low detection limit of 5.09 ppb. The color and fluorescence two-output signals are preferable for qualitative and quantitative detection of hydrazine with high sensitivity and specificity. More importantly, the probe is also successfully applied to visualize hydrazine in living both cells and mice.

Visible light photoactivatable CO releasing manganese (I) tricarbonyl complexes: Experimental and DFT studies

The reaction between [Mn(CO)5Br] and antibacterial drugs (Nifuroxazide (NIF) and Nitazoxanide (NTZ)) afforded fac-[MnBr(CO)3(NIF)] (1) and fac-[MnBr(CO)3(NTZ)2] (2) complexes, which were characterized by different analytical and spectral tools. The photo-induced CO releasing properties and the ability of the metal carbonyls to drive CO to the myoglobin solution were investigated. The green-light (525 nm) illumination of the DMSO solution of 1 for 20 min gave rise to the maximum amount of CO. Compared with the DMSO solution of 1, the rate of CO release from the 80% aqueous-DMSO mixed media is slower, and the plateau is reached upon the illumination for 140 min. On the contrast, a clear isosbestic point is developed during the storing of the DMSO solution of 2 for 6 h, which reflects its instability. To drive CO from 2 to myoglobin solution, the energy of the green light was not sufficient. However, the grown of the bands corresponding to the formation of carboxy-myoglobin species is stopped after 50 min of the irradiation at 468 nm and 36 min at 410 nm. The electronic structures and the related electronic transitions have been calculated by TD-DFT/B3LYP (or CAM-B3LYP)/LANL2DZ methods. The spectra calculated by the hybrid exchange-correlation functional CAM-B3LYP with a long range correction term are more closed to the experimental findings. The remarkable toxicity of 2 against the tested microbes, with respect to the free drug, may be attributed to the change of the lipophilicity of free NTZ upon the complex formation as well as the releasing of CO upon the incubation in dark.

A reversible “turn-on” fluorescent sensor for selective detection of Zn2+

A novel Schiff-base oxime sensor H2L with the ONON chelating moieties was designed and synthesized based on condensation reaction. Different methods such as elemental analyses, FT-IR, 1H and 13C NMR were fully utilized in characterizing the structure of sensor H2L. The UV–vis spectra of the Zn2+ complex exhibit four clear isosbestic points which may be assigned to the ONON moieties binding to Zn2+. Sensor H2L operates on the CHEF and PET mechanisms, and the tendency of Zn2+ to strongly enhance fluorescence of sensor H2L was explored. Fluorescence measurements show that sensor H2L has excellent fluorescent selectivity for Zn2+ over many other metal ions based on intramolecular charge-transfer, and the results demonstrate that the binding between sensor H2L and Zn2+ is chemically reversible. The crystal structure of the Zn2+ complex has been characterized by X-ray crystallography.

Structural Evolution of Metastable Protein Aggregates in the Presence of Trivalent Salt Studied by (V)SANS and SAXS.

We present a study of the structural evolution of protein aggregates formed in solutions of a globular protein, β-lactoglobulin (BLG), in the presence of YCl3. These aggregates are often observed before crystallization starts and they are metastable with respect to the crystalline phase. Here we focus on the characterization of the hierarchical structure of this intermediate phase and its temperature dependent structure evolution using a combination of (very) small angle neutron and X-ray scattering (VSANS, SANS, and SAXS). Results show that the hierarchical structure ranges from nanometer scale protein monomer, dimer and compact protein clusters to micrometer scale fractal protein aggregates. Upon cooling, the overall hierarchical structure is preserved, but the evolution of the internal structure within the aggregates is clearly visible: the monomer-monomer correlation peak reduces its intensity and disappears completely at lower temperatures, whereas the cluster-cluster correlation is enhanced. At a larger length scale, the fractal dimension of protein aggregates increases. The kinetics of the structure change during a temperature ramp was further investigated using time-resolved SAXS. The time dependent SAXS profiles show clear isosbestic points and the kinetics of the structural evolution can be well described using a two-state model. These dynamic properties of protein aggregates on a broad length scale may be essential for being the precursors of nucleation.

Supramolecular coordination polymer formed from artificial light-harvesting dendrimer.

We report the formation of supramolecular coordination polymers formed from multiporphyrin dendrimers (PZnPM; M = FB or Cu), composed of the focal freebase porphyrin (PFB) or cupper porphyrin (PCu) with eight zinc porphyrin (PZn) wings, and multipyridyl porphyrins (PyPM; M = FB or Cu), PFB or PCu with eight pyridyl groups, through multiple axial coordination interactions of pyridyl groups to PZns. UV-vis absorption spectra were recorded upon titration of PyPFB to PZnPFB. Differential spectra, obtained by subtracting the absorption of PZnPFB without guest addition as well as the absorption of PyPFB, exhibited clear isosbestic points with saturation binding at 1 equiv addition of PyPFB to PZnPFB. Job's plot analysis also indicated 1:1 stoichiometry for the saturation binding. The apparent association constant between PZnPFB and PyPFB (2.91 × 10(6) M(-1)), estimated by isothermal titration calorimetry, was high enough for fibrous assemblies to form at micromolar concentrations. The formation of a fibrous assembly from PZnPFB and PyPFB was visualized by atomic force microscopy and transmission electron microscopy (TEM). When a 1:1 mixture solution of PZnPFB and PyPFB (20 μM) in toluene was cast onto mica, fibrous assemblies with regular height (ca. 2 nm) were observed. TEM images obtained from 1:1 mixture solution of PZnPFB and PyPFB (0.1 wt %) in toluene clearly showed the formation of nanofibers with a regular diameter of ca. 6 nm. Fluorescence emission measurement of PZnPM indicated efficient intramolecular energy transfer from PZn to the focal PFB or PCu. By the formation of supramolecular coordination polymers, the intramolecular energy transfer changed to intermolecular energy transfer from PZnPM to PyPM. When the nonfluorescent PyPCu was titrated to fluorescent PZnPFB, fluorescence emission from the focal PFB was gradually decreased. By the titration of fluorescent PyPFB to nonfluorescent PZnPCu, fluorescence emission from PFB in PyPFB was gradually increased due to the efficient energy transfer from PZn wings in PZnPCu to PyPFB.

Study of Dithienylcyclopentene with Substituents at 4-Position of Thiophene

1-(2,5-Dimethylthien-3-yl)-2-(4-acetyl-2,5-dimethylthien-3-yl) cyclopentene has been synthesized by a six-step procedure, such as Vilsmeier reaction, Wolff-Kishner-Huang reduction reaction, The Friedel-Crafts acylation, McMurry reaction etc. Upon alternate irradiation with 297 and 500 nm light upon 1-(2,5-Dimethylthien-3-yl)-2-(4-acetyl-2,5-dimethylthien-3-yl) cyclopentene, the absorption spectrum changes reversibly and a clear isosbestic point appears. These results suggest that the compound can undergo photochromism and do not decompose during the irradiation.

Factors affecting hydrogen-tunneling contribution in hydroxylation reactions promoted by oxoiron(IV) porphyrin π-cation radical complexes.

Hydrogen atom transfer with a tunneling effect (H-tunneling) has been proposed to be involved in aliphatic hydroxylation reactions catalyzed by cytochrome P450 and synthetic heme complexes as a result of the observation of large hydrogen/deuterium kinetic isotope effects (KIEs). In the present work, we investigate the factors controlling the H-tunneling contribution to the H-transfer process in hydroxylation reaction by examining the kinetics of hydroxylation reactions at the benzylic positions of xanthene and 1,2,3,4-tetrahydronaphthalene by oxoiron(IV) 5,10,15,20-tetramesitylporphyrin π-cation radical complexes ((TMP(+•))Fe(IV)O(L)) under single-turnover conditions. The Arrhenius plots for these hydroxylation reactions of H-isotopomers have upwardly concave profiles. The Arrhenius plots of D-isotopomers, clear isosbestic points, and product analysis rule out the participation of thermally dependent other reaction processes in the concave profiles. These results provide evidence for the involvement of H-tunneling in the rate-limiting H-transfer process. These profiles are simulated using an equation derived from Bell's tunneling model. The temperature dependence of the KIE values (k(H)/k(D)) determined for these reactions indicates that the KIE value increases as the reaction temperature becomes lower, the bond dissociation energy (BDE) of the C-H bond of a substrate becomes higher, and the reactivity of (TMP(+•))Fe(IV)O(L) decreases. In addition, we found correlation of the slope of the ln(k(H)/k(D)) - 1/T plot and the bond strengths of the Fe═O bond of (TMP(+•))Fe(IV)O(L) estimated from resonance Raman spectroscopy. These observations indicate that these factors modulate the extent of the H-tunneling contribution by modulating the ratio of the height and thickness of the reaction barrier.

Microenvironment Effects on the Kinetics of the Alkaline Hydrolysis of Bispyridinium Conformers

ABSTRACTThe alkaline hydrolysis of a series of homologous reactants constituted by two reactive centers bridged by a methylene spacers chain, the 1,n‐bis(2‐azidepyridinium)alkanes (n = 3, 4, 5, 6, and 8), is investigated. The reaction under pseudo–first‐order condition was followed by ultraviolet–visible spectrophotometry. The presence of clear isosbestic point suggests the absence of stable intermediates. However, the intermediates 1‐(2‐azidepyridinium), n‐(2‐pyridone)alkanes (monocationic compounds), were isolated and characterized as well the reaction end products 1,n‐(2‐pyridone)alkanes (noncharged compounds). The kinetic analysis fitted to a two‐step consecutive reaction, where the k1/k2 values demonstrate the larger reactivity of the first step over the second one, especially for shorter bridged reactants. The OH− reaction order is one for each step. Although Debye–Hückel law was obeyed, the experimental point at ionic strength zero is much higher than the extrapolated one. In addition, the k1 values substantially decrease as KCl is added especially for shorter homologous whereas the effect on k2 is almost negligible. Simple charge density effects as a function of the spacer's length do not explain the observations. On the other hand, from the pronounced anion selectivity inhibition effects on k1 for the shorter derivatives, the existence of an equilibrium involving a conformer, a “sandwich‐type” complex with the OH− between the two pyridinium rings, with an “open‐stretched” conformer is proposed. For short‐bridged reactants, the complex conformer prevails whereas for long‐bridged compounds the stretched structure prevails. The results in acetonitrile/water seem to reflect changes in the equilibrium. The reaction is controlled by the balance between enthalpy and entropy. The contribution of the activation entropy in a short‐bridged reactant is higher than the long‐bridged one, which suggests an activated complex with high structural organization in the first reactants.

Synthesis and Photochromism of Bis(thien-2-yl)Cyclopentene Derivatives

Bis (thien-2-yl) cyclopentene derivatives have been synthesized in the two routes. Upon irradiation with 313 and 405 nm light, the absorption spectrum of compound 3O and 4O did not change and clear isosbestic points appeared. These results suggest that the compounds do not decompose during the irradiation and are suitable as switches

Benzthiazole-based multifunctional chemosensor: fluorescent recognition of Fe3+ and chromogenic recognition of [formula omitted

A benzthiazole-based multifunctional chemosensor was designed with siderophores like binding sites for recognition of Fe3+ and both hydrogen bond donor and acceptor binding sites for oxyanion HSO4−. The sensor recognizes Fe3+ selectively through enhancement of fluorescence emission intensity at 440 nm and recognizes HSO4− through a bathochromic shift in UV–vis spectra from 315 nm to 365 nm with clear isosbestic points at 345 nm and 395 nm. The recognition of both Fe3+ and HSO4− is free from the interference from the other cations and anions, respectively.

Observation of Exceptionally Low-Lying π–π* Excited States in Oxidized Forms of Quadruple-Decker Phthalocyanine Complexes

Spectroscopic investigations have been performed for the oxidized forms of two quadruple-decker phthalocyanine complexes in order to clarify the electronic structures of multiply stacked π-systems. Up to three-electron-oxidized species were isolated by using phenoxathiin hexachloroantimonate as the oxidant. As the oxidations proceed, the Q-bands in the visible region shift bathochromically along with the clear isosbestic points. The one- and three-electron-oxidized species exhibited typical π-radical signals in the ESR spectra, while the neutral and two-electron oxidized species gave no indication of the presence of π-radicals. The electronic transitions observed for the oxidized species reach even into the so-called fingerprint region in IR spectroscopy (~1000 cm(-1)). With the aid of theoretical calculations, these bands can be assigned to the π-π* transitions. Our results provide new insights into π-electronic systems having exceptionally small MO energy gaps.

Absolute Stereochemical Determination of Chiral Carboxylates Using an Achiral Molecular Tweezer

A new type of molecular tweezer (1) has been synthesized for the direct determination of the absolute configuration of chiral carboxylates without analyte derivatization. Upon the addition of diamine and anionic guests, 1 exhibited shifts in its absorption spectrum with clear isosbestic points. The continuous variation method indicated that both the diamine and anionic guests form 1:1 host-guest complexes with 1 with very high binding affinity. When Boc-L-Ala (BLA) as a form of tetrabutylammonium salt was added to 1, a weak negative CD signal was observed. This weak CD signal was dramatically changed to a strong positive CD couplet upon addition of achiral 1,12-diaminododecane. Such a positive CD couplet was observed for all of the tested L-amino acid derivatives, while the D-amino acid derivatives gave the opposite signals. As a result of these unique characteristics of 1, it can be utilized as a highly sensitive probe for the absolute stereochemical determination of chiral carboxylates.

Ultrafast Studies of Excess Electrons in Liquid Acetonitrile: Revisiting the Solvated Electron/Solvent Dimer Anion Equilibrium

We examine the ultrafast relaxation dynamics of excess electrons injected into liquid acetonitrile using air- and water-free techniques and compare our results to previous work on this system [Xia, C. et al. J. Chem. Phys. 2002, 117, 8855]. Excess electrons in liquid acetonitrile take on two forms: a "traditional" solvated electron that absorbs in the near-IR, and a solvated molecular dimer anion that absorbs weakly in the visible. We find that excess electrons initially produced via charge-transfer-to-solvent excitation of iodide prefer to localize as solvated electrons, but that there is a subsequent equilibration to form the dimer anion on an ~80 ps time scale. The spectral signature of this interconversion between the two forms of the excess electron is a clear isosbestic point. The presence of the isosbestic point makes it possible to fully deconvolute the spectra of the two species. We find that solvated molecular anion absorbs quite weakly, with a maximum extinction coefficient of ~2000 M(-1)cm(-1). With the extinction coefficient of the dimer anion in hand, we are also able to determine the equilibrium constant for the two forms of excess electron, and find that the molecular anion is favored by a factor of ~4. We also find that relatively little geminate recombination takes place, and that the geminate recombination that does take place is essentially complete within the first 20 ps. Finally, we show that the presence of small amounts of water in the acetonitrile can have a fairly large effect on the observed spectral dynamics, explaining the differences between our results and those in previously published work.

Raman spectroscopic study of the effects of dissolved NaCl on water structure

Raman spectroscopy was utilized to investigate the effects of dissolved NaCl on water structure. For aqueous NaCl solutions, the difference spectra indicate a clear isosbestic point at 3345cm−1 and a weak isosbestic point around 3625cm−1. According to our explanation on Raman OH stretching band of water, it can be inferred that the addition of NaCl primarily breaks the tetrahedral hydrogen bonding and promotes formation of the donor hydrogen bonding in water, and slightly lowers the amount of free OH bonds. This is different from the effects of pressure and temperature on water structure. For liquid water, a water molecule interacts with neighboring water molecules through various local hydrogen bonded networks. Additionally, the enthalpy change of hydrogen bonding in water can be determined to be 11.35kJ/mol.