Synthesis of luminescent 2-(2'-hydroxyphenyl)benzoxazole (HBO) borate complexes

Complexation of boron trifluoride by a series of electron donor/acceptor substituted 2-(2'-hydroxy phenyl)benzoxazole (HBO) derivatives yields luminescent B(III) complexes with an emission wavelength ranging from 385 to 425 nm in dichloromethane or toluene. Appropriate chemical functionalization of these new dyes allows connection to different photoactive subunits (Boranil, BODIPY), endowing an efficient cascade energy transfer.     

Aggregation-induced emission enhancement of 2-(2'-hydroxyphenyl)benzothiazole-based excited-state intramolecular proton-transfer compounds

A novel class of 2-(2'-hydroxyphenyl)benzothiazole-based (HBT-based) excited-state intramolecular proton-transfer (ESIPT) compounds, N,N'-di[3-Hydroxy-4-(2'-benzothiazole)phenyl]isophthalic amide (DHIA) and N,N'-di[3-Hydroxy-4-(2'-benzothiazole)phenyl]5-tert-butyl-isophthalic amide (DHBIA) has been feasibly synthesized and the properties of their nanoparticles in THF/H2O mixed solvent were investigated. Both compounds were found to exhibit aggregation-induced emission enhancement (AIEE) due to restricted intramolecular motion and easier intramolecular proton transfer in solid state. On identical experimental conditions, the emission of DHBIA aggregates increased more remarkably than that of DHIA. Different aggregation forms of these two organic compounds, due to the steric hindrance of a single tert-butyl group, could be responsible for the notably different degrees of the fluorescence enhancement. Their aggregation modes were investigated on the basis of time-dependent absorption, scanning electron microscope (SEM) images, and molecular modeling with theoretical calculation. The photophysical dynamics were also depicted based on the extremely fast ESIPT four-level cycle.     

Revealing the ionization ability of binding site I of human serum albumin using 2-(2'-hydroxyphenyl)benzoxazole as a pH sensitive probe

The ability of site I of human serum albumin (HSA) to bind medium sized molecules is important for the distribution, metabolism, and efficacy of many drugs. Herein, we show that this binding site has the ionization ability that may alter the drug structure during the process of its delivery. We reveal this ability by employing 2-(2'-hydroxyphenyl)benzoxazole (HBO) as a pH sensitive probe. Binding of HBO in site I is studied here at physiological pH 7.2 using steady-state and lifetime spectroscopic measurements, molecular docking and molecular dynamics (MD) simulation methods. The complex photophysics of HBO and the unique fluorescence signature of its anionic form indicate that, upon binding with HSA, the molecule exists in equilibrium between the anionic and the syn-keto forms. The position of HBO inside the binding site was determined experimentally by measuring the fluorescence quenching of W214, the sole tryptophan residue in HSA. The ionization degree of HBO inside the binding site was estimated to be close to the ionization degree of HBO in an aqueous solution of pH 10. This was concluded by comparing the fluorescence behavior of bound HBO to that of HBO in different solvents and in aqueous solutions of different pH values. Molecular docking and MD simulations show that HBO binds in site I close to W214, confirming the experimental results, and pinpoint the dominant role of hydrophobic interactions in the binding site. The formation of the anionic form is proposed to be due to through-space interaction between the OH group of HBO and both R222 and I290 with a binding mode similar to that of warfarin in site I. Comparison of the results with those of HBO mixed with key amino acids in solution indicates the importance of through-space interaction in the formation of the anion, similar to enzymatic reactions.     

Complexing properties of 2-(2'-hydroxyphenyl)8-hydroxyquinoline

The reactions of the new ligand, 2-(2'-hydroxyphenyl)-8-hydroxyquinoline with selected metal ions have been studied. With Co(II), Ni(II), Cu(II), Zn(II) and Cd(II), 1:1 complexes are formed in solution which are considerably more stable than the corresponding complexes of 8-hydroxyquinoline. With AI(III) stable 1:1 and 2:1 complexes are formed. With the aid of molecular models, the enhanced stability is attributed to the formation of a strain-free 5,6 bicyclic chelate-ring system.     

Gravimetric determination of palladium with 2-(o-Hydroxyphenyl) benzoxazole

Summary A method has been developed for the direct gravimetric determination of palladium. The method saves time and is reproducible and accurate. Of the several cations and anions studied, only a few of the cations interfered seriously with the determination of palladium.     

Fluorescence spectroscopy and amplified spontaneous emission (ASE) of phenylimidazoles: predicted vibronic coupling along the excited-state intramolecular proton transfer in 2-(2'-hydroxyphenyl)imidazoles

Methylation at the 1N position of 2-phenylimidazole provides the shortest wavelength for a liquid-state laser dye reported to date; that is, the 1-methyl-2-phenylimidazole molecule in cyclohexane solution yields amplified spontaneous emission (ASE) with a peak wavelength at 314.5 nm and a constant laser gain value of 5 cm(-1) from 310 to 317 nm. Methyl substitution in this case favors the appearance of laser action (owing to a torsion-vibrational mechanism) in cyclohexane as compared with the nonmethylated species which does not exhibit ASE in this solvent. The 2-(2'-hydroxyphenyl)imidazole molecules give rise to ASE with high gain values (ca. 9 cm(-1)) at 450 and 466 nm. The mechanism of population inversion is understood in terms of a vibronic coupling between the hydroxyl stretching motion and the torsional vibration of the phenyl and imidazole rings. The proton-transfer spectroscopy of 2-(2'-hydroxyphenyl)imidazoles is studied in dioxane, cyclohexane, dimethyl sulfoxide, methanol, and water. The greater the acidity of the solvent the greater the disruption of the intramolecular hydrogen bond; solvent acidity is the main parameter which favors formation of the open-form species in the ground electronic state. Methyl substitution at the 1N position favors formation of the open species for 2-hydroxyphenylimidazoles in the ground electronic state, which decreases their own capacity to undergo ASE. Low-temperature absorption spectroscopy confirms aggregation processes for 2-(2'-hydroxyphenyl)imidazoles in solution. In accordance with X-ray analyses in the solid phase, these molecules form associations through intermolecular chains of the type N-H...O or O-H...N.     

Ultrafast branching of reaction pathways in 2-(2'-hydroxyphenyl)benzothiazole in polar acetonitrile solution

In a combined study on the photophysics of 2-(2'-hydroxyphenyl)-benzothiazole (HBT) in polar acetonitrile utilizing ultrafast infrared spectroscopy and quantum chemical calculations, we show that a branching of reaction pathways occurs on femtosecond time scales. Apart from the excited-state intramolecular hydrogen transfer (ESIHT) converting electronically excited enol tautomer into the keto tautomer, known to be the dominating mechanism of HBT in nonpolar solvents such as cyclohexane and tetrachloroethene, in acetonitrile solution twisting also occurs around the central C-C bond connecting the hydroxyphenyl and benzothiazole units in both electronically excited enol and keto tautomers. The solvent-induced intramolecular twisting enables efficient internal conversion pathways to both enol and keto tautomers in the electronic ground state. Whereas relaxation to the most stable enol tautomer with twisting angle Θ = 0° implies full ground state recovery, a small fraction of HBT molecules persists as the keto twisting conformer with the twisting angle Θ = 180° for delay times extending beyond 120 ps.     

Modeling the Tautomeric Equilibrium and Absorption Spectrum of 4,5-Dimethyl-2-(2'-hydroxyphenyl)imidazole

Russian Journal of Physical Chemistry A - The potential of a new interface for molecular dynamics simulations with potentials of combined quantum mechanics/molecular mechanics is demonstrated...     

Zn2+-triggered excited-state intramolecular proton transfer: a sensitive probe with near-infrared emission from bis(benzoxazole) derivative

Near-infrared (NIR) emission can offer distinct advantages for biological applications. A fluorescent sensor, Zinhbo-1, based on bis(benzoxazole) ligand with 2,2'-dipicolylamine (DPA) as receptor, was synthesized. In aqueous solution, Zinhbo-1 demonstrates high sensitivity and selectivity for sensing Zn(2+) with about 10-fold enhancement and nanomolar sensitivity (K(d) = 0.29 nM). Moreover, sensor Zinhbo-1 can detect Zn(2+) in near-infrared region (over 700 nm) with large Stokes shift (ca. 230 nm) attributing to the Zn(2+)-induced excited state intramolecular proton transfer (ESIPT).     

Matching-pursuit split-operator Fourier-transform simulations of excited-state intramolecular proton transfer in 2-(2'-hydroxyphenyl)-oxazole

The excited-state intramolecular proton-transfer dynamics associated with the keto-enolic tautomerization reaction in 2-(2(')-hydroxyphenyl)-oxazole is simulated according to a numerically exact quantum-dynamics propagation method and a full-dimensional excited-state potential energy surface, based on an ab initio reaction surface Hamiltonian. The reported simulations involve the propagation of 35-dimensional wave packets according to the recently developed matching-pursuit/split-operator-Fourier-transform (MP/SOFT) method by Wu and Batista. The underlying propagation scheme recursively applies the time-evolution operator as defined by the Trotter expansion to second order accuracy in dynamically adaptive coherent-state expansions. Computations of time-dependent survival amplitudes, photoabsorption cross sections, and time-dependent reactant(product) populations are compared to the corresponding calculations based on semiclassical approaches, including the Herman-Kluk semiclassical initial value representation method. The reported results demonstrate the capabilities of the MP/SOFT method as a valuble computational tool to study ultrafast reaction dynamics in polyatomic systems as well as to validate semiclassical simulations of complex (nonintegrable) quantum dynamics in multidimensional model systems.     

Spectrofluorimetric determination of cysteine by 5-maleimidyl-2-(m-methylphenyl)benzoxazole

A new thiol weak-fluorescence probe, 5-maleimidyl-2-(m-methylphenyl)benzoxazole (MMPB), gives a highly fluorescence product in the presence of Cys. In this paper, MMPB has been developed for the fluorimetric determination of cysteine (Cys). At lambda(ex)/lambda(em) = 305.6/425.6 nm, the linear range is from 0 to 3.3 x 10(-7) mol l(-1) and the detection limit (sigma = 3) of 6.2 x 10(-10) mol l(-1). The main advantage of this method lies in the relative high selectivity compared with the methods using other N-substituted maleimide type of thiol reagents, in which 0.15-fold (molar ratio) of GSH is allowed and most of other amino acids at 100-fold (molar ratio) level had no obvious effect on the results. The proposed method has been applied to the determination of Cys in real samples.     

Effect of the aggregate state on the conjugation in the 2-(2′-quinolyl)benzoxazole system

It was demonstrated by x-ray diffraction analysis, electronic spectroscopy, and mass spectrometry that the phase state and a decrease in the temperature of the solution have a substantial effect on the dihedral angle between the planes of the rings and the conjugation in the 2-(2-quinolyl)benzoxazole molecule. The molecule is planar in the crystalline state, in the gas phase, and in solution at low temperature. The conjugation is maximal in these cases. The conjugation decreases when the compound is dissolved, and this is reflected in the character of the electronic absorption and emission spectra.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 775–779, June, 1982.     

2－（4－联苯基）苯并噁唑衍生物的合成及其光学性能

合成了７种２－（４－联苯基）苯并　唑衍生物，其中５种为未知物．经元素分析、ＩＲ、ＵＶ、１ＨＮＭＲ和ＭＳ表征，并测定了上述化合物的荧光发射光谱、荧光量子产率和激光转换效率，结果发现５，７－二异丙基－２－（４－联苯基）苯并　唑的激光转换效率高，具有应用价值．     

Size-tunable emission from 1,3-diphenyl-5-(2-anthryl)-2-pyrazoline nanoparticles

Organic nanoparticles of 1,3-diphenyl-5-(2-anthryl)-2- pyrazoline (DAP) ranging in average diameters from 40 to 160 nm were prepared through the reprecipitation method. The average diameters of the particles were controlled by variation of the aging time. We found that DAP nanoparticles exhibit the size-dependent optical properties. The absorption transitions of the nanoparticles at the lower-energy side experience a bathochromic shift with an increase in the particle size as a result of the increased intermolecular interactions, while the higher-energy bands of anthracene split possibly due to the electronic coupling between the pyrazoline ring of one molecule and the anthracene moiety of the neighboring molecule. Most interestingly, the nanoparticle emission in the blue light region from pyrazoline chromophore shifts to shorter wavelengths with an increase in the particle size, accompanied with a relatively gradual dominance of the emission at about 540 nm from an exciplex between the pyrazoline ring of one molecule and the anthracene moiety of the neighboring molecule. The hypsochromic shift in the emission of DAP nanoparticles was identified as originating from the pronounced decrease in the Stokes shift due to the restraint of vibronic relaxation and the configuration reorganization induced by the increased intermolecular interaction.     

The role of tautomeric and rotameric species in the photophysics of 2-(2′-hydroxyphenyl)benzoxazole

The complex photophysical behaviour of 2-(2′-hydroxyphenyl)benzoxazole (HBO) has been investigated using both steady-state absorption and fluorescence and time-resolved (picosecond) emission spectroscopy, and found to be consistent with the existence of various tautomeric and rotameric species in equilibrium in the ground state. The natures of these species are discussed. Molecular-orbital calculations (PPP), performed for all species, gave results in good agreement with experiment. Excited-state intramolecular proton transfer was found to occur with a rate constant exceeding 1 × 10¹¹ s^?1.     

The effect of hydrogen bonding on the excited-state proton transfer in 2-(2'-hydroxyphenyl)benzothiazole: a TDDFT molecular dynamics study

The dynamics of the excited-state proton transfer (ESPT) in a cluster of 2-(2'-hydroxyphenyl)benzothiazole (HBT) and hydrogen-bonded water molecules was investigated by means of quantum chemical simulations. Two different enol ground-state structures of HBT interacting with the water cluster were chosen as initial structures for the excited-state dynamics: (i) an intramolecular hydrogen-bonded structure of HBT and (ii) a cluster where the intramolecular hydrogen bond in HBT is broken by intermolecular interactions with water molecules. On-the-fly dynamics simulations using time-dependent density functional theory show that after photoexcitation to the S(1) state the ESPT pathway leading to the keto form strongly depends on the initial ground state structure of the HBT-water cluster. In the intramolecular hydrogen-bonded structures direct excited-state proton transfer is observed within 18 fs, which is a factor two faster than proton transfer in HBT computed for the gas phase. Intermolecular bonded HBT complexes show a complex pattern of excited-state proton transfer involving several distinct mechanisms. In the main process the tautomerization proceeds via a triple proton transfer through the water network with an average proton transfer time of approximately 120 fs. Due to the lack of the stabilizing hydrogen bond, intermolecular hydrogen-bonded structures have a significant degree of interring twisting already in the ground state. During the excited state dynamics, the twist tends to quickly increase indicating that internal conversion to the electronic ground state should take place at the sub-picosecond scale.     

Phenylboronic acid catalyzed-cyanide promoted, one-pot synthesis of 2-(2-hydroxyphenyl)benzoxazole derivatives

A novel, one-pot, phenylboronic acid catalyzed, cyanide promoted synthesis of 2-(2-hydroxyphenyl)benzoxazoles from salicylaldehydes and o-aminophenols is described. The synthesis is characterized by mild conditions, short reaction times, and simple workup of crystalline, high purity products.     

Efficient synthesis of 2-(2′-hydroxyphenyl)benzoxazole by palladium(II)-catalyzed oxidative cyclization

An efficient method for the synthesis of 2-(2′-hydroxyphenyl)benzoxazole has been developed by using palladium-mediated oxidative cyclization.