Molecular probe FET: Long-wavelength fluorescence excitation effect and proton transfer reaction

We studied the steady-state fluorescence spectra of solutions of FET (4′-(diethylamino)-3-hydroxyflavone) in acetonitrile that were excited at different temperatures by quanta with different energies located in the range of the main absorption band and in its long-wavelength wing. We found that, at room temperature, the emission intensity ratio of the bands of the normal and tautomeric forms, which are located at 505 and 570 nm, respectively, depends on the excitation wavelength. In the range of the main absorption band 300–360 nm, this ratio remains nearly the same, i.e., 1.45, while, upon excitation in the range of the long-wavelength wing 360–380 nm of the main band, it decreases to 1.33 at a wavelength of 460 nm. In this same range, a long-wavelength excitation effect that is unusual for liquid inviscid solvents at room temperature, i.e., a bathochromic shift of the entire short-wavelength emission band by 11 nm, manifests itself. We propose to explain these dependences using energy diagrams, which take into account the dependence of free energy on the orientational polarization of the polar solvent. The observed effect of the long-wavelength shift of the fluorescence spectrum with increasing excitation wavelength is explained in terms of the inhomogeneous broadening of electronic spectra of polar solutions, and it should be described using the scheme of energy states that takes into account sublevels of orientational broadening due to orientational dipole-dipole interactions of the fluorophore with nearest molecules of the polar solvent, as well as the relation between the fluorophore lifetime in the excited state and the dielectric relaxation time of solvent molecules in the field of the fluorophore dipole.     

Aggregation of trichogin analogs in weakly polar solvents: PELDOR and ESR studies

The technique of pulsed electron-electron double-resonance is used to study the self-aggregation of spin labeled trichogin GA IV analogs in weakly polar solvents. The dipole-dipole spinspin relaxation of spin labels has been experimentally studied in glassy solutions of spin-labeled peptides frozen to 77 K in the mixtures of chloroform-toluene, chloroform-decalin, tetrachloromethanetoluene, dichloroethan-toluene depending on the label position in peptide and the structure of terminal groups. It is shown that the studied trichogin analogs in weakly polar solvents form aggregates whose structure depends on solvent properties and peptide structure. It is also demonstrated that the distances between spin labels which can be measured in aggregates amount to the values of 2.3, 2.6 and 3.3 nm. The lower estimate is given for the average number of peptide molecules in aggregates to within 3.1–4.3 depending on peptide structure and solvent composition.     

Combined effect of linear and nonlinear dipole-dipole interactions on the electronic spectra of van der Waals complexes in solutions

A new semiempirical approach previously proposed by the author for describing the regularities of the shift of absorption bands in the IR spectra of ultimately ordered van der Waals complexes is extended to the case of three-component solutions of dyes in binary solvents consisting of neutral (nonpolar) and active (polar) components. This approach is based on the analysis of the specific features of the combined manifestation of nonlinear (complex formation) and linear (solvation) intermolecular forces of dipole-dipole nature in the change in the electrical parameters of interacting molecules. It is shown that the concepts developed enable correct quantitative interpretation of a large amount of available spectral-luminescent data for complexes of 1: 1 composition between molecules of amino-substituted derivatives of phthalimide and molecules of the active component of a binary solvent, with these complexes being solvated by molecules of the nonpolar component of the solvent.     

Application of the method of selective fluorescence excitation to investigation of complexation and solvation processes of polar organic dye molecules in binary solvents

It is proposed to use the method of selective fluorescence excitation to find absorption spectra (fluorescence excitation spectra) of 1 : 1 primary solvated complexes between polar molecules of an organic dye and the active component of a binary solvent, whose neutral component is a nonpolar or low-polarity liquid. The technique was tested with diluted solutions of 4-dimethylaminochalcone (4-DMC) in mixtures of ethylbenzene with dimethyl formamide at extremely low contents of the latter. It is shown that the experimental absolute shift of the long-wavelength vibronic absorption band of three-component DMC solutions is in a good quantitative agreement with the analogous shift obtained independently based on the semi-empirical theory describing the joint effect of nonlinear (complexation) and linear (solvation) dipole-dipole interactions on the shift of spectral bands.     

Mechanisms of specific effects of polar solvent in optical absorption spectra

We measured the optical absorption spectra of para-benzoquinone and duroquinone in polar (methanol) and nonpolar (n-hexane) solvents. We find that the specific effect of the polar solvent, which manifests itself here as a bathochromic shift of one of π-π* bands, is caused by the formation of hydrogen bonds between solvent molecules and the molecule under study and, as a consequence, by a decrease in the energy gap between the corresponding occupied (π) and unoccupied (π*) molecular orbitals. This result is obtained by TDDFT B3LYP/6-311+G(d, p)-calculations of electronic spectra, which, in the case of isolated para-benzoquinone and duroquinone molecules, reproduce experimental optical absorption spectra of the corresponding compounds in n-hexane and, in the case where these molecules form complexes with methanol molecules by means of hydrogen bonds, reproduce spectra measured in methanol.     

Quantum-Chemical Calculations of the Excited Electronic States of the Prodan Molecule and Its Complexes in Water

A quantum-chemical calculation of the prodan molecule and its complexes in water for the geometry of the ground and fluorescent states is carried out. To describe the fluorescent state, changes in the electronic state (population) on bonds and atoms during transition of the molecule into an excited state are taken into account. A model of interaction of the prodan molecule with a polar proton-donor solvent (water) is suggested. It is shown that interaction with the ionic forms (H₃O₊ provides an explanation for the sensitivity of prodan to the solvent (displacement of the fluorescence bands). The nature of the electronic excited states of the prodan molecule and its complexes has been investigated. The constants of the rates of radiative and nonradiative processes and the fluorescence quantum yields have been calculated. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 3, pp. 330–334, May–June, 2005.     

Solvatochromic and Fluorescence Behavior of Sulfisoxazole

The Fluorescence spectroscopic and solvatochromic behavior of Sulfisoxazole, a sulfa drug with antimicrobial activities, in various pure solvents of different polarity and hydrogen bonding capability is reported. The fluorescence emission spectrum of sulfisoxazole was found to be solvent polarity dependent, where a notable red shift in emission maximum was observed with increasing solvent polarity as well as hydrogen bonding capability. The effects of the latter two solvent parameters were quantitatively investigated using the methods of Lippert–Mataga and solvatochromic comparison method (SCM) that is based on the Kamlet-Taft equation. Particularly, the Lippert–Mataga method was applied to estimate the dipole moment of the excited state (μ_e) upon plotting Stokes shift versus solvent polarizability (Δf), where a value of 11.54 Debye was obtained. On the other hand, applying the multiple regression analysis to the SCM method revealed that solvent polarizability (π*) and hydrogen-bond donor capability (α) approximately equally stabilize sulfisoxazole in the excited state with minor destabilization contribution by the hydrogen-bond acceptor capability (β). These findings revealed that the excited state of sulfisoxazole is stabilized by polar solvents, indicating that this drug molecules exhibit larger dipole moment in the excited state than in the ground state, which in turn implies that a potential intramolecular charge transfer (ICT) occurs after excitation.     

Investigation of Dual Fluorescence in 4-Dimethylaminobenzonitrile

A quantum-chemical study of the fluorescent properties of a 4-dimethylaminobenzonitrile molecule is performed with allowance for the interaction with a polar solvent. It is shown that two radiative states are formed and no significant changes occur in the geometry of the molecule. The mechanism of formation of anomalous dual emission for polar molecules is discussed. It is suggested that the interaction between a polar molecule and a polar solvent results in a strong hydrogen bond and a molecule-polar solvent complex where two radiative processes of different nature are possible.     

Rotational Diffusion of Coumarins in Alcohols: A Dielectric Friction Study

The rotational dynamics of three structurally similar polar molecules viz., coumarin 440, coumarin 151 and coumarin 450 has been studied in alcohols at room temperature using steady-state fluorescence depolarization method and time correlated single photon counting technique. The observed reorientation times of all the three probes are found to be longer than those predicted by Stokes–Einstein–Debye (SED) hydrodynamic theory with stick boundary condition and a deviation towards super-stick behavior is noted. Dielectric friction theories of Nee–Zwanzig and van der Zwan–Hynes, which treat the solute as a point dipole, overestimate the dielectric friction contribution exhibited by all the three coumarins in alcohols. Results are discussed in the light of theoretical models and the possibility of hydrogen bonding between the amino group of the probe molecules and the hydroxyl group of the alcohols.     

Intramolecular charge transfer and formation of luminescence centers of 4-nitro-4-dimethylaminobutadiene in binary solvents

Absorption and luminescence spectra of a 4-nitro-4-dimethylaminobutadiene molecule in binary solvents have been experimentally obtained and investigated. Formation of the absorption spectra is associated with a chromophore grouping of the molecule in a bent state, while the bathochromic shift of the band in the absorption spectrum is consistent with the theory of dipole-dipole interaction of a bipolar molecule with solvent molecules. The fluorescence spectrum and its form and bathochromic shift depend not only on the solvent polarity but also on the proportion and kind of binary mixture of solvents. For a certain proportion of the binary solvent, radiation has been generated at λ=610 nm in pumping by a nitrogen laser. Reported at the International Conference on Luminescence, November, 22–24, 1994, Moscow. Belarusian State University, 4, F. Skorina Ave., Minsk, 220080, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 119–121, January–February, 1997.     

Structural Dynamic Models and Vibrational Spectra of Nitrobenzene and Nitropyridines

Using the DFT/B3LYP method with the basis sets 6-31G*(**), we analyzed the vibrational spectra of nitrobenzene and nitropyridines for free molecules and their solutions in water, ethanol, and benzene. The influence of a solvent on the behavior of the characteristic spectral parameters of nitrobenzene is revealed, and the assignment of vibrations for nitropyridines is given. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 4, pp. 450–453, July–August, 2005.     

Determination of the rate constant of oxygen quenching of the excited states of molecules from the singlet oxygen luminescence

We have pioneered a method of determining the rate constant of quenching of the excited electronic states of molecules by molecular oxygen from measurements of the kinetics of photosensitized luminescence of singlet molecular oxygen (^lδ_g). The method can be used in the case where the lifetime of the excited electronic state in an air-saturated solution is comparable with or larger than the luminescence time of the singlet molecular oxygen in the given solvent. It is shown that this situation is implemented on quenching, by molecular oxygen, of the excited triplet states associated with the biopolymers of tetrapyrrole molecules in aqueous (H₂O and D₂O) solutions. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 3, pp. 401–404, May–June, 2000.     

Specifics of C<Subscript>60</Subscript> Fullerene Cluster Formation in a Solvent Mixture of Toluene and <Emphasis Type="Italic">N</Emphasis>-Methyl-2-Pyrollidone

The two-level organization of C₆₀ fullerene clusters in solutions of C₆₀ in pure toluene and toluene/N-methyl-2-pyrrolidone solvent mixtures is studied by small-angle X-ray scattering (SAXS). The SAXS data for freshly prepared solutions are compared with the data obtained by dynamic light scattering, a technique we use to assess the temporal stability of the prepared solutions. For all solutions in the mixed solvent, regardless of the ratio between the polar and nonpolar components, the UV?Vis absorption spectra tend to become featureless with time. We attribute this behavior primarily to the presence of the polar component in the molecular environment of fullerene molecules.     

Phase transitions and dielectric relaxation in (1 − <Emphasis Type="Italic">x</Emphasis>)SrTiO<Subscript>3</Subscript>-<Emphasis Type="Italic">x</Emphasis>BiFeO<Subscript>3</Subscript> (0 ≤ <Emphasis Type="Italic">x</Emphasis> ≤ 0.04)

The dielectric and acoustic properties of (1 − x)SrTiO₃-xBiFeO₃ (0 ≤ x ≤ 0.04) solid solutions have been studied in the temperature range 10–300 K. The polar state exhibiting permittivity dispersion and dielectric hysteresis loops has been revealed at temperatures of 40–100 K. At 20–40 K, we have observed one more dielectric relaxation, which is not associated with the polar state and vanishes at a concentration of the second solid-solution component x = 0.04. The antiferrodistorsive transition has been found to vary with increasing concentration x. At temperatures below the antiferrodistorsive transition point, the polar (relaxor) state has been shown to persist in all the measured solid solutions.     

Microphase separation in polymer solutions containing surfactants

A microphase separation in solutions containing a polymer and a mixture of two solvents, one of which consists of amphiphilic molecules (surfactant), is considered theoretically in the weak-segregation regime. A surfactant molecule is described as a dimer consisting of hydrophobic and polar parts. The energy gain due to the orientation of surfactant molecules can lead to the appearance of non-homogeneities in the solution, where density fluctuations cause the orientational ordering of surfactant molecules. The difference in the interaction energies of hydrophobic and polar groups of a surfactant with solvent is considered as a main reason for orienting surfactant molecules. The free energy is calculated for various morphologies (lamellar, cylindrical hexagonal, spherical particles arranged at different cubic lattices). The phase diagrams are presented. With worsening the solvent quality, the transitions from disordered to a macro-separated state at low polymer and surfactant concentrations or to a body-centered-cubic, then hexagonal, and then lamellar structure at high polymer and surfactant concentrations are predicted. The amphiphilicity degree of surfactant molecules should exceed a certain critical value to make a microstructure formation possible. The period of the lamellar microstructure decreases with increasing the surfactant and polymer concentrations.     

Solution electronic spectra of brassinosteroid and a synthesized conjugate of a steroid and a fluorescent label

Electronic absorption spectra of 24-epicastasterone and 24-epibrassinolide solutions have been recorded and analyzed. The long-wavelength band in solution spectra of the former steroid with maxima in the region 292–286 nm is assigned to the electronic nπ*-transition of a carbonyl chromophore. The latter steroid contains an O-C=O group, for which the extinction coefficient in this spectral region is approximately one order of magnitude less. These differences can be used to identify the studied steroids. A method to synthesize the conjugate of 24-epicastasterone with a fluorescent label, dansylhydrazine, which shows intense fluorescence and absorption, is described. Absorption spectra of the label and conjugate in diethylether, tetrahydrofuran, ethanol, and acetonitrile solutions are practically independent of the solvent properties. The exceptions are solutions of the conjugate in hexane and water. All investigated solutions of label and conjugate show intense fluorescence. As the dielectric constant of the solvent increases, the fluorescence spectra shift to long-wavelength. The fluorescence energetic and time characteristics of both the label and conjugate are similar, which enables the conjugate to be used in developing a method for immunofluorescent analysis of the brassinosteroid 24-epicastasterone. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 74–78, January–February, 2008.     

Fluorescent properties and spontaneous Raman spectroscopy of new ketocyanine probes in organic solvents

We have used fluorescence spectroscopy and spontaneous Raman spectroscopy to study the characteristics of two ketocyanine dyes: 2,5-di[(E)-1-(4-diethylaminophenyl)methylidene]-1-cyclopentanone (CPET) and 2-[(E)-1-(4-diethylaminophenyl)methylidene]-5-{(E)-1-[4-(4,7,10,13-tetraoxa-1-azacyclopentadecalin) phenyl]methylidene}-1-cyclopentanone (CPMR) in organic solvents. The position of their electronic spectra depends strongly on the polarity of the solvent. We measured the dipole moments of the dyes in the equilibrium ground state and the Franck-Condon excited state. In mixtures of neutral nonpolar toluene with aprotic polar dimethylsulfoxide, we observe inhomogeneous broadening of the electronic spectra for the indicated compounds, due to fluctuations in solution of the intermolecular interaction energy. The time-resolved characteristics of fluorescence obtained suggest formation of an intermolecular hydrogen bond between the dye and the surrounding medium in a toluene-ethanol mixture. We measured the Raman spectra of CPET and CPMR in different organic solvents. The most intense lines in the 1582–1591 cm⁻¹ region can be assigned to stretching of the phenyl rings of the molecules; the lines in the 831–842 cm⁻¹ region can be assigned to a cyclopentanone ring mode; the lines at 1186–1195 cm⁻¹ can be assigned to stretching of the =C-C-bond of the phenyl ring and rocking of the H atoms of the phenyl ring. We have observed that the position and width of the lines for the stretching vibrations of the ketocyanines depend substantially on the polarity of the surrounding medium. The studied dyes can be used as probes for studying different biological systems by site-selective laser spectroscopy and Raman spectroscopy. The fact that these two methods can be used simultaneously for diagnostics of biosystems is an important advantage of ketocyanine dyes compared with other known probes. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 763–769, November–December, 2006.     

Facile synthesis,growth mechanism,and optical properties of CdSe nanoparticles in self-assembled micellar media and their efficient conjugation with proteins

This article demonstrates the influence of various surfactants of different polarities—anionic, sodium dodecyl sulfate, cationic, hexadecyltrimethylammonium bromide and non-ionic, and polyoxyethylene iso-octyl phenyl ether (TX-100)—on the formation of CdSe nanoparticles in aqueous solutions. The surfactant-stabilizing effect has been monitored using transmission electron microscopy. Spectral properties of CdSe nanoparticles have been investigated; the structure of the long-wave edge of the fundamental absorption band of CdSe nanoparticles has been analyzed. It has been shown that the variation of the synthesizing conditions (stabilizer’s nature and concentration, CdSe concentration, etc.) allows the tailoring of the CdSe nanoparticle size in the range of 8–17 nm. Lifshitz–Slyrzov–Wagner kinetic analysis has also been performed using the size variation according to ripening temperature and time period. The differences in the stabilization ability of tested substances are discussed with respect to their structure and possible mechanism of the surface interaction with the nanoparticles. The flexible surface chemistry of the CdSe-micelles causes them to be water soluble and allows their further conjugation with protein molecules through electrostatic attraction. The interaction between functionalized CdSe nanoparticles with protein molecules have been investigated using fluorescence spectroscopy.     

Fluorescence of 3-monomethylamino-N-methylphthalamide in mixtures of polar and nonpolar solvents

Modifications occurring in the fluorescence and excitation spectra of 3-monoethylamino-N-methylphthalimide (3 MANP) dissolved in nonpolar solvents (-hexane and methylcyclohexane) as a function of added n-butanol and n-propanol between -90 and 90°C were measured. The modifications were due to the presence of two types of centres resulting from the interaction of the solvent molecules with the nonpolar solvent on the one hand, and with the polar solvent on the other. The mixed complex character of the fluorescence band was not affected by the concentration of the solution in the range 10^-7?5 x 10^-4 mol/l. An attempt was made to establish a relationship between the emission due to the centres resulting from the interaction with the alcohol molecules and the association degree of the respective molecules.