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.     

Effects of solvent on nonlinear absorption properties of tetraphenylporphyrin compounds in the nanosecond regime

The nonlinear absorption properties of tetraphenylporphyrins (TPP) in different solvents and excitation intensities are investigated with nanosecond pulses by the Z-scan technique. The nonlinear absorption behaviour observed presents a distinct difference between in solvent mixtures and in pure solvents. A crossover from reverse saturable absorption (RSA) to saturable absorption (SA) and then again to RSA is observed with the increases of excitation intensity in chloroform and pyridine mixed solvents. However, porphyrin molecules in pure solvents show good RSA behaviour under the studied intensity range, no saturation absorption is observed with nanosecond excitation. Excited state absorption parameters and two-photon absorption coefficients are obtained by theoretical fit using rate equations for population densities in a seven-level energy scheme.     

Spectrophotometric Characteristics of New Pyridylindolizine Derivatives Solutions

Three new pyridylindolizine derivatives, 1, 2, 3-tricarbometoxi-7-(4-pyridyl)-pyrrolo[1, 2-a]pyridine (I), 1,2-dicarboethoxy-3-(4-bromobenzoyl)-7-(4-pyridyl)-pyrrolo[1,2-a]pyridine (II) and its isomer 1,2-dicarboethoxy-3- (4-bromobenzoyl) -5- (2-pyridyl) -pyrrolo[1, 2-a]pyridine (III) have been investigated in different solutions by UV-VIS absorption, steady-state, and time-resolved fluorescence methods. The effects of the substituent and solvent on the spectroscopic properties have been demonstrated. The fluorescence decay data could be fitted to a single-exponential function. The lifetime values are higher in protic polar than in aprotic apolar solvents for compound I. In the case of compounds II and III the fluorescence intensities and lifetimes are very low, with the exception of III in aprotic solvents. The absorption and fluorescence properties of the compounds showed a solvent dependence.     

Fluorescent properties of thiochrome in solvents of different polarity

Spectral and fluorescent properties of thiochrome in solvents of different polarity were studied. It was found that the pK_a value of the transition between the cationic and neutral forms of thiochrome in aqueous solutions increased from ∼5.5 to 9.7 upon photoexcitation. It is supposed that protonation takes place in the excited state of the molecule resulting in fluorescence quenching of the thiochrome neutral form in aqueous solutions at neutral pH values. The fluorescence quantum yield of thiochrome increased by ∼2.2 times upon the transition from aqueous solutions to alcohols or polar aprotic solvents. It was found that an increase of the solvent polarity led to an increase in the Stokes shift from 3200 to 4200 cm⁻¹ for the thiochrome neutral form emission. The change in the dipole moment upon excitation into the S1-state was estimated to be less than 3D.     

Hypersensitivity in the Luminescence and 4f?C4f Absorption Properties of Mono- and Dinuclear EuIII and ErIII Complexes Based on Fluorinated ??-Diketone and Diimine/ Bis-Diimine Ligands

The results of our investigation on the sensitized luminescence properties of three Eu(III) ??-diketonate complexes of the form [Eu₂(fod)₆(??-bpm)], [Eu(fod)₃(phen)] and [Eu(fod)₃(bpy)] and 4f?C4f absorption properties of their Er(III) analogues ( fod = anion of 6,6,7,7,8,8,8- heptafluoro-2,2-dimethyl-3,5-octanedione, bpm = 2,2??-bipyrimidine, phen = 1,10-phenanthroline and bpy = 2,2??-bipyridyl) in a series of non-aqueous solvents are presented. The Eu(III) complexes are highly luminescent and their luminescence properties (intensity and band shape) are sensitive to the changes in the inner coordination sphere of the Eu(III) ion. The luminescence intensity of the mononuclear complexes in pyridine is drastically decreased. The coordination structure of the complexes in pyridine is transformed into a more symmetrical one which results into a slow radiative rate of the emission from the complexes. The ancillary ligands, phen and bpy are found better co-sensitizers as compared to the bpm to sensitize Eu(III)-luminescence. The 4f?C4f absorption properties (oscillator strength and band shape) of the Er(III) complexes demonstrate that ⁴G_11/2 ?? ⁴I_11/2 and ²H_11/2 ?? ⁴I_15/2 hypersensitive transitions of Er(III) are very sensitive in some coordinating solvents which reflects complex?Csolvent interaction in solution. The hypersensitive transitions of [Er(fod)₃(phen)] remain unaffected in any of the solvents and this complex retains its bulk composition in solution. The erbium complexes as well as the Er(fod)₃ chelate are invaded by DMSO. This solvent enters the inner coordination sphere by replacing heterocyclic ligand and the complexes acquire similar structure [Er(fod)₃(DMSO)₂] in this solvent. The results reveal that the luminescence and absorption properties of lanthanide complexes in solution can be controlled by tuning the coordination structure through ancillary ligands and donor solvents. This work shall prove useful in designing new biological applications with such probes.     

Raman spectral analysis of the intermolecular hydrogen bond in bromoform

The maximum of the perpendicular polarized component of the Raman spectrum of the C−H mode in liquid bromoform shifts by 1.5 cm⁻¹ toward higher frequencies from the maximum of the parallel polarized component. A high-frequency shift of this band is noted in solution with nitromethane and nitrobenzene, and the shape of the band becomes more complex. The result is attributed to superposition of the associate lines in the mixture (the high-frequency component) and the monomer line. There are indications that the 537 cm⁻¹ band of bromoform in a mixture with the same solvents and with pyridine has a doublet character. When CHBr₃ is dissolved in a neutral solvent such as heptane, both bands exhibit normal behavior, the widths of the lines decreasing presumably because of an increase in the vibrational relaxation time. A. Navoi State University, Samarkand. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 90–94, December, 1998.     

Beiträge zur Fluoreszenz und Energieübertragung in reinen Lösungsmitteln und flüssigen Mischsystemen organischer Szintillatoren

The intensity and the decay time of pure solvents and mixtures of two different solvents with and without fluorescent molecules have been investigated by way of excitation with electron beams and ultraviolet light. In the solutions Anthracene/Benzene and o-o-Diphenol/Benzene the characteristic transfer constanta=1/τ _t ·c has been determined in two different ways. The investigation of the intensity and the decay time of a solution with two solvents in dependence of the ratio of the components at constant concentration of the fluorescent molecules leads to the conclusion, that the energy transfer from solvent to solute via other solvent molecules is insignificant.     

Association of rhodamine 6G in light and heavy water solutions

The absorption characteristics of rhodamine 6G molecules in light and heavy water solutions are studied. The association efficiencies of dye molecules and the structure and the binding energy of complexes formed are determined in relation to the solvent composition. The structure of H₂O +D₂O solutions is studied using correlation and low-frequency Raman spectroscopy. The distribution of dissolved complex molecules in the water matrix is shown to have inhomogeneities, which are responsible for a high association efficiency. The characteristic size of these inhomogeneities is estimated. The fractal dimension of the structural formations of light and heavy water molecules is determined as a function of their concentration.     

The effect of anti-hydrogen bond on Fermi resonance: A Raman spectroscopic study of the Fermi doublet ν1—ν12 of liquid pyridine

The effects of anti-hydrogen bond on the ν₁—ν₁₂ Fermi resonance (FR) of pyridine are experimentally investigated by using Raman scattering spectroscopy. Three systems, pyridine/water, pyridine/formamide, pyridine/carbon tetrachloride, provide varying degrees of strength for the diluent-pyridine anti-hydrogen bond complex. Water forms a stronger anti-hydrogen bond with pyridine than with formamide, and in the case of adding non-polar solvent carbon tetrachloride, which is neither a hydrogen bond donor nor an acceptor and incapable of forming hydrogen bond with pyridine, the intermolecular distance of pyridine will increase and the interaction of pyridine molecules will reduce. The dilution studies are performed on the three systems. Comparing with the values of Fermi coupling coefficient W of the ring breathing mode ν ₁ and triangle mode ν ₁₂ of pyridine at different volume concentrations, which are calculated according to the Bertran equations, in three systems, we find that the solution with the strongest anti-hydrogen bond, water, shows the fastest change in the ν₁—ν₁₂ Fermi coupling coefficient W with the volume concentration varying, followed by the formamide and carbon tetrachloride solutions. These results suggest that the stronger anti-hydrogen bond-forming effect will cause a greater reduction in the strength of the ν₁—ν₁₂ FR of pyridine. According to the mechanism of the formation of anti-hydrogen bond in the complexes and the FR theory, a qualitative explanation for the anti-hydrogen bond effect in reducing the strength of the ν₁—ν₁₂ FR of pyridine is given.     

Deuterium NMR of pyridine intercalated cadmium chalcogenophosphate

Deuterium NMR spectra of the intercalation complexes of lamellar Cd₂P₂S₆ with various deuterated pyridine molecules are reported. The measurements indicate that the pyridine molecules lie within the chalcogen van der Waals gaps with their molecular planes parallel to the chalcogen layers. Between room temperature and −60°C the pyridine molecules reorient rapidly (on the NMR time scale) about an axis perpendicular to their molecular plane.     

Influence of the Structure of Water–Alcohol Solutions of Dyes on Their Molecular Association

The spectral and luminescence characteristics of water and water–ethanol solutions of dyes have been investigated. The efficiency of the process of association of dye molecules and the structure of the complexes formed depending on their concentration and the solvent composition have been established. The presence of inhomogeneities in the distribution of dissolved complex molecules in the water matrix which determine the high efficiency of the association processes in water solutions as compared with other solvents is shown. The characteristic sizes of these inhomogeneities have been estimated.     

Raman Spectra and Solvent-Solute Interaction of Mercury(II)-Thiocyanate Complexes in Some Aprotic Donor Solvents

Raman spectra of the species Hg(SCN)₂, [Hg(SCN)₃], and [Hg(SCN)₄]^2- in solution of ten aprotic donor solvents have been investigated in the region of the Hg-S vibration. Observed frequency values of measured band of Hg(SCN)₂ and [Hg(SCN)₃] in different solutions correlate well with donor strength of the solvents. There is a linearity between Hg-S frequency decreasing and increasing of the interaction of the solvent molecules with the mercury (II) ion in the thiocyanate complexes. No significant frequency changes have be found for [Hg(SCN)₄]^2-. Evidence based on the frequency shifts and depolarization ratios in various solvents supports the view that the Hg(SCN)₂ in solution undergos departure from linearity as a result of increasing solvent coordination to the mercury (II) ion. Almost constant frequencies of Hg-S vibration of [Hg(SCN)₄]^2- in all investigated solvents suggest a regular tetrahedral structure of the ion in solution having much larger radius than corresponding HgX₄ (X=Cl,Br,I) ions.     

The role of environment in the nonradiative relaxation of the triplet state of naphthalene derivatives in solid solutions at 77 K

The nonradiative relaxation of the triplet states of oxy-and amino-derivatives of naphthalene and conjugated ions is studied by luminescent and kinetic methods (measurements of the phosphorescence decay and of phosphorescence excitation and phosphorescence spectra) in solid solutions of ethanol-h ₆, ethanol-d ₆, and mixtures of toluene-h ₈ with piperidine-h ₁₁ at 77 K. It is found that, along with intramolecular factors, a microsolvate surrounding a molecule or an ion substantially affects the nonradiative relaxation. The contribution of this factor in ethanol increases in the series cation-polar molecule-anion and in mixtures of toluene with piperidine—with increasing piperidine concentration. The results are interpreted assuming the inductive-resonance dipole-dipole transfer of the triplet energy to the dipole acceptors of intramolecular bonds and bonds with molecules of the environment. The relative arrangement of hydrogen atoms of OH groups of ethanol molecules in microsolvates of cations and anions estimated using the inductive-resonance model agrees with the difference in the structure of solvates of oppositely charged ions, which is caused by the electrostatic charge-dipole interaction and the distribution of the electron density in the ground state of the corresponding emitting center. The inductive-resonance model was used for studying the features of solvation processes involving polar molecules. It is shown that the difference between the structures of microsolvates of 2-oxy-naphthalene molecules in solvents with close dipole moments (ethanol and piperidine) is mainly caused by the different ability of these solvent molecules to form associates. The structure of microsolvates of oxy-derivatives of naphthalene in the associated amphoteric solvent is found to depend on the number and position of substituents. In oxy-derivatives of naphthalene with spatially separated OH groups in ethano-d ₆, deuteroexchange occurs in both substituents, whereas in the naphthalene derivative with adjacent OH groups this occurs only in one of the groups. Comparison of the phosphorescence spectra of hydrogen-bond complexes and proton-transfer complexes in nonpolar solvents at 77 K revealed the existence of molecular naphthol entities in the triplet state that were formed from ionized entities in the singlet state.     

Interactions of triiodide cluster ion with solvents

An equilibrium molecular dynamics model is developed to investigate the interactions of triiodide cluster ion with solvents. The internal dynamics of the triiodide ion is described by a valence bond model which responds to the field of the classical solvent molecules. The solvent molecules were described by standard classical models with rigid molecules, fixed partial charges on atomic sites and site-site Lennard-Jones interactions. One finds the solvent effects on the I^-₃ are unusually strong as it is a very polarizable species. Protic solvents such as water, ethanol, and methanol that can form hydrogen bonds to lead to the I^-₃ geometry with two unequal bonds and an asymmetric distribution of charges. But for the solvents such as xenon, tetrahydrofuran, methyltetrahydrofuran, and acetonitrile, the I^-₃ only illustrates a geometry with two equal bonds. We find that structure changing is induced by local electrostatic attraction between solvent molecules.     

Formation of Zero Density Regions During the Dissolving of C60 and C70

Based on a comprehensive review of the calculated and experimental data obtained in this and previous articles by the authors, a phenomenological model of the supramolecular structure of fullerene solutions is proposed. The model suggests the development in the fullerene solutions of some zero density regions due to poor packing of spheroidal fullerene molecules C₆₀ and C₇₀, on the one hand, and columnar or ribbon supramolecular structures formed by solvent molecules, on the other. It is assumed that the source of the long-range energy of fullerenes is a physical vacuum, concentrated not only in the cavities of the fullerene molecules but also in new regions forming outside the fullerenes during their dissolving in single-ring aromatic solvents.     

Variation of Dipole Moment Function of O-H Bond of t-Butanol with the Nature of Solvent and Its Temperature

The intermolecular influence on dipole moment function is evaluated for O-H bond of t-butanol in different nonpolar solvents at temperatures ranging from 10° to 60°C employing IR band intensities of fundamental and first overtone bands. Two sets of dipole moment derivatives have been calculated corresponding to – and +- combinations of the transition moment matrix elements R₁₀ and R₂₀, the values for ++ and -+ combinations are equal in magnitude to those for - and +- combinations, respectively and opposite in signs. In general the dipole moment derivatives increase on lowering the temperature as well as with increasing molecular interactions with the solvent molecules. Dipole moment plots with dimensionless coordinate ξ [=(r-r_e)/r_e, where r and r_e are internuclear distances during vibration and at equilibrium, respectively] are reported for various systems considered. It is found that for +- combination the dipole moment maximum shifts to higher internuclear distances when polarisation of the solute molecules is increased by lowering of temperature or increase in molecular interactions between solute and solvent molecules. A reverse trend is observed for – combination.- The OH band of t-butanol vapor has been measured.     

Intramolecular Proton Transfer Effects on 2,6-diaminopyridine

The photophysical behaviour of 2,6-diaminopyridine (DAP) has been studied in solvents of different polarity, pH, β-cyclodextrin (β-CD) and compared with 2-amino pyridine (2AP). The inclusion complex of both molecules with β-CD are analysed by UV-visible, fluorimetry, FT-IR, ¹H NMR, SEM and AM1 methods. The solvent studies shows i) DAP gives more red shifted absorption and emission maxima than 2AP molecule and ii) addition of amino group in 2AP effectively increase the resonance interaction in the pyridine ring. A regular red shift observed in acidic pH solutions suggests intramolecular proton transfer (IPT) present in both molecules. β-CD studies indicates i) in pH  ~ 7, a regular red shifted absorption and emission maxima observed in AP molecules suggests pyridine ring encapsulated in to the β-CD cavity (1:1 inclusion complex formed) and ii) in pH ~ 1, a blue shifted absorption maxima noticed in 2AP, is due to protonated amino group deeply encapsulated in to the hydrophobic part of the β-CD cavity.     

The Correlation Between <Emphasis Type="Italic">f–f</Emphasis> Absorption and Sensitized Visible Light Emission of Luminescent Pr(III) Complexes: Role of Solvents and Ancillary Ligands on Sensitivity

The electronic absorption, excitation and sensitized visible light emission studies of three praseodymium (III) complexes: [Pr(fod)₃(bpy)], [Pr(fod)₃(phen)] and [Pr(fod)₃(bpm)]_n (fod = anion of 6,6,7,7,8,8,8- heptafluoro-2,2-dimethyl-3,5-octanedione; bpy = 2,2'-bipyridyl, phen = 1,10-phenanthroline, bpm = 2,2'-bipyrimidine) in a series of non–aqueous solvents is presented. The f–f absorption transitions of Pr(III) are environment sensitive which is reflected by the change in the intensity (oscillator strength) and band shape (stark splitting) upon change in the solvent and/or the ligands. The sensitization of intense Pr(III) emission, in the visible region, of the complexes in solution upon excitation into the ligand centered π→π* absorption band is remarkable. The planar phen has pronounced impact and increases considerably the emission intensity of Pr(III) luminescence than the flexible bpy while bpm has been found least effective in promoting the emission intensity. The intensity of the f-f absorption and sensitized emission are correlated with the nature of the solvents. The donor solvent pyridine enhances the emission intensity of the [Pr(fod)₃(phen)] drastically and of [Pr(fod)₃(bpy)] marginally while the luminescent intensity of [Pr(fod)₃(bpm)]_n is decreased. The combined photophysical studies demonstrate that entry of the solvent molecule(s) to inner coordination sphere (complex–solvent interaction) is governed by the structure and basicity of the ancillary heterocyclic ligand attached to the Pr(III) complex. The strong donor DMSO transforms the three complexes into a similar species, [Ln(fod)₃(DMSO)₂], which results in similar electronic absorption and emission properties of the complexes in this solvent. The results demonstrate that highly luminescent praseodymium chelates can be designed with ligands containing suitable energy levels and their emission properties can be further modulated through suitable ancillary ligands and donor solvents, thus opening perspectives for applications in electroluminescent devices and luminescent probes.     

Effect of the medium and the formation of nanostructures on deexcitation of electronic excitation of Eu(III) and Tb(III) chelates

The intensity I _lum and lifetime τ_lum of the luminescence of complexes of Eu(III) and Tb(III) ions with β-diketones and o-phenanthroline in water-ethanol solutions of these ligands have been analyzed as functions of the concentrations of ligand, luminescing lanthanide ions, and added ions causing columinescence and of the solvent deuteration. It is shown that the formation of nanostructures from Ln complexes and their coarsening leads to an increase in τ_lum of Eu(III) and Tb(III) and that this increase is due to the suppression of both photochemical deexcitation of these ions and transfer of their electronic excitation energy to OH vibrations of water molecules. The disappearance of the dependence of I _lum of Eu(III) on deuteration of water-ethanol solutions of n-methoxybenzoyltrifluoracetone + o-phenanthroline caused by adding Gd(III) ions is explained by the shift of the equilibrium of formation of complexes of Ln chelates to neutral hydrophoblic forms corresponding to the formation of nanostructures of these chelates in the solution. The differences in effect of La(III) and Gd(III) ions on I _lum and τ_lum of Eu(III) and Tb(III) complexes are explained. It is shown that the widely discussed effect of columinescence not only results from the energy migration in mixed structures of Eu or Tb complexes and Gd complexes but is also due to a large extent to the decrease in τ_lum of Eu(III) or Tb(III) caused by their incorporation into nanostructures.     

Ionic interactions in solutions of electrolytes as studied by ultra-violet spectroscopy

Spectrophotometric evidence for the formation of contact ion-pairs in solutions of tetra-alkylammonium iodides in certain solvents of poor ion-solvating power, such as carbon tetrachloride, is presented.

Since the band ascribed to contact ion-pairs is not detected for solutions in other solvents, such as chloroform, it is concluded that the ion-pairs detectable by other techniques in such solvents are separated by solvent molecules.