Nature of the influence of a solvent on the fluorescence quantum yield of solutions of the fluorescent probe 4-dimethylaminochalcone

The fluorescence quantum yields of solutions of 4-dimethylaminochalcone (DMC) in a series of individual solvents of different chemical nature have been measured using a technique based on studying the kinetics of fluorescence decay. A new quantitative regularity, namely, the dependence of the fluorescence quantum yield on the absolute solvation shift in the fluorescence spectrum due to the gas-solution phase transition, has been revealed. The opinion has been advanced and justified that the revealed regularity has a fundamental character, which is determined by the dominant contribution from the universal intermolecular interactions to the formation of the properties of the solvated DMC molecule in the excited electronic state.     

Quantum-chemical study of relation of spectral and luminescent properties of positively solvatochromic malononitrile-based merocyanine dyes with their structure

The electronic structure, the spectra, and the efficiency of photophysical processes of energy deactivation are calculated by a semiempirical method for three positively solvatochromic merocyanine dyes with different polymethine chain lengths. The electronic structure and spectra calculated by different modifications of the INDO method at different molecular geometries are compared, and the optimum geometry of the isolated molecule is chosen. The absorption spectra of dyes are interpreted, including the short-wave-length bands related to transitions to highest excited states. The possibility of a specific electrophilic solvation of these compounds in the ground and fluorescent states, as well as the contribution of specific intermolecular interactions to the total interaction with the medium, is estimated. The role played by the trans-cis isomerization of isolated merocyanine molecules in the deactivation of their excited states is considered.     

Fluorescence excitation spectra of jet-cooled carbazole complexes with monohydric alcohols

We have analyzed the fluorescence excitation spectra of carbazole complexes with a single molecule of methyl, deuterated methyl, ethyl, and propyl (1-propanol and 2-propanol) alcohols, cooled in a supersonic jet. We have determined the shifts in the fluorescence excitation spectra of the complexes relative to the frequency of the purely electronic transition of unbound carbazole. They occur as a result of formation of hydrogen bonds between the N-H group of the carbazole and the OH group of the alcohols. The frequencies of stretching vibrations of the hydrogen bonds with different alcohols vary within the range 150–157 cm⁻¹, while the frequencies of the bending vibrations vary in the range 21–22.9 cm⁻¹. From the shape of the rotational contours of the bands for the purely electronic and vibronic transitions of the complexes, we determined that they belonged to rotational conformers. We calculated the equilibrium configurations of the complexes in the ground state. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 6, pp. 756–760, November–December, 2007.     

Nonspecific solvation and oscillator strengths for the long-wavelength vibronic absorption band of 4-dimethylaminochalcone solutions

The 4-dimethylaminochalcone molecule is one of the most efficient spectral-luminescent probes widely used in various medical and biological studies. Experimental data on the oscillator strengths in the long-wavelength vibronic band of 4-dimethylaminochalcone solutions in a series of individual solvents of different chemical nature are analyzed using the earlier derived refined equation describing the influence of nonspecific solvation processes on the matrix elements of the dipole moment of a purely electronic transition in organic dye molecules. It is shown that the semiempirical theory developed gives a satisfactory quantitative explanation to experimentally observed regularities indicative of a relatively weak influence of the solvent reaction field (orientation and induction forces) and dispersion interactions on the integrated intensity of the studied band belonging in the category of intramolecular charge transfer bands.     

A new generalized spectral luminescent method for determining dipole moments of molecules in the ground and excited states

An original spectral luminescent method for determining dipole moments of molecules in the ground and excited states was proposed, and its preliminarily tests were made. It is based on a combined quantitative analysis of experimental data on the linear (solvation) and nonlinear (complex formation) components of the orientation shift of electronic and vibrational spectra of molecules in two-and three-component solutions. To realize the method in practice, one should make a series of standard measurements of the position of absorption bands or the stationary luminescence of diluted solutions of a substance under study in several rationally chosen liquid individual and binary solvents at the room temperature. The distinctive feature of the method is that it is based on the use of spectroscopic data only and does not require that the equilibrium value of the effective radius of the intermolecular interaction (Onsager radius of a molecule of a diluted substance) be specified and the angles between the moments of ground and excited states be known.     

Role of electronic versus atomic relaxations in Stokes shifts at defects in solids

Redshifts of luminescence relative to optical absorption bands (Stokes shifts) of molecules and of defects in solids are universally attributed to slow atomic relaxations on the grounds that electronic transitions are fast (Franck-Condon principle). Here we report a novel phenomenon that can occur only in the solid state: Stokes shifts caused by slow electronic relaxations. We demonstrate that the phenomenon occurs in the nonbridging oxygen defect in amorphous SiO2. We predict that another defect (OH group), which can exist in either crystalline or amorphous SiO2, has a similar Stokes shift, but it arises from a mix of lattice and electronic relaxations with manifest differences in the two phases.     

On the mechanism of formation and broadening of the electronic and vibrational spectra of vapors of octaethylporphyrin metal complexes

Optical absorption spectra of vapors of metal complexes of octaethylporphyrins known from the literature are analyzed on the basis of notions about sequences as basic elements of the formation of electronic-vibrational bands. According to this analysis, major features of these spectra are explained. These features include considerable half-widths of the electronic-vibrational bands of rarefied vapors, when intermolecular interactions virtually do not affect the broadening; a Lorentzian intensity distribution over the contour of the 0-0 spectral band; long-wavelength shifts of the intensity maxima of bands with increasing temperature; and the origin of a fine structure of the electronic and vibrational spectra at low temperatures. It was concluded that sequential transitions form the basis of the mechanism of formation and broadening of the electronic and vibrational spectra of metal complexes of octaethylporphyrins as representatives of complex biomolecules.     

Franck-Condon transitions and thermodynamics of photoinduced processes of orientational relaxation in polar solutions

Polar luminescence probe+polar medium subsystems are considered. True minimization parameters of the free energy F are obtained. It is shown that Franck-Condon transitions between F-terms of different electronic states are generally not “vertical”. A complete thermodynamic classification of photoinduced processes of orientational relaxation in the excited S₁ and ground S₀ states is presented. Depending on the excitation conditions they can be either exo- or endothermic, spontaneous or induced (not spontaneous), and accompanied by an increase or decrease in entropy. Indifferent processes also take place. Regularities of shifts of fluorescence spectral bands and burned holes in time-resolved experiments are discussed. Institute of Molecular and Atomic Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 753–765, November–December, 1997.     

Effect of relaxation of the geometric structure on the solvatochromic shift in electronic spectra of molecules

The influence of relaxation effects on the solvatochromic shifts observed in the electronic spectra of organic compounds in solvents is investigated using quantum-chemical methods. The calculations are performed in the framework of the time-dependent density functional theory. It is revealed that, in 40% of the cases under consideration, the relaxation contribution accounts for more than 20% of the total solvatochromic shift. The most pronounced effects are associated with the observed changes in the dihedral angles between molecular fragments.     

Influence of energy transfer on the intensity pattern of vibronic excitation studied by reduced density-matrix theory

Intensity pattern of the vibronic transitions of a molecular dimer consisting of two molecules interacting through the Coulombic coupling is theoretically studied using a reduced density-matrix approach. The monomeric molecules are assumed to be electronic two-state systems. A single vibration mode with a high frequency and a continuous distribution of low-frequency phonons represented by the Ohmic spectral density are coupled to the electronic transition of the respective molecules. The spin-Boson model is employed to include the effect of electron-vibration and electron-phonon couplings. The intermolecular Coulombic coupling is assumed to be weak (inducing the Förster type of energy transfer process). It is found that, in addition to the well-known excitonic shifts, the intensity of the vibronic side bands reduces with the intermolecular coupling strength in the J-aggregate type of dimer while it increases in the H-aggregate type. When the vibronic bands are blurred by the broadening resulting from the coupling of the electrons to the continuous distribution of the phonons, the absorption line shape shows a wide range of variation depending on the strength of the intermolecular coupling.     

Molecular motion and solvent effects in several Cr5+ compounds using nonstationary nuclear-electron resonance

Dynamic proton polarization in several diols Cr⁵⁺ complex ion solutions has been studied at various magnetic field strengths and temperatures. Normally, in most organic radical solutions the dynamic enhancements of solvent proton resonances decay with increasing field and decreasing temperature due to the dipole-dipole intermolecular interactions, modulated by translational diffusion of individual molecules. In contrast to free radical, a strong solvation effect has been discovered in paramagnetic Cr⁵⁺ compounds. Drastic departures from the usual DNP behaviour have been observed. An explanation of the observed effects is proposed by considering the occurrence of hydrogen-bridged solvates. The important motional mechanism that modulates the proton-electron interaction switches from “translation” to “rotation” of the complex molecules. A new type of pulse double resonance equipment is used.     

Electronic structure of a complex with charge transfer and carrier energy bands in thin films of semiconductors containing carbazole

The electronic structures of hole and electron carrier energy bands and complexes with charge transfer (CCT) in films of semiconductors containing carbazole and sensitized with TNP were determined by the methods of photoelectron and electron spectroscopy as well as by studying the wavelength dependence of the quantum yield of photogeneration at temperatures of 295 and 80 K. The hole carrier bands are determined by the top three filled electronic levels of the carbazole nuclei, while the electron carrier bands are determined by the bottom free electron level of TNP. The first three excited states of CCT are determined, respectively, by electronic transitions from the first three top filled electron levels of the carbazole nuclei to the bottom free electronic level of TNP. Photogeneration of current carriers is observed with excitation of CCT in the region of the second and third electronic transitions.Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 7, pp. 47–51, July, 1989.     

Molecular dynamics study of the vibrational relaxation of OCl and OCl in the bulk and the surface of water and acetonitrile

The vibrational relaxation of OCl and OCl⁻ in the bulk and the liquid/vapor interface of water and acetonitrile is studied by molecular dynamics computer simulations. Both equilibrium calculations of the vibrational friction and non-equilibrium simulation of the energy relaxation are used to elucidate the factors that influence the rate of energy relaxation in systems that represent polar ionic and non-ionic solutes in polar protic and non-protic solvents. We find that, in agreement with previous experimental and theoretical studies, the relaxation of the ionic solute is much faster than that of the non-ionic solute in both the solvents. However, while the relaxation is slowed down considerably when the non-ionic solute is transferred from the bulk to the interface, no such surface effect is found in the case of the ionic solute. This behavior can be explained by noting that the ionic solute is able to keep its first solvation shell intact upon transfer to the interface and that the main contribution to the friction is due to the Lennard-Jones part of the intermolecular potential.     

Angular dependence of the relaxation times of <Superscript>139</Superscript>La nuclei in La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript>

Relaxation of the magnetization of ¹³⁹La nuclei is considered in lanthanum manganites, which are materials with anisotropic interactions of localized electronic spins, namely, the Dzyaloshinski?-Moriya interactionand the interaction with a crystal field. Expressions are derived for the relaxation times of the longitudinal and transverse components of the nuclear magnetization, and the angular dependences of these relaxation times are found for the La_0.95Sr_0.05MnO₃ compound. In contrast to electronic relaxation, the anisotropy of nuclear relaxation contains a contribution from the shift in the electron Zeeman frequency. The theoretically calculated numerical values of the nuclear relaxation times and their ratios correspond to the range of experimental values in the compounds studied. The results can be of importance for designing devices based on these materials and for further investigation.     

Solid state solvation in amorphous organic thin films

The photoluminescence (PL) of the red laser dye DCM2, doped into blended thin films of polystyrene (PS) and the polar small molecule camphoric anhydride (CA), redshifts as the CA concentration increases. The DCM2 PL peaks at 2.20 eV (lambda=563 nm) for pure PS films and shifts to 2.05 eV (lambda=605 nm) for films with 24.5% CA (by mass). The capacitively measured electronic permittivity also increases from epsilon=2.4 to epsilon=5.6 with CA concentration. These results are consistent with the theory of solvatochromism developed for organic molecules in liquid solvents. To our knowledge, this work is the first application of a quantitative theory of solvation to organic molecules in amorphous thin films with continuously controllable permittivity, and demonstrates that "solid state solvation" can be used to predictably tune exciton energies in organic thin film structures.     

Spectral Manifestations of Anion-Cation Interactions of Water-Soluble Porphyrins

The influence of anion-cation interactions on the structure and electronic absorption spectra of cationic 5,10,15,20-tetrakis-(4-N-methylpyridyl)porphyrin and anionic 5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin has been investigated by absorption spectroscopy and molecular modeling. It is shown that the shifts of bands in the electronic absorption spectra appearing when passing from aqueous solutions to solutions in organic solvents are due to the interaction of ionized peripheral substituents of the porphyrin macrocycle with counterions. The bathochromic shift of the Q _x transition is due to the influence of counterions on the energy characteristics of the molecule itself, primarily, as a result of Coulomb interactions changing the relative position of the orbitals and the distribution of electron density on them. The hypsochromic shift of the Q _y transition is a consequence of the increase in the dihedral angle between the plane of the porphyrin macrocycle and the aromatic rings with ionized groups due to the decrease in the -electron interaction between them under the action of counterions.     

Electronic,structural and chemical effects of charge-transfer at organic/inorganic interfaces

During the last decade, interest on the growth and self-assembly of organic molecular species on solid surfaces spread over the scientific community, largely motivated by the promise of cheap, flexible and tunable organic electronic and optoelectronic devices. These efforts lead to important advances in our understanding of the nature and strength of the non-bonding intermolecular interactions that control the assembly of the organic building blocks on solid surfaces, which have been recently reviewed in a number of excellent papers. To a large extent, such studies were possible because of a smart choice of model substrate-adsorbate systems where the molecule-substrate interactions were purposefully kept low, so that most of the observed supramolecular structures could be understood simply by considering intermolecular interactions, keeping the role of the surface always relatively small (although not completely negligible). On the other hand, the systems which are more relevant for the development of organic electronic devices include molecular species which are electron donors, acceptors or blends of donors and acceptors. Adsorption of such organic species on solid surfaces is bound to be accompanied by charge-transfer processes between the substrate and the adsorbates, and the physical and chemical properties of the molecules cannot be expected any longer to be the same as in solution phase. In recent years, a number of groups around the world have started tackling the problem of the adsorption, self- assembly and electronic and chemical properties of organic species which interact rather strongly with the surface, and for which charge-transfer must be considered. The picture that is emerging shows that charge transfer can lead to a plethora of new phenomena, from the development of delocalized band-like electron states at molecular overlayers, to the existence of new substrate-mediated intermolecular interactions or the strong modification of the chemical reactivity of the adsorbates. The aim of this review is to start drawing general conclusions and developing new concepts which will help the scientific community to proceed more efficiently towards the understanding of organic/inorganic interfaces in the strong interaction limit, where charge-transfer effects must be taken into consideration.     

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.     

MOLECULAR STRUCTURE AND PHOTOPHYSICS OF N-QUATERNARY DIARYLOXAZOLIUM SALTS

ABSTRACT

The concern of this work is to study molecular structure, electronic absorption and emission spectra of several N-quaternary salts of the well-known diaryloxazole scintillating compounds: 2,5-diphenyloxazole, para-, meta- and ortho- isomers of bis-2-(5-phenyl-oxazolyl)-benzene (POPOP). All of them were obtained from the initial aryloxazoles by their methylation with dimethylsulfate.

We found, that N-methylation manifests itself in arising of sterical hindrance in the molecules of diaryloxazolium salts, which results in distortion of their planarity. On the contrary to the ground state, the investigated molecules become more planar in their lowest singlet excited state. As a result of such an excited state flattening, fluorescence Stokes shifts values of the diaryloxazolium salts exceed 9000–10,000 cm^?1. The excited state flattening rate constants, estimated for the studied oxazolium compounds, are of the 10¹⁰ s^?1 range. No considerable increase of radiationless losses, induced by the excited state structural relaxation, was found. Owing to these facts, N-quaternary diaryloxazolium salts may be considered as effective abnormally high Stokes shift organic luminophores.     

Rotational dependence of Franck-Condon factors for OH+, NH+, SiH,MgH+, SiH+, and NO+

The rotational dependence of Franck-Condon factors has been calculated for OH⁺, NH⁺, SiH, MgH⁺, SiH⁺, and NO⁺ using the rotating Morse oscillator model. A rotational dependence is noticed in the electronic bands of each of these molecular species; this dependence may make a significant contribution in the determination of rotational temperatures, abundances and opacity distribution functions.