Improvement of the parametric method of the theory of vibronic spectra of polyatomic molecules: Absorption spectra and the structure of butadiene, hexatriene, and octatetraene in the excited state

The structure of the excited states and absorption spectra of butadiene, hexatriene, and octatetraene are calculated by the parametric method of the theory of vibronic spectra using models of the first-and second-order approximations. It is shown that these molecular models adequately reflect the molecular structure and allow one to predict quantitatively the shape and fine vibrational structure of the absorption spectra. When passing to the second-order approximation, only two additional (angular) parameters are used. These parameters are transferable in the series of polyenes. Compared to the first-order approximation model, the second-order approximation model more accurately takes into account the angular deformations of polyenes upon their excitation and describes the intensity distribution in the vibrational spectrum, including weak lines. In addition, the calculations also quantitatively predict spectral variations in the molecular series. The parametric method is more efficient for modeling polyatomic molecules in the excited states and their vibrational spectra compared to other semiempirical and ab initio methods.     

Calculation and interpretation of vibronic absorption and fluorescence spectra of the first electronic nπ* transitions of pyridine and pyrimidine

We have calculated vibronic spectra of the first electronic nπ* transitions of pyridine and pyrimidine in the isolated state using the DFT method in the Franck-Condon approximation. Vibrational spectra for the ground and excited states have been calculated in the anharmonic approximation, which allowed us to refine the assignment of normal vibrations of pyridine and pyrimidine. We have done a complete interpretation of the vibrational structure of the absorption and fluorescence spectra of pyridine and pyrimidine. It has been shown that Fermi resonances between fundamental and combination vibrations and overtones 12 and 16b + 4, 6a and 2 × 16b affect the formation of the vibrational structure of electronic spectra of pyrimidine. Good agreement between calculated and experimental spectra confirms the correctness of the models of the two molecules in their ground and excited states, which makes it possible to use the models in further investigations of various properties of these molecules in electronically excited states, e.g., tautomerism of pyrimidine bases of nucleic acids.     

On the stability of parameters of molecular models in the parametric method of the theory of vibronic spectra

We have studied the stability of the system of parameters used in the parametric method of the theory of vibronic spectra of polyatomic molecules with respect to whether the initial structural-dynamic model of the ground state of a molecule is formed either by the fragmentary method or by direct quantum-chemical calculation. Modeling of excited states and spectra of the stilbene-h ₁₂ and stilbene-d ₁₂ molecules showed that, although the initial models of the ground states are significantly different, calculated changes in the geometry that occur upon excitation and electronic-vibrational spectra of the models are close and quantitatively agree with experiment. This indicates that the parameters of the method are stable and are applicable for performing predictive calculations of vibronic spectra based on models of the ground states created by different methods. We show that the fragmentary method has a considerable advantage, since models created by this method take into account the continuity of the structure in series of related compounds and calculations can be easily performed for molecules of arbitrarily high complexity. We show that direct quantum-chemical calculations are important in the case of molecules with unknown structural-dynamic models of fragments in the ground states.     

Vibronic fluorescence and fluorescence excitation spectra of diphenyl oxazolyl benzene and its oxadiazol analogue

The fine-structure fluorescence and fluorescence excitation spectra of conjugated chain compounds—1,4-di(5-phenyl-2-oxazolyl) benzene and its oxadiazol analogue—are obtained at 4.2 K in an n-octane matrix using the Shpolskii method. The spectra are modeled by representing the band of each of the vibronic transitions as the sum of a zero-phonon line and a phonon wing with certain parameters (half-widths, Debye-Waller factors). The spectra calculated in this fashion coincide with the experimental spectra. This makes it possible to determine the relative intensities of vibronic transitions and refine the frequencies of the normal vibrations in the S ₀ and S*₁ states. The parameters of the Franck-Condon and Herzberg-Teller interactions in the molecules considered are determined. The influence of the replacement of one of the CH groups in the heterocycles with a N atom on the parameters determining the formation of vibronic spectra is considered.     

Donor-Acceptor Conjugated Linear Polyenes: A Study of Excited State Intramolecular Charge Transfer,Photoisomerization and Fluorescence Probe Properties

Numerous studies of donor-acceptor conjugated linear polyenes have been carried out with the goal to understand the exact nature of the excited state electronic structure and dynamics. In this article we discuss our endeavours with regard to the excited state intramolecular charge transfer, photoisomerization and fluorescence probe properties of various donor-acceptor substituted compounds of diphenylpolyene [Ar(CH = CH) _n Ar] series and ethenylindoles.     

Study of vibronic interactions in impurity centers by conjugate fluorescence and absorption spectra with a poorly resolved vibrational structure

The conjugate fluorescence and fluorescence excitation spectra of recently synthesized substituted arylpolyene (C₆H₅-[CH=CH]₂-C₆H₄-NH₂) are studied in solid n-octane at a temperature of 4.2 K. The spectra exhibit a weakly pronounced vibrational structure. A method of determination of the vibronic interaction parameters responsible for the shape of the spectra is developed. The method is based on the modeling of the spectra by series of vibronic bands, each of which consists of a narrow zero-phonon line and a broad phonon wing (phonon sideband). This makes it possible to calculate the fluorescence and fluorescence excitation spectra with the weakly pronounced vibrational structure and compare them with the measured spectra. The deviations from the mirror symmetry between the measured fluorescence and fluorescence excitation conjugate spectra are explained by the combined effect of the Franck-Condon and Herzberg-Teller interactions. The parameters of these interactions are determined.     

Calculation of the vibronic spectra and structure of methylstyrene in the excited state by the parametric method

Calculations of the excited-state structure and of the absorption and radiation spectra of trans-β-methylstyrene are performed by the parametric method for models of the first and second approximations. Only three of the parameters, being constant in a series of related compounds, are used for molecular fragments. In the calculation, the main regularities observed in electronic spectra are reproduced, and an analysis and interpretation of their fine vibrational structure are carried out. The use of the model of the second approximation with a single additional parameter allows more accurate reproduction of angular deformations of the molecule under excitation and of the characteristic changes observed in the low-frequency spectral region in the series of diphenyl polyenes, stilbene, styrene, and methylstyrene molecules. It is shown that the parameters of the second approximation (just as of the first one) possess the transference property in the homological series of molecules. In modeling complex molecules in excited states and their vibronic spectra, the parametric method is more efficient than the ab initio one. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 148–153, March–April, 2000.     

Molecular Rydberg transitions: The lowest-energy Rydberg transitions of s-type in CH3X and CD3X,X = Cl,Br, and I

The absorption spectra associated with transitions to the lowest-energy s-type Rydberg states of CH₃X and CD₃X, X = Cl, Br, and I, have been measured and analyzed. The spectra of the bromides and iodides consist, individually, of four electronic origins of s-excitation type. The vibrational frequency of a given normal mode is more or less identical in all four excited states of any one molecule; and the excited state/ground state ratios of the frequencies of any given normal vibrational mode are essentially identical for all four molecules (i.e., for 16 states, four for each of two bromides and two iodides). The spectra of the chlorides are amenable to a number of different vibronic analyses, none of them unique; these analyses are discussed.     

Fluorescence and fluorescence excitation spectra of jet-cooled carbazole

The fluorescence excitation spectra of jet-cooled carbazole molecules at vibrational temperatures of 55 and 80 K and the fluorescence spectrum of these molecules excited by radiation at the frequency of a pure electronic transition are measured. As the vibrational temperature increases, the excitation spectra exhibit a series of lines of the same symmetry, which are caused by the interaction of the active vibration with a subensemble of optically inactive vibrations. The final symmetry of the totally and nontotally symmetric vibrations is determined from the shape of the rotational contours of the lines of vibronic transitions. The values of a decrease in the frequency of the nontotally symmetric vibrations in the first excited electronic state S ₁ due to their interaction with the electronic state S ₂ are calculated to be up to 100 cm^?1. The frequencies of the pure electronic transitions in the absorption and fluorescence spectra coincide with each other and are equal to 30809 cm^?1, the frequencies of vibrations in the ground state S ₀ exceeding the frequencies of the corresponding vibrations in the excited state S ₁. The degree of polarization of the integral fluorescence is determined for a series of vibronic transitions of the a ₁ and b ₂ final symmetry that are observed in the fluorescence excitation spectra, and the contribution of the intensity with the borrowed polarization θ_⊥ to the integral fluorescence is calculated. It is found that the intensity θ_⊥ is higher for the transitions of the b ₂ symmetry and can reach ≈50%.     

Simulation of time-resolved vibronic spectra and the possibility of analyzing molecules with similar spectral properties

The possibility of using time-resolved vibronic spectroscopy for spectral analysis of mixtures of chemical compounds with similar optical properties, when traditional methods (based on stedy-state spectra) are inefficient, is demonstrated by using the method of computer simulation. The analysis is carried out by the example of molecules of a series of polyenes (butadiene, hexadiene, octatetraene, decapentaene, and decatetraene), their various cis-and trans-rotational isomers, and phenyl-substituted polyenes. Ranges of relative concentrations of molecules similar in their spectral properties, where reliable interpretation of time-resolved spectra of mixtures and both qualitative and quantitative analyses are possible, are determined. The use of computer simulation methods for optimizing full-scale experiments in femtosecond spectroscopy is shown to hold much promise.     

Two-photon vibronic spectroscopy of allene at 7.0–10.5 eV: experiment and theory

2?+?1 resonance-enhanced multiphoton ionization (REMPI) spectra of allene at 7.0–10.5?eV have been observed. The excited vibronic symmetry has been determined from polarization-ratio measurements. Based on the vibronic energies and peak intensities calculated using ab initio MO and time-dependent density functional theory, the very congested REMPI spectra have been assigned as due to π*?←?π, 3p?←?π, 4s?←?π, 4p?←?π, and 4d?←?π transitions. Vibrational progressions related to the CH₂ twisting (ν₄ ～770?cm^?1) have been observed for several excited electronic states. Calculated Franck–Condon factors also confirm that CH₂ twisting is the most active mode in the vibronic spectra of allene. In this study, theoretical calculations of two-photon intensities and polarization ratios have been made through the ab initio computed one-photon transition dipole moments to various electronic states as intermediates. As a starting point to interpret the complicated vibronic spectrum of allene, the theoretical approach, without vibronic couplings, has been applied to predict the peak positions, spectral intensities, and polarization ratios of Rydberg states, and qualitatively shows a considerable agreement with experimental observations.     

Magnetic and optical properties of nitroxide radicals and their lanthanide complexes

This paper reports the structural, magnetic and optical properties of three series of lanthanide complexes [Ln(radical)₄](ClO₄)₃, [Ln(radical)₂(NO₃)₃] and [Ln(radical)(hfac)₃] (Ln=Gd(III), La(III) or Eu(III)) with nitronyl or imino nitroxide radicals.The magnetic properties of the gadolinium complexes were studied. Along the series, most gadolinium(III) complexes exhibit antiferromagnetic Gd^III-radical interaction. These results are discussed.The full absorption and luminescence spectra of some lanthanide complexes and their uncoordinated free radical ligands were measured. The rich vibronic structure in luminescence and absorption spectra indicates that several excited states define the absorption spectra between 400 and 800 nm. Qualitative trends can be established between magnetic ground state properties and the energies and vibronic structure of the title compounds.     

Quantitative analysis of intramolecular interactions of all-trans-α,ω-diphenylpolyenes with one to four double bonds

We have analyzed intensity distributions in fine structure fluorescence spectra of three all-trans-diphenylpolyenes with number of double bonds n = 1?4 in solutions in n-paraffins at 4.2 K. Modeling of spectra by representing each of the vibronic transitions by a zero-phonon line and a phonon wing with certain parameters (width, Debye-Waller factor) made it possible to determine the relative intensities of vibronic transitions. We have calculated and comparatively analyzed the parameters of intramolecular interactions that form fine structure spectra of stilbene and investigated compounds.     

Intracavity Laser Spectroscopy of NiCl System G: Identification of a [13.0] Π3/2 State

The (0,0) vibronic band of NiCl system G with a bandhead near 12 961 cm⁻¹ was recorded at high resolution in absorption using intracavity laser spectroscopy (ILS). For the ILS absorption spectra, the NiCl molecules were produced in a nickel hollow cathode, operated with a small amount of CCl₄, and line positions were referenced to iodine spectra. Fourier transform (FT) emission spectroscopy was used to record an extensive region of the spectrum used in a vibronic analysis of system G. For the FT spectra, excited NiCl molecules were produced in a high-temperature King-type carbon tube furnace. We show that this transition is the (0,0) vibronic band associated with a newly identified ²Π_3/2 excited state and the X²Π_3/2 ground state. The molecular constants for the new ²Π_3/2 electronic state are derived from the rotational analysis. Improved vibronic constants for the band are obtained from analysis of the FT spectra.     

Modeling of the time-resolved vibronic spectra of polyatomic molecules: The formulation of the problem and analysis of kinetic equations

A semiempirical parametric method is proposed for modeling three-dimensional (time-resolved) vibronic spectra of polyatomic molecules. The method is based on the use of the fragment approach in the formation of molecular models for excited electronic states and parametrization of these molecular fragments by modeling conventional (one-dimensional) absorption and fluorescence spectra of polyatomic molecules. All matrix elements that are required for calculations of the spectra can be found by the methods developed. The time dependences of the populations of a great number (>10³) of vibronic levels can be most conveniently found by using the iterative numerical method of integration of kinetic equations. Convenient numerical algorithms and specialized software for PC are developed. Computer experiments showed the possibility of the real-time modeling three-dimensional spectra of polyatomic molecules containing several tens of atoms.     

Effect of adsorption of vapours on the electrical conductivity of a series of some naphthyl polyenes: Adsorption and desorption kinetics

This paper deals with the change in semiconductivity of a series of naphthyl polyenes of the type R-(CHCH)_n-R, where R stands for the naphthyl group and n = 1–6, on adsorption of vapours of some organic solvents at a constant sample temperature. Appreciable enhancement in the conductivity is observed. Such enhancement depends on the chemical nature of the solvent and also on the vapour pressure. The desorption is much slower than that of adsorption. Both the adsorption and desorption kinetics follow the modified Roginsky-Zeldovich relation. Kinetics for the compounds having odd-n is distinctly different from those with even-n. The results show that in all these vapour-semiconductor systems, the adsorption is a two-stage process.     

Vibronic Structures of the Ground and Excited Singlet Electronic States of Dimethylnaphthalenes Cooled in a Supersonic Jet

Abstract

Fluorescence excitation and dispersed fluorescence spectra of jet-cooled naphthalene and 2,6-, 2,7-dimethylnaphthalenes have been measured. The frequencies of optical active vibrations in the ground and first excited singlet states have been determined. The new technique for calculation of planar vibration frequencies of polycyclic benzenoid hydrocarbons in the excited electronic states has been developed. The vibration frequencies in the ground and first excited singlet states of these molecules were calculated using the developed technique and the Ohno's model. The interpretation of vibronic spectral lines based on the comparison of the calculated and experimental data was made. The calculation rms errors for the vibration frequencies in the ground electronic states of the investigated molecules do not exceed 20 cm^?1 and are approximately 1.5 times higher for excited states without additional adjustment of parameters for individual molecules.     

cis Acrolein: Mixing of the S0 and S1 electronic states by the (a″) skeletal torsional vibration

Large effects of vibronic coupling upon vibrational levels of the ground (¹A′) and first excited (¹A″) singlet electronic states of cis acrolein (2-propenal) are successfully modeled. Some implications for CH₂CHCHO spectroscopy and photophysics are discussed briefly.     

Exciton and phonon spectra of acoustooptic Tl3AsS3 crystals

The λ-modulated exciton reflection spectra of Tl₃AsS₃ crystals are investigated at 8 and 77 K, in which the ground (n=1) and excited (n=2, 3) exciton states are revealed. Taking into account the spatial dispersion, the shapes of λ-modulated reflection spectra of the n=1 line are calculated and the basic parameters of excitons and bands are determined (the translational and reduced masses of excitons and the effective masses of electrons and light and heavy holes). The one-phonon reflection spectra are studied in the region from 50 to 500 cm^?1 in polarizations E ∥ c and E ⊥ c. The shapes of one-phonon reflection spectra are calculated and the parameters of vibrational modes E and A ₂ are determined.