Q Branch Excited Polarized Fluorescence of Free Polyatomic Molecules

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It is shown that the effect of intramolecular lability in complex molecules on the degree of fluorescence polarization of rarified vapors is mainly mediated through perturbations of rotational motion in the region of separatrixes of A-and C-forms of rotation stereo dynamics. It is most pronounced for the orientation of optical transition dipole moment components along the axis of the molecule average moment of inertia (B) and manifests itself in all types of molecular tops. Such lability for dipole orientations along the A and C axes usually causes the degree of anisotropy to decrease by an order of magnitude and more. So it is advisable to use optical transitions with dipole moments oriented along the A or C axes for optical polarization analysis of molecules in the gaseous state, for example in analytical high-temperature polarized fluorescence.Introduction. Many molecular spectroscopy methods are based on the anisotropy of the optical transition molecular dipole cross section. Polarized spectroscopy of polyatomic molecules in the vapor has been developing for more than three decades. The gas phase has several advantages for obtaining basic information about the molecular structure, its dynamics, and the dynamics of intermolecular interactions because they are manifested in free molecules in a pure form unperturbed by constant external influences. This is just as important for applied analysis of the molecular structure and properties that are difficultly accessible or generally unavailable in the condensed phase. For example, a method was developed to determine the quenching efficiency of excited states of complex molecules by oxygen in a single collision without invoking such difficultly calculated or measured characteristics such as the collision cross section [1,2]. The use of complex molecules cooled extensively in a gas stream expanding into a vacuum was previously shown to be the most fruitful method of studying free molecules. For example, the molecular rotating quantum echo made it possible to analyze the structure of molecules and their complexes with atoms and molecules [3-6].However, it turned out that intramolecular motion of the nuclei even under highly cooled conditions could have a noticeable effect on the observed average anisotropy of the transition dipole cross sections. For example, excitation of various vibrations in stream-cooled molecules causes changes of different magnitude in the fluorescence anisotropy that depend considerably on the intramolecular orientation of the transition dipole moment [7-10]. It was observed experimentally that heating leads to complete fluorescence depolarization if the transition dipole moment is oriented along the axis of the average moment of inertia at temperatures of hundreds of degrees where the motion of the nuclei is high. This phenomenon was observed in hot vapor of anthracene, five 3,6-disubstituted phthalimides, pyrene, and carbazole, the fluorescence transition dipole moments of which were directed along the average axis of inertia of the molecules...

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confidence: 99%

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confidence: 99%

Effect on optical anisotropy of intramolecular lability in various molecular rotational configurations

Tolkachev

Блохин

2009

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“…As follows from [15], for molecules that can be modeled by rigid tops, the shape of the rotational contour of the vibronic transition line is determined by the ratio of the principal moments of inertia I A /I B , I C /I B and the orientation of the oscillators with absorption and emission on the axes of the principal moments of the inertia for the molecule. For such a model, the Q branch is a narrow line [16], while the model allows us to compare the intensity of the Q branch only with the intensity of the Q branch for other types of tops and not with the intensities of the P and R branches of the contour. The calculated directions of the oscillators for the electronic S 0 -S 1 transitions are oriented along the axis of the I A principal moment of inertia for the cis form of POPOP and TOPOT, or are close to the I A orientations for PPO (2,5-diphenyloxazole) and the trans forms of POPOP and TOPOT.…”

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confidence: 99%

Spectral characteristics of heterocyclic compounds with a chain structure, cooled in an ultrasonic jet

Поведайло

Yakovlev

2006

the fluorescence spectra of POPOP, under conditions where the molecules were cooled in an ultrasonic helium jet. A line structure is observed in the spectra of POPOP and TOPOT; for the BPO molecules, whose configuration changes considerably during electronic excitation, vibrational structure is apparent only in the low-frequency region of the excitation spectrum, and a diffuse spectrum is recorded starting from ν 0 0 + 200 cm -1 . For all the compounds, in the spectra we recorded vibrations with frequencies up to 100 cm -1 , arising due to the flexibility of the molecular structure. The rotational contours of the lines for the electronic and vibronic transitions of the POPOP molecules (T rot = 10.5 K) and TOPOT molecules (T rot = 15 K) are structureless and bell-shaped. The degree of polarization of the fluorescence P fl for the jet-cooled POPOP molecules for excitation of vibrations along the absorption band up to 2000 cm -1 above ν 0 0 is practically constant (,8.4%) and matches P fl for high-temperature vapors.Introduction. In complex organic molecules with a chain structure, consisting of phenyl and oxazole rings linked by >C-C< single bonds, in the ground state these rings may be rotated relative to each other under the influence of electrostatic repulsion of adjacent nuclei of the ring hydrogen atoms. During electronic excitation, in such molecules substantial conformational rearrangements may occur which are manifested in the fluorescence excitation spectra as low-frequency vibrations of a phenyl group relative to the rest of the molecule. Such flexibility should affect the inertial characteristics of the molecule and consequently the shape of the rotational contour of the vibronic transition lines.Studies to date on the low-frequency vibrations and their potential energy surfaces have been conducted for a number of molecules under jet-cooling conditions: biphenyl [1], 9,9′-bianthryl [2], 9-phenylanthracene [3], 2-phenylindole [4], etc., for which the equilibrium configuration is nonplanar in the ground state while generally electronic excitation leads to a more planar structure.In this work, we have studied the effect of the flexibility of heterocyclic molecules with a chain structure on the fluorescence excitation spectra, the fluorescence spectra, the rotational contours of the bands for purely electronic and vibronic transitions, and the polarization of fluorescence for such molecules cooled in an ultrasonic jet. We have also carried out quantum chemical calculations for the structure of the molecules. As the objects of investigation, we selected 2-phenyl-5-(4-diphenylyl)oxazole (BPO), 1,4-di[2-(5-phenyloxazolyl)]benzene (POPOP), and 1,4-di[2-(5-n-tolyloxazolyl)]benzene (TOPOT), which are known as scintillators and are active media of lasers in gas and condensed phases [5].Experimental procedure. The fluorescence excitation and fluorescence spectra were measured using the spectrometric system described in [6]. The ultrasonic jet was formed by discharge of a mixture of vapor of the organic

“…For nonrigid molecules, a closer value of the rotational temperature is obtained when comparing the distances between the P and R branches. The intensity of the Q branch in the case when the dipole moment for the S 0 -S 1 transition is oriented orthogonal to the B axis, close to the direction of the A axis, is determined by the ratio of the principal moments of inertia [7,8] and should be rather high [9]. The absence of a Q branch on the rotational contour of the line for the purely electronic transition of β-binaphthylene oxide even cooled down to ≈15 K indicates that the molecular structure is not rigid.…”

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confidence: 99%

Fluorescence polarization of helical molecules cooled in a supersonic jet

Поведайло

Yakovlev

2006

The β-binaphthylene oxide molecules studied under supersonic cooling conditions have a number of specific properties due to their nonplanarity. Low-frequency vibrations of the molecules in the excited S 1 state are higher than the frequencies for the S 0 state, and conversely the high-frequency vibrations have lower frequencies. The S 0 -S 2 fluorescence excitation spectrum is structureless. The absence of a Q branch in the rotational contour of the line for the purely electronic transition indicates that it is substantially broadened and shifted toward shorter wavelengths as a result of rotational perturbations of the helical structure of the molecule. Multiplet lines in the spectra of β-binaphthylene oxide complexes with argon, krypton, and xenon correspond to different isomeric complexes. Their bond energies are below those observed previously for planar polycyclic molecules such as perilene, fluorene, and carbazole. The greater number of isomers with xenon is due to strengthening of the bond in the van der Waals complex and the nonequivalence of the position of the xenon atoms on the outside and inside of the helical molecule.Introduction. Within a series of similar heterocyclic compounds differing only in the hetero atom in the bridge of the five-membered ring, binaphtho[2,1-b:1′,1′-d]furan (β-binaphthylene oxide) has special properties [1]. The helical structure of the molecules and their mobility at ordinary temperatures and especially in high-temperature vapor has made it difficult to establish the effect of these properties on the spectral polarization characteristics. Therefore we need to study β-binaphthylene oxide at low temperatures, when the molecules are more stable.In this paper, we present the results of studies of the fluorescence excitation and fluorescence spectra of β-binaphthylene oxide, and also van der Waals complexes of the molecule with argon, krypton, and xenon when cooled in a supersonic jet. The spectra were measured on a spectrometric system described in [2]. The supersonic jet was formed by discharge into a vacuum chamber of a mixture of β-binaphthylene oxide vapor at a temperature of 460 K and helium at a pressure of 1 atm. The cooled molecules were excited in the jet at a distance of 18 mm from the nozzle by the second harmonic emission of a wavelength-tunable dye laser, pumped by the second harmonic of a YAG:Nd 3+ laser. Van der Waals complexes with inert gases (Ar, Kr, or Xe) were formed by discharge of saturated β-binaphthylene oxide vapor with helium at a temperature of 515 K and a helium pressure of 2 atm. Its concentration