N. A. Borisevich

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 S1 due to their interaction with the electronic state S2 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 S0 exceeding the frequencies of the corresponding vibrations in the excited state S1. The degree of polarization of the integral fluorescence is determined for a series of vibronic transitions of the a1 and b2 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 b2 symmetry and can reach ≈50%.

Specific Fluorescence Properties and Picosecond Transient Absorption of 8-Azasteroids

The specific fluorescence properties as well as picosecond transient absorption features have been studied for two 8-azasteroids. It is shown that at various excitation wavelengths the essentially different final excited electronic states are realized. Because of the multicenter character of 8-azasteroids the spectroscopic data obtained may be analyzed on a basis of the “mesomeric tautomerism” model taking into account the dynamic combination of cis- and trans-configurations. The dependence of fluorescence spectral characteristics on the solvent nature is a manifestation of intermolecular H-bond interactions.

Exciton luminescence of 8-azasteroid microcrystals

Luminescence of microcrystals of 2,3-methoxy-8-azagon-1,3,5(10),13-tetraene-12,17-dion of the class of molecules of biologically active steroids is detected at room temperature (293 K). It represents fast fluorescence of free and self-localized excitons and prolonged phosphorescence of triplet excitons.

Femtosecond dynamics of geminate recombination of radicals upon photodissociation of aromatic disulfides

The dynamics of the geminate recombination of thiyl radicals formed upon photodissociation of aromatic disulfides and the effect of the intramolecular relaxation on this recombination are studied using pico-and femtosecond kinetic spectroscopy. It is shown that, in terms of a phenomenological model, the geminate recombination of phenylthiyl radicals in neutral solvents can be satisfactorily described by a biexponential dependence. The model suggests the occurrence of primary geminate recombination in a solvent cage formed around an original molecule and secondary recombination controlled by the diffusional motion of the radicals of a pair. The primary geminate recombination, whose characteristic time (9 ps) is close to the characteristic times of intermolecular vibrational relaxation of complex molecules in solvents at room temperatures, masks the manifestation of thermalization processes in the time kinetics. The direct geminate recombination of aminophenylthiyl radicals with the formation of original molecules virtually does not occur because of the intramolecular charge transfer. In connection with this, the intermolecular vibrational relaxation manifests itself in the kinetics of the induced optical density in the region of the absorption band maximum of radicals as a growing component with a characteristic time of 6 ps.

Polarized fluorescence of jet-cooled molecular iodine

The degree of polarized fluorescence of molecular iodine 127I2 cooled in a supersonic jet under rotationally selective excitation in the electronic transition B3Π0u+-X1Σg+ has been measured and calculated. It was found that the interaction of the angular momentum of the molecule with the nuclear spins of iodine atoms leads to a considerable depolarizing effect. This effect is most pronounced for small rotational quantum numbers of the angular momentum that are comparable with the total nuclear spin of the iodine molecule, which is equal to 5.

Spectral‐Luminescent Properties of Oxazoles and Oxadiazoles in a Gas Phase on Excitation by Electrons

The energy‐loss spectra of electrons, fluorescence excitation functions, and the fluorescence spectra on excitation of the vapors of a number of oxazoles and oxadiazoles by monokinetic beams of electrons of various energies are determined. In contrast to optical absorption spectra, in the energy‐loss spectra of a number of studied substances a band associated with the S0–T1 singlet‐triplet transition is observed. The π–π*‐type transitions are fixed up to S0–S5 on excitation of molecules by high‐energy electrons, including the region of vacuum ultraviolet. The cross sections of elastic and inelastic collisions of electrons of different energies with POPOP molecules have been measured. The dependences obtained differ substantially from those calculated in the Born approximation. The cross section of elastic scattering is in a rather good correspondence with the geometric section of the molecule.

Spectral‐Luminescence Characteristics of Polyphenyls and Polyacenes in the Gas Phase in Electron‐Beam Excitation

The spectra of electron‐energy loss, excitation functions, and fluorescence spectra in excitation of the vapor of polyphenyls and polyacenes by electron beams of different energies are determined. The influence of successive complication of the molecules under study on these spectral‐luminescence characteristics is tracked. Unlike the optical absorption spectra, in the spectra of electron‐energy loss of all the substances studied one observes a band which is related to the singlet‐triplet transition S0–T1. The transitions up to S0–S5 are recorded in excitation of the molecules by high‐energy electrons, including the region of vacuum ultraviolet. From the functions of fluorescence excitation the authors have determined the excitation thresholds that correlate with the energies of the S1 levels, except for pyrene in which the S0–S1 transition is forbidden and does not show up not only in photon excitation but also in electron‐beam excitation, although the intercombination forbiddenness in the latter case is removed and the S0–T1 band is observed.

Losses in the Energy of Low-Energy Electrons in Fluorene, Fluorenone, and Diiodofluorenone Vapors

Electron energy loss spectra (EELS) of fluorene, fluorenone, and diiodofluorenone vapors excited by monokinetic electrons of energies 15–50 eV have been obtained. The singlet and triplet absorption bands of these molecules have been calculated. Comparison of these bands with the experimental EELSs and optical absorption spectra has shown that the forbiddenness of singlet-triplet transitions is not completely removed in the process of interaction of molecules with electrons. The presence of heavy iodine atoms in the diiodofluorenone molecule enhances singlet-triplet transitions. Bands of overtones of stretching vibrations of the CH groups of the benzene rings have been detected near the peak of elastic scattering of electrons of the molecules studied.

Electron Energy Loss Spectra of the Organic Complexes of Europium

In the electron energy loss spectra (EELS) of the organic europium complexes Eu3+ (BTFA)3TPPO and Eu3+(Br‐BTFA)3TPPO in a gas phase obtained on excitation by monokinetic beams of electrons of different energies in the range 12–50 eV, we have identified the bands associated with the electron transitions S0–S1, S0–S2, and S0–S3. The connection of these transitions with the structural groups of the complexes is established. The addition of the bromine atom to the phenyl ring of β‐diketonate leads to the rise in the relative intensity of the S0–S2 band. The singlet‐triplet transitions manifest themselves in the region 2.5–3.2 eV and contribute to the S0–S2 band of the electron energy loss spectra.

Electronic Structure of Immunoactive Molecules of 8–Azagona–12,17–Dione

Using the semiempirical method of partial neglect of differential overlap (PNDO), we have calculated the wave functions, energies, orbital configurations of electronic states, oscillator strengths of transitions, electronic density distributions, and dipole moments for the molecule of biologically active 8–azagona–12,17–dione, containing a conformationally rigid α–acyl–β–aminovinylcarbonyl fragment. It has been established that as to their orbital nature the excited lower and higher singlet electronic states of this molecule are of the nπ*– and ππ* type respectively. The results of the theoretical analysis are in good qualitative agreement with the spectral data on absorption and luminescence. The calculations of the intermolecular interaction of the compound under consideration with a medium show that the molecular systems under consideration can possess a dynamic multicenter structure.