Long-lived luminescence of complex molecules

Results of investigations of all types of long-lived luminescence of organic molecules in the gas phase and condensed media are presented. Methods for identification and separation of contributions of phosphorescence and thermally activated fluorescence in long-lived luminescence of organic-molecule vapors are proposed. Energy transfer and migration processes in the case of the inductive-resonant mechanism of intermolecular interaction leading to the appearance of sensitized phosphorescence and annihilation-induced delayed fluorescence are considered. Experimental results on energy migration obtained for solid solutions of organic compounds are analyzed within the framework of the concepts of percolation theory with account for the microscopic inhomogeneity of the systems under investigation and the fractal properties of the clusters of activator molecules. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, F. Skorina Ave., 70, Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 662–674, September–October, 1998.     

Infrared multiphoton excitation of polyatomic molecules

Results of investigations of the process of multiphoton excitation of polyatomic molecules by CO₂-laser radiation are presented. The mechanism of formation of the profiles of IR absorption bands of polyatomic molecules is discussed. New experimental methods of investigation of relaxation processes at high levels of vibrational excitation of molecules in the ground and triplet states are considered. For vapors of polyatomic molecules and their mixtures with foreign gases, the quantitative characteristics of the collisional exchange and the vibrational-energy transfer as well as the rates of intercombinational conversion ⇛ and triplet-triplet transfer are presented and their dependences on the vibrational-excitation level are discussed. Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 675–693, September–October, 1998.     

Rapidly progressing photophysical and photochemical relaxation processes in complex organic molecules

Using methods of high-speed kinetic laser spectroscopy, we investigated the dynamics and mechanisms of rapidly progressing relaxation processes in multiatomic molecules. We separated intra- and intermolecular channels of relaxation of the vibrational energy of excitation by the rate of transformation of nonstationary absorption spectra in a picosecond range of times. The role of highly excited electronic states in the process of electronic-vibrational relaxation in molecules of the class of phenazines was ascertained. Applying femtosecond light pulses in a real time scale, we recorded the dynamics of the decay of optically induced anisotropy in rarefied vapors of organic compounds. Several mechanisms of transfer of a hydrogen atom in the process of formation of free radicals in photoreduction of ketones were established. The dynamics of formation of inter- and intramolecular exciplexes in binary gas-phase systems was investigated. We determined the mechanisms of intramolecular dissociation of a C–O chemical bond in spiropyrans and xanthene dyes. Photodissociation of S–S bonds in molecules of disulfides was studied. The possibility of the existence of states with intramolecular charge transport with a twisted configuration for organic free radicals is shown. To whom correspondence should be addressed. Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 635–661, September–October, 1998.     

Properties of lasers based on vapors and solutions of complex organic compounds

We present the most important results of investigations of lasers based on vapors and solutions of complex organic compounds. We investigated the effect of a foreign gas, the duration and intensity of the exciting pulse, the temperature of the active medium, and the spectroscopic parameters of the material on the generation characteristics of complex molecules in the gas phase. The special features of generation of radiation by a laser with distributed feedback in transition from a condensed to the gas phase in heating the active medium are considered. It is suggested that the polarization characteristics of the radiation generated by a vapor laser be used to investigate the laws that govern the relaxation of optically induced anisotropy of excited molecules of the active medium. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 709–720, September–October, 1998.     

Fluorescence-polarization detection of the scattering of molecular beams by a surface

We investigate analytically the effect of the forces that act both in the plane of a scattering surface and normally to it on the polarized fluorescence of reflected molecules in forward scattering. We show that as the velocity of the beam of incident molecules increases, the polarization degree of the fluorescence of reflected molecules becomes substantially different from that of free and/or desorbed molecules. Therefore, the detection of the scattering of molecular beams by a surface using the polarization degree of fluorescence can serve as a rather reliable means of diagnosing surfaces and determining the modes of scattering. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 205–212, March–April, 1999.     

Energy yield of fluorescence of complex organic compounds upon electron excitation

With use of electron energy loss spectra for vapors of anthracene, perylene, 1,4[di(2,5-phenyloxazolyl)] benzene, and paratherphenyl the energy yields of fluorescence are calculated for electron collision. It is shown that they are considerably smaller than those for optical excitation conditions. Deceased Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 1, pp. 146–148, January–February, 1998.     

Excited states, relaxation processes, and coordination reactions in the active centers of heme proteins

Based on the results of studying hemoglobin and myoglobin molecules using the methods of laser absorption spectroscopy of superhigh time resolution, we described photophysical and spectral properties of excited states, and intramolecular electronic and thermal relaxation processes. A mechanisms of the photodissociation reaction of oxy forms of these proteins is proposed and substantiated. Reported at the VIIIth International Conference on Spectroscopy of Porphyrins and Their Analogs, Minsk, September 22–26, 1998. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 479–482, July–August, 1999.     

Polarized fluorescence of radicals in photodissociation of disulfides in the gas phase

The polarization of the fluorescence of radicals formed in laser-induced photodissociation of some disulfides in the gas phase is measured. Experimental data obtained in the study are interpreted within the free recoil model. It is shown that polarization experiments carried out to investigate photodecay in vapors allow researchers to uniquely the intramolecular direction of the dipole moments of transitions with absorption and emission. Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 54–59, January–February, 1999.     

Quenching of the fluorescence of solutions of complex molecules by nonradiative transfer of electronic-excitation energy to a saturable acceptor

The study puts toward a theoretical model describing the fluorescence parameters for concentrated solid solutions consisting of complex molecules of two types (donor-acceptor) with nonradiative transfer of electronic-excitation energy with intense pulse excitation. Consideration is given to the dynamics of quenching of the luminescence of the donor molecules due to transfer of electronic-excitation energy to the acceptor molecules. Numerical results are presented in the form of approximation formulas. To whom correspondence should be addressed. Belarusian State University, 4, F. Skorina Ave., Minsk, 220050, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 5, pp. 652–656, September–October, 1999.     

Luminescence of complex organic molecules upon electron excitation

Pesults of investigation of the luminescence of complex organic compounds upon excitation with electrons of various energies are reported. Data on spectra of energy losses of electrons, fluorescence excitation cross sections and functions, fluorescence emission spectra, fluorescence kinetics, polarization, and energy yield, effects of impurity gases, and theoretical analysis of the processes taking place in the system are presented. Results of investigation of the luminescence of complex molecules in electric discharges (dc, pulsed, high-frequency, non-self-maintained, and plasma-beam) and in a condensed state (solutions, molecular crystals, and thin solid films) are discussed. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk 220072, Belarus. Transtated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 694–708, September–October, 1998.     

Kinetics of fluorescence polarization for solid bichromophore solutions in intense pulse excitation

The kinetics of fluorescence polarization in intense pulse excitation of solid disordered solutions of bichromophores that consist of complex molecules of two types between which there can be inductiveresonance transfer of electron-excitation energy is theoretically investigated. Variants of fluorescence excitation by single pulses and pulse trains are considered. The lifetime of the fluorescence of a given solution increases with the intensity of the exciting pulses. The possibility of controlling the duration of fluorescence attenuation for donor molecules incorporated into the bichromophores by the action of luminescence radiation at the frequency of acceptor-molecule absorption on the solution is demonstrated. Belarusian State University, 4, F. Skorina Ave., Minsk, 220050, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii Vol. 65, No. 4, pp. 546–550, July–August, 1998.     

Theory of polarized fluorescence of polyatomic radicals upon photodissociation of complex molecules in the gas phase

The effect of rotational predissociation on the polarization response of an ensemble of photofragments is investigated within the framework of the model with free scatter of fragments for the most general case when both original molecules and fragments are asymmetric rotators. Photoinduced decomposition reactions with single- and two-photon excitation of fragment fluorescence are analyzed. Second- and third-order orientational correlation functions determining the polarization degree of fragment fluorescence are calculated. The behavior of these correlation functions depending on the shape of original molecules and fragments, directions of dipole moments, and characteristic times of the process are analyzed. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 6, pp. 908–914, November–December, 1998.     

Electronic deactivation and energy transfer in doped oligophenylenevinylene nanoparticles

Suspensions of oligophenylenevinylene (nPV) nanoparticles withn = 2 vinylene units are doped with nPVs of longer chainlengths,n = 3–5. Absorption and fluorescence spectroscopy and steady-state and time-resolved fluorescence anisotropy measurements are used to determine the photo-physical properties of the suspensions. Undoped nanoparticles form highly oriented H-aggregates with low fluorescence quantum yields (Φ_F ≈ 0.1). Introduction of bulky substituents into the particle constituting molecules perturbs the intermolecular orientation. Upon doping, efficient energy transfer to the dopants is found, changing the color and leading to enhancement of the fluorescence quantum yields up to Φ_F = 0.6. The intermolecular orientation is not changed upon doping.     

Dynamic and stationary characteristics of nonlinear luminescence in molecular layers with electronic-excitation energy transfer

We carry out an analysis of the characteristics of nonlinear luminescence of a multicomponent layer system consisting of layers of molecules of a sensitizer, donor, and acceptor with inductive-resonance transfer of energy between the components of the molecular ensemble. We show that in a stationary regime bistable dependences of the populations of excited states of molecules on excitation intensity are realized without external feedback. For pulse pumping, we show the possibility for forming response-luminescence pulses with widely variable characteristics: shape, duration, intensity, and delay. Belorussian State University, 4, F. Skorina Ave., Minsk, 220050, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 190–196, March–April, 1999.     

Concentration self-quenching in solid solutions of complex organic molecules under intense excitation

A theoretical study was carried out to investigate concentration self-quenching which is nonlinear in intensity in a molecular system caused by nonradiative electronic excitation energy transfer to relatively long-lived acceptors that are formed in a solution and whose function is performed by molecules that have acquired the triplet state. It is shown that at ratios of constants of radiative and nonradiative transitions typical of complex molecules at concentrations of ∼10⁻³–10⁻² mole/liter substantial (2- to 10-fold) fluorescence takes place at intensities of excitation of ∼10⁴–10³ of the intensity of saturation for a singlet subsystem. Polarization characteristics of the system are analyzed. Belarusian State University, 4, F. Skorina Ave., 220050, Minsk, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 729–733, November–December, 1997.     

Kinetics of triplet-triplet absorption of anionic water-soluble porphyrins

It was established that in aqueous solutions of anionic porphyrins the processes of triplet-triplet (T-T) annihilation are absent (K₂<2.5·10⁵ M⁻¹ sec⁻¹), while a delayed annihilation fluorescence can be detected for these porphyrins in methanol (K₂≊3.5·10⁷ M⁻¹ sec⁻¹) and for their lipophilic analogs in organic solutions (K ₂≊2.5·10⁹ M⁻¹ sec⁻¹), and the kinetics of absorption of the latter porphyrins in triplet states is dependent on the intensity of the exciting radiation. The extremely small constant of T-T annihilation of anionic porphyrins can be due to the formation of a relatively dense solvate shell consisting of water molecules around the charged groups of the porphyrin molecules, which increases steric barriers and thus prevents an exchange-resonance transfer of energy between triplet excited molecules in aqueous solutions. It is also shown that porphyrin molecules can electrostatically repel one another when in their structure there are charged groups. To whom correspondence should be addressed. Deceased. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70 F. Skorina Ave., Minsk, 220072, Belarus; e-mail: llum@imaph.bas-net.by. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 6, pp. 791–795, November–December, 1999.     

Method for analytical calculation of the nonlinear optical properties of polyatomic molecules in the MO LCAO approximation

Analytical expressions are obtained for any orders of the polarizabilities of polyatomic molecules as functions of the dipole integrals and derivatives of the matrix for the orders of the bonds over the field components. A procedure for calculating these polarizabilities is suggested. Belorussian State University of Information Science and Radioelectronics, 6, P. Brovka St., Minsk, 220600, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 3, pp. 436–438, May–June, 1998.     

Efficiency of the structure analysis of complex molecules in the gas phase by their optical rotation

It was shown in the dipole approximation of optical rotation that in the general case only in orientationally anisotropic vapors is the rotational force dependent on the intramolecular orientation of both the electric and magnetic dipole moments. Expressions relating the optical rotational force to the intramolecular orientation of these moments, the orientational distribution in an anisotropic ensemble, and the configuration of a measurement have been obtained. Calculated dependences of the rotational force on the intramolecular orientation of the magnetic moment at a fixed electric moment and “rotational force excitation spectra” obtained for different types of rigid asymmetric top molecules and rotational contours are presented. It is proposed to measure the intramolecular orientation of the electric and magnetic dipole moments with the use of the rotational force normalized to that detected in the case of observation at a “magic” angle to the direction of the exciting light electric vector. Institute of Molecular and Atomic Physics of the Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 6, pp. 843–849, November–December, 1998.     

Luminescence properties of humic substances

A detailed analysis of luminescence properties of humic substances and fulvic acids extracted from peat soil is carried out for the first time. Their fluorescence excitation and emission spectra, fluorescence lifetimes, phosphorescence excitation and emission spectra, and phosphorescence lifetimes at room temperature are investigated. The nature of chromophores of humic substances is discussed. Presented at the International Symposium on Peat Organic Substances, May 15–19, 1995, Minsk, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 4, pp. 489–493, July–August, 1997.     

Structure of molecular associates of fluorescent probes in solutions of human serum albumin

Spectral-luminescent characteristics and molecular association processes in solutions of human serum albumin are analyzed at different pH values for three fluorescent probes (eosin, erythrosin, and fluorescein). Common features for all three probes in protein solutions are quenching of the fluorescence, a red shift of the fluorescence maximum, a decrease in the degree of association, and an increase in the angle between dipole moments of dye molecules in dimers. This being so, differences between fluorescein and its halogen derivatives (eosin and erythrosin) are observed in the pH dependences of fluorescence, degree of association, and the angle between dipole moments of probe molecules in dimers. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 6, pp. 782–787, November–December, 2008.