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.     

Photophysics ofβ-carotene and related compounds (review)

A review of data on the photophysics of carotenoids is presented. Results of investigations of spectroscopic, temporal, and energy parameters of excited S₁ and S₂ singlet states of β-carotene and related compounds are critically examined. These states give rise to extremely high probabilities (10¹¹–10¹³ sec⁻¹) of radiationless deactivation of the electronic excitation energy in carotenoids. Results of investigations of photophysical properties of triplet states of carotenoids are considered mainly from the standpoint of quenching of singlet oxygen and triplet states of organic molecules by carotenoids. 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. 1, pp. 5–19, January–February, 1997.     

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.     

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.     

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.     

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.     

Features of particle excitation in high-frequency capacitive discharge

The efficiencies of excitation of different atomic and molecular levels of a gaseous medium by a low-current high-frequency capacitive discharge (HFCD) and a dc discharge are compared theoretically. It is shown that for transitions between lower vibrational levels of molecules an increase in spectral line intensity by a factor of 1.2–1.6 is observed, and for neon transitions from high-lying levels, by a factor of 1.5–3.0. The stability of low-current HFCD is experimentally investigated for different configurations of gas-discharge gaps. The results of these investigations are used in developing compact planar and coaxial HFCD lasers. B. I. Spepanov Institute of Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220071. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 291–296, May–June, 1997.     

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.     

Collisional Energy Transfer from Vibrationally Excited Polyatomic Molecules

Abstract

Intensities and decay rates of delayed fluorescence initiated by CO₂ laser excitation of the triplet-state molecules are used to probe collisional relaxation of vibrationally excited polyatomic molecules. Collisional efficiencies for large polyatomic molecules are found not to exceed the value of 10^?2-10^?3 even in most favourable case of vibrational energy exchange in collisions between parent molecules. At intermediate levels of excitation (1500—12000 cm^?1) the average energies <ΔE> transferred per collision with polyatomic molecules increase as vib>^rn, where m≥2, and decrease with increasing numbers of atoms in the excited molecules.     

Absorption and fluorescence of 9,10-diiodoanthracene in undegassed solutions

A procedure for producing 9,10-diiodoanthracene that is an alternative to the known procedure is described. The spectra of absorption and fluorescence in hexane, carbon tetrachloride, and polymethylmethacrylate are measured at 22°C. The quantum yield of undegassed liquid solutions in hexane and carbon tetrachloride in excitation at the maximum of the 0←0 band is practically the same and is equal to 0.018. In excitation in shorter-wave vibrational bands, it is different for different solvents and decreases noticeably as the frequency of the exciting light increases. The decrease in the yields is explained by the existence of a fast nonradiative process from hot (Franck-Condon) vibrational S₁-states capable of competing with vibrational relaxation. A possible scenario of this approach is discussed. Institute of Molecular Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 197–200, March–April, 1999.     

Photophysical properties of mesonitrophenyl-substituted porphyrins and their dimers at 295 K

It is shown that at 295 K in meso-orthonitrophenyl-substituted octaethylporphyrins and their chemical dimers the steric interactions of the nitro-group and the volume substituents at β-positions of the pyrrole rings favor direct overlapping of molecular orbitals in a donor-acceptor pair. The efficient quenching of fluorescence of the nitroporphyrins in toluene is attributed to direct nonadiabatic electron transfer from the S₁-level of a porphyrin to the lower-lying state with charge transfer by the “through-space” mechanism. Quenching of the T₁-states is related with heat-stimulated transmission to the higher-lying states with charge transfer of the ion-radical pair as well as with enhancement of the probability of the nonradiative T₁→S₀-transition. To whom correspondence should be addressed. 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. 531–535, July–August, 1999.     

Spectral energy characteristics of 6,γ- and 7,γ-dimethyl ethers of chlorine 6 in water-alcohol solutions

Absorption spectra and dispersion of optical rotation (DOR) are measured for monomers of 6,γ- and,7,γ-dimethyl ethers of chlorin e₆ (Ch-6,γ-DME and Ch-7,γ-DME) in 96% ethanol and for associates of them that are polymers and colloidal particles. The optical activity of Ch-6,γ-DME monomers is higher than that of Ch-7,γ-DME monomers, which is linked with more pronounced distortion of the molecular macrocycle for Ch-6,γ-DME in the γ-C₆ local region. The structural and energy parameters of dimeric unit cells of Ch-6,γ-DME and Ch-7,γ-DME polymers are discussed from the standpoint of the Davydov-Tinoko exciton theories. Longwave absorption spectra and DOR are resolved into individual components with the aid of a modified Alentsev-Fock method. Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 68, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 3, pp. 331–337, May–June, 1999.     

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.     

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.     

Gain and luminescence spectra of broadband emitters based on asymmetric quantum-well heterostructures

Laser diodes and amplifiers of a new type based on asymmetric quantum-well heterostructures having a set of effective layers of different thickness are considered. In distinction to conventional laser heterostructures, for such modified quantum-well systems the gain spectrum and the set of lasing frequencies can be varied over a wide range by choosing the widths and the component composition of quantum wells and barrier regions. The transformation of the amplification bands for the TE and TM modes with the excitation current was studied. Calculations were performed for the GaAs−Al_xGa_1−xAs system. In a model of direct transitions the subbands of heavy and light holes as well as changes in the polarization factor with the frequency of light are considered. A number of questions associated with spectral broadening of lines and determination of radiation localization and distribution of quasi-Fermi levels in the effective layers are discussed. B. I. Stepanov Institute of Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Belarusian State University. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 221–227, March–April, 1997.     

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.     

Optoacoustic gas analyzer based on a13C16O2 laser for multicomponent pollution of air

A computer-aided optoacoustic gas analyzer based on a continuous¹³C¹⁶C₂ laser for multicomponent pollution of atmospheric air is described. The analyzer has the ability to detect absorption of radiation by detected substances at the level of ∼1·10⁻⁹ cm⁻¹ at a time resolution of 30 sec. Results of an experiment on simultaneous detection of H₂O, CO₂, NO₂, NH₃, HNO₃, OCS, and C₂H₄ in the atmospheric air using 40 laser lines are presented. B. I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, 68, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 3, pp. 345–350, May–June, 1999.     

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.     

Fine fluorescence and absorption spectra of hypericin in a polymer matrix at a temperature of 4.2 K

Monochromatic photoexcitation and selective registration in bands of purely electronic S₁↔S₀ transitions resulted in fine fluorescence and absorption (fluorescence excitation) spectra of hypericin in polyvinylbutyral at a temperature of 4.2 K. Vibration analysis of the spectra is done and conclusions are made about the form of many normal vibrations. It is found that the formation of intramolecular hydrogen bonds in the hypericin structure has a substantial effect on the relative position of the electronic vibrational levels of the pigment molecule. Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina ave., Minsk 200072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 4, pp. 539–545, 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.