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The mechanism of the nonlinear optical response of colloidal solutions of Ag2S quantum dots passivated with thioglycolic acid molecules is considered. Appearance of nonlinear absorption and nonlinear refraction is shown by the Z-scan method using 10-ns pulses of the second harmonic of a Nd:YAG laser. The absorption coefficient dependence was found to saturate with increasing incident radiation intensity. It is suggested that nonlinear absorption occurs with participation of the localized states of structural defects in Ag2S, including the defect luminescence center, which is confirmed by femtosecond transient absorption spectroscopy. A broad structureless transient absorption band is observed in the range of 500–1000 nm, which decays on a time scale of picoseconds. It is concluded that the transient absorption signal is caused by fast (fractions of picoseconds) capture of charge carriers by localized states, which then induce the nonlinear absorption of 10-ns pulses. [ABSTRACT FROM AUTHOR]

The spectroscopic and photophysical properties of the biologically important plant antioxidant quercetin in organic solvents, polymer films of polyvinyl alcohol, and a buffer solution at pH 7.0 are studied by stationary luminescence and femtosecond laser spectroscopy at room temperature and 77 K. The large magnitude of the dipole moment of the quercetin molecule in the excited Franck-Condon state μ= 52.8 C m indicates the dipolar nature of quercetin in this excited state. The transient induced absorption spectra S → S in all solvents are characterized by a short-wave band at λ = 460 nm with exponential decay times in the range of 10.0-20.0 ps. In the entire spectral range at times of >100 ps, no residual induced absorption was observed that could be attributed to the triplet-triplet transitions Т → Т in quercetin. In polar solvents, two-band fluorescence was also recorded at room temperature, which is due to the luminescence of the initial enol form of quercetin (~415 nm) and its keto form with a transferred proton (550 nm). The short-wave band is absent in nonpolar 2-methyltetrahydrofuran (2-MTHF). The spectra of fluorescence and fluorescence excitation exhibit a low dependence on the wavelength of excitation and detection, which may be related to the solvation and conformational changes in the quercetin molecule. Decreasing the temperature of a glassy-like freezing quercetin solution in ethanol and 2-MTHF to 77 K leads to a strong increase in the intensity (by a factor of ~100) of both bands. The energy circuits for the proton transfer process are proposed depending on the polarity of the medium. The main channel for the exchange of electronic excitation energy in the quercetin molecule at room temperature is the internal conversion S ⇝ S , induced by the state with a proton transfer. [ABSTRACT FROM AUTHOR]

Using the spectral methods of induced absorption, luminescence, and photostimulated luminescence flash, we have experimentally investigated processes of decay of electronic excitations in CdS colloidal quantum dots and in CdS/ZnS “core-shell” systems synthesized in gelatin by the sol-gel method. It has been shown that the decay of electronic excitations in colloidal quantum dots of this type is predominantly related to a fast localization of nonequilibrium charge carriers on surface defects and their subsequent recombination during times on the order of units and tens of picoseconds. The passage to core-shell systems eliminates, to a large extent, surface defects of the core, some of which are luminescence centers. However, upon using the sol-gel synthesis, a noticeable fraction of luminescence centers are formed in the interior of the CdS quantum dot, which, as well as in the case of CdS/ZnS systems, ensures localization of exciton, blocks its direct annihilation, and maintains recombination radiation. [ABSTRACT FROM AUTHOR]

Using methods of steady state luminescence and femtosecond spectroscopy, we have studied the mechanism of intramolecular proton transfer in synthesized 3,7-dihydroxy-2,8-di(4-methoxyphenyl)-4H,6H-pyrano[3,2- g]chromen-4,6-dion in polar and nonpolar solutions, films, and polycrystals at 293 and 77 K. In an excited singlet state, intramolecular proton transfer occurs in two stages. At the first stage, a tautomer with one transferred proton (OTP tautomer) is formed from the Franck-Condon state within τ = 0.6 ps. At the second stage, the second proton is transferred within τ = 3.1 ps and a tautomer with two transferred protons (TTP tautomer) is formed, which fluoresces in toluene at 293 K with a high quantum yield, Φ = 0.66, and the fluorescence spectrum of which is characterized by a large Stokes shift, 9900 cm. At 293 K, polar solvents (dimethylformamide, dimethyl sulfoxide, ethanol, etc.) solvate the BFV molecule in the ground state, while, in the excited state, an OTP tautomer is mainly formed. In polar ethanol at 77 K, a dual fluorescence spectrum is observed, which is caused by the fluorescence emission of polysolvates with λ = 460 nm and TTP phototautomers at λ= 610 nm. [ABSTRACT FROM AUTHOR]

The kinetics of the relaxation processes in silver iodide nanocrystals synthesized in a nanoporous silicate glass matrix and excited by femtosecond laser pulses has been experimentally studied. A short-lived characteristic transmission band is found to be formed in the spectral range of exciton absorption of nanocrystals. The specific features of photoexcited-carrier relaxation, which are due to the energy redistribution of captured carriers over levels in the band gap of silver iodide nanocrystals, are considered. The energy transfer relaxation rates are estimated and possible mechanisms of the energy transfer are discussed. [ABSTRACT FROM AUTHOR]

The kinetics of photoinduced absorption spectra of CdF crystals with bistable indium and gallium centers under femtosecond pulsed excitation has been experimentally investigated. Based on the example of indium ions, it is shown that the transmission band in the absorption spectrum of deep centers is formed for 0.8-1 ps, which significantly exceeds the photon absorption time. This process is interpreted as a result of displacement of indium ion from the initial interstitial position to a site of neighboring unit cell; the displacement velocity is estimated to be 200-250 m/s, a value close to the thermal velocity of this ion at room temperature. The times characteristic of the formation of free polarons as a result of the displacement of neighboring lattice ions have been experimentally estimated for the first time at a level of 0.8-1.2 ps. The capture times of free polarons by trivalent gallium and indium ions are estimated (5 and 10 ps, respectively), as well as the corresponding cross sections (2 × 10 and 8 × 10 cm). [ABSTRACT FROM AUTHOR]

The spectral and kinetic properties of an immunoactive 8-azasteroid and a product of its photochemical transformation are studied by the methods of steady-state and transient spectroscopy. It is found that the spectral features of the studied compounds in solvents of different nature are determined by a high sensitivity of the electronic structure of the aminovinyldicarbonyl fragment to intermolecular interactions of the donor-acceptor type. This leads to the existence in solutions of several mesomeric-tautomeric forms that are in a dynamic equilibrium controlled by the experimental conditions. Based on the analysis of the dynamics of the transient spectra of induced absorption and of the spectral and luminescent properties of the extracted photoproduct, the conclusion is made on the direction of the photochemical transformation of 2,3-dimethoxy-8-azagona-1,3,5(10),13-tetraene-12,17-dione and the final product is identified as 6,7-dehydroderivative of the parent compound. [ABSTRACT FROM AUTHOR]

The spectral and luminescent properties of compounds from the class of immunoactive 8-azaster-oids, 2,3-dimethoxy-8-aza-D-homogona-1,3,5(10),13-tetraene-12,17 a-dione and its 9,11 dehydroderivative, are studied by the methods of steady-state spectroscopy. It is found that the specific features of the fluorescent properties of the compounds under investigation in different solvents are determined by a high sensitivity of their electronic structure to intermolecular interactions of the donor-acceptor type. This leads to the existence in solutions of several mesomeric-tautomeric forms that are in a dynamic equilibrium controlled by the experimental conditions. A photoproduct that is responsible to a large extent for the fluorescent properties of solutions of aminovinyldicarbonyl 8-azasteroids is not a 9,11 dehydroderivative of the initial compounds. [ABSTRACT FROM AUTHOR]

The effect of acid-base interactions on the photophysical properties of 6,7-dimethoxy-3,4-dihydroisoquinoline in protic solvents is studied by the methods of steady-state and picosecond spectroscopy. It is found that the specific features of the spectral and luminescent properties of solutions of 6,7-dimethoxy-3,4-dihydroisoquinoline are connected with the presence of emission centers of two types—solvated initial molecules and their protonated cationic forms. Considerable long-wavelength shifts observed in the electronic absorption and fluorescence spectra of the cationic form of the molecule as compared to the spectra of its initial form are caused by the elongation of a conjugated chain present in the fragment of the molecule that separates the nitrogen atom and the oxygen atoms of the methoxy groups. The transition of the cationic form of the molecule to an excited electronic state is not accompanied by a change in its dipole moment. [ABSTRACT FROM AUTHOR]

The spectral and kinetic properties of singlet excited states of two tetrahydrobenzo[ c]phenanthridines—3,3-dimethyl-1,2,3,4-tetrahydrobenzo[ c]phenanthridine-1-one (DTP) and 3,3-dimethyl-1,2,3,4-tetrahydrobenzo[ c]phenanthridine-1-ol (DTP1)—are studied by picosecond absorption and luminescence spectroscopy in protic and aprotic solvents at 293 and 77 K. Based on the differences in the spectral and luminescent properties of DTP and DTP1, it was shown that the oxygen atom involved in the conjugation system of DTP is responsible for the formation of hydrogen-bonded complexes with alcohols. In the picosecond spectra of the induced absorption of DTP in n-hexane and acetonitrile, two bands peaked at 480 and 600 nm are observed, which are caused by the T k ← T 1-and S n ← S 1 absorption of DTP molecules solvated by the solvent by means of universal intermolecular interactions. In alcohols (methanol, ethanol, and n-propanol), an additional band is observed at 850 nm, which is caused by the S m ← S 1 absorption of hydrogen-bonded complexes. From analysis of the kinetics of the rise and decay of the induced absorption of DTP in n-hexane, acetonitrile, and alcohols, it is suggested that the main nonradiative channel in DTP is intersystem crossing. [ABSTRACT FROM AUTHOR]

The influence of the polarity of a medium on the spectral and luminescent properties of 2-[(2E,4E)-6-(1,3,3-trimethyl-2,3-dihydro-1H-2-indolyliden)-2,4-hexadienyliden]malononitrile (THDM) in solutions and polymer matrices is studied at room temperature under conditions of steady-state and pulsed laser excitation. A large bathochromic shift of the absorption spectra observed upon an increase in the polarity of a solvent is caused by a strong increase in the molecular dipole moment μ due to a transition of molecules from the ground state (μg = 7.6 D) to an excited Franck–Condon state (μFC = 33.5 D). Based on the solvatochromic data, the quadratic polarizability was calculated to be β = (3.2±0.6) ×10–28 esu, which is close to the experimentally determined value βex = (3.9±0.2) ×10–28 esu. A strong narrowing of the fluorescence spectra in comparison with the absorption spectra is observed upon an increase in the solvent polarity. This narrowing is explained by a decrease in the bond length alternation parameter and by weakening of vibronic interactions in the singlet excited state. The dynamic solvatofluorochromism of THDM in the picosecond range is caused by reorientations of molecules of the polar environment occurring during a time period consistent with the dielectric relaxation time of these molecules. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

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. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

The dynamics of relaxation processes in meso-ortho-nitrophenyl-substituted octaethylporphyrins OEP-Ph(o-NO[sub 2]) and PdOEP-Ph(o-NO[sub 2]) occurring with the participation of the S[sub 1] and T[sub 1] states is studied in polar (dimethylformamide) and nonpolar (toluene) media at 295 and 77 K using pico- and nanosecond laser kinetic spectroscopy. It is shown that, at 295 K, the steric interactions between β-alkyl substituents in porphyrins and ortho-nitro groups of meso-phenyl in these compounds create optimal conditions for overlapping of molecular orbitals of the donor and the acceptor, which ensures an efficient photoinduced electron transfer (PET). It is ascertained that, in free-base OEP-Ph(o-NO[sub 2]), the PET occurs only via the S[sub 1] state of porphyrin in time intervals of 40 ps (dimethylformamide) and 125 ps (toluene), whereas the competing intersystem crossing S[sub 1][functional_relationship]T[sub 1] is unlikely. In the case of the metal complex PdOEP-Ph(o-NO[sub 2]), the PET involves both S[sub 1] and T[sub 1] states. In the PdOEP-Ph(o-NO[sub 2]) molecule, direct PET from the T[sub 1] state to the charge-transfer state also occurs in the picosecond range (20 and 46 ps for dimethylformamide and toluene, respectively, at 295 K), with the rate constants being 3–5 times smaller in comparison with the corresponding values found for the PET occurring via the S[sub 1] state of this compound. For both compounds, the experimentally observed long-lived component in the decay kinetics of the induced T[sub 1]–T[sub n] absorption (250–700 ns) is due to the recombination processes in ion–radical pairs whose lifetime decreases with an increase in the polarity of the medium and is almost independent of the presence of molecular oxygen in the solution. The PET is completely absent at 77 K in the compounds under investigation when they are dissolved in rigid solutions. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

The spectral-luminescent properties and electronic-level structure of a derivative of oxotetrahydrobenzo[c]phenanthridine-3,3-dimethyl-1,2,3,4-tetrahydrobenzo[c]phenanthridine-1-one (DTP)-in polar and nonpolar solutions are studied by the luminescence and picosecond spectroscopy techniques at 293 and 77 K. It is shown that DTP exhibits ππ* fluorescence only in polar proton-containing methanol, ethanol, and n-propanol and that this fluorescence is associated with a chromophore + alcohol complex. Solutions of DTP inn-propanol and the nonpolar mixture methylcyclohexane + toluene (6:1) at 77 K display ππ* phosphorescence with a quantum yield Φ[SUBPh] = 0.1, a lifetime τ[SUBPh] = 1.9 s, and a frequency of the 0-0 transition equal to 21550 cm[SUP-1]. The fact that DTP fluorescence is absent in aprotic solvents and exists in alcohols is explained by the inversion of the energy levels of the S[SUBππ*] and T[SUBnπ*] states, as a result of which the energy level of the fluorescing S[SUBππ*] state in alcohol becomes the lowest. The picosecond spectra and kinetics of the induced S[SUBn](ππ*) ← S[SUB1](ππ*) and T[SUBk](ππ*) ← T[SUB1](ππ*) absorption made it possible to estimate the lifetime of the S[SUB1](ππ*) state of DTP inn-pro-panol at 293 K (20 ± 10 ps) and the position of the triplet-triplet absorption peak (495 nm). [ABSTRACT FROM AUTHOR]