S. A. Tikhomirov

Picosecond dynamics of singlet excited states of oxotetrahydrobenzo[c]phenanthridine in protic and aprotic solvents

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 Tk ← T1-and Sn ← S1 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 Sm ← S1 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.

Decay of Electronic Excitations in CdS and CdS/ZnS Colloidal Quantum Dots: Spectral and Kinetic Investigations

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.

Photophysical and second-order nonlinear properties of push-pull fluorinated 4-(dicyanomethylene)-pyranes

The photophysical and nonlinear properties of some push-pull fluorinated 4-(dicyanomethylene)- pyranes using the steady- sate and time-resolved picosecond spectroscopies with an electric-field-induced second-harmonic-generation technique have been investigated. Strong fluorescence quenching and large shifts of time-resolved transient picosecond spectra on going form nonpolar to polar solvents are connected with effective charge transfer interactions of polar singlet excited molecules with a polar environment. The high second- order polarizabilty (beta) equals 260 X 10-30 esu for one of the investigated push-pull pyranes allows to propose using such kind of molecules in the development of second- harmonic-generation elements.

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.

Influence of the Polarity of a Medium on the Photonics of a Merocyanine Dye with a High Quadratic Polarizability

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.

Optical detection of picosecond processes of formation of free carriers and primary photolysis products in AgBr(J) nanocrystals

The fastest picosecond processes occurring in the electronic stage of photolysis in photosensitive AgBr(J) nanocrystals—the generation of free carriers and their initial capture by lattice defects and impurity centers with subsequent transition of captured carriers to deeper levels—were identified and quantitatively studied by methods of optical detection for the first time. The experimental characteristic times of these processes for AgBr(J) nanocrystals were shorter than 5 ps in the first stage and about 30 and 300 ps (for the generation and the capture of free carriers, respectively) in subsequent stages. The minimum durations of pulses of destructive nonactinic radiation at which dephotolysis processes can be reliably detected were estimated to be 20 ps for simultaneous illumination by pulses of destructive and exciting radiation and about 5 ps in the case of successive illumination. The decisive role of two-photon absorption in nonlinear processes of formation of hidden images under the action of picosecond pulses of nonactinic radiation was shown experimentally.

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.

Dynamics of collisional relaxation of the optically induced anisotropy of para-quaterphenyl in the gas phase

The decay kinetics of the optically induced anisotropy of para-quaterphenyl molecules is theoretically and experimentally studied in a wide range of pressures of buffer gases, pentane and argon. The anisotropy of absorption is measured in real time using a femtosecond spectrometer. Comparison with the theoretical models of orientational relaxation shows that the collisional interruption of the angular momentum corresponds to a situation that is intermediate between the situations described by the J-diffusion model and by the model of the Fokker-Planck equation and is closer to the latter situation. In terms of the Keilson-Storer model, such a regime corresponds to the collision efficiency parameter γ ≈ 0.78 for argon and 0.66 for pentane; i.e., 4.5 collisions with argon or 2.9 collisions with pentane are necessary to completely randomize the angular momentum of para-quaterphenyl molecules.

Spectral and luminescent properties of 2,3-dimethoxy-8-aza-D-homogona-1,3,5(10),13-tetraene-12,17a-dione and its Δ9,11 derivative

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-12,17a-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.