M.G. Kuzmin

Donor-acceptor complexes of singlet excited states of aromatic hydrocarbons with aliphatic amines

Abstract Aliphatic amines form the donor-acceptor complexes with the singlet excited states of benzene, p-terphenyl, naphthalene and some other aromatic compounds. The excited complex of triethylamine with naphthalene has a dipole moment equal to 11 D . In non-polar solvents this complex is in equilibrium with the excited singlet state of they hdyrocarbon.

Set of luminescence pressure sensors for aerospace research

Abstract The application of luminescent sensor paint for quantitative pressure measurement on the surface of an aircraft model in wind tunnels is discussed. The technique employed is based on the phenomenon of dynamic quenching of luminescence by oxygen of the air using a specially synthesized resin (applied to the model surface). The main features of this optical sensor are: fast response time (0.005-0.5 s for 99.5% response); thermo-, photo- and mechanical stabilities; homogeneity of properties; low dependence of indicated pressure on temperature (temperature coefficient in the range 0–60 °C is less than 0.3%/°C); and high intensity of luminescence. Results obtained for quantitative pressure-measurement experiments with a sensing surface up to 1 m2 in a transonic wind tunnel (cross section 2.75 m × 2.75 m, T-128 TsAGI wind tunnel) at Mach number 0.4–2 are also discussed. A comparison of the results with the pressure tap technique shows that the variations in the data obtained by optical sensors with the data obtained by the traditional method do not exceed 2%. The sensors are also used for semi-quantitative pressure measurements in shock wind tunnels at Mach number 8, presenting information which was previously inaccessible.

A photoelectrochemical effect at the interface of immiscible electrolyte solutions

Abstract A photoelectrochemical effect (i.e. the generation of a photocurrent or photopotential) was found upon irradiation of an ITIES (aqueous solution of NaCl/1,2-dichloroethane solution of tetrabutylammonium tetraphenylborate), when the organic solution contains an electron donor and an electron acceptor. A photochemical reaction in the organic solution yields ionic species which are transferred into the aqueous phase and produce an electric current across the interface. Two kinds of photochemical systems were used: protoporphyrin+quinones (excitation 540–580 nm) and quinones + tetraphenylborate (excitation 313–365 nm), in which the charge carriers are of different nature. The photocurrent quantum yield lies within the range 10 −3 –10 −1 and depends on the structure of the quinone and the potential drop at the interface. A mathematical theory of the charge phototransfer across the liquid/liquid interface is developed on the basis of the diffusion kinetics.

Spectroscopy, photophysics and photochemistry of 1,3-diketoboronates: IV: Luminescence spectroscopic investigations of 2-naphthyl-substituted 1,3-diketoboronates

Abstract Aryl-substituted 1,3-diketoboronates exhibit high fluorescence quantum yields. In the case of 2-naphthyl-substituted 1,3-diketoboronates, excitation-wavelength-dependent luminescence spectra and luminescence-wavelength-dependent excitation spectra were recorded. In polar solvents the compounds exhibit large Stokes shifts and the fluorescence lifetimes depend on the excitation and fluorescence wavelengths. The multiple fluorescence behaviour is interpreted by assuming the existence of excited non-equilibrated rotamers with different photophysical properties and high dipole moments in the excited state. The relaxation of polar solvents around these high polar species contributes to the broadening of the fluorescence bands and to the fluorescence-wavelength dependence of the lifetimes caused by the fluorescence of incompletely solvent-relaxed species.

Exciplex mechanism of the fluorescence quenching in polar media

The formation of exciplexes (non-emitting or poorly emitting) is suggested as one of the causes for deviations of experimental data on fluorescence quenching in polar solvents from the classical model of excited-state electron transfer yielding radical ion pairs. Several evidences for the formation of such exciplexes were found for fluorescence quenching of aromatic compounds by weak electron donors and acceptors. For cyano-substituted anthracenes exciplex emission can be observed in the presence of quenchers even in polar solvents. In other systems, indirect evidences of exciplex formation were observed: nonlinear dependence of the inverse value of excited pyrene lifetime on the concentration of the quencher; very small and, in some cases, even negative experimental activation energies of pyrene fluorescence quenching, which are much less than activation energies, calculated from the experimental values of the quenching rate constants etc.

Photoprotolytic dissociation in micellar solutions

Abstract Rate constants of proton transfer of several hydroxyaromatic compounds in non-ionic, anionic and cationic surfactant solutions are determined. The data are extrapolated to the limiting case of complete solubilization. The photoprotolytic dissociation rate constants in non-ionic micelles of Brij-56 decrease approximately by an orde of magnitude, and are 15, 30 and 60 times lower in sodium decylsulphate, sodium dodecylsulphate and sodium hexadecylsulphate respectively and change little in cetyltri-methylammonium bromide micelles in comparison with aqueous solutions. Photoprotolytic dissociation does not need the exit of the reactant molecules from micelles and excited products are formed initially within the micellar phase. Rate constants of the exit of the anions thus formed from the micelles are estimated. The exit from cationic and non-ionic micelles is much slower than deactivation of the excited anions but is quite effective in anionic micelles. Exit rate constants increase with the growth of the surfactant hydrocarbon chain.

Exciplex mechanism of excited state electron transfer reactions in polar media

Abstract Experimental data provide strong evidence for the transient formation of exciplexes in excited state electron transfer reactions, even in polar media. The exciplex mechanism of excited state electron transfer involves the reversible formation of low polarity exciplexes from excited molecules at positive or slightly negative free energies of electron transfer (i.e. in the kinetic region). This means that the mechanism of electron transfer involves a gradual electron shift from electron donor to acceptor and conjugated simultaneous reorganization of the reactants and media, rather than an electron jump after preliminary reorganization. The apparent quenching rate depends on the equilibrium constant of formation and lifetime of the exciplex rather than on the reorganization energy of the reactants and media. Transient exciplex formation provides new pathways for excitation decay (internal conversion and intersystem crossing) which compete with the formation of the products of electron transfer and lead to a decrease in their yield and, at the same time, produce an increase in the total rate of excited state quenching.

Excited-state, proton-transfer reactions of substituted naphthols in micelles. Comparative study of reactions of 2-naphthol and its long-chain alkyl derivatives in micellar solutions of cetyltrimethylammonium bromide

Abstract Excited-state, proton-transfer (ESPT) reactions of 2-naphthol and its long-chain alkyl derivatives (6-hexadecyl-2-naphthol and 1-octadecyl-2-naphthol) were investigated in micellar solutions of cetyltrimethylammonium bromide (CTAB) and in ethanol solutions. Significant slowing down (by a factor of five to eight) of the photodissociation of 2-naphthol containing alkyl groups in comparison with non-substituted 2-naphthol was observed in CTAB micelles, in contrast with ethanol solutions where the rate constants of ESPT reactions with the acetate anion for 2-naphthol and its long-chain alkyl derivatives are similar. Experimental results are rationalized in terms of the effect of the specific localization of the naphthol and naphthol moieties of different hydrophobicity in the micelles and of the relationship between the kinetic and thermodynamic parameters of ESPT.

Nature of the processes of charge-carrier generation at ITIES by the photoexcitation of porphyrins

Abstract The kinetics of the photocurrent through the interface between two immiscible electrolyte solutions (ITIES) was investigated under high intensity irradiation when the organic phase was a concentrated solution of tetraphenylporphine or protoporphyrin. A number of different processes of charge-carrier generation are analysed. The reaction of the excited porphyrin molecule with ground state porphyrin and with oxidized forms of porphyrins was found to generate charge carriers in the volume near the interface. The mathematical description of the generation, diffusion and transformation of charge carriers, as well as relationships between these processes, is presented in the form of a system of differential equations. Numerical simulation has shown that the depletion of the irradiated zone by the oxidized forms of porphyrins strongly affects the kinetics of the photocurrent.

Evidence for diffusion-controlled electron transfer in exciplex formation reactions. Medium reorganisation stimulated by strong electronic coupling

Diffusion-controlled rates of formation were found from the temperature dependence of apparent quenching rate constants for exciplexes, when the driving force of excited-state electron transfer -0.1 < deltaG(ET)* < +0.1 eV. This is inconsistent with the conventional mechanism of electron-transfer reactions, involving preliminary reorganisation of the medium and reactants, and provides strong support for the mechanism of medium reorganisation stimulated by strong electronic coupling of locally excited and charge-transfer states.