N.A. Sadovskii

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.

The donor-acceptor properties of exciplexes. Comparison of fluorescence quenching of excited aromatic molecules and their exciplexes

Abstract Exciplex reactivity in electron transfer reactions is investigated. Methods are proposed for the calculation of exciplex ionization potentials and electron affinities. Exciplex quenching rate constants have been measured. It is shown that exciplex quenching follows the same laws as that of excitcd molecules. Conditions of specific exciplex quenching are considered.

Exciplex quenching as a method of investigating photochemical reaction mechanisms

Abstract The application of exciplex quenching to the investigation of photochemical reaction mechanisms is investigated. Kinetic analyses of exciplex fluorescence quenching and photochemical reaction quenching are presented. The product of a photochemical reaction is generally formed by one of the following three routes: (a) the product is formed in parallel with the exciplex; (b) the product and the exciplex are both formed from a common non-relaxed charge transfer state; (c) the product is formed from the fluorescent (relaxed) state of the exciplex or the reaction proceeds via the exciplex as an intermediate. Expressions enabling the partial quantum yields of routes (a), (b) and (c) to be determined are obtained. The quenching method is used to analyse the mechanism of halogen photoelimination in 9,10-dichloroanthracene in the presence of diethylaniline. The reaction is shown to proceed via both the non-relaxed and relaxed states of the exciplex.

Temperature effects on radical ion and triplet quantum yields in excited-state electron transfer reactions of 1,12-benzperylene in acetonitrile

Abstract Activation energies of 1,12-benzperylene fluorescence quenching ( E Q ) in acetonitrile by a series of electron donors and acceptors and of radical ion ( E R ) and triplet state ( E T ) formation were determined from Arrhenius plots of the quantum yields. The quenchers were chosen in the range of Gibbs energies of electron transfer Δ G et =−100 to 6 kJ mol −1 . Surprisingly low E Q , E R and E T values were obtained when Δ G et >−5 kJ mol −1 . The effects of temperature are discussed from the point of view of two alternative mechanisms: the conventional mechanism of direct electron transfer and that of transient exciplex formation. Evidence for the latter is given.

Exciplex mechanism of 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.The proposed model explains the difference between theoretical and experimental dependencies of logkQ vs. Gibbs energy of electron transfer ‡DGET and other experimental features known for fluorescence quenching by electron donors and acceptors. This model states that the exciplex is in equilibrium with the encounter complex and apparent quenching rate constants are controlled by two main factors — the lifetime of the exciplex and the enthalpy of its formation. Experimentally observed dependence of apparent quenching rate constant on ‡GET is caused by the dependence of the exciplex formation enthalpy on ‡GET, which is quite different from the dependence of electron transfer activation energy on ‡GET predicted by the theoretical models. Simulations of the dependencies of logkQ vs. ‡GET according to the exciplex formation model confirms its agreement with the experimental data.Electronic structure of the exciplex involved may be close to contact radical-ion pair only at ‡GET < 0, when the rate of quenching is limited mainly by the diffusion, but for ‡GET> 0, the structure of the exciplex should be much less polar.

Fluorescence of pyrene intramolecular excimers in Langmuir-Blodgett films

Abstract Fluorescence spectra and fluorescence decay kinetics of intramolecular excimers of di(1-pyrenylmethylene)adipate (DPA) and di(1-pyrenylmethylene)ether (DPE) were studied in Langmuir-Blodgett (LB) monolayers consisting of stearic acid (SA) and dipalmitoylphosphatidylcholine (DPPC). Two types of excimers with different lifetimes and different lifetime dependences on temperature were found for both of the compounds. No quenching of excimer fluorescence by oxygen was observed in LB films with SA as the matrix compound. When DPPC was used as a matrix only one type of excimer was quenched by oxygen. Photolysis of DPA and DPE LB films resulted in a dramatic decrease in excimer fluorescence, but negligible changes in the pyrene absorption band.

Different properties of exciplexes and excited states of CT complexes in the solid phase

Abstract The fluorescence, excitation and absorption spectra and fluorescence decay kinetics of charge-transfer (CT) complexes and exciplexes ofheteroaromatic cations and some other electron acceptors were investigated in ethanol at 77 K. Experimental data indicate that the fluorescence state of the exciplex may differ from that of the CT complex.

Quenching of excimers by electron donors

Abstract The quenching of excimers of pyrene and 9,10-dichloroanthracene by electron donors in toluene, ethanol and acetonitrile has been investigated. The excimer quenching rate constants are substantially lower than those of the parent excited molecules. The decrease in the reactivity of the excited molecules arising from excimer formation is related to the enthalpy of formation of the excimer. The rate constants for quenching of the excimers were found to be affected only slightly by the polarity of the medium.