Roman I. Personov

Luminescence analysis of complex organic materials based on resolved line spectra produced by selective laser excitation

Abstract Selective methods of obtaining the resolved line spectra of the solutions of complex molecules, along with the peculiarities of these spectra, are considered. At sufficiently low temperatures (4.2 K), broad-band absorption and luminescence spectra of the solutions of organic compounds are often broadened, mainly inhomogeneously. Excitation of the luminescence by tunable lasers can eliminate this broadening. The spectra obtained consist of dozens of narrow (Δv ≈ 1 cm-1) vibrational lines. Some examples of selective excitation for the spectrofluorimetric analysis of complex organic materials are given: identification (at the 10-11 g ml-1 level) and determination of individual polynuclear hydrocarbons in certain materials without preliminary separation is demonstrated by the determination of benzopyrene and perylene in gasoline and the detection of perylene in a solid paraffin.

Delayed luminescence of the ladder-type methyl-poly(para-phenylene)

Abstract The delayed luminescence of a film of ladder-type π-conjugated polymer – the methyl-substituted poly( para -phenylene) (MeLPPP) – has been detected and investigated at 6, 77 and 295 K. The delayed emission consists of a delayed fluorescence (DF) in the region of 450–550 nm, which decreases by a factor of 10 during 200 ns (at 77 K), and a phosphorescence in the region of 600–750 nm with a much slower decay rate. The latter decreases by a factor of 10 during about 100 μs. The DF spectrum is the superposition of the intrinsic fluorescence emission and an excimer-type emission. The analysis of the experimental data provides evidence for a geminate pair recombination being the origin of both types of DF.

Line-narrowed spectra and kinetics of laser-excited delayed fluorescence of complex molecules in solid solutions

Abstract Delayed fluorescence (DF) of coronene, phenanthrene, triphenylene and carbazole in organic glasses upon laser excitation has been investigated. As different from previous works on DF for solid solutions, an early stage of the DF decay has been investigated and a detailed theoretical analysis has been made taking into account the dependence of a T-T annihilation rate on the distance between the molecules in pairs. At the initial DF decay stages (10 −4 τ T t T ) an essentially nonexponential law is revealed for all the objects: I DF ( t )∞ t −ga , where α=0.8–0.9 in a wide range of concentrations and of the temperature (4.2–90 K). It is shown that the above law is due to an exchange type interaction of two triplet-excited molecules. The possibility of pair-size selecting has been established for various stages of the DF decay. Fine structure DF spectra upon selective laser excitation have been obtained for the first time. As the experimental data analysis shows pairs with the size r ≳ 20–30 A are revealed in the DF under our experimental conditions. The paper also discusses new possibilities for fine spectral and kinetic investigations of bimolecular interactions via selective spectroscopic methods.

Concentration effects on spectra and persistent hole-burning of porphyrin-doped Langmuir-Blodgett films

Abstract The influence of impurity concentration on excitation and emission spectra, fluorescence decay rates and persistent spectral hole-burning parameters is reported for porphyrin dye molecules in a Langmuir—Blodgett film host. A shift of the fluorescence band and non-exponential radiative decay in heavily doped samples provide evidence of energy transfer. Hole widths in the heavily doped samples are strongly dependent on wavelength, becoming broader with higher excitation energy. No wavelength dependence of hole width and depth is observed for weakly doped samples. An analysis of the effect of Forster energy transfer on hole width and depth rules out this mechanism as being the whole explanation for the concentration and wavelength dependence. The effect of impurity aggregation is also inconsistent with our results.

Stark effect on holes in the absorption spectra of phthalocyanine and the aluminum-phthalocyanine complex in polymer matrices

The effect of an external electric field on narrow spectral holes in S1←S0 absorption bands of molecules of free-base phthalocyanine and its complex with Al in polyvinyl butyral films at 4.2 K was studied. It was found that the average values of the change in the electric dipole moment due to phototransitions in the free-base phthalocyanine and its complex amount to 0.135 and 0.145 D, respectively. Both these values considerably exceed the relevant values for phthalocyanine complexes with Zn and Mg in the same matrix. Possible causes of the observed effects are discussed. Most probably, they are related to the differences in the structure of the central part of phthalocyanine-type molecules.