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Сообщение автор Admin в Вт Фев 25 2014, 18:02

Docent Department of Common and Biological Chemistry
Poltava State Agrarian Academy, Yu.E. Sakhno
Postdoctoral researcher of Universita degli Studi di Torino
Dipartimento di Chimica Inorganica, Fisica e dei Materiali


At the last years, in the world wide develop the research concerning to the exploration and technical developments ways to use unconventional energy sources. The alternative exists - is the energy of solar radiation, the use of which is possible with the luminescent solar concentrators (LSC). An LSC is typically made from a plate of a transparent solid containing a fluorescent material. The fluorescent material absorbs sunlight and then re-radiates isotropically, ideally with a high quantum efficiency the irradiated light being trapped in the sheet (layer) by the internal reflection. Some progress in development of LSC is associated with the manufacture of polymeric structures with several plates which contain the different dyes. The spectral properties of these dyes embedded in polymeric matrix can not always be provided, based on experimental results. From a wide variety of dyes used in the LCS, we chose coumarins, which are currently widely studied chromophores and therefore widely used in various technical fields, and as a model compound for research. In this context, coumarins are compounds, which are ubiquitous in nature, extensively studied due to their practical applications and understanding of spectral-luminescence properties helps not only in the design of new materials but also for the best performance for specific application, such as wavelength shifter in scintillators or luminescence solar concentrators. Furthermore, data chromophores were selected because of their absorption and emission spectra are located in different frequency bands.
Earlier by experimental results and quantum chemical calculations (at various theoretical levels) the strong dependence of quantum yields of fluorescence same coumarins derivatives on temperature was founded. A mechanism of the temperature quenching of fluorescence are proposed at the first time [1]. But, it is known that the electronic spectra of these molecules are influenced by their immediate environment [2, 3]. Among the major environmental factors influencing the electronic spectra, solvent effects are of particular importance.
The aim of this theoretically review is to examine the influence of nature of matrix on the photophysical properties same coumarin derivatives: coumarin-3.4-benzocoumarin-coumarin 504 and prediction of the properties of this chromophores embedded into the polymer matrix. Since information about the energy performance of coumarins in various polymer matrices is limited, the theoretical study and their interpretation indicates the relevance of our research. One way of decision of this problem is the theoretical modeling of the behavior of dyes in a polymer matrix, whose properties are modeled by solvent with the corresponding value of the dielectric constant. The dependence of absorption maxima on solvent polarity was explored quantitatively using the solvent parameters Kiekwood-Onzager.
The quantum-chemical calculations of the energy parameters of the model compounds were performed employing the program package GAUSSIAN 03W within density functional theory (TD-DFT) B3LYP method and basis set 6-31G (d). On the basis of the computation results we discussed effects of solvent polarity on electronic characteristics of coumarins containing polymers. The dependence of energy levels electronic-excited states of investigated compounds on solvent polarity was explored quantitatively using the solvent parameters Kiekwood-Onzager. The correlation between solvent polarity and electronic structure of coumarins is established.

As follows from the plots, the increase in solvent polarity is accompanied by an increase of energy nπ- levels in the molecule coumarin and in chloroform should expect the appearance of fluorescence due to the inversion Snπ- and Sππ- levels. However, with the increasing the solvent polarity, the energy gap between Sππ- Tnπ- levels is decreases, and thus, it becomes possible deactivation Sππ-state in the process of nonradiative ISC to the Tnπ- triplet state. Molecules of 3,4-benzocoumarin in nonpolar solvents does not radiates, due to the possibility of ISC between Sππ- and Tnπ-levels. A further increase in solvent polarity leads to inversion of the levels and the appearance of fluorescence. The absorption spectrum of coumarin 504 is shifting bathochromic on the transition from heptane to acetonitrile. In the gas phase molecule does not fluorescence due to the energy proximity of Sππ- and Tnπ-levels. In the all studied solvents, due to the large mobility of Tnπ-level the fluorescence can be observed. The comparison of the calculated position of the absorption spectra of coumarin 504 in ethanol and chloroform, indicating a minor shift , which agrees well with the experimental data.
Based on the presented material, in many cases it is possible to predict theoretically, not only the spectral properties of the coumarin derivatives, but any others of chromophores when embedded in a polymeric matrix. The represented results determine the ways of further research towards a photostability polymer compositions in which the chromophores units can be including coumarin derivatives.

References: 1. Sakhno T.V., Korotkova I.V., Khakhel′ O.A. // Functional Materials. – 1996. – V.3, № 4, P.502-503. 2. Hrdlovic P., Donovalova J., Stankovicova H., Gaplovsky A.// Molecules. – 2010. – V.15, P.8915-8932. 3. Raikar U., Renuka C., Nadaf Y. // Spectrochimica Acta Part A. – 2006. – V.65, P.673-677.


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