The data of the theoretical and experimental investigations of the luminescence and absorption spectra of different organic compounds (aromatic hydrocarbons and their derivatives, phthalocyanines and porphyrins, polyenes, etc.) in frozen polycrystal solutions are summarized and systematized.The recent data prove that quasi-line spectra are the sets of zero-phonon lines accompanied by their vibrational repetitions when the impurity arrangement provides insignificant heterogeneous line broadening and the electronphonon interaction is sufficiently small. Extensive experimental data make it possible to conclude that in the organic systems under consideration such conditions may be realised by means of a matrix modification near the impurity centres.The characteristics of quasi-line spectra (namely, the well-resolved vibrational structure and the sensitivity to internaland external perturbations) are shown to be of use in solving different problems of molecular and crystal spectroscopy.Quasi-linear fluorescence and absorption spectra of condensed aromatic hydrocarbons were discovered more than twenty years ago 1 • In the first papers devoted to the study of these spectra it was shown that they were high-resolved vibronic spectra of polyatomic organic molecules • 3 •To obtain quasi-linear spectra the crystallinity of the matrix is a prercquisite. Originally the choice of solvents was limited to normal saturated hydrocarbons (n-paraffins), which were crystallized on freezing. Later, howcver, solvents of other classes were used in a number of studies. Structural spectra of each compound studied can only be obtained in a limited number of solvents.Temperature is a significant factor in thc observation of a quasi-linear spectrum. Thus, in some cases it is sufficient to decrease the temperature to 77 K (for instance, n-paraffin solutions of a large number of aromatic hydrocarbons) while in others it is necessary to cool the solution tested to 20 Kor 4 K (for instance, chlorophyll 4 , diphenylpolyene 5 and benzophenone 6 solutions). At present quasi-linear spectra of about 500 compounds have been obtained and analysed. This means that we are dealing with a reasonably common phenomenon: a phenomenon which is consistent with the standard concept which is based on the discretc system of electronic and vibrational Ievels, of vibronic spectra of polyatomic molecules. Therefore, the problern 183 Unauthenticated Download Date | 5/11/18 6:28 PM

Spectra of Para-Halogen Substituted Stilbenes. The stilbene molecule possesses a unified system of 14 Tr-eteetrons and is the first member of the diphenylpolyene series. The spectral properties of stilbene vapor [1], solutions [2][3][4][5], and crystals [6, 7], and also of stilbene impurities in various crystalline solids [6, 8] over a wide temperature range have been the subject of numerous investigations.Use of the quasilinear spectra method [9], which permits highly accurate measurements of the O -O transition frequency and the molecular frequencies of complex organic compounds to be made not only in the ground (So) but also in the excited ($1) states, makes it possible to consider the detailed vibrational structure of the stilbene molecule [10, 11], as well as those of some other members of the diphenylpolyene series [10] in the S o and S~ states. Introduction of a substituent causes perturbation of the v-electron cloud, changes the state of the sigma bonds, and increases the molecular mass. In an earlier study [5] absorption and fluorescence spectra were measured at room temperature of heptane and toluene solutions of various stilbene derivatives. However the diffuse nature of the spectra only allowed an approximate estimation of the shift of the band maxima to be made. Investigation of the absorption anf fluorescence spectra of stilbene and of its para-, mono-F-, CI-, and Br--derivatives present as impurities in dibenzyl at 20~K [8] showed that under these eonitions only the spectra of stilbene and 4-F-stilbene were sufficiently sharp for carrying out vibrational analysis.Quasilinear electronic-vibrational spectra of molecules are able to yield information regarding changes in the first electronic transition energy (O-O frequencies), molecular vibrational frequencies in the ground and excited states, caused by introduction of a substituent and depending on the properties of the original molecule and of the substituent. The quasilinear spectra method is particularly sensitive to different intramolecular interactions [12]. In the present work this method has been used to study the effect of a substituent in the para-positionof the stilbene molecule, on the frequency of the first electronic transition and the molecular frequencies in the ground and first excited states. The accuracy of the frequency determinations for most of the spectral lines (~ S0~) was 5-10 cm -~. The frequency interval ~ 200 cm -~ combining with the other frequencies which appear as overtones, was determined with an accuracy of 2-5 cm -1.We have carried out measurements of the absorption and fluorescence spectra of different n-alkane (from hexaae to undecane) solutions of 4-F-, 4-C1-, 4,4' -diCl-, 4-Br-, and 4-I-derivatives of stilbene at 77~The concentrations of the solutions were ~10 -4 g/ml.The spectra were recorded on a DFS-8 spectrograph with an inverse linear dispersion of 6.25 A/mm. For fluorescence excitation the ~313 nm region of the DKSSh-1000 lamp emission was used. The same lamp was used for measuring the absorption spect...

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