Modern trends in quasi-linear spectra studies

Abstract

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

The nature of solution spectra. Inhomogeneous broadening and phonon effects in frozen solutions

“…Moreover, the narrow-line spectra arising from these different sites are quite similar, apart from the overall shift in frequency. 14 In particular, the vibrational spacings are the same to within 2-5 cm.-1 Here, as in the case of the Shpol'skii multi-plets15, 16 and some mixed crystals,17 the different local environments appear to shift only the electronic contribution to the The absorption and emission spectra of tetracene in a 2-methyltetrahydrofuran glass at 4.2 K. The absorption and emission mirror one another and show a small Stokes shift (~90 cm-1).…”

Comment on ‘On Saltiel's isopolarizability approach and its applicability to diphenylpolyenes’ by J. Catalán, Chem. Phys. Lett. 635 (2015) 56–59

“…3-Methylpentane, the one hydrocarbon in the set, at 77 K, a T well below its m.p. (110.3 K [26]), as a glass or a solid, presents Shpolskii like conditions to the solute [27]. Even the rate of cooling can significantly alter the appearance of a solute spectrum in such cases [28,29].…”

Absorption and Fluorescence Spectra of some 1,3,4-Oxadiazole Derivatives in n-Paraffins at 77 K

“…Quasilinear absorption and luminescence spectra of simple aromatic and N-heterocyclic compounds in n-paraffins [1] and in similar solvents [2,3], and of more complicated compounds such as oxazole and oxadiazole derivatives [4 -6] Spectroscopic-grade n-hexane, decane and nonane were used as solvents. The apparatus used was similar to the one described in Ref.…”