Abstract
The energetic structures and conformations of trimethine cyanine dye molecules were investigated. For research, group theoretical and quantum chemical calculation methods were used. The theoretical group analysis of electronic and vibrational structure of molecules was carried out. Also, the energetic structures and conformations of the molecule of this dye were studied. Research shows that the investigated molecule may reside in three different conformational states, one of which is highly symmetric (symmetry C2v) and the other two with low symmetry. The third conformer is characterized by lowering of binding energy of the electronic system by 0.23 eV, and the long-wavelength absorption band is shifted to lower energies. Also the group theoretical analysis of the trimethine cyanine molecule had allowed systematizing the vibrational and electronic quantum transitions and identifying the bands in the absorption spectra. It is shown that the excitation of the molecule in S1-state causes trans-cis-isomerization. The presence of the barrier of ~0.1 eV allows the fluorescence process to compete with isomerization process, but isomerization causes a decrease in the fluorescence quantum yield of the dye.
Highlights
The absorption spectra and fluorescence spectra of solutions of polymethine dyes have been studied quite extensively and described in scientific papers [1,2,3,4,5,6], since this class of dyes is widely used in practice [7, 8]
We present the structure of four molecular orbital (MO) of those listed in Table 3
We have considered above the totally symmetric vibrations of molecule (I), which will give the structure of longwavelength band in the absorption spectrum
Summary
The absorption spectra and fluorescence spectra of solutions of polymethine dyes have been studied quite extensively and described in scientific papers [1,2,3,4,5,6], since this class of dyes is widely used in practice [7, 8]. The energy structure of the molecules of these dyes was not enough studied, which does not allow us to definitely interpret the fluorescence spectra of the higher excited states that are typical of this class of dyes. The conformational structure and way of the transition of the molecule from one conformation to another which affect the quantum yield of fluorescence were not described in detail. Aims of this work are to describe the results of the investigation of excited states and conformational transformations, for example, model of the molecule, Dioksazoltrimethinecyanine cation (I) (see Scheme 1)
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