Dichroism of absorption and polarization of phosphorescence of molecules of the closed form of nitrobenzospirothiopyran

Abstract

The spectral-polarization characteristics of absorption and phosphorescence of molecules of the initial form of nitro-substituted indolinospirobenzothiopyran were studied in oriented polyethylene fdms and in solutions with different polarity. An oscillator model of the electron transitions responsible for the formation of absorption and luminescence spectra was suggested. It was established that the principal differences in the spectral and photophysical properties of the compound studied and its oxygen-containing analog are associated with the fact that the electronegativity of the S atom is lower than that of the O atom.Key words: spirobenzopyrans, phosphorescence, polarization, dichroism.The 1,3,3-trimethyl-6 "-nitrospiro(indoline-2,2 "-[2H-1]benzothiopyran) (STP) molecule exhibits pronounced photochromic properties. When irradiated with UV light, for example, the STP molecule is transformed from the initial state A into the colored form B, which absorbs light intensely in the visible region of the spectrum. In studies of STP, the colored form B has recieved the most attention, 1-5 because this form is characterized by a substantially larger bathochromic shift of the maximum of the long-wavelength absorption than the oxygen-containing analog, namely, nitrospirobenzopyran (SBP). However, replacement of the O atom in the benzopyran fragment of the spiro compound should also affect the photophysical and photochemical properties of the initial form A, which, in turn, should be seen in a change in the characteristics of the luminescence spectrum compared to those of the SBP molecule. The absorption and luminescence spectra of nitro-substituted spiro compounds consist generally of structureless overlapping bands, which are difficult to analyze. Therefore, the results of studies of the anisotropy of light absorption and emission are of importance. Based on these results, one can infer the nature of electron transitions and the orientations of their moments relative to the atomic core of the molecule as well as the mechanism of the spine Me NO 2 STP orbital interaction. In this work, we studied for the first time the dichroism of absorption of STP and the polarization of phosphorescence of the closed form of STP molecules in oriented polyethylene films and in solutions with different polarity, and compared our results with the data on its oxygen-containing analog, namely, SBP.S-s Experimental 1,3,3 -Trimethyl-6 "-nitrospiro(indoline-2,2 ' -[2/-/-I ]benzothiopyran) was synthesized and purified according to the known procedure. 4 Polyethylene was purified by extraction in boiling chloroform. The purities of the samples were confirmed by the absence of impurity luminescence. STP molecules were introduced into polyethylene according to a procedure reported previously. 9 Stretching of polyethylene ffdrns, determination of the degree of dichroism, and calculations of the directions of the moments of electron transitions were carried out analogously to the procedure reported in Ref. 8 in which the dichroism of absorption of ...