The study of phase transitions in organic crystals as a step to tunable modification of their physicochemical properties is in the center of modern crystal engineering approaches. Organic crystals with polyiodide anions and S,N-containing heterocycles, such as the substituted thiazolo(azino)quinolinium salts [1-3] open possibility for the design of wide row of nonlinear optical and semiconductor materials and serve as components of dye-sensitized solar cell devices. Variety of crystal structures, available for material engineering is due to their ability to form different types of non-covalent interactions with iodine participation. The methodology of the present work includes consistent analysis of X-Ray diffraction and Raman spectroscopy data accompanied by the results of periodic quantum-chemical calculations in order to reveal the changes in crystal packing, non-covalent interactions feature and spectral properties as a result of undergoing phase transition under non-ambient conditions. In the center of the present work is the case of two newly obtained crystal structures of substituted thia- and oxazinoquinolinium iodides with the typical I…I halogen bonds. Substituted thiazinoquinolinium monoiodide undergoes low-temperature phase transition with decrease of symmetry from P21/c to P-1 registered by X-Ray diffraction and Raman spectroscopy. The interpretation of the observed changes in the Raman spectra is made on the basis of theoretic spectra in low wavenumber region.

Acknowledgments: This work was supported by the Ministry of Science and High Education of the Russian Federation, FENU 2020-0019

1. Bartashevich E.V.; Yushina I.D.; Stash A.I.; Tsirelson V.G. Crystal Growth & Design, 2014, 14 (11), P. 5674-5684

2. Yushina I.D.; Kolesov B.A.; Bartashevich E.V. New Journal of Chemistry, 2015, 39 (8), P. 6163-6170

3. Yushina I.D., Tarasova N.M., Kim D.G., Sharutin V.V., Bartashevich E.V. Crystals, 2019, 9, P. 506.