The influence of an N-substituent in the indoline fragment of pyrano-pyridine spiropyran salts on their crystalline structure and photochromic properties

The crystalline structure of spiro[indoline-pyranopyridinium] salt (SP(I)+I−) has been determined by X-ray method at temperatures 300 and 200 K. By using powder diffraction method, variations of the intensity of X-ray peaks have been observed. They are referred to the re-arrangement in the crystalline structure, which occurs during the reverse photochemical reaction in the crystalline phase. The influence of substituents on the crystalline structure of salts SP+X− and their photochromic properties has been studied. Replacement of N1–Me in the indoline fragment of salt SP(II)+I− by N1-i-Pr (to yield salt SP(I)+I−) was shown to result in the essential change of the crystalline structure of SP+I− salts. SP(II)+I− has a denser packing of a parquet type than SP(I)+I−, which has a layer-by-layer packing with planar pyrano-pyridine fragments. SP(I)+I− structure provides a higher mobility of the pyran ring, this being apparent in increasing the constant of dark bleaching and cyclicity of SP(I)+I− as compared to SP(II)+I−.