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 N-1-Me in the indoline fragment of salt SP(II)I-+(-) by N-1-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-+(-). (c) 2006 Elsevier B.V. All rights reserved.