The molecular and crystal structure of single crystalline 5´ hydroxymethyl 1,3,3,7´,8´ pentamethylspiro[indoline 2,2´ [2H ] pyrano[3,4 b]pyridinium] iodide (1) was studied. The crystal structure is composed of cation layers separated by iodide ions and provides a sufficient volume for the phototransformation of compound 1. The spectroscopic and photochromic properties of this compound in the crystalline state were investigated. Upon irradiation with light at 270-370 nm, up to 600 layers of single crystals are subjected to photochromic transfor mations. The quantum yield of formation of the open form is ~0.1, and the quantum yield of decoloration is 0.04. After ten reversible photocyclizations, the absorption of the colored form was decreased by 10%. The photochromic properties were related to the single crystal struc ture.Key words: spiropyran salt, spiro[indolinepyranopyridinium] iodide, single crystal, photo chromic properties.In recent years, solid state photochromic transforma tions of spiropyrans have attracted considerable interest not only from the viewpoint of the design of new materi als for optical information storage and processing 1,2 but also for the preparation of new polyfunctional materials combining different properties in a crystal lattice. 3,4 In these crystals, photoinduced transformations can give rise to new functions (for example, photoswitching of nonlin ear optical properties 5 ). However, photochromic trans formations in crystals of spiropyrans and spirooxazines are rather hindered due to the close packing of the mol ecules in the crystal lattice and also because these phototransformations, which are associated with the C spiro -O bond cleavage followed by isomerization of the molecule, require a large free volume. As a rule, spiro pyrans and spirooxazines in microcrystalline powders do not exhibit the photochromic properties under conditions of stationary irradiation. However, the use of intense femtosecond laser irradiation leads to the formation of a short lived photocolored form in microcrystals, whose absorption spectrum is identical to the spectra of the photo forms of spiropyran and spirooxazine in solutions. 6 Recently, pyridinium containing salts of spiropyrans and spirooxazines, which possess the photochromic prop erties in the crystalline state, have been synthesized. 3,7,8 Study of ultrathin pressed microcrystalline samples dem onstrated that they become colored under continuous UV irradiation. The constant of thermal decoloration of the resulting photo form (k) is 10 -4 -10 -8 s -1 . The unusual behavior of the crystals of spiropyrans and spirooxazines was attributed to the electron withdrawing effect of the positively charged N methylpyridinium ring and/or the characteristic features of the crystal packing of salts of spiropyran and spirooxazine. To correctly interpret the properties of these compounds, it is necessary to study the structures of their single crystals and photochromic trans formations occurring in the crystals.In the present study, we synthesized cationic sp...
