Structure and photochromic properties of a single-crystalline spiro[indolinepyranopyridinium] salt

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 transformations. The quantum yield of formation of the open form is similar to 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 structure.