Structure and photochromic properties of single crystals of spiropyran salts

Molecular and crystalline structure has been studied for the single crystals of neutral spiropyran (SP) 1,3,3,6′-tetramethyl-spiro[indoline-2,2′-[2H]pyrano[3,2-b]pyridine] (Ia) and its salts, 1,3,3,5′,6′-pentamethyl-spiro[indoline-2,2′-[2H]pyrano[3,2-b]pyridinium] iodide (I) and 5′-hydroxymethyl-1,3,3,7′,8′-pentamethyl-spiro[indoline-2,2′-[2H]pyrano[3,4-b]pyridinium] iodide (II). The crystalline structure of salts I and II differs from that of the neutral SP being formed by the cationic layers separated by I− ions, thus providing a sufficient volume for spiropyran phototransformations. Photochromism of I and II single crystals has been first studied. The quantum yield of the open form formation is about 0.1, and of bleaching 0.04. Ten-fold reversible photocycling reduces absorption of the colored form by 10–15%. Photochromic properties have been correlated with the single crystals structure.