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 (1) and 5'-hydroxymethyl-1,3,3,7',8'-pentamethyl-spiro[indoline-2,2'-[2H]pyrano[3,4-b]pyridinium] iodide (11). The crystalline structure of salts I and 11 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 11 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. (c) 2005 Elsevier B.V. All rights reserved.