Elongated Lifetime of Unstable Colored Species by Intermolecular Hydrogen Bond Formation in Photochromic Crystals

The crystallization of N-3,5-di-tert-butylsalicylidene-3-carboxyaniline (1) from a methanol solution gave three crystal forms, 1 alpha, 1 beta and 1 gamma. The colorless 1 alpha and pale yellow 1 beta turned red when they were irradiated with UV light whereas orange 1 gamma showed no color change. The lifetimes of the colored crystals were estimated to be 17 and 780 min at room temperature for 1 alpha and 1 beta, respectively, from IR spectra. The structure of the colored 1 alpha crystal was analyzed by X-ray at 93 K. In addition to the original enol form of 1, the trans-keto form appeared on the difference electron density map with an occupancy factor of 0. 117(2). This indicates that the color change was caused by the structural change from the enol to trans-keto form, which is the same as the color change previously reported in the crystal of N-3,5-di-tert-butylsalicylidene-3-nitroaniline (2) (lifetime, 1200 min). It was made clear from the structures of the trans-keto forms in the crystals of 1 alpha, 1 beta, and 2 that the lifetime of the unstable colored form should be determined by the possibility and the strength of the intermolecular hydrogen bond formation of the N-H group of the trans-keto form to the neighboring molecules.