Structures of Two Spiropyrans in the Open and Closed Form

The crystal structure of the photochromism compound 3-{3,3-dimethyl-6'-nitrospiro[2'H-chromene-2,2'-(2,3-dihydro-1H-indole)]-1-yl}-propanoic acid, SP1 , has been characterized by X-ray crystallography. We compared it with the crystal structure of 3-{2-[2-(3,5-dinitro-2-oxidophenyl)ethyl]-3,3-dimethyl-3H-indol-1-ium-1-yl}-propanoic acid. SP2 solvated with two DMF molecules that was obtained earlier. We found that great differences existed between them. Such differences can also be easily found by their [ 1 H]NMR and IR spectra. Their photochromic behaviors in solutions were investigated by UV-vis absorption with the aid of ultraviolet irradiation, leading to the conclusion that SP1 tended to the closed form while SP2 tended to be the open form both in solution and in the solid state.