Photoisomerisation in single molecules of nitroprusside embedded in mesopores of xerogels

Photoswitchable hybrid materials are successfully prepared by embedding guanidinium nitroprussides (GuNP, (CN₃H₆)₂Fe(CN)₅NO]) into mesopores of transparent xerogel monoliths. The such prepared hybrid materials exhibit a higher photostability than the corresponding GuNP solutions, whereby the chemical stability of the Fe(CN)₅NO]²⁻-anion in titania gel is nearly infinite. By irradiation with light in the blue-green spectral range one nitrosyl isomer is formed by a 180° rotation of the NO ligand changing the Fe–NO into a Fe–ON coordination (SI), which is detected by the shift of the ν(NO) stretching vibration from 1945 cm⁻¹ (Fe–NO) to 1820  cm⁻¹ (Fe–ON). Consequently there is enough space around the NO-ligand for such movement in xerogel mesopores. The embedding in silica xerogels increases the achievable population of the isomeric nitrosyl configuration to about 15% with respect to a single crystalline powder where only 9% are reached.