Solid-State Photoswitching of Hydrazones Based on Excited-State Intramolecular Proton Transfer

The development of new photochromic systems is motivated by the possibility of controlling the properties and functions of materials with high spatial and temporal resolution in a reversible manner. While there are several classes of photoswitches operating in solution, the design of systems efficiently operating in the solid state remains highly challenging, mainly due to limitations related to confinement effects. Triaryl-hydrazones represent a relatively new subclass of bistable hydrazone photoswitches exhibiting efficient Z/E photochromism in solution. As "large volume" photoswitches, they have been anticipated to display only limited solid-state photoswitching. Here, we show that the Z isomers of newly prepared triaryl-hydrazones containing a perfluorinated hydrazine phenyl ring (PHZs) exhibit impressive solid-state photochromism with an unexpected light-induced red-shift of the absorption maximum. Based on (time-dependent) density functional theory calculations, a photoswitching reaction mechanism involving the excited state intramolecular proton transfer has been proposed, which rationalizes the observed red-shift in absorption by the formation of a metastable proton transfer structure. Advanced experimental techniques including X-ray diffraction, solid-state NMR and EPR spectroscopy, and confocal Raman microscopy corroborated the suggested mechanism and revealed that the observed photochromism is a superficial phenomenon. This atypical photochromic behavior of PHZs can also be realized by using visible light and in the form of thin films, which manifests their potential use in optics and optoelectronics.