Is it possible to maintain photomechanical properties of crystalline diarylethenes after thermal amorphization?

Abstract

The photomechanical properties of the photochromic 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene (DTE) crystals, largely documented since the discovery by Irie et al. of its reversible photoswitching of crystallographic plane morphology, is renewed with a novel approach consisting on suppressing gradually dynamical effects through thermal amorphization. In the crystalline case, the reversibility of UV/Visible induced plastic deformations have been observed as expected. In addition, a number of large jumps for both cycles are reported for the first time for this molecule in contrast to being observed only during the first UV irradiation cycle for other DTEs. In the amorphous case, several striking results have been obtained: (i) the still present reversible photoinduced plastic deformations mimicking the crystalline case are interpreted in terms of apparent polyamorphism; (ii) the total vanishing of the jumps brings the evidence of the crucial role of the crystallinity for the propelling surface in contact with the substrate; (iii) the discovery of a never reported material jump during the first visible cycle. This last result is rationalized combining two features –a thermal one, a glass transition temperature very close to the room temperature (Tg ≈ 34°C) and –a photochemical one, an inefficient photoreversion allowing the other vibrational relaxation pathway to proceed inducing the required temperature rise to overpass the phase transition (Solid → Liquid) responsible for the jump.