Light-induced molecular motion of azobenzene-containing molecules: a random-walk model

International audienceSince it was first evidenced in 1995, light-induced mass motion in layers of azobenzene-containing molecules has led to diverging interpretations, and it remains partly unexplained. In this paper, we discuss a light-driven random-walk model where moving chromophores drag the molecule to which they are grafted. It consists in a diffusion motion of the azobenzene functions where each random step follows an isomerizing absorption. After a summary of the main characteristics of the motion, we present the hypotheses of the model and we show how it suits the experimental observations reported. In the frame of this model, where each azobenzene function is put in motion by light, we assess the distance over which an azobenzene-containing molecule can be dragged. We also estimate the energetic output of this dragging process. Finally, we discuss the microscopic origin of these molecular motors and we compare it to the model of thermal ratchets introduced by Feynman and extensively resorted to in Biology nowadays. (Some figures in this article are in colour only in the electronic version) In 1995, Rochon et al [1] showed that a thin layer of an azobenzene-containing material subjected to a low-intensity modulated blue/green light pattern is significantly altered: an initially flat film at the nanometre scale can reach a corrugation whose amplitude may be up to fifty per cent of the initial thickness. This suggests that the molecules are put in motion upon illumination. The most commonly used setup to induce mass motion is an interference device with which a sine modulated intensity pattern is cast on a layer at the micron scale. As a result, the free surface is deformed into a surface relief grating, the periodicity of which is equal to that of the interference pattern. Figure 1 is an AFM image of a surface relief grating inscribed on a PMMA DR1 layer. Ten years after this light-induced motion was evidenced, its origins remain a confused and divisiv