Electrical Transduction of Phase-Transition Mechanochromism in Fluorenylidene–Acridane Organic Thin-Film Dielectric Devices

A pressure-sensitive device in the field of mechanochromic materials is accomplished by dimethoxyfluorenylidene–octylphenylacridane exhibiting phase-transition mechanochromism, enabling electrical transduction of mechanical stimuli in organic thin films. One of the derivatives shows visual mechanochromism from blue in the crystalline state to green in the amorphous state upon applying mechanical pressure. The mechanochromism is attributed to the anisotropic molecular orientation after characterization by techniques such as ultraviolet–visible spectroscopy, theoretical calculation, X-ray diffraction, and birefringence observation. Furthermore, multiple-angle incidence resolution spectroscopy measurements reveal that the molecules in the blue thin film adopted an edge-on orientation, which is different from the molecular orientation in the green thin film. Organic dielectric devices with a blue or green thin film are fabricated and their electrical properties are assessed by alternating-current impedance measurements. According to Cole–Cole plots, the electrical resistance of the blue thin film is lower than that of the green thin film. The electrical resistance of the green thin-film device approaches that of the blue thin-film device as the applied mechanical pressure increased. Within the investigated thickness range (2–4 μm), the impedance response showed no substantial dependence on film thickness. XRD and Raman measurements indicate that pressure induces short-range molecular ordering in the green film, although complete conversion to the blue crystalline phase was not established. This study provides the first demonstration regarding the pressure-sensitive devices using mechanochromic materials. Also, this study demonstrates a proof-of-concept for electrically readable, mechanosensitive organic thin films and suggests their potential use as an abnormity sensor in a pressure vessel or a recording indicator in high-pressure processing.