Mechanical characteristics and mechanism of the triboluminescence of fluorescent molecular crystals

Abstract

The mechanical characteristics of triboluminescence (TL) from coumarin, acenaphthene, resorcinol, phenanthrene, and m-aminophenol crystals are studied. The TL spectra of these crystals resemble their fluorescence spectra. The TL appears concurrently with the sudden force decreases in the fracture regions of the force vs compression curves of the crystals. The decay time of TL from the motion of a single crack is of the order of a microsecond. The dependence of the TL intensity on time, impact velocity, the applied stress, and the crystal dimensions is discussed quantitatively using a phenomenological theory based on the movement and interaction of cracks in the crystals. No systematic correlation of the normalized TL intensity with the photoluminescence efficiency, charge produced during crystal fracture, or the experimental fracture stress is found. No considerable differences are found between the TL intensities of crystals grown in argon and in nitrogen atmospheres when they are crushed inside argon-stripped liquid or in air. Triboluminescence is an intrinsic property of the crystals and is discussed in terms of the piezoelectrification produced during the movement of cracks in the crystals.