Recently, the use of high pressure has proven to be an effective approach to uncover the relationship between the crystal structure and material properties. In organic molecules, simple compression can effectively alter the molecular packing through changing molecular distances, conformations, and crystal structures. Such alterations impact intermolecular interaction of the organic molecules and the strength of intermolecular interactions determine the physical properties of those materials. Here, we introduce recent high-pressure investigation results of a large Polycyclic Aromatic Hydrocarbons (PAHs), where we employed various spectroscopic technique, structure analysis, and physical property measurements to obtain deeper insight of the structure-property relationships.
     
PAHs are known as both carbon-rich and hydrogen-rich organic molecules and their outstanding tuneability of crystal structure and molecular packing have been attracting continuous research interest for their possibilities for technological application, such as organic optoelectronic and photovoltaic devices, semiconductors, and nanoelectronics. In general, PAHs are wide band-gap insulators and their band-gap can be reduced via different ways, for instance, enhancement of intermolecular interactions under simple compression. We observed that the perturbation to the electronic band-gap energy of the electronic transition in PAH molecules directly influence physical properties. For instance, the optical band-gap energy of the coronene (C₂₄H₁₂) at ambient pressure is 2.85 eV, which can be reduced to 1.35 eV upon compression, which was accompanied by full-color piezochromic luminescence under UV-light (Figure below). During compression, we discovered that the tuneable piezochromic luminescence of coronene is strictly related to the intermolecular interactions, that is, crystal structure transitions observed are associated with each emission color changes from blue to green, and to red. We combined in-situ photoluminescence, UV-visible absorption, Raman, infrared spectroscopy, and synchrotron powder XRD measurements to demonstrate their outstanding tuneability and high stability of coronene crystal structure allows significant modifications of chromaticity of their emission.  
   
The information obtained during high pressure investigations not only sheds light on the structure-optical property relationships of hydrocarbon, but also paves the way to develop a single molecular system that exhibit RGB emission even at ambient conditions. We developed chemical modification of the band-gap energy and excited-state dynamics of coronene to realize RGB emission without applying high pressure. Single-component materials with such optical properties, which can be prepared without complex and costly organic synthesis procedures, are an important discovery for application in various optoelectronic materials.
 

high pressure,Hydrocarbon,wide bandgap,photoluminescence,organic carbon