Structural Evolution And Optical Properties Of Fluorenone Based Organic Luminescent Materials Under High Pressure

Pressure is an efficient tool to regulate the molecular configurations,the molecular arrangements and the intermolecular interactions.Applying high pressure to organic molecular crystal materials is a new method to explore their special properties and structure-property relationships.Recently,fluorenone based luminescent materials have gained much attention for their diversity of crystal structures and ease of modification.In this research,we combined high-pressure techniques with fluorenone based luminescent materials to explore the new pressure-responsive optical system.By applying high pressure to diphenylfluorenone crystals,we found that their UV-vis absorption and photoluminescence spectra exhibited consecutive,extensive and reversible switching behaviors under high pressure in the entire visible light region.In particular,the photoluminescence spectra revealed a redshift of over 300 nm with the emission peak shifted to the near infrared region under 13.3 GPa.In-situ high-pressure infrared spectra and synchrotron X-ray diffraction analysis showed that diphenylfluorenone undergone a reversible ? to ? phase transition at 3 GPa.The packing modes of phenyl groups in the crystal structure changed from a vertical way to a parallel way.Time-dependent density functional theory calculation indicated that the molecular orbitals of the ?-phase exhibited obvious overlap after the phase transition,which caused the enhancement of the intermolecular interaction and thus the large-scale redshift of the absorption and emission.In order to further study the modulation effect of the pressure on the crystal structure and optical properties of organic crystal,we applied high pressure to dinaphthalenyfluorenone with longer conjugated structures.The in-situ UV-vis absorption and photoluminescence spectra of 2,7-di(naphthalen-1-yl)-fluorenone(1-DNFO)showed the continuous,large-scale and reversible redshift under high pressure.Synchrotron X-ray diffraction analysis suggested that the 1-DNFO did not experience any phase transition under high pressure.The redshift of its absorption and emission came from the reduction of the unit cell volume as well as the intramolecular dihedral angle and the enhancement of the intermolecular interaction.Compared to 1-DNFO,its analogue2,7-di(naphthalen-2-yl)-fluorenone(2-DNFO)showed the smaller-scale redshift of absorption and emission,due to the difference in the molecular packing modes.By time-dependent density functional theory calculation,the molecular orbitals of 1-DNFO exhibited the strong intermolecular interaction and obvious intermolecular energy transfer under high pressure,which was responsible for the large-scale redshift of absorption and emission.This research reveals the structure-property relationship of organic molecular crystals under high pressure and highlights the uniqueness of the pressure in adjusting the properties.Especially,we find that the crystal structure of organic molecules with effective ?···? packing modes are particularly sensitive to the pressure in modulating the properties,which provides guidelines for future design and development of optical devices based on molecular materials pressure sensors.