Research On The Structure Change Of Several Conjugated Molecules Under High Pressure By Fluorescence And Raman Spectroscopies

A new type of luminescent materials with fluorescent color or intensity changingupon the stimulation of external pressure is called piezochromic material. What’smore, some piezochromic materials also have aggregation-induced emission (AIE) oraggregation-induced emission enhancement (AIEE) property. This material that hasboth piezochromism property and AIE/AIEE property is called PAIE material.Recently, comparing the change of the fluorescence emission before and aftergrinding is major research on the piezochromism property of material, and themechanism of mechanofluorochromism is attributed to the phase transition fromcrystalline to amorphous state upon grinding due to the disappearance of XRDpatterns. However, there are few reported studies on the change of the molecularstructure and fluorescence emission under higher pressure. So in this article, weresearch on the different change of molecular structure and fluorescence emission ofseveral typical organic conjugated molecules via fluorescence and Ramanspectroscopies under shearing force and hydrostatic pressure. The main researchcontents include the following aspects:1. Tetraphenylethylene (TPE) has only AIE property, tetra(4-methoxyphenyl)ethylene (TMOE), a methoxy-substituted tetraphenylethylenederivative, has AIE and piezochromism properties. We compared the differentfluorescence change mechanism of TPE and TMOE under shearing force andhydrostatic pressure, and research the molecular structure change TMOE viahigh-pressure Raman spectra and theoretical simulation Raman spectra. We foundthat the methoxy in TMOE molecular is the fundamental reason for the differentfluorescence change of TPE and TMOE under pressure, and the change of thedihedral angle is the main factors resulting in the piezochromism property of TMOE.2. Triphenylamine (TPA) is a typical nonplanar molecular, compared with otherelectron donor materials, TPA has a higher HOMO energy level, thus, TPAderivatives are a kind of important organic light-emitting materials. We chose TPA asa model molecule to study different molecular configuration and fluorescence changesunder different external stimuli (such as solution, mechanical grinding and hydrostaticpressure). We found that the fluorescence intensity of TPA appeared obviousenhancement induced by the restricted free rotation of benzene ring under bothaggregation state and hydrostatic pressure.3. Some TPA derivatives with a D-A twisted molecular structure have a newemission state: hybridized local and charge transfer state (HLCT) inducing TPAderivatives to have high fluorescence emission intensity. And there is a directrelationship between the presence of HLCT and the dihedral angle in molecular. Wechose three TPA derivatives with different chromophore (TPA-AN, TPA-PA,TPA-NA) as the model molecules to study fluorescence and Raman spectroscopyunder different external stimuli (such as solution, mechanical grinding and hydrostaticpressure). We found that the distribution of charge transfer state is the main reason offluorescence emission intensity and peak position of TPA derivatives rather than thebenzene ring free rotation. And the distribution of charge transfer state depends on thedihedral angle between the TPA and substituent.