Large Steric Substituent-Entangled Luminescence Enhancement under High Pressure

Pressure-induced emission enhancement is a new and effectivewayto manipulate the color of organic materials. However, the specificfluorescence enhancement mechanism and the causes of the color changehave not been previously well explored. Herein, two rofecoxib derivatives,MOX2 and Y7, were synthesized with methyl sulfonyl and cyan as theterminal groups on the para-position of the benzenering (Ph-), and their fluorescence behavior was investigatedat 0-20 GPa. The two compounds exhibit opposite pressure-tunedfluorescent behavior in the 0-3.1 GPa range. Theoretical calculationindicates that the -SO2CH3 group of MOX2directs the dihedral angle between the Ph- and Ph-ring planes to an optimized steric position, producing a large sterichindrance and neutralizing the influence of pressure on the benzenering, which results in the invariance of the fluorescence peak position.The change in this dihedral angle is mainly caused by the gradualformation of hydrogen bonds between the O atoms on -SO2 and the H atoms of the benzene ring, resulting in a 6.3-foldincrease in the fluorescence intensity. Conversely, the luminescenceintensity of Y7 is monotonously suppressed as the pressure is increaseddue to the smaller size of the -C VN substituent. Thisstudy provides a reference for regulating the luminescence propertiesof organic molecules.