Large Steric Substituent-Entangled Luminescence Enhancement under High Pressure

Pressure-induced emission enhancement is a new and effective way to manipulate the color of organic materials. However, the specific fluorescence enhancement mechanism and the causes of the color change have not been previously well explored. Herein, two rofecoxib derivatives, MOX2 and Y7, were synthesized with methyl sulfonyl and cyan as the terminal groups on the para-position of the benzene ring ② (Ph-②), and their fluorescence behavior was investigated at 0–20 GPa. The two compounds exhibit opposite pressure-tuned fluorescent behavior in the 0–3.1 GPa range. Theoretical calculation indicates that the −SO2CH3 group of MOX2 directs the dihedral angle between the Ph-② and Ph-③ ring planes to an optimized steric position, producing a large steric hindrance and neutralizing the influence of pressure on the benzene ring, which results in the invariance of the fluorescence peak position. The change in this dihedral angle is mainly caused by the gradual formation of hydrogen bonds between the O atoms on −SO2 and the H atoms of the benzene ring, resulting in a 6.3-fold increase in the fluorescence intensity. Conversely, the luminescence intensity of Y7 is monotonously suppressed as the pressure is increased due to the smaller size of the −C≡N substituent. This study provides a reference for regulating the luminescence properties of organic molecules.