Origin of Nonmonotonical Variation of Luminescence Efficiency under Pressure in Organic Molecule

The nonmonotonical variation of luminescence efficiency under extra pressure occurs frequently in organic molecules; however, the mechanism behind this is still elusive. Using a theoretical protocol combining thermal vibration function rate formalism coupled quantum mechanics/molecular mechanics models, we explored the relationship between extra pressure, molecular packing, and fluorescent quantum efficiency (FQE) of the representative 1,2,3,4-tetraphenyl-1,3-cyclopentadiene (TPC). It is found that the first increase and then decrease of FQE in TPC crystalline aggregates upon pressurization is cooperatively attributed to the continuous reduction of the radiative decay rate constant and nonmonotonical change of the nonradiative decay rate constant (kic). The initial decrease of kic originates from the effective suppression of electron-vibration coupling and the Duschinsky rotation effect by extra pressure, whereas the following increase of kic comes from the surge of nonadiabatic electronic coupling and the reduction of adiabatic excitation energy upon further compression. This study can provide a theoretical basis for the rational design and performance control of the piezochromic luminescent materials.