Pressure-Induced Enhancement of Room-Temperature Phosphorescence in Heavy Halogen-Containing Imide and Polyimide.

To investigate the correlation between the aggregated state and photoluminescence (PL) mechanism of dual fluorescent (FL) and phosphorescent (PH) polyimides (PIs), the photophysical processes of FL-type , PH-type , and their corresponding imide model compounds ( and ) dispersed in poly(methyl methacrylate) (PMMA) films were analyzed at elevated pressures up to 8 GPa using a diamond anvil cell. Dibromo-substituted demonstrated a shorter wavelength absorption than owing to the larger dihedral angle in the biphenyl moiety. Both MCs exhibited red-shifts in their absorption spectra with increasing pressure, indicating planarization occurred at the biphenyl moieties associated with the compression of the free volume in PMMA. The PL intensity of increased with increasing pressure, while its quantum yield (Φ) decreased sharply due to the enhanced energy transfer via the Förster mechanism. In contrast, the PH quantum yield (Φ) of monotonically increased at lower pressures, while it showed excitation wavelength-dependent behaviors at higher pressures: Φ remained unchanged under excitation at 340 nm but gradually increased under excitation at 365 nm. This fact suggests that, at higher pressures, 365 nm excitation promoted intersystem crossing (ISC) from excited singlet states at higher energy levels. Using this phenomenon, a significant pressure-induced PH enhancement (PIPE) was observed for up to 0.9 GPa upon excitation at 365 nm, which is a rare phenomenon for organic polymers. This study indicates that even in colorless and optically transparent amorphous polymers, an enhancement of PH due to restricted molecular motion and intensified ISC outweighs the deactivation due to intermolecular energy transfer under certain pressures, leading to an increase in Φ.