Distinct mechanochromic and piezochromic behaviors of thermally activated delayed fluorescence 4CzTPN crystal

Incorporating mechano-luminescence into thermally activated delayed fluorescence (TADF) molecules represents a promising strategy for developing multifunctional mechanoluminescent materials. However, the controllable utilization of the versatile properties inherent in TADF molecules remains challenging due to the inherent difficulties in system design. Herein, we investigate the pressure modulation of the photophysical properties of 2,3,5,6-tetrakis(carbazol-9-yl)-1,4-dicyanobenzene (4CzTPN), a representative TADF emitter, through in situ high-pressure photoluminescence (PL), time-resolved PL, UV-visible spectra and infrared spectroscopy. The different mechanisms of a blue shift by grinding crystals and of a red shift under hydrostatic pressure are fully investigated. High-pressure time-resolved PL measurements show that the short fluorescence lifetime decreases and the lifetime of delayed fluorescence disappears beyond 3.0 GPa. Infrared spectra indicate the pattern of relative intensity for the infrared absorption peak of ν(C-H) transform significantly when the pressure is greater than 3.0 GPa, which would be the cause of the disappearance of delayed fluorescence. This study enriches the insights into mechanochromic and multifunctional materials.