Mechanoluminescence of Polymer–Organic Composites under Strain and Hydrostatic Pressure

The development of underwater-based Internet of Things has drawn much attention to fluorescence sensors for crucial technological advancement. Therefore, the technologies for underwater mechanoluminescence (ML) sensing have created various applications for sensors and self-powered waterproof displays. However, developing single-molecule-based intrinsically adaptive materials capable of responding to multiple stimuli with high sensitivity remains a challenge. Herein, a flexible sensor based on the fluorescent ligand DHN was fabricated using a PVDF polymer matrix to form PVDH, enabling the exploration of external-stimuli-responsive fluorescence enhancement under mechanical strain and underwater pressure. For PVDH, the transition from crystalline to amorphous state by mechanical stimuli boosted its photoluminescence by ∼75%, and stretching (strain ∼16%) the sensor boosted its photoluminescence by ∼145%, which is due to the formation of molecular aggregates in the amorphous state. Additionally, the increase in ML correlated with an output voltage of ∼4 V of the fabricated device under mechanical stress. The in-depth density functional theory (DFT) calculations further support the experimental observation by studying the charge distribution and orbital overlapping in the DHN ligand. Furthermore, the film shows clear visualization when subject to underwater pressure and stable fluorescence upon exposure to different impurities. Therefore, this study reveals the use of fluorescence and mechanochromic properties of the ligand for designing advanced underwater sensors for visualization and communication.