Ion-Specific Freezing-Induced NIR Phosphorescence: Interfacial Synergy Enables Imaging of "Invisible Ice"

Icing threatens the safety of aviation, power-transmission and wind-energy systems, yet concealed or transparent ice remains difficult to detect. Here we report a freezing-induced near-infrared (NIR) phosphorescence (FIP) imaging strategy based on aryl-substituted pyrrolo[3,2-b]pyrrole probes PP4P-X (X = F^-, Br^-, I^-, NO₃ ^-, and SCN^-). Across the PP4P-X series, freezing broadly amplifies the steady-state emission, whereas a NIR phosphorescence band at 750 nm enables deep-penetration, low-background imaging with pronounced counterion dependence. The FIP turn-on is strongest for PP4P-F, followed by PP4P-Br, switching from undetectable emission to intense phosphorescence. Mechanistic investigations reveal that specific adsorption of F^-/Br^- at the ice-water interface induces dense aggregation at the freezing front, strengthening molecular interactions to promote intersystem crossing and suppress triplet non-radiative decay. Leveraging this interfacial regulation, PP4P-F enables high-contrast, centimeter-scale ice imaging in diverse frozen media, with a 152-fold increase in signal-to-background ratio (SBR). In wind-tunnel aircraft icing tests, FIP imaging accurately maps the onset, thickness evolution, and downstream propagation of ice along the wing leading edge and correlates with laser-measured ice thickness. Overall, this work establishes a noncontact, in situ, and quantitative approach for "invisible ice" detection and provides a framework for NIR phosphorescent probes in frozen-phase monitoring.