Ion‐Specific Freezing‐Induced NIR Phosphorescence: Interfacial Synergy Enables Imaging of “Invisible Ice”

Abstract

ABSTRACT 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 − , NO 3 − , 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.