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

TY - JOUR

T1 - Ion-Specific Freezing-Induced NIR Phosphorescence

T2 - Interfacial Synergy Enables Imaging of “Invisible Ice”

AU - Cao, Yanyan

AU - Wu, Jiahui

AU - Zhang, Chuanbiao

AU - Yan, Yifei

AU - Zhang, Kai

AU - Li, Gengchen

AU - Zhu, Zeyu

AU - Xu, Ye

AU - Zhang, Wenqiang

AU - Dong, Yuping

AU - Chen, Xiaofei

AU - Cai, Zhengxu

AU - He, Zhiyuan

N1 - Publisher Copyright: © 2026 Wiley-VCH GmbH.

PY - 2026/5/25

Y1 - 2026/5/25

N2 - 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.

AB - 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.

KW - freezing-induced near-infrared phosphorescence

KW - ice-interfacial interactions

KW - invisible ice detection

KW - ion-specificity

KW - ion-π interactions

UR - https://www.scopus.com/pages/publications/105035689506

U2 - 10.1002/anie.7443864

DO - 10.1002/anie.7443864

M3 - Article

AN - SCOPUS:105035689506

SN - 1433-7851

VL - 65

JO - Angewandte Chemie - International Edition

JF - Angewandte Chemie - International Edition

IS - 22

M1 - e7443864

ER -