Conjugated polymer intrachain donor-acceptor reconfiguration-tailored off-on NIR-II photoacoustic probes with ultralow background for high-fidelity imaging of immunotherapy-associated macrophage reprogramming.

High-fidelity imaging of pro-tumorigenic M2 macrophages reprogramming into anti-tumorigenic M1 phenotypes during tumor immunotherapy is crucial for guiding personalized treatment. Off-on probes, switching from low "off" state signals to high "on" signals upon biomarker activation, enable visualization of this process. However, most existing off-on probes fail to deliver high-fidelity imaging, as even low "off" state probe signals are amplified by extensive intratumoral accumulation without biomarker activation, producing a high probe background that is indistinguishable from biomarker-triggered signals and prone to false-positive diagnoses. Here we report semiconducting polymer intrachain donor-acceptor reconfiguration-tailored off-on NIR-II (1000-1700 nm) photoacoustic probes (BDPNP) with ultralow background for high-fidelity imaging of immunotherapy-associated macrophage reprogramming. BDPNP undergoes polymer intrachain donor-acceptor reconfiguration-induced large absorption redshift from a short wavelength (no NIR-II photoacoustic background) to a long wavelength (robust NIR-II photoacoustic signals) under the macrophage reprogramming biomarker nitric oxide. BDPNP shows ultralow background even after extensive tumor accumulation without immunotherapy, but signals amplify 21.4-fold upon immunotherapy. The design clearly distinguishes biomarker-triggered signals from probe background in vivo, enabling high-fidelity mapping of macrophage reprogramming in tumor immune microenvironment. This work establishes a general strategy for ultralow-background off-on NIR-II photoacoustic probes and provides a powerful tool for high-fidelity imaging of cancer immunotherapy response.