Investigation on the fluorescence detection mechanism of NIR fluorescent probes based on intramolecular spiro cyclization.

Introduction: This study focuses on the detection mechanisms of recently developed NIR fluorescent probes that depend on ring formation and opening processes. A novel class of polymethine dyes (NIRII-RTs) serves as the core fluorescent moiety of these probes, which exhibit bright, stable, and anti-solvent quenching NIR-II emission, accompanied by large Stokes shifts. Methods: Quantum chemical calculation methods were employed to systematically analyze the light absorption and emission processes of three target-specific probes: NIR-pH (targeting H Results: The results demonstrated that the probes exhibit weak fluorescence in the closed spiro cyclization state. This weak emission is attributed to the interrupted π-electron distribution at the C-N bond of the reaction site, which facilitates electron transfer from the ground state to the excited state and restricts excitation to the benzene ring region. Upon reaction with target analytes, the spiro cyclization structure is disrupted, transitioning to a linear chain configuration. Discussion: The consistency between the calculated optical parameters and experimental data validates the proposed detection mechanism centered on spiro cyclization/ring-opening processes and associated changes in π-electron conjugation. This mechanism clarifies how the structural flexibility of the probes (driven by analyte binding) regulates their fluorescence properties, providing a theoretical basis for the rational design of high-performance NIR-II fluorescent probes with tunable optical responses. Future work may leverage this mechanism to develop probes for a broader range of analytes, further advancing their utility in biological imaging and environmental monitoring.