Stepwise Conformational Restriction of Acylhydrazone‐Based Molecules for High Signal‐to‐Noise Fluorescent Recognition of Synthetic Cannabinoid FUB‐INACA

ABSTRACT Establishing explicit correlations among material dimensionality, luminescent properties, and sensing performance is of critical importance for the rational design of sensing materials with superior detection performance. Herein, employing 1 H ‐imidazole‐4, 5‐dicarbohydrazide (IDA) and triphenylamine (TPA) derivatives as the fundamental building blocks, we propose a stepwise conformational restriction strategy to construct acylhydrazone‐based discrete molecules (0D), linear polymers (1D), and covalent organic framework (COF, 2D) with progressively reduced conformational freedom. Upon interaction with FUB‐INACA through synergistic multiple non‐covalent interactions and hydrophobic effects, photoinduced electron transfer (PET) or intramolecular charge transfer (ICT) processes are activated, resulting in pronounced fluorescence modulation and enabling selective recognition. The higher‐dimensional TFPA‐IDA COF, featuring the most restricted conformational environment and the lowest baseline emission, delivers rapid (<1 s), sensitive (LOD, 1.3 nM), and high signal‐to‐noise detection with negligible interference from structurally analogous species. Furthermore, the reliability and applicability of TFPA‐IDA COF were validated through fabricating a portable microfluidic sensing chip, thereby confirming that the 2D framework holds considerable potential for the detection of residual FUB‐INACA in practical scenarios. Collectively, this work establishes dimension‐regulated conformational restriction as a generalizable model for background‐suppressed fluorescent recognition, offering a guiding principle for the rational design of next‐generation high‐fidelity sensing materials.