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

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 1H-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.