Demystifying Conformational Preorganization in Pyrene-Imidazopyridine Probes: Complementary Roles of π-Spacer Integration and Zn2+ Templating in Sequential Cu2+ Recognition

The analytical performance of fluorescent chemosensors is fundamentally governed by the interplay among intramolecular charge transfer efficiency, coordination-site accessibility, and receptor−analyte binding affinity. However, in compact fluorophore−receptor scaffolds, planar fluorophores and sterically demanding receptor units can impose torsional strain, attenuating donor−acceptor electronic transitions while restricting analyte access to the coordination cavity. To delineate this structure−performance relationship, a sterically congested D−A probe was investigated as a structural counterpart to its phenyl π-spacer-integrated D−π−A analogue. The π-spacer serves as an internal preorganization unit that relieves steric strain, preserves near-global coplanarity, improves receptor accessibility, and promotes ICT characteristics. As a result, the D−π−A probe exhibited stronger affinity toward Cu2+ and Zn2+ ions with superior detection limits of 1.86 nM and 0.85 nM, respectively. Complementing the spacer-mediated internal preorganization, Zn2+ coordination provides an external supramolecular template that organizes the receptor into a binding-competent geometry for sequential Cu2+ recognition. This template effect was more pronounced for the sterically congested D−A probe, increasing its apparent Cu2+ binding constant by nearly 1 order of magnitude (1.59 × 105 M−1) with an improved detection limit of 1.06 nM. Nevertheless, the D−π−A + Zn2+ complex remained the most sensitive system, achieving an enhanced detection limit of 0.72 nM. Furthermore, the practical applicability of the probes was demonstrated by their reliable performance in mixed aqueous media, sensitive recognition of Cu-GHK, spike-recovery studies, and robust performance in both fluorescent paper-strip assays and live-cell imaging across mammalian and plant cells. Overall, this work establishes π-spacer-mediated internal preorganization and Zn2+ template-assisted external preorganization as complementary strategies for designing efficient fluorescent chemosensors.