NH2–BDC-Pillared Ho-MOFs with/without Viologen Encapsulation: Fluorescence Sensing, Multistimulus Response, and PVDF Composite Film

Viologen-encapsulated metal–organic frameworks (MOFs) have attracted significant interest owing to their responsiveness to multiple external stimuli, which enhances their potential applications in sensing and environmental monitoring. Here, we synthesized the novel viologen-encapsulated Ho-MOF material (pbpy)·[Ho2(NH2–BDC)4(H2O)]·3H2O (complex 1) via an in situ method using 2-aminoterephthalic acid (NH2–H2BDC) and the viologen derivative dichloro-1,1′-(1,4-benzylidene)-di(4,4′-bipyridine) (pbpy·2Cl), along with the control material [Ho4(NH2–BDC)5(DMA)2(μ3-OH)2]·3H2O (complex 2) without a viologen for comparison. Complex 1 exhibited dual detection capability for Th(IV) and nitroaromatic compounds (NACs), demonstrating significant advantages in sensing performance compared to complex 2. These results highlight the critical role of viologen incorporation in improving sensor functionality. Moreover, we prepared a MOF-based PVDF composite film material capable of rapidly detecting Th(IV) and NACs, demonstrating potential for practical sensing applications. In addition, thanks to the presence of a viologen inside the channels, complex 1 could achieve rapid and visual detection of N2H4·H2O. EPR results indicate that the color change is due to the generation of viologen radicals. These properties make the material a promising candidate for multianalyte detection, advancing the development of multistimulus responsive materials.