The impact of halogen substitution quantities on the fluorescence intensity ratio of lanthanide Schiff base complexes.

The signal intensity ratio (SIR) is a crucial factor in advancing probe technology due to its direct impact on sensitivity and precision, particularly in applications such as medical imaging, environmental monitoring, and food safety testing. However, the development of high-SIR probes is challenged by complexities in fabrication, cost, and mechanical stability. In this study, we address these limitations by investigating the role of halogen atom substitutions in modulating the intermolecular binding energy and aggregation behavior of Ce-Salen Schiff base complexes. We synthesized a novel Schiff base pH probe, Ce-3,5-Cl-Salpn (3,5-Cl-Salpn = N, N'-bis (3,5-dichlorosalicylidene)ethylene-1,3-diaminopropane), and introduced its analogues Ce-5-Cl-Salpn (5-Cl-Salpn = N, N'-bis (5-chlorosalicylidene)ethylene-1,3-diaminopropane) and Ce-Salpn (Salpn = N, N'-bis (salicylidene)ethylene-1,3-diaminopropane) for comparative analysis. Through fluorescence measurements, single-crystal analysis, and theoretical calculations, we demonstrate that halogen substitution leads to significant modulation of fluorescence intensity and SIR in the pH range of 6.0 to 7.0. Notably, Ce-3,5-Cl-Salpn exhibited the highest SIR, with a 182.5-fold increase, compared to the non-halogenated variant's 9.2-fold rise. Frontier molecular orbital (FMO) analysis revealed a reduction in the HOMO-LUMO energy gap as halogen substitution increased, resulting in enhanced optical properties and more efficient electronic transitions. Additionally, binding energy calculations confirmed that halogen atoms strengthen intermolecular interactions, thereby improving molecular stability and aggregation-caused quenching effects.