Spacer-Length-Dependent Photophysical Properties and Metal-Ion-Induced Fluorescence Response of Two NBD-Based Fluorophores

Two previously reported fluorophores based on 4-amino-7-nitrobenzo[c][1,2,5]oxadiazole (NBD), containing N,N-dimethylaminoethyl (NBD1) and N,N-dimethylaminopropyl (NBD2) moieties, were prepared using an optimized procedure in ethyl acetate, affording isolated yields of 89% and 82%, respectively, without chromatographic purification. Their electronic structures and photophysical properties were comparatively investigated using spectroscopic measurements and DFT calculations, with particular emphasis on the effect of a single methylene group in the aliphatic spacer. Although NBD1 and NBD2 exhibit similar absorption characteristics, their fluorescence properties differ substantially. Depending on the solvent, the fluorescence quantum yield of NBD2 is approximately 5–36 times higher than that of NBD1, despite the relatively low absolute quantum yields of both compounds. Protonation of the terminal tertiary amino group produces fluorescence enhancement factors of approximately 58 and 19.8 for NBD1 and NBD2, respectively, with apparent pKa values of 6.65 and 7.46. Pronounced differences were also observed in their fluorescence responses to metal ions. Fe(III), Zn(II), and Sn(II) produced the strongest fluorescence enhancement for NBD1, with enhancement factors reaching approximately 65, whereas the corresponding values for NBD2 remained below 4 for all investigated metal ions. Overall, these results demonstrate that the insertion of a single methylene group into the spacer can markedly influence fluorescence efficiency, protonation-dependent behavior, and metal-ion-induced fluorescence response without substantially altering the absorption characteristics of the NBD chromophore. The findings highlight spacer-length modification as a simple structural strategy for tuning the photophysical properties of NBD-based fluorophores.