Tuning of Emission Properties of Luminol-Linked ZIF-90 Revealed by Two-Dimensional Photoluminescence Spectroscopy

초록

In this work, a zeolitic imidazolate framework (ZIF), which is a subclass of metal-organic frameworks (MOF), was synthesized to play dual roles of nanofirtilezers in regulating plant growth and reactive oxygen species-responsive sensors. The synthesized ZIF, termed as ZIF-90, can serve as a sensor in a wide range of applications, including biomedicine, owing to its exceptional structural stability and intrinsic luminescent properties. In addition, we synthesized a luminol-conjugated derivative of ZIF-90, termed ZIF-90L, to enhance and manipulate the luminescent behavior of the parent framework (Fig. 1a). In particular, we compared the performance of ZIF-90L with that of the parent ZIF-90. Two-dimensional photoluminescence spectroscopy was employed to analyze their luminescent characteristics. ZIF-90 exhibited emission at 460 nm upon excitation at 310 nm, as can be seen in Fig. 1c. This luminescence originates from ligand-centered π → π* transitions, instead of the metal center [1]. In contrast, as can be seen in Fig. 1d, the emission from ZIF-90L was shifted in both its excitation and emission wavelengths compared with that of ZIF-90, presumably due to the covalent linkage of luminol; specifically, its major emission peak was located at 435 nm under 350 nm excitation. Notably, the emission at 435 nm suggests that the emissive origin from the luminol molecule (see Fig. 1b) is preserved in ZIF-90L. However, the excitation wavelengths (300 nm and 350 nm) to induce this emission are significantly blue shifted compared with those of free luminol (320 nm and 380 nm). This change is considered to arise from photoinduced electron transfer (PET) between the ZIF ligand and luminol. In conclusion, the emission properties of ZIF-90L demonstrates that a simple covalent linkage of a fluorophore to a MOF scaffold can protect and modulate emission characteristics?allowing for the fine-tuning of both wavelength and intensity?and thus offers a versatile design strategy for