Abstract
Metal–organic frameworks (MOFs) are emerging materials for luminescent and photochemical applications. Armed with femto to millisecond spectroscopies, and fluorescence microscopy, the photobehaviors of two Ce‐based MOFs are unravelled: Ce‐NU‐1000 and Ce‐CAU‐24‐TBAPy. It is observed that both MOFs show ligand‐to‐cluster charge transfer reactions in ≈100 and ≈70 fs for Ce‐NU‐1000 and Ce‐CAU‐24‐TBAPy, respectively. The formed charge separated states, resulting in electron and hole generation, recombine in different times for each MOF, being longer in Ce‐CAU‐24‐TBAPy: 1.59 and 13.43 µs than in Ce‐NU‐1000: 0.64 and 4.91 µs. The linkers in both MOFs also undergo a very fast intramolecular charge transfer reaction in ≈160 fs. Furthermore, the Ce‐NU‐1000 MOF reveals excimer formation in 50 ps, and lifetime of ≈14 ns. The lack of this interlinkers event in Ce‐CAU‐24‐TBAPy arises from topological restriction and demonstrates the structural differences between the two frameworks. Single‐crystal fluorescence microscopy of Ce‐CAU‐24‐TBAPy shows the presence of a random distribution of defects along the whole crystal, and their impact on the observed photobehavior. These findings reflect the effect of linkers topology and metal clusters orientations on the outcome of electronic excitation of reticular structure, key to their applicability in different fields of science and technology, such as photocatalysis and photonics.
Highlights
Metal–organic frameworks (MOFs) are emerging materials for luminescent cleaning, drug delivery, and bioimaging, to cite some of their applications.[1,2,3,4,5,6,7,8] In and photochemical applications
It should be noted that, while we cannot determine if the number and nature of defects are different in these Ce-based MOFs, there is a small difference in the metal-clusters orientation in both MOFs: the metal clusters in Ce-CAU-24-TBAPy are all oriented in the same direction, which is not the case for Ce-NU-1000 with 3 different orientations
In this work, armed with the potential of fast and ultrafast spectroscopies, we have shown and discussed the photobehavior of two Ce-based MOFs having pyrene as the main part of the linker moiety: Ce-NU-1000 and Ce-CAU-24-TBAPy
Summary
To explore the photodynamics of these MOFs, we first studied the emission decays of the free linker in acetone. The sample was excited at 370 nm (IRF = 70 ps) and the fluorescence decays were recorded from 430 to 525 nm. Illustration of the ultrafast processes and their time constants occurring within the excited two Ce-based MOFs in acetone suspensions: ICT (intramolecular-charge transfer, blue), LCCT (ligand-to-cluster charge transfer, green), and excimer formation (red). Www.advancedscience.com component (≈40 ps) increases at longer wavelengths (Figure 2D and Table 1). We believe that this lifetime is of species that underwent twisting or rotation events of the phenyl carboxylate moieties, enhancing the nonradiative processes (Scheme 1 and Scheme S1A, Supporting Information)
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