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Issue 40, 2016
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Photochemistry of Zr-based MOFs: ligand-to-cluster charge transfer, energy transfer and excimer formation, what else is there?

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Abstract

Understanding of the photocatalytic behaviour of Zr-based MOFs is fundamental for the improved design of new and more efficient photocatalysts. The present work describes steady-state and photodynamical studies on the behavior of two MOFs: Zr-2,6-naphthalene dicarboxylate (Zr-NDC) and Zr-4-amino-2,6-naphthalene dicarboxylate (Zr-NADC, 65% NDC/35% NADC) in dichloromethane (DCM) and N,N-dimethylformamide (DMF) suspensions. In the DMF suspension, the Zr-NDC MOF exhibits excimer formation in 280 ps due to the interactions between neighboring linkers. Using the femtosecond transient absorption (fs-TA) technique we have found that the Zr-NADC MOF exhibits a ligand-to-cluster charge transfer (LCCT) event in ∼170 and <100 fs in DMF and DCM, respectively. The Zr-NDC MOF shows similar LCCT in <100 fs in both solvents. The Zr-NADC MOF in DMF suspension shows an energy transfer (ET) from the excited NDC linkers to the NADC ones in 1.1 ps. Flash photolysis experiments demonstrate dominant radiative electron–hole (e–h+) recombination from two different trap states in 310 ns and 1.3 μs, and 32 ns and 1.0 μs in the photoexcited Zr-NADC and Zr-NDC MOFs in DMF, respectively. Excitation of the NDC linkers in the Zr-NADC and Zr-NDC MOFs in DCM allows for the characterization of the dynamics associated with the charge-separated state and the non-radiative recombination from the trap states (45 ns and 0.3 μs). Direct excitation of the NADC linkers (410 nm) in the Zr-NADC MOF in DCM suspension produces a radiative (e–h+) recombination in 3.5 μs. Further experiments in the presence of electron donor (N,N,N′,N′-tetramethyl-p-phenylenediamine, TMPD) and acceptor (methyl viologen, MV2+) molecules corroborate the formation of charge separated states in these MOFs. These findings are relevant for understanding the photocatalytic and photonic behaviours of these MOFs.

Graphical abstract: Photochemistry of Zr-based MOFs: ligand-to-cluster charge transfer, energy transfer and excimer formation, what else is there?

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Supplementary files

Article information


Submitted
01 Jun 2016
Accepted
08 Aug 2016
First published
09 Aug 2016

Phys. Chem. Chem. Phys., 2016,18, 27761-27774
Article type
Paper

Photochemistry of Zr-based MOFs: ligand-to-cluster charge transfer, energy transfer and excimer formation, what else is there?

M. Gutierrez, B. Cohen, F. Sánchez and A. Douhal, Phys. Chem. Chem. Phys., 2016, 18, 27761
DOI: 10.1039/C6CP03791G

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