title: Photochemistry of Zr-based MOFs: ligand-to-cluster charge transfer, energy transfer and excimer formation, what else is there?
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.
authors.affiliation: Departamento de Química Física, Facultad de Ciencias Ambientales y Bioquímica, and INAMOL, Universidad de Castilla-La Mancha, Avenida Carlos III, S.N., 45071 Toledo, Spain. abderrazzak.douhal@uclm.es.
authors.forename: Mario
authors.identifier: http://orcid.org/0000-0001-9222-3647
authors.initials: M
authors.lastname: Gutierrez
authors.affiliation: Departamento de Química Física, Facultad de Ciencias Ambientales y Bioquímica, and INAMOL, Universidad de Castilla-La Mancha, Avenida Carlos III, S.N., 45071 Toledo, Spain. abderrazzak.douhal@uclm.es.
authors.forename: Boiko
authors.initials: B
authors.lastname: Cohen
authors.affiliation: Instituto de QuímicaOrgánica, CSIC, Juan de la Cierva, 3, 28006 Madrid, Spain.
authors.forename: Félix
authors.initials: F
authors.lastname: Sánchez
authors.affiliation: Departamento de Química Física, Facultad de Ciencias Ambientales y Bioquímica, and INAMOL, Universidad de Castilla-La Mancha, Avenida Carlos III, S.N., 45071 Toledo, Spain. abderrazzak.douhal@uclm.es.
authors.forename: Abderrazzak
authors.initials: A
authors.lastname: Douhal