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, MV) 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.

Molecular Porous Photosystems Tailored for Long‐Term Photocatalytic CO2 Reduction

“…The steady‐state emission spectra highlight an important difference in the population of those photoexcited states between PerBpyCMP and PyBpyCMP materials, respectively. Elucidating the role of the charge separated states formed upon photoexcitation of conjugated backbones is of great interest to elucidate and optimize the photocatalytic activity of porous materials …”

“…Thes teady-state emission spectra highlight an important difference in the population of those photoexcited states between PerBpyCMP and PyBpyCMP materials, respectively.E lucidating the role of the charge separated states formed upon photoexcitation of conjugated backbones is of great interest to elucidate and optimize the photocatalytic activity of porous materials. [45] In the case of Cp*Rh@PerBpyCMP,and also of the parent PerBpyCMP,t he spectrum shows the typical vibronic progression of the fluorescence of the perylene molecule (Figure 4a). [46,47] Thus,t he emission mainly stems from al ocal excited (LE) state on the perylene moiety (see also the excitation spectra in Figure 4a).…”

Unravelling Why and to What Extent the Topology of Similar Ce‐Based MOFs Conditions their Photodynamic: Relevance to Photocatalysis and Photonics

“…[9][10][11][12][13][14][15][16][17] Therefore, recent advances in experiments and theory have shed light on photoprocesses occurring in MOFs both in suspension and in solid state. [8,[18][19][20][21] For example, for Zr-based MOFs, studies have elucidated the ultrafast dynamics related to excimer formation, ligand-to-cluster charge transfer (LCCT), and energy transfer to encapsulated guests. [18,[22][23][24][25] Other studies have explored the dynamics of Zr-based MOFs having the same linkers and metal clusters but different crystalline phases, and thus different distances and angles between their neighboring linkers.…”

“…[8,[18][19][20][21] For example, for Zr-based MOFs, studies have elucidated the ultrafast dynamics related to excimer formation, ligand-to-cluster charge transfer (LCCT), and energy transfer to encapsulated guests. [18,[22][23][24][25] Other studies have explored the dynamics of Zr-based MOFs having the same linkers and metal clusters but different crystalline phases, and thus different distances and angles between their neighboring linkers. [26][27][28][29] These reports have shown the key role of the structural parameters in the interchromophoric interactions that lead to the formation of excimers in a specific network, and to their absence in others even having the same composition, but different structure.…”

Understanding metal–organic frameworks for photocatalytic solar fuel production

“…70 In the case of very weak absorbance but reasonable fluorescence, this is usually done in an emission fashion. 71,72 Here, time-correlated single-photon counting (TCSPC) is the most popular method, measuring picosecond emission decays. For the case of femtosecond emission transients, the fluorescence up-conversion technique is often used.…”