Theoretical Investigation of Charge Transfer in Metal Organic Frameworks for Electrochemical Device Applications

Abstract

For electrochemical device applications metal organic frameworks (MOFs) must exhibit suitable conduction properties. To this end, we have performed computational studies of intermolecular charge transfer in MOFs consisting of hexa-Zr IV nodes and tetratopic carboxylate linkers. This includes an examination of the electronic structure of linkers that are derived from tetraphenyl benzene 1, tetraphenyl pyrene 2 and tetraphenyl porphyrin 3 molecules. These results are used to determine charge transfer propensities in MOFs, within the framework of Marcus theory, including an analysis of the key parameters (charge transfer integral t, reorganization energy λ and free energy change ∆G 0 ), and evaluation of figures of merit for charge transfer based on the chemical structures of the linkers. This qualitative analysis indicates that delocalization of the HOMO/LUMO on terminal substituents increases t and decreases λ, while weaker binding to counterions decreases ∆G 0 , leading to better charge transfer propensity. Subsequently, we study hole transfer in the linker 2 containing MOFs, NU-901 and NU-1000, in detail and describe mechanisms (hopping and superexchange) that may be operative under different electrochemical conditions. Comparisons with experiment are provided whereavailable. Based on the redox and catalytic activity of nodes and linkers, we propose three possible schemes for constructing electrochemical devices for catalysis. We believe that the results of this study will lay the foundation for future experimental work on this topic.

Fluorescent Imaging-Guided Chemotherapy-and-Photodynamic Dual Therapy with Nanoscale Porphyrin Metal-Organic Framework

“…TCPP fluorescence was observed at 651 nm with a weaker shoulder peak at 710 nm when excited at 514 nm (Figure b). However, the NPMOFs showed only one emission peak at 689 nm (with a 38 nm redshift), which was attributed to the orbital hybrid between the Zr ions and TCPP as the d orbital of the Zr ions decreases the gap between the HOMO and LUMO of TCPP . The strong far‐red emission and long Stokes shift of NPMOFs led to a high signal‐to‐noise ratio in the fluorescent images because of the low excitation interference, autofluorescence, and scattering light from the biological tissue …”

Exploiting redox activity in metal–organic frameworks: concepts, trends and perspectives

“…This approach been valuable for modelling the degree of CT in [(Zn(DMF)) 2 (TTFTC)(DPNI)], 62 where d was predicted to be 0.6 (i.e., near the mixed-valence regime discussed in Section 2.2) and in the ferrocene functionalised NU-1000 MOF. 93 In the latter case, the Marcus equation was used to describe the rate of intermolecular charge transfer in a chemically relevant fragment from the extended framework crystal structure, and charge transfer was subsequently shown to occur via a superexchange mechanism. 38 In addition to modelling the intrinsic charge transfer properties of the MOF, this study also demonstrated the application of computational methods to model MOFs for electrochemical device applications where additional components including the solvent, catalyst and contacts must also be considered.…”

“…38 In addition to modelling the intrinsic charge transfer properties of the MOF, this study also demonstrated the application of computational methods to model MOFs for electrochemical device applications where additional components including the solvent, catalyst and contacts must also be considered. 93 In the case of [(Zn(DMF)) 2 (TTFTC)(DPNI)] 62 and the ferrocene functionalised NU-1000 MOF, 93 the systems lie at the weak coupling limit and involve localised CT in which charge is likely to hop discretely between sites. 94 While theories pioneered by Hush 95 and Marcus 96,97 may be relevant to charge transfer in the localised limit, they are likely to be inadequate for strongly interacting systems where the classical treatment of nuclear motion is not realistic.…”

“…The potential to alter the absorption and luminescence properties 93,94 of MOFs by exploiting the different characteristics of the redox states is a subject of great interest owing to their potential applications in optoelectronic devices, and as chemical sensors. 118,149 For example, a Mn(II) coordination solid based on the tris(4-(pyridin-4-yl)phenyl)amine ligand was found to exhibit switchable 'on'/'off' fluorescence (using chemical oxidation) based on the redox state of the ligand which can exist in its radical cation ('off') or neutral state ('on').…”

Metal–Organic Frameworks Toward Electrocatalytic Applications

“…Although NU-901 is electrically insulating [38], by electrochemically oxidizing the pyrene-based linkers in the presence of counter ions in the electrolyte, more than 90% of the pyrene units within the micrometer-thick MOF thin film can be electrochemically addressable; the resulting MOF thin film exhibits remarkable electrochromism with reversibility. Such redox-hopping strategies have been widely utilized in a range of MOF thin films for electrochemical purposes and their mechanistic insights have been investigated during recent years [31,[39][40][41]. Very recently, S. Ott et al found that the thin films of a Zr-MOF constructed from NDI-based linkers can perform stable redox-hopping behavior in aqueous electrolytes [42].…”