Titanium(IV) Inclusion as a Versatile Route to Photoactivity in Metal–Organic Frameworks

Abstract

Titanium-containing metal-organic frameworks (MOFs) are known to perform light-promoted chemical transformations. Formation of these frameworks, accessed either natively or via transmetallation, can instill beneficial photocatalytic properties in previously photo-inert scaffolds. Band edge diagrams coupled with their density of states illustrate the persistent nature of the accessible titanium d-states at the conduction band edge, independent of linker identity. In essentially all of these Ti-containing frameworks, the valence band edge localizes on the organic component, permitting facile modulation of the band gap. In sum, the spatial separation of photogenerated excitons will be observed in both the native and titanium(IV)-substituted MOFs, and this exciton pair is useful in photocatalysis.

Metal–Organic Frameworks: Molecules or Semiconductors in Photocatalysis?

“…The postsynthetic exchange of metal ions in the node can introduce new bands, typically vacant ones, into a desirable energy range. In principle, the band gap can then be controlled by low‐lying, vacant orbitals of the dopant, such as 3d orbitals of Ti [39, 40] …”

“…Important work, certainly from the application perspective, involves the development of new Ti‐based MOFs that bring an improved stability in catalytic reactions. Predicted in computation by Mancuso and Hendon, there is some robustness to the Ti 3d band alignment with respect to linker identity [40] . One way to improve stability is to employ stronger metal–ligand bonds.…”

On the limit of proton-coupled electronic doping in a Ti(iv)-containing MOF

“…20 Combining the best of both approaches, metalorganic frameworks (MOFs) merge the benefits of heterogeneous catalysis and synthetic modularity. [21][22][23][24] Previous studies in MOF catalysis have primarily focused on thermally driven reactions at either the linker or node using intrinsic [25][26][27] and extrinsic [28][29][30] catalytically active metals. Conversely, MOF photoredox chemistry [31][32][33] typically focuses on electron transfer between the inorganic and organic MOF components 34 , shuttling electrons to interstitial molecular catalysts [35][36][37] or guests [38][39][40][41] .…”

“…EPR later validated the mechanism of Ti III formation. 50 Paired with electronic structure calculations, Ti III -formation was rationalized by identifying the conduction band minimum (CBM) origin as vacant Ti d-states in the pristine MOF — a general property extending to all reported Ti IV -containing MOFs, 25 and likely a general consideration when designing photoactive MOFs with inorganic-centered conduction bands.…”

Metal–Organic Frameworks: Molecules or Semiconductors in Photocatalysis?

“…Changing Bands with Metal Substitution Thepostsynthetic exchange of metal ions in the node can introduce new bands,t ypically vacant ones,i nto ad esirable energy range.I np rinciple,t he band gap can then be controlled by low-lying,v acant orbitals of the dopant, such as 3d orbitals of Ti. [39,40] Thef irst case that employed such postsynthetic metal exchange for photocatalytic application came by Sun et al,w ho exchanged Zr 4+ for Ti 4+ in aminated UiO-66. [41] DFT calculations confirmed that states from Ti 4+ were indeed responsible for al owering of the gap.…”