Sonia Castellanos

Co@NH2-MIL-125(Ti): cobaloxime-derived metal–organic framework-based composite for light-driven H2 production

We present a synthetic strategy for the efficient encapsulation of a derivative of a well-defined cobaloxime proton reduction catalyst within a photoresponsive metal–organic framework (NH2-MIL-125(Ti)). The resulting hybrid system Co@MOF is demonstrated to be a robust heterogeneous composite material. Furthermore, Co@MOF is an efficient and fully recyclable noble metal-free catalyst system for light-driven hydrogen evolution from water under visible light illumination.

Photoswitchable metal organic frameworks: turn on the lights and close the windows

The ability of modulating the properties of metal–organic frameworks (MOF) on demand by external light-stimuli is a most appealing pathway to enhance their performance in storage and separation and to render novel advanced applications. Photoswitchable linkers of different nature have been inserted in several MOF structures either as integral parts of their scaffolds or as guests in the pores. In this highlight we analyse the different strategies and expose some aspects that should be considered in the design of new generations of photoswitchable MOFs.

Structural Effects in Visible-Light-Responsive Metal–Organic Frameworks Incorporating ortho-Fluoroazobenzenes

The ability to control the interplay of materials with low-energy photons is important as visible light offers several appealing features compared to ultraviolet radiation (less damaging, more selective, predominant in the solar spectrum, possibility to increase the penetration depth). Two different metal-organic frameworks (MOFs) were synthesized from the same linker bearing all-visible ortho-fluoroazobenzene photoswitches as pendant groups. The MOFs exhibit different architectures that strongly influence the ability of the azobenzenes to isomerize inside the voids. The framework built with Al-based nodes has congested 1D channels that preclude efficient isomerization. As a result, local light-heat conversion can be used to alter the CO2 adsorption capacity of the material on exposure to green light. The second framework, built with Zr nodes, provides enough room for the photoswitches to isomerize, which leads to a unique bistable photochromic MOF that readily responds to blue and green light. The superiority of green over UV irradiation was additionally demonstrated by reflectance spectroscopy and analysis of digested samples. This material offers promising perspectives for liquid-phase applications such as light-controlled catalysis and adsorptive separation.

Gating Charge Recombination Rates through Dynamic Bridges in Tetrathiafulvalene–Fullerene Architectures

Conversion of light into chemical energy is a process that nature has optimized over eons in photosynthetic organisms, such as bacteria, plants, and algae. However, the search for non-natural systems that mimic the complex overall process of photosynthesis has remained a challenge. In particular, the key step of the initial light-induced charge separation between electron donors and acceptors is hampered by its inherent microscopic reversibility, that is, competing charge recombination. Well-defined molecular model systems typically comprise a donor (D) and an acceptor (A) covalently linked by a bridge (B). In the resulting D–B–A structures, the role of the bridge is ideally to facilitate the desired initial photoinduced charge separation, yet, to slow down the undesired charge recombination. Among the many combinations of donors and acceptors that have been explored, those consisting of proaromatic tetrathiafulvalene (TTF) and fullerene derivatives, such as C60, have shown outstanding results. The exceptional electron donating and accepting properties originate from the aromatic stabilization of the formed TTF radical cation and from C60 s unique three dimensional delocalized p-electron system, respectively. This last feature leads to low reorganization energies upon the reduction to the C60 radical anion and allows for the uptake of up to six additional electrons. The photophysical properties of various TTF–C60 conjugates featuring different p-conjugated molecular bridges have been investigated and charge-separated states with lifetimes ranging from a few nanoseconds up to hundreds of microseconds have been realized. Of particular interest are conjugates with p-extended TTF derivatives, in which a conjugated p-quinoid anthracene moiety is placed between the TTF s two 1,3-dithiole rings. Nevertheless, the design of such D–B–A architectures features inherent drawbacks. For example, even with optimized donors and acceptors, the bridge needs to play two opposing roles. On the one hand, it should enhance the coupling between D and A to facilitate the initial charge separation. On the other hand, once the charge-separated state has been formed it should prevent charge recombination by decoupling DC and AC . Clearly, conventional, static bridges have to be a compromise of these two demands. However, if D and A are connected by a dynamic bridge, which can be switched between a coupled and a decoupled form, prolonged charge-separated state lifetimes could potentially be attained without compromising the initial charge separation. Such improved molecular design requires a switch entity that adopts two electronically distinct forms and allows for precise timing of the switching, that is, when the bridge is being coupled or decoupled. Dithienylethenes (DTE) are ideal candidates as switchable bridges as they reversibly interconvert between their ring-open (decoupled) and ring-closed (p-conjugated) forms upon irradiation with light of specific wavelengths. Adopting this new strategy, we prepared four novel D–DTE–A structures connecting either TTF or exTTF acting as D and with C60 functioning as A by photochromic dithienylperhydrocyclopentene or perfluorocyclopentene bridges (Scheme 1). Some researchers have used photochromic units as lightresponsive electronic traps that allow or prevent intramolecular electron transfer from D to A depending on the adopted isomeric form. There are also some examples, in which the electron transfer kinetics are clearly altered by the structural modification of the bridging units by chemical inputs (chelation) or, in mechanically interlocked D and A units, by topological changes. However, herein, we show for the first time that in (ex)TTF–DTE–C60 architectures, the lifetime of the charge-separated state can be significantly shortened or prolonged by performing light-induced structural changes in the bridging unit. [*] Dr. S. Castellanos, Dr. J. Moreno, Prof. Dr. S. Hecht Department of Chemistry Humboldt-Universit t zu Berlin Brook-Taylor-Strasse 2, 12489 Berlin (Germany) E-mail: sh@chemie.hu-berlin.de

Organic Linker Defines the Excited-State Decay of Photocatalytic MIL-125(Ti)-Type Materials.

Recently, MIL-125(Ti) and NH2 -MIL-125(Ti), two titanium-based metal-organic frameworks, have attracted significant research attention in the field of photocatalysis for solar fuel generation. This work reveals that the differences between these structures are not only based on their light absorption range but also on the decay profile and topography of their excited states. In contrast to MIL-125(Ti), NH2 -MIL-125(Ti) shows markedly longer lifetimes of the charge-separated state, which improves photoconversion by the suppression of competing decay mechanisms. We used spectroelectrochemistry and ultrafast spectroscopy to demonstrate that upon photoexcitation in NH2 -MIL-125(Ti) the electron is located in the Ti-oxo clusters and the hole resides on the aminoterephthalate unit, specifically on the amino group. The results highlight the role of the amino group in NH2 -MIL-125(Ti), the electron donation of which extends the lifetime of the photoexcited state substantially.

Illuminating the nature and behavior of the active center: the key for photocatalytic H2 production in Co@NH2-MIL-125(Ti)

Advanced atomically resolved characterization methods unveil the mechanism of a promising photocatalytic Co@MOF(Ti) system for H2 production. The combination of X-ray absorption spectroscopy (XAS) and electron paramagnetic resonance (EPR) experiments allows for the characterization of atomic and electronic rearrangements in the photoinduced species. This information provides the basis for the optimization of photocatalyst design.