Upon light excitation MOF-5 behaves as a semiconductor and undergoes charge separation (electrons and holes) decaying in the microsecond time scale. The actual conduction band energy value was estimated to be 0.2 V versus NHE with a band gap of 3.4 eV. Photoinduced electron transfer processes to viologen generates the corresponding viologen radical cation, while holes of MOF-5 oxidizes N,N,N',N'-tetramethyl-p-phenylenediamine. One application investigated for MOF-5 as a semiconductor has been the shape-selective photocatalyzed degradation of phenol in aqueous solutions.

Light excitation powers the reversible shuttling movement of the ring component of a rotaxane between two stations located at a 1.3-nm distance on its dumbbell-shaped component. The photoinduced shuttling movement, which occurs in solution, is based on a ''four-stroke'' synchronized sequence of electronic and nuclear processes. At room temperature the deactivation time of the high-energy charge-transfer state obtained by light excitation is Ϸ10 s, and the time period required for the ring-displacement process is on the order of 100 s. The rotaxane behaves as an autonomous linear motor and operates with a quantum efficiency up to Ϸ12%. The investigated system is a unique example of an artificial linear nanomotor because it gathers together the following features: (i) it is powered by visible light (e.g., sunlight); (ii) it exhibits autonomous behavior, like motor proteins; (iii) it does not generate waste products; (iv) its operation can rely only on intramolecular processes, allowing in principle operation at the single-molecule level; (v) it can be driven at a frequency of 1 kHz; (vi) it works in mild environmental conditions (i.e., fluid solution at ambient temperature); and (vii) it is stable for at least 10 3 cycles. molecular machine ͉ nanoscience ͉ photochemistry ͉ rotaxane ͉ supramolecular chemistry T he miniaturization race is encouraging scientists to design and construct motors on the nanometer scale, that is, at the molecular level (1-5). Such a daring goal finds its scientific origin in the existence of natural molecular motors (6-9).Natural molecular motors, however, are extremely complex, and any attempt to construct systems of such a complexity, using the bottom-up molecular approach (10), would be challenging. What can be done, at present, is to construct simple prototypes of artificial molecular motors and machines (1-5, 11-19), consisting of a few components capable of moving in a controllable way, and to investigate the associated problems posed by interfacing them with the macroscopic world (20-25), particularly as far as energy supply is concerned.Natural motors are ''autonomous'': they keep operating, in a constant environment, as long as the energy source is available. By contrast, apart from a few recent examples (26-28), the fuel-powered artificial motors described so far are not autonomous because, after the mechanical movement induced by a chemical input, they need another, opposite chemical input to reset, which also implies generation of waste products. Addition of a fuel, however, is not the only means by which energy can be supplied to a chemical system. In fact, nature shows that, in green plants, the energy needed to sustain the machinery of life is ultimately provided by sunlight. Energy inputs in the form of photons can indeed cause mechanical movements by reversible chemical reactions without formation of waste products (13,14,16,17).In a previous work (29), we reported on the rotaxane 1 6ϩ (Scheme 1) that was carefully designed and synthesized to perform as a linear molecular...

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A periodic structured organosilica solid of the MCM-41 type containing 4,4A-bipyridinium presumably inserted on the walls that is stable after surfactant removal has been prepared; the ability of bipyridinium units to act as electron acceptor termini has been demonstrated by observation of the radical cation in the photochemical and thermal activation of the as-synthesized material.

Metal–organic frameworks (MOFs) have been frequently used as photocatalysts for the hydrogen evolution reaction (HER) using sacrificial agents with UV–vis or visible light irradiation. The aim of the present review is to summarize the use of MOFs as solar-driven photocatalysts targeting to overcome the current efficiency limitations in overall water splitting (OWS). Initially, the fundamentals of the photocatalytic OWS under solar irradiation are presented. Then, the different strategies that can be implemented on MOFs to adapt them for solar photocatalysis for OWS are discussed in detail. Later, the most active MOFs reported until now for the solar-driven HER and/or oxygen evolution reaction (OER) are critically commented. These studies are taken as precedents for the discussion of the existing studies on the use of MOFs as photocatalysts for the OWS under visible or sunlight irradiation. The requirements to be met to use MOFs at large scale for the solar-driven OWS are also discussed. The last section of this review provides a summary of the current state of the field and comments on future prospects that could bring MOFs closer to commercial application.

A series of mixed-metal NH 2-UiO-66(Zr/Ti) with different percentages of exchanged Ti have been prepared and studied by transient absorption spectroscopy (TAS). The photo-generated transients from mixed NH 2-UiO-66(Zr/Ti) exhibit at short time scales two defined absorption bands, evolving to a continuous absorption band expanding from 300 nm to 700 nm at longer time scales. The observed spectral changes are compatible with an initial formation of Ti 3+-O-Zr 4+ and its further transformation to Ti 4+-O-Zr 3+ via metal-metal electron exchange, thus, providing support to the role of substituted Ti as mediator to facilitate electron transfer from excited ligand to the (Zr/Ti) 6 O 4 (OH) 4 nodes in mixed NH 2-UiO-66(Zr/Ti). The slow recombination of photo-generated electrons and holes in the mixed NH 2-UiO-66(Zr/Ti) has been advantageously used for the construction of a photovoltaic cell fabricated with the mixed NH 2-UiO-66(Zr/Ti), reaching higher photon-to-current efficiency than NH 2-UiO-66(Zr).

The presence of organic linkers in MOFs allows introducing response in the solid upon chemical, electrochemical, or photochemical excitation of these units. In the present study, we report the intrinsic photoresponse of four commercially available MOFs as well as after incorporation of some organic guests. Laser flash photolysis measurements have allowed us to detect transient species upon irradiation of commercial Al2(BDC)3. The signal has been rationalized as derived from the photochemical generation of charge separated states. In contrast to Al2(BDC)3, the other three commercial MOFs tested did not exhibit any signal. The photoinduced charge separation in Al2(BDC)3 can be modulated by inclusion of organic guests that can act as traps of electrons or holes increasing the lifetime of charge separation. When the oxidation potential of the organic guest is low, as in the case of 1,4-phenylendiamine, PDA, we have been able to observe spontaneous charge separation even for Fe-BTC and Cu3(BTC)2. The basic understanding of the photoresponse has been applied to build photovoltaic cells using Al2(BDC)3 as semiconductor. The best performing device was the one constructed with 1,4-dimethoxybenzene, DMB, DMB@ Al2(BDC)3 with the lowest thickness, which more than doubles the efficiency of the Al2(BDC)3 cells prepared without DMB. Overall, our report exemplifies how understanding of the basic photochemistry can be used for developing new applications of MOFs.

A series of periodic mesoporous silicas of MCM-41 type containing varying amounts (5−50%) of chiral binaphthyl and cyclohexadiyl moieties occupying framework positions have been synthesized, and their characteristic MCM-41 features were observed by powder XRD and porosity measurements. The covalent bonding of the organics to the silicate framework was confirmed by 1H and 29Si MAS NMR experiments. Direct measurement of the optical activity demonstrates that the solids are able to rotate the angle of plane-polarized light. Also, a certain degree of chiral discrimination has been observed for the asymmetric enhancement of the binaphthyl fluorescence by adding enantiomerically pure 1,2-cyclohexadiamine.