Mechanistic insights into two-photon-driven photocatalysis in organic synthesis.

A mechanism based on the sequential absorption of two photons by the components of a redox couple has been recently proposed for catalysis of the energetically demanding reduction of aryl halides. Here, we analyze the suggested photochemical mechanism of this reaction, which employs perylenediimide (PDI) as a photocatalyst, on the basis of spectroscopic, electrochemical and electron paramagnetic resonance data. Our results indicate that the photoexcited PDI radical anion (*PDI˙-) cannot play the role of a photosensitizer in the aforementioned process. Instead, the reduction of 4'-bromoacetophenone likely involves *PDI˙- decomposition products. The extremely short lifetime of the photoexcited transient species, as *PDI˙-, is a major general limitation for photocatalytic schemes based on sequential two-photon excitation. In order to better understand the potential of such schemes, we discuss them in the context of the Z-scheme in natural photosynthesis.

Photoredox Catalysis: The Need to Elucidate the Photochemical Mechanism.

The photocatalytic mechanism reported in a recent Communication to produce the radical anion of pyrenes postulates a highly endergonic electron transfer process. An analysis of the thermodynamics is reported together with the proposal of an alternative thermodynamically feasible mechanism.

Solar Electricity and Solar Fuels: Status and Perspectives in the Context of the Energy Transition.

The energy transition from fossil fuels to renewables is already ongoing, but it will be a long and difficult process because the energy system is a gigantic and complex machine. Key renewable energy production data show the remarkable growth of solar electricity technologies and indicate that crystalline silicon photovoltaics (PV) and wind turbines are the workhorses of the first wave of renewable energy deployment on the TW scale around the globe. The other PV alternatives (e.g., copper/indium/gallium/selenide (CIGS) or CdTe), along with other less mature options, are critically analyzed. As far as fuels are concerned, the situation is significantly more complex because making chemicals with sunshine is far more complicated than generating electric current. The prime solar artificial fuel is molecular hydrogen, which is characterized by an excellent combination of chemical and physical properties. The routes to make it from solar energy (photoelectrochemical cells (PEC), dye-sensitized photoelectrochemical cells (DSPEC), PV electrolyzers) and then synthetic liquid fuels are presented, with discussion on economic aspects. The interconversion between electricity and hydrogen, two energy carriers directly produced by sunlight, will be a key tool to distribute renewable energies with the highest flexibility. The discussion takes into account two concepts that are often overlooked: the energy return on investment (EROI) and the limited availability of natural resources-particularly minerals-which are needed to manufacture energy converters and storage devices on a multi-TW scale.

Light: A Very Peculiar Reactant and Product.

See the light of day: Light is the fastest way of transferring energy and information through space, and in chemistry it can perform the dual role of reactant and product. Sunlight, a really unique reactant, represents our ultimate energy source. Chemists are engaged in designing systems for the conversion of light into electrical or chemical energy and vice versa to create a more sustainable way of life.

The beauty of chemistry in the words of writers and in the hands of scientists.

Chemistry is a central science because all the processes that sustain life are based on chemical reactions, and all things that we use in everyday life are natural or artificial chemical compounds. Chemistry is also a fantastic world populated by an unbelievable number of nanometric objects called molecules, the smallest entities that have distinct shapes, sizes, and properties. Molecules are the words of matter. Indeed, most of the other sciences have been permeated by the concepts of chemistry and the language of molecules. Like words, molecules contain specific pieces of information that are revealed when they interact with one another or when they are stimulated by photons or electrons. In the hands of chemists, molecules, particularly when they are suitably combined or assembled to create supramolecular systems, can play a variety of functions, even more complex and more clever than those invented by nature. The wonderful world of chemistry has inspired scientists not only to prepare new molecules or investigate new chemical processes, but also to create masterpieces. Some nice stories based on chemical concepts (1) show that there cannot be borders on the Earth, (2) underline that there is a tight connection among all forms of matter, and (3) emphasize the irreplaceable role of sunlight.

The legacy of fossil fuels.

Currently, over 80% of the energy used by mankind comes from fossil fuels. Harnessing coal, oil and gas, the energy resources contained in the store of our spaceship, Earth, has prompted a dramatic expansion in energy use and a substantial improvement in the quality of life of billions of individuals in some regions of the world. Powering our civilization with fossil fuels has been very convenient, but now we know that it entails severe consequences. We treat fossil fuels as a resource that anyone anywhere can extract and use in any fashion, and Earth's atmosphere, soil and oceans as a dump for their waste products, including more than 30 Gt/y of carbon dioxide. At present, environmental legacy rather than consistence of exploitable reserves, is the most dramatic problem posed by the relentless increase of fossil fuel global demand. Harmful effects on the environment and human health, usually not incorporated into the pricing of fossil fuels, include immediate and short-term impacts related to their discovery, extraction, transportation, distribution, and burning as well as climate change that are spread over time to future generations or over space to the entire planet. In this essay, several aspects of the fossil fuel legacy are discussed, such as alteration of the carbon cycle, carbon dioxide rise and its measurement, greenhouse effect, anthropogenic climate change, air pollution and human health, geoengineering proposals, land and water degradation, economic problems, indirect effects on the society, and the urgent need of regulatory efforts and related actions to promote a gradual transition out of the fossil fuel era. While manufacturing sustainable solar fuels appears to be a longer-time perspective, alternatives energy sources already exist that have the potential to replace fossil fuels as feedstocks for electricity production.

The hydrogen issue.

Hydrogen is often proposed as the fuel of the future, but the transformation from the present fossil fuel economy to a hydrogen economy will need the solution of numerous complex scientific and technological issues, which will require several decades to be accomplished. Hydrogen is not an alternative fuel, but an energy carrier that has to be produced by using energy, starting from hydrogen-rich compounds. Production from gasoline or natural gas does not offer any advantage over the direct use of such fuels. Production from coal by gasification techniques with capture and sequestration of CO2 could be an interim solution. Water splitting by artificial photosynthesis, photobiological methods based on algae, and high temperatures obtained by nuclear or concentrated solar power plants are promising approaches, but still far from practical applications. In the next decades, the development of the hydrogen economy will most likely rely on water electrolysis by using enormous amounts of electric power, which in its turn has to be generated. Producing electricity by burning fossil fuels, of course, cannot be a rational solution. Hydroelectric power can give but a very modest contribution. Therefore, it will be necessary to generate large amounts of electric power by nuclear energy of by renewable energies. A hydrogen economy based on nuclear electricity would imply the construction of thousands of fission reactors, thereby magnifying all the problems related to the use of nuclear energy (e.g., safe disposal of radioactive waste, nuclear proliferation, plant decommissioning, uranium shortage). In principle, wind, photovoltaic, and concentrated solar power have the potential to produce enormous amounts of electric power, but, except for wind, such technologies are too underdeveloped and expensive to tackle such a big task in a short period of time. A full development of a hydrogen economy needs also improvement in hydrogen storage, transportation and distribution. Hydrogen and electricity can be easily interconverted by electrolysis and fuel cells, and which of these two energy carriers will prevail, particularly in the crucial field of road vehicle powering, will depend on the solutions found for their peculiar drawbacks, namely storage for electricity and transportation and distribution for hydrogen. There is little doubt that power production by renewable energies, energy storage by hydrogen, and electric power transportation and distribution by smart electric grids will play an essential role in phasing out fossil fuels.

Light-powered molecular devices and machines.

One century ago Giacomo Ciamician predicted that photochemistry would have had a wealth of useful applications, starting from the conversion of solar energy into fuels. Most of Ciamician's predictions have not yet been achieved, but in the last decade outstanding progress concerning the interaction between light and molecules has led to the creation of artificial photochemical molecular devices and machines capable of using light as an energy supply (to sustain energy-expensive functions) or as an input signal (to be processed and/or stored). This paper illustrates (i) the principles of photochemical molecular devices for information processing, with a few examples of memories, logic functions, and encoding/decoding systems; (ii) the operational mechanisms of light-powered molecular machines, with some examples of rotary motors, shuttles, valves, and switchable boxes; and (iii) the recent progress made in the design and construction of the components of artificial photosynthetic systems. The use of photons to convert abundant low energy molecules into high energy valuable compounds, and to read, write, and erase smart molecular and supramolecular systems for information processing is likely to play a fundamental role for the progress of mankind.

A light-harvesting antenna resulting from the self-assembly of five luminescent components: a dendrimer, two clips, and two lanthanide ions.

We have investigated the self-assembly of three luminescent species in CH(3)CN/CH(2)Cl(2), namely: 1) a polylysin dendrimer (D) composed of 21 aliphatic amide units and 24 green luminescent dansyl chromophores at the periphery, 2) a molecular clip (C) with two blue luminescent anthracene sidewalls and a benzene bridging unit that bears two sulfate groups in the para position, and 3) a near infrared (NIR)-emitting Nd(3+) ion. For purposes of comparison, analogous systems have also been investigated in which Gd(3+) replaced Nd(3+). The dendrimer and the clip can bind Nd(3+) ions with formation of [D.2Nd(3+)] and [C.Nd(3+)] complexes, in which energy transfer from dansyl and, respectively, anthracene to Nd(3+) ion takes place with 65 and 8% efficiency, in air-equilibrated solution. In the case of [C.Nd(3+)], the energy-transfer efficiency is quenched by dioxygen, thereby showing that the energy donor is the lowest triplet excited state of anthracene. In [D.2Nd(3+)] the intrinsic emission efficiency of Nd(3+) is much higher (ca. 5 times) than in [C.Nd(3+)] because of a better protection of the excited lanthanide ion towards nonradiative deactivation caused by interaction with solvent molecules. By mixing solutions of D, Nd(3+), and C with proper concentrations, a supramolecular structure with five components of three different species, [D.2Nd(3+).2C], is formed. The excitation light absorbed by the clips is transferred with 100% efficiency to the dansyl units of the dendrimer and then to the Nd(3+) ions with 65% efficiency either in the presence or absence of dioxygen. These results show that the [D.2Nd(3+).2C] complex is able to efficiently harvest UV light by the 24 dansyl units of the dendrimer and the four anthracene chromophores of the two clips, and efficiently transfer it to the encapsulated Nd(3+) ions that emit in the NIR spectral region.

Light powered molecular machines.

The bottom-up construction and operation of mechanical machines of molecular size is a topic of high interest for nanoscience, and a fascinating challenge of nanotechnology. Like their macroscopic counterparts, nanoscale machines need energy to operate. Although most molecular motors of the biological world are fueled by chemical reactions, light is a very good choice to power artificial molecular machines because it can also be used to monitor the state of the machine, and makes it possible to obtain systems that show autonomous operation and do not generate waste products. By adopting an incrementally staged design strategy, photoinduced processes can be engineered within multicomponent (supramolecular) species with the purpose of obtaining light-powered molecular machines. Such an approach is illustrated in this tutorial review by describing some examples based on rotaxanes investigated in our laboratories.

Adducts between dansylated poly(propylene amine) dendrimers and anthracene clips mediated by Zn(II) ions: highly efficient photoinduced energy transfer.

A family of poly(propylene amine) dendrimers, decorated at their periphery with 4, 16, and 32 dansyl units and a molecular clip, composed of two anthracene sidewalls and a disulfate benzene bridging unit, show intense UV absorption and strong fluorescence in the visible region when in a CH(3)CN/CH(2)Cl(2) (1:1, v/v) solvent mixture. Both these classes of compounds are good ligands for Zn(II) ions, as demonstrated by the changes in the absorption and fluorescence spectra upon addition of metal ions. These coordinating properties have been exploited in the self-assembly of complex structures in which the interaction between a dansylated dendrimer and anthracene-functionalized clips is mediated by Zn(II) ions. The self-assembly process is reversible and the number of metal ions and molecular clips associated with each dendrimer increases with the generation number. In these adducts, an energy transfer process from the anthracene to yield the fluorescent excited state of dansyl takes place with almost unitary efficiency.

Dendrimers with a pentaphenylene core: a photophysical study.

Novel dendrimers G2PC and G4PC consisting of a p-pentaphenylene core (PC) appended in the para position with two second-generation (G2) or two fourth-generation (G4) sulfonimide branches and two n-octyl chains, as well as a model compound of the pentaphenylene core (G0PC), are prepared. The photophysical properties (absorption, emission, and excitation spectra; fluorescence decay lifetime; and fluorescence anisotropy spectra) of the three compounds are investigated under different experimental conditions (dichloromethane solution and solid state at 293 K, dichloromethane/methanol rigid matrix at 77 K). In the absorption spectra contributions from both the branches and the core can be clearly identified. The fluorescence spectra show only the characteristic fluorescence of the pentaphenylene unit with lambda(max) around 410 nm in fluid solution and 420 nm in the solid state. In solution the fluorescence quantum yields are 0.78, 0.76, and 0.72 for G0PC, G2PC, and G4PC, respectively, and the fluorescence lifetime is about 0.7 ns in all cases. Energy transfer from the chromophoric groups of the dendrimer branches to the core does not occur. The three compounds show the same, high steady-state anisotropy value (0.35) in dilute rigid-matrix solution at 77 K. In dichloromethane at 293 K, the increasing anisotropy values along the series G0PC (0.17), G2PC (0.27), and G4PC (0.32), with increasing molecular volume of the three compounds, show that depolarization takes place by molecular rotation. In the solid state the anisotropy is very low (0.015, 0.017, and 0.035 for G0PC, G2PC, and G4PC, respectively), probably because of fast depolarization via energy migration.

Polysulfurated pyrene-cored dendrimers: luminescent and electrochromic properties.

We have synthesized a novel class of dendrimers, consisting of a polysulfurated pyrene core with appended poly(thiophenylene) dendrons (PyG0, PyG1, and PyG2, see Scheme 1), which exhibit remarkable photophysical and redox properties. In dichloromethane or cyclohexane solution they show a strong, dendron-localized absorption band with a maximum at around 260 nm and a band in the visible region with a maximum at 435 nm, which can be assigned to the pyrene core strongly perturbed by the four sulfur substituents. The dendrimers exhibit a strong (Phi=0.6), short-lived (tau=2.5 ns) core-localized fluorescence band with maximum at approximately 460 nm in cyclohexane solution at 293 K. A strong fluorescence is also observed in dichloromethane solution at 293 K, in dichloromethane/chloroform rigid matrix at 77 K, and in the solid state (powder) at room temperature. The dendrimers undergo reversible chemical and electrochemical one-electron oxidation with formation of a strongly colored deep blue radical cation. A second, reversible one-electron oxidation is observed at more positive potential values. The photophysical and redox properties of the three dendrimers are finely tuned by the length of their branches. The strong blue fluorescence and the yellow to deep blue color change upon reversible one-electron oxidation can be exploited for optoelectronic devices.

Shape-persistent macrocycles functionalised with coumarin dyes: acid-controlled energy- and electron-transfer processes.

We have investigated the spectroscopic properties (absorption spectra, emission spectra, emission lifetimes) of three triads in CH(2)Cl(2): C2-M-C2, C343-M-C343, and C2-M-C343, in which M is a shape-persistent macrocyclic hexagonal backbone composed of two 2,2'-bipyridine (bpy) units embedded in opposing sides, and C2 and C343 are coumarin 2 and coumarin 343, respectively. All the components are strongly fluorescent species (Phi=0.90, 0.79, and 0.93 for M, C2, and C343, respectively, as established by investigating suitable model compounds). In each triad excitation of M leads to almost quantitative energy transfer to the lowest coumarin-localised excited state. Upon addition of acid, the two bpy units of the M component undergo independent protonation leading to monoprotonated (e.g., C2-MH(+)-C2) and diprotonated (e.g., C2-M2 H(+)-C2) species. Further addition of acid leads to protonation of the coumarin component so that each triad is involved in four protonation equilibria. Protonation causes strong (and reversible, upon addition of base) changes in the absorption and fluorescence properties of the triads because of inversion of the excited-state order and/or the occurrence of electron-transfer quenching processes.

Polyviologen dendrimers as hosts and charge-storing devices.

Two dendrimers were designed and synthesized that contain a 1,3,5-trisubstituted benzenoid core and incorporate 9 and 21 viologen (4,4'-bipyridinium) units in their branches in addition to hydrophilic (aryloxy) terminal groups. For comparison purposes, model compounds containing one and two viologen units were also studied. These polycationic dendrimers form strong host-guest complexes with the dianionic form of the red dye eosin in dilute CH(2)Cl(2) solutions. Titration experiments, based on fluorescence measurements, showed that each viologen unit in the dendritic structures becomes associated with an eosin dianion. Electrochemical (in MeCN) and photosensitization (in CH(2)Cl(2)) experiments revealed that only a fraction of the viologen units present in the dendritic structures can be reduced. This fraction corresponds to the number of viologen units present in the outer shells of the dendrimers. The reasons for incomplete charge pooling are discussed. Comparison with the behavior of polyviologen dendrimers that are terminated with bulky tetraarylmethane groups and were studied previously enabled the role played by the terminal groups in the redox and hosting properties to be elucidated.

Photochemical conversion of solar energy.

Energy is the most important issue of the 21st century. About 85% of our energy comes from fossil fuels, a finite resource unevenly distributed beneath the Earth's surface. Reserves of fossil fuels are progressively decreasing, and their continued use produces harmful effects such as pollution that threatens human health and greenhouse gases associated with global warming. Prompt global action to solve the energy crisis is therefore needed. To pursue such an action, we are urged to save energy and to use energy in more efficient ways, but we are also forced to find alternative energy sources, the most convenient of which is solar energy for several reasons. The sun continuously provides the Earth with a huge amount of energy, fairly distributed all over the world. Its enormous potential as a clean, abundant, and economical energy source, however, cannot be exploited unless it is converted into useful forms of energy. This Review starts with a brief description of the mechanism at the basis of the natural photosynthesis and, then, reports the results obtained so far in the field of photochemical conversion of solar energy. The "grand challenge" for chemists is to find a convenient means for artificial conversion of solar energy into fuels. If chemists succeed to create an artificial photosynthetic process, "... life and civilization will continue as long as the sun shines!", as the Italian scientist Giacomo Ciamician forecast almost one hundred years ago.

Molecular clips with extended aromatic sidewalls as receptors for electron-acceptor molecules. Synthesis and NMR, photophysical, and electrochemical properties.

We have synthesized molecular clips 1 comprising (i) two benzo[k]fluoranthene sidewalls and (ii) a dimethylene-connected benzene bridge that carries two acetoxy (1a), hydroxy (1b), or methoxy (1c) substituents in the para position. Their NMR spectra, single-crystal structures, and photophysical (fluorescence intensity, lifetime, depolarization) and electrochemical properties are discussed. For the purpose of comparison, similar compounds (2 and 3) containing only one benzo[k]fluoranthene unit have been prepared and studied. The strongly fluorescent clips 1 form stable complexes with electron-acceptor guests because of a highly negative electrostatic potential on the inner van der Waals surface of their cavity. The complexation constants in chloroform solution for a variety of guests, determined by NMR and fluorescence titration, are much larger than those of the corresponding anthracene and naphthalene clips (4 and 5), particularly in the case of extended aromatic guests. The effect of the substituents in the para position of the benzene spacer unit of clips 1 is discussed on the basis of the host-guest complex structures obtained by X-ray analysis and molecular mechanics simulations. In the case of 9-dicyanomethylene-2,4,7-trinitrofluorene (TNF) guest, complex formation with clip 1a causes dramatic changes in the photophysical and electrochemical properties: (i) a new charge-transfer band at 600 nm arises, (ii) a very efficient quenching of the strong benzo[k]fluoranthene fluorescence takes place, (iii) shifts of both the first oxidation (clip-centered) and reduction (TNF-centered) potentials are observed, and (iv) reversible disassembling of the complex can be obtained by electrochemical stimulation.