Birch‐Type Photoreduction of Arenes and Heteroarenes by Sensitized Electron Transfer

The direct reduction of arenes and heteroarenes by visible‐light irradiation remains challenging, as the energy of a single photon is not sufficient for breaking aromatic stabilization. Shown herein is that the energy accumulation of two visible‐light photons allows the dearomatization of arenes and heteroarenes. Mechanistic investigations confirm that the combination of energy‐transfer and electron‐transfer processes generates an arene radical anion, which is subsequently trapped by hydrogen‐atom transfer and finally protonated to form the dearomatized product. The photoreduction converts planar aromatic feedstock compounds into molecular skeletons that are of use in organic synthesis.     

Visible‐Light Photochemical Activity of Nanoporous Carbons under Monochromatic Light

By using monochromatic light the ability of semiconductor‐free nanoporous carbons to convert the low‐energy photons from the visible spectrum into chemical reactions (i.e. phenol photooxidation) is demonstrated. Data shows that the onset wavelength of the photochemical activity can be tuned by surface functionalization, with enhanced visible‐light conversion upon introducing N‐containing groups.     

Upconversion of Reductants

The many applications of photon upconversion—conversion of low‐energy photons into high‐energy photons—raises the question of the possibility of “electron upconversion”. In this Review, we illustrate how the reduction potential can be increased by using the free energy of exergonic chemical reactions. Electron (reductant) upconversion can produce up to 20–25 kcal mol^?1 of additional redox potential, thus creating powerful reductants under mild conditions. We will present the two common types of electron‐upconverting systems—dissociative (based on unimolecular fragmentations) and associative (based on the bimolecular formation of three‐electron bonds). The possible utility of reductant upconversion encompasses redox chain reactions in electrocatalytic processes, photoredox cascades, design of peroxide‐based medicines, firefly luminescence, and reductive repair of DNA photodamage.     

Catalytic Enantioselective Radical Transformations Enabled by Visible Light

Visible light has been recognized as an economical and environmentally benign source of energy that enables chemoselective molecular activation of chemical reactions and hence reveal a new horizon for the design and discovery of novel chemical transformations. On the other hand, asymmetric catalysis represents an economic method to satisfy the increasing need for enantioenriched compounds in the chemical and pharmaceutical industries. Therefore, combining visible light photocatalysis with asymmetric catalysis creates a wider range of opportunities for the development of mechanistically unique reaction schemes. However, there arise two main problems like undesirable photochemical background reactions and difficulties in controlling the stereochemistry with highly reactive photochemical intermediates which can pose a serious challenge to the development of asymmetric visible light photocatalysis. In recent years, several methods have been developed to overcome these challenges. This review summarizes the recent advances in visible light‐induced enantioselective reactions. We divide our discussion into four categories: Asymmetric photoredox organocatalysis, asymmetric transition metal photoredox catalysis, asymmetric photoredox Lewis acid catalysis and asymmetric photoinduced energy transfer catalysis. Special emphasis has been given to different catalytic activation modes that enable the construction of challenging carbon‐carbon and carbon‐heteroatom bond in an enantioselective fashion. A brief analysis of substrate scope and limitation as well as reaction mechanism of these reactions has been included.     

Photon cascade luminescence in CaAl12O19:Pr, Cr

In this report, the Cr³⁺ ion was chosen as a co-dopant to modify the unpractical photon cascade emission properties of Pr³⁺-doped CaAl₁₂O₁₉ phosphors, which emit one near-UV photon from the ¹S₀ state followed by visible photons from the ³P₀ state, into phosphors that emits two visible photons through energy transfer. The photon cascade emission process via energy transfer and the transfer mechanisms were systemically investigated by luminescence spectra and dynamics using synchrotron radiation as one of the excitation sources. The internal visible quantum efficiency of CaAl₁₂O₁₉:Pr, Cr was estimated and compared with CaAl₁₂O₁₉:Pr and the drawback to obtain the visible quantum efficiency higher than unit for CaAl₁₂O₁₉:Pr, Cr as a practical VUV phosphor was also discussed.     

Light‐Harvesting Ytterbium(III)–Porphyrinate–BODIPY Conjugates: Synthesis,Excitation‐Energy Transfer,and Two‐Photon‐Induced Near‐Infrared‐Emission Studies

Based on a donor–acceptor framework, several conjugates have been designed and prepared in which an electron‐donor moiety, ytterbium(III) porphyrinate (YbPor), was linked through an ethynyl bridge to an electron‐acceptor moiety, boron dipyrromethene (BODIPY). Photoluminescence studies demonstrated efficient energy transfer from the BODIPY moiety to the YbPor counterpart. When conjugated with the YbPor moiety, the BODIPY moiety served as an antenna to harvest the lower‐energy visible light, subsequently transferring its energy to the YbPor counterpart, and, consequently, sensitizing the Yb^III emission in the near‐infrared (NIR) region with a quantum efficiency of up to 0.73 % and a lifetime of around 40 μs. Moreover, these conjugates exhibited large two‐photon‐absorption cross‐sections that ranged from 1048–2226 GM and strong two‐photon‐induced NIR emission.     

Visible‐Light‐Induced Organic Photochemical Reactions through Energy‐Transfer Pathways

Visible‐light photocatalysis is a rapidly developing and powerful strategy to initiate organic transformations, as it closely adheres to the tenants of green and sustainable chemistry. Generally, most visible‐light‐induced photochemical reactions occur through single‐electron transfer (SET) pathways. Recently, visible‐light‐induced energy‐transfer (EnT) reactions have received considerable attentions from the synthetic community as this strategy provides a distinct reaction pathway, and remarkable achievements have been made in this field. In this Review, we highlight the most recent advances in visible‐light‐induced EnT reactions.     

Visible‐Light Sensitization of Vinyl Azides by Transition‐Metal Photocatalysis

Irradiation of vinyl and aryl azides with visible light in the presence of Ru photocatalysts results in the formation of reactive nitrenes, which can undergo a variety of C? N bond‐forming reactions. The ability to use low‐energy visible light instead of UV in the photochemical activation of azides avoids competitive photodecomposition processes that have long been a significant limitation on the synthetic use of these reactions.     

Pump‐Pump‐Probe Spectroscopy of a Molecular Triad Monitoring Detrimental Processes for Photoinduced Charge Accumulation

Controlling light‐induced accumulation of electrons or holes is desirable in view of multi‐electron redox chemistry, for example for the formation of solar fuels or for photoredox catalysis in general. Excitation with multiple photons is usually required for electron or hole accumulation, and consequently pump‐pump‐probe spectroscopy becomes a valuable spectroscopic tool. In this work, we excited a triarylamine‐Ru(bpy)₃²⁺‐anthraquinone triad (bpy = 2,2′‐bipyridine) with two temporally delayed laser pulses of different color and monitored the resulting photoproducts. Absorption of the first photon by the Ru(bpy)₃²⁺ photosensitizer generated a triarylamine radical cation and an anthraquinone radical anion by intramolecular electron transfer. Subsequent selective excitation of either one of these two radical ion species then induced rapid reverse electron transfer to yield the triad in its initial (ground) state. This shows in direct manner that after absorption of a first photon and formation of the primary photoproducts, the absorption of a second photon can lead to unproductive electron transfer events that counteract further charge accumulation. In principle, this problem is avoidable by careful excitation wavelength selection in combination with good molecular design.     

Photoinduced water splitting with oxotitanium porphyrin: a computational study

The photochemistry of the hydrogen-bonded oxotitanium porphyrin-water complex (TiOP-H(2)O) has been explored with electronic-structure calculations. It is shown that intramolecular charge-transfer processes, which are initiated by the excitation of the Soret band of TiOP, accumulate electronic charge on the oxygen atom of TiOP, which in turn abstracts a hydrogen atom from water by an exoenergetic and essentially barrierless hydrogen-transfer reaction, resulting in the TiPOH˙-OH˙ biradical. About 75% of the absorbed photon energy is thus stored as chemical energy in two ground-state radicals. Absorption of a second photon by TiPOH˙ can result in the detachment of the H˙ radical and recovery of the photocatalyzer TiOP. Again, about 75% of the photon energy is stored in the dissociation energy of TiPOH˙. Overall, a water molecule is decomposed into H˙ and OH˙ radicals by the absorption of two visible photons. Exoenergetic radical recombination reactions can yield molecular hydrogen, molecular oxygen or hydrogen peroxide as closed-shell products.     

Sensitized Two‐NIR‐Photon Z→E Isomerization of a Visible‐Light‐Addressable Bistable Azobenzene Derivative

Two‐NIR‐photon‐triggered Z→E isomerization of an azobenzene was accomplished by covalently linking a two‐photon‐harvesting triarylamine antenna to a thermally stable ortho‐fluorinated azobenzene derivative. The obtained photoswitch is fully addressable with visible and NIR light by using one‐photon and two‐photon excitation, respectively, with the latter offering enhanced penetration depth and improved spatial resolution.     

Discovery of a New Light–Molecule Interaction: Supracence Reveals What Is Missing in Fluorescence Imaging

The currently understood principles about light–molecule interactions are limited, and thus scientific scope beyond current theories is rarely harvested. Herein we demonstrate supracence phenomena, in which the emitted photons have more energy than the absorbed photons. The extra energy comes from couplings of the absorbed and emitted photon to molecular phonons, whose potentials are constantly exchanging with molecular quantum energy and the environment. Thus, supracence is a linear optical process rather than a nonlinear optical process, such as second harmonic generation. Because supracence results in cooled molecular phonons and thus cooled molecules, behavior opposite to that of hot fluorescing emitters is expected. This report reveals certain supracence principles while contrasting fluorescence with supracence in high‐resolution imaging.     

Light‐Driven Electron Accumulation in a Molecular Pentad

Accumulation and temporary storage of redox equivalents with visible light as an energy input is of pivotal importance for artificial photosynthesis because key reactions, such as CO₂ reduction or water oxidation, require the transfer of multiple redox equivalents. We report on the first purely molecular system, in which a long‐lived charge‐separated state (τ≈870 ns) with two electrons accumulated on a suitable acceptor unit can be observed after excitation with visible light. Importantly, no sacrificial reagents were employed.     

[2+2] Cycloaddition of 1,3‐Dienes by Visible Light Photocatalysis

[2+2] Photocycloadditions of 1,3‐dienes represent a powerful yet synthetically underutilized class of reactions. We report that visible light absorbing transition metal complexes enable the [2+2] cycloaddition of a diverse range of 1,3‐dienes. The ability to use long‐wavelength visible light is attractive because these reaction conditions tolerate the presence of sensitive functional groups that might be readily decomposed by the high‐energy UVC radiation required for direct photoexcitation of 1,3‐dienes. The resulting vinylcyclobutane products are poised for a variety of further diversification reactions, and this method is consequently expected to be powerfully enabling in the synthesis of complex organic targets.     

Energy Relay from an Unconventional Yellow Dye to CdS/CdSe Quantum Dots for Enhanced Solar Cell Performance

A new design for a quasi‐solid‐state Forster resonance energy transfer (FRET) enabled solar cell with unattached Lucifer yellow (LY) dye molecules as donors and CdS/CdSe quantum dots (QDs) tethered to titania (TiO₂) as acceptors is presented. The Forster radius is experimentally determined to be 5.29 nm. Sequential energy transfer from the LY dye to the QDs and electron transfer from the QDs to TiO₂ is followed by fluorescence quenching and electron lifetime studies. Cells with a donor–acceptor architecture (TiO₂/CdS/CdSe/ZnS‐LY/S^2?‐multi‐walled carbon nanotubes) show a maximum incident photon‐to‐current conversion efficiency of 53 % at 530 nm. This is the highest efficiency among Ru‐dye free FRET‐enabled quantum dot solar cells (QDSCs), and is much higher than the donor or acceptor‐only cells. The FRET‐enhanced solar cell performance over the majority of the visible spectrum paves the way to harnessing the untapped potential of the LY dye as an energy relay fluorophore for the entire gamut of dye sensitized, organic, or hybrid solar cells.     

Excited‐State Charge Separation in the Photochemical Mechanism of the Light‐Driven Enzyme Protochlorophyllide Oxidoreductase

The unique light‐driven enzyme protochlorophyllide oxidoreductase (POR) is an important model system for understanding how light energy can be harnessed to power enzyme reactions. The ultrafast photochemical processes, essential for capturing the excitation energy to drive the subsequent hydride‐ and proton‐transfer chemistry, have so far proven difficult to detect. We have used a combination of time‐resolved visible and IR spectroscopy, providing complete temporal resolution over the picosecond–microsecond time range, to propose a new mechanism for the photochemistry. Excited‐state interactions between active site residues and a carboxyl group on the Pchlide molecule result in a polarized and highly reactive double bond. This so‐called “reactive” intramolecular charge‐transfer state creates an electron‐deficient site across the double bond to trigger the subsequent nucleophilic attack of NADPH, by the negatively charged hydride from nicotinamide adenine dinucleotide phosphate. This work provides the crucial, missing link between excited‐state processes and chemistry in POR. Moreover, it provides important insight into how light energy can be harnessed to drive enzyme catalysis with implications for the design of light‐activated chemical and biological catalysts.     

Infrared-enhanced multiphoton dissociation of iodobenzene cations

When gas-phase iodobenzene cations in an ion cyclotron resonance spectrometer are irradiated simultaneously with visible 610 nm and IR 977 cm⁻¹ photons, the visible two-photon process is enhanced by a factor of up to three. The experimental pressure and intensity dependences can be modeled using a set of rate equations for the evolution of the populations of different internal energy levels. Modeling suggests that the major mechanism for the mixed visible/IR process is the absorption of several IR photons, followed by the absorption of one visible photon which brings about dissociation: that the visible photoexcitation cross section decreases with increasing ion internal energy: that IR pumping does not compete successfully with relaxation processes near dissociation threshold: and that the IR radiative relaxation time is constant ≈ 500 ms.     

From One‐Photon to Two‐Photon Probes: “Caged” Compounds,Actuators, and Photoswitches

Molecular systems that can be remotely controlled by light are gaining increasing importance in cell biology, physiology, and neurosciences because of the spatial and temporal precision that is achievable with laser microscopy. Two‐photon excitation has significant advantages deep in biological tissues, but raises problems in the design of “smart” probes compatible with cell physiology. This Review discusses the chemical challenges in generating suitable two‐photon probes.     

Highly Efficient,Conjugated‐Polymer‐Based Nano‐Photosensitizers for Selectively Targeted Two‐Photon Photodynamic Therapy and Imaging of Cancer Cells

Two‐photon photodynamic therapy (2P‐PDT) is a promising noninvasive treatment of cancers and other diseases with three‐dimensional selectivity and deep penetration. However, clinical applications of 2P‐PDT are limited by small two‐photon absorption (TPA) cross sections of traditional photosensitizers. The development of folate receptor targeted nano‐photosensitizers based on conjugated polymers is described. In these nano‐photosensitizers, poly{9,9‐bis[6′′‐(bromohexyl)fluorene‐2,7‐ylenevinylene]‐co‐alt‐1,4‐(2,5‐dicyanophenylene)}, which is a conjugated polymer with a large TPA cross section, acts as a two‐photon light‐harvesting material to significantly enhance the two‐photon properties of the doped photosensitizer tetraphenylporphyrin (TPP) through energy transfer. These nanoparticles displayed up to 1020‐fold enhancement in two‐photon excitation emission and about 870‐fold enhancement in the two‐photon‐induced singlet oxygen generation capability of TPP. Surface‐functionalized folic acid groups make these nanoparticles highly selective in targeting and killing KB cancer cells over NIH/3T3 normal cells. The 2P‐PDT activity of these nanoparticles was significantly improved, potentially up to about 1000 times, as implied by the enhancement factors of two‐photon excitation emission and singlet oxygen generation. These nanoparticles could act as novel two‐photon nano‐photosensitizers with combined advantages of low dark cytotoxicity, targeted 2P‐PDT with high selectivity, and simultaneous two‐photon fluorescence imaging capability; these are all required for ideal two‐photon photosensitizers.     

Energy up-conversion by low-power excitation: new applications of an old concept

Energy up-conversion is the process by which two absorbed photons are converted in one photon of higher energy. Such a process can be conveniently performed by low-power excitation through sensitized triplet-triplet annihilation and it is now an emerging technique with possible applications in different fields, including photovoltaic devices and bioimaging.