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Silicon (Si) nanocrystals are relatively strong light emitters, but are weak light absorbers as a result of their indirect band gap. One way to enhance light absorption is to functionalize the nanocrystals with chromophores that are strong light absorbers. By designing systems that enable efficient energy transfer from the chromophore to the Si nanocrystal, the brightness of the nanocrystals can be significantly increased. There have now been a few experimental systems in which covalent attachment of chromophores, efficient energy transfer and significantly increased brightness have been demonstrated. This review discusses progress on these systems and the remaining challenges.

Combinations of suitably chosen metal ions and dendritic ligands may lead to giant metal complexes that display interesting spectroscopic properties, particularly as far as luminescence is concerned. This paper reviews some recent results obtained with metal complexes of dendrimers capable of exhibiting: (i) metal-centered, (ii) metal-to-ligand charge-transfer, or (iii) ligand-centered luminescence. The study of the luminescent properties reveals that these compounds can be useful for several purposes, including light harvesting, sensing with signal amplification, and conversion of the absorbed light into emitted light of quite different wavelength (e.g., UV light into IR light).

A regioselective vinylation of aromatic and aliphatic aldehydes promoted by the merging of photoredox and nickel catalysis is here reported. A comprehensive investigation on the reaction conditions allowed the disclosure of a valid and reproducible protocol based on a nickel-mediated reductive coupling approach under visible light irradiation. The employment of 3CzClIPN (2,4,6-tris(carbazol-9-yl)-5-chloro-isophthalonitrile) as the photocatalyst and Hantzsch's ester as the sacrificial organic reductant replace the use of boron-, silicon- or zinc-based reducing agents, making this method a worthy alternative to the already known protocols. The developed mild reaction conditions allow the access to a wide range of substituents decorating both the aldehyde and the alkyne. Moreover, careful photophysical investigations shed light on the mechanism of the reaction.

Presented herein are two fully characterized gold tetrathiocyanocorroles representing a potentially significant new class of NIR-emissive 5d-metallocorroles. The four SCN groups on the bipyrrole unit of the corrole exert a powerful electron-withdrawing effect, upshifting both the oxidation and reduction potentials by roughly half a volt relative to their unsubstituted counterparts. That said, the upshift of the LUMO is somewhat higher than that of the HOMO so these complexes also exhibit a smaller HOMO–LUMO gap, as evinced in both electrochemical measurements and Q band energies (∼595 nm relative to ∼571 nm for their SCN-free counterparts). The new compounds exhibit NIR phosphorescence under ambient conditions with emission maxima around 900 nm (compared with 790 nm for simple Au triarylcorroles), phosphorescence quantum yields around 0.3%, phosphorescence lifetimes around 10 μs, and singlet oxygen sensitization with a quantum yield of around 50 ± 5% in solution, together signifying wide-ranging potential applications as triplet photosensitizers in oxygen sensing and photodynamic therapy.

BODIPYs are renowned fluorescent dyes with strong and tunable absorption in the visible region, high thermal and photo-stability and exceptional fluorescence quantum yields. Transition metal complexes are the most commonly used triplet photosensitisers, but, recently, the use of organic dyes has emerged as a viable and more sustainable alternative. By proper design, BODIPY dyes have been turned from highly fluorescent labels into efficient triplet photosensitizers with strong absorption in the visible region (from green to orange). In this perspective, we report three design strategies: (i) halogenation of the dye skeleton, (ii) donor–acceptor dyads and (iii) BODIPY dimers. We compare pros and cons of these approaches in terms of optical and electrochemical properties and synthetic viability. The potential applications of these systems span from energy conversion to medicine and key examples are presented., BODIPYs offer a versatile platform to build organic triplet photosensitisers for PDT, TTA upconversion and photocatalysis. Tuning their properties provides the opportunity of replacing heavy-metal complexes and can lead to improved sustainability.

Tetraphenylmethane appended with four pyridylpyridinium units works as a scaffold to self-assemble four ruthenium porphyrins in a tetrahedral shape-persistent giant architecture. The resulting supramolecular structure has been characterised in the solid state by X-ray single crystal analysis and in solution by various techniques. Multinuclear NMR spectroscopy confirms the 1 : 4 stoichiometry with the formation of a highly symmetric structure. The self-assembly process can be monitored by changes of the redox potentials, as well as by modifications in the visible absorption spectrum of the ruthenium porphyrin and by a complete quenching of both the bright fluorescence of the tetracationic scaffold and the weak phosphorescence of the ruthenium porphyrin. An ultrafast photoinduced electron transfer is responsible for this quenching process. The lifetime of the resulting charge separated state (800 ps) is about four times longer in the giant supramolecular structure compared to the model 1 : 1 complex formed by the ruthenium porphyrin and a single pyridylpyridinium unit. Electron delocalization over the tetrameric pyridinium structure is likely to be responsible for this effect.

We report the use of microfluidics to functionalize suspended reduced graphene oxide flakes through the addition of aryl radical, generated in situ by reaction between aryl amines and isopentyl nitrite. Microfluidic enabled a tight control of temperature, reaction times and stoichiometric ratios, making it possible to tune the growth of oligomers on the surface of the flakes, which in turn affects the interactions of the functional material with the surrounding environment. The results suggest that shear stress phenomena within the reactor may play a role in the chemistry of graphene materials, by providing a constant driving force towards exfoliation of the layered structures. Scale up of the functionalization process is also reported, along with the grafting of dyes based on squaric acid cores. Photophysical characterization of the dye-modified flakes proves that the microfluidic approach is a viable method towards the development of new materials with tailored properties.

The herein reported visible-light-activated catalytic asymmetric [3+2] photocycloadditions between cyclopropanes and alkenes or alkynes provide access to chiral cyclopentanes and cyclopentenes, respectively, in 63–99 % yields and with excellent enantioselectivities of up to >99 % ee. The reactions are catalyzed by a single bis-cyclometalated chiral-at-metal rhodium complex (2–8 mol %) which after coordination to the cyclopropane generates the visible-light-absorbing complex, lowers the reduction potential of the cyclopropane, and provides the asymmetric induction and overall stereocontrol. Enabled by a mild single-electron-transfer reduction of directly photoexcited catalyst/substrate complexes, the presented transformations expand the scope of catalytic asymmetric photocycloadditions to simple mono-acceptor-substituted cyclopropanes affording previously inaccessible chiral cyclopentane and cyclopentene derivatives.

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.

Photoluminescence (PL) spectra were measured for dodecene-capped Si nanocrystals with a wide range of average diameters, from 1.8 to 9.1 nm. Nanocrystals larger than 3 nm exhibited relatively high PL quantum yields of 30%−45%. Smaller nanocrystals exhibited lower quantum yields that decreased significantly with reduced size. Because smaller nanocrystals also have lower optical absorption there is a significant biasing of the PL spectra by the larger nanocrystals. We show that with proper accounting of polydispersity and size- dependent quantum yields and optical absorption the effective mass approximation (EMA) accurately estimates the average diameter of silicon (Si) nanocrystals from experimentally determined PL emission peak energies. A finite confinement model is presented that explains the decreased PL quantum yields of the smaller diameter nanocrystals.

A shape-persistent molecule, featuring four bipyridinium units, has been synthesized that upon reduction undergoes intermolecular pimerization because of the rigid architecture of the molecule. The pimerization process has been investigated by a variety of techniques, such as absorption measurements, EPR spectroscopy, as well as gamma and pulse radiolysis, and compared with the behavior of a model compound. Computational studies have also been performed to support the experimental data. The most interesting feature of the tetramer is that pimerization occurs only above a threshold concentration of monoreduced species, on the contrary to the model compound. Furthermore, there is an increase of the apparent pimerization constant by increasing the concentration of reduced bipyridinium units. These results have been interpreted by the fact that pimerization is favored in the tetrahedrally shaped molecule because of a cooperative mechanism. Each multiply reduced molecule can indeed undergo multiple intermolecular interactions that enhance the stabilization of the system, also leading to hierarchical supramolecular growth. The resulting supramolecular system formed by such intermolecular pimerization should exhibit a diamond-like structure, as suggested by a simplified modeling approach. The intermolecular nature of the pimerization process occurring in the tetramer has been demonstrated by measuring the corresponding bimolecular rate constant by pulsed radiolysis experiments.

Photocatalysis is an important branch of catalysis and much more than that. To understand the potential applications and the working mechanisms of photocatalysis, it is necessary to know some important concepts of photochemistry, the branch of science that deals with the interaction of light and matter: (1) light excitation with a photon of suitable energy promotes a molecule or a semiconductor from the ground state to an electronically excited state that exhibits its own chemical and physical properties; (2) the most relevant consequence from the viewpoint of photocatalysis is that the excited state is both a better oxidant and a better reductant than the ground state; (3) some molecules or semiconductors can serve as photosensitizers, i.e., they can absorb light and then make available the excited state energy to promote reactions of non-absorbing species. Photosensitization and photocatalysis play an important role in nature and technology and they may take place in homogeneous or heterogeneous phase. Such processes can use sunlight (1) to convert solar energy into chemical or electrical energy, (2) to perform organic synthesis that cannot be achieved by thermal activation, and (3) to remedy pollution. Water splitting using sunlight and suitable photosensitizers and catalysts (artificial photosynthesis) is perhaps one of the most thoroughly investigated chemical processes. Breakthrough in this area can contribute to solve the energy and climate crisis, but substantial technological development is still needed.

Photoluminescence of silicon nanocrystals (SiNCs) in the presence of a series of quinone electron acceptors and ferrocene electron donors is quenched by oxidative and reductive electron transfer dynamic processes, respectively. The rate of these processes is investigated as a function of (a) the thermodynamic driving force of the reaction, by changing the reduction potentials of the acceptor or donor molecules, (b) the dimension of SiNCs (diameter = 3.2 or 5.0 nm), (c) the surface capping layer on SiNCs (dodecyl or ethylbenzene groups), and (d) the solvent polarity (toluene vs dichloromethane). The results were interpreted within the classical Marcus theory, enabling us to estimate the position of the valence and conduction bands, as well as the reorganization energy (particularly small, as expected for quantum dots) and electronic transmission coefficients. The last parameter is in the range 10-5-10-6, demonstrating the nonadiabaticity of the process, and it decreases upon increasing the SiNC dimensions: this result is in line with a larger number of excitons generated in the inner silicon core for larger SiNCs and thus resulting in a lower electronic coupling with the quencher molecules.

The synthesis, the structural characterization and the luminescence spectroscopy of new Ln(III) nitrate complexes (Ln = La, Eu, Gd, Tb and Lu) with the quinoline-based N, N’-bis(2-quinolylmethylidene)- 1,2-(R,R+S,S)-cyclohexanediamine, L1 and N, N’-bis(2-quinolylmethyl)- 1,2-(R,R+S,S)-cyclohexanediamine, L2 are presented. All the L1Ln(NO3)3 complexes and L2Ln(NO3)3 with Ln= La, Eu and Tb are isostructural. Metal ions such as La(III), Gd(III) and Lu(III) are able to promote intra-ligand (IL) fluorescence from L1 and L2. Gd(III) and Lu(III) in addition, are also able to stimulate IL phosphorescence at room temperature from the L2 ligand, by means of “paramagnetic effect” and/or “heavy-atom effect”. Both ligands are able to sensitize the typical f-f Eu(III) luminescence and only L2 the one of Tb(III) (green emission). The Eu(III) emission features for both complexes [L1Eu(NO3)3 and L2Eu(NO3)3] are in agreement with their structural properties. All the complexes under investigation reveal promising application in the field of luminescent devices and biomedicine. In particular, excitation at 360 nm, in the case of L1-based complexes, enable the use of glass optics and minimize interfering excitation by chromophores in biological media.

Here we report the application of readily prepared and available coumarin dyes for photoredox catalysis, which are able to mimic powerful reductant [Ir(iii)] complexes. Coumarin derivatives 9 and 10 were employed as photoreductants in pinacol coupling and in other reactions, in the presence of Et3N as a sacrificial reducing agent. As the electronic, photophysical, and steric properties of coumarins could be varied, a wide applicability to several classes of photoredox reactions is predicted.

Two families of strongly luminescent silicon nanocrystals (SiNCs) cofunctionalized with dodecyl chains and pyrene chromophores perform as light-harvesting antennae and interact with carbon allotropes. The energy harvested in the SiNC core can thus be used to populate, for example, the lowest triplet excited state of C-60.

Phosphorescence is generally considered a property of metal complexes; conversely, organic molecules are usually phosphorescent only at 77 K, where non-radiative decays are slowed down and quenching by dioxygen is inhibited. In this issue of Chem , Tang and colleagues report on rational guidelines for the design of organic chromophores exhibiting long-lived and bright phosphorescence at ambient conditions.

none 5 si A tetrahedral shape-persistent molecule equipped with four terpyridine ligands can form an oligomeric structure upon complexation with metal ions, in which two terpyridine units are intermolecularly connected via a metal ion. The most interesting results were obtained upon binding of lanthanide ions, whose luminescence is sensitized by the ligand. In particular, dual luminescent probes were obtained upon concomitant complexation of two different lanthanide ions, such as EuIII and NdIII. Molloy, Jennifer K; Pillai, Zeena; Sakamoto, Junji; Ceroni, Paola; Bergamini, Giacomo Molloy, Jennifer K; Pillai, Zeena; Sakamoto, Junji; Ceroni, Paola; Bergamini, Giacomo

General discussion on natural and artificial photosynthesis.

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.

Photocatalysis enables the cascade reactions of indoles and CBr4 in MeOH through a C(sp(2) )H functionalization/methanolysis sequence. The title reaction provides an efficient access to indole 2- and 3-carboxylates in a single operation (no preinstallation of protecting as well as directing groups was required) with good yields under mild reaction conditions.

We have investigated the complex formation between dendritic guests and a molecular tweezer host by NMR, absorption, and emission spectroscopy as well as electrochemical techniques. The dendrimers are constituted by an electron-acceptor 4,4'-bipyridinium core appended with one (DnB(2+)) or two (Dn(2)B(2+)) polyaryl-ether dendrons. Tweezer T comprises a naphthalene and four benzene components bridged by four methylene groups. Medium effects on molecular recognition phenomena are discussed and provide insight into the conformation of dendrimers: change in solvent polarity from pure CH(2)Cl(2) to CH(2)Cl(2)/CH(3)CN mixtures and addition of tetrabutylammonium hexafluorophosphate (NBu(4)PF(6), up to 0.15 M), the supporting electrolyte used in the electrochemical measurements, have been investigated. The association constants measured in different media show the following trend: (i) they decrease upon increasing polarity of the solvent, as expected for host-guest complexes stabilized by electron donor-acceptor interactions; (ii) no effect of generation and number of dendrons (one for the DnB(2+) family and two for the Dn(2)B(2+) family) appended to the core is observed in higher polarity media; and (iii) in a low-polarity solvent, like CH(2)Cl(2), the stability of the inclusion complexes is higher for DnB(2+) dendrimers than for Dn(2)B(2+) ones, while within each dendrimer family it increases by decreasing dendron generation, and upon addition of NBu(4)PF(6). The last result has been ascribed to a partial dendron unfolding. Kinetic investigations performed in lower polarity media evidence that the rate constants of complex formation are slower for symmetric Dn(2)B(2+) dendrimers than for the nonsymmetric DnB(2+) ones, and that within the Dn(2)B(2+) family, they decrease by increasing dendron generation. The dependence of the rate constants for the formation and dissociation of the complexes upon addition of NBu(4)PF(6) has also been investigated and discussed.

Chemistry can contribute, in many different ways to solve the challenges we are facing to modify our inefficient and fossil-fuel based energy system. The present work was motivated by the search for efficient photoactive materials to be employed in the context of the energy problem: materials to be utilized in energy efficient devices and in the production of renewable electricity and fuels. We presented a new class of copper complexes, that could find application in lighting techhnologies, by serving as luminescent materials in LEC, OLED, WOLED devices. These technologies may provide substantial energy savings in the lighting sector. Moreover, recently, copper complexes have been used as light harvesting compounds in dye sensitized photoelectrochemical solar cells, which offer a viable alternative to silicon-based photovoltaic technologies. We presented also a few supramolecular systems containing fullerene, e.g. dendrimers, dyads and triads.The most complex among these arrays, which contain porphyrin moieties, are presented in the final chapter. They undergo photoinduced energy- and electron transfer processes also with long-lived charge separated states, i.e. the fundamental processes to power artificial photosynthetic systems.

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.

We have investigated the spectroscopic and electrochemical behavior of symmetric and unsymmetric first-, second-, and third-generation dendrimers comprising an electron-acceptor 4,4'-bipyridinium core (viologen type) and electron-donor 1,3-dimethyleneoxybenzene (Frechet-type) dendrons. The quite strong fluorescence of the symmetrically and unsymmetrically disubstituted 1,3-dimethyleneoxybenzene units of the dendrons is completely quenched as a result of donor-acceptor interactions that are also evidenced by a low-energy tail in the absorption spectrum. In dichloromethane solution, the 4,4'-bipyridinium cores of the investigated dendrimers are hosted by a molecular tweezer comprising a naphthalene and four benzene components bridged by four methylene units. Host-guest formation causes the quenching of the tweezer fluorescence. The association constants, as measured from fluorescence and (1)H NMR titration plots, (i) are of the order of 10(4) M(-1), (ii) decrease on increasing dendrimer generation, and (iii) are slightly larger for the unsymmetric than for the symmetric dendrimer of the same generation. The analysis of the complexation-induced shifts of the temperature-dependent (1)H NMR signals of the host and guest protons confirms that the bipyridinium core is positioned inside the tweezer cavity and allows the conclusions that (i) shuttling of the tweezer from one to the other pyridinium ring is fast (DeltaG < 10 kcal/mol), (ii) in the case of the unsymmetric dendrimers, the less substituted pyridinium ring is preferentially complexed in apolar solvents, and (iii) complexation of the 4,4'-bipyridinium core proceeds by clipping for the symmetric dendrimers and by threading in the case of unsymmetric ones. Host-guest formation causes a displacement of the first reduction wave of the 4,4'-bipyridinium unit toward more negative potential values, whereas the second reduction wave is unaffected. These results show that the host-guest complexes between the tweezer and the dendrimers are stabilized by electron donor-acceptor interactions and can be reversibly assembled/disassembled by electrochemical stimulation.

Three dendrimers consisting of a dimethoxybenzil core and branches that contain two (G0), four (G1), and eight (G2) naphthalene units at the periphery and zero (G0), two (G1), and six (G2) dimethoxybenzene units in the branches have been synthesized and their photophysical, photochemical, and electrochemical properties have been investigated. For comparison purposes, the properties of dimethoxybenzil (MB) and of a dendron containing four naphthalene and three dimethoxybenzene units (D2) have also been studied. The properties of the dendrimers in the ground state (absorption spectra and electrochemical behavior) are those expected for their noninteracting component units. The excited state properties, however, are substantially controlled by electronic interactions between the dimethoxybenzil core and the naphthalene units contained in the branches. In dichloromethane–chloroform 1 : 1 (v/v) solution at 298 K, energy transfer from the lowest excited state (S1) of the naphthalene units to the lower lying S1 excited state of the dimethoxybenzil core takes place with high efficiency. In a rigid matrix at 77 K, selective excitation of the dimethoxybenzil chromophore yields an emission band that exhibits a spectral evolution: in the millisecond time scale it shows a spectral profile very similar to the dimethoxybenzil phosphorescence, whereas in the second time scale it is very similar to the naphthalene-type phosphorescence. Energy transfer from the T1 excited state of the dimethoxybenzil core to the T1 excited state of the naphthalene units takes place at 77 K, but not at 298 K, because the T1 excited state of the dimethoxybenzil core moves to energy lower than that of the naphthalene chromophore. The photochemical results show that the dimethoxybenzil core maintains its intrinsic photoreactivity toward dioxygen, and that on increasing dendrimer generation a photoreaction between core and branches predominates.

The efficient transformation of the hexadentate bispidinol 1 into carbamate derivatives yields functional bispidines for the convenient functionalization for targeted imaging. The BODIPY-substituted bispidine 3 combines the coordination site for metal ions (e.g., radioactive 64CuII) with a fluorescent unit. Product 3 was thoroughly characterized by standard analytical methods, single crystal diffraction, radiolabeling and photophysical analysis. The luminescence of ligand 3 was found to be strongly dependent on metal ion coordination: CuII quenches the BODIPY fluorescence, while NiII and ZnII ions do not affect it. It follows that, in imaging applications with the positron emitter 64CuII, residues of its origin from enriched 64Ni and the decay products 64NiII and 64ZnII, efficiently restore the fluorescence of the ligand. This allows for monitoring of the emitted radiation as well as the fluorescence signal. The stability of the 64CuII-3 complex was investigated by transmetalation experiments with ZnII and NiII, using fluorescence and radioactivity detection and the results confirm the high stability of 64CuII-3. In addition, metal complexes of ligand 3 with the lanthanide ions TbIII, EuIII and NdIII are shown to exhibit emission of the BODIPY ligand and the lanthanide ion, thus enabling dual emission detection.

The synthesis, solution behavior and photophysical properties of several heteronuclear bimetallic d/f complexes that utilise a Ru-II bis-terpyridine moiety as the sensitiser for Eu-III, Nd-III and Yb-III luminescence are reported and compared to a Gd-III analogue. The former compounds display sensitised emission in the visible and near-infrared (NIR) regions depending on the choice of the Ln(III) cation. We illustrate that sensitised lanthanide emission can operate by two distinctly different pathways that involve either the triplet ligand-centred ((LC)-L-3) excited state of the organic ligand or the triplet metal-to-ligand charge transfer ((MLCT)-M-3) excited state of the Ru-II complex. Owing to the electronic structure of this antenna and the higher density of metal-centred accepting states, a higher sensitization efficiency with Nd-III was observed than with Yb-III and Eu-III. Lastly, we have observed unusual sensitised emission from the upper (F-4(5/2), H-2(9/2)) excited state of Nd-III at approximately 800 nm, which we report here for the first time from a molecular species in solution.