Highly efficient light-driven hydrogen evolution utilizing porphyrin-based nanoparticles.

We developed dye-sensitized photocatalytic systems (DSPs) by utilizing porphyrins as a photosensitizer (PS) or as a photosensitizer-catalyst (PS/CAT) upon their chemisorption onto platinum-doped titanium dioxide nanoparticles (Pt-TiO2 NPs). The DSPs coated with Pt-Tc3CP (PS/CAT entity) exhibited a record-high stability (25 500 TONs) and H2 evolution activity (707 mmol g-1 h-1) compared to similar DSPs in the literature.

Light-induced hydrogen production from water using nickel(II) catalysts and N-doped carbon-dot photosensitizers: catalytic efficiency enhancement by increase of catalyst nuclearity.

Solar energy conversion to chemical energy via light-induced H2O splitting to O2 and H2 is considered to be a promising solution to meet the growing global energy demands. To make this transformation economically viable, it is necessary to develop sustainable photocatalytic systems. Herein, we present an efficient photocatalytic H2 production system which relies on components comprised of low-cost and high-abundance elements. In particular, a series of mononuclear complexes [Ni(LNS)3]- and [Ni(N^N)(LNS)2] and a hexanuclear complex [Ni(LNS)2]6 (N^N = diimine and LNS- = heterocyclic thioamidate with different group-substituents) were synthesized and utilized as catalysts, in combination with N-doped carbon dots as photosensitizer, for efficient H2 evolution from aqueous protons. Differences in H2 production efficiency were observed among the studied Ni(II) catalysts, with complexes bearing ligands with stronger electron-donating ability exhibiting higher catalytic activity. A remarkable catalytic efficiency enhancement was observed for the hexanuclear complex, with catalyst loadings lower than those of the mononuclear Ni(II) complexes, affording TONs >1550 (among the highest values reported for photocatalytic systems of similar type operating in H2O). These data provide an indication of catalytic cooperativity between the metal centers of the hexanuclear complex, and demonstrate the crucial role of atomically precise polynuclear Ni(II) catalysts in light-induced H2 production, a result that can guide future catalyst design towards the development of highly efficient, low-cost and environmentally benign photocatalytic systems.

Porphyrins and phthalocyanines as biomimetic tools for photocatalytic H2 production and CO2 reduction.

The increasing energy demand and environmental issues caused by the over-exploitation of fossil fuels render the need for renewable, clean, and environmentally benign energy sources unquestionably urgent. The zero-emission energy carrier, H2 is an ideal alternative to carbon-based fuels especially when it is generated photocatalytically from water. Additionally, the photocatalytic conversion of CO2 into chemical fuels can reduce the CO2 emissions and have a positive environmental and economic impact. Inspired by natural photosynthesis, plenty of artificial photocatalytic schemes based on porphyrinoids have been investigated. This review covers the recent advances in photocatalytic H2 production and CO2 reduction systems containing porphyrin or phthalocyanine derivatives. The unique properties of porphyrinoids enable their utilization both as chromophores and as catalysts. The homogeneous photocatalytic systems are initially described, presenting the various approaches for the improvement of photosensitizing activity and the enhancement of catalytic performance at the molecular level. On the other hand, for the development of the heterogeneous systems, numerous methods were employed such as self-assembled supramolecular porphyrinoid nanostructures, construction of organic frameworks, combination with 2D materials and adsorption onto semiconductors. The dye sensitization on semiconductors opened the way for molecular-based dye-sensitized photoelectrochemical cells (DSPECs) devices based on porphyrins and phthalocyanines. The research in photocatalytic systems as discussed herein remains challenging since there are still many limitations making them unfeasible to be used at a large scale application before finding a large-scale application.

Shape dependent photocatalytic H2 evolution of a zinc porphyrin.

Hydrogen is regarded as a promising molecular fuel in order to produce clean energy, thus it is of great importance to produce and store H2 in order to replace fossil fuels and to resolve the global energy and environmental problems. One strategy to produce hydrogen is the photocatalytic splitting of water. In this study different supramolecular architectures of a Zn(II) porphyrin, showing "flower", octahedral and "manta ray" shaped structures, were obtained using the "good-bad" solvent self-assembly protocol. More specifically, the bad solvent (methanol) was retained and the good solvent was alerted obtaining diverse assemblies. The different structures were studied by scanning electron microscopy, PXRD, UV-Vis and IR spectroscopies. The prepared structures were capable of proton reduction and production of molecular H2 in the presence of 5% w/w Pt-nanoparticles as catalysts and ascorbic acid as a sacrificial electron donor. Moreover, depending on the structure of the chromophore that is formed the amount of H2 produced varies. The maximum H2 production was obtained with the octahedral structures (185.5 µmol g-1 h-1).

Carbon Nanodots as Electron Transport Materials in Organic Light Emitting Diodes and Solar Cells.

Charge injection and transport interlayers play a crucial role in many classes of optoelectronics, including organic and perovskite ones. Here, we demonstrate the beneficial role of carbon nanodots, both pristine and nitrogen-functionalized, as electron transport materials in organic light emitting diodes (OLEDs) and organic solar cells (OSCs). Pristine (referred to as C-dots) and nitrogen-functionalized (referred to as NC-dots) carbon dots are systematically studied regarding their properties by using cyclic voltammetry, Fourier-transform infrared (FTIR) and UV-Vis absorption spectroscopy in order to reveal their energetic alignment and possible interaction with the organic semiconductor's emissive layer. Atomic force microscopy unravels the ultra-thin nature of the interlayers. They are next applied as interlayers between an Al metal cathode and a conventional green-yellow copolymer-in particular, (poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-{2,1',3}-thiadiazole)], F8BT)-used as an emissive layer in fluorescent OLEDs. Electrical measurements indicate that both the C-dot- and NC-dot-based OLED devices present significant improvements in their current and luminescent characteristics, mainly due to a decrease in electron injection barrier. Both C-dots and NC-dots are also used as cathode interfacial layers in OSCs with an inverted architecture. An increase of nearly 10% in power conversion efficiency (PCE) for the devices using the C-dots and NC-dots compared to the reference one is achieved. The application of low-cost solution-processed materials in OLEDs and OSCs may contribute to their wide implementation in large-area applications.

Controlling Solar Hydrogen Production by Organizing Porphyrins.

In this study, a highly efficient photocatalytic H2 production system is developed by employing porphyrins as photocatalysts. Palladium and platinum tetracarboxyporphyrins (PdTCP and PtTCP) are adsorbed or coadsorbed onto TiO2 nanoparticles (NPs), which act as the electron transport medium and as a scaffold that promotes the self-organization of the porphyrinoids. The self-organization of PdTCP and PtTCP, forming H- and J-aggregates, respectively, is the key element for H2 evolution, as in the absence of TiO2 NPs no catalytic activity is detected. Notably, J-aggregated PtTCPs are more efficient for H2 production than H-aggregated PdTCPs. In this approach, a single porphyrin, which self-organizes onto TiO2 NPs, acts as the light harvester and simultaneously as the catalyst, whereas TiO2 serves as the electron transport medium. Importantly, the concurrent adsorption of PdTCP and PtTCP onto TiO2 NPs results in the most efficient catalytic system, giving a turnover number of 22,733 and 30.2 mmol(H2 ) g(cat)-1 .

Photochemical hydrogen production and cobaloximes: the influence of the cobalt axial N-ligand on the system stability.

We report on the first systematic study of cobaloxime-based hydrogen photoproduction in mixed pH 7 aqueous/acetonitrile solutions and demonstrate that H2 evolution can be tuned through electronic modifications of the axial cobalt ligand or through introduction of TiO2 nanoparticles. The photocatalytic systems consist of various cobaloxime catalysts [Co(dmgH)2(L)Cl] (L = nitrogen-based axial ligands) and a water soluble porphyrin photosensitizer. They were assayed in the presence of triethanolamine as a sacrificial electron donor. Optimal turnover numbers related to the photosensitizer are obtained with electron-rich axial ligands such as imidazole derivatives (1131 TONs with N-methyl imidazole). Lower stabilities are observed with various pyridine axial ligands (443 TONs for para-methylpyridine), especially for those containing electron-acceptor substituents. Interestingly, when L is para-carboxylatopyridine the activity of the system is increased from 40 to 223 TONs in the presence of TiO2 nanoparticles.

Artificial hemes for DSSC and/or BHJ applications.

Lately solar to power energy conversion devices have attracted a lot of attention and there is an effort to synthesize novel light-harvesting organic materials. Inspired by nature and due to their remarkable role in photosynthesis, porphyrins have been extensively used for DSSC and BHJ applications. This review provides a summary of the developments and approaches that were used in our group in order to improve the power conversion efficiency of DSSCs and BHJs based on porphyrin hybrid derivatives. Aiming to extend the light absorption range of the sensitizing dyes and to achieve enhanced solar cell performances, functionalized porphyrins with carboxylic acid anchoring groups were synthesized. These include monomeric, dimeric and trimeric porphyrins with one or two anchoring groups and in the presence of an additional chromophore in some examples. Also, porphyrins with pyridyl anchoring groups were studied, in order to examine the influence on the power conversion efficiency of a different anchoring group. The photovoltaic properties improved after the reduction of dye-aggregation by means of chemical co-adsorbents, and by using co-sensitization, modified photoanodes and electrolyte additives. Finally, some of the above chromophores were used as electron donors for the preparation of solution processed BHJ organic solar cells in the presence of various electron acceptors.

Meso-substituted porphyrin derivatives via palladium-catalyzed amination showing wide range visible absorption: synthesis and photophysical studies.

In recent years, there has been a growing interest in the design and synthesis of chromophores, which absorb in a wide region of the visible spectrum, as these constitute promising candidates for use as sensitizers in various solar energy conversion schemes. In this work, a palladium-catalyzed coupling reaction was employed in the synthesis of molecular triads in which two porphyrin or boron dipyrrin (BDP) chromophores are linked to the meso positions of a central Zn porphyrin (PZn) ring via an amino group. In the resulting conjugates, which strongly absorb over most of the visible region, the electronic properties of the constituent chromophores are largely retained while detailed emission experiments reveal the energy transfer pathways that occur in each triad.

Urea porphyrins as simple receptors for sugars.

Urea-functionalized porphyrins with amino acid substituents bind sugar derivatives strongly in non-polar solution.