The influence of the polypyridyl ligand on the photophysics of fac-[Re(CO)3(NN)(bpa)](+), bpa = 1,2-bis-(4-pyridyl)ethane and NN = 1,10-phenanthroline (phen), pyrazino[2,3-f][1,10]-phenanthroline (dpq), and dipyrido[3,2-a:2'3'-c]phenazine (dppz) has been investigated by steady state and time-resolved emission spectroscopy combined with theoretical calculations using time-dependent density functional theory (TD-DFT). The fac-[Re(CO)3(phen)(bpa)](+) is a typical MLCT emitter in acetonitrile with ϕ = 0.11 and τ = 970 ns. The emission lifetime and quantum yield decrease significantly in fac-[Re(CO)3(dpq)(bpa)](+) (ϕ = 0.05; τ = 375 ns) due to the presence of a close lying dark charge transfer state located at the pyrazine ring of dpq, as indicated by TD-DFT data. The luminescence of these complexes is quenched by hydroquinone with kq = (2.9 ± 0.1) × 10(9) and (2.6 ± 0.1) × 10(9) L mol(-1) s(-1), respectively, for NN = phen or dpq. These values are increased respectively to (4.6 ± 0.1) × 10(9) and (4.2 ± 0.1) × 10(9) L mol(-1) s(-1) in the 1 : 1 H2O-CH3CN mixture. In this medium Stern-Volmer constants determined by steady-state and time-resolved measurements differ from each other, which is indicative of static quenching, i.e. the pre-association of hydroquinone and the complexes through hydrogen bonding between the remote N-atom in the bpa ligand (KA ≅ 1-2 × 10(1) L mol(-1)), followed by a concerted proton-electron transfer. In contrast to other investigated complexes, fac-[Re(CO)3(dppz)(bpa)](+) is weakly emissive in acetonitrile at room temperature (ϕ ≅ 10(-4)) and does not exhibit a rigidochromic effect. This photophysical behaviour as well as TD-DFT data indicate that the lowest lying triplet excited state can be described as (3)ILdppz. The results provide additional insight into the influence of the polypyridyl ligand on the photophysical properties of Re(I) complexes.

Reactions of p-toluenesulfohydrazide with R-isothiocyanates afford new ligands containing both the sulfonamide and thiosemicarbazide moieties (L R : R= cyclohexyl (Cy) or phenyl (Ph)). L R reacts with AgNO 3 in EtOH in a 2:1 M ratio leading to formation of colourless Ag(I) complexes of general formula [Ag(L R ) 2 ]NO 3 . The crystal structure of a representative complex was determined by crystallographic studies and shows the Ag(I) coordinated by two thiosemicarbazide ligands in a Smonodentate coordination mode along with a significant interaction with a nitrate counter-ion in a bent geometry.Photophysical studies show that both the free ligands and complexes are essentially non-luminescent at room temperature. At 77 K and glassy media, the silver complexes exhibit a luminescence band in the visible-light region. Besides, experimental and theoretical analyses suggest the population of the triplet state of these complexes after electronic excitation at around 250 nm. Overall, it was verified that the observed emission of the Ag(I) complexes can be attributed to 3 MLCT radiative decay and that the silver complex with L Ph shows a higher intensity emission than the complex with L Cy .

The large and continuous use of fossil fuels as a primary energy source has led to several environmental problems, such as the increase of the greenhouse effect. In order to minimize these problems, attention has been drawn to renewable energy production. Solar energy is an attractive candidate as renewable source due to its abundance and availability. For this, it is necessary to develop devices able to absorb sunlight and convert it into fuels or electricity in a economical, technical and sustainable way. The so-called artificial photosynthesis has called the attention of researchers due to the possibility of using solar photocatalysts in converting water and CO2 into fuels. This manuscript presents a review of the recent developments of hybrid systems based on molecular photocatalysts immobilized on semiconductor surfaces for solar fuel production through water oxidation and CO2 reduction and also discusses the current challenges for the potential application of these photocatalyst systems.

In the present work the influence of the distance of the protonatable site of different ancillary ligands to the metal center on the luminescence quenching of Re I polypyridyl complexes by hydroquinone are evaluated by means of experimental and theoretical studies. In these systems, it is expected the occurrence of proton-coupled electron transfer (PCET) reactions upon excitation, which is a key process in solar-to-fuels energy conversion. The series fac-[Re(CO) 3 (2,2-bpy)(L)]PF 6 , L = pyridine, 1,4-pyrazine, 4,4'-bipyridyl, 1,2-bis-(4-pyridyl)ethane were synthesized and the luminescence quenching rate constant (k q ) by hydroquinone in CH 3 CN and 1:1 CH 3 CN/H 2 O were determined by steady-state and lifetime measurements. In bare acetonitrile, the 1,4-pyrazine exhibits the higher k q (3.49 ± 0.02) × 10 9 L mol -1 s -1 among the species investigated, followed by 4,4'-bipyridyl (k q = 2.50 ± 0.02) × 10 9 L mol -1 s -1. In 1:1 CH 3 CN/H 2 O, the k q values for all complexes are very similar evidencing the role of water molecules as proton acceptor following the reductive quenching of the complexes by hydroquinone. In CH 3 CN, the proton release for the solvent is not spontaneous and the higher basicity of the coordinated 1,4-pyrazine and 4,4'-bipyridyl in relation to 1,2-bis-(4-pyridyl)ethane after metal-to-ligand charge transfer (MLCT) excitation contributes to the proton transfer step. These results are corroborated by time-dependent density functional theory (TD-DFT) calculations. Moreover, the low H/D kinetic isotope effect (KIE) in 3:1 CH 3 CN/X 2 O (X = H or D) confirms that the major PCET pathway is the electron transfer followed by proton transfer, but for 1,4-pyrazine and 4,4'-bipyridyl the concerted proton-electron transfer seems to play a role at high hydroquinone concentrations.

COORDINATION CHEMISTRY AND SOLAR FUEL PRODUCTION. Life on earth depends on the absorption and conversion of solar energy into chemical bonds, i.e. photosynthesis. In this process, sun light is employed to oxidize water into oxygen and reducing equivalents used to produce fuels. In artificial photosynthesis, the goal is to develop relatively simple systems able to mimic photosynthetic organisms and promote solar-to-chemical conversion. The aim of the present review was to describe recent advances in the application of coordination compounds as catalysts in some key reactions for artificial photosynthesis, such as water splitting and CO 2 reduction.Keywords: solar fuels; artificial photosynthesis; coordination chemistry; photochemistry. INTRODUÇÃOAtualmente, o tópico energia tem despertado o interesse de diversos setores da sociedade devido ao dilema de suprir a crescente demanda sem aumentar os danos ambientais já causados pela exploração dos combustíveis fósseis.1-5 O incentivo a novas tecnologias para energia renovável ganhou ainda mais força com estudos recentes que mostram o aumento da concentração de CO 2 na atmosfera devido à queima de combustíveis fósseis, o que causa o chamado efeito estufa. 6,7 Nesse contexto, o aproveitamento da energia solar torna-se extremamente atraente devido ao seu baixo impacto ambiental e à grande oferta de energia. Anualmente, o fluxo de radiação sobre a terra chega a 3,4×10 24 J, o que supera em milhares de vezes o consumo de energia atual do mundo.7 De fato, o sol é nossa fonte primária de energia, responsável pela formação da biomassa, dos ventos, da maré, enfim, da vida no planeta.O aproveitamento da energia solar envolve sua conversão em outras formas de energia. Atualmente, já se encontram amplamente difundidos nas residências sistemas para aquecimento de água a partir da luz solar. Outra possibilidade envolve a conversão da energia luminosa em eletricidade por meio de sistemas fotovoltaicos. Tais sistemas são baseados em células solares tradicionais de silício ou, mais recentemente, em novas gerações de dispositivos como as DSCs (dye-sensitized solar cells) e as OPVs (organic photovoltaics), cuja eficiência e estabilidade têm avançado constantemente. [8][9][10][11][12][13][14][15] A energia solar pode ainda ser convertida e armazenada na forma de ligações químicas, como ocorre nos organismos fotossintéticos, em que espécies químicas de alto conteúdo energético (carboidratos) são formadas a partir de CO 2 e H 2 O, equação 1.(1) Biologicamente, a fotossíntese promove a quebra de água em oxigênio e prótons, enquanto a respiração combina essas espécies de forma controlada e eficiente para produzir metabólitos. Assim, a síntese desses metabólitos representa a fixação de hidrogênio e consequente armazenamento da energia solar na forma de ligações químicas. Estima-se que os organismos fotossintéticos produzem mais de 100 bilhões de toneladas de biomassa seca anualmente, o que equivale a 100 vezes o peso de toda a população humana na Terra e representa um estoque de energia de ap...