Estudo anatômico e palinológico de Antônia ovata Pohl (Loganiaceae)

Nesta comunicação o autor considera a anatomia do caule, pecíolo, lâmina foliar e madeira, além dos aspectos morfológicos externo e palinológico, de espécimes de Antonia ovata, ocorrentes na floresta da região do rio Jarí (Estado do Pará) e nos cerrados da Amazônia e do Brasil Central; nomeia os espécimes da mata como sendo uma variedade nova para a ciência: Antonia ovata Pohl var. excelsa Paula.In this paper the author studies extern morphological, palinological and anatomical aspects, aiming to put an end to the doubts in the taxonomic studies of the specimens of Antonia ovata Pohl (or aiming make clear the taxonomy of the specimens of Antonia ovata. Specimens of Antonia ovata from the woods of the region of Jarí river (Amazônia) are considered by the author as a new variety. With its description, the number of varieties of Antonia ovata rose to three: pilosa, ovata and excelsa (new variety). The extern morphological aspect is found among the individuals from three habitats: "cerrados" of Amazônia, Brasil Central and forest of the region Jarí river. The identification of the three varieties is based on the following characteristic. Presence or lack of hairs on the leaves and branches; microscopic structure of wood (see comparative table); height and diameter of the specimens; and finally the habitat. Pollen grains of these two varieties excelsa and ovata present polymorphism. The leaf of that species has structure of a higrophyllous plants. The stem is rich in mucilaginous cells; vascular bundles are bicollateral; the leafknot is bilacunar, and the trace is formed by two vascular bundles

Visible Light Driven Bromide Oxidation and Ligand Substitution Photochemistry of a Ru Diimine Complex

The complex [Ru­(deeb)­(bpz)₂]²⁺ (<b>RuBPZ</b>²⁺, deeb = 4,4′-diethylester-2,2′-bipyridine, bpz = 2,2′-bipyrazine) forms a single ion pair with bromide, [<b>RuBPZ</b>²⁺, Br^–]⁺, with <i>K</i>_eq = 8400 ± 200 M^–1 in acetone. The <b>RuBPZ</b>²⁺ displayed photoluminescence (PL) at room temperature with a lifetime of 1.75 μs. The addition of bromide to a <b>RuBPZ</b>²⁺ acetone solution led to significant PL quenching and Stern–Volmer plots showed upward curvature. Time-resolved PL measurements identified two excited state quenching pathways, static and dynamic, which were operative toward [<b>RuBPZ</b>²⁺, Br^–]⁺ and free <b>RuBPZ</b>²⁺, respectively. The single ion-pair [<b>RuBPZ</b>²⁺, Br^–]⁺* had a lifetime of 45 ± 5 ns, consistent with an electron transfer rate constant, <i>k</i>_et = (2.2 ± 0.3) × 10⁷ s^–1. In contrast, <b>RuBPZ</b>²⁺* was dynamically quenched by bromide with a quenching rate constant, <i>k</i>_q = (8.1 ± 0.1) × 10¹⁰ M^–1 s^–1. Nanosecond transient absorption revealed that both the static and dynamic pathways yielded <b>RuBPZ</b>⁺ and Br₂^•– products that underwent recombination to regenerate the ground state with a second-order rate constant, <i>k</i>_cr = (2.3 ± 0.5) × 10¹⁰ M^–1 s^–1. Kinetic analysis revealed that <b>RuBPZ</b>⁺ was a primary photoproduct, while Br₂^•– was secondary product formed by the reaction of a Br^• with Br^–, <i>k</i> = (1.1 ± 0.2) × 10¹⁰ M^–1 s^–1. Marcus theory afforded an estimate of the formal reduction potential for E⁰(Br^•/–) in acetone, 1.42 V vs NHE. A ¹H NMR analysis indicated that the ion-paired bromide was preferentially situated close to the Ru^II center. Prolonged steady state photolysis of <b>RuBPZ</b>²⁺ and bromide yielded two ligand-substituted photoproducts, <i>cis</i>- and <i>trans</i>-Ru­(deeb)­(bpz)­Br₂. A photochemical intermediate, proposed to be [Ru­(deeb)­(bpz)­(κ¹-bpz)­(Br)]⁺, was found to absorb a second photon to yield <i>cis</i>- and <i>trans</i>-Ru­(deeb)­(bpz)­Br₂ photoproducts

Bromide Photo-oxidation Sensitized to Visible Light in Consecutive Ion Pairs

The titration of bromide into a [Ru­(deeb)­(bpz)₂]²⁺ (Ru²⁺, deeb = 4,4′-diethylester-2,2′-bipyridine; bpz = 2,2′-bipyrazine) dichloromethane solution led to the formation of two consecutive ion-paired species, [Ru²⁺, Br^–]⁺ and [Ru²⁺, 2Br^–], each with distinct photophysical and electron-transfer properties. Formation of the first ion pair was stoichiometric, <i>K</i>_eq 1 > 10⁶ M^–1, and the second ion-pair equilibrium was estimated to be <i>K</i>_eq 2 = (2.4 ± 0.4) × 10⁵ M^–1. The ¹H NMR spectra recorded in deuterated dichloromethane indicated the presence of contact ion pairs and provided insights into their structures and were complimented by density functional theory calculations. Static quenching of the [Ru­(deeb)­(bpz)₂]^2+* photoluminescence intensity (PLI) by bromide was observed, and [Ru²⁺, Br^–]^+* was found to be nonluminescent, τ < 10 ns. Further addition of bromide resulted in partial recovery of the PLI, and [Ru²⁺, 2Br^–]* was found to be luminescent with an excited-state lifetime of τ = 65 ± 5 ns. Electron-transfer products were identified as the reduced complex, [Ru­(deeb)­(bpz)₂]⁺, and dibromide, Br₂^•–. The bromine atom, Br^•, was determined to be the primary excited-state electron-transfer product and was an intermediate in Br₂^•– formation, Br^• + Br^– → Br₂^•–, with a second-order rate constant, <i>k</i> = (5.4 ± 1) × 10⁸ M^–1 s^–1. The unusual enhancement in PLI for [Ru²⁺, 2Br^–]* relative to [Ru²⁺, Br^–]^+* was due to a less favorable Gibbs free energy change for electron transfer that resulted in a smaller rate constant, <i>k</i>_et = (1.5 ± 0.2) × 10⁷ s^–1, in the second ion pair. Natural atomic charge analysis provided estimates of the Coulombic work terms associated with ion pairing, Δ<i>G</i>_w, that were directly correlated with the measured change in rate constants

Iodide Ion Pairing with Highly Charged Ruthenium Polypyridyl Cations in CH₃CN

A series of three highly charged cationic ruthenium­(II) polypyridyl complexes of the general formula [Ru­(deeb)_3–<i>x</i>(tmam)_<i>x</i>]­(PF₆)_2<i>x</i>+2, where deeb is 4,4′-diethyl ester-2,2′-bipyridine and tmam is 4,4′-bis­[(trimethylamino)­methyl]-2,2′-bipyridine, were synthesized and characterized and are referred to as <b>1</b>, <b>2</b>, or <b>3</b> based on the number of tmam ligands. Crystals suitable for X-ray crystallography were obtained for the homoleptic complex <b>3</b>, which was found to possess <i>D</i>₃ symmetry over the entire ruthenium complex. The complexes displayed visible absorption spectra typical of metal-to-ligand charge-transfer (MLCT) transitions. In acetonitrile, quasi-reversible waves were assigned to Ru^III/II electron transfer, with formal reduction potentials that shifted negative as the number of tmam ligands was increased. Room temperature photoluminescence was observed in acetonitrile with quantum yields of ϕ ∼ 0.1 and lifetimes of τ ∼ 2 μs. The spectroscopic and electrochemical data were most consistent with excited-state localization on the deeb ligand for <b>1</b> and <b>2</b> and on the tmam ligand for <b>3</b>. The addition of tetrabutylammonium iodide to the complexes dissolved in a CH₃CN solution led to changes in the UV–vis absorption spectra consistent with ion pairing. A Benesi–Hildebrand-type analysis of these data revealed equilibrium constants that increased with the cationic charge <b>1</b> < <b>2</b> < <b>3</b> with <i>K</i> = 4000, 4400, and 7000 M^–1. ¹H NMR studies in CD₃CN also revealed evidence for iodide ion pairs and indicated that they occur predominantly with iodide localization near the tmam ligand(s). The diastereotopic H atoms on the methylene carbon that link the amine to the bipyridine ring were uniquely sensitive to the presence of iodide; analysis revealed that an iodide “binding pocket” exists wherein iodide forms an adduct with the 3 and 3′ bipyridyl H atoms and the quaternized amine. The MLCT excited states were efficiently quenched by iodide. Time-resolved photoluminescence measurements of <b>1</b> revealed a static component consistent with rapid electron transfer from iodide in the “binding pocket” to the Ru metal center in the excited state, <i>k</i>_et > 10⁸ s^–1. The possible relevance of this work to solar energy conversion and dye-sensitized solar cells is discussed

Chloride Oxidation by Ruthenium Excited-States in Solution

Photodriven HCl splitting to produce solar fuels is an important goal that requires strong photo-oxidants capable of chloride oxidation. In a molecular approach toward this goal, three ruthenium compounds with 2,2′-bipyrazine backbones were found to oxidize chloride ions in acetone solution. Nanosecond transient absorption measurements provide compelling evidence for excited-state electron transfer from chloride to the Ru metal center with rate constants in excess of 10¹⁰ M^–1 s^–1. The Cl atom product was trapped with an olefin. This reactivity was promoted through pre-organization of ground-state precursors in ion pairs. Chloride oxidation with a tetra-cationic ruthenium complex was most favorable, as the dicationic complexes were susceptible to photochemical ligand loss. Marcus analysis afforded an estimate of the chlorine formal reduction potential <i>E</i>°(Cl^•/–) = 1.87 V vs NHE that is at least 300 meV more favorable than the accepted values in water

Rhodamine-Platinum Diimine Dithiolate Complex Dyads as Efficient and Robust Photosensitizers for Light-Driven Aqueous Proton Reduction to Hydrogen

Three new dyads consisting of a rhodamine (RDM) dye linked covalently to a Pt diimine dithiolate (PtN₂S₂) charge transfer complex were synthesized and used as photosensitizers for the generation of H₂ from aqueous protons. The three dyads differ only in the substituents on the rhodamine amino groups, and are denoted as <b>Pt-RDM1</b>, <b>Pt-RDM2</b>, and <b>Pt-RDM3</b>. In acetonitrile, the three dyads show a strong absorption in the visible region corresponding to the rhodamine π–π* absorption as well as a mixed metal-dithiolate-to-diimine charge transfer band characteristic of PtN₂S₂ complexes. The shift of the rhodamine π–π* absorption maxima in going from <b>Pt-RDM1</b> to <b>Pt-RDM3</b> correlates well with the HOMO–LUMO energy gap measured in electrochemical experiments. Under white light irradiation, the dyads display both high and robust activity for H₂ generation when attached to platinized TiO₂ nanoparticles (Pt-TiO₂). After 40 h of irradiation, systems containing <b>Pt-RDM1</b>, <b>Pt-RDM2</b>, and <b>Pt-RDM3</b> exhibit turnover numbers (TONs) of 33600, 42800, and 70700, respectively. Ultrafast transient absorption spectroscopy reveals that energy transfer from the rhodamine ¹π–π* state to the singlet charge transfer (¹CT) state of the PtN₂S₂ chromophore occurs within 1 ps for all three dyads. Another fast charge transfer process from the rhodamine ¹π–π* state to a charge separated (CS) RDM^(0•)-Pt^(+•) state is also observed. Differences in the relative activity of systems using the RDM-PtN₂S₂ dyads for H₂ generation correlate well with the relative energies of the CS state and the PtN₂S₂ ³CT state used for H₂ production. These findings show how one can finely tune the excited state energy levels to direct excited state population to the photochemically productive states, and highlight the importance of judicious design of a photosensitizer dyad for light absorption and photoinduced electron transfer for the photogeneration of H₂ from aqueous protons

Photoacidic and Photobasic Behavior of Transition Metal Compounds with Carboxylic Acid Group(s)

Excited state proton transfer studies of six Ru polypyridyl compounds with carboxylic acid/carboxylate group(s) revealed that some were photoacids and some were photobases. The compounds [Ru^II(btfmb)₂(LL)]²⁺, [Ru^II(dtb)₂(LL)]²⁺, and [Ru^II(bpy)₂(LL)]²⁺, where bpy is 2,2′-bipyridine, btfmb is 4,4′-(CF₃)₂-bpy, and dtb is 4,4′-((CH₃)₃C)₂-bpy, and LL is either dcb = 4,4′-(CO₂H)₂-bpy or mcb = 4-(CO₂H),4′-(CO₂Et)-2,2′-bpy, were synthesized and characterized. The compounds exhibited intense metal-to-ligand charge-transfer (MLCT) absorption bands in the visible region and room temperature photoluminescence (PL) with long τ > 100 ns excited state lifetimes. The mcb compounds had very similar ground state p<i>K</i>_a’s of 2.31 ± 0.07, and their characterization enabled accurate determination of the two p<i>K</i>_a values for the commonly utilized dcb ligand, p<i>K</i>_a1 = 2.1 ± 0.1 and p<i>K</i>_a2 = 3.0 ± 0.2. Compounds with the btfmb ligand were photoacidic, and the other compounds were photobasic. Transient absorption spectra indicated that btfmb compounds displayed a [Ru^III(btfmb^–)­L₂]²⁺* localized excited state and a [Ru^III(dcb^–)­L₂]²⁺* formulation for all the other excited states. Time dependent PL spectral shifts provided the first kinetic data for excited state proton transfer in a transition metal compound. PL titrations, thermochemical cycles, and kinetic analysis (for the mcb compounds) provided self-consistent p<i>K</i>_a* values. The ability to make a single ionizable group photobasic <i>or</i> photoacidic through ligand design was unprecedented and was understood based on the orientation of the lowest-lying MLCT excited state dipole relative to the ligand that contained the carboxylic acid group(s)

Synthesis of Panchromatic Ru(II) Thienyl-Dipyrrin Complexes and Evaluation of Their Light-Harvesting Capacity

Ru­(II) complexes with 5-(3-thienyl)-4,6-dipyrrin (3-TDP), containing 2,2′-bipyridine (bpy) or 4,4′-bis­(methoxycarbonyl)-2,2′-bipyridine (dcmb) as coligands, have been prepared and extensively characterized. Crystal structure determination of [Ru­(bpy)₂(3-TDP)]­PF₆ (<b>1a</b>) and [Ru­(bpy)­(3-TDP)₂] (<b>2</b>) reveals that the 3-thienyl substituent is rotated with respect to the plane of the dipyrrinato moiety. These complexes, as well as [Ru­(dcmb)₂(3-TDP)]­PF₆ (<b>1b</b>), act as panchromatic light absorbers in the visible range, with two strong absorption bands observable in each case. A comparison to known Ru­(II) complexes and quantum-chemical calculations at the density functional theory (DFT) level indicate that the lower-energy band is due to metal-to-ligand charge transfer (MLCT) excitation, although the frontier occupied metal-based molecular orbitals (MOs) contain significant contributions from the 3-TDP moiety. The higher energy band is assigned to the π–π* transition of the 3-TDP ligand. Each complex exhibits an easily accessible one-electron oxidation. According to DFT calculations and spectroelectrochemical experiments, the first oxidation takes place at the Ru^II center in <b>1a</b>, but is shifted to the 3-TDP ligand in <b>1b</b>. An analysis of MO energy diagrams suggests that complex <b>1b</b> has potential to be used for light harvesting in the dye-sensitized (Grätzel) solar cell

A Distance Dependence to Lateral Self-Exchange across Nanocrystalline TiO₂. A Comparative Study of Three Homologous Ru^III/II Polypyridyl Compounds

Self-exchange intermolecular Ru^III/II electron transfer, a process commonly referred to as “hole-hopping”, is of great interest as it provides a means of charge transport across the surface of nanocrystalline (anatase) TiO₂ mesoporous thin films without the loss of free energy. This process was characterized by cyclic voltammetry and chronoabsorptometry for three homologous Ru diimine compounds of the general form [Ru­(LL)₂(dcbH₂)]­(PF₆)₂, where LL is 2,2′-bipyridine (<b>bpy</b>), 4,4′-dimethyl-2,2′-bipyridine (<b>dmb</b>), or 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine (<b>dtb</b>) and dcbH₂ is 2,2′-bipyridyl-4,4′-dicarboxylic acid. Apparent electron diffusion coefficients, <i>D</i>, abstracted from this data increased with <b>dtb</b> < <b>bpy</b> < <b>dmb</b>. Both techniques were consistent with this trend, despite differences in the magnitude of <i>D</i> between the two methods. Temperature dependent measurements revealed an activation barrier for electron self-exchange of 250 ± 50 meV that was within this error the same for all three diimine compounds, suggesting the total reorganization energy, λ, was also the same. Application of Marcus theory, with the assumption that the 900 ± 100 meV total reorganization energy for self-exchange electron transfer was independent of the Ru compound, revealed that the electronic coupling matrix element, <i>H</i>_AB, followed the trend <b>dtb</b> (0.02 meV) < <b>bpy</b> (0.07 meV) < <b>dmb</b> (0.10 meV). The results indicate that insulating side groups placed on redox active molecules can be utilized to tune the electronic coupling and hence self-exchange rate constants without significantly altering the reorganization energy for electron transfer on TiO₂ surfaces