Eduardo Schott

A Trisheteroleptic Cyclometalated RuII Sensitizer that Enables High Power Output in a Dye-Sensitized Solar Cell†

The low embodied energy and high power-conversion efficiency (h) over disparate light intensities renders the dyesensitized solar cell (DSSC) a promising alternative to conventional photovoltaic technologies. Significant penetration of the DSSC into the photovoltaic market, however, is hindered predominantly by the long-term stability of dyes and electrolytes under practical conditions. The instability of champion (i.e., h> 10%) dyes (which, until recently, all were derivatives of [Ru(dcbpy)2(NCS)2] (N3 ; dcbpy= 4,4’dicarboxy-2,2’-bipyridine)) in the DSSC is caused primarily by desorption of the dyes from the surface and/or liberation of the NCS ligands from the metal centre. While the rate of dye desorption from TiO2 can be manipulated by replacing the CO2H moiety with other anchoring groups, this strategy typically compromises electron injection into the TiO2. [8] An alternative approach is to replace the dcbpy ligands that comprise N3 with bidentate ligands bearing aliphatic substituents (e.g., Scheme 1a), which serve to hinder water from reaching the surface to hydrolytically cleave the TiO2–dye ester linkage. These groups provide the additional benefit of suppressing recombination between the electrolyte and the electrons in TiO2, thus leading to higher efficiencies (Scheme 1a). Chemical strategies for avoiding the labile Ru NCS bond have been realized recently; indeed, we and others have now documented remarkably high h values for DSSCs containing NCS -free Ru sensitizers. Cyclometalated Ru complexes such as [Ru(dcbpy)2(ppy)] 1+ (ppy= 2-phenylpyridine) provide a versatile platform in this respect because: 1) the highest occupied molecular orbital (HOMO) is extended over the metal and anionic ring thus enabling its modulation through judicious installation of substituents at the R2 site in Scheme 1b; and 2) the low-lying excited states, which contain orbital character that resides on the p* framework of the dcbpy ligand(s), are poised for electron injection into the TiO2. [10,11,15–18] This scenario leaves open the opportunity to replace one dcbpy with a bidentate ligand capable of suppressing recombination and enhancing the optical properties as per the aforementioned protocol (Scheme 1). While we recently demonstrated synthetic access to trisheteroleptic Ru sensitizers (e.g., 1 and 2 ; Scheme 2), we learned that removing the acid linkers

Interrogation of electrocatalytic water oxidation mediated by a cobalt complex

Examination of the aqueous electrochemistry of a Co(II) complex bearing a pentadentate ligand suggests that the catalytic current corresponding to water oxidation is molecular in origin, and does not emanate exclusively from Co-oxide phases formed in situ.

Theoretical Study of Sensitizer Candidates for Dye-Sensitized Solar Cells: Peripheral Substituted Dizinc Pyrazinoporphyrazine–Phthalocyanine Complexes

We have carried out a theoretical study of the geometrical and electronic structures of a family of planar dimers constituted by zinc(II) pyrazinoporphyrazine and zinc(II) phthalocyanine with peripheral electron-donating and electron-withdrawing substituents R [where R = -OH (1), -C(CH(3))(3) (2), -CH(3) (3), -C(6)H(5) (4), -H (5), -CO(2)H (7), -NO(2) (7), and -PO(3)H(2) (8)]. The complexes are connected by varying the bridge (B) ligand, where, in 1-9, B is -CH= and, in 10-12, B is -N=, -O-, and -S-, respectively. The -CO(2)H group was included in complexes identified as 9-12. This was done because of the known properties of this group in acting as an anchor to adsorb a dye onto a semiconductor oxide. The aim of this work was to provide a useful theoretical basis for the design and screening of new potential dye candidates to be used in these devices, based on the properties of the dyes suitable for their good performance in solar cells, such as frontier molecular orbital spatial distributions; charge-separated states in the electronic transitions in the visible region of the spectrum; and importantly, the energy diagram of the frontier MOs of these dyes and the conduction band (CB) of the semiconductor, where the LUMO energy levels that are above of the CB suggest which dyes are capable of electron injection into TiO(2). In this sense, it is expected that complexes 1-5 and 9-12 should be very promising dyes to act as sensitizers. Finally, a linear correlation was found between the HOMO and LUMO energies of all of the systems and the Hammett constants, where these molecular orbitals become more stable when R is more electron-withdrawing.

Intramolecular and lateral intermolecular hole transfer at the sensitized TiO2 interface

Characterization of the redox properties of TiO2 interfaces sensitized to visible light by a series of cyclometalated ruthenium polypyridyl compounds containing both a terpyridyl ligand with three carboxylic acid/carboxylate or methyl ester groups for surface binding and a tridentate cyclometalated ligand with a conjugated triarylamine (NAr3) donor group is described. Spectroelectrochemical studies revealed non-Nernstian behavior with nonideality factors of 1.37 ± 0.08 for the Ru(III/II) couple and 1.15 ± 0.09 for the NAr3(•+/0) couple. Pulsed light excitation of the sensitized thin films resulted in rapid excited-state injection (k(inj) > 10(8) s(-1)) and in some cases hole transfer to NAr3 [TiO2(e(-))/Ru(III)-NAr3 → TiO2(e(-))/Ru(II)-NAr3(•+)]. The rate constants for charge recombination [TiO2(e(-))/Ru(III)-NAr3 → TiO2/Ru(II)-NAr3 or TiO2(e(-))/Ru(II)-NAr3(•+) → TiO2/Ru(II)-NAr3] were insensitive to the identity of the cyclometalated compound, while the open-circuit photovoltage was significantly larger for the compound with the highest quantum yield for hole transfer, behavior attributed to a larger dipole moment change (Δμ = 7.7 D). Visible-light excitation under conditions where the Ru(III) centers were oxidized resulted in injection into TiO2 [TiO2/Ru(III)-NAr3 + hν → TiO2(e(-))/Ru(III)-NAr3(•+)] followed by rapid back interfacial electron transfer to another oxidized compound that had not undergone excited-state injection [TiO2(e(-))/Ru(III)-NAr3 → TiO2/Ru(II)-NAr3]. The net effect was the photogeneration of equal numbers of fully reduced and fully oxidized compounds. Lateral intermolecular hole hopping (TiO2/Ru(II)-NAr3 + TiO2/Ru(III)-NAr3(•+) → 2TiO2/Ru(III)-NAr3) was observed spectroscopically and was modeled by Monte Carlo simulations that revealed an effective hole hopping rate of (130 ns)(-1).

The Nature of the Donor Motif in Acceptor-Bridge-Donor Dyes as an Influence in the Electron Photo-Injection Mechanism in DSSCs

The combination and balance of acceptor(A)-bridge-donor(D) architecture of molecules confer suitable attributes and/or properties to act as efficient light-harvesting and sensitizers in dye sensitized solar cells (DSSCs). An important process in a DSSC performance is the electron photoinjection (PI) mechanism which can take place either via type I (indirect), that consists in injecting from the excited state of the dye to the semiconductor, or type II (direct), where the PI is from the ground state of the dye to the semiconductor upon photoexcitation. Here, we present a computational study about the role of the donor motif in the PI mechanisms displayed from a family of 11 A-bridge-D structured dyes to a (TiO2)15 anatase cluster. To this end, different donor motifs (D1-D11) were evaluated while the A and bridge motifs remained the same. All the computations were carried out within the DFT framework, using the B3LYP, PW91, PBE, M06L and CAM-B3LYP functionals. The 6-31G(d) basis set was employed for nonmetallic atoms and the LANL2DZ pseudopotential for Ti atoms. The solvation effects were incorporated using the polarized continuum model (PCM) for acetonitrile. As benchmark systems, alizarin and naphthalenediol dyes were analyzed, as they are known to undergo Type I and Type II PI pathways in DSSCs, respectively. Donors in the studied family of dyes could influence to drive Type I or II PI since it was found that D2 could show some Type II PI route, showing a new absorption band, although with CAM-B3LYP this shows a very low oscillator strength, while the remaining dyes behave according to Type I photoinjectors. Finally, the photovoltaic parameters that govern the light absorption process were evaluated, as the use of these criteria could be applied to predict the efficiency of the studied dyes in DSSCs devices.

Synthesis, characterization and computational studies of (E)-2-{[(2-aminopyridin-3-yl)imino]-methyl}-4,6-di-tert-butylphenol

(E)-2-{((2-Aminopyridin-3-yl)imino)-methyl}-4,6-di-tert-butyl-phenol (3), a ligand containing an intramolecular hydrogen bond, was prepared according to a previous literature report, with modifications, and was characterized by UV-vis, FTIR, 1H-NMR, 13C-NMR, HHCOSY, TOCSY and cyclic voltammetry. Computational analyses at the level of DFT and TD-DFT were performed to study its electronic and molecular structures. The results of these analyses elucidated the behaviors of the UV-vis and electrochemical data. Analysis of the transitions in the computed spectrum showed that the most important band is primarily composed of a HOMO→LUMO transition, designated as an intraligand (IL) charge transfer.

FACILE SYNTHESIS OF ISOXAZOLES AND PYRAZOLES FROM ß-DIKETOHYDRAZONES

New 3,5-dimethyl-4-[(E)-4-(R1-phenyl)diazenyl]isoxazoles and 3,5-dimethyl-1-(R2-phenyl)-4-[(E)-(R1-phenyl)diazenyl]-1H-pyrazoles may be obtained by reaction of 3-[2-(R1-phenyl)hydrazono)]pentane-2,4-dione with H2NOH-HCl and R2-4-C6H4-NHNH2, respectively. The reactions were performed in ethanol as solvent and catalyzed by glacial acetic acid.

Experimental and theoretical studies of the ancillary ligand (E)-2-((3-amino-pyridin-4-ylimino)-methyl)-4,6-di-tert-butylphenol in the rhenium(I) core

The fac-[Re(CO)3(deeb)L]+ complex (C2) where L is the (E)-2-((3-amino-pyridin-4-ylimino)-methyl)-4,6-di-tert-butylphenol ancillary ligand, which presents an intramolecular hydrogen bond, has been synthesized and characterized using UV-vis, 1H-NMR, FT-IR, cyclic voltammetry and DFT calculations. The UV-vis absorption and emission properties have been studied at room temperature and the results were compared with TDDFT calculations including spin–orbit effects. We report an alternative synthesis route for the fac-Re(CO)3(deeb)Br (C1) complex where deeb = (4,4′-diethanoate)-2,2′-bpy. Besides, we have found that the C1 shows a red shift in the emission spectrum due to the nature of the ancillary electron donating ligand, while the C2 complex shows a blue shift in the emission spectrum suggesting that the ancillary ligand L has electron withdrawing ability and the importance of the intramolecular hydrogen bond. The calculations suggest that an experimental mixed absorption band at 361 nm could be assigned to MLCT and LLCT transitions. The electron withdrawing nature of the ancillary ligand in C2 explains the electrochemical behavior, which shows the oxidation of ReI at 1.83 V and the reduction of deeb at −0.77 V.

BODIPY-phenylacetylene macrocycle motifs for enhanced light-harvesting and energy transfer applications

A supramolecular BODIPY molecule functionalized with a phenylacetylene macrocycle is reported. Light absorption by the macrocycle results in energy transfer to the BODIPY core as evidenced by fluorescence studies. This supramolecular motif demonstrates an effective strategy for panchromatic light-harvesting.

A family of substituted hydrazonoisoxazolones with potential biological properties

The synthesis, characterization and biological study of a new 3,4,5-trisubstituted isoxazolones have been reported, whereby a series of (Z)-3-methyl-4-(2-(R-phenyl)hydrazinylidene)isoxazol-5(4H)-ones were prepared by the reaction of a β-diketohydrazone with hydroxylammonium chloride. All the products were characterized using EA, UV-Vis, FT-IR, 1H-NMR, 13C-NMR spectroscopy and HMBC. The crystalline and molecular structures of three compounds were solved by X-ray diffraction methods. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were performed to obtain a better explanation of the observed experimental behaviour of these newly synthetized compounds. Furthermore, the reports of cytotoxicity and an antiproliferative effect in human promyelocytic leukaemia cells, HL-60, was tested by the MTT reduction method, showing that most of the newly synthetized compounds had important antineoplastic activity. The most active isoxazolones were used in reverse transcription polymerase chain reaction (RT-PCR) experiments to determine the effect on the expression levels on mRNA encoding using the anti-apoptotic, Bcl 2, pro-apoptotic, Bax, and the proliferation inhibition, p21WAF-1, proteins. Therefore, it was possible to fully characterize the complete library of 15 isoxazolones and to show that most of them are antineoplastic trough an apoptotic pathway.