Kiyoshi C D Robson

The University of Calgary, Calgary, Alberta, Canada

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Publications (16)96.29 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: It is reported herein that the co-adsorption of chenodeoxycholic acid (CDCA) with the D–π–A organic dyes (E)-3-(5-(4-(bis(4-(hexyloxy)phenyl)amino)phenyl)thiophen-2-yl)-2-cyanoprop-2-enoic acid (Dye-O) or (E)-3-(5-(4-(bis(4-(hexylthio)phenyl)amino)phenyl)thiophen-2-yl)-2-cyanoprop-2-enoic acid (Dye-S) has a significant influence on the ground state absorption spectra of sensitized TiO2 thin films. In the absence of CDCA, evidence for aggregation was observed at even the lowest measured surface coverages. The presence of CDCA also had a significant and very beneficial influence on the unwanted charge recombination reaction between TiO2(e−) and oxidized cobalt mediators.
    Thin Solid Films 01/2014; 560:49–54. · 1.87 Impact Factor
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    ABSTRACT: Characterization of the redox properties of TiO2 interfaces sensitized to visible light by a series of cyclometalated ruthenium polypyridyl compounds bis-ligated to 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 of the surface anchored compounds revealed non-Nernstian behavior with non-ideality factors of 1.37 + 0.08 for the RuIII/II couple and 1.15 + 0.09 for the NAr3•+/0 couple. Pulsed light excitation of the sensitized thin films immersed in 0.5 M LiClO4/CH3CN resulted in rapid excited state injection, kinj > 108 s-1, and, in some cases, hole transfer to the NAr3 group (TiO2(e-)/RuIII-NAr3 → TiO2(e-)/RuII-NAr3•+). The quantum yield for intramolecular-hole transfer was tuned from < 0.02 to 0.74 by peripheral substituents, the surface anchoring groups, and the interfacial proton concentration. Charge recombination (TiO2(e-)/RuIII-NAr3 → TiO2/RuII-NAr3) or (TiO2(e-)/RuII-NAr3•+ → TiO2/RuII-NAr3) rate constants 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 Debye. Visible light excitation under conditions where the RuIII centers had been electrochemically oxidized resulted in injection (TiO2/RuIII-NAr3 + hν → TiO2(e-)/RuIII-NAr3•+) into TiO2 with a quantum yield of 0.1 followed by rapid back interfacial electron transfer to another oxidized compound that had not undergone excited state injection (TiO2(e-)/RuIII-NAr3 → TiO2/RuII-NAr3). This back interfacial electron transfer reaction occurred over 1000 times faster than when the numbers of injected electrons and oxidized dyes were the same. The net effect was the photogeneration of equal numbers of fully reduced and fully oxidized compounds. Lateral intermolecular hole hopping (TiO2/RuII-NAr3 + TiO2/RuIII-NAr3•+ → 2 TiO2/RuIII-NAr3) was observed spectroscopically and was modeled by Monte Carlo simulations that revealed a hole hopping rate of (130 ns)-1. The relevance of these findings for solar energy conversion is addressed.
    Journal of the American Chemical Society 12/2013; · 10.68 Impact Factor
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    ABSTRACT: A series of heteroleptic bis(tridentate) Ru(II) complexes featuring N^C^N-cyclometalating ligands is presented. The 1,2,3-triazole-containing tridentate ligands are readily functionalized with hydrophobic side chains by means of click chemistry and the corresponding cyclometalated Ru(II) complexes are easily synthesized. The performance of these thiocyanate-free complexes in a dye-sensitized solar cell was tested and a power conversion efficiency (PCE) of up to 4.0 % (Jsc =8.1 mA cm(-2) , Voc =0.66 V, FF=0.70) was achieved, while the black dye ((NBu4 )3 [Ru(Htctpy)(NCS)3 ]; Htctpy=2,2':6',2''-terpyridine-4'-carboxylic acid-4,4''-dicarboxylate) showed 5.2 % (Jsc =10.7 mA cm(-2) , Voc =0.69 V, FF=0.69) under comparable conditions. When co-adsorbed with chenodeoxycholic acid, the PCE of the best cyclometalated dye could be improved to 4.5 % (Jsc =9.4 mA cm(-2) , Voc =0.65 V, FF=0.70). The PCEs correlate well with the light-harvesting capabilities of the dyes, while a comparable incident photon-to-current efficiency was achieved with the cyclometalated dye and the black dye. Regeneration appeared to be efficient in the parent dye, despite the high energy of the highest occupied molecular orbital. The device performance was investigated in more detail by electrochemical impedance spectroscopy. Ultimately, a promising Ru(II) sensitizer platform is presented that features a highly functionalizable "click"-derived cyclometalating ligand.
    Chemistry 10/2013; 19(42):14171-14180. · 5.93 Impact Factor
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    ABSTRACT: A family of seven organic triphenylamine-based dyes suitable for dye-sensitized solar cell (DSSC) applications is reported. The donor portion of these dyes has been systematically modified using polymerisable thienyl subunits. The physicochemical properties and device performance are discussed with device efficiencies ranging from 5.51 to 6.65%.
    Organic & Biomolecular Chemistry 09/2013; · 3.57 Impact Factor
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    ABSTRACT: Two donor-acceptor organic dyes have been synthesized that differ only by a two-heteroatom change from oxygen to sulfur within the donor unit. The two dyes, (E)-3-(5-(4-(bis(4-(hexyloxy)phenyl)amino)phenyl)thiophen-2-yl)-2-cyanoprop-2-enoic acid (Dye-O) and (E)-3-(5-(4-(bis(4-(hexylthio)phenyl)amino)phenyl)thiophen-2-yl)-2-cyanoprop-2-enoic acid) (Dye-S), were tested in solar cell devices employing both I3-/I- based and [Co(bpy)3]3+/2+ redox mediators. Power conversion efficiencies (PCE) over 6% under simulated AM 1.5 illumination (1 Sun) were achieved in both electrolytes. Despite similar optical and redox properties for the two dyes, a consistently higher open-circuit voltage (Voc) was measured for Dye-S relative to Dye-O. The improved efficiency observed with Dye-S in an iodide redox mediator is against the commonly held view that sulfur atoms promote charge recombination attributed to inner-sphere interactions Detailed mechanistic studies revealed that this is a consequence of a 25-fold enhancement of the regeneration rate constant that enhances the regeneration yield under open circuit conditions. The data shows that a high short circuit photocurrent does not imply optimal regeneration efficiency as is often assumed.
    Journal of the American Chemical Society 01/2013; · 10.68 Impact Factor
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    ABSTRACT: The recent finding that cyclometalated Ru(II) chromophores can generate high power conversion efficiencies (η) in the dye-sensitized solar cell (DSSC) has ushered in a new era of dye development. This class of dyestuff not only leaves open the opportunity to circumvent the labile monodentate NCS− groups that constitute conventional high performance dyes (e.g., [Ru(dcbpy)2(NCS)2]; dcbpy = 4,4′-dicarboxy-2,2′-bipyridine (N3) and derivatives thereof), but also offers unprecedented control of the frontier molecular orbitals for DSSC dyes. Indeed, complexes bearing aromatic cyclometalating ligands enable the manipulation of both the ground- and excited state reduction potentials through the judicious installation of substituents. This review outlines how the new and rapidly expanding class of Ru-based sensitizers bearing bidentate and tridentate cyclometalating ligands provide a conduit for potentially enhancing the stability and performance of the DSSC, gaining new insight into interfacial electron-transfer and dye regeneration processes, and utilizing new redox mediators in the DSSC.
    Coordination Chemistry Reviews 08/2012; 256(s 15–16):1438–1450. · 11.02 Impact Factor
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    ABSTRACT: We report the first case of Ru(II) dipyrrinates employed as dyes in dye-sensitized solar cells. These complexes exhibit panchromatic light harvesting that results in significant DSSC current densities, rendering them promising for photovoltaic applications. Adjustment of the lowest excited state energy is required to boost the power conversion efficiency.
    Chemical Communications 07/2012; 48(70):8790-2. · 6.38 Impact Factor
  • Kiyoshi C D Robson, Paolo G Bomben, Curtis P Berlinguette
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    ABSTRACT: A divergence from the conventional approach to chromophore design has led to the establishment of many exciting new benchmarks for the dye-sensitized solar cell (DSSC), including the first documented power conversion efficiency in excess of 12% at 1 sun illumination [Yella et al., Science 2011, 334, 629]. Paramount to these advances is the deviation from polypyridyl ruthenium dyes bearing NCS(-) ligands, such as [Ru(dcbpy)(2)(NCS)(2)] (N3; dcbpy = 4,4'-dicarboxy-2,2'-bipyridine). While metal-free and porphyrin dyes have demonstrated much promise, the discovery that the NCS(-) ligands of N3 can be replaced by anionic, chelating cyclometalating ligands without compromising device efficiencies has ushered in a new era of ruthenium dye development. A particularly appealing feature of this class of dyestuff is that they offer acute control of the frontier molecular orbitals to enable the precise attenuation of both the ground and excited state redox potentials through judicious chemical modification of the aryl ring. This Perspective summarizes very recent developments in the field, and demonstrates how the new and rapidly expanding class of Ru-based sensitizers provides a conduit for enhancing the performance (and potentially the stability) of the DSSC.
    Dalton Transactions 05/2012; 41(26):7814-29. · 3.81 Impact Factor
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    ABSTRACT: Three ruthenium compounds with triphenyl amine donors were anchored to nanocrystalline TiO(2) thin films for interfacial electron-transfer studies. Molecular tuning of reduction potentials enabled the extent of hole transfer from the photo-oxidized ruthenium center to the triphenyl amine to be tuned from zero to unity. Kinetic data revealed two new insights into the unwanted interfacial recombination reaction of the injected electrons with the oxidized compounds. First, recombination was highly sensitive to the concentration of oxidized compounds present at the interface. Second, a significant enhancement of the open circuit photovoltage was realized without a change in the recombination kinetics, behavior attributed to translation of the hole away from the interface thereby generating a larger surface dipole.
    Journal of the American Chemical Society 05/2012; 134(20):8352-5. · 10.68 Impact Factor
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    ABSTRACT: The syntheses and physical properties of cyclometalated Ru(II) complexes containing a triphenylamine (TPA) unit bearing aliphatic groups are reported. Each member of the series consists of an octahedral Ru(II) center coordinated by a tridentate polypyridyl ligand and a tridentate cyclometalating ligand. One of the chelating ligands contains electron-deficient methyl ester groups, while a TPA unit is attached to the central ring of the adjacent chelating ligand through a thiophene bridge. This study builds on our previous work (Inorg. Chem. 2011, 50, 6019-6028; Inorg. Chem. 2011, 50, 5494-5508) by (i) outlining a synthetic protocol for installing aliphatic groups on the TPA substituents, (ii) examining the role that terminal -O-hexyl and -S-hexyl groups situated on the TPA have on the electrochemical properties, and (iii) demonstrating the potential benefit of installing the TPA on the neutral chelating ligand rather than the anionic chelating ligand. The results reported herein provide important synthetic advances for our broader goal of developing bis-tridentate cyclometalated Ru(II) complexes for light-harvesting applications.
    Inorganic Chemistry 02/2012; 51(3):1501-7. · 4.59 Impact Factor
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    ABSTRACT: A series of bis-tridentate Ru(II) complexes consisting of trimethyl-4,4′,4″-tricarboxylate-2,2′:6′,2″-terpyridine (Me3tctpy) and derivatized 6-phenyl-2,2′-bipyridine (pbpy) ligands are reported. Each complex is attached to a terminal triphenylamine (TPA) substituent at the central ring of pbpy through a thiophene bridge to benefit light absorption, while the anionic ring of pbpy is functionalized with substituents to modulate the metal-based redox potential. The cyclometalation step was found to favor the isomer where the electron-donating groups (EDGs; i.e., −OEt, −SEt) are situated ortho to the organometallic bond rather than the sterically favored para position, while the para isomer is formed in exclusivity when electron-withdrawing groups (e.g., −CF3) are installed on the anionic ring. Moreover, the distribution of the isomeric products is affected by the identity of the chalcogen: ortho:para = 1:0 and 3:1 where EDG = −OEt and −SEt, respectively. Because our molecular scaffold rules out certain cyclometalation pathways (e.g., oxidative addition, agostic interactions, σ-bond metathesis), we are able to experimentally establish that the observed regioselectivity is in accordance with an electrophilic metalation where the relative stabilities of the products and carbanionic intermediates govern the ratio of the isomers formed.
    Organometallics 12/2011; 30(24):6628–6635. · 4.15 Impact Factor
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    ABSTRACT: The syntheses and physicochemical properties of nine bis-tridentate ruthenium(II) complexes containing one cyclometalating ligand furnished with terminal triphenylamine (TPA) substituents are reported. The structure of each complex conforms to a molecular scaffold formulated as [Ru(II)(TPA-2,5-thiophene-pbpy)(Me(3)tctpy)] (pbpy = 6-phenyl-2,2'-bipyridine; Me(3)tctpy = trimethyl-4,4',4''-tricarboxylate-2,2':6',2''-terpyridine), where various electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) are installed about the TPA unit and the anionic ring of the pbpy ligand. It is found that the redox chemistry of the Ru center and the TPA unit can be independently modulated by (i) placing EWGs (e.g., -CF(3)) or EDGs (e.g., -OMe) on the anionic ring of the pbpy ligand (substituted sites denoted as R(2) or R(3)) and/or (ii) installing electron-donating substituents (e.g., -H, -Me, -OMe) para to the amine of the TPA group (i.e., R(1)). The first oxidation potential is localized to the TPA unit when, for example, EDGs are placed at R(1) with EWGs at R(2) (e.g., the TPA(•+)/TPA(0) and Ru(III)/Ru(II) redox couples appear at +0.98 and +1.27 V vs NHE, respectively, when R(1) = -OMe and R(2) = -CF(3)). This situation is reversed when R(3) = EDG and R(1) = -H: TPA-based and metal-centered oxidation waves occur at +1.20 and +1.11 V vs NHE, respectively. The UV-vis spectrum for each complex is broad (e.g., absorption bands are extended from the UV region to beyond 800 nm in all cases) and intense (e.g., ε ∼ 10(4) M(-1)·cm(-1)) because of the overlapping intraligand charge-transfer and metal-to-ligand charge-transfer transitions. The information derived from this study offers guiding principles for modulating the physicochemical properties of bichromic cyclometalated ruthenium(II) complexes.
    Inorganic Chemistry 07/2011; 50(13):6019-28. · 4.59 Impact Factor
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    ABSTRACT: The syntheses and the electrochemical spectroscopic properties of a suite of asymmetrical bistridentate cyclometalated Ru(II) complexes bearing terminal triphenylamine (TPA) substituents are reported. These complexes, which contain structural design elements common to both inorganic and organic dyes that exhibit superior power conversion efficiencies in the dye-sensitized solar cell (DSSC), are broadly formulated as [Ru(II)(L-2,5'-thiophene-TPA-R(1))(L-R(2))](+) [L = tridentate chelating ligand (e.g., 2,2':6',2''-terpyridine (tpy); deprotonated forms of 1,3-di(pyridin-2-yl)benzene (Hdpb) or 6-phenyl-2,2'-bipyridine (Hpbpy)); R(1) = -H, -Me, -OMe; R(2) = -H, -CO(2)Me, -CO(2)H]. The following structural attributes were systematically modified for the series: (i) electron-donating character of the terminal substituents (e.g., R(1) = -H, -Me, -OMe) placed para to the amine of the "L-2,5'-thiophene-TPA-R(1)" ligand framework; (ii) electron-withdrawing character of the tridentate chelate distal to the TPA-substituted ligand (e.g., R(2) = -H, -CO(2)Me, -CO(2)H); and (iii) position of the organometallic bond about the Ru(II) center. UV-vis spectra reveal intense and broad absorption bands arising from a collection of metal-to-ligand charge-transfer (MLCT) and TPA-based intraligand charge-transfer (ILCT) transitions that, in certain cases, extend beyond 800 nm. Electrochemical data indicate that the oxidative behavior of the TPA and metal chelate units can be independently modulated except in cases where the anionic phenyl ring is in direct conjugation with the TPA unit. In most cases, the anionic character of the cyclometalating ligands renders a metal-based oxidation event prior to the oxidation of the TPA unit. This situation can, however, be reversed with an appropriately positioned Ru-C bond and electron-rich R(1) group. This finding is important in that this arrangement confines the highest occupied molecular orbital (HOMO) to the TPA unit rather than the metal, which is optimal for sensitizing TiO(2); indeed, a remarkably high power conversion efficiency (η) in the DSSC (i.e., 8.02%) is measured for the TPA-substituted pbpy(-) chelate where R(1) = -OMe. These results provide a comprehensive strategy for improving the performance of bistridentate Ru sensitizers devoid of NCS(-) groups for the DSSC.
    Inorganic Chemistry 06/2011; 50(12):5494-508. · 4.59 Impact Factor
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    ABSTRACT: The photophysical properties of a family of heteroleptic [Ru(tpy)(2)](2+) (tpy = 2,2':6',2''-terpyridine) complexes modified with triphenylamine donor units with different bridging units are reported.
    Inorganic Chemistry 06/2010; 49(12):5335-7. · 4.59 Impact Factor
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    ABSTRACT: The response of the metal-to-ligand charge-transfer (MLCT) band to variability in terminal substituents within a related set of tridentate polypyridyl and cyclometalated Ru(II) complexes is reported. These complexes are formulated as [Ru(tpy-R(1))(tpy-R(2))](PF(6))(2) (1-6; tpy = 2,2':6',2''-terpyridine; R(1) = -H, -2-furyl, or -OMe; R(2) = -H, -2-furyl, or -CO(2)H) and [Ru(tpy-R(2))(dpb-R(1))]PF(6) (7-10; Hdpb = 1,3-di(pyridin-2-yl)benzene; R(2) = -H or -2-furyl; R(1) = -H or -OMe). Absorption spectra for the [Ru(tpy-R(1))(tpy-R(2))](2+) series highlight the sensitivity of the MLCT band to the indicated substituents at the 4' position of one or both tpy ligands (e.g., a bathochromic shift up to 24 nm coupled with a 2-fold increase in absorption intensity). Similar observations are made for the [Ru(tpy-R(2))(dpb-R(1))](+) series, where a single Ru-N dative bond is replaced by a Ru-C sigma-bond to form a cyclometalated complex. The reduced symmetry at the metal center within this series results in a broadening of the lowest-energy MLCT band, while an additional set of transitions at higher energies emerges that involves an excited state localized on the cyclometalating ligand. These MLCT transitions collectively render a broad absorption envelope of substantial intensity at wavelengths longer than ca. 525 nm. Optimal results are obtained for compound 10 (R(1) = -OMe; R(2) = -2-furyl), where a strong electron-donating group is situated para to the Ru-C bond (lambda(max) = 523 nm; epsilon = 2.6 x 10(4) M(-1) cm(-1)). This approach imparts substantial polarization within the molecule, which should benefit excited-state electron-transfer reactions for photosensitizing applications (e.g., dye-sensitized solar cells). Spectroscopic data are corroborated by electrochemical and TD-DFT measurements for all compounds.
    Inorganic Chemistry 10/2009; 48(20):9644-52. · 4.59 Impact Factor
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    ABSTRACT: The effects of replacing a single polypyridyl ligand with an analogous anionic cyclometalating ligand were investigated for a set of three structurally related series of Ru(II) compounds formulated as [Ru(bpy)(2)(L)](z), [Ru(tpy)(L)](z), and [Ru(tpy)(L)Cl](z), where z = 0, +1, or +2, and L = polypyridyl (e.g., bpy = 2,2'-bipyridine, tpy = 2,2':6',2''-terpyridine) or cyclometalating ligand (e.g., deprotonated forms of 2-phenylpyridine or 3-(2-pyridinyl)-benzoic acid). Each of the complexes were synthesized and characterized by (1)H NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and/or elemental analyses (EA). Cyclic voltammetry reveals that cyclometalation causes a shift of the first oxidation and reduction potentials by -0.5 to -0.8 V and -0.2 to -0.4 V, respectively, relative to their polypyridyl congeners. These disparate shifts have the effect of inducing a bathochromic shift of the lowest-energy absorption bands by as much as 90 nm. With the aid of time-dependent density functional theory (DFT), the lowest-energy bands (lambda(max) = 500-575 nm) were assigned as predominantly metal-to-ligand charge-transfer (MLCT) transitions from Ru to the polypyridyl ligands, while Ru-->C(wedge)NN (or C(wedge)N(wedge)N or N(wedge)C(wedge)N) transitions are found within the absorption bands centered at ca. 400 nm. The properties of a series of compounds furnished with carboxylic acid anchoring groups at various positions are also examined for applications involving the sensitization of metal-oxide semiconductors. It is determined that the thermodynamic potentials of many of these compounds are appropriate for conventional photoelectrochemical cells (e.g., dye-sensitized solar cells) that utilize a titania electrode and iodide-based electrolyte.
    Inorganic Chemistry 09/2009; 48(20):9631-43. · 4.59 Impact Factor