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Publications (21)

  • Guocan Li · Ke Hu · Kiyoshi C. D. Robson · [...] · Michael Shatruk
    [Show abstract] [Hide abstract] ABSTRACT: Two novel tris-heteroleptic Ru–dipyrrinates were prepared and tested as sensitizers in the dye-sensitized solar cell (DSSC). Under AM 1.5 sunlight, DSSCs employing these dyes achieved power conversion efficiencies (PCEs) of 3.4 and 2.2 %, substantially exceeding the value achieved previously with a bis-heteroleptic dye (0.75 %). As shown by electrochemical measurements and DFT calculations, the improved PCEs stem from the synthetically tuned electronic structure, which affords more negative excited state redox potentials and favorable electron injection into the TiO2 conduction band. Electron injection was quantified by nanosecond transient absorption spectroscopy, which revealed that the highest injection yield is achieved with the dye that acts as the strongest photoreductant.
    Article · Nov 2014 · Chemistry - A European Journal
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    [Show abstract] [Hide abstract] ABSTRACT: A series of three bis(tridentate) cycloruthenated sensitizers with furyl, thiophene, or selenophene units attached to the cyclometalated ligand were designed to examine how chalcogen atoms effect interfacial electron transfer events that occur following the absorption of visible light by the sensitizers when attached to mesoporous titania thin films immersed in CH3CN electrolytes. Spectroelectrochemistry established that the RuIII/II reduction potentials were confined to the 0.954-0.965 V vs NHE range for the series and that the density of TiO2 acceptor states were sensitizer-independent. Pulsed light excitation into the metal-to-ligand charge transfer band of the sensitized thin films resulted in rapid excited state injection, kinj > 108 s-1. Charge recombination (RuIII/TiO2(e-) → RuII/TiO2) rate constants were insensitive to the identity of the cyclometalated compound, yet the open circuit photovoltages were markedly lower for the compound containing selenophene. These lower photovoltages appear to be a manifestation of a 4-fold-larger, second-order rate constant measured for the reaction between triiodide and TiO2(e-) in the case that the selenophene comprises the donor fragment of the dye. Adduct formation between oxidized iodide(s) and the selenophene moiety of the sensitizer is implicated.
    Full-text available · Article · Jul 2014 · The Journal of Physical Chemistry C
  • Ke Hu · Kiyoshi C.D. Robson · Curtis P. Berlinguette · Gerald J. Meyer
    [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.
    Article · Jun 2014 · Thin Solid Films
  • Christopher J. Wood · Kiyoshi C. D. Robson · Paul I. P. Elliott · [...] · Elizabeth A. Gibson
    [Show abstract] [Hide abstract] ABSTRACT: A pair of ruthenium based donor-π-chromophore sensitizers (K1 and K2) have been synthesized for use in NiO based p-type dye sensitized solar cells (p-DSCs). The optical and electrochemical properties of these dyes were determined experimentally and interpreted by DFT modelling. NiO p-DSC devices incorporating these dyes gave photocurrents of 2.9 for K1 and 2.0 mA cm−2 for K2 (IPCE of 14% and 9%); this is a vast improvement on the photocurrents of p-DSC devices incorporating ‘traditional’ ruthenium sensitizers.
    Article · Jan 2014 · RSC Advances
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    Ke Hu · Kiyoshi C D Robson · Evan E Beauvilliers · [...] · Gerald John Meyer
    [Show abstract] [Hide abstract] ABSTRACT: 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 center dot+/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 center dot+]. The rate constants for charge recombination [TiO2(e(-))/Ru-III-NAr3 -> TiO2/Ru-II-NAr3 or TiO2(e(-))/Ru-II-NAr3 center dot+ -> 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 (Delta mu = 7.7 D). Visible-light excitation under conditions where the Ru-III centers were oxidized resulted in injection into TiO2 [TiO2/Ru-III-NAr3 + h nu -> TiO2(e(-))/Ru-III-NAr3 center dot+] 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 center dot+ -> 2TiO(2)/Ru-III-NAr3) was observed spectroscopically and was modeled by Monte Carlo simulations that revealed an effective hole hopping rate of (130 ns)(-1).
    Full-text available · Article · Dec 2013 · Journal of the American Chemical Society
  • Benjamin Schulze · Douglas G Brown · Kiyoshi C D Robson · [...] · Ulrich S Schubert
    [Show abstract] [Hide abstract] 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.
    Article · Oct 2013 · Chemistry - A European Journal
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    [Show abstract] [Hide abstract] 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%.
    Full-text available · Article · Sep 2013 · Organic & Biomolecular Chemistry
  • Kiyoshi C D Robson · Ke Hu · Gerald John Meyer · Curtis P Berlinguette
    [Show abstract] [Hide abstract] 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.
    Article · Jan 2013 · Journal of the American Chemical Society
  • [Show abstract] [Hide abstract] 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.
    Article · Aug 2012 · Coordination Chemistry Reviews
  • Guocan Li · Paolo G Bomben · Kiyoshi C D Robson · [...] · Michael Shatruk
    [Show abstract] [Hide abstract] 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.
    Article · Jul 2012 · Chemical Communications
  • Kiyoshi C D Robson · Paolo G Bomben · Curtis P Berlinguette
    [Show abstract] [Hide abstract] 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.
    Article · May 2012 · Dalton Transactions
  • Ke Hu · Kiyoshi C.D. Robson · Patrik G Johansson · [...] · Gerald J Meyer
    [Show abstract] [Hide abstract] 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.
    Article · May 2012 · Journal of the American Chemical Society
  • Kiyoshi C D Robson · Bryan D Koivisto · Curtis P Berlinguette
    [Show abstract] [Hide abstract] 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.
    Article · Feb 2012 · Inorganic Chemistry
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    [Show abstract] [Hide abstract] 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.
    Full-text available · Article · Dec 2011 · Organometallics
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    Kiyoshi C D Robson · Barbora Sporinova · Bryan D Koivisto · [...] · Curtis P Berlinguette
    [Show abstract] [Hide abstract] 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.
    Full-text available · Article · Jul 2011 · Inorganic Chemistry
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    Kiyoshi C D Robson · Bryan D Koivisto · Aswani Yella · [...] · Curtis P Berlinguette
    [Show abstract] [Hide abstract] 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.
    Full-text available · Article · Jun 2011 · Inorganic Chemistry
  • Kiyoshi C D Robson · Bryan D Koivisto · Terry J Gordon · [...] · Curtis P Berlinguette
    [Show abstract] [Hide abstract] 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.
    Article · Jun 2010 · Inorganic Chemistry
  • Kiyoshi C D Robson · Cory D Phillips · Brian O Patrick · W Stephen McNeil
    [Show abstract] [Hide abstract] ABSTRACT: Condensation of 1-phenyl-1,3-butanedione with various substituted anilines affords N-aryl substituted beta-ketoamines PhC(O)CHC(CH(3))Naryl, which, when deprotonated and reacted with Co(OAc)(2).4H(2)O yields a series of bis(beta-ketoaminato)cobalt(II) complexes 1b to 6b (aryl = Ph, 1b; p-CH(3)C(6)H(4), 2b; 2,6-(CH(3))(2)C(6)H(3), 3b; 3,5-(CH(3))(2)C(6)H(3), 4b; p-CF(3)C(6)H(4), 5b; p-CH(3)OC(6)H(4), 6b). All six cobalt compounds were characterised by (1)H NMR, elemental analysis, magnetic susceptibility, and X-ray crystallography, indicating a uniform tetrahedral geometry in all cases. Electrochemical oxidation potentials indicate sensitivity to aryl substitution at the ortho- and para- positions, but not to meta-substitution, a conclusion supported by DFT calculations.
    Article · Mar 2010 · Dalton Transactions
  • Bryan D Koivisto · Kiyoshi C D Robson · Curtis P Berlinguette
    [Show abstract] [Hide abstract] 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.
    Article · Oct 2009 · Inorganic Chemistry
  • [Show abstract] [Hide abstract] 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.
    Article · Sep 2009 · Inorganic Chemistry