RochfordLab

Publications

Turning on the Protonation-First Pathway for Electrocatalytic CO2 Reduction by Manganese Bipyridyl Tricarbonyl Complexes

Ken T. Ngo, Meaghan McKinnon, Bani Mahanti, Remya P. Narayanan, David C. Grills*, Mehmed Z. Ertem*, and Jonathan Rochford*

J. Am. Chem. Soc., 2017, 139 (7), pp 2604–2618 DOI: 10.1021/jacs.6b08776

Electrocatalytic reduction of CO2 to CO is reported for the complex, {fac-MnI([(MeO)2Ph]2bpy)(CO)3(CH3CN)}(OTf), containing four pendant methoxy groups, where [(MeO)2Ph]2bpy = 6,6’-bis(2,6-dimethoxyphenyl)-2,2’-bipyridine. In addition to a steric influence similar to that previously established for the 6,6’-dimesityl-2,2’-bipyridine ligand in [fac-MnI(mes2bpy)(CO)3(CH3CN)](OTf), which prevents Mn0–Mn0 dimerization, the [(MeO)2Ph]2bpy ligand introduces an additional electronic influence combined with a weak allosteric hydrogen bonding interaction that significantly lowers the activation barrier for C–OH bond cleavage from the metallocarboxylic acid intermediate. This provides access to the thus far elusive protonation-first pathway, minimizing the required overpotential for electrocatalytic CO2 to CO conversion by Mn(I) polypyridyl catalysts, while concurrently maintaining a respectable turnover frequency. Comprehensive electrochemical and computational studies here confirm the positive influence of the [(MeO)2Ph]2bpy ligand framework on electrocatalytic CO2 reduction and its dependence upon the concentration and pKa of the external Brønsted acid proton source (water, methanol, trifluoroethanol, and phenol) that is required for this class of manganese catalyst. Linear sweep voltammetry studies show that both phenol and trifluoroethanol as proton sources exhibit the largest protonation-first catalytic currents in combination with {fac-MnI([(MeO)2Ph]2bpy)(CO)3}−, saving up to 0.55 V in overpotential with respect to the thermodynamically-demanding reduction-first pathway, while bulk electrolysis studies confirm a high product selectivity for CO formation. To gain further insight into catalyst activation, time-resolved infrared (TRIR) spectroscopy combined with pulse-radiolysis (PR-TRIR), infrared spectroelectrochemistry, and density functional theory calculations were used to establish the v(CO) stretching frequencies and energetics of key redox intermediates relevant to catalyst activation.
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Engineering of Ruthenium(II) Photosensitizers with Non-Innocent Oxyquinolate and Carboxyamidoquinolate Ligands for Dye-Sensitized Solar Cells

Ken T. Ngo, Nicholas A. Lee, Sashari D. Pinnace, Jonathan Rochford*

Chemistry - A European Journal, 2017, 23, 7497–7507.

An alternative approach towards the replacement of the isothiocyantate ligands of the N3 photosensitizer with light-harvesting bidentate ligands is investigated for application in dye-sensitized solar cells. An in depth theoretical analysis has been applied to inform on the optical and redox properties of four non-innocent ligand platforms which is corroborated with experiment. Taking advantage of the 5- and 7-positions of 8-oxyquinolate, or the carboxyaryl ring system of the N-arylcarboxy-8-amidoquinolate ligand, fluorinated aryl substituents are demonstrated as an effective means to tune complex redox potentials and light harvesting properties. The non-innocent character derived from covalent mixing of both the central metal d(pi) and ligand-(pi) manifolds generates hybrid metal-ligand frontier orbitals that play a major role in contributing to the redox properties and visible electronic transitions, and promote an improved power conversion efficiency in a Ru non-innocent ligand sensitized DSSC device.
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Coordination Chemistry and Reactivity of Bis(aldimino)pyridine Nickel Complexes in Four Different Oxidation States

Blake R. Reed, Richard L. Lord, Jonathan Rochford,* and Stanislav Groysman*

Organometallics, 2017, 36 (3), pp 582–593 DOI: 10.1021/acs.organomet.6b00793

A series of nickel complexes with potentially redox active bis(aldimino)pyridine ligands [NNN] ([NNN] = 1,1′-(pyridine-2,6-diyl)bis(N-arylmethanimine), where aryl = 2,6-diisopropylphenyl, mesityl, 4-methoxyphenyl, 4-trifluoromethylphenyl, and 3,5-bis(trifluoromethyl)phenyl) were synthesized, and their properties and reactivities were investigated as a function of the overall oxidation state of the system. (Ni[NNN])2+ complexes of ligands featuring bulky electron-rich substituents (1a-Br2 and 1b-Br2, [NNN] = 1,1′-(pyridine-2,6-diyl)bis(N-(2,6-diisopropylphenyl)methanimine) and 1,1′-(pyridine-2,6-diyl)bis(N-mesitylmethanimine), respectively) demonstrated five electrochemical reduction events, the first three of which were quasi-reversible. In contrast, only two quasi-reversible reductions were observed for the less bulky and electron-deficient N-aryl substituents 4-(trifluoromethyl)phenyl and 3,5-bis(trifluoromethyl)phenyl. Chemical reduction of 1a-Br2 and 1b-Br2 with 1 equiv of KC8 or CoCp*2 forms (Ni[NNN])+ complexes of the general formula Ni[NNN]Br (2a-Br and 2b-Br). Structural, spectroscopic, and theoretical studies reveal that these complexes feature significant unpaired spin density on the metal, consistent with “nickel(I)” character. This behavior is in contrast with previously reported bis(ketimino)pyridine systems, in which at the (Ni[NNN])+ state the unpaired electron resided exclusively in the ligand. Further reduction forms a series of (Ni[NNN])0 complexes, in which all of the potentially tridentate [NNN] ligands bind via only one iminopyridine unit; the second arm is left unbound in most complexes. Variable temperature NMR spectroscopy demonstrates that bound and unbound arms exchange via a postulated tridentate intermediate. Electrochemical reduction, via three sequential one-electron reductions, of 1a-Br2 and 1b-Br2 in the presence of CO2/H+ forms an active catalyst for H2 evolution at a glassy-carbon electrode surface, again emphasizing the unique redox chemistry of the bulky bis(aldimino)pyridine nickel complexes.
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Ambiguous electrocatalytic CO2 reduction behaviour of a nickel bis(aldimino)pyridine pincer complex

Narayanan, R.; McKinnon, M.; Reed, B. E.; Ngo, K. T.; Groysman, S.*; Rochford, J.*

Dalton Trans., 2016,45, 15285-15289. DOI: 10.1039/C6DT01872F

The electrochemical properties of two Ni(NNN)X2 pincer complexes are reported where X = Cl or Br and NNN is N,N’-2,6-diisopropylphenyl)bis-aldiminopyridine. Cyclic voltammetry under 1 atm of CO2 suggests electrocatalytic CO2 reduction activity, however, bulk electrolysis shows a poor Faradaic efficiency for CO evolution with a high Faradaic yield for H2 evolution.
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Probing the noninnocent π-bonding influence of N-carboxyamidoquinolate ligands on the light harvesting and redox properties of ruthenium polypyridyl complexes

Ngo, K.T.; Lee, N.A.; Pinnace, S.D.; Szalda, D.A.; Weber, R.T.; Rochford, J.*

Inorganic Chemistry 2016, 55, 2460–2472. DOI: 10.1021/acs.inorgchem.5b02834.

Electronic and photophysical characterization is presented for a series of bis-heteroleptic [Ru(bpy)2(R-CAQN)]+ complexes where CAQN is a bidentate N-(carboxyaryl)amidoquinolate ligand and the aryl substituent R = p-tolyl, p-fluorobenzene, p-trifluoromethylbenzene, 3,5-bis(trifluoromethyl)benzene, or 4-methoxy-2,3,5,6-tetrafluorobenzene. Characterized by a strong noninnocent Ru(dπ)–CAQN(π) bonding interaction, density functional theory (DFT) analysis is used to estimate the contribution of both atomic Ru(dπ) and ligand CAQN(π) manifolds to the frontier molecular orbitals of these complexes. UV–vis absorption and emission studies are presented where the noninnocent Ru(dπ)–CAQN(π) bonding scheme plays a major role in defining complex electronic and photophysical properties. Oxidation potentials are tuned over a range of 0.92 V with respect to the [Ru(bpy)3]2+ reference system, hereafter referred to as 12+, by varying the degree of R-CAQN fluorination while maintaining consistently strong and panchromatic visible absorption properties. Electron paramagnetic resonance (EPR) spectroscopy is employed to experimentally map delocalization of the unpaired electron/electron–hole within the delocalized Ru(dπ)–CAQN(π) singly occupied valence molecular orbital of the one-electron oxidized complexes. EPR data is complemented experimentally by UV–vis–NIR spectroelectrochemistry, and computationally by molecular orbital Mulliken contributions and spin-density analysis. It is ultimately demonstrated that the CAQN ligand framework provides a simple yet broad synthetic platform in the design of redox-active transition metal chromophores with a range of electronic and spectroscopic characteristics hinting at the diversity and potential of these complexes toward photochemical and catalytic applications.
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Photoelectrochemical Properties of Porphyrin Dyes with a Molecular Dipole in the Linker

Ngo, K.T.; Rochford, J.; Hao, F.; Batarseh, A.; Chitre, K.; Rangan, S.; Bartynski, R.A.; Galoppini, E.

Faraday Discussions 2015, 185, 497-506.

The electronic properties of three porphyrin–bridge–anchor photosensitizers are reported with (1a, 1e, 3a and 3e) or without (2a and 2e) an intramolecular dipole in the bridge. The presence and orientation of the bridge dipole is hypothesized to influence the photovoltaic properties due to variations in the intrinsic dipole at the semiconductor–molecule interface. Electrochemical studies of the porphyrin–bridge–anchor dyes self-assembled on mesoporous nanoparticle ZrO2 films, show that the presence or direction of the bridge dipole does not have an observable effect on the electronic properties of the porphyrin ring. Subsequent photovoltaic measurements of nanostructured TiO2 semiconductor films in dye sensitized solar cells show a reduced photocurrent for photosensitizers 1a and 3a containing a bridge dipole. However, cooperative increased binding of the 1a + 3a co-sensitized device demonstrates that dye packing overrides any differences due to the presence of the small internal dipole.
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Dye-sensitized solar cell performance of a cobalt(III/II) redox mediator with the 2,6-bis(8-quinolinyl)pyridine ligand

Lee, N.A.; Frenzel, B.A.; Rochford, J.; Hightower S.E.

European Journal of Inorganic Chemistry, 2015, 23, 3843–3849.

Optical and electrochemical properties of the homoleptic cobalt(III/II) complexes mer,mer-[Co(tpy-κ3N,N′,N″)]n+ and mer,mer-[Co(bqp-κ3N,N′,N″)]n+ (n = 3+ or 2+) are presented [tpy = 2,2′:6′,2″-terpyridine; bqp = 2,6-bis(8′-quinolinyl)pyridine]. Both [Co(tpy)2]3+/2+ and [Co(bqp)2]3+/2+ systems have been used to formulate redox-mediator electrolytes for application in dye-sensitized solar-cell (DSSC) devices with the MK-2 dye TiO2 sensitizer. The [Co(tpy)2]3+/2+-based electrolyte shows a slightly higher percentage power-conversion efficiency (η = 4.47 %) in a DSSC device relative to the [Co(bqp)2]3+/2+-based electrolyte (η = 3.73 %). Although the [Co(bqp)2]3+/2+ electrolyte shows a slightly higher open-circuit potential (VOC = 0.79 V), it suffers from a reduced short-circuit current (JSC = 6.21 mA cm–2) relative to that of the [Co(tpy)2]3+/2+ system (JSC = 10.84 mA cm–2). Tafel and electrochemical impedance analysis (EIS) of symmetric FTO/Pt||Pt/FTO cells, as well as EIS and chronocoulometry of the functional DSSC device are presented. Collectively, this data points toward a decreased turnover of the CoIII/II redox couple at the Pt counter electrode for the [Co(bqp)2]3+/2+ system concomitant with an increased resistance to mass-transfer diffusion.
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Rigid triarylamine donor–π–acceptor porphyrin dyes and their application in dye-sensitized solar cells

Zhou, Y.; Lee, N.A.; Ngo, K.T.; Peng, X.; Feng, Y.; Rochford, J.

RSC Advances 2015, 5, 41193-41202.

Three donor–π–acceptor porphyrin dyes bearing a variety of rigid triarylamine donor groups were synthesized for application as photosensitizers in TiO2 based dye-sensitized solar cells (DSSCs). Compared with the “naked” porphyrin ZnP, i.e. having no triarylamine moiety, broadened and red-shifted spectral features were exhibited by the triaylamine porphyrin sensitizers ISB-ZnP, CZ-ZnP, and IDB-ZnP where ISB = 5-phenyliminostilbene, CZ = 5-phenylcarbazole, and IDB = 5-phenyliminodibenzyl. Percentage power conversion efficiencies (η) and incident photon-to-current conversion efficiencies (%IPCE) in DSSC devices show the trend ISB-ZnP < ZnP < CZ-ZnP < IDB-ZnP. Inferior performance of the ISB-ZnP dye is attributed to its weaker adsorption to the TiO2 film, which is roughly half that of ZnP. In contrast, introduction of the CZ and IDB electron donors is demonstrated to promote a better performance than the “naked” ZnP porphyrin. The best performance was observed for the IDB-ZnP device reaching a power conversion efficiency of η = 3.62% under AM 1.5 irradiation conditions with a corresponding %IPCE maximizing at 48% for both Soret band (450 nm) and Q band (570 nm) photoexcitation.
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A BODIPY-luminol chemiluminescent resonance energy-transfer (CRET) cassette for imaging of cellular superoxide

Bag, S.; Tseng, J.-C.; Rochford, J.

Organic & Biomolecular Chemistry 2015, 13, 1763-1767.

Spectroscopic and in cellulo studies are here reported on the very first BODIPY-luminol chemiluminescent resonance energy-transfer (CRET) cassette where the luminol CL agent is covalently linked to the BODIPY energy-transfer acceptor in a molecular dyad. The efficiency of intramolecular CRET investigated for the BODIPY-luminol dyad was found to be 64% resulting in a dual emissive response. Successful in cellulo biochemiluminescence via CRET was achieved in PMA activated splenocytes.
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Synthesis, electronic and photophysical characterization of pi-conjugated meso-ferrocenyl-porphyrin fluorescent redox switches

Zhou, Y.; Ngo, K.T.; Zhang, B.; Feng, Y.; Rochford, J.

Organometallics 2014, 33, 7078–7090.

A series of meso-ferrocenyl-porphyrin dyads linked by four different π-conjugated bridging units (directly bound, vinyl, ethynyl, and phenyl) have been synthesized to investigate the influence of the conjugated linker on both the electronic and photochemical properties of the porphyrin chromophore. The basic structure consists of 5-(Fc)-15-(4-methylbenzoate)-10,20-diphenylporphyrin zinc(II), where Fc = ferrocene, vinylferrocene, ethynylferrocene, or phenylferrocene. Upon introduction of the various electron-donating ferrocenyl moieties at the meso-position of the porphyrin ring, Soret and Q-band electronic transitions of the resultant dyads are red-shifted compared with those of the nonferrocenyl reference porphyrin system 15-(4-methylbenzoate)-10,20-diphenylporphyrin zinc(II). The electronic properties of these systems have been investigated by electrochemical (cyclic voltammetry) and computational (DFT/TDDFT) methods, while UV/vis absorption and fluorescence emission spectroscopic analysis is also presented. Collectively, electronic and photophysical analysis indicate a strong electronic communication between the porphyrin macrocycle and directly bound ferrocenyl, vinylferrocenyl, and ethynylferrocenyl dyads. The presence of a phenyl spacer acts to inhibit such electronic communication due to the orthogonal geometry of the bridging phenyl ring at the meso-position of the porphyrin macrocycle. In addition to electronic factors, and in particular for the directly bound 5-(ferrocenyl)-15-(4-methylbenzoate)-10,20-diphenylporphyrin zinc(II) dyad, computational analysis suggests that a significant ruffling of the porphyrin macrocyle from planarity is required to facilitate the bulky ferrocene group directly at the meso-position. Of particular note for each of the meso-ferrocenyl-porphyrin dyads is how fluorescence emission derived from the porphyrin S1 (π–π*) excited state is quantitatively quenched due to photoinduced charge-transfer from the ferrocene unit onto the excited state porphyrin. Spectroelectrochemical studies demonstrate redox off/on switching of the porphyrin fluorescence emission via ferricenium/ferrocene redox cycling. Interestingly, it was found that the S0 ← S1 fluorescence emission is also switched-on following titration with the metal ions Ce(IV), Cu(II), and Fe(III) in acetonitrile.
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Shining light on the dark side of imaging: Excited state absorption enhancement of a bis-styryl-BODIPY photoacoustic contrast agent

Frenette, M.; Hatamimoslehabadi, M.; Laoui, S.; Bag, S.; Bellinger-Buckley, S.; La, J.; Mallidi, S.; Bouma, B.; Hasan, T.; Yelleswarapu, C.; Rochford. J.

Journal of the American Chemical Society 2014, 136, 15853−15856.

A first approach toward understanding the targeted design of molecular photoacoustic contrast agents (MPACs) is presented. Optical and photoacoustic Z-scan spectroscopy was used to identify how nonlinear (excited-state) absorption contributes to enhancing the photoacoustic emission of the curcuminBF2 and bis-styryl (MeOPh)2BODIPY dyes relative to Cy3.
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Synthesis, structures, and reactivity of copper(I) complexes supported by a rigid dinucleating ligand

Bheemaraju, A.; Beattie, J. W.; Danylyuk, Y.; Rochford, J.*; Groysman, S.

European Journal of Inorganic Chemistry 2014, 34, 5865-5873

Herein, we report the diverse CuI chemistry of a dinucleating ligand L that features two iminopyridine chelates bridged by a 1,8-diaminoxanthene linker {L = (1E,1′E)-N,N′-(2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-diyl)bis[1-(pyridin-2-yl)methanimine]}. The ligand L leads to the formation of discrete bimetallic species, or metallopolymers, as a result of syn-parallel or anti-parallel mutual disposition of the chelating units. The reaction of L with [Cu(NCMe)4]+ forms the discrete bimetallic complexes [Cu2(L)(NCMe)2]2+ (1) or [Cu2(L)2]2+ (2), depending on the solvent and the ligand-to-metal ratio. In contrast, the reaction of L with CuX (X = Cl, Br) forms the metallopolymers [Cu2(L)(Br)2]n (3) and {[Cu3(L)2Cl2][CuCl2]}n (4). The compounds were characterized by X-ray crystallography, NMR spectroscopy, mass spectrometry, electrochemistry, and DFT calculations. The reaction of compound 1 with O2 in CH2Cl2 was found to lead to a mixture of CuII products of which tetrametallic [Cu4(L)4(OH)3(Cl)]4+ and [Cu4(L)2(OH)4]4+ were identified by X-ray crystallography.
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Controlled CO release using photochemical, thermal and electrochemical approaches from the amino carbene complex, (CO)5CrC(NC4H8)CH3

McMahon, S.; Rochford, J.; Manton, J. C.; Harvey, E. C.; Greetham, G. M.; Clark, I. P.; Rooney, D. A.; Long, C.; Pryce, M. T.

Phys. Chem. Chem. Phys. 2014, 16, 21230-21233.

Multimodal photo, thermal and electrochemical approaches toward CO release from the amino carbene complex [(CO)5CrC(NC4H8)CH3] is reported. Picosecond time resolved infrared spectroscopy was used to probe the photo-induced early state dynamics leading to CO release, and DFT calculations confirmed that CO release occurs from a singlet excited state.
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Nonlinear optical properties of multipyrrole dyes

Frenette M.; Hatamimoslehabadi M.; Bellinger-Buckley S.; Laoui S.; Bag S.; Dantiste O.; Rochford J.; Yelleswarapu C.

Chemical Physics Letters 2014, 608, 303-307.

The nonlinear optical properties of a series of pyrrolic compounds consisting of BODIPY and aza-BODIPY systems are investigated using 532 nm nanosecond laser and the Z-scan technique. Results show that 3,5-distyryl extension of BODIPY to the red shifted MeO2BODIPY dye has a dramatic impact on its nonlinear absorption properties changing it from a saturable absorber to an efficient reverse saturable absorbing material with a nonlinear absorption coefficient of 4.64 × 10−10 m/W. When plotted on a concentration scale per mole of dye in solution MeO2BODIPY far outperforms the recognized zinc(II) phthalocyanine dye and is comparable to that of zinc(II) tetraphenylporphyrin.
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Tuning oxyquinolate non-innocence at the ruthenium polypyridyl core

Zhao, H.C.; Huang Y.-T.; Schweinfurth D.; Harney J.P.; Sarkar B.; Tsai, M.-K.; Rochford, J.

European Journal of Inorganic Chemistry 2013, 2013, 4410 – 4420.

Tuning oxyquinolate non-innocence at the ruthenium polypyridyl core

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An investigation of monomeric versus dimeric fac-Re(I) tricarbonyl systems containing the non-innocent 8-oxyquinolate ligand

Zhao, H.C.; Mello, B.; Fu, B.-L.; Chowdhury, H.; Szalda, D.J.; Tsai, M.-K.; Grills, D.C.; Rochford, J.

Organometallics 2013, 32, 1832-1841.

Synthesis and characterization of the dimeric [fac-Re(R-OQN)(CO)3]2 and monomeric fac-Re(R-OQN)(CO)3(CH3CN) complexes are reported where R = unsubstituted, 2-methyl, 5,7-dimethyl, or 5-fluoro and OQN = 8-oxyquinolate. Facile solvolysis of the dimeric systems is observed in coordinating media quantitatively yielding the monomer complexes in situ. Due to poor synthetic yields of the dimeric precursors, a direct synthetic strategy for isolation of the acetonitrile monomer complexes with an improved yield was developed. The fac-Re(CH3CN)2(CO)3Cl complex was easily generated in situ as a convenient intermediate to give the desired products in quantitative yield via reaction with the appropriately substituted 8-hydroxyquinoline and tetramethylammonium hydroxide base. Key to the success of this reaction is the precipitation of the product with triflic acid, whose conjugate triflate base is here noncoordinating. Furthermore, isolation of the solvated single crystal [fac-Re(FOQN)(CO)3](μ-Cl)[fac-Re(FHOQN)(CO)3]·CH3C6H5 has allowed a unique opportunity to access a possible reaction intermediate, giving insight into the formation of the [fac-Re(R-OQN)(CO)3]2 dimeric systems. Spectroscopic features (UV–vis, FTIR, and 1H NMR) of both monomeric and dimeric structures are discussed in terms of the π-electron-donating ability of the oxyquinolate ligand. Interpretation of these electronic effects and the associated steric properties is aided by single-crystal X-ray diffraction, electrochemical, and DFT/TD-DFT computational studies.
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Convergent synthesis of meso-ferrocenyl porphyrins for TiO2 sensitization

Harney, J. P.; Dransfield, T.; Rochford, J.

Tetrahedron Letters 2012, 53, 4700–4703.

Electron-donating ferrocenyl moieties have been incorporated into supramolecular carboxyporphyrin architectures Zn(II)-5-ferrocenyl-10,20-bistolyl-15-(4-methylbenzoate)porphyrin (trans-Fc-ZnP-CO2Me), Zn(II)-5-ferrocenyl-10,15,20-tris(4-methylbenzoate)porphyrin [trans-Fc-ZnP-(CO2Me)3], and Zn(II)-5,15-bisferrocenyl-10,20-bis(4-benzoate)porphyrin [trans-Fc2-ZnP-(CO2Me)2] for self-assembly on metal oxide nanoparticles. Efficient and economic synthesis has required a convergent strategy toward reduced symmetry trans-A2B2 and trans-AB2C substitution patterns about the porphyrin macrocycle minimizing the production of porphyrin side products and increasing yields of the target ferrocenylporphyrins. Preliminary spectroscopic data in solution and in the solid state bound to mesoporous TiO2 films are discussed.
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Evaluation of a ruthenium oxyquinolate architecture for dye sensitized solar cells

Zhao, H. C.; Harney, J. P.; Huang, Y.-T.; Yum, J.-H.; Nazeeruddin, Md. K.; Grätzel, M.; Tsai, M.-K.; Rochford, J.

Inorganic Chemistry 2012, 51, 1-3.

The synthesis of the [Ru(dcbpy)2(OQN)]+ complex is reported in which dcbpy and OQN− are the bidentate 4,4′-dicarboxy-2,2′-bipyridyl and 8-oxyquinolate ligands, respectively. Spectroscopic, electrochemical, and theoretical analyses are indicative of extensive Ru(OQN) molecular orbital overlap due to degenerate Ru d(π) and OQN p(π) mixing. [Ru(dcbpy)2(OQN)]+ displays spectroscopic properties remarkably similar to those of the N3 dye, making it a promising candidate for application in dye-sensitized solar cell devices. However, its solar power conversion efficiency requires further optimization.
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Water oxidation by a mononuclear ruthenium catalyst: characterization of the intermediates

Polyansky, D. E.; Muckerman, J. T.; Rochford, J.; Zong, R.; Thummel, R. P.; Fujita, E.

Journal of the American Chemical Society 2011, 133, 14649-14665.

A detailed characterization of intermediates in water oxidation catalyzed by a mononuclear Ru polypyridyl complex [RuII–OH2]2+ (Ru = Ru complex with one 4-t-butyl-2,6-di-(1′,8′-naphthyrid-2′-yl)-pyridine ligand and two 4-picoline ligands) has been carried out using electrochemistry, UV–vis and resonance Raman spectroscopy, pulse radiolysis, stopped flow, and electrospray ionization mass spectrometry (ESI-MS) with H218O labeling experiments and theoretical calculations. The results reveal a number of intriguing properties of intermediates such as [RuIV═O]2+ and [RuIV–OO]2+. At pH > 2.9, two consecutive proton-coupled one-electron steps take place at the potential of the [RuIII–OH]2+/[RuII–OH2]2+ couple, which is equal to or higher than the potential of the [RuIV═O]2+/[RuIII–OH]2+ couple (i.e., the observation of a two-electron oxidation in cyclic voltammetry). At pH 1, the rate constant of the first one-electron oxidation by Ce(IV) is k1 = 2 × 104 M–1 s–1. While pH-independent oxidation of [RuIV═O]2+ takes place at 1420 mV vs NHE, bulk electrolysis of [RuII–OH2]2+ at 1260 mV vs NHE at pH 1 (0.1 M triflic acid) and 1150 mV at pH 6 (10 mM sodium phosphate) yielded a red colored solution with a Coulomb count corresponding to a net four-electron oxidation. ESI-MS with labeling experiments clearly indicates that this species has an O–O bond. This species required an additional oxidation to liberate an oxygen molecule, and without any additional oxidant it completely decomposed slowly to form [RuII–OOH]+ over 2 weeks. While there remains some conflicting evidence, we have assigned this species as 1[RuIV–η2-OO]2+ based on our electrochemical, spectroscopic, and theoretical observations alongside a previously reported analysis by T. J. Meyer’s group (J. Am. Chem. Soc. 2010, 132, 1545–1557).
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Influence of the electron-cation interaction on electron mobility in dye-sensitized ZnO and TiO2 Nanocrystals: A study using ultrafast terahertz spectroscopy

Nemec, H.; Rochford, J.; Taratula, O.; Galoppini, E.; Kuzel, P.; Polivka, T.; Yartsev, A.; Sundstrom,V.

Physical Review Letters 2010, 104, 197401-197405.

Charge transport and recombination in nanostructured semiconductors are poorly understood key processes in dye-sensitized solar cells. We have employed time-resolved spectroscopies in the terahertz and visible spectral regions supplemented with Monte Carlo simulations to obtain unique information on these processes. Our results show that charge transport in the active solar cell material can be very different from that in nonsensitized semiconductors, due to strong electrostatic interaction between injected electrons and dye cations at the surface of the semiconductor nanoparticle. For ZnO, this leads to formation of an electron-cation complex which causes fast charge recombination and dramatically decreases the electron mobility even after the dissociation of the complex. Sensitized TiO2 does not suffer from this problem due to its high permittivity efficiently screening the charges.
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Thienyl appended porphyrins: Their synthesis, photochemical and electrochemical properties and applications

Boyle, N. M.; Rochford, J.; Pryce, M.

Coordination Chemistry Reviews 2010, 254, 77-102.

This review focuses on the synthesis, photophysical and electrochemical properties of thienyl porphyrins where processes such as electron transfer, energy transfer and electropolymerisation are discussed. The purpose of this review is to examine the influence of the thienyl ring, whether it be directly connected (via meso and β positions) or indirectly connected (via a covalent linker or axial coordination) on the ground and excited state electronic properties of the porphyrin macrocycle. Additionally, the importance of the electronic properties of a bridging oligothiophene between the porphyrin and another centre in supramolecular systems is discussed. Also included are applications of thienyl porphyrins in such areas as catalysis, therapeutics, (opto)electronics and electron-transfer/light-harvesting systems.
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Ruthenium complexes with non-innocent ligands: Electron distribution and implications for catalysis

Boyer, J.; Rochford, J.; Tsai, M.-K.; Muckerman, J. T.; Fujita, E.

Coordination Chemistry Reviews 2010, 254 (3-4), 309-330.

Ruthenium complexes with the non-innocent ligands (NILs) benzoquinone, iminobenzoquinone and benzoquinonediimine and their redox derivatives exhibit intriguing electronic properties. With the proper ligand set the NIL π* orbitals mix extensively with the ruthenium dπ orbitals resulting in delocalized electron distributions and non-integer oxidation states, and in most of these systems a particular ruthenium oxidation state dominates. This review critically examines the electronic structure of Ru–NIL systems from both an experimental and computational (DFT) perspective. The electron distribution within these complexes can be modulated by altering both the ancillary ligands and the NIL, and in a few cases the resultant electron distributions are exploited for catalysis. The Ru–NIL systems that perform alcohol oxidation and water oxidation catalysis are discussed in detail. The Tanaka catalyst, an anthracene-bridged dinuclear Ru complex, is an intriguing example of a Ru–NIL framework in catalysis. Unlike other known ruthenium water oxidation catalysts, the two Ru atoms remain low valent during the catalytic cycle according to DFT calculations, some experimental evidence, and predictions based on the behavior of the related mononuclear species.
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Oxidation state characterization of ruthenium 2−iminoquinone complexes through experimental and theoretical studies

Rochford, J.; Tsai, M.-K.; Szalda, D. J.; Boyer, J. L.; Muckerman, J. T.; Fujita, E.

Inorganic Chemistry 2010, 49 (3), 860-869.

The oxidation states of [Ru(trpy)(NIL)(OAc)]n complexes (NIL = 2-iminoquinone and its derivatives, n = +1, 0), as well as of the previously reported [RuIII(trpy)(2-iminosemiquinone)(Cl)]+ complex, are investigated by spectroscopic, electrochemical, and theoretical methods resulting in all the complexes being assigned as RuII−NILOx. Evidence for the presence of two structural isomers was found for all complexes, and crystal structures for both isomers of the [RuII(trpy)(2-imino-4-tert-butylquinone)(Cl)]ClO4 complex are reported.
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Energy level alignment of a zinc(II) tetraphenylporphyrin dye adsorbed onto TiO2(110) and ZnO(1120) surfaces

Rangan, S.; Katalinic, S.; Thorpe, R.; Bartynski, R.; Rochford, J.; Gallopini, E.

Journal of Physical Chemistry C 2010, 114 (2), 1139-1147.

The electronic structure of the Zn(II)-5-(3,5-dicarboxyphenyl)-10,15,20-triphenylporphyrin dye (ZnTPP-Ipa), chemisorbed onto ZnO(112̅0) and TiO2(110) single-crystal surfaces, has been investigated by means of density functional theory (DFT) and by electron spectroscopy methods in an ultra-high-vacuum environment. The core levels (Ti 2p and Zn 2p) as well as the valence band have been probed using X-ray and ultraviolet photoemission spectroscopies, whereas the conduction band has been evaluated from inverse photoemission spectroscopy. The calculated density of states for the gas phase molecule compares well to the experimentally determined electronic structure, allowing both a simple understanding of the adsorbate electronic properties and a direct determination of the ZnTPP-Ipa frontier orbitals with respect to the substrates’ band edges.
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Characterization of redox states of Ru(OH2)(Q)(tpy)2+ (Q = 3,5-di-tert-butyl-1,2-benzoquinone, tpy = 2,2’:6’,2”-terpyridine) and related species through experimental and theoretical studies

Tsai, M.-K.; Rochford, J.; Polyansky, D. E.; Tanaka, K.; Fujita, E.; Muckerman, J. T.

Inorganic Chemistry 2009, 48 (10), 4372.

The redox states of Ru(OH2)(Q)(tpy)2+ (Q = 3,5-di-tert-butyl-1,2-benzoquinone, tpy = 2,2′:6′,2″-terpyridine) are investigated through experimental and theoretical UV−vis spectra and Pourbaix diagrams.
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Photoelectrochemical behavior of polychelate porphyrin chromophores and titanium dioxide nanotube arrays for dye-sensitized solar cells

de Tacconi, N. R.; Chanmanee, W.; Rajeshwar, K.; Rochford, J.; Galoppini, E.

Journal of Physical Chemistry C 2009, 113 (7), 2926.

The influence of TiO2 nanotube array (NTA) morphology on dye-sensitized solar cell performance was studied with five porphyrin chromophores containing up to four carboxylic acid anchor groups. The TiO2 NTAs were anodically grown on Ti foil in NH4F electrolyte containing additives such as poly(ethylene glycol) (PEG 400), ethylene glycol, and glycerol. The NTAs grown in the presence of PEG 400 had the largest nanotube diameter (∼120 nm) and also yielded the best photoresponse with all five porphyrins in iodide/polyiodide electrolyte. Interestingly, nanoporous TiO2 “control” films, anodically grown on Ti foil in the absence of the organic additive, yielded an inferior photoresponse under the same conditions. Two of the porphyrins (P3 and P5), envisioned to have a spider-like configuration on the TiO2 NTA surface, yielded the best photoresponse among the five candidates in the Q(1,0) porphyrin spectral region. This result underlines that the molecular structure of the chromophore affects the directionality of electron transfer and its consequent ability to inject electrons (by the photoexcited porphyrin) into the oxide nanotube host. Finally, some perspectives on the use of TiO2 NTAs and porphyrins in DSSCs are given against the backdrop of the above data.
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Photophysical and electrochemical properties of meso-substituted thien-2-yl porphyrins

Rochford, J.; Brady, C.; Botchway, S.; McGarvey, J. J.; Rooney, A. D.; Pryce, M. T.

Journal of Physical Chemistry A, 2008, 112, 11611.

The influence of the thiophene ring on the ground and excited state properties of the porphyrin ring is investigated, when substituted at the meso-position. A series of mono-, di-, tri-, and tetra-meso-thien-2-yl porphyrins are studied and discussed with respect to the reference compounds zinc(II)-5,10,15,20-tetra(thien-2′-yl)porphyrin (1a) and zinc(II)-5,10,15,20-tetraphenylporphyrin (ZnTPP). The extended conjugated system zinc(II)-5-(5′-(5′′-ethynyl-2′′-thiophenecarboxaldehyde)thien-2′-yl)-10,15,20-triphenylporphyrin (4d) is also studied and shows enhanced charge transfer character due to the presence of the terminal aldehyde accepting group. A detailed analysis of ground and excited state UV−vis absorption, steady-state and time-resolved fluorescence, laser flash photolysis, and electrochemical data all point toward substantial electronic communication between the central Zn(II) porphyrin ring and the meso-thien-2-yl substituents, which is evident from excited state charge transfer character.
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Zn(II) tetraarylporphyrins anchored to TiO2, ZnO and ZrO2 nanoparticle films through rigid-rod linkers

Rochford, J.; Gallopini, E.

Langmuir, 2008, 24, 5366.

A series of six Zn(II) tetraphenylporphyrins (ZnTPP), with a phenyl (P) or oligophenyleneethynylene (OPE = (PE)n) rigid-rod bridge varying in length (9−30 Å) and terminated with an isophthalic acid (Ipa) anchoring unit, were prepared as model dyes for the study of sensitization processes on metal oxide semiconductor nanoparticle surfaces (MOn = TiO2, ZnO, and insulating ZrO2). The dyes were designed such that the electronic properties of the central porphyrin chromophore remained consistent throughout the series, with the rigid-rod anchoring unit allowing each porphyrin unit to be located at a fixed distance from the metal oxide nanoparticle surface. Electronic communication between the porphyrin and the rigid-rod unit was not desired. Rigid-rod porphyrins ZnTPP−Ipa, ZnTPP−P−Ipa, ZnTPP−PE−Ipa, ZnTPP−(PE)2−Ipa, ZnTPP−(PE)3−Ipa, and ZnTMP−Ipa (with mesityl substituents on the porphyrin ring) were synthesized using combinations of mixed aldehyde condensations and Pd-catalyzed cross-coupling reactions. Their properties, in solution and bound, were compared with that of Zn(II) 5,10,15,20-tetra(4-carboxyphenyl)porphyrin (p-ZnTCPP) as the reference compound. Solution UV–vis and steady-state fluorescence spectra for all six rigid-rod−Ipa porphyrins were almost identical to each other and to that of p-ZnTCPP. Cyclic voltammetry and differential pulse voltammetry scans of the methyl ester derivatives of the six rigid-rod−Ipa porphyrins, recorded in dichloromethane/electrolyte, exhibited redox behavior typical of ZnTPP porphyrins, with the first oxidation in the range +0.99 to 1.09 V vs NHE. All six rigid-rod−Ipa porphyrins and p-ZnTCPP were bound to metal oxide (MOn = TiO2, ZnO, and insulating ZrO2) nanoparticle films. The Fourier transform infrared attenuated total reflectance spectra of all compounds bound to MOn films showed a broad band at 1553−1560 cm−1 assigned to the v(CO2−) asymmetric stretching mode. Splitting of the Soret band into two bands at 411 and 423 nm in the UV–vis spectra of the bound compounds, and broadening and convergence of both fluorescence emission bands in the fluorescence spectra of the porphyrins bound to insulating ZrO2 were also observed. Such changes were less evident for ZnTMP−Ipa, which has mesityl substituents on the porphyrin ring to prevent aggregation. Steady-state fluorescence emission of rigid-rod−Ipa porphyrins bound to TiO2 and ZnO through the longest bridges (>14 Å) showed residual fluorescence emission, while fluorescence quenching was observed for the shortest compounds.
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Redox control of meso-Zn(II) ferrocenylporphyrin based fluorescence switches

Rochford, J.; Botchway, S.; Rooney, A. D.; Pryce, M. T.

Inorganic Chemistry 2007, 46 (18), 7247-7249.

The switching on and off of porphyrin fluorescence is achieved through the use of the ferrocene/ferrocenium redox couple. This approach makes use of the quenching of the porphyrin fluorescence by excited-state electron transfer from the ferrocene to the porphyrin, a process that, in the present systems, can be “switched on”, reversibly, by oxidation to the ferrocenium ion.
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Tetrachelate porphyrin chromophores for metal oxide semiconductor sensitization: The effect of spacer length and anchoring group position

Rochford, J.; Chu, D.; Hagfeldt, A.; Galoppini, E.

Journal of the American Chemical Society 2007, 129 (15), 4655-4665.

Four Zn(II)-tetra(carboxyphenyl)porphyrins in solution and bound to metal oxide (TiO2, ZnO, and ZrO2) nanoparticle films were studied to determine the effect of the spacer length and anchoring group position (para or meta) on their binding geometry and photoelectrochemical and photophysical properties. The properties of three types of anchoring groups (COOH and COONHEt3) for four Zn(II)-porphyrins (Zn(II)-5,10,15,20-tetra(4-carboxyphenyl)porphyrin (p-ZnTCPP), Zn(II)-5,10,15,20-tetra(3-carboxyphenyl)porphyrin (m-ZnTCPP), Zn(II)-5,10,15,20-tetra(3-(4-carboxyphenyl)phenyl)porphyrin (m-ZnTCP2P), and Zn(II)-5,10,15,20-tetra(3-ethynyl(4-carboxyphenyl)phenyl)porphyrin (m-ZnTC(PEP)P)) were compared. In m-ZnTCPP, m-ZnTCP2P, and m-ZnTC(PEP)P the four anchoring groups are in the meta position on the meso-phenyl rings of the porphyrin macrocycle, thus favoring a planar binding mode to the metal oxide surfaces. The three meta-substituted porphyrin salts have rigid spacer units of increasing length (phenyl (P), biphenyl (P2), and diphenylethynyl (PEP)) between the porphyrin ring and the carboxy anchoring groups, thus raising the macrocycle from the metal oxide surface. All porphyrins studied here, when bound to TiO2 and ZnO, exhibited quenching of the fluorescence emission, consistent with electron injection into the conduction band of the semiconductor. Steady-state UV−vis and fluorescence studies of p-ZnTCPP on insulating ZrO2 showed evidence of aggregation and exciton coupling. This was not observed in any of the meta-substituted porphyrins. The photoelectrochemical properties (IPCE, Voc, and Isc) of the porphyrins bound to TiO2 films in solar cells have been measured and rationalized with respect to the sensitizer binding geometry and distance from the surface.
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Fast electron transport in MOCVD-grown dye-sensitized ZnO nanorod solar cells

Galoppini, E.; Rochford, J.; Chen, H.; Saraf, G.; Lu, Y.; Hagfeldt, A.; Boschloo, G.

Journal of Physical Chemistry B 2006, 110 (33), 16159-16161.

The electron transport in dye-sensitized solar cells with a MOCVD (metal organic vapor deposition)-grown ZnO nanorod array (ZnO-N) or a mesoporous film prepared from ZnO colloids (ZnO-C) as the working electrode was compared. The electrodes were of similar thickness (2 μm) and sensitized with zinc(II) meso-tetrakis(3-carboxyphenyl)porphyrin, while the electrolyte was I-/I3- in 3-methoxypropionitrile. Electron transport in the ZnO-C cells was comparable with that found for colloidal TiO2 films (transport time ∼ 10 ms) and was light intensity dependent. Electron transport in solar cells with ZnO-N electrodes was about 2 orders of magnitude faster (∼30 μs). Thus, the morphology of the working ZnO electrode plays a key role for the electron transport properties.
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Pyrene terminated phenylenethynylene rigid linkers anchored to metal oxide nanoparticles

Taratula, O.; Rochford, J.; Piotrowiak, P.; Galoppini, E.; Carlisle, R. A.; Meyer, G. J.

Journal of Physical Chemistry B 2006, 110, 15734.

Phenylenethynylene (PE) rigid linkers (para and meta) were used to anchor pyrene to the surface of TiO2 (anatase) and ZrO2 nanoparticle thin films through the two COOH groups of an isophthalic acid (Ipa) unit. Four chromophore-linker models were studied in solution and bound. Two are novel meta-pyrene−PE linker systems:  dimethyl 5-(3-(1-pyrenylethynyl)phenylethynyl)-isophthalate, carrying one pyrene, and dimethyl 5-(bis-3,5-(1-pyrenylethynyl)phenylethynyl)-isophthalate, carrying two. These were compared with para rigid-rods dimethyl 5-(1-pyrenylethynyl)isophthalate and dimethyl 5-(4-(1-pyrenylethynyl)phenylethynyl)-isophthalate, each carrying one pyrene but varying in length. The length of the PE linkers and the para or meta substitution influence the photophysical properties of the compounds. The extinction coefficient increased, and the long wavelength absorbance of the pyrene chromophore was shifted to the red with increasing conjugation. Compared to unsubstituted pyrene, the pyrene-linker systems were characterized by short fluorescence lifetimes (τ ∼ 2 ns in tetrahydrofuran solutions), but quantum yields were close to unity. ZINDO/S CI calculations attribute this effect to a switching in the order of the two lowest-lying singlet states of pyrene. High surface coverages, ∼10-8 mol/cm2, and carboxylate binding modes on nanostructured TiO2 films were obtained in all cases. The appearance of a pyrene excimer emission on ZrO2, an insulator, indicates that the pyrene-linker system is closely packed (Py−Py < 4 Å) on the surface. The fluorescence emission on TiO2 was completely quenched, consistent with quantitative and rapid electron injection into the semiconductor indicating that the pyrene excimer acts as a sensitizer. Photoelectrochemical studies in regenerative solar cells with I3-/I- as the redox mediator indicated near-quantitative conversion of absorbed photons into an electrical current.
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