John Fielden

Emory University, Atlanta, Georgia, United States

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Publications (38)136.62 Total impact

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    ABSTRACT: Visible light driven water oxidation has been demonstrated at near-neutral pH using photoanodes based on nanoporous films of TiO2, polyoxometalate (POM) water oxidation catalyst [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10- (1), and both known photosensitizer [Ru(bpy)2(H4dpbpy)]2+ (P2) and the novel crown ether functionalized dye [Ru(5-crownphen)2(H2dpbpy)] (H22). Both triads, containing catalyst 1, and catalyst-free dyads, produce O2 with high faradaic efficiencies (80 to 94%), but presence of catalyst enhances quantum yield by up to 190% (maximum 0.39%). New sensitizer H22 absorbs light more strongly than P2, and increases O2 quantum yields by up to 270%. TiO2-2 based photoelectrodes are also more stable to desorption of active species than TiO2-P2: losses of catalyst 1 are halved when pH > TiO2 point-of-zero charge (pzc), and losses of sensitizer reduced below the pzc (no catalyst is lost when pH < pzc). For the triads, quantum yields of O2 are higher at pH 5.8 than at pH 7.2, opposing the trend observed for 1 under homogeneous conditions. This is ascribed to lower stability of the dye oxidized states at higher pH, and less efficient electron transfer to TiO2, and is also consistent with the 4th 1-to-dye electron transfer limiting performance rather than catalyst TOFmax. Transient absorption reveals that TiO2-2-1 has similar 1st electron transfer dynamics to TiO2-P2-1, with rapid (ps timescale) formation of long-lived TiO2(e-)-2-1(h+) charge separated states, and demonstrates that metallation of the crown ether groups (Na+/Mg2+) has little or no effect on electron transfer from 1 to 2. The most widely relevant findings of this study are therefore: (i) increased dye extinction coefficients and binding stability significantly improve performance in dye-sensitized water splitting systems; (ii) binding of POMs to electrode surfaces can be stabilized through use of recognition groups; (iii) the optimal homogeneous and TiO2-bound operating pHs of a catalyst may not be the same; and (iv) dye-sensitized TiO2 can oxidize water without a catalyst.
    No preview · Article · Jun 2015 · Chemical Science
  • Yanqun Tang · Wanhong He · Yanluo Lu · John Fielden · Xu Xiang · Dongpeng Yan
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    ABSTRACT: Host–guest photofunctional materials have received much attention recently due to their potential applications in light emitting diodes, polarized emission, and other optoelectronic fields. In this work, we report the encapsulation of a photoactive ruthenium-based complex (4,4′-diphosphonate-2,2′-bipyridine) into the biphenyl-based metal–organic framework (MOF) as a host–guest material toward potential photofunctional applications. The resulting material (denoted as Ru@MOF) presents different two-color blue/red luminescences at the crystal interior and exterior as detected by three-dimensional confocal fluorescence microscopy. Additionally, up-conversion emission and an enhanced photoluminescence lifetime relative to the pristine Ru-based complex can also be observed in this Ru@MOF system. Upon attaching on the rutile TiO2 nanoarray, the Ru@MOF also exhibits alternated photoelectrochemical properties relative to the pristine complex. Moreover, a density functional theoretical calculation was performed on the Ru@MOF structure to provide understanding of the host–guest interactions. Based on the combination of experimental and theoretical studies on the Ru@MOF system, the aim of this work is to deeply investigate how the host–guest materials can present different photofunctionalities and optoelectronic properties compared with those of the individual components, and to give detailed information on the potential host–guest energy/electronic transfer between the MOF and the complex.
    No preview · Article · Nov 2014 · The Journal of Physical Chemistry C
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    ABSTRACT: In an effort to develop robust molecular sensitizers for solar fuel production, the electronic structure and photodynamics of transition-metal-substituted polyoxometalates (POMs), a novel class of compound in this context, was examined. Experimental and computational techniques including femtosecond (fs) transient absorption spectroscopy have been used to study the cobalt-containing Keggin POMs, [CoIIW12O40]6− (1 a), [CoIIIW12O40]5− (2 a), [SiCoII(H2O)W11O39]6− (3 a), and [SiCoIII(H2O)W11O39]5− (4 a), finding the longest lived charge transfer excited state so far observed in a POM and elucidating the electronic structures and excited-state dynamics of these compounds at an unprecedented level. All species exhibit a bi-exponential decay in which early dynamic processes with time constants in the fs domain yield longer lived excited states which decay with time constants in the ps to ns domain. The initially formed states of 1 a and 3 a are considered to result from metal-to-polyoxometalate charge transfer (MPCT) from CoII to W, while the longer-lived excited state of 1 a is tentatively assigned to a localized intermediate MPCT state. The excited state formed by the tetrahedral cobalt(II) centered heteropolyanion (1 a) is far longer-lived (τ=420 ps in H2O; τ=1700 ps in MeCN) than that of 3 a (τ=1.3 ps), in which the single CoII atom is located in a pseudo-octahedral addendum site. Short-lived states are observed for the two CoIII-containing heteropolyanions 2 a (τ=4.4 ps) and 4 a (τ=6.3 ps) and assigned solely to O→CoIII charge transfer. The dramatically extended lifetime for 1 a versus 3 a is ascribed to a structural change permitted by the coordinatively flexible central site, weak orbital overlap of the central Co with the polytungstate framework, and putative transient valence trapping of the excited electron on a single W atom, a phenomenon not noted previously in POMs.
    No preview · Article · Apr 2014 · Chemistry - A European Journal
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    ABSTRACT: The viable production of solar fuels requires a visible-light-absorbing unit, a H2O (or CO2) reduction catalyst (WRC), and a water oxidation catalyst (WOC) that work in tandem to split water or reduce CO2 with H2O rapidly, selectively, and for long periods of time. Most catalysts and photosensitizers developed to date for these triadic systems are oxidatively, thermally, and/or hydrolytically unstable. Polyoxometalates (POMs) constitute a huge class of complexes with extensively tunable properties that are oxidatively, thermally, and (over wide and adjustable pH ranges) hydrolytically stable. POMs are some of the fastest and most stable WOCs to date under optimal conditions. This Microreview updates the very active POM WOC field; it reports the application of POMs as WRCs and initial self-assembling metal oxide semiconductor–photosensitizer–POM catalyst triad photoanodes. The complexities of investigating these POM systems, including but not limited to the study of POM-hydrated metal-ion–metal-oxide speciation processes, are outlined. The achievements and challenges in POM WOC, WRC, and triad research are outlined.
    No preview · Article · Feb 2014 · Berichte der deutschen chemischen Gesellschaft
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    ABSTRACT: Triadic photoanodes were prepd. based on nanoporous films of the metal oxides ZrO2, TiO2, and SnO2, sensitizer [Ru(bpy)2(dpbpy)]2+ (P2), and polyoxometalate water oxidn. catalyst [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10- (I) and investigated for their potential utility in water-splitting dye-sensitized photoelectrochem. cells. Transient visible and mid-IR absorption spectroscopic studies were carried out to investigate the charge sepn. dynamics of these systems, indicating that electron transfer from photoexcited P2 to TiO2 and SnO2 is still the main excited state quenching pathway in the presence of I. Furthermore, the accelerated recovery of the P2 ground state bleach in the presence of I results from ultrafast (nanosecond) electron transfer from catalyst to oxidized dye. Catalyst loading appears to depend largely on the point of zero charge of the supporting oxide and as such is significantly lower on SnO2 than on TiO2: nonetheless, the rate of recovery of the ground state bleach is similar in both TiO2-P2-I and SnO2-P2-I films. Spectral evidence for the formation of long-lived charged sepd. states is provided by the observation of signals persisting beyond 0.5 μs which are attributed to Stark effect induced change of the P2 spectrum and/or formation of oxidized I. Photoelectrochem. measurements on TiO2-P2 and TiO2-P2-I photoanodes under visible light irradn. indicate a ca. 100% photocurrent enhancement in the presence of I, suggesting light-driven water oxidn. by the TiO2-P2-I system with an internal quantum efficiency of ca. 0.2%. The fast formation and long lifetime of the photo-oxidized catalyst suggest that photoanodes of this type may reward further optimization through the introduction of faster catalysts and stabilization of the binding of the dye to the electrode. [on SciFinder(R)]
    No preview · Article · Jan 2013 · The Journal of Physical Chemistry C
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    John Fielden · Kyle Quasdorf · Leroy Cronin · Paul Kögerler
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    ABSTRACT: An unusual PFO(3)(2-)-templated "inverse Keggin" polyanion, [Mo(12)O(46)(PF)(4)](4-), has been isolated from the degradation reaction of an {Mo(132)}-type Keplerate to [PMo(12)O(40)](3-) by [Cu(MeCN)(4)](PF(6)) in acetonitrile. (31)P-NMR studies suggest a structure-directing role for [Cu(MeCN)(4)](+) in the formation of the highly unusual all-inorganic inverse Keggin structure.
    Full-text · Article · Jul 2012 · Dalton Transactions
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    ABSTRACT: Late-first row transition metal nitrate complexes of the tetradentate N-donor ligand cis-3,5-bis[(2-pyridinyleneamino]-trans-hydroxycyclohexane (DDOP) adopt a mono-cationic [M(DDOP)(H(2)O)(NO(3))](+) structure (M = Co, 1; Cu, 2; Zn, 3) in which the DDOP ligand occupies the equatorial plane. The complexes are essentially isostructural and isomorphous, allowing the Co(II) and Cu(II) complexes to co-crystallize in mixed-metal solid solutions with the formula [Co(x)Cu(1-x)(DDOP)(NO(3))(H(2)O)](NO(3)), where x = 0.4 (4), 0.1 (5), and 0.7 (6). For 4, structural and magnetochemical analysis indicate that the geometry of the octahedral Co(II) complex distorts to match that of the dominant Jahn-Teller distorted Cu(II) center. Magnetic susceptibility data of octahedral Co(II) are sensitive to ligand geometry distortions and have been analyzed accordingly, comparing 4 to the reference systems 1 and 2. Bond valence calculations have been used to estimate the relative stabilities of the six hydrogen bonded networks, suggesting that the stretching of the Co(II) coordination sphere 4 in is assisted by adoption of the most stable hydrogen bonded network; but that in 6 this is overcome by a higher loading of Co. This family of complexes therefore represent predictable metal-based tectons which can help probe the influence of secondary non-covalent interactions over metal coordination geometries and properties.
    Full-text · Article · Mar 2012 · Dalton Transactions
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    ABSTRACT: The homochiral iron(III) wheels [Fe(6){(S)-pedea}(6)Cl(6)] and [Fe(6){(R)-pedea)}(6)Cl(6)] [(R)- and (S)-2; pedea = phenylethylaminodiethoxide] exhibit high optical activities and antiferromagnetic exchange. These homochiral products react with each other, producing the centrosymmetric, crystallographically characterized [Fe(6){(S)-pedea}(3){(R)-pedea}(3)Cl(6)] diastereomer [(RSRSRS)-2]. (1)H NMR and UV-vis studies indicate that exchange processes are slow in both homo- and heterochiral systems but that, upon combination, the reaction between (R)- and (S)-2 occurs quickly.
    Full-text · Article · Mar 2012 · Inorganic Chemistry
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    ABSTRACT: The effect of thermal fluctuations on the dynamics of a gapped quantum magnet is studied using inelastic neutron scattering on copper nitrate, a model material for the spin-1/2, one-dimensional (1D) bond alternating Heisenberg chain. A large, highly deuterated, single-crystal sample of copper nitrate is produced using a solution growth method and measurements are made using the high-resolution backscattering spectrometer OSIRIS at the ISIS Facility. Theoretical calculations and numerical analysis are combined to interpret the physical origin of the thermal effects observed in the magnetic spectra. The primary observations are (1) a thermally induced central peak due to intraband scattering, which is similar to Villain scattering familiar from soliton systems in 1D, and (2) the one-magnon quasiparticle pole is seen to develop with temperature into an asymmetric continuum of scattering. We relate this asymmetric line broadening to a thermal strongly correlated state caused by hard-core constraints and quasiparticle interactions. These findings are a counter example to recent assertions of the universality of line broadening in 1D systems and are applicable to a broad range of quantum systems.
    No preview · Article · Jan 2012 · Physical review. B, Condensed matter
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    ABSTRACT: Four new dipolar cations have been synthesized, containing ferrocenyl electron donor groups and diquaternized 2,2′-bipyridyl (diquat) acceptors. To our knowledge, these are the first organometallic diquat derivatives to be reported and have been characterized as their PF6– salts by using various techniques including 1H NMR and electronic absorption spectroscopies and cyclic voltammetry. UV–vis spectra show multiple intramolecular charge-transfer bands, and three reversible redox processes are observed for each compound. Molecular quadratic nonlinear optical (NLO) responses have been determined by using hyper-Rayleigh scattering at 1064 nm and Stark (electroabsorption) spectroscopic studies on the intense π → π* intraligand and d → π* metal-to-ligand charge-transfer bands. The most active compounds have estimated, Stark-derived β0 values approaching that of the chromophore in the technologically important material (E)-4′-(dimethylamino)-N-methyl-4-stilbazolium tosylate. Single-crystal X-ray structures have been obtained for three of the salts, with one adopting the orthorhombic space group Aba2 and having potential for bulk NLO behavior due to its polar structure. Attempted crystallizations of the remaining chromophore revealed that it undergoes an unusual intermolecular formal Michael cycloaddition between an activated methyl group and a double bond, forming a dimeric species. This diastereomeric cyclic complex has also been characterized via single-crystal X-ray diffraction.
    No preview · Article · Oct 2011 · Organometallics
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    ABSTRACT: Six new dicationic 2D nonlinear optical (NLO) chromophores with pyrazinyl-pyridinium electron acceptors have been synthesized by nucleophilic substitutions of 2,6-dichloropyrazine with pyridyl derivatives. These compounds have been characterized as their PF(6)(-) salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. Large red shifts in the intense, π → π* intramolecular charge-transfer (ICT) transitions on replacing -OMe with -NMe(2) substituents arise from the stronger π-electron donor ability of the latter. Each compound shows a number of redox processes which are largely irreversible. Single crystal X-ray structures have been determined for five salts, including two nitrates, all of which adopt centrosymmetric packing arrangements. Molecular first hyperpolarizabilities β have been determined by using femtosecond hyper-Rayleigh scattering at 880 and 800 nm, and depolarization studies show that the NLO responses of the symmetric species are strongly 2D, with dominant "off-diagonal" β(zyy) components. Stark (electroabsorption) spectroscopic measurements on the ICT bands afford estimated static first hyperpolarizabilities β(0). The directly and indirectly derived β values are large, and the Stark-derived β(0) response for one of the new salts is several times greater than that determined for (E)-4'-(dimethylamino)-N-methyl-4-stilbazolium hexafluorophosphate. These Stark spectroscopic studies also permit quantitative comparisons with related 2D, binuclear Ru(II) ammine complex salts.
    Full-text · Article · Nov 2010 · The Journal of Organic Chemistry
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    ABSTRACT: Inelastic neutron scattering (INS) in variable magnetic field and high-field magnetization measurements in the millikelvin temperature range were performed to gain insight into the low-energy magnetic excitation spectrum and the field-induced level crossings in the molecular spin cluster {Cr(8)}-cubane. These complementary techniques provide consistent estimates of the lowest level-crossing field. The overall features of the experimental data are explained using an isotropic Heisenberg model, based on three distinct exchange interactions linking the eight Cr(III) paramagnetic centers (spins s = 3/2), that is supplemented with a relatively large molecular magnetic anisotropy term for the lowest S = 1 multiplet. It is noted that the existence of the anisotropy is clearly evident from the magnetic field dependence of the excitations in the INS measurements, while the magnetization measurements are not sensitive to its effects.
    Full-text · Article · Nov 2010 · Journal of Physics Condensed Matter
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    ABSTRACT: A large series of complexes has been synthesized with two chelating, Schiff base azobenzene derivatives connected linearly by coordination to a central nickel(II) or palladium(II) ion. These compounds have the general formulas M(II)(OC(6)H(3)-2-CHNR-4-N═NC(6)H(4)-4-CO(2)Et)(2) [M = Ni; R = n-Bu (3c), n-C(6)H(13) (3d), n-C(8)H(17) (3e), n-C(12)H(25) (3f), Ph (3g), OH (3h), C(6)H(4)-4-CO(2)Et (3i). M = Pd; R = i-Pr (4a), t-Bu (4b), n-Bu (4c), n-C(6)H(13) (4d), n-C(8)H(17) (4e), n-C(12)H(25) (4f), Ph (4g)], M(II)[OC(6)H(3)-2-CHN(n-C(8)H(17))-4-N═NC(6)H(4)-4-CO(2)(n-C(8)H(17))](2) [M = Ni (9), Pd (10)], M(II)[OC(6)H(3)-2-CHN(n-C(8)H(17))-4-N═NC(6)H(4)-4-C(6)H(4)-4-O(n-C(7)H(15))](2) [M = Ni (14), Pd (15)], and M(II)[OC(6)H(3)-2-CHN(CMe(2))-4-N═NC(6)H(4)-4-CO(2)Et](2) [M = Ni (17), Pd (18); the CMe(2) groups are connected]. These compounds have been characterized by using various physical techniques including (1)H NMR spectroscopy and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. Single-crystal X-ray structures have been obtained for two pro-ligands and five complexes (3e, 4e, 14, 15, and 17). The latter always show a strictly square planar arrangement about the metal center, except for the Ni(II) complex of a salen-like ligand (17). In solution, broadened (1)H NMR signals indicate distortions from square planar geometry for the bis-chelate Ni(II) complexes. Electronic absorption spectroscopy and ZINDO_S (Zerner's intermediate neglect of differential overlap) and TD-DFT (time-dependent density functional theory) calculations show that the lowest energy transition has metal-to-ligand charge-transfer character. The λ(max) of this band lies in the range of 409-434 nm in dichloromethane, and replacing Ni(II) with Pd(II) causes small blue-shifts. Dichroic ratios measured in various liquid crystal hosts show complexation-induced increases with Ni(II), but using Pd(II) has a detrimental effect.
    No preview · Article · Oct 2010 · Inorganic Chemistry
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    ABSTRACT: A series of chromophoric salts has been prepared in which 4-(diphenylamino)phenyl (Dpap) electron donor groups are connected to electron-accepting diquaternized 2,2'-bipyridyl (diquat) units. The main aim is to combine large quadratic and cubic nonlinear optical (NLO) effects in potentially redox-switchable molecules with 2D structures. The chromophores have been characterized as their PF(6)(-) salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. The visible absorption spectra are dominated by intense π → π* intramolecular charge-transfer (ICT) bands, and all of the compounds show two reversible or quasireversible diquat-based reductions and partially reversible Dpap oxidations. Single crystal X-ray structures have been obtained for one salt and for the precursor compound (E)-4-(diphenylamino)cinnamaldehyde, both of which adopt centrosymmetric space groups. First hyperpolarizabilities β have been measured by using hyper-Rayleigh scattering (HRS) with a 800 nm laser, and Stark (electroabsorption) spectroscopy of the ICT bands affords estimated static first hyperpolarizabilities β(0). The directly and indirectly derived β values are large and generally increased substantially for the bis-Dpap derivatives when compared with their monosubstituted analogues. Polarized HRS studies show that the NLO responses of the disubstituted species are dominated by "off-diagonal" β(zyy) components. Lengthening the diquaternizing alkyl unit lowers the electron-acceptor strength and therefore increases the ICT energies and decreases the E(1/2) values for diquat reduction. However, compensating increases in the ICT intensity prevent significant decreases in the Stark-based β(0) responses. Cubic NLO properties have been measured by using the Z-scan technique over a wavelength range of 520-1600 nm, revealing relatively high two-photon absorption cross-sections of up to 730 GM at 620 nm for one of the disubstituted chromophores.
    No preview · Article · Oct 2010 · The Journal of Physical Chemistry A
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    ABSTRACT: Six new nonlinear optical (NLO) chromophores with pyrazinyl-pyridinium electron acceptors have been synthesized by complexing a known pro-ligand with electron donating {Ru(II)(NH(3))(5)}(2+) or trans-{Ru(II)(NH(3))(4)(py)}(2+) (py = pyridine) centers. These cationic complexes have been characterized as their PF(6)(-) salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. The visible d → π* metal-to-ligand charge-transfer (MLCT) absorptions gain intensity on increasing the number of Ru(II) centers from one to two, but remain at constant energy. One or two Ru(III/II) redox processes are observed which are reversible, quasi-reversible, or irreversible, while all of the ligand-based reductions are irreversible. Molecular first hyperpolarizabilities β have been determined by using hyper-Rayleigh scattering (HRS) at 1064 nm, and depolarization studies show that the NLO responses of the symmetric species are strongly two-dimensional (2D) in character, with dominant "off-diagonal" β(zyy) components. Stark (electroabsorption) spectroscopic measurements on the MLCT bands also allow the indirect determination of estimated static first hyperpolarizabilities β(0). Both the HRS and the Stark-derived β(0) values increase on moving from mono- to bimetallic complexes, and substantial enhancements in NLO response are achieved when compared with one-dimensional (1D) and 2D monometallic Ru(II) ammine complexes reported previously.
    Full-text · Article · Oct 2010 · Inorganic Chemistry
  • John Fielden · Arkady Ellern · Paul Kögerler
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    ABSTRACT: The Cr(4)O(4) hetero-cubane-centered octachromium(III) cluster [Cr(8)(PhCO(2))(16)O(4)] crystallizes from fluorobenzene-acetonitrile as dodeca-mu(2)-benzoato-tetrabenzoatotetra-mu(4)-oxido-octachromium(III) acetonitrile tetrasolvate dihydrate, [Cr(8)(C(7)H(5)O(2))(16)O(4)].4C(2)H(3)N.2H(2)O, (I). Crystals produced by this method are significantly more stable than the originally published dichloromethane pentasolvate, [Cr(8)(PhCO(2))(16)O(4)].5CH(2)Cl(2) [Atkinson et al. (1999). Chem. Commun. pp. 285-286], leading to a significantly higher quality structure and allowing the production of large quantities of high-quality nondeuterated and deuterated material suitable for inelastic neutron scattering (INS) measurements. Compound (I) reveals a higher symmetry structure in which the cluster sits on a twofold rotation axis, and is based on an asymmetric unit containing four crystallographically independent Cr positions, two oxide ligands, eight benzoate ligands, two acetonitrile solvent molecules and one disordered water molecule. All the Cr atoms are six-coordinate, with an octahedral geometry for the inner cubane and a more highly distorted coordination environment in the outer positions. Despite the higher symmetry, the coordination geometries observed in (I) are largely similar to the dichloromethane pentasolvate structure, indicating that crystal-packing effects have little influence on the molecular structure of [Cr(8)(PhCO(2))(16)O(4)]. Close structural analysis reveals that the high magnetic anisotropy observed in the INS measurements is a consequence of the distorted coordination geometry of the four outer Cr atoms.
    No preview · Article · Sep 2010 · Acta Crystallographica Section C Crystal Structure Communications
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    ABSTRACT: In this article, we present a detailed study of structure-activity relationships in diquaternized 2,2'-bipyridyl (diquat) derivatives. Sixteen new chromophores have been synthesized, with variations in the amino electron donor substituents, pi-conjugated bridge, and alkyl diquaternizing unit. Our aim is to combine very large, two-dimensional (2D) quadratic nonlinear optical (NLO) responses with reversible redox chemistry. The chromophores have been characterized as their PF(6)(-) salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. Their visible absorption spectra are dominated by intense pi --> pi* intramolecular charge-transfer (ICT) bands, and all show two reversible diquat-based reductions. First hyperpolarizabilities beta have been measured by using hyper-Rayleigh scattering with an 800 nm laser, and Stark spectroscopy of the ICT bands affords estimated static first hyperpolarizabilities beta(0). The directly and indirectly derived beta values are large and increase with the extent of pi-conjugation and electron donor strength. Extending the quaternizing alkyl linkage always increases the ICT energy and decreases the E(1/2) values for diquat reduction, but a compensating increase in the ICT intensity prevents significant decreases in Stark-based beta(0) responses. Nine single-crystal X-ray structures have also been obtained. Time-dependent density functional theory clarifies the molecular electronic/optical properties, and finite field calculations agree with polarized HRS data in that the NLO responses of the disubstituted species are dominated by 'off-diagonal' beta(zyy) components. The most significant findings of these studies are: (i) beta(0) values as much as 6 times that of the chromophore in the technologically important material (E)-4'-(dimethylamino)-N-methyl-4-stilbazolium tosylate; (ii) reversible electrochemistry that offers potential for redox-switching of optical properties over multiple states; (iii) strongly 2D NLO responses that may be exploited for novel practical applications; (iv) a new polar material, suitable for bulk NLO behavior.
    Full-text · Article · Aug 2010 · Journal of the American Chemical Society
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    ABSTRACT: The research towards more efficient organic molecules for second-order nonlinear optical (NLO) applications has resulted in substantial improvements of the molecular nonlinear polarizabilities. Different strategies for increasing NLO responses at the molecular level in ionic chromophores are reviewed. However, only a small subset of the highly efficient non-centrosymmetric molecules also forms non-centrosymmetric crystals at the bulk level. Examples of success in achieving polar crystals of molecular salts are presented. Such crystals with a high number density of aligned dipolar chromophores are promising materials for efficient second-order NLO applications, such as terahertz generation via difference frequency mixing.
    No preview · Article · Aug 2010 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: We describe a series of nine new complex salts in which electron-rich Ru(II) or Fe(II) centers are connected via pi-conjugated bridges to six electron-accepting N-methyl-/N-arylpyridinium groups. This work builds upon our previous preliminary studies (Coe , B. J. J. Am. Chem. Soc. 2005, 127, 13399-13410; J. Phys. Chem. A 2007, 111, 472-478), with the aims of achieving greatly enhanced NLO properties and also combining large quadratic and cubic effects in potentially redox-switchable molecules. Characterization has involved various techniques, including electronic absorption spectroscopy and cyclic voltammetry. The complexes display intense, visible d --> pi* metal-to-ligand charge-transfer (MLCT) bands, and their pi --> pi* intraligand charge-transfer (ILCT) absorptions in the near-UV region show molar extinction coefficients as high as ca. 3.5 x 10(5) M(-1) cm(-1). Molecular quadratic nonlinear optical (NLO) responses beta have been determined by using hyper-Rayleigh scattering at 800 and 1064 nm and also via Stark (electroabsorption) spectroscopic studies. The directly and indirectly derived beta values are very large, with the Stark-based static first hyperpolarizabilities beta(0) reaching as high as ca. 10(-27) esu, and generally increase on extending the pi-conjugation and enhancing the electron-accepting strength of the ligands. Cubic NLO properties have also been measured by using the Z-scan technique, revealing relatively high two-photon absorption cross sections of up to 2500 GM at 750 nm.
    No preview · Article · Feb 2010 · Journal of the American Chemical Society
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    John Fielden · D. L. Long · Leroy Cronin · P Kogerler
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    ABSTRACT: Reaction of (NBu4)(2)[Mo2O7] with [Cu(CH3CN)(4)](PF6) in acetonitrile results in isolation of the orange beta-octamolybdate [Cu(CH3CN)(4)](2)[Mo8O26Cu2(CH3CN)(4)] (1) along with the colourless alpha-octamolybdate [Cu(CH3CN)(4)](4)[Mo8O26]center dot 2CH(3)CN (2). Both products decompose rapidly upon removal from their mother liquors, forming an insoluble, dark brown coloured phase with the composition Cu-4[Mo8O26]center dot 0.6CH(3)CN center dot 16H(2)O (3). The copper(I) acetonitrile derivatised isopolyanion in 1 thus represents an intermediate structure between the simple, underivatised octamolybdate 2 and fully condensed, polymeric phase 3. .
    Full-text · Article · Sep 2009 · Polyhedron

Publication Stats

489 Citations
136.62 Total Impact Points

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Institutions

  • 2013-2015
    • Emory University
      • Department of Chemistry
      Atlanta, Georgia, United States
  • 2014
    • University of East Anglia
      • School of Chemistry
      Norwich, England, United Kingdom
  • 2012
    • RWTH Aachen University
      Aachen, North Rhine-Westphalia, Germany
  • 2006-2012
    • Iowa State University
      • • Ames Laboratory
      • • Department of Physics and Astronomy
      Ames, Iowa, United States
  • 2004-2012
    • University of Glasgow
      • School of Chemistry
      Glasgow, Scotland, United Kingdom
  • 2010-2011
    • The University of Manchester
      • School of Chemistry
      Manchester, England, United Kingdom