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ALTUN, ZİKRİ

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    PublicationOpen Access
    Stochastic enhancement of high-order harmonic generation
    (2012-01-01) ALTUN, ZİKRİ; YAVUZ, İLHAN; Yavuz İ., Bleda E. A., Altun Z.
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    Complexes with Tunable Intramolecular Ferrocene to Ti-IV Electronic Transitions: Models for Solid State Fe-II to Ti-IV Charge Transfer
    (AMER CHEMICAL SOC, 2016) ALTUN, ZİKRİ; Turlington, Michael D.; Pienkos, Jared A.; Carlton, Elizabeth S.; Wroblewski, Karlee N.; Myers, Alexis R.; Trindle, Carl O.; Altun, Zikri; Rack, Jeffrey J.; Wagenknecht, Paul S.
    Iron(II)-to-titanium(IV) metal-to-metal-charge transfer (MMCT) is important in the photosensitization of TiO2 by ferrocyanide, charge transfer in solid-state metal-oxide photocatalysts, and has been invoked to explain the blue color of sapphire, blue kyanite, and some lunar material. Herein, a series of complexes with alkynyl linkages between ferrocene (Fc) and Ti-IV has been prepared and characterized by UV-vis spectroscopy and electrochemistry. Complexes with two ferrocene substituents include Cp2Ti(C(2)Fc)(2), Cp*Ti-2(C(2)Fc)(2), and Cp2Ti(C(4)Fc)(2). Complexes with a single ferrocene utilize a titanocene with a trimethylsilyl derivatized Cp ring, Cp-TMS, and comprise the complexes (Cp2Ti)-Cp-TMS(C(2)Fc)(C2R), where R = C6H5, p-C6H4CF3, and CF3. The complexes are compared to Cp2Ti(C2Ph)(2), which lacks the second metal. Cyclic voltammetry for all complexes reveals a reversible Ti-IV/III reduction wave and an Fe-II/III oxidation that is irreversible for all complexes except (Cp2Ti)-Cp-TMS-(C(2)Fc)(C2CF3). All of the complexes with both Fc and Ti show an intense absorption (4000 M-1 cm(-1) < epsilon < 8000 M-1 cm(-1)) between 540 and 630 nm that is absent in complexes lacking a ferrocene donor. The energy of the absorption tracks with the difference between the Ti-IV/III and Fe-III/II reduction potentials, shifting to lower energy as the difference in potentials decreases. Reorganization energies, lambda, have been determined using band shape analysis (2600 cm(-1) < lambda < 5300 cm(-1)) and are in the range observed for other donor-acceptor complexes that have a ferrocene donor. Marcus-Hush-type analysis of the electrochemical and spectroscopic data are consistent with the assignment of the low-energy absorption as a MMCT band. TD-DFT analysis also supports this assignment. Solvatochromism is apparent for the MMCT band of all complexes, there being a bathochromic shift upon increasing polarizability of the solvent. The magnitude of the shift is dependent on both the electron density at Ti-IV and the identity of the linker between the titanocene and the Fc. Complexes with a MMCT are photochemically stable, whereas Cp2Ti(C2Ph)(2) rapidly decomposes upon photolysis.
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    PublicationOpen Access
    Phosphorescence Tuning through Heavy Atom Placement in Unsymmetrical Difluoroboron beta-Diketonate Materials
    (WILEY-V C H VERLAG GMBH, 2018-02-06) ALTUN, ZİKRİ; Liu, Tiandong; Zhang, Guoqing; Evans, Ruffin E.; Trindle, Carl O.; Altun, Zikri; DeRosa, Christopher A.; Wang, Fang; Zhuang, Meng; Fraser, Cassandra L.
    Difluoroboron beta-diketonates (BF(2)bdks) show both fluorescence (F) and room-temperature phosphorescence (RTP) when confined to a rigid matrix, such as poly(lactic acid). These materials have been utilized as optical oxygen sensors (e.g., in tumors, wounds, and cells). Spectral features include charge transfer (CT) from the major aromatic donor to the dioxaborine acceptor. A series of naphthyl-phenyl dyes (BF(2)nbm) (1-6) were prepared to test heavy-atom placement effects. The BF(2)nbm dye (1) was substituted with Br on naphthyl (2), phenyl (3), or both rings (4) to tailor the fluorescence/phosphorescence ratio and RTP lifetimeimportant features for designing O-2 sensing dyes by means of the heavy atom effect. Computational studies identify the naphthyl ring as the major donor. Thus, Br substitution on the naphthyl ring produced greater effects on the optical properties, such as increased RTP intensity and decreased RTP lifetime compared to phenyl substitution. However, for electron-donating piperidyl-phenyl dyes (5), the phenyl aromatic is the major donor. As a result, Br substitution on the naphthyl ring (6) did not alter the optical properties significantly. Experimental data and computational modeling show the importance of Br position. The S-1 and T-1 states are described by two singly occupied MOs (SOMOs). When both of these SOMOs have substantial amplitude on the heavy atom, passage from S-1 to T-1 and emission from T-1 to S-0 are both favored. This shortens the excited-state lifetimes and enhances phosphorescence.
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    Studies on spin state preferences in Fe(II) complexes
    (ELSEVIER, 2015) BLEDA, ERDİ ATA; Bleda, Erdi A.; Trindle, Carl; Altun, Zikri
    We report modeling studies of high spin and low spin forms of the test set of problematic cases of pseudo-octahedral Fe(II) complexes defined by Ye and Neese. These include the most thoroughly studied spin crossover compound Fe(SCN)(2)(Phen)(2) and a more recently discovered Fe(II)-2,13-dimethyl-6,9-dio xa-3,12,18-triaza-bicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentene. Reliable estimates of the often small high-spin to low-spin energy gap require balanced treatment of electronic structure in very different bonding environments. Density functionals differ in their bias toward low spin or high spin configurations. For the test set defined by Ye and Neese, we describe the estimates of their energy gaps between high-spin and low-spin forms for 75 functionals; geometries are obtained by optimization with B3LYP with zero-order relativistic approximation (ZORA) in a triple-zeta quality basis. Fairly reliable results for high-spin to low-spin energy gaps are obtained by functionals IMP, HCTH, OPBE, and B3LYP*, and with the omega B97XD functional expressed in the cc-pVTZ basis, about half the size of the def2-QZVPP used by Ye and Neese. The best functionals are still biased systematically toward the high spin by about 20 kJ/mol. We apply these methods in a new study of a pseudo C-3v four coordinate crossover species reported by Scepaniak et al. Their four coordinate spin crossover complex of Fe with aryl-substituted scorpionate and N = P(Ph)(3) ligands is modeled by a truncated structure lacking aromatic substituents. These substituents appear to stabilize the lower spin. Reproducing the 80 K crossover temperature requires an empirical correction of about 12 kJ/mot in favor of the singlet for this truncated model system. (C) 2015 Elsevier B.V. All rights reserved.
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    Photophysical and Analyte Sensing Properties of Cyclometalated Ir(III) Complexes
    (SPRINGER/PLENUM PUBLISHERS, 2012) ALTUN, ZİKRİ; Leavens, Bethany B. H.; Trindle, Carl O.; Sabat, Michal; Altun, Zikri; Demas, James N.; DeGraff, B. A.
    The synthesis of some heteroleptic, cyclometalated iridium(III) complexes is described. The utility of these [Ir(ppy)(2)(N-N)]Cl (ppy=2-phenylpyridine and N-N=substituted bipyridine, biquinoline, or phenanthroline) complexes as luminescence-based sensors is assessed. The emission intensity of an Ir(III) complex featuring the 3,3'-H(n)dcbpy ligand (H(n)dcbpy=dicarboxylic acid-2,2'-bipyridine; n=0,1,2 to indicate deprotonated, mono-and diprotonated species, respectively) is seen to increase in the presence of Pb(II). Insight into the structure and analyte-sensing capability is achieved by X-ray crystallography in conjunction with computational modeling. Complexes incorporating carboxylic acid-functionalized bipyridine and biquinoline as the polypyridyl ligand show pH sensitivity while similar phenanthroline complexes do not.
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    Thermochemistry of N-Heterocyclic Carbenes with 5-, 4-, 3-, and 2-Membered Rings
    (WILEY, 2014) BLEDA, ERDİ ATA; Altun, Zikri; Bleda, Erdi A.; Trindle, Carl; Wang, Jason
    N-heterocyclic carbenes (NHCs) based on imidazole-2-ylidene (1) or the saturated imidazolidine-2-ylidene (2) scaffolds are long-lived singlet carbenes. Both benefit from inductive stabilization of the sigma lone pair on carbon by neighboring N atoms and delocalization of the N pi lone pairs into the nominally vacant p-pi atomic orbital at the carbene carbon. With thermochemical schemes G4 and CBS-QB3, we estimate the relative thermodynamic stabilization of smaller ring carbenes and acyclic species which may share the keys to NHC stability. These include four-membered ring systems incorporating the carbene center, two trivalent N centers, and either a boron or a phosphorus atom to complete the ring. Amino-substituted cyclopropenylidenes have been reported but three-membered rings containing the carbene center and two N atoms are not known. Our calculations suggest that amino-substituted cyclopropenylidenes are comparable in stability to the four-membered NHCs but that diazacyclopropanylidenes would be substantially less effectively stabilized. Concluding the series are acyclic carbenes with and without neighboring N atoms and a series of two-membered ring azapropadienenylidene cations of form :CNW with W = an electron-withdrawing agent. We have studied W = NO2, CH2(+), CF2(+), and (CN)(2)C(+). Although these systems display a degree of stabilization and carbene-like electronic structure, the stability of the NHCs is unsurpassed. (c) 2014 Wiley Periodicals, Inc.
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    Enhancement of high-order harmonic generation in the presence of noise
    (IOP PUBLISHING LTD, 2011) YAVUZ, İLHAN; Yavuz, I.; Altun, Z.; Topcu, T.
    We report on our simulations of the generation of high-order harmonics from atoms driven by an intense femtosecond laser field in the presence of noise. We numerically solve the non-perturbative stochastic time-dependent Schrodinger equation and observe how varying noise levels affect the frequency components of the high harmonic spectrum. Our calculations show that when an optimum amount of noise is present in the driving laser field, roughly a factor of 45 net enhancement can be achieved in high-order harmonic yield, especially, around the cut-off region. We observe that, for a relatively weak noise, the enhancement mechanism is sensitive to the carrier-envelope phase. We also investigate the possibility of generating ultra-short intense attosecond pulses by combining the laser field and noise and observe that a roughly four orders of magnitude enhanced isolated attosecond burst can be generated.
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    PublicationOpen Access
    Electron-Impact Ionization of Se16+ and Ser(17+) Ions
    (IOP PUBLISHING LTD, 2015-09-07) BLEDA, ERDİ ATA; Bleda, Erdi A.; Tikman, Yavuz; Altun, Zikri; Diaz, C; Rabadan, I; Garcia, G; Mendez, L; Martin, F
    Electron-impact ionization cross sections for Se16+ and Se17+ ions have been calculated using semirelativistic configuration-average distorted-wave (CADW) method for both the direct and indirect ionization contributions. The excitation-autoionization contributions originating from the excitations of 2s and 2p subshels were calculated using a semirelativistic level to level distorted-wave (LLDW) method. Direct ionization cross section calculations involving 2s,2p,3s, and 3p subshells were also calculated using the LANL collisional atomic code and comparisons are made.
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    PublicationOpen Access
    Picosecond to Nanosecond Manipulation of Excited-State Lifetimes in Complexes with an Fe-II to Ti(IV )Metal-to-Metal Charge Transfer: The Role of Ferrocene Centered Excited States
    (AMER CHEMICAL SOC, 2019-11-18) ALTUN, ZİKRİ; Livshits, Maksim Y.; Turlington, Michael D.; Trindle, Carl O.; Wang, Lei; Altun, Zikri; Wagenknecht, Paul S.; Rack, Jeffrey J.
    Time-resolved transient absorption spectroscopy and computational analysis of D-pi-A complexes comprising Fe-II donors and Ti-IV acceptors with the general formula (Cp2Ti)-Cp-R(C(2)Fc)(2) (where RCp = Cp*, Cp, and Cp-MeOOC) and (Cp2Ti)-Cp-TMS(C(2)Fc)(C2R) (where R = Ph or CF3) are reported. The transient absorption spectra are consistent with an Fe-III/Ti-III metal-to-metal charge-transfer (MMCT) excited state for all complexes. Thus, excited-state decay is assigned to back-electron transfer (BET), the lifetime of which ranges from 18.8 to 41 ps. Though spectroscopic analysis suggests BET should fall into the Marcus inverted regime, the observed kinetics are not consistent with this assertion. TDDFT calculations reveal that the singlet metal-to-metal charge-transfer ((MMCT)-M-1) excited state for the Fe-II/Ti-IV complexes is not purely MMCT in nature but is contaminated with the higher-energy (1)Fc (d-d) state. For the diferrocenyl complexes, (Cp2Ti)-Cp-R(C(2)Fc)(2), the ratio of MMCT to Fc centered character ranges from 57:43 for the Cp* complex to 85:15 for the Cp-MeOOC complex. For the diferrocenyl and monoferrocenyl complexes investigated herein, the excited-state lifetimes decrease with increased (1)Fc character. The effect of Cu-I coordination was also analyzed by time-resolved transient absorption spectroscopy and reveals the elongation of the excited-state lifetime by 3 orders of magnitude to 63 ns. The transient spectra and TDDFT analysis suggest that the long-lived excited state in Cp2Ti(C(2)Fc)(2).CuX (where X is Cl or Br) is a triplet iron species with an electron arrangement of Ti-IV-Fe-3(II)-Cu-I.
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    Titanocene as a New Acceptor (A) for Arylamine Donors (D) in D-pi-A Chromophores
    (AMER CHEMICAL SOC, 2016) ALTUN, ZİKRİ; Pienkos, Jared A.; Agakidou, A. Danai; Trindle, Carl O.; Herwald, D. Wolfgang; Altun, Zikri; Wagenknecht, Paul S.
    Charge transfer (CT) transitions are relevant in the fields of solar energy conversion and nonlinear optical materials. Herein, a series of complexes with an alkynyl linkage between an aryl amine donor and a Ti-IV (titanocene) acceptor is reported. Each complex displays a strong (15000 < epsilon < 24,000 M(-1)cm(-1)), low-energy (520 < lambda < 560 nm) absorption ascribed to an amine to Ti-IV ligand-to-metal CT. This characterization is supported by UV-vis spectroscopy, cyclic voltammetry, and TD-DFT calculations. These complexes are not photostable; therefore, an alternate architecture, wherein the amine donor is appended to the titanocene cyclopentadienyl ligand, has been designed. The molar absorptivity of the amine to Ti-IV CT in this latter architecture is lower (2100 M-1 cm(-1)), indicating weaker donor-acceptor coupling. This architecture is indeed much more photostable.