Home About us Contact
|
Home About us Contact | ||
|
|
||
Singlet
Kinds of Singlet Terms modified by Singlet Selected AbstractsExciton,Exciton Annihilation in Mixed-Phase Polyfluorene FilmsADVANCED FUNCTIONAL MATERIALS, Issue 1 2010Paul E. Shaw Abstract Singlet,singlet annihilation is studied in polyfluorene (PFO) films containing different fractions of , -phase chains using time-resolved fluorescence. On a timescale of >15,ps after excitation, the results are fitted well by a time-independent annihilation rate, which indicates that annihilation is controlled by 3D exciton diffusion. A time-dependent annihilation rate is observed during the first 15,ps in the glassy phase and in the , -phase rich films, which can be explained by the slowdown of exciton diffusion after excitons reach low-energy sites. The annihilation rate in the mixed-phase films increases with increasing fraction of , -phase present, indicating enhanced exciton diffusion. The observed trend agrees well with a model of fully dispersed, -phase chromophores in the surrounding glassy phase with the exciton diffusion described using the line-dipole approximation for an exciton wavefunction extending over 2.5,nm. The results indicate that glassy and, -phase chromophores are intimately mixed rather than clustered or phase-separated. [source] Singlet,triplet splitting and stability of divalent five-membered ring C4H4M, C4H6M, and C4H8M (M = C, Si, Ge, Sn, and Pb)HETEROATOM CHEMISTRY, Issue 3 2008E. Vessally The sum of electronic and thermal free energy differences between singlet and triplet states (, Gt-s) is calculated for C4H4M, C4H6M, and C4H8M (M = C, Si, Ge, Sn, and Pb) at B3LYP/6-311++G (3df,2p) level. Singlet,triplet splitting (, Gt-s) is compared for three analogs C4H4M, C4H6M, and C4H8M. The change order of , Gt-s is (except for M = C) C4H6M > C4H8M > C4H4M. The results of homodesmotic reaction energies show the most stability for singlet state of C4H6M with respect to C4H4M and C4H8M. In contrast, the triplet state of C4H4M (except for M = C) is the most stable with respect to C4H6M and C4H8M. © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:245,251, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20428 [source] Theory of chemical bonds in metalloenzymes XIII: Singlet and triplet diradical mechanisms of hydroxylations with iron-oxo species and P450 are revisitedINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2009Kizashi Yamaguchi Abstract Electronic structures of the Compound I (CpdI) in P450 are investigated on the basis of spin coupling forms of iron-oxo (Fe(IV)O) cores and radical ligand (,L) groups to generalize previous singlet and triplet diradical (TD) mechanisms for oxygenations of alkanes with Fe( IV)O. Orbital interaction schemes for four lower-lying spin configurations of CpdI with HC bond of substrate are examined to elucidate how magnetic coupling modes correlate with radical reaction pathways for hydroxylation reactions on the basis of the broken symmetry (BS) molecular orbital (MO) model. The configuration correlation diagrams for the four configurations model are depicted on the basis of the isoelectronic analogy among O, O2, and Fe( IV)O, in addition to Coulomb exchange energy on the iron site, which determines its local spin configuration. Important role of ligand spin (,L) of CpdI for regulation of hydroxylation mechanisms is clarified with the aid of the spin coupling forms. Transition states for one quartet and three doublet configurations under the BS MO approximation are examined on the basis of potential curve crossings along reaction pathways. The four transition structures and corresponding radical intermediates for methane and trimethyl methane with CpI are located by the BS hybrid Kohn,Sham density functional theory (DFT) (B3LYP) method to confirm the orbital interaction schemes. Spin density populations obtained by the BS B3LYP calculations are found to be consistent with the theoretical predictions based on the four configurations model. The configuration and state correlation diagrams by BS B3LYP before and after spin projection are also consistent with the BS MO interaction schemes, which provide local SD and TD mechanisms of hydroxylation with CpdI. The present BS MO-theoretical framework is useful for systematic understanding of a lot of recent BS hybrid DFT computational results for hydroxylation reactions with CpdI and configuration correlation diagrams reported by several groups. Implications of the present theoretical and computational results are discussed in relation to several experimental characteristics of hydroxylation reactions with iron-oxo species and P450. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] White-Light Emission from a Single Polymer with Singlet and Triplet Chromophores on the BackboneMACROMOLECULAR RAPID COMMUNICATIONS, Issue 24 2006Hongyu Zhen Abstract Summary: A strategy to generate an efficient white-light emission has been developed by mixing fluorescence and phosphorescence emission from a single polymer. Fluorene is used as the blue-emissive component, benzothiadiazole (BT) and the iridium complex [(btp)2Ir(tmd)] are incorporated into a polyfluorene backbone, respectively, as green- and red-emissive chromophores by Suzuki polycondensation. By changing the contents of BT and [(btp)2Ir(tmd)] in the polymer, the electroluminescence spectrum from a single polymer can be adjusted to achieve white-light emission. A white polymeric light-emitting diode (WPLED) with a structure of ITO/PEDOT:PSS/PVK/PFIrR1G03/CsF/Al shows a maximum external quantum efficiency of 3.7% and the maximum luminous efficiency of 3.9 cd,·,A,1 at the current density of 1.6 mA,·,cm,2 with the CIE coordinates of (0.33, 0.34). The maximum luminance of 4,180 cd,·,m,2 is achieved at the current density of 268 mA,·,cm,2 with the CIE coordinates of (0.31, 0.32). The white-light emissions from such polymers are stable in the white-light region at all applied voltages, and the electroluminescence efficiencies decline slightly with the increasing current density, thus indicating that the approach of incorporating singlet and triplet species into the polymer backbone is promising for WPLEDs. Structure of PFIrR1G04 and the EL spectra of its devices under various voltages. Device structure: ITO/PEDOT:PSS/PVK/polymer/CsF/Al. [source] Radical Cation Generation from Singlet and Triplet Excited States of All-trans-Lycopene in Chloroform,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2004Rui-Min Han ABSTRACT On direct photoexcitation, subpicosecond time-resolved absorption spectroscopy revealed that the 1Bu -type singlet excited state of all- trans -lycopene in chloroform was about seven times more efficient than all-trans-,-carotene in generating the radical cation. The time constant of radical cation generation from the 1Bu -type state was found to be ,0.14 ps, a value that was comparable for the two carotenoids. On anthracene-sensitized triplet excitation, radical cation generation was found to be much less efficient for lycopene than for ,-carotene. A slow rising phase (20-30 ,s) in the bleaching of ground-state absorption was common for both lycopene and ,-carotene in chloroform and was ascribed to an efficient secondary reaction with a solvent radical leading to the formation of carotenoid radical cations. The reverse ordering in the tendency of the excited states of different multiplicities for the two carotenoids to generate radical cations is discussed in relation to the two carotenoids as scavengers of free radicals. [source] Excited Singlet (S1)-state Interactions of Nile Red with Aromatic Amines,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2003J. Mohanty ABSTRACT Both steady-state (SS) and time-resolved (TR) studies show that the fluorescence of the dye Nile red (NR) is quenched by various aromatic amines (ArA). Bimolecular quenching constants (kq) from both SS and TR measurements are observed to match well, indicating that the interaction is dynamic in nature. The quenching interaction in the present systems has been attributed to electron transfer (ET) from ArA to excited NR, based on the variations in the kq values with the oxidation potentials of the amines. The kq values calculated within the framework of Marcus' outer-sphere ET theory at different free-energy changes (,G0) of the ET reactions match well with the experimental ones, supporting the ET mechanism in the systems studied. The reorganization energy (,) estimated from the correlation of the experimental and the calculated kq values is quite similar to the solvent reorganization energy (,s), calculated on the basis of the solvent dielectric continuum model along with the assumption that the reactants are the effective spheres. Although a modest error is involved in this ,s calculation, the similarity in , and ,s values suggests that the solvent reorganization plays a dominant role in governing the ET dynamics in the present systems. [source] Theoretical Study of HNXH (X: N, P, As, Sb, and Bi) Isomers in the Singlet and Triplet States.CHEMINFORM, Issue 26 2003Chin-Hung Lai No abstract is available for this article. [source] ChemInform Abstract: A Novel cis,trans Photoisomerization of Vinylidenecyclopropanes via Excited Singlet and Triplet States.CHEMINFORM, Issue 6 2001Kazuhiko Mizuno Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Theoretical Exploration of the Oxidative Properties of a [(trenMe1)CuO2]+ Adduct Relevant to Copper Monooxygenase Enzymes: Insights into Competitive Dehydrogenation versus Hydroxylation Reaction PathwaysCHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2008Aurélien de, Lande Dr. Abstract Singlet and triplet H-transfer reaction paths from CH and NH bonds were examined by means of DFT and spin-flip TD-DFT computations on the [(trenMe1)CuO2]+ adduct. The singlet energy surfaces allow its evolution towards H2O2 and an imine species. Whereas NH cleavage appears to be a radical process, CH rupture results in a carbocation intermediate stabilized by an adjacent N atom and an electrostatic interaction with the [CuIOOH] metal core. Upon injection of an additional electron, the latter species straightforwardly forms a hydroxylated product. Based on these computational results, a new mechanistic description of the reactivity of copper monooxygenases is proposed. [source] Singlet,Singlet Annihilation Leading to a Charge-Transfer Intermediate in Chromophore-End-Capped PentaphenylenesCHEMPHYSCHEM, Issue 9 2007Eduard Fron Dr. Abstract The excited-state properties of two peryleneimide chromophore end-capped pentaphenylene compounds were investigated in detail using femtosecond transient absorption and single-photon timing experiments. Singlet,singlet annihilation was found to promote one chromophore into a higher excited state and results in the formation of an ultra-short-living intermediate charge-transfer (CT) state in the Sn,S1 deactivation pathway. In low-polarity solvents, this CT state is found to be energetically higher than the first excited state and thus cannot be populated via one-photon excitation. The observed CT state decays with a time constant of about 1 ps to form the lowest singlet excited state. These results demonstrate the potential use of the singlet,singlet annihilation as a novel tool in studying reactions occurring in states that are energetically above the S1. [source] Origin of the Paramagnetic Properties of the Mixed-Valence Polyoxometalate [GeV14O40]8, Reduced by Two Electrons: Wave Function Theory and Model Hamiltonian CalculationsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 34 2009N. Suaud Abstract The aim of the work is to give an explanation of the magnetic properties of a mixed-valence [GeV14O40]8, polyoxometalate reduced by two electrons, which, in contrast to what happens in other two-electron-reduced polyoxometalates, does not show any magnetic coupling between the two unpaired electrons. For this purpose, a quantitative evaluation of the microscopic electronic parameters (electron transfer, magnetic coupling, magnetic orbital energy, and Coulomb repulsion) of the mixed-valence polyoxometalate cluster is performed. The parameters are extracted from valence-spectroscopy large configuration interaction (CI) calculations on embedded fragments. Then, these parameters are used in an extended t - J model Hamiltonian suited to model the properties of the whole anion. The analysis of the wave functions of the lowest singlet and triplet states and of the microscopic parameters emphasizes that the electron delocalization in this mixed-valence cluster is such that each unpaired electron is almost trapped in a different half of the polyoxovanadate, thus disabling any exchange interaction between them.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Beyond the Icosahedron: A Density Functional Theory Study of 14-Atom Germanium ClustersEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 25 2008R. Bruce King Abstract Density functional theory (DFT) at the hybrid B3LYP level has been applied to the germanium clusters Ge14z (z = ,8, ,6, ,4, ,2, 0, +2, +4) starting from seven different initial configurations. An Oh omnicapped cube structure is the most stable for Ge142, followed by a hexagonal antiprism structure with a relative energy of 42.7 kcal/mol. The lowest-energy structure for neutral Ge14 is a triplet omnicapped cube with full Oh symmetry followed by a singlet omnicapped cube compressed to D4h symmetry through Jahn,Teller distortion. The lowest energy Ge142+ structure is also an Oh structure derived from the omnicapped cube through elongation of the 12 edges of the underlying cube to give a rhomboidal dodecahedron with 12 rhombus faces. The lowest-energy Ge124+ structure is a bicapped icosahedron. Some D6h hexagonal wheel structures at higher energies are also found for the hypoelectronic systems Ge14, Ge142+, and Ge144+. The lowest-energy structures for the hyperelectronic Ge144,, Ge146,, and Ge148, are relatively unsymmetrical not readily recognizable open structures typically with some pentagonal or hexagonal faces. The D6d bicapped hexagonal antiprism found in 14-vertex C2B12 carborane and M2C2B10 dimetallacarborane structures is not the lowest-energy structure for any of the Ge14z clusters.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Comparison of Isoelectronic Heterometallic and Homometallic Binuclear Cyclopentadienylmetal Carbonyls: The Iron,Nickel vs. the Dicobalt SystemsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 8 2008Jun D. Zhang Abstract The heterometallic binuclear cyclopentadienylironnickel carbonyl compounds Cp2FeNi(CO)n (n = 3, 2, 1; Cp = ,5 -C5H5) have been studied by density functional theory (BP86) for comparison with the isoelectronic homometallic dicobalt derivatives Cp2Co2(CO)n. The FeNi tricarbonyl is shown to be the doubly bridged isomer Cp2Fe(CO)Ni(,-CO)2 with an Fe,Ni distance of 2.455 Å (BP86), in accord with experiment and in contrast to Cp2Co2(CO)3 where singly and triply bridged but not doubly bridged isomers are found. The dicarbonyl compounds Cp2FeNi(,-CO)2 and Cp2Co2(,-CO)2 both have analogous doubly bridged structures with M=M distances around 2.35 Å, suggesting formal M=M double bonds. The monocarbonyl compounds have analogous singly bridged axial structures Cp2FeNi(,-CO) and Cp2Co2(,-CO) with metal,metal distances in the range 2.05 Å (M2 = Co2) to 2.12 Å (M2 = FeNi) consistent with the formal M,M triple bonds required for the favored 18-electron configuration. Open-shell states of Cp2FeNi(,-CO) are found to have even lower energies than the closed-shell structure, which indicates that the ground state of Cp2FeNi(,-CO) might be a high spin structure. However, the global minimum for the monocarbonyl is found to be a singlet "hot dog" perpendicular Cp2NiFe(CO) structure with a terminal CO group bonded to the iron atom. Other higher energy perpendicular structures are also found for Cp2FeNi(CO)n (n = 3, 2, 1) with terminal CO groups and bridging Cp rings. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] The Reductive Elimination of Methane from ansa -Hydrido(methyl)metallocenes of Molybdenum and Tungsten: Application of Hammond's Postulate to Two-State ReactionsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 15 2005José-Luis Carreón-Macedo Abstract The energetic profile of the methane reductive elimination from a selected number of hydrido(methyl)molybdenocene and -tungstenocene derivatives has been calculated by DFT methods. The calculations were carried out for the CH2(C5H4)2M (a -M), SiH2(C5H4)2M (a -H2Si,M), and SiMe2(C5Me4)2M (a -Me2Si,M*) ansa -metallocene systems for M = Mo, W. They include the full optimization of minima [the hydrido(methyl) starting complexes, M(H)(CH3), the intermediate methane complexes, M(CH4), and the metallocene products in the singlet and triplet configurations, (3M and 1M)], transition states (for the methyl hydride reductive elimination, M,TSins, and for the hydrogen exchange, M,TSexch), and the minimum energy crossing point (M,MECP) leading from the singlet methane complexes to the corresponding triplet metallocenes. The results are compared with those previously obtained for the simpler (C5H5)2M (Cp2M) systems (J. C. Green, J. N. Harvey, and R. Poli, J. Chem. Soc., Dalton Trans.2002, 1861). The calculated energy profiles, notably the relative energies of M,TSins and M,MECP, are in agreement with available experimental observations for the a -Me2Si,M* systems. The comparison of the energies and geometries of the rate-determining M,TSins and M,MECP structures with those of the thermodynamically relevant minima for the various systems show the applicability of Hammond's postulate to two-state reactions. However, one notable exception serves to show that the principle is only quantitatively reliable when all the potential energy surfaces for the set of analogous reactions have similar shapes. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Orbital Splitting and Pairing Energy in Open-Shell Organometallics: A Study of Two Families of 16-Electron Complexes [Cp2M] (M = Cr, Mo, W) and [CpM(PH3)] (M = Co, Rh, Ir)EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 12 2005Rinaldo Poli Abstract The singlet,triplet gap for two families of 16-electron organometallic complexes has been examined in detail by DFT calculations at the B3LYP level with polarized basis sets on both metal and ligands. For the first family, the group 6 metallocenes (Cp2M with Cp = ,5 -C5H5 and M = Cr, Mo, W), the singlet,triplet gap (ES , ET) is always positive and decreases continuously on going from Cr to Mo to W. For the family of group 9 CpM(PH3), on the other hand, there is a decrease on going from Co to Rh, followed by a slight increase on going further to Ir. These trends have been analyzed in qualitative monoelectronic terms as a function of the competition between the pairing energy and the orbital gap. While the pairing energy decreases as expected in the order 3d >> 4d > 5d, the orbital gap varies in a different way for the two families and, though quantitatively less important, is responsible for the different trends. It is argued that changes in orbital gap are system-dependent for open-shell organometallic systems, thus it is not possible to establish a universal trend of singlet,triplet gaps for a homologous series of complexes with a group of transition metals. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Close Proximity Dibenzo[a,c]phenazine,Fullerene Dyad: Synthesis and Photoinduced Singlet Energy TransferEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 18 2010Rajeev K. Dubey Abstract A dibenzo[a,c]phenazine,fullerene (DBPZ-C60) dyad in which two chromophores are linked in close proximity to each other has been synthesized and studied in detail by optical spectroscopy to explore a new energy donor,acceptor system. The dyad was prepared by Prato reaction between 11-formyldibenzo[a,c]phenazine and fullerene. 3,5-Di- tert -butylbenzyl group was introduced onto the fulleropyrrolidine unit to achieve adequate solubility of the dyad. A thorough study of the photophysical properties of the dyad and relevant reference compounds, performed by means of steady state and time resolved spectroscopic measurements, has revealed the presence of highly efficient (ca. 98,%) and extremely fast (ken = 5,×,1011 s,1) intramolecular photoinduced singlet,singlet energy-transfer process from singlet excited state of the DBPZ moiety to fullerene. In both polar and nonpolar environment transduction of singlet excited state energy governs the excited state deactivation, but the efficiency and rate of energy transfer were found to be higher in nonpolar solvents in comparison to polar. The DBPZ singlet excited state decays within 2 and 4.7 ps in toluene andbenzonitrile, respectively, via singlet,singlet energy transfer to produce a fullerene singlet excited state which decays with a life time of 1.5 ns to give a very long-lived fullerene triplet state as final populated excited state. [source] Off the Back or on the Side: Comparison of meso and 2-Substituted Donor-Acceptor Difluoroborondipyrromethene (Bodipy) DyadsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 15 2010Andrew C. Benniston Abstract The preparation of several difluoroborondipyrromethene (Bodipy) dyads is described incorporating covalently attached hydroquinone/quinone groups at the 2-position (BD-SHQ, BD-SQ, BD-SPHQ, BD-SPQ). The compounds, currently under investigation as chemical sensors for reactive oxygen species, show various levels of fluorescence depending on the oxidation state of the appended group. The 19F NMR spectrum for BD-SHQ in CDCl3 at room temperature reveals the two fluorines are inequivalent on the NMR timescale. In contrast, the 19F NMR spectrum for the counterpart quinone compound, BD-SQ, is consistent with two equivalent fluorine atoms. The two results are interpreted as the quinone is free to rotate around the connector bond, whereas this motion is restricted for the hydroquinone group and makes the fluorines chemically inequivalent. Cyclic voltammograms recorded for all derivatives in CH2Cl2 electrolyte solution are consistent with typical Bodipy-based redox chemistry; the potentials of which depend on factors such as presence of the phenylene spacer and oxidation state of the appended group. A comparison of the electrochemical behaviour with the counterpart meso derivatives reveals some interesting trends which are associated with the location of the HOMO/LUMOs. The absorption profiles for the compounds in CH3CN are again consistent with Bodipy-based derivatives, though there are some subtle differences in the band-shapes of the closely-coupled systems. In particular, the absorption spectra for the dyad, BD-SQ, in a wide range of solvents are appreciably broader than for BD-SHQ. Femtosecond transient absorption spectroscopy performed on the hydroquinone derivatives, BD-SHQ and its meso analogue is interpreted as electron transfer occurs from the hydroquinone unit to the first-excited singlet (S1) state of the Bodipy center, followed by ultrafast charge recombination to reinstate the ground state. The coupling of OH vibrations to the return electron transfer process is invoked to explain the lack of clear identification of the charge-separated state in the transient records. [source] A Novel Bis(zinc,porphyrin),Oxoporphyrinogen Donor,Acceptor Triad: Synthesis, Electrochemical, Computational and Photochemical StudiesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 3 2006Jonathan P. Hill Abstract The first example of a porphyrin-quinonoid donor,acceptor triad featuring (tetraphenylporphinato)zinc(II) moieties covalently attached to an oxoporphyrinogen through its macrocyclic nitrogen atoms is reported. This arrangement of chromophores results in an interesting interplay between the electron-donating zinc,porphyrin(s) and the electron/energy accepting oxoporphyrinogen. The optical absorption of the triad reveals features corresponding to both the donor and acceptor entities. The geometry and electronic structure of the triad deduced from B3LYP/3-21G(*) calculations reveal an absence of inter-chromophoric interactions and localization of the HOMO on one zinc,porphyrin group and the LUMO on the oxoporphyrinogen scaffold. The electrochemical redox states of the triad were established from a comparative electrochemistry of the triad and the reference compounds. Both steady-state and time-resolved emission studies revealed quenching of the singlet excited state of zinc,porphyrin in the triad, and the free-energy calculations performed using Weller's approach indicate the possibility of electron transfer from the singlet excited zinc,porphyrin group to the oxoporphyrinogen in polar solvents. Time-resolved fluorescence studies reveal excited state energy transfer from zinc,porphyrin to oxoporphyrinogen in nonpolar solvents, while nanosecond transient absorption studies combined with time-resolved fluorescence studies in polar solvents are indicative of the occurrence of photoinduced charge separation from the singlet excited zinc,porphyrin to the oxoporphyrinogen. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Radical Cation and Dication Derived from 4,8-Diethylbenzo[1,2- d:4,5- d,]bis[1,2,3]trithiole [DEBBT]: Change of Electronic State from Singlet-State Dication DEBBT(2+)- S to Triplet-State Dimer 2DEBBT(2+)- T in D2SO4 and CD3CN SolutionsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 24 2003Takeshi Kimura Abstract 4,8-Diethylbenzo[1,2- d:4,5- d,]bis[1,2,3]trithiole [DEBBT] was oxidized using concentrated D2SO4, leading to the generation of the radical cation DEBBT(·+) which was verified by ESR spectroscopy. DEBBT(·+) in the solution was further oxidized to produce the dication DEBBT(2+), as determined by 1H and 13C NMR spectroscopy. DEBBT(2+) was also prepared by treating DEBBT 1-oxide [DEBBT 1-O] with concentrated D2SO4, and was verified by 1H and 13C NMR spectroscopy. The 13C NMR chemical shifts of DEBBT(2+), calculated by the density functional theoretical (DFT) method at the B3LYP6-31G** level, correlated well with those obtained experimentally. The ESR signal of DEBBT(2+) generated from DEBBT 1-O was observed in solution, which implies that the singlet-state dication DEBBT(2+)- S isomerizes to the triplet-state dication DEBBT(2+)- T, and that two molecules of DEBBT(2+)- T further form a spin pair at one trithiole ring with significant distance between the two radical centers. The oxidation of DEBBT with one or two equivalents of single-electron oxidizing reagents produced DEBBT(·+) and DEBBT(2+), and the salts were isolated in a stable form. However, the DEBBT(2+) that was prepared by oxidation with NOPF6 proved silent for NMR in CD3CN, while ESR was active. The stability, electronic state, and NMR and ESR spectroscopy of the dication are affected by solvation with D2SO4 and CD3CN. The optimized structures and the total energy of the singlet- and triplet-state dication were calculated using the DFT method at the B3LYP6-31G** level, which shows that the structures of the singlet- and triplet-state dications have a completely planar form with 1.7 kcal/mol as the total energy difference between them. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Effects of singlet oxygen on membrane sterols in the yeast Saccharomyces cerevisiaeFEBS JOURNAL, Issue 6 2000Till Böcking Photodynamic treatment of the yeast Saccharomyces cerevisiae with the singlet oxygen sensitizer toluidine blue and visible light leads to rapid oxidation of ergosterol and accumulation of oxidized ergosterol derivatives in the plasma membrane. The predominant oxidation product accumulated was identified as 5,,6,-epoxy-(22E)-ergosta-8,22-dien-3,,7,-diol (8-DED). 9(11)-dehydroergosterol (DHE) was identified as a minor oxidation product. In heat inactivated cells ergosterol is photooxidized to ergosterol epidioxide (EEP) and DHE. Disrupted cell preparations of S. cerevisiae convert EEP to 8-DED, and this activity is abolished in a boiled control indicating the presence of a membrane associated enzyme with an EEP isomerase activity. Yeast selectively mobilizes ergosterol from the intracellular sterol ester pool to replenish the level of free ergosterol in the plasma membrane during singlet oxygen oxidation. The following reaction pathway is proposed: singlet oxygen-mediated oxidation of ergosterol leads to mainly the formation of EEP, which is enzymatically rearranged to 8-DED. Ergosterol 7-hydroperoxide, a known minor product of the reaction of singlet oxygen with ergosterol, is formed at a much lower rate and decomposes to give DHE. Changes of physical properties of the plasma membrane are induced by depletion of ergosterol and accumulation of polar derivatives. Subsequent permeation of photosensitizer through the plasma membrane into the cell leads to events including impairment of mitochondrial function and cell inactivation. [source] The Energy of Charge-Transfer States in Electron Donor,Acceptor Blends: Insight into the Energy Losses in Organic Solar CellsADVANCED FUNCTIONAL MATERIALS, Issue 12 2009Dirk Veldman Abstract Here, a general experimental method to determine the energy ECT of intermolecular charge-transfer (CT) states in electron donor,acceptor (D,A) blends from ground state absorption and electrochemical measurements is proposed. This CT energy is calibrated against the photon energy of maximum CT luminescence from selected D,A blends to correct for a constant Coulombic term. It is shown that ECT correlates linearly with the open-circuit voltage (Voc) of photovoltaic devices in D,A blends via eVoc,=,ECT,,,0.5,eV. Using the CT energy, it is found that photoinduced electron transfer (PET) from the lowest singlet excited state (S1 with energy Eg) in the blend to the CT state (S1,,,CT) occurs when Eg,,,ECT,>,0.1,eV. Additionally, it is shown that subsequent charge recombination from the CT state to the lowest triplet excited state (ET) of D or A (CT,,,T1) can occur when ECT,,,ET,>,0.1,eV. From these relations, it is concluded that in D,A blends optimized for photovoltaic action: i) the maximum attainable Voc is ultimately set by the optical band gap (eVoc,=,Eg,,,0.6,eV) and ii) the singlet,triplet energy gap should be ,EST,<,0.2,eV to prevent recombination to the triplet state. These favorable conditions have not yet been met in conjugated materials and set the stage for further developments in this area. [source] Amplified Spontaneous Emission of Poly(ladder-type phenylene)s , The Influence of Photophysical Properties on ASE Thresholds,ADVANCED FUNCTIONAL MATERIALS, Issue 20 2008Frédéric Laquai Abstract Amplified spontaneous emission (ASE) of a series of blue-emitting poly(ladder-type phenylene)s (LPPP)s has been studied in thin film polymer waveguide structures. The chemically well-defined step-ladder polymers consist of an increasing number of bridged phenylene rings per monomer unit starting from fully arylated poly(ladder-type indenofluorene) up to poly(ladder-type pentaphenylene). The ASE characteristics of the polymers including the onset threshold values for ASE, the gain and loss coefficients as well as the photoluminescence (PL) properties, i.e., the solid state fluorescence lifetimes, decay kinetics and solid state quantum efficiencies have been studied by time-resolved PL spectroscopy. A fully arylated polyfluorene has been synthesized and its photophysical properties were compared to the step-ladder polymers. Steady-state photoinduced absorption and ultrafast transient absorption spectroscopy have been used to study excited state absorption of singlet and triplet states and polarons present in the solid state. The results demonstrate a minimum regarding the onset threshold value of ASE for a fully arylated poly(ladder-type indenofluorene) and a successive increase of the ASE threshold for the step-ladder polymers with more bridged phenylene rings. In particular, carbazole-containing step-ladder LPPPs exhibit significantly increased ASE threshold values as compared to their carbazole-free analogues due to a pronounced overlap of stimulated emission (SE) and photoinduced absorption (PA). [source] Covalent Functionalization of Carbon Nanohorns with Porphyrins: Nanohybrid Formation and Photoinduced Electron and Energy Transfer,ADVANCED FUNCTIONAL MATERIALS, Issue 10 2007G. Pagona Abstract The covalent attachment of carbon nanohorns (CNHs) to ,-5-(2-aminophenyl)-,-15-(2-nitrophenyl)-10,20-bis(2,4,6-trimethyl-phenyl)-porphyrin (H2P) via an amide bond is accomplished. The resulting CNH,H2P nanohybrids form a stable inklike solution. High-resolution transmission electron microscopy (HRTEM) images demonstrate that the original dahlia-flowerlike superstructure of the CNHs is preserved in the CNH,H2P nanohybrids. Steady-state and time-resolved fluorescence studies show efficient quenching of the excited singlet state of H2P, suggesting that both electron and energy transfer occur from the singlet excited state of H2P to CNHs, depending on the polarity of the solvent. In the case of electron transfer, photoexcitation of H2P results in the reduction of the nanohorns and the simultaneous oxidation of the porphyrin unit. The formation of a charge-separated state, CNH,,,H2P,+, has been corroborated with the help of an electron mediator, hexyl-viologen dication (HV2+), in polar solvents. Moreover, the charge-separated CNH,,,H2P,+ states have been identified by transient absorption spectroscopy. [source] Photochemical Reactions of Regioisomeric 2,2-Dimethyl-5,5-diphenyl- and 5,5-Dimethyl-2,2-diphenyl-Substituted Diazo Ketones of a Tetrahydrofuran SeriesHELVETICA CHIMICA ACTA, Issue 10 2009Ludmila Abstract The principal direction of conventional photolysis of the regioisomeric 2,2-dimethyl-5,5-diphenyl- and 5,5-dimethyl-2,2-diphenyl-substituted 4-diazodihydrofuran-3(2H)-ones 1a and 1b, respectively, is the Wolff rearrangement, while other photochemical processes, which are giving rise to the formation of CH-insertion, 1,2-alkyl- or -aryl-shifts, as well as H-atom-abstraction products occur to a much lower degree (Schemes,2 and 3). The ratio of similar reaction products from both regioisomers 1a and 1b is essentially independent of their structure, and a substantial effect of the relative position of the Ph and diazo group to each other on the yield of CH-insertion products does not occur. Based on stereochemical considerations, the Wolff rearrangement of diazodihydrofuran-3(2H)-ones apparently proceeds in a concerted manner, whereas the appearance in the reaction mixture of 1,2-shift and H-atom-abstraction products points to the parallel generation during photolysis of singlet and triplet carbenes (Schemes,4 and 5). [source] Ligand Effects on the Mechanisms of Thermal Bond Activation in the Gas-Phase Reactions NiX+/CH4,Ni(CH3)+/HX (X=H, CH3, OH, F).HELVETICA CHIMICA ACTA, Issue 12 2008Short Communication Abstract The thermal ion-molecule reactions NiX++CH4,Ni(CH3)++HX (X=H, CH3, OH, F) have been studied by mass spectrometric methods, and the experimental data are complemented by density functional theory (DFT)-based computations. With regard to mechanistic aspects, a rather coherent picture emerges such that, for none of the systems studied, oxidative addition/reductive elimination pathways are involved. Rather, the energetically most favored variant corresponds to a , -complex-assisted metathesis (, -CAM). For X=H and CH3, the ligand exchange follows a ,two-state reactivity (TSR)' scenario such that, in the course of the thermal reaction, a twofold spin inversion, i.e., triplet,singlet,triplet, is involved. This TSR feature bypasses the energetically high-lying transition state of the adiabatic ground-state triplet surface. In contrast, for X=F, the exothermic ligand exchange proceeds adiabatically on the triplet ground state, and some arguments are proposed to account for the different behavior of NiX+/Ni(CH3)+ (X=H, CH3) vs. NiF+. While the couple Ni(OH)+/CH4 does not undergo a thermal ligand switch, the DFT computations suggest a potential-energy surface that is mechanistically comparable to the NiF+/CH4 system. Obviously, the ligands X act as a mechanistic distributor to switch between single vs. two-state reactivity patterns. [source] Anomalous Spin Transition Observed in Bis(2,6-bis(pyrazol-3-yl)pyridine)iron(II) Thiocyanate Dihydrate,ADVANCED FUNCTIONAL MATERIALS, Issue 11 2003A. Bhattacharjee Abstract Bis(2,6-bis(pyrazol-3-yl)pyridine)iron(II) thiocyanate dihydrate undergoes a two-step singlet (1A1) , quintet (5T2) transition in which both steps are associated with thermal hysteresis. Thermal cycling of the sample results in its conversion to a second phase which displays a single-step transition with a very narrow hysteresis loop. This second phase slowly reverts to the initial phase on standing at 300 K. The interconversions are completely reversible. The spin state changes have been monitored by measurement of magnetism and Mössbauer spectra and by differential scanning calorimetry (DSC) studies. [source] Photophysical and Photochemical Studies of PyridoxamineHELVETICA CHIMICA ACTA, Issue 10 2003Claudio Bueno The absorption and fluorescence emission of pyridoxamine were studied as function of pH and solvent properties. In the ground state, pyridoxamine exhibits different protonated forms in the range of pH,1.5,12. Fluorescence studies showed that the same species exist at the lowest singlet excited state but at different pH ranges. The phenol group is by ca. 8,units more acidic in the excited state than in the ground state. On the other hand, the pyridine N-atom is slightly more basic in the lowest excited state than in the ground state. Excitation spectra and emission decays in the pH range of 8,10 indicate the protonation of the pyridine N-atom by proton transfer from the amine group, in the ground and singlet excited states. Spectroscopic studies in different solvents showed that pyridoxamine in the ground or excited states exhibits intramolecular proton transfer from the pyridine N-atom to the phenol group, which is more favorable in solvents of low hydrogen-bonding capacity. The cationic form with the protonated phenolic group, which emits at shorter wavelength, is the dominant species in nonprotic solvents, but, in strong proton-donor solvents, both forms exist. The fluorescence spectra of these species exhibit blue shift in protic solvents. These shifts are well-correlated with the polarity and the H-donor ability of the solvent. [source] Effect of the Media on the Quantum Yield of Singlet Oxygen (O2(1,g)) Production by 9H -Fluoren-9-one: Solvents and Solvent MixturesHELVETICA CHIMICA ACTA, Issue 2 2003Claudia We have investigated the effect of a series of 18 solvents and mixtures of solvents on the production of singlet molecular oxygen (O2(1,g), denoted as 1O2) by 9H -fluoren-9-one (FLU). The normalized empirical parameter E derived from ET(30) has been chosen as a measure of solvent polarity using Reichardt's betaine dyes. Quantum yields of 1O2 production (,,) decrease with increasing solvent polarity and protic character as a consequence of the decrease of the quantum yield of intersystem crossing (,ISC). Values of ,, of unity have been found in alkanes. In nonprotic solvents of increasing polarity, ,ISC and, therefore, ,, decrease due to solvent-induced changes in the energy levels of singlet and triplet excited states of FLU. This compound is a poor 1O2 sensitizer in protic solvents, because hydrogen bonding considerably increases the rate of internal conversion from the singlet excited state, thus diminishing ,, to values much lower than those in nonprotic solvents of similar polarity. In mixtures of cyclohexane and alcohols, preferential solvation of FLU by the protic solvent leads to a fast decrease of ,, upon addition of increasing amounts of the latter. [source] Singlet,triplet splitting and stability of divalent five-membered ring C4H4M, C4H6M, and C4H8M (M = C, Si, Ge, Sn, and Pb)HETEROATOM CHEMISTRY, Issue 3 2008E. Vessally The sum of electronic and thermal free energy differences between singlet and triplet states (, Gt-s) is calculated for C4H4M, C4H6M, and C4H8M (M = C, Si, Ge, Sn, and Pb) at B3LYP/6-311++G (3df,2p) level. Singlet,triplet splitting (, Gt-s) is compared for three analogs C4H4M, C4H6M, and C4H8M. The change order of , Gt-s is (except for M = C) C4H6M > C4H8M > C4H4M. The results of homodesmotic reaction energies show the most stability for singlet state of C4H6M with respect to C4H4M and C4H8M. In contrast, the triplet state of C4H4M (except for M = C) is the most stable with respect to C4H6M and C4H8M. © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:245,251, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20428 [source] Phosphorescent Light-Emitting Transistors: Harvesting Triplet ExcitonsADVANCED MATERIALS, Issue 48 2009Ebinazar B. Namdas Phosphorescent light-emitting transistors, in which light emission from singlet and triplet energy levels is harvested using solution-processed materials (see figure), are presented. While a green phosphorescent dendrimer exhibits an external quantum efficiency of 0.45% at 480,cd m,2, a red polymer/phosphorescent small-molecule blend produces a brightness exceeding 30,cd m,2 with a relatively high hole mobility of 2.5,×,10,2,cm2 V,1 s,1. [source] | |||||||||||||||||||||||||||||||||||||