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  • 1.
    Jansson, Emil
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Electronic characterization of molecules with application to organic light emitting diodes2007Licentiate thesis, comprehensive summary (Other scientific)
    Abstract [en]

    The presented thesis is devoted to the field of organic light emitting

    diodes (OLEDs). Time-dependent Kohn-Sham density functional theory

    (TDDFT) is applied in order to eludicate optical properties such as fluorescence and

    phosphorescence for some of the most important materials. The accuracy of TDDFT

    is evaluated with respect to the calculated absorption and emission spectra for

    commonly used light emitting polymers. A continuation of this work is devoted to

    Polyfluorene as this polymer has proven to be very promising. In this study the

    chain length dependence of its singlet and triplet excited states is analyzed as well

    as the excited state structures.

    Understanding the phosphorescence mechanism of tris(2-phenylpyridine)Iridium is

    of importance in order to interpret the high efficiency of OLEDs

    containing these specimens. The mechanism is analyzed by calculating

    the electric transition dipole moments by means of TDDFT using

    quadratic response functions. As not only the optical properties are essential for

    effective devices, electron transfer properties are addressed. The electron

    transfer capability of the sulfur and nitrogen analogues of Oxadiazole

    is evaluated through their internal reorganization energy.

  • 2.
    Jansson, Emil
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Jha, Prakash Chandra
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Chain length dependence of singlet and triplet excited states of oligofluorenes: A density functional study2007In: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 336, no 2-3, p. 91-98Article in journal (Refereed)
    Abstract [en]

    Using time dependent density functional theory, we investigate the chain length dependence of the energies of excited states of a series of conjugated 9,9-dihexylfluorene-2,7-diyl oligomers. Excited state optimization reveals that upon excitation the dihedral angle between two adjacent monomer units moves towards zero, forming a planar structure within the oligomer. The calculated energies of the optical transitions in absorption, fluorescence, phosphorescence and triplet-triplet absorption are compared with recently reported experimental data. The calculated as well as experimentally reported energies involved seem to saturate very fast as the chain length increases. The energy dispersion and saturation indicates that the triplet ground state is somewhat more confined than the first singlet excited state. Our calculated energies agree well with the experimental findings where available, showing small but systematic deviations.

  • 3.
    Jansson, Emil
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Jha, Prakash Chandra
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Density functional study of triazole and thiadiazole systems as electron transporting materials.2006In: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 330, no 02-jan, p. 166-171Article in journal (Refereed)
    Abstract [en]

    Density functional theory has been used for the calculation of electronic structures, vertical electron affinities and intramolecular reorganization energies for bis-aryl substituted triazole and thiadiazole. The results obtained on the basis of the theoretical calculations indicate that the HOMO and LUMO energies of the substituted molecules can be tuned by changing the substituents as well as by changing the center atom. These changes lead to energy shifts in the order of 2-2.5 eV. The calculation and comparison of vertical electron affinities and intramolecular reorganization energies confirm that thiadiazole systems are interesting for electron transport properties. Taking a lesson from these substitutions, we further model the systems by twisting the molecular units along the central dihedral angle starting from the ideal structure and compare their HOMO-LUMO gap, electron affinity and reorganization energy. We find that by having simple substituents at proper positions one can control the reorganization energy, which in turn indicates that electron transport properties can be tuned.

  • 4.
    Jansson, Emil
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Minaev, Boris
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Schrader, Sigurd
    echnische Fachhochschule Wildau, University of Applied Sciences.
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Time-dependent density functional calculations of phosphorescence parameters for fac-tris(2-phenylpyridine) iridium2007In: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 333, no 03-feb, p. 157-167Article in journal (Refereed)
    Abstract [en]

    fac-Tris(2-phenylpyridine) iridium [fac-Ir(PPY)(3)] produces strong phosphorescence and has therefore been used as materials in organic light emitting diodes to overcome the efficiency limit imposed by the formation of triplet excitons. Accounting for this circumstance we present in this paper a theoretical study of phosphorescence in the Ir(PPY)(3) complex. The spin-orbit coupling effects and the radiative lifetime in the high temperature limit (T) are calculated by time-dependent density functional theory using quadratic response technology in order to elucidate the main mechanism of the phosphorescence. It is found that the orbital structure of the T, state has a localized character and that the T1 -> S0 transition is determined mostly by charge transfer from one of the ligands to the metal. At the vertical S-0-T-1 excitation the triplet state is highly delocalized among all three ligands and has a mixed pi pi* and metal-to-ligand charge transfer character. The intensity borrowing from the S-0 to S-5 transitions is mostly responsible for the strong phosphorescence emission from the x and y spin sublevels. Our results concord with the experimental data on temperature and magnetic field dependence of the phosphorescence kinetics. The calculated radiative lifetime in the high temperature limit agrees well with the measured decay times (2 mu s) accounting for negligible non-radiative quenching of the lowest triplet state.

  • 5.
    Jansson, Emil
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512).
    Norman, Patrick
    Linköpings Universitet.
    Minaev, Boris
    KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512).
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512).
    Evaluation of low-scaling methods for calculation of phosphorescence parameters2006In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 124, no 11Article in journal (Refereed)
    Abstract [en]

    In order to find a methodology that is a compromise between favorable computational scaling and tolerable errors, a series of nonrelativistic approaches for the calculation of radiative phosphorescence lifetimes are benchmarked against fully relativistic four-component results. The study of the a (3)A(2)-X (1)A(1) transition intensity in the series of H2CX molecules, where X is a chalcogene atom, X={O,S,Se,Te}, indicates a general good agreement between fully relativistic four-component and nonrelativistic perturbation-theoretical calculations. Among the nonrelativistic approaches, the scaled-charge spin-orbit operator approach is recognized as to provide transition matrix elements that are in good agreement with those obtained with the more elaborate Breit-Pauli and atomic mean field spin-orbit operators. This finding supports phosphorescence calculations using the available linear scaling technology for large complexes and, together with effective-core potentials, large complexes including heavy elements.

  • 6.
    Jha, Prakash Chandra
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Jansson, Emil
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Triplet energies of pi-conjugated polymers2006In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 424, no 03-jan, p. 23-27Article in journal (Refereed)
    Abstract [en]

    We report time-dependent density functional theory calculations of singlet-triplet splittings (S-0-T-1) and triplet-triplet (T-1-T-n) excitation energies of a range of conjugated polymers with relevance as light emitting diode materials. The errors obtained from the computed results are discussed in terms of maximum and mean deviation values in comparison with experimental data. The theoretical methodology shows in general a sufficient agreement to warrant its use for prediction of polymer (S-0-T-1) and (T-0-T-1) excitation energies. We confirm that spin contamination of the triplet state is not a serious problem when computing triplet-triplet spectra of organic oligomers.

  • 7. Minaev, Boris
    et al.
    Jansson, Emil
    KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512).
    Lindgren, Mikael
    Application of density functional theory for studies of excited states and phosphorescence of platinum(II) acetylides2006In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 125, no 9, p. 094306-Article in journal (Refereed)
    Abstract [en]

    The electronic states of different conformations of platinum acetylides Pt(PH3)(2)(C C-Ph)(2) and Pt(PH3)(2)(C C-PhC C-Ph)(2) (PE1 and PE2) were calculated with density functional theory (DFT) using effective core potential basis sets. Time dependent DFT calculations of UV absorption spectra showed strong dependence of the intense absorption band maxima on mutual orientation of the phenyl rings with respect to the P-Pt-P axis. Geometry optimization of the first excited triplet state (T-1) indicates broken symmetry structure with the excitation being localized in one ligand. This splits the two substitution ligands into a nondistorted aromatic ring with the C C-Ph bonds for one side and into a quinoid structure with a cumulenic C = C = C link on the other side. Quadratic response (QR) calculations of spin-orbit coupling and phosphorescence radiative lifetime (tau(R)) indicated a good agreement with experimental tau(R) values reported for solid PE1 and PE2 and PE2 capped with dendrimers in tetrahydrofuran solutions. The QR calculations reproduced an increase of tau(R) upon prolongation of pi chain of ligands and concommittant redshift of the phosphorescence. Moreover, it is shown how the phosphorescence borrows intensity from sigma ->pi(*) transitions localized at the C C-Pt-P fragments and that there is no intensity borrowing from delocalized pi ->pi(*) transitions.

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