Conformational disorder of conjugated polymers. - In: The journal of chemical physics, ISSN 1089-7690, Bd. 125 (2006), 15, 154903, insges. 7 S.
Conformational disorder of conjugated polymers is an important issue to be understood and quantified. In this paper we present a new method to assess the chain conformation of conjugated polymers based on measurements of intrachain energy transfer. The chain conformation is modeled on the basis of monomer-monomer interactions, such as torsion, bending, and stretching of the connecting bond. The latter two potentials are assumed to be harmonic, while the torsional potential was calculated by density functional theory using B3-LYP functional with the SVP basis set. The energy transfer dynamics of excitons on these chains are quantitatively simulated using Förster-type line-dipole energy transfer. This allows us to compare the simulated ground state conformation of single polymer chains to ultrafast depolarization experiments of poly [3-(2,5-dioctylphenyl)thiophene] in solution. We identify torsional rotation as the main contributor to conformational disorder and find that this disorder is mainly controlled by the energy difference between syn and anti bonds.
https://doi.org/10.1063/1.2358682
Anomalous energy transfer dynamics due to torsional relaxation in a conjugated polymer. - In: Physical review letters, ISSN 1079-7114, Bd. 97 (2006), 16, 166804, insges. 4 S.
In isolated conjugated polymers two explanations are in discussion for the redshift of the emission on a picosecond time scaleexciton energy transfer (EET) between conjugated segments along the chains and conformational changes of these segments themselves, i.e., torsional relaxation. In order to resolve this question we perform femtosecond time-resolved transient absorption measurements of the energy relaxation of poly[3-(2,5-dioctylphenyl)thiophene] in toluene solution. We show that torsional relaxation can be distinguished from EET by site-selectively exciting low-energy conjugated segments. We present a unified model that integrates EET and torsional dynamics. In particular, comparison to ultrafast depolarization measurements shows that torsional dynamics cannot be neglected when analyzing EET dynamics and furthermore reveals that the exciton extends itself by about 2 monomer units during torsional relaxation.
https://doi.org/10.1103/PhysRevLett.97.166804
Comparison of the electronic structure of PPV and its derivative DIOXA-PPV. - In: Chemical physics, Bd. 327 (2006), 2/3, S. 474-484
Electronic structure of two conjugated polymers, PPV and its derivative DIOXA-PPV, are studied by quantum-chemistry methods. Good agreement between the calculations and available experimental data validates the applied methods and enables us to draw conclusions about properties of excitons and polarons in these polymers. In particular, simultaneous use of different correlation plots in atomic site representation and transition densities in real space representation reveals electronic structure and spatial localization of the elementary excitations. The properties of the DIOXA-PPV are discussed in context of possible applications in molecular electronics devices.
http://dx.doi.org/10.1016/j.chemphys.2006.05.031
Interpretation of near-field images of semiconductor nanostructures. - In: Applied physics. Lasers and optics. - Berlin : Springer, 1981- , ISSN: 1432-0649 , ZDB-ID: 1458437-2, ISSN 1432-0649, Bd. 84 (2006), 1/2, S. 103-110
We discuss near-field wave function imaging, introducing a model for high spatial resolution photoluminescence imaging of semiconductor nanostructures. The model is applied to optically bright and dark exciton and biexciton states in different quantum dot systems, explicitly taking the experimental imaging configuration into account. Our results show that direct imaging of the exciton density is only possible in collection mode experiments with nonresonant excitation in the high-resolution limit. For other geometries and for biexcitonic states, the images reflect not only the size and shape of the wave function and the spatial resolution of the near-field probe but also in particular the inherent optical nonlinearity of the imaging process. Different examples for the effects of this nonlinearity are discussed, providing new insight into the interpretation of existing experiments, and guidelines for designing novel experiments.
http://dx.doi.org/10.1007/s00340-006-2215-0
Spectral functions and optical conductivity of spinless fermions on a checkerboard lattice. - In: Physical review. Condensed matter and materials physics / American Physical Society. - College Park, Md. : APS, 1970-2015 , ISSN: 1550-235X , ZDB-ID: 1473011-X, ISSN 1550-235X, Bd. 73 (2006), 12, S. 125121, insges. 9 S.
We study the dynamical properties of spinless fermions on the checkerboard lattice. Our main interest is the limit of large nearest-neighbor repulsion V as compared with hopping |t|. The spectral functions show broad low-energy excitations which are due to the dynamics of fractionally charged excitations. Furthermore, it is shown that the fractional charges contribute to the electrical current density.
http://dx.doi.org/10.1103/PhysRevB.73.125121
Classical correlations of defects in lattices with geometrical frustration in the motion of a particle. - In: Physical review. Condensed matter and materials physics / American Physical Society. - College Park, Md. : APS, 1970-2015 , ISSN: 1550-235X , ZDB-ID: 1473011-X, ISSN 1550-235X, Bd. 73 (2006), 17, S. 174417, insges. 5 S.
We map certain highly correlated electron systems on lattices with geometrical frustration in the motion of added particles or holes to the spatial defect-defect correlations of dimer models in different geometries. These models are studied analytically and numerically. We consider different coverings for four different lattices: square, honeycomb, triangular, and diamond. In the case of a hard-core dimer covering, we verify the existing results for square and triangular lattices and obtain new ones for the honeycomb and diamond lattices while in the case of a loop covering we obtain new numerical results for all the lattices and use the existing analytical Liouville field theory for the case of a square lattice. The results show power-law correlations for the square and honeycomb lattices, while exponential decay with distance is found for the triangular lattice and exponential decay with the inverse distance on the diamond lattice. We relate this fact to the lack of bipartiteness of the triangular lattice and in the latter case to the three dimensionality of the diamond. The connection of our findings to the problem of fractionalized charge in such lattices is pointed out.
http://dx.doi.org/10.1103/PhysRevB.73.174417
Control of emission by intermolecular fluorescence resonance energy transfer and intermolecular charge transfer. - In: The journal of physical chemistry, ISSN 1520-5215, Bd. 110 (2006), 19, S. 6324-6328
Control of emission by intermolecular fluorescence resonant energy transfer (IFRET) and intermolecular charge transfer (ICT) is investigated with the quantum-chemistry method using two-dimensional (2D) and three-dimensional (3D) real space analysis methods. The work is based on the experiment of tunable emission from doped 1,3,5-triphenyl-2-pyrazoline (TPP) organic nanoparticles (Peng, A. D.; et al. Adv. Mater. 2005, 17, 2070). First, the excited-state properties of the molecules, which are studied (TPP and DCM) in that experiment, are investigated theoretically. The results of the 2D site representation reveal the electron-hole coherence and delocalization size on the excitation. The results of 3D cube representation analysis reveal the orientation and strength of the transition dipole moments and intramolecular or intermolecular charge transfer. Second, the photochemical quenching mechanism via IFRET is studied (here "resonance" means that the absorption spectrum of TPP overlaps with the fluorescence emission spectrum of DCM in the doping system) by comparing the orbital energies of the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) of DCM and TPP in absorption and fluorescence. Third, for the DCM-TPP complex, the nonphotochemical quenching mechanism via ICT is investigated. The theoretical results show that the energetically lowest ICT state corresponds to a pure HOMO-LUMO transition, where the densities of the HOMO and LUMO are strictly located on the DCM and TPP moieties, respectively. Thus, the lowest ICT state corresponds to an excitation of an electron from the HOMO of DCM to the LUMO of TPP.
https://doi.org/10.1021/jp060275m
Quantum mechanical magnetic-field-gradient drift velocity: an analytically solvable model. - In: Physical review. Condensed matter and materials physics / American Physical Society. - College Park, Md. : APS, 1970-2015 , ISSN: 1550-235X , ZDB-ID: 1473011-X, ISSN 1550-235X, Bd. 72 (2005), 16, S. 161308, insges. 4 S.
http://dx.doi.org/10.1103/PhysRevB.72.161308
Surface reaction of C60 with atomic hydrogen: formation of a protecting hydrocarbon layer. - In: Fullerenes, nanotubes & carbon nanostructures, ISSN 1536-4046, Bd. 13.2005, Suppl. 1, S. 131-138
The interaction of a single domain C60/InP(0 0 1)-(2x4) film during exposure to atomic hydrogen was investigated. At low hydrogen dosing, a high reactivity of the C60 molecules was detected. Presumably, the cracking of fullerene cages follows from a fast destruction of the molecular order. High dosing leads, by chemical modification, to the formation of a stable coverage of hydrocarbons, which is promising for future technological applications.
http://dx.doi.org/10.1081/FST-200039235
Near-field wave-function spectroscopy of excitons and biexcitons. - In: Physical review. Condensed matter and materials physics / American Physical Society. - College Park, Md. : APS, 1970-2015 , ISSN: 1550-235X , ZDB-ID: 1473011-X, ISSN 1550-235X, Bd. 71 (2005), 3, S. 035347, insges. 5 S.
Improvements in spatial resolution of near-field spectroscopy have recently allowed for wave-function imaging of single excitons and biexcitons localized in semiconductor quantum wells. Surprisingly, the apparent extent of the biexciton wave function was found to be considerably reduced compared to a single exciton. We analyze theoretically the image contrast in general near-field wave-function mapping, taking into account the finite spatial resolution of the experiment. In particular, we show that due to the nonlinear nature of biexciton spectroscopy, a smaller biexciton image size is expected even if the spatial resolution is insufficient to resolve the wave functions.
http://dx.doi.org/10.1103/PhysRevB.71.035347