203 research outputs found
Optical absorption in the substituted phenylene-based conjugated polymers: theory and experiment
Journal ArticleWe investigate theoretically and experimentally the effects of (2,5) chemical substitution on the optical absorption in the phenylene-based conjugated polymers. Theoretically, substitution destroys both the charge-conjugation symmetry and spatial symmetry that characterize the unsubstituted materials. Within Coulomb-correlated theoretical models, the effect of broken charge-conjugation symmetry alone on the underlying electronic structure and on the absorption spectrum is rather weak. When both broken spatial symmetry and broken charge-conjugation symmetry are taken into account, a strong effect on the electronic structure of polyphenylene derivatives is found. In spite of the strong effect of the broken symmetries on the electronic structure, the effect on the optical-absorption spectrum is weak
Excitons in poly(para-phenylenevinylene)
Journal ArticleWe show that one-electron band theory fails to describe optical absorption in PPV and the absorption spectrum can be described only within a Coulomb-correlated model. The lowest optical state is an exciton, whose binding energy is estimated theoretically to be 0.90 ± 0 . 1 5 e V Measurements of the photoconductivity quantum efficiency in poly[2-methoxy,5-(2'-ethyl-hexyloxy)-l,4 phenylenevinylene] reveal a binding energy in good agreement with the theoretical value
Comment on ``Density-matrix renormalization-group method for excited states''
In a Physical Review B paper Chandross and Hicks claim that an analysis of
the density-density correlation function in the dimerised Hubbard model of
polyacetylene indicates that the optical exciton is bound, and that a previous
study by Boman and Bursill that concluded otherwise was incorrect due to
numerical innacuracy. We show that the method used in our original paper was
numerically sound and well established in the literature. We also show that,
when the scaling with lattice size is analysed, the interpretation of the
density-density correlation function adopted by Chandross and Hicks in fact
implies that the optical exciton is unbound.Comment: RevTeX, 10 pages, 4 eps figures fixed and included now in tex
Density functional theory and DFT+U study of transition metal porphines adsorbed on Au(111) surfaces and effects of applied electric fields
We apply Density Functional Theory (DFT) and the DFT+U technique to study the
adsorption of transition metal porphine molecules on atomistically flat Au(111)
surfaces. DFT calculations using the Perdew-Burke-Ernzerhof (PBE) exchange
correlation functional correctly predict the palladium porphine (PdP) low-spin
ground state. PdP is found to adsorb preferentially on gold in a flat geometry,
not in an edgewise geometry, in qualitative agreement with experiments on
substituted porphyrins. It exhibits no covalent bonding to Au(111), and the
binding energy is a small fraction of an eV. The DFT+U technique, parameterized
to B3LYP predicted spin state ordering of the Mn d-electrons, is found to be
crucial for reproducing the correct magnetic moment and geometry of the
isolated manganese porphine (MnP) molecule. Adsorption of Mn(II)P on Au(111)
substantially alters the Mn ion spin state. Its interaction with the gold
substrate is stronger and more site-specific than PdP. The binding can be
partially reversed by applying an electric potential, which leads to
significant changes in the electronic and magnetic properties of adsorbed MnP,
and ~ 0.1 Angstrom, changes in the Mn-nitrogen distances within the porphine
macrocycle. We conjecture that this DFT+U approach may be a useful general
method for modeling first row transition metal ion complexes in a
condensed-matter setting.Comment: 14 pages, 6 figure
Long-Range Excitons in Optical Absorption Spectra of Electroluminescent Polymer Poly(para-phenylenevinylene)
The component of photoexcited states with large spatial extent is
investigated for poly(para-phenylenevinylene) using the intermediate exciton
theory. We find a peak due to long-range excitons at the higher-energy side of
the lowest main feature of optical spectra. The fact that the onset of
long-range excitons is located near the energy gap is related to the mechanisms
of large photocurrents measured in such energy regions. We show that a large
value of the hopping integral is realistic for characterizing optical
excitations.Comment: To be published in J. Phys. Soc. Jpn. (Letters
Theory of excited state absorptions in phenylene-based -conjugated polymers
Within a rigid-band correlated electron model for oligomers of
poly-(paraphenylene) (PPP) and poly-(paraphenylenevinylene) (PPV), we show that
there exist two fundamentally different classes of two-photon A states in
these systems to which photoinduced absorption (PA) can occur. At relatively
lower energies there occur A states which are superpositions of one
electron - one hole (1e--1h) and two electron -- two hole (2e--2h) excitations,
that are both comprised of the highest delocalized valence band and the lowest
delocalized conduction band states only. The dominant PA is to one specific
member of this class of states (the mA). In addition to the above class of
A states, PA can also occur to a higher energy kA state whose 2e--2h
component is {\em different} and has significant contributions from excitations
involving both delocalized and localized bands. Our calculated scaled energies
of the mA and the kA agree reasonably well to the experimentally
observed low and high energy PAs in PPV. The calculated relative intensities of
the two PAs are also in qualitative agreement with experiment. In the case of
ladder-type PPP and its oligomers, we predict from our theoretical work a new
intense PA at an energy considerably lower than the region where PA have been
observed currently. Based on earlier work that showed that efficient
charge--carrier generation occurs upon excitation to odd--parity states that
involve both delocalized and localized bands, we speculate that it is the
characteristic electronic nature of the kA that leads to charge generation
subsequent to excitation to this state, as found experimentally.Comment: Revtex4 style, 2 figures inserted in the text, three tables, 10 page
Prediction of infrared light emission from pi-conjugated polymers: a diagrammatic exciton basis valence bond theory
There is currently a great need for solid state lasers that emit in the
infrared, as this is the operating wavelength regime for applications in
telecommunications. Existing --conjugated polymers all emit in the visible
or ultraviolet, and whether or not --conjugated polymers that emit in the
infrared can be designed is an interesting challenge. On the one hand, the
excited state ordering in trans-polyacetylene, the --conjugated polymer
with relatively small optical gap, is not conducive to light emission because
of electron-electron interaction effects. On the other hand, excited state
ordering opposite to that in trans-polyacetylene is usually obtained by
chemical modification that increases the effective bond-alternation, which in
turn increases the optical gap. We develop a theory of electron correlation
effects in a model -conjugated polymer that is obtained by replacing the
hydrogen atoms of trans-polyacetylene with transverse conjugated groups, and
show that the effective on-site correlation in this system is smaller than the
bare correlation in the unsubstituted system. An optical gap in the infrared as
well as excited state ordering conducive to light emission is thereby predicted
upon similar structural modifications.Comment: 15 pages, 15 figures, 1 tabl
Large scale numerical investigation of excited states in poly(phenylene)
A density matrix renormalisation group scheme is developed, allowing for the
first time essentially exact numerical solutions for the important excited
states of a realistic semi-empirical model for oligo-phenylenes. By monitoring
the evolution of the energies with chain length and comparing them to the
experimental absorption peaks of oligomers and thin films, we assign the four
characteristic absorption peaks of phenyl-based polymers. We also determine the
position and nature of the nonlinear optical states in this model.Comment: RevTeX, 10 pages, 4 eps figures included using eps
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