Stoyan Karabunarliev

Spin-dependent electron-hole capture kinetics in luminescent conjugated polymers

The recombination of electron-hole pairs injected in extended conjugated systems is modeled as a multistep interconversion relaxation in monoexcited electronic state space, mediated by electron-phonon coupling. The computed ratio of triplet-to-singlet exciton formation times r=tau(T)/tau(S) increases from 0.9 for a model dimer to 2.5 for a 32-unit chain, in good agreement with recent experiments. We rationalize the conjugation-length dependence of r in terms of spin-specific energetics and mutual vibronic coupling of the excited states.

Franck–Condon spectra and electron-libration coupling in para-polyphenyls

Proceeding from quantum-chemical potential energy surfaces, we compute the absorption and fluorescence spectra of conventional and ladder-type para-phenylene oligomers (OPP and OLPP) with up to 7 benzene rings. Electronically excited states are addressed by means of extended configuration interaction within a standard molecular all-valence-electron semiempirical Hamiltonian. Adiabatic excitation energies, interstate distortions and normal modes are used to compute Franck–Condon band shapes with rigorous consideration of vibrational structure. Theoretical spectra agree with the experiment and rationalize the striking disparities in the linear optical response of OPP and OLPP. Whereas electron–phonon coupling in OLPP is essentially restricted to the carbon–carbon bond-stretching modes, photoexcitation, and emission processes in OPP are followed by significant relaxations in ring-torsional degrees of freedom. The broadening of spectra of OPP, especially pronounced in absorption, and the large Stokes shift be...

Rigorous Franck-Condon Absorption and Emission Spectra of Conjugated Oligomers from Quantum Chemistry

A harmonic Condon approach is used to calculate excitation and emission band shapes for the lowest dipole-allowed electronic transitions in conjugated oligomers: polyenes, oligorylenes, and para-phenylenevinylenes. Ground- and excited-state adiabatic energies, equilibrium structures, and vibrational modes are obtained within standard all-valence-electron molecular Hamiltonian incorporating extended configuration interaction. The interstate distortion is cast in normal coordinates and used to calculate transition probabilities from the zero-phonon initial state to the vibrational manifold of the final state. Spectral profiles are obtained as a superposition of Lorentzian line shapes. Theoretical band shapes reproduce prominent features in the absorption and fluorescence spectra of the oligomers in question. The strength of the bond-stretching vibronic progression increases with oligomeric length in polyenes, but decreases in para-phenylenevinylenes. In line with experiment, absorption and emission band sha...

Exciton dissociation dynamics in model donor-acceptor polymer heterojunctions. I. Energetics and spectra

In this paper we consider the essential electronic excited states in parallel chains of semiconducting polymers that are currently being explored for photovoltaic and light-emitting diode applications. In particular, we focus upon various type II donor-acceptor heterojunctions and explore the relation between the exciton binding energy to the band offset in determining the device characteristic of a particular type II heterojunction material. As a general rule, when the exciton binding energy is greater than the band offset at the heterojunction, the exciton will remain the lowest-energy excited state and the junction will make an efficient light-emitting diode. On the other hand, if the offset is greater than the exciton binding energy, either the electron or hole can be transferred from one chain to the other. Here we use a two-band exciton to predict the vibronic absorption and emission spectra of model polymer heterojunctions. Our results underscore the role of vibrational relaxation and suggest that intersystem crossings may play some part in the formation of charge-transfer states following photoexcitation in certain cases.

Polaron–excitons and electron–vibrational band shapes in conjugated polymers

The neutral excitations in poly(p-phenylenevinylene) are studied in conjunction with the vibronic structure of the lowest optical transitions. Combining the configuration interaction of Wannier-localized electron–hole pairs with an empirical description of electron–phonon coupling, we obtain the potential energy surfaces of monoexcited states and the Condon electron–vibrational spectra in absorption and emission. The S1→S0 luminescence band shape is found compatible with self-localization of S1 within about 10 monomers, driven exclusively by electron–phonon coupling. The singlet and triplet polaron–excitons are exchange–split by about 1 eV and differ substantially in terms of average electron–hole separation.

Band structure of quasi-one-dimensional polycondensed aromatic hydrocarbons I. Poly(periacene)s

Abstract The energy spectrum of a group of poly(periacene)s (PPAcs) is investigated theoretically. The topological ( Δ top ), geometrical ( Δ geom ) and correlation ( Δ corr ) contributions to the energy gap ΔE ∞ are examined, with ΔE ∞ = [Δ corr 2 +(Δ top +Δ geom ) 2 ] 1 2 . If the number of benzene rings n within the elementary unit reaches infinity ( n a ∞ ), the bandgap vanishes, but the ΔE ∞ ( n ) values do not decrease in a monotonic fashion. It appears that PPAc can be classified into the following two series: the (3 n +2) and the (3 n + p ) series with p = 1, 3; in each series ΔE ∞ decreases monotonically. The frontier MOs of the two series, which determine the energy gap, belong to different irreducible representations of the symmetry group D (2 m ) h . The MOs of the (3 n +2) series are symmetric with respect to reflection in the mirror plane σ h , while the MOs of the (3 n + p ) series are symmetric. The connection between the symmetry of the frontier MOs and the energy spectrum of the PPAcs is discussed.

Dissipative dynamics of spin-dependent electron–hole capture in conjugated polymers

Spin-dependent electron–hole (e–h) recombination in poly(p-phenylenevinylene) chains is modeled by the dissipative dynamics of the multilevel electronic system coupled to the phonon bath. The underlying Hamiltonian incorporates the Coulomb and exchange interactions of spin-singlet and spin-triplet monoexcitations in Wannier-orbital basis and their coupling to the prominent Franck–Condon active modes. In agreement with experiment, we obtain that the ratio of singlet versus triplet exciton formation rates is strongly conjugation-length dependent and increasing on going from the model dimer to the extended chain. The result is rationalized in terms of a cascade interconversion mechanism across the electronic levels. In parallel to the direct formation of spin-dependent excitons, e–h capture is found to generate long-lived charge-transfer states, whose further phonon-mediated relaxation to the bottom of the density of states is hindered by the near e–h symmetry of conjugated hydrocarbons. Being nearly spin in...

A class of narrow-band high-spin organic polymers II. Polymers with indirect exchange interaction☆

Abstract It is shown that indirect exchange magnetic interaction may be crucial for the ground state spin multiplicity of a class of organic polymers. Such polymer chains possess a system of extended π-conjugation and each elementary unit contains at least one radical substituent R . with a localized partially occupied molecular orbital (LPOMO): The occurrence of LPOMOs is caused either by (i) steric hindrance which inhibits the π-π interaction of LPOMOs and the π-system along the polymer chain by making the two planes of π-conjugation almost orthogonal or (ii) specific topology of the π-network of the radical substituent R . preserving its π-POMO even when linked to the polymer chain. The almost perfect localization of POMOs results in vanishing direct (Hund) exchange interaction between unpaired electrons. These uncompensated spins still induce spin polarization of the overall system of π-conjugation which may break the spin degeneracy. An approach similar to the s-d(s-f) model of indirect exchange is applied. The ground state spin multiplicity depends on the exchange constants of the corresponding effective spin Hamiltonian. The latter constants are calculated for several model polymers in order to establish the type (ferro- or antiferromagnetic) and magnitude of the indirect exchange interaction.

BOND ALTERNATION AND ELECTRONIC-STRUCTURE OF POLY(PERINAPHTHALENE)

Abstract It is shown on the Hartree-Fock level of theory that Peierls distortions of two different bond alternation patterns are possible for poly(perinaphthalene). In the structurally related oligomeric systems, the stabilization of a given type of bond length distortion results from the boundary conditions as imposed by different end groups. For the polymer the π-π electron interaction is found to give preference to the bond alternation observed in oligorylenes. Nevertheless, the adiabatic potential energy surface has a double-well shape, with two almost degenerate minima separated by a low potential barrier.

Band structure of quasi-1D polycondensed hydrocarbons II. Energy spectra of ribbon polymers in relation to graphite☆

Abstract The energy spectra of three classes of polyconjugated hydrocarbon polymers with ribbon structure (A n , B n and C n ) have been studied. The relative contributions of the topological, geometrical and electron correlational factors to the width of the energy gap ΔE have been determined. An analytical expression has been derived for the energy gap of the A n and B n polymers, with an arbitrary number of ribbons L . For these polymers, ΔE tends monotonically towards zero ( ΔE for graphite) when the number of ribbons reaches infinity ( L → ∞). For C n polymers the asymptotic behaviour depends on the value of L . For C n polymers with L = 3 r ( r = 1, 2, …), ΔE is equal to zero. This means that they are ideal 1D models for graphite. For the other C n polymers ( L = 3 r + 1 or L = 3 r + 2, r = 0, 1, 2, …) ΔE tends monotonically towards zero. An analytical expression has been proven (valid for A n and B n polymers) which determines the general dependence of the one-electron components of ΔE on the geometric characteristics of the elementary unit of the polymers.