Comparative theoretical study of the doping of conjugated polymers: Polarons in polyacetylene and polyparaphenylene

Abstract

Defect-state calculations on all-trans polyacetylene and polyparaphenylene have been performed in the framework of the adiabatic Huckel Hamiltonian with o. -bond compressibility. In polyacetylene, the study of the energetics of the separation of the radical (neutral defect) -ion (charged defect) pair induced upon doping indicates that the two defects tend to remain in close proximity, resulting in the formation of a polaron. The binding energy of the polaron is estimated to be about 0.05 eV with this model. Absorption spectra at low doping levels are shown to be compatible with polaron formation, thus demonstrating the nonuniqueness of the previously proposed soliton model in explaining these absorption data. At higher doping levels, interaction between polarons leads to the formation of charged solitons carrying no spin. In polyparaphenylene, defects are always correlated in pairs due to the absence of a degenerate ground state. At low doping, polarons with a binding energy estimated at 0.03 eV are formed on ionization of polyparaphenylene. The related deformation of the lattice is relatively soft, in agreement with crystallographic data on biphenyl anions, and extends over about five rings. Increasing the doping level leads to the formation of bipolarons (doubly charged defects) that require a stronger deformation of the lattice and carry no spin. The possibility of a conduction mechanism in polyparaphenylene involving motion of bipolarons is consistent with magnetic data indicative of very low Pauli susceptibility in the metallic regime of SbF5-doped polyparaphenylene.

Theoretical Investigation of Excited States of Large Polyene Cations as Model Systems for Lightly Doped Polyacetylene

“…For systems longer than C 41 H 43 , MP2 becomes prohibitively expensive, and DFT-hybrid geometries can be used without loss of accuracy. For example in Figure 3 Since more localized defects are obtained with HF, MP2/ 6-31G, and with semiempirical methods 7,16,20,23,29,35 and since forces during the geometry optimizations are very small, it seemed plausible that the differences are caused by very flat potential energy surfaces rather than the failure of certain methods. To investigate how big the energy lowering due to localization is, the counterions were removed, and single point energies were calculated on the geometries optimized in the presence of counterions.…”

“…[16][17][18] Theoretical calculations, mostly employing semiempirical and Hartree-Fock methods, have confirmed the presence of self-localization in the presence and in the absence of counterions. [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] At higher levels of theory, the size of the defects in the absence of counterions tends to increase 35 and bipolarons become unstable with respect to two polarons. [36][37][38] As an alternative to bipolaron formation, π-dimers of radical cations have been shown to be stable when solvent effects are included in the theoretical treatment.…”

Electrochemical materials science: calculation vs. experiment as predictive tools in tailoring intrinsically conducting polythiophenes

“…In heterocyclic polymers, where two non-degenerate regions are separated by a topological defect, the formation of single solitons like polyacetylene is energetically unfavorable [27], and paired sites are formed [28]. This is the case for polythiophene, which can be drawn with either aromatic (Fig.…”

Designing of Nano Composites of Conducting Polymers for EMI Shielding

“…These polymers do not support soliton-like defects because the ground state energy of the quinoid form is substantially higher than the aromatic benzenoid structure figure, 5. As a result, the charge defects on these polymers are different [38]. As an example, consider the oxidation of polypyrrole ( Figure 6).…”