Theory of excited-state absorption in phenylene-basedπ-conjugated polymers

Abstract

Within a rigid-band correlated electron model for oligomers of poly-͑paraphenylene͒ ͑PPP͒ and poly͑paraphenylenevinylene͒ ͑PPV͒, we show that there exist two fundamentally different classes of two-photon A g states in these systems to which photoinduced absorption ͑PA͒ can occur. At relatively lower energies there occur A g 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 g ). In addition to the above class of A g states, PA can also occur to a higher energy kA g state whose 2e -2h component is different and has significant contributions from excitations involving both delocalized and localized bands. Our calculated scaled energies of the mA g and the kA g agree reasonably well to the experimentally observed low-and high-energy PA's in PPV. The calculated relative intensities of the two PA's are also in qualitative agreement with experiment. In the case of ladder-type PPP and its oligomers, we predict from our theoretical work an intense PA at an energy considerably lower than the region where PA's 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 g that leads to charge generation subsequent to excitation to this state, as found experimentally.

Photoinduced absorption in disubstituted polyacetylenes: Comparison of theory with experiments

“…In a recent theoretical analysis of the 1B u PA spectra of PPV, which also has two prominent bands labeled PA1 and PA2, we indeed found that the first and the most intense peak ͑PA1͒ contributing to the spectra was indeed the mA g state of the polymer. 21 …”

“…Comparing these states to the ones contributing resonant peaks to the TPA and THG spectra of oligo-PDPA's calculated by us earlier, 16,17 we conclude that these states correspond to nothing but the mA g states of the corresponding oligo-PDPA. Recalling that the mA g is the state with a very strong dipole coupling to the 1B u state, which makes its presence prominent in both the TPA and THG spectra of several conjugated polymers, 13,21 it should not at all be surprising that it leads to an intense peak in their 1B u PA spectra as well. Indeed, in a recent calculation performed on oligomers of PPP and PPV, 21 we identified the first feature of their 1B u PA spectra with their mA g states.…”

“…Recalling that the mA g is the state with a very strong dipole coupling to the 1B u state, which makes its presence prominent in both the TPA and THG spectra of several conjugated polymers, 13,21 it should not at all be surprising that it leads to an intense peak in their 1B u PA spectra as well. Indeed, in a recent calculation performed on oligomers of PPP and PPV, 21 we identified the first feature of their 1B u PA spectra with their mA g states. Upon examining the dipole coupling of the mA g states of oligo-PDPA's to their 1B u states, we find this ͑a͒ to be mainly directed along the longitudinal direction, and that ͑b͒ in terms of magnitude the coupling of this state to the 1B u state to be the largest among all other A g -type excited states.…”

“…The Ohno parametrization initially was carried out for small molecules, and, therefore, it is possible that the Coulomb parameters for the polymeric samples could be somewhat smaller due to interchain screening effects. 19 In various calculations performed on phenylenebased conjugated polymers including PDPA's, [9][10][11]16,17,20,21 we notice that "screened parameters" with U = 8.0 eV and i,i = 1.0, and i,j = 2.0, otherwise, proposed by Chandross and Mazumdar,19 lead to a much better agreement with the experiments, as compared to the "standard parameters" with U = 11.13 eV and i,j = 1.0, as proposed originally by Ohno. Therefore, in the present work we compare the results of our screened-parameter-based calculations with the experiments, while the results of our standard-parameter-based calculations are compared with similar calculations performed on corresponding polyenes.…”

A Fortran 90 Hartree–Fock program for one-dimensional periodic π-conjugated systems using Pariser–Parr–Pople model

“…In recent years, our group and collaborators(author?) [28,29,30,31,32,33,34,35,36,37,38,39,40], along with numerous other groups(author?) [1,2,3,41,42,43,44,45,46], have used the P-P-P model to study the electronic structure and optical properties of conjugated molecules and oligomers.…”