Electronic excitations and alternation of conjugated polymers

Abstract

The electronic excitations and fluorescence of conjugated polymers are related to large or small alternation δ of the transfer integrals t(1±δ) along the backbone. The fluorescence of polysilanes (PSs) and poly (para-phenylenevinylene (PPV) is linked to large δ, which places the one-photon gap E_g below the lowest two-photon gap E_a and reduces distortions due to electron-phonon (e-p) coupling. In contrast to small δ~0.1 in π-conjugated polymers, such as polyacetylene (PA), para-conjugated phenyls lead to an extended π-system with increased alternation, to states localized on each ring and to charge-transfer excitations between them. Surprisingly good agreement is found between semiempirical parametric method 3 (PM3) bond lengths and exact Pariser-Parr-Pople (PPP) π-bond orders for trans-stilbene, where the PPV bipolarons are confined to two phenyls. Stilbene spectra are consistent with increased alternation and small e-p distortions.