ONE PHOTON AND 2-PHOTON EXCITATIONS OF SIGMA-CONJUGATED CHAINS

The Sandorfy C model of alternating transfer integrals t(1 ± δ) for σ conjugation in (SiRR′)n polymers is extended to include Coulomb interactions. The resulting Pariser-Parr-Pople (PPP) models are solved exactly for oligomers with n≤6 and extrapolated to polymers. Increasing alternation δ reverses the position of the one-photon excitation to 1 1B and two-photon excitation to 2 1A for PPP parameters appropriate to either polyenes with δ≤0.10 or polysilanes (PS) with δ ≈ 1 3. The PS excitation to 1 1B is about 1 eV below the 2 1A level due to exciton formation. The corresponding states are essentially degenerate in Hubbard models restricted to on-site interactions U=2t for δ>0.10. The dimer limit (δ ≈ 1) relates PPP or Hubbard models and their microscopic parameters to bond, charge-transfer, and triplet excitons and rationalizes the numerical results for arbitrary δ. © 1990.