MOLECULAR-EXCITON APPROACH TO SPIN-CHARGE CROSSOVERS IN DIMERIZED HUBBARD AND EXCITONIC CHAINS

The crossover from band to correlated states in half-filled quantum cell models is studied in a molecular-exciton framework based on a chain of dimers. Crystal states with one or several excited dimers yield analytical excitation energies to first order in interdimer Coulomb interactions V(p,p') for excitonic chains or interdimer electron transfer t-=t(1-δ) for Hubbard chains. Molecular-exciton analysis of excitations and transition moments rationalizes exact numerical solutions of oligomers with arbitrary intradimer correlations U, V1, and electron transfer t+=t(1+δ), including the number, positions, and transition moments of low-lying excitations. Short correlation lengths of infinite chains with large alternation δ≥0.6 lead to converged crystal states for oligomers containing N=4-7 dimers. The present approach provides a detailed picture of excited-state crossovers with increasing U, V1, and V(p,p'). Quite generally, the lowest singlet excitation S1 is one-photon allowed (1B) on the band side of the spin-charge crossover and two-photon allowed (2A) on the correlated side. Intermediate correlations and large δ reveal different crossovers in Hubbard chains, where 1B involves charge transfer between dimers, and excitonic chains, where 1B has an excited dimer. We also obtain two-photon transition moments M and extend vanishing M(2A) in the band limit up to U=2t+, the δ∼1 crossover of Hubbard chains. We find finite M(2A) on the correlated side, however, where 2A contains two triplet dimers in either alternating Hubbard or excitonic chains. Their different spin-charge crossovers appear as an abrupt and continuous increase, respectively, of two-photon intensity on going from the correlated to the band side. The greater delocalization (δ∼0.07-0.33) realized in conjugate polymers is consistent with excitonic chains. The potential V(p,p') in the Pariser-Parr-Pople model for conjugated hydrocarbons distinguishes strongly fluorescent polymers with S1=1B from others with S1=2A. We also relate our results at large δ to other approximations for nonlinear optical spectra of conjugated polymers. © 1995 The American Physical Society.