Quantum-confinement effects on the ordering of the lowest-lying excited states in conjugated chains

Abstract

The symmetrized density-matrix renormalization-group approach is applied within the extended Hubbard-Peierls model (with parameters U/t, V/t, and bond alternation δ) to study the ordering of the lowest one-photon (1¹B_u^-) and two-photon (2¹A_g⁺) states in one-dimensional conjugated systems with chain lengths N up to N=80 sites. Three different types of crossovers are studied, as a function of U/t, δ, and N. The "U crossover" emphasizes the larger ionic character of the 2A_g state compared to the lowest triplet excitation. The "δ crossover" shows strong dependence on both N and U/t. the "N crossover" illustrates the more localized nature of the 2A_g excitation relative to the 1B_u excitation at intermediate correlation strengths.