Charge-transfer excitons in mixed-stack donor—acceptor compounds: A variety of solitonic states

Abstract

Charge-transfer excitons (CTEs) strongly coupled to lattice phonons in quasi-1-d neutral mixed-stack compounds may self-trap, forming various large-radius solitonic states. Two types of symmetric solitons (denoted by S o −, S π +) and dimerized ones ( D) are described together with the areas of crystal parameters at which they exist. The exact discrete spectra Ω n , n = 0, 1,…, of local optical vibrations bound to S-solitons are found. Phonons around S π + always have Ω n > ω , ω being the bare frequency, while Ω n < ω for S o − and the soft optical mode Ω 0 → 0 may appear indicating thus S o − → D transformation below a certain ratio of electron-hole motion energy and polaronic shift. The possibility of unusual S K ≃ −- solitons shifted down below arbitrary points k = K ≃ of exciton bands is demonstrated for “multicomponent” states. A general problem of several exciton states mixing in quasi-1-d systems in the presence of the strong exciton-phonon interaction is also discussed for the examples of site excitations in coupled-chain-composed substances and mixed CTE-Frenkel excitons. Known estimates and experimental data distinguish CTEs as the most appropriate candidate for soliton formation compared to other molecular excitations in quasi-1-d.