Stark effect and Franz-Keldysh effect of a quantum wire realized by conjugated polymer chains of a diacetylene 3NPh2

Abstract

Conjugated polymer chains diluted in monomer single crystals of a diacetylene, 1,10-bis(diphenylamino)-4,6-decadiyne, represent quantum wires of long-range coherence. They differ from most polydiacetylenes by their repeat unit that consists of two ${\text{C}}_{4}$ units with twisted molecular planes. The resulting modification of exciton and band states is studied by electroabsorption which resolves an increase in the exciton transition energy to 2.399 eV and of the free-electron gap to 3.158 eV. The Stark shift of the exciton reveals a significant reduction in the exciton radius by about 40% to $6\ifmmode\pm\else\textpm\fi{}1\text{ }\text{\AA{}}$ which is consistent with a similar increase in the exciton binding energy to 759 meV. The line shape of electroabsorption spectra at the free-electron gap is for fields larger than 30 kV/cm in accordance with the one-dimensional Franz-Keldysh effect and yields a reduced mass of $0.07{m}_{0}$ larger than in other polydiacetylene (PDA). Spectra at smaller fields are distorted by contributions of a weak dipole allowed and a forbidden exciton 38 and 12 meV, respectively, below the band gap. Coherent coupling of free electrons and vibrational modes leads to replicas of the Franz-Keldysh effect. Since light polarized parallel to the conjugated chain excites two propagating modes in the monomer crystals the quantitative analysis was restricted to electroabsorption spectra taken with perpendicular polarization in samples which transmit only the weakly absorbed polariton mode.