A high energy exciton in polydiacetylene chains, involving electrons localized on the triple bond

U-V spectra of polydiacetylene (PDA) chains in their monomer single crystal matrix show an absorption near 4 eV and above, predominantly polarized perpendicular to the chain direction. It is shown by combining absorption and electroabsorption studies that the spectrum corresponds to an excitonic transition. This exciton has polarizability and binding energy comparable to those of the well-known exciton near 1.9 eV in PDAs, but an oscillator strength smaller by more than two orders of magnitude. It is assigned to the excitation of a pi electron localized on an acetylenic bond to the well delocalized pi* states. The transition gains intensity via resonant coupling to states of the continuum associated to the 1.9 eV exciton. (C) 2004 Elsevier B.V. All rights reserved.