Variation with disorder of absorption and electroabsorption spectra of a π-conjugated polymer:: 4BCMU

Absorption and electroabsorption spectra of a polydiacetylene with alkyl-urethane sidegroup (4BCMU) are compared in states of decreasing order: polymer chains diluted and oriented in a single crystal of the monomer, different types of polymer single crystals and spin cast films as a highly disordered material. The EA spectra of the most perfectly oriented chains serve as reference to determine the relevant states and to evaluate quantitatively the characteristic properties of the different field effects: the quadratic Stark effect of an exciton of B-u, symmetry which dominates the absorption spectrum and the Franz-Keldysh effect of a continuum of states which is also involved in the excitonic Stark effect. The Stark effect implies that the exciton must be delocalized because its size of about 2.5 nm is much larger than the polymer repeat unit. This exciton is well separated from other states. Exciton and continuum states have different sensitivity to disorder which is used to estimate the range of order: more than 20 nm in the chains oriented in a single crystalline monomer matrix and 10 nm or less in polymer crystals. Disorder on a shorter range introduces asymmetric charge distribution resulting from boundary conditions for the plane wave part of the eigenstates which contributes an additional component to the EA spectra of second derivative. Homogeneous chain segments in these films are shorter than 5 nm but still larger than the size of the exciton. (C) 1998 Elsevier Science B.V.