Intrachain annihilation and spatial extension of electronic excitations in a ladder-type conjugated polymer probed via hole-burning

The method of transient hole-burning using a triplet "bottle neck" is applied to study the intrinsic properties of electronic excitations on isolated chains of a ladder-type pi-conjugated poly(para-phenylene) in solutions. Transient spectral changes of absorption comprising holes and anti-holes were studied. The dependence of the hole area on the burning power reveals a pronounced saturation at a relative level of ca. 10% of the total absorbance, independent on the concentration of solutions. This saturation effect is rationalized in terms of strong exciton annihilation arising when two excitations are simultaneously present on one chain. The saturated depth of holes and spectral location of anti-holes are a sensor of the size of the conjugated segments of the polymer chains. The experimental results support the view that the extent of the wave function of singlet excitations is restricted by severe topological defects of the chain rather than by a relatively weak energetic disorder imposed by an environment. (C) 2002 Published by Elsevier Science B.V.