Triplet photoexcitations in a polythiophene with a high degree of energetic disorder

We report a photophysical study of a polythiophene with a wide distribution of conjugation lengths, obtained by the random copolymerization of 3-butyl and 3,4-butyl thiophenes in a 1/3 ratio. The photoinduced absorption spectra shift in energy upon varying the excitation wavelength, showing a localization threshold within the absorption tail. The data are explained in terms of triplet energy migration to the longest conjugated segments.