Broad and narrow bands in the photoluminescence spectrum of solid-state oligothiophenes: Two marks of an intrinsic emission

The optical properties of quaterthiophene (4T) films have been studied and related to the presence of perfectly crystalline and orientationally disordered domains. The translational order is always retained, however, since 4T molecules lack a reflection symmetry plane perpendicular to the long molecular axis, each molecule can assume two nonequivalent orientations. The electronic state energies are almost configuration independent, but the selection rules for the transition from the first excited state towards the ground state are completely different in well-oriented and orientationally disordered domains. All observed photoluminescence spectral features of high-quality films are interpreted in terms of intrinsic emissions. The model of chiral and achiral pinwheel aggregates provides theoretical support to this picture.