Temperature-dependent emission in disordered herringbone aggregates of conjugated oligomers

A model accounting for the simultaneous presence of static and dynamic disorder is developed for herringbone aggregates and thin films of conjugated oligomers such as oligo-phenylene vinylene and oligothiophene. Orientational point defects in which a molecule is rotated 180 deg about its long axis away from its normal minimal energy position are considered in detail within the motional narrowing regime. Temperature effects are included under the fast scattering approximation leading to rapid exciton thermalization. In the thermodynamic limit, the b-polarized 0-0 line strength scales inversely with temperature for low defect concentrations but becomes independent of temperature at high defect concentrations. In marked contrast the primarily ac-polarized 0-1 line strength is insensitive to both temperature and defect concentration. The ratio of the 0-0 to 0-1 line strengths is used to determine the exciton coherence size.