Excited-State Absorption of Conjugated Polymers in the Near-Infrared and Visible: A Computational Study of Oligofluorenes

Excited-state properties of conjugated polymers play a central role in applications ranging from organics-based photovoltaics to nonlinear photonics. From a theoretical and computational point of view, however, an accurate first-principles description poses a formidable task. Typical molecule sizes go well beyond the size limits for which highly reliable wave function based electronic-structure methods can be applied. In the present work, we demonstrate that nonlinear-response density functional theory can be used to accurately model the excited state absorption process in an important class of conjugated materials. We compute transitions between up to 100 excited states for fluorene oligomers containing up to about 100 conjugated atoms. Furthermore, we demonstrate that this approach can explain the nature of absorption bands in the ESA in near-infrared and visible spectral range. These systems are large enough that we approach the polymer limit in terms of electronic properties of excited states. The results obtained are in good agreement with available experimental data.