Relating the Physical Structure and Optoelectronic Function of Crystalline TIPS‐Pentacene

Theory and experiment are combined to investigate the nature of low‐energy excitons within ordered domains of 6,13‐bis(triisopropylsilylethynyl)‐pentacene (TIPS‐PEN) polycrystalline thin films. First‐principles density functional theory and many‐body perturbation theory calculations, along with polarization‐dependent optical absorption spectro‐microscopy on ordered domains, show multiple low‐energy absorption peaks that are composed of excitonic states delocalized over several molecules. While the first absorption peak is composed of a single excitonic transition and retains the polarization‐dependent behavior of the molecule, higher energy peaks are composed of multiple transitions with optical properties that can not be described by those of the molecule. The predicted structure‐dependence of polarization‐dependent absorption reveals the exact inter‐grain orientation within the TIPS‐PEN film. Additionally, the degree of exciton delocalization can be significantly tuned by modest changes in the solid‐state structure and the spatial extent of the excitations along a given direction is correlated with the degree of electronic dispersion along the same direction. These findings pave the way for tailoring the singlet fission efficiency of organic crystals by solid‐state structure.