Charge-Transfer Coupling of an Embedded Pentacene Dimer with the Surrounding Crystal Matrix

Singlet exciton fission id pentacene is commonly described in terms of the dimer model, containing some adjustable parameters that implicitly account for the influence of the crystalline environment: Here we use state-of-the-art results published by other authors to base the dimer model on strictly ab initio input and consistently extend it, explicitly including the interaction with the surrounding Crystal matrix. In the multiscale approach that ensues, the molecular pair residing at the center of the model cluster is identified with the dimer for which the ab initio results are available. The main environmental effects are attributed to electrostatic stabilization of the dimer charge transfer (CT) states and to CT coupling between the dimer and the adjacent molecules. According to our calculations, this latter coupling stabilizes the lowest Frenkel state so that its energy becomes lower than that of the triplet-pair (tt) state, in dramatic contrast to the situation in an isolated dimer. The interpretationally successful level order assumed in semiempirical approaches may be restored by extending the purely electronic approach of the original ab initio calculations to include vibronic effects: The results show that extreme Caution must be exercised when extrapolating dimer results to draw conclusions concerning the situation in the crystal.