Dynamics of the Excitonic Coupling in Organic Crystals

We show that the excitonic coupling in molecular crystals undergoes a very large fluctuation at room temperature as a result of the combined thermal motions of the nuclei. This observation dramatically affects the description of exciton transport in organic crystals and any other phenomenon (like singlet fission or exciton dissociation) that originates from an exciton in a molecular crystal or thin film. This unexpected result is due to the predominance of the short-range excitonic coupling mechanisms (exchange, overlap, and charge-transfer mediated) over the Coulombic excitonic coupling for molecules in van der Waals contact. To quantify this effect we develop a procedure to evaluate accurately the short-range excitonic coupling (via a diabatization scheme) along a molecular dynamics trajectory of the representative molecular crystals of anthracene and tetracene.

Figure 1

(a) Chemical structure and (b) crystal arrangement for anthracene with the labeling of the nonequivalent nearest neighbors (molecular dimers $A$ and $B$). (c) Time evolution of the excitonic coupling due to the thermal motions (300 K) for the two anthracene dimers $A$ and $B$ and (d) distribution of the excitonic couplings.

Figure 2

Fourier transform of the autocorrelation function of the excitonic coupling between the first excited states of the neighboring molecules for anthracene dimers $A$ (top) and $B$ (bottom), at 300 K.