Coherent photoexcitation of entangled triplet pair states

The functional properties of organic semiconductors are defined by the interplay between optically bright and dark states. Organic devices require rapid conversion between these bright and dark manifolds for maximum efficiency, and one way to achieve this is through multiexciton generation (S₁→¹TT). The dark state ¹TT is typically generated from bright S₁ after optical excitation; however, the mechanistic details are hotly debated. Here we report a ¹TT generation pathway in which it can be coherently photoexcited, without any involvement of bright S₁. Using <10-fs transient absorption spectroscopy and pumping sub-resonantly, ¹TT is directly generated from the ground state. Applying this method to a range of pentacene dimers and thin films of various aggregation types, we determine the critical material properties that enable this forbidden pathway. Through a strikingly simple technique, this result opens the door for new mechanistic insights into ¹TT and other dark states in organic materials. (Figure presented.)