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 (S→TT). The dark state TT is typically generated from bright S after optical excitation; however, the mechanistic details are hotly debated. Here we report a TT generation pathway in which it can be coherently photoexcited, without any involvement of bright S. Using <10-fs transient absorption spectroscopy and pumping sub-resonantly, TT 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 TT and other dark states in organic materials.