Coexistence of Ion Pairs and Charge-Transfer Complexes and Their Impact on Pentacene Singlet Fission

Pentacene (PEN) is a prototypical small-molecule organic semiconductor and a singlet fission (SF) material, but the details of its charge-transfer (CT) interactions with electron acceptors are still under debate. Here, we revisit the CT interactions in thin film blends of PEN and 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (F4-TCNQ) and find experimental evidence for the coexistence of integer CT (ICT) and charge-transfer complex (CTC) formation, with the relative amount depending on the mixing ratio. In contrast, in blends of PEN and 2,2'-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ), only ICT is found, illustrating the special behavior of PEN/F4-TCNQ at the border between CTC formation and ICT. We further study the photophysics of these donor/acceptor blends with respect to the importance of PEN as a SF material and find that SF is robust against moderate amounts of acceptors in the blends. Triplet pair formation and separation, as well as CT from PEN triplets to CTCs and acceptor molecules, is observed.