Mixed stack organic semiconductors: The anomalous case of the BTBT-TCNQFx series

We present a detailed characterization of three mixed stack organic semiconductors, where benzothienobenzothiophene electron donor is associated with fluorinesubstituted TCNQ of increasing electron affinity. BTBTTCNQ and BTBT-TCNQF2are isomorphous, whereas the three-dimensional packing of BTBT-TCNQF4is different. Rather surprisingly, we found that the degree of charge transfer is almost constant along the series, at variance with what has been found in analogous cocrystal series in which TCNQFxare involved. We explain this finding in terms of the Madelung energy resulting from interstack packing of the cocrystals, and of the fact that both the HOMO and HOMO-1 of BTBT are involved in the charge transfer mechanism. We also propose that the difference in interstack packing resulting from increasing F substitution is a consequence of electrostatic interactions.