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Mixed Stack Organic Semiconductors: The Anomalous Case of the BTBT-TCNQF<sub><i>x</i></sub> Series
We present a detailed
characterization of three mixed stack organic
semiconductors, where benzothieno-benzothiophene electron donor is
associated with fluorine-substituted TCNQ of increasing electron affinity.
BTBT-TCNQ and BTBT-TCNQF<sub>2</sub> are isomorphous, whereas the
three-dimensional packing of BTBT-TCNQF<sub>4</sub> is 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 TCNQF<sub><i>x</i></sub> are 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