Role of intermolecular charge delocalization and its dimensionality in efficient band-like electron transport in crystalline 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane (F2-TCNQ)

Theoretical understanding of charge transport in organic semiconductors is exclusively important for organic electronics, but still remains a subject of debate. The recently discovered record-high band-like electron mobility in single crystals of 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane (F-2-TCNQ) is challenging from the theoretical viewpoint. First, the very small size of the F-2-TCNQ molecule implies high reorganization energy that seems incompatible with efficient charge transport. Second, it is not clear why the crystals of a similar compound, 7,7,8,8-tetracyanoquinodimethane (TCNQ), show an inefficient hopping electron transport mechanism. To address these issues, we apply DFT and QM/MM calculations to the F-n-TCNQ (n = 0,2,4) crystal series. We show that multidimensional intermolecular charge delocalization is of key importance for efficient charge transport in materials consisting of smallsized molecules, and commonly used guidelines for the search for high-mobility organic semiconductors are to be corrected.