Fine-Tuning of Crystal Packing and Charge Transport Properties of BDOPV Derivatives through Fluorine Substitution

Molecular packing
in organic single crystals greatly influences
their charge transport properties but can hardly be predicted and
designed because of the complex intermolecular interactions. In this
work, we have realized systematic fine-tuning of the single-crystal
molecular packing of five benzodifurandione-based oligo­(p-phenylenevinylene) (BDOPV)-based small molecules through incorporation
of electronegative fluorine atoms on the BDOPV backbone. While these
molecules all exhibit similar column stacking configurations in their
single crystals, the intermolecular displacements and distances can
be substantially modified by tuning of the amounts and/or the positions
of the substituent fluorine atoms. Density functional theory calculations
showed that the subtle differences in charge distribution or electrostatic
potential induced by different fluorine substitutions play an important
role in regulating the molecular packing of the BDOPV compounds. Consequently,
the electronic couplings for electron transfer can vary from 71 meV
in a slipped stack to 201 meV in a nearly cofacial antiparallel stack,
leading to an increase in the electron mobility of the BDOPV derivatives
from 2.6 to 12.6 cm2 V–1 s–1. The electron mobility of the five molecules did not show a good
correlation with the LUMO levels, indicating that the distinct difference
in charge transport properties is a result of the molecular packing.
Our work not only provides a series of high-electron-mobility organic
semiconductors but also demonstrates that fluorination is an effective
approach for fine-tuning of single-crystal packing modes beyond simply
lowering the molecular energy levels.