Fluorinated Arylethynyl Substituents in Zn(II) Complexes of Azadipyrromethene Guide Self-Assembly in Crystals

Zn(II) complexes of 2,8-arylethynyl 1,3,7,9-tetraphenyl azadipyrromethenes are promising molecular semiconductors with strong visible-NIR absorption and a nonplanar distorted tetrahedral geometry around the Zn(II) center, but their charge transport properties are limited by the orientation and strength of intermolecular π–π stacking interactions. To optimize their intermolecular π–π stacking interactions, we explored substitution of the pyrrolic phenylethynyl groups in Zn(L1)2 at the phenyl meta positions with either fluorines (Zn(L1-m-2F)2) or trifluoromethyls (Zn(L1-m-2CF3)2). Fluorination had minimal effects on optical properties and slightly deepened frontier orbital energy levels. On the other hand, fluorination strongly affected the complexes’ molecular packing: while Zn(L1-m-2F)2 had the same C2/c space group as Zn(L1)2, the −CF3 groups in Zn(L1-m-2CF3)2 promoted a higher ordered space group of Fddd. Further, in Zn(L1)2 and Zn(L1-m-2F)2, the phenylethynyl groups stack with the core phenyls of neighboring molecules, whereas in Zn(L1-m-2CF3)2, the bis(trifluoromethyl)-substituted phenylethynyl groups of neighboring molecules stack with each other with a shorter π–π stacking distance than in the other two molecules. Computations revealed that fluorination decreases the internal reorganization energy for electron transfer and increases overlap integrals for charge transfer. Fluorination is thus a promising strategy to tune molecular packing and parameters relevant to charge transport.