Monofluorination of Naphthyls Promotes the Cofacial π–π Stacking and Increases the Electron Mobility of Non-Planar Zinc(II) Complexes of Di(naphthylethynyl)azadipyrromethene

The
homoleptic zinc­(II) complex of [2,8-di­(1-naphthylethynyl) 3,7-diphenyl
1,9-(4-hexylphenyl)­azadipyrromethene (ZnL2)2] is a promising
non-planar non-fullerene acceptor for organic photovoltaic applications,
but it has a relatively low electron mobility that may limit its performance.
Here, we explored the fluorination of peripheral aryl groups to increase
intermolecular cofacial π–π stacking interactions,
which are desirable for electron transport. Complexes with fluorine
on the distal phenyls [Zn­(1F-L2)2], on the naphthyls [Zn­(2F-L2)2], and on both [Zn­(3F-L2)2] were synthesized and
characterized. All three complexes had similar optical and electrochemical
properties. The crystal packing structure of Zn­(2F-L2)2 and Zn­(3F-L2)2 revealed cofacial parallel-displaced π–π
stacking between the fluorinated 1-naphthylethynyl groups. Such a
cofacial orientation was not observed in Zn­(L2)2 crystals,
suggesting that fluorination of the naphthyl groups promotes the cofacial
π–π stacking orientation. The hole mobility increased
from 1.0 × 10–4 cm2 V–1 s–1 for Zn­(L2)2 to 0.8–1.0 ×
10–3 cm2 V–1 s–1 for the fluorinated complexes. Fluorination on the
naphthyl groups increased the electron mobility from 4.2 × 10–5 cm2 V–1 s–1 for Zn­(L2)2 and Zn­(1F-L2)2 to 2.0 × 10–4 cm2 V–1 s–1 for Zn­(2F-L2)2 and Zn­(3F-L2)2, consistent
with cofacial π–π stacking being favorable for
electron transport. The three complexes were tested in OPVs using
regioregular poly­(3-hexylthiophene) (P3HT) as the p-type material, and the best power conversion efficiencies were 5.2,
5.4, and 5.8% for Zn­(2F-L2)2, Zn­(1F-L2)2, and
Zn­(3F-L2)2, respectively, compared to 5.5% for Zn­(L2)2. The fluorination combination found in Zn­(3F-L2)2 resulted in the best device performance. This study points to a
viable strategy to increase the electron mobility and performance
of non-planar zinc­(II) complexes of azadipyrromethene.