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

Abstract

The homoleptic zinc(II) complex of [2,8-di(1naphthylethynyl) 3,7-diphenyl 1,9-(4-hexylphenyl)azadipyrromethene (ZnL2) 2 ] is a promising non-planar nonfullerene 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 cm 2 V −1 s −1 for Zn(L2) 2 to 0.8−1.0 × 10 −3 cm 2 V −1 s −1 for the fluorinated complexes. Fluorination on the naphthyl groups increased the electron mobility from 4.2 × 10 −5 cm 2 V −1 s −1 for Zn(L2) 2 and Zn(1F-L2) 2 to 2.0 × 10 −4 cm 2 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.

Tuning the charge transport properties of non-planar zinc(ii) complexes of azadipyrromethenes using solubilizing groups

“…Here, we further analyse the crystal packing of these molecules, with the goal of gaining insights into charge-transport properties. 20,39 Tables S1-S4 (ESI †) shows calculated hole and electron overlap integrals (V h+ and V eÀ , absolute values) for all distinguishable pairs with |overlap integral| 4 0.1, and Fig. 3 shows the adjacent pairs that have the largest calculated overlap integrals.…”

“…This includes pyramidal subphthalocyanines such as Cl-BsubPc, used in vacuum deposited OPVs with an electron mobility ( μ e ) of ∼10 −5 cm 2 V −1 s −1 , 17 octahedral Alq3 used as an electron transport layer material for OLEDs with a low μ e of ∼10 −5 cm 2 V −1 s −1 , 18 and the distorted tetrahedral homoleptic metal( ii ) complexes of azadipyrromethenes (ADP), which we have used as electron acceptors in OPVs. 6 Interestingly, our zinc( ii ) complexes of ADP can have a relatively high μ e of ∼10 −4 to 10 −3 cm 2 V −1 s −1 , 19,20 perhaps because these non-planar molecules are fully conjugated thanks to four intramolecular π–π stacking interactions between the two ligands. 5…”

“…[22][23][24][25][26] We showed that fluorination can also be beneficial for non-planar systems: fluorination of the naphthylethynyl groups of Zn(L2) 2 promotes co-facial interactions between the naphthylethynl groups of two adjacent molecules, leading to greater order and increase of both electron and hole mobility by an order of magnitude to 1 Â 10 À4 cm 2 V À1 s À1 and 2 Â 10 À3 cm 2 V À1 s À1 , respectively. 20 Another common method to guide film self-assembly and optimize charge transport of solution-processable p-conjugated materials is by optimizing the solubilizing groups nature, length, branching, and placement on the conjugated moiety, also known as side chain engineering. [27][28][29] Side-chain engineering can be used to control steric hindrance, which affects solubility, conformation and molecular packing in films.…”

“…22–26 We showed that fluorination can also be beneficial for non-planar systems: fluorination of the naphthylethynyl groups of Zn(L2) 2 promotes co-facial interactions between the naphthylethynl groups of two adjacent molecules, leading to greater order and increase of both electron and hole mobility by an order of magnitude to 1 × 10 −4 cm 2 V −1 s −1 and 2 × 10 −3 cm 2 V −1 s −1 , respectively. 20…”

Photophysical, thermal, and DFT studies on a tetraaryl-azadipyrromethene ligand and its zinc(II) complex

“…The transition-metal coordination chemistry of these ligands, which are noted for their intense absorption in the red region of the spectrum, has remained underexplored. Coordination of azadipyrromethenes with Zn(II) gives M(L) 2 -type homoleptic complexes with distorted tetrahedral geometry [ 18 , 19 ]. In the search for beneficial photophysical features, thermal stability, photostability, and singlet oxygen production properties, a new homoleptic azadipyrromethene metal complex was synthesized.…”