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Organic electronics have attracted much attention as they promise great potential for applications as lightweight and exible electronic devices. In order to improve the performance of such devices, the development of improved charge─ transporting materials is of paramount importance. The molecular design of such materials should aim to control not only the electronic structure, but also the molecular orientation of these compounds in the solid state. For the development of materials with improved charge─ transporting properties, we focused on compounds with a quasiplanar structure, i.e. a structure that is slightly twisted from planarity. As a model skeleton, we designed and synthesized partially oxygen─ bridged triarylamines. On the basis of their quasiplanar skeletons as key units, we have developed hole─ transporting materials (HTMs) that were subsequently used in organic electronics devices, such as OLEDs and perovskite solar cells, and led to improved performances. This account article illustrates our endeavors regarding the development of such functional π ─ conjugated materials, with a focus on facile and ef cient synthetic strategies. Figure 1. Typical device structures of organic electronics and direction of the charge transport required therein. ( 100 ) J. Synth. Org. Chem., Jpn. 1128 OFETs. In the solid state, pentacene adopts a herringbone structure with a tilted molecular arrangement, which is not only due to π ─ π interactions, but also to CH─ π interactions (electrostatic interactions). 4 In order to obtain a better packing motif with facilitated charge mobility, introducing substituents at the peri ─ position of acenes or increasing the C/H ratio in the ring─ fused aromatic compounds should represent effective molecular design strategies, which could prevent CH─ π interactions. 2,5 However, strategies that allow the effective control over the molecular orientation in the solid state still remain limited. Conversely, triarylamines such as N,N ’─ diphenyl─ N,N ’─ di(m ─ tolyl)benzene (TPD) 6a and 4,4─ bis[N ─ (1─ naphtyl)─ N ─ phenylamino]biphenyl (α ─ NPD) 6b with a propeller─ like structures, are representative examples of π ─ skeletons used for hole─ transporting materials (HTMs) in OLEDs. These materials exhibit good amorphous stability, and are hence used as amorphous lms. In order to develop improved charge─ transporting materials, we focused on partially bridged triarylamines (Figure 2). In these amines, three phenyl groups are constrained in a quasiplanar fashion by two oxygen bridges, which results in a structure that is an intermediate between the planar structure of acenes and the propeller structure of triarylamines. We envisioned that the use of a quasiplanar structure as a key scaffold should: 1) afford enhanced planarity compared to triarylamines with propeller─ like structures, and thus lead to delocalized π ─ conjugation, and 2) induce dense π ─ stacking in the solid state on account of the slightly twisted molecular structure. The latter is based on the notion that the dispersion in the direction of the C─ H bonds at the lateral positions of the quasiplanar molecules should decrease the contribution of the CH─ π interactions between neighboring molecules, which could facilitate on─ top π ─ stacking in the solid state. As model compounds for oxygen─ bridged triphenylamines, we designed and synthesized dimers 1─ 4, 7 as well as the π ─ extended derivatives 5─ 7, which contain an azulene core. 8 The utility of these compounds as HTMs was demonstrated by their incorporation in OLEDs (1─ 4) 9 and perovskite solar cells (5─ 7) 8 (Figure 3). Two highly important points to consider during the molecular design and development of model compounds for organic electronics applications are structural simplicity and intrinsically high stability. These properties are crucial for the utility of such compounds in materials applications, given that such applications require ef cient synthetic methodologies that can be easily scaled. In this account, we summarize our research chemistry on oxygen─ bridged triphenylamine derivatives, especially with respect to their ef cient synthesis, their fundamental properties, their use as charge─ transporting materials, and their device performance. 2. Hole─ transporting Materials for Applications in OLEDs 2.1 Synthesis of Oxygen─ bridged Triphenylamine Derivatives Okada and co─ workers previously reported a synthetic methodology, in which doubly oxygen─ bridged triphenylamines were synthesized by aromatic intermolecular nucleophilic substitution of 2,6─ dichloronitrobenzene with o ─ bromophenol, followed by a reductive transformation of the nitro group into an amino group, and a twofold intramolecular amination. 10 Unfortunately, the overall yield in these reactions was moderate, due to the low yield of the twofold intramolecular amination. Thus, we developed an upgrade of this synthetic strategy, using a twofold intramolecular nucleophilic aromatic substitution as the key reaction step in order to furnish partially oxygen─ bridged triarylamines (Scheme 1). For example, 10a can be prepared in 66% yield by Ullmann reactions 11 of commercially available iodoanisole 8a and di uoroaniline 9a. After the removal of the protecting methyl groups in 10a with BBr 3, a treatment with K 2CO 3 in DMF at 100 °C resulted in the formation of cyclized product 11a in 95% yield. This synthetic route is straightforward and can be easily scaled

Partially Oxygen-Bridged Triphenylamines with a Quasiplanar Structure as a Key Scaffold for Hole-Transporting Materials