The past couple of years have witnessed a remarkable burst in the development of organic field-effect transistors (OFETs), with a number of organic semiconductors surpassing the benchmark mobility of 10 cm2/(V s). In this perspective, we highlight some of the major milestones along the way to provide a historical view of OFET development, introduce the integrated molecular design concepts and process engineering approaches that lead to the current success, and identify the challenges ahead to make OFETs applicable in real applications.

Integrated materials design of organic semiconductors for field-effect transistors

Nanographenes, or extended polycyclic aromatic hydrocarbons, have been attracting renewed and more widespread attention since the first experimental demonstration of graphene in 2004. However, the atomically precise fabrication of nanographenes has thus far been achieved only through synthetic organic chemistry. The precise synthesis of quasi-zero-dimensional nanographenes, i.e. graphene molecules, has witnessed rapid developments over the past few years, and these developments can be summarized in four categories: (1) non-conventional methods, (2) structures incorporating seven- or eight-membered rings, (3) selective heteroatom doping, and (4) direct edge functionalization. On the other hand, one-dimensional extension of the graphene molecules leads to the formation of graphene nanoribbons (GNRs) with high aspect ratios. The synthesis of structurally well-defined GNRs has been achieved by extending nanographene synthesis to longitudinally extended polymeric systems. Access to GNRs thus becomes possible through the solution-mediated or surface-assisted cyclodehydrogenation, or “graphitization,” of tailor-made polyphenylene precursors. In this review, we describe recent progress in the “bottom-up” chemical syntheses of structurally well-defined nanographenes, namely graphene molecules and GNRs.

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New advances in nanographene chemistry

The advantages of organic field-effect transistors, such as low cost, mechanical flexibility and large-area fabrication, make them potentially useful for electronic applications such as flexible switching backplanes for video displays, radio frequency identifications and so on. A large amount of molecules were designed and synthesized for electron transporting (n-type) and ambipolar organic semiconductors with improved performance and stability. In this review, we focus on the advances in performance and molecular design of n-type and ambipolar semiconductors reported in the past few years.

25th Anniversary Article: Recent Advances in n‐Type and Ambipolar Organic Field‐Effect Transistors

Does the dehydrogenative coupling of aromatic compounds mediated by AlCl3 at high temperatures and also by FeCl3, MoCl5, PIFA, or K3[Fe(CN)6] at room temperature proceed by the same mechanism in all cases? With the growing importance of the synthesis of aromatic compounds by double C-H activation to give various biaryl structures, this question becomes pressing. Since some of these reactions proceed only in the presence of non-oxidizing Lewis acids and some only in the presence of certain oxidants, the authors venture the hypothesis that, depending on the electronic structure of the substrates and the nature of the "catalyst", two different mechanisms can operate. One involves the intermediacy of a radical cation and the other the formation of a sigma complex between the acid and the substrate. The goal of this Review is to encourage further mechanistic studies hopefully leading to an in-depth understanding of this phenomenon.

Comparison of Oxidative Aromatic Coupling and the Scholl Reaction

Azaacenes have been known for a very long time, either as N,N′-dihydro compounds or in their oxidized form as 4 n+2π systems, but only recently have processable and charcterizable derivatives been sought. In the last three years synthetic routes to large N-heteroacenes have been developed. In particular, the Pd-catalyzed coupling of aromatic diamines with activated aromatic dihalogenides has enabled simple access to numerous new azaacenes. Since 2010, azapentacene and stabile oligoazahexacene have been synthesized, as well as a symmetrical tetraazapentacene, which acts as an excellent electron-transport material for thin-film transistors.

Large N‐Heteroacenes: New Tricks for Very Old Dogs?

An exploratory study on novel silylethynylated N-heteropentacenes, which have their N atoms on the terminal rings of the pentacene backbone, is reported. This study leads to both p- and n-channel organic thin-film transistors with high field-effect mobility and also reveals that the position of the N atoms plays an important role in tuning the structures and properties of organic semiconductors based on N-heteropentacenes.

The Position of Nitrogen in N‐Heteropentacenes Matters

Reported here are two types of curved π-molecules that are π-isoelectronic to planar hexabenzocoronene (HBC) but are forced out of planarity either by an embedded seven-membered ring or by atom crowding at the fjord region. Embedding a heptagon in HBC leads to a novel saddle-shaped molecule 1, whose π-backbone is slightly less curved than the previously reported [7]circulene in terms of the average Gauss curvature, but surprisingly much more rigid than [7]circulene. Overcrowded fjord regions in novel derivatives of hexabenzoperylene (HBP) 2a,b lead to both chiral twisted and antifolded conformers. The successful synthesis of 1 and 2a,b is related to introducing alkoxyl groups to unprecedented positions of hexaphenylbenzenes. It is found that the red twisted isomer of 2b isomerizes at elevated temperature to the yellow anti-folded conformer. This finding along with the study on the thermodynamics and kinetics of the thermal isomerization has improved the early understandings on the conformation of HBP. In ...

Renxin Mao

Papers

The Position of Nitrogen in N‐Heteropentacenes Matters

Curved Polycyclic Aromatic Molecules That Are π-Isoelectronic to Hexabenzocoronene

Hydrogen-Bonded Dihydrotetraazapentacenes

Highly electron-deficient hexaazapentacenes and their dihydro precursors.