Thienoisoindigo (TII)‐Based Quinoidal Small Molecules for High‐Performance n‐Type Organic Field Effect Transistors

Abstract

A novel quinoidal thienoisoindigo (TII)-containing small molecule family with dicyanomethylene end-capping units and various alkyl chains is synthesized as n-type organic small molecules for solution-processable organic field effect transistors (OFETs). The molecular structure of the 2-hexyldecyl substituted derivative, TIIQ-b16, is determined via single-crystal X-ray diffraction and shows that the TIIQ core is planar and exhibits molecular layers stacked in a "face-to-face" arrangement with short core intermolecular distances of 3.28 Å. The very planar core structure, shortest intermolecular N•••H distance (2.52 Å), existence of an intramolecular non-bonded contact between sulfur and oxygen atom (S•••O) of 2.80 Å, and a very low-lying LUMO energy level of −4.16 eV suggest that TIIQ molecules should be electron transporting semiconductors. The physical, thermal, and electrochemical properties as well as OFET performance and thin film morphologies of these new TIIQs are systematically studied. Thus, air-processed TIIQ-b16 OFETs exhibit an electron mobility up to 2.54 cm 2 V −1 s −1 with a current ON/OFF ratio of 10 5-10 6 , which is the first demonstration of TII-based small molecules exhibiting unipolar electron transport characteristics and enhanced ambient stability. These results indicate that construction of quinoidal molecule from TII moiety is a successful approach to enhance n-type charge transport characteristics.

Quinoidal Small Molecule Containing Ring-Extended Termini for Organic Field-Effect Transistors

“…To establish a quinoidal structure, electron-withdrawing groups (EWGs) should be used at termini. A few EWGs are used for the terminal groups of quinoidal molecules such as dicyanomethylene (DCM), quinone, carbonyl, and para -quinodimethane, as shown in Figure . − The DCM group is generally used for quinoidal oligothiophenes with high performance in OFET devices. − However, the DCM-terminated quinoidal molecules require rather complex procedures, including the preparation of dibrominated aromatic compounds, Pd-catalyzed Takahashi coupling reaction, and oxidation of bis­(dicyanomethyl)-precursors by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. , …”

Balanced Ambipolar Charge Transport in Phenacene/Perylene Heterojunction-Based Organic Field-Effect Transistors

“…Although the search for n-type organic compounds for high-mobility and air-stable transistors continues to progress, as demonstrated by recent publications, , PDIF-CN 2 remains a well-known and commercially available n-type organic semiconductor combining excellent self-assembling properties and remarkable stability in air. , Thanks to the presence of cyano (−CN) groups in the bay region and of the fluoroalkyl (−CH 2 C 3 F 7 ) side chains, the PDIF-CN 2 surface is highly hydrophobic, and the transistor bearing PDIF-CN 2 active channels can work even in a liquid environment . In the recent past, PDIF-CN 2 was also used, in combination with rubrene, to study the intriguing behavior of single-crystal organic heterojunctions…”

End-Capping π-Conjugated Naphthodithiophene Diimide (NDTI)-Based Triads with Noncovalent Intramolecular S···O Interactions: A Route towards High-Performance Solution-Processable Air-Stable n-Type Semiconductors

“…Much evidence shows that the noncovalent S···O interactions are usually contained in thiophene and alkoxy chains or carbonyls containing oxygen atoms. In recent years, these noncovalent bond forces have been applied for the exploitation of high-performance π-conjugated small-molecule and polymer semiconductors for organic solar cells (OSCs) − and OFETs (Figure a,b). − In 2009, Guo et al reported the first example of introducing the noncovalent S···O interactions into PhBT copolymers for OFETs, with a hole mobility of 0.17 cm 2 V –1 s –1 in air (Figure a ( PhBT12 )). In 2012, Marks and co-workers further proposed the noncovalent S···O “conformational locks” concept to construct highly planar π-systems (Figure a ( P3 ) and Figure b ( SM1 )) for enhancing FET carrier mobilities .…”

“…Up to now, a variety of small-molecule and polymer semiconductors with different transmission properties for OFETs have been obtained by this method. However, as far as we know, n-type small-molecule semiconductors constructed by this method are mainly concentrated in quinone-based molecular systems with the presence of noncovalent intramolecular S···S or S···O interactions …”

“…However, as far as we know, n-type small-molecule semiconductors constructed by this method are mainly concentrated in quinonebased molecular systems with the presence of noncovalent intramolecular S•••S or S•••O interactions. 44 Naphthodithiophene diimide (NDTI) as a potential electron-deficient π-conjugation functional module has characteristic features such as a planar configuration, a low LUMO energy level, and chemical diversity; 46 small-molecule and polymer semiconductors with different transport properties have been widely constructed by chemical modifications of two active α-positions of lateral thiophene in NDTI. 47−52 As we know, in NDTI, the two sulfur atoms exposed to the outer end on the two thiophene groups provide the possibility of constructing the noncovalent intramolecular S•••O locks with the groups containing oxygen atoms.…”