A Trapezoidal Octacyanoquinoid Acceptor Forms Solution and Surface Products by Antiparallel Shape Fitting with Conformational Dipole Momentum Switch

Mechanistic studies on single-electron transfer in frustrated Lewis pairs and its application to main-group chemistry

Advances in the field of frustrated Lewis pair (FLP) chemistry have led to the discovery of radical pairs, obtained by a single-electron transfer (SET) from the Lewis base to the Lewis acid. Radical pairs are intriguing for their potential to enable cooperative activation of challenging substrates (e.g., CH4, N2) in a homolytic fashion, as well as the exploration of novel radical reactions. In this review, we will cover the two known mechanisms of SET in FLPs-thermal and photoinduced-along with methods (i.e., CV, DFT, UV-vis) to predict the mechanism and to characterise the involved electron donors and acceptors. Furthermore, the available techniques (i.e., EPR, UV-vis, transient absorption spectroscopy) for studying the corresponding radical pairs will be discussed. Initially, two model systems (PMes3/CPh3+ and PMes3/B(C6F5)3) will be reviewed to highlight the difference between a thermal and a photoinduced SET mechanism. Additionally, three cases are analysed to provide further tools and insights into characterizing electron donors and acceptors, and the associated radical pairs. Firstly, a thermal SET process between LiHMDS and [TEMPO][BF4] is discussed. Next, the influence of Lewis acid complexation on the electron acceptor will be highlighted to facilitate a SET between (pBrPh)3N and TCNQ. Finally, an analysis of sulfonium salts as electron acceptors will demonstrate how to manage systems with rapidly decomposing radical species. This framework equips the reader with an expanded array of tools for both predicting and characterizing SET events within FLP chemistry, thereby enabling its extension and application to the broader domain of main-group (photo)redox chemistry.

Indandione-Terminated Quinoidal Compounds for Low-Bandgap Small Molecules with Strong Near-Infrared Absorption: Effect of Conjugation Length on the Properties

Low-bandgap organic semiconductors have attracted much attention for their multiple applications in optoelectronics. However, the realization of narrow bandgap is challenging particularly for small molecules. Herein, we have synthesized four quinoidal compounds, i. e., QSN3, QSN4, QSN5 and QSN6, with electron rich S,N-heteroacene as the quinoidal core and indandione as the end-groups. The optical bandgap of the quinoidal compounds is systematically decreased with the extension of quinoidal skeleton, while maintaining stable closed-shell ground state. QSN6 absorbs an intense absorption in the first and second near-infrared region in the solid state, and has extremely low optical bandgap of 0.74 eV. Cyclic voltammetry analyses reveal that the lowest unoccupied molecular orbital (LUMO) energy levels of the four quinoidal compounds all lie below -4.1 eV, resulting in good electron-transporting characteristics in organic thin-film transistors. These results demonstrated that the combination of π-extended quinoidal core and end-groups in quinoidal compounds is an effective strategy for the synthesis of low-bandgap small molecules with good stability.