2,6-bis(arylethynyl)anthraquinones: Small-molecule acceptors for organic semiconductors

In recent years there have been a number of advances in the field of organic semiconductors (OSCs). Organic semiconductors are lightweight, flexible and inexpensive to use in devices such as organic field effect transistors (OFETs), organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs). There are two main criteria for organic molecules to work as semiconductors: They should have a small HOMO-LUMO band gap (1.5-3 eV) and should exhibit good charge mobility.;Bulk-heterojunction (BHJ) OPVs are designed with a donor and an acceptor. Most of the donors are conjugated polymers, while the acceptors are usually fullerene derivatives. Among the acceptors, phenyl C61 butyric acid methyl ester (PCBM) and its C60 homologues are dominant due to their favorable LUMO energies and reversible reduction properties. Several research groups are working on the development of small-molecule acceptors to replace PCBMs. Aromatic quinones are well-known acceptors, so the Revell group is interested in synthesizing some 2,6-diaryl-ethynyl anthraquinones.;There are some natural and synthetic anthraquinone derivative dyes like carmic acid and vat dyes that are reduced during the process of dyeing. Evaluation of several vat dyes shows a HOMO/LUMO band gap of 1.5-2.5 eV with good charge mobilities. These studies suggest that anthraquinones may be useful as acceptors in BHJ devices. This research is focused on the synthesis and opto-electrical characterization of compound 3a. as shown in Figure 1.;Compound 1 (2,6-dibromoanthraquinone) is the starting material to which compound 2a (deprotected trimethylsilyl (TMS) acetylene) was coupled by means of Sonogashira coupling in the presence of PdCl 2 (PPh3)2 and CuI catalysts, THF solvent, and triethylamine base to produce compound 3a. Compound 2a was synthesized from anthrone using TMS acetylene, n-BuLi, and a solvent mixture of THF : hexanes to give an intermediate compound 4a. It was then deprotected and coupled to compound 1.;Compound 3a was analyzed by proton NMR, MALDI-MS, UV-Vis, cyclic voltammetry and differential pulse voltammetry. Its presence can be confirmed from NMR spectra and MALDI data, but they are not clear due to impurities. UV-Vis spectra shows a maxima at 468 nm, cyclic voltammetry and differential pulse voltammetry shows a single electron reduction peak at -0.8 V using Ferrocene/Ferrocenium + as reference, and HOMO-LUMO values are calculated as -5.8 eV and -3.5 eV respectively, which is higher when compared to HOMO-LUMO values of other electron acceptors such as PCBMs (-6.1 eV and -3.75 eV). This higher LUMO value discourages compound 3a as a good electron acceptor for organic semiconductors.;Several attempts were made to derivatize 2a by addition of alkyl and thiol groups in order to manipulate the solubility and morphology of anthracyl ethynyl compounds, but these were each unsuccessful. For the better results, the LUMO energy of 2,6-bis (arylethynyl) anthraquinones should be lowered by adding electron-withdrawing groups to compound 2a.