Recent Advances towards the Synthesis and Material Applications of Indoloindoles

An important class of N-heteroacenes is indoloindoles which are air-stable, electron-rich and possess many tuneable properties. Initially, indoloindoles were explored for potential biological applications but current interest is based on their performance as photovoltaic materials. With growing applications of indoloindoles across multiple facets as organic functional materials, the need for efficient methods to synthesize and functionalize indoloindoles has taken a centre stage. Over the years, synthetic routes leading to indoloindoles have evolved from multistep protocols to one-pot multicomponent synthesis. Present literature boasts of a variety of reports that employ metals such as Cu, Ru, Rh, Pd, or Au to mediate the reaction towards indoloindoles. As alternatives to such metal-mediated methods, researchers have also developed metal-free and catalyst-free conditions. Indoloindoles, which are fundamentally fused-indoles, are often synthesized by transforming indole derivatives but methods that employ anilines or arynes as the starting substrates are equally abundant. The present review highlights the rich diversity and versatility of recent literature for the synthesis of indolo[3,2-b]indoles, indolo[2,3-b]indoles, indolo[7,6-g]indoles, and indolo[5,4-e]indoles. This review discusses protocols that were explicitly designed to obtain the above-mentioned indoloindoles and also explores several other methods that can be adapted to access said heteroacenes. Available mechanistic details pertaining to novel transformations have been detailed for the readers. Various applications where indoloindoles function as organic light-emitting diodes, organic field-effect transistors, solar cells, etc. have also been delved into before concluding with an outlook on future research.