Design, Synthesis, and Optoelectronic Properties of π-Expanded Indoloindolizines.

The development of stable and tunable polycyclic aromatic compounds (PACs) is crucial for advancing organic optoelectronics. Conventional polycyclic aromatic hydrocarbons (PAHs), such as acenes, often suffer from poor stability due to photooxidation and oligomerization, which are linked to their frontier molecular orbital energy levels. To address these limitations, we have designed and synthesized a new class of π-expanded indoloindolizines by merging indole and indolizine moieties into a single polycyclic framework. Guided by the Glidewell-Lloyd rule—which predicts that in fused polycyclic systems, larger rings lose aromaticity in favor of smaller ones—we achieved precise modulation of the electronic structure by controlling the aromaticity of specific rings. Benzannulation at targeted positions allowed fine-tuning of the HOMO-LUMO gap, leading to distinct shifts in optoelectronic properties. We developed a scalable synthetic protocol to produce a wide range of π-expanded derivatives. The structural, electronic, and optical properties of these compounds were extensively characterized. Single-crystal X-ray diffraction confirmed their molecular structure, while theoretical calculations provided insights into the observed experimental trends. These indoloindolizines exhibit vivid colors and fluorescence across the visible spectrum, and their enhanced stability against photooxidation compared to acenes makes them promising candidates for practical applications in optoelectronic devices. Reactivity studies demonstrated high regioselectivity in electrophilic substitutions, highlighting the indole-like behavior of these compounds and opening avenues for further functionalization. Overall, our work establishes indoloindolizines as a promising platform for the development of stable, tunable organic materials for optoelectronic applications. By leveraging rational molecular design guided by the Glidewell-Lloyd rule, we offer a new pathway for molecular design in organic electronics, potentially enhancing device performance and longevity.