Extended Tetrathiafulvalenes with Fluoreno[3,2‐b]fluorene and Diindeno[1,2‐b : 1′,2′‐i]anthracene Cores

In one‐dimensional polycyclic aromatic hydrocarbons (PAHs) containing five‐ and six‐membered rings fused together, one key question is whether the structures possess a quinoidal or aromatic diradical character. Here, we generate such PAHs by reversible oxidation of PAH‐extended tetrathiafulvalenes (TTFs). Extended TTFs were thus prepared and studied for their geometrical properties (crystallography), redox properties, and UV/Vis/NIR/EPR characteristics as a function of charge state. The EPR measurements of radical cations showed unique features for each PAH‐TTF. The dications, formally composed of fluoreno[3,2‐b]fluorene and diindeno[1,2‐b:1′,2′‐i]anthracene cores, were experimentally found to exhibit singlet ground states. For the latter, calculations reveal the closed shell, quinoid singlet state to be isoenergetic with the open shell singlet diradical. Each charge state exhibited unique optical properties with radical cations absorbing strongly in the NIR region with signatures from π‐dimers for the large core. The experimental results were paralleled and supported by detailed computations, including spin density distribution calculations, EPR simulations, and nucleus independent chemical shift (NICS) xy scans. Extended tetrathiafulvalenes (TTFs) with fluoreno[3,2‐b]fluorene and diindeno[1,2‐b:1′,2′‐i]anthracene cores were prepared and their redox and optical properties studied in comparison to the known extended TTF with an indeno[1,2‐b]fluorene core. The dications of these compounds were found to be singlet states that may correspond to closed shell quinoid structures or open shell singlet diradical structures ‐ for the diindeno[1,2‐b:1′,2′‐i]anthracene core, these structures are isoenergetic. The various charge states were differentiated by unique UV/Vis/NIR absorptions.