Ladder Oligo(m-aniline)s: Derivatives of Azaacenes with Cross-Conjugated π-Systems

We describe the synthesis and electronic properties of ladder oligomers of poly­(m-aniline) that may be considered as derivatives of azaacenes with cross-conjugated π-systems. Syntheses of ladder oligo­(m-aniline)­s with 9 and 13 collinearly fused six-membered rings employed Pd-catalyzed aminations and Friedel–Crafts-based ring closures. Structures were confirmed by either X-ray crystallography or correlations between DFT-computed and experimental spectroscopic data such as 1H, 13C, and 15N NMR chemical shifts and electronic absorption spectra. All compounds have planar “azaacene” moieties. The experimental band gaps Eg ≈ 3.5–3.65 eV, determined by the UV–vis absorption onsets, were in agreement with the TD-DFT-computed vertical excitation energies to the S1 state. Fluorescence quantum yields of up to 20% were found. Electrochemically estimated HOMO energies of −4.8 eV suggested propensity for a facile one-electron oxidation and just sufficient environmental stability toward oxygen (O2). For two oligomers with “tetraazanonacene” moieties, potentials of E4+/3+ ≈ 1.6–1.7 V vs SCE were determined for four-electron oxidation to the corresponding tetraradical tetracations.