Peroullazines: Synthesis, Structure, and Optoelectronic Properties

Replacement of benzenoid subunits by isoelectronic heteroaromatic motifs, N-doping, and π-extension provide complementary strategies for modulating the electronic structure and physical properties of polycyclic aromatic π-systems. Ullazine (indolizino[6,5,4,3-aij]quinoline) can be regarded as a π-isoelectronic heteroaromatic analogue of pyrene in which one six-membered benzenoid ring is replaced by a five-membered pyrrolic ring. Ullazine derivatives are of considerable interest in materials chemistry because this structural modification substantially alters their electronic properties relative to pyrene. Herein, we report peroullazine (PU), a longitudinally π-extended ullazine analogue of peropyrene, together with the pyridinic azaperoullazines 7-APU and 8-APU. Their synthesis combines Pd-catalyzed cross-coupling with two- and fourfold alkyne benzannulation. Replacing a pyrene substructure with an ullazine unit renders these π-systems markedly more electron-rich than the corresponding peropyrene and azaperopyrene counterparts, as reflected in substantially lower oxidation potentials. Most strikingly, this pyrene-to-ullazine replacement produces strongly red-shifted optical transitions and optical gaps approaching those of considerably more π-extended teropyrenes, despite the peropyrene-sized π-system. Pyridinic nitrogen incorporation further produces pronounced position-dependent effects, with substitution at the 7-position exerting a substantially stronger influence than substitution at the 8-position on the optical, electrochemical, and protonation responses.