Protonation-induced cyclization of 1,8-bis(arylethynyl)anthraquinones: Monopyrylium salt formation and intensification of donor-acceptor interaction

We previously reported protonation-induced double cyclization reaction of 1,4-Ar₂Aq and 1,5-Ar₂Aq (Ar₂Aq: bis(arylethynyl) anthraquinone) with strong acid HX that generated the corresponding dipyrylium salts [1,4-Ar₂Pyl₂]X₂ and [1,5-Ar₂Pyl₂]X₂. In this communication we disclose the protonation reactions of 1,8-Fc₂Aq (Fc: ferrocenyl), 1,8-Am₂Aq (Am: 4-N,N-bis(4-methoxyphenyl)aminophenyl), and 1,8-AmFcAq, which is the first example of heterodonor molecules in the Ar ₂Aq series, and synthesized by means of stepwise SonogashiraHagihara cross-coupling reactions. In contrast to the 1,4-Ar₂Aq and 1,5-Ar₂Aq series, 1,8-Ar₂Aq undergoes protonation-induced single cyclization, so that it is converted into the corresponding monopyrylium salt [1,8-Ar₂Pyl]X. [1,8-Ar₂Pyl]X features an extremely small HOMOLUMO gap (0.50-0.73 V), ascribable to the significant lowering of the LUMO level upon the pyrylium formation.