Double Protonation of 1,5-Bis(triarylaminoethynyl)anthraquinone To Form a Paramagnetic Pentacyclic Dipyrylium Salt

Protonation-induced intramolecular cyclization reactions of new donor (D)-acceptor (A) and D-A-D conjugated molecules 1-triarylaminoethynylanthraquinone (1-AmAq) and 1,5-bis(triarylaminoethynyl)anthraquinone (1,5-Am(2)Aq), respectively, were achieved. The former undergoes monoprotonation with bis(trifluoromethanesulfone)imide acid (TFSIH) to give pyrylium salt [1-AmPyl]TFSI, whereas the latter undergoes a novel double proton cyclization reaction to yield 1,5-bis(triarylamino)dipyrylium salt [1,5Am(2)Pyl(2)](TFSI)(2) with a new pentacyclic backbone. This divalent cationic salt can be reduced to give the neutral species 2,8-bis(triarylamino)benzo[de]isochromeno[1,8-gh]chromene ([1,5-Am(2)Pyl(2)](0)), which maintains the planar pentacyclic backbone. The obtained condensed-ring compounds show unique optical, electrochemical, and magnetic properties due to the extremely narrow HOMO-LUMO gap. In particular, the dication [1,5-Am(2)Pyl(2)](2+) shows paramagnetic behavior with two spins centered on two triarylamine moieties through valence tautomerization with the pentacyclic backbone.