Synthesis of a Highly Fluorescent Quinoxalino[2,3- b ]quinoxaline Polycyclic Derivative via Intramolecular Michael Addition to a Squaramide Ring

In the presence of TBAOH, bis-indolylsquaramide (1) converts into the highly emissive, novel bis(3H-pyrrolo[1,2,3-de]quinoxaline) exa-cyclic derivative 2. This compound was fully characterized in solution and the solid state, with emission properties supported by DFT calculations. Derivative 2 exhibits high quantum yield in solution with aggregation-induced quenching, and a large spectral shift between solid state and solution. Reaction conditions were optimized, and a mechanism involving a double intramolecular Michael addition triggered by deprotonation, oxidation and photodecarbonylation was proposed on the basis of DFT calculations and LC-MS measurements. In addition to reporting a novel, highly π-conjugated emissive compound, this manuscript highlights an unprecedented squaramide reactivity under basic conditions, resulting in the first example of intramolecular quinoxaline moiety formation from a squaramide derivative.