Acenaphthylene-fused ullazines: fluorescent π-extended monopyrroles with tunable electronic gaps

pi-Extended dibenzoullazines containing an acenaphthylene subunit were designed and synthesized. Two different synthetic strategies were employed: route A, based on Pd-catalyzed cyclodehydrohalogenation of alpha,alpha-disubstituted N-arylpyrroles, and route B, using a dipolar 1,3-cycloaddition reaction of azomethine ylides (PAMYs) to functionalized acenaphthylenes. Molecules of the resulting ullazines are almost flat, leading to strong pi-pi interactions in the solid state. The new ullazines are highly fluorescent (with a quantum yield of up to 0.89 for the naphthalimide-fused system), and show moderate solvatochromism with no fluorescence quenching in polar solvents. Stepwise two-electron oxidation of the ullazines is possible, yielding reversibly the corresponding ullazine radical cations and dications. Edge expansion of the ullazine core with methylene bridges is additionally shown to produce an ullazine analogue containing two seven-membered rings in its structure, which is characterized by axial chirality and can be resolved into enantiomers.

Automated summary

In this study, a series of pi-extended dibenzoullazines containing an acenaphthylene subunit were successfully synthesized using two different synthetic strategies. These compounds exhibit strong fluorescence and moderate solvatochromism, making them potentially useful in various applications. Additionally, a chiral ullazine analogue with axial chirality was discovered and successfully resolved into enantiomers.