Nonplanar Butterfly-Shaped π-Expanded Pyrrolopyrroles

Large aza-analogues of curved polycyclic aromatic hydrocarbons with a double-helicene structure present unique features for molecular photonics. We present the preparation and characterization of three such structures. The synthesis of these heterocyclic nanographenes involves only a few high-yield steps that use readily available starting materials. X-ray analysis revealed that each of these new dyes has three conformational isomers: one diastereoisomer in a meso form and two enantiomers in twisted forms [(P,P)] and [(M,M)]. The low energy barriers between the conformers, however, prevent their separation by using chiral HPLC, and the NMR spectra show only one set of signals for each of these curved compounds. Density functional theory (DFT) calculations quantify the small energy difference and the small energy barriers between the chiral and meso forms, which fully supports the experimental results. Their optical absorption lacks any sensitivity to the solvent environment, whereas their fluorescence features exhibit pronounced solvatochromism. This rarely observed solvatofluorochromism of centrosymmetric molecules without either electron-withdrawing groups or -donating substituents was probed by using time-resolved spectroscopy. These studies suggest that, similar to 9,9-bianthryl, the nonpolar locally excited state shows negligible solvatochromism, whereas the charge-transfer state is sensitive to solvent polarity.