Nonalternant Nanographenes Containing N‑Centered Cyclopenta[ef]heptalene and Aza[7]Helicene Units

Introducing helical subunits into negatively curved π-systems has a significant effect on both the molecular geometry and photophysical properties; however, the synthesis of these helical π-systems embedded with nonbenzenoid subunits remains challenging due to the high strain deriving from both the curvature and helix. Here, we report a family of nonalternant nanographenes containing a nitrogen (N)-doped cyclopenta­[ef]­heptalene unit. Among them, CPH-2 and CPH-3 can be viewed as hybrids of benzoannulated cyclopenta­[ef]­heptalene and aza[7]­helicene. The crystal structures revealed a saddle geometry for CPH-1, a saddle-helix hybrid for CPH-2, and a twist-helix hybrid for CPH-3. Experimental measurements and theoretical calculations indicate that the saddle moieties in CPHs undergo flexible conformational changes at room temperature, while the aza[7]­helicene subunit exhibits a dramatically increased racemization energy barrier (78.2 kcal mol–1 for CPH-2, 143.2 kcal mol–1 for CPH-3). The combination of the nitrogen lone electron pairs of the N-doped cyclopenta­[ef]­heptalene unit with the twisted helix fragments results in rich photophysics with distinctive fluorescence and phosphorescence in CPH-1 and CPH-2 and the similar energy fluorescence and phosphorescence in CPH-3. Both enantiopure CPH-2 and CPH-3 display distinct circular dichroism (CD) signals in the UV–vis range. Notably, compared to the reported fully π-extended helical nanographenes, CPH-3 exhibits excellent chiroptical properties with a |g abs| value of 1.0 × 10–2 and a |g lum| value of 7.0 × 10–3; these values are among the highest for helical nanographenes.