Nonalternant Nanographenes Containing N-Centered Cyclopenta[<i>ef</i>]heptalene and Aza[7]Helicene Units

Abstract

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[<i>ef</i>]heptalene unit. Among them, <b>CPH-2</b> and <b>CPH-3</b> can be viewed as hybrids of benzoannulated cyclopenta[<i>ef</i>]heptalene and aza[7]helicene. The crystal structures revealed a saddle geometry for <b>CPH-1</b>, a saddle-helix hybrid for <b>CPH-2,</b> and a twist-helix hybrid for <b>CPH-3</b>. Experimental measurements and theoretical calculations indicate that the saddle moieties in <b>CPHs</b> 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 <b>CPH-2</b>, 143.2 kcal mol^-1 for <b>CPH-3</b>). The combination of the nitrogen lone electron pairs of the N-doped cyclopenta[<i>ef</i>]heptalene unit with the twisted helix fragments results in rich photophysics with distinctive fluorescence and phosphorescence in <b>CPH-1</b> and <b>CPH-2</b> and the similar energy fluorescence and phosphorescence in <b>CPH-3</b>. Both enantiopure <b>CPH-2</b> and <b>CPH-3</b> display distinct circular dichroism (CD) signals in the UV-<i>vis</i> range. Notably, compared to the reported fully π-extended helical nanographenes, <b>CPH-3</b> exhibits excellent chiroptical properties with a |<i>g</i>_abs| value of 1.0 × 10^-2 and a |<i>g</i>_lum| value of 7.0 × 10^-3; these values are among the highest for helical nanographenes.