Nonalternant Nanographenes Containing N-Centered Cyclopenta[ef]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[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.

Benzo-Extended Heli(aminoborane)s: Inner Rim BN-Doped Helical Molecular Carbons with Remarkable Chiroptical Properties

“…However, the g lum value of B has not been reported in the literature. The “laterally π-extended helicenes”, with simultaneous lateral and longitudinal extensions of the helical structures, or 3D π-extended helicenes have shown to have higher values of g lum than their parent single-stranded helicene derivatives. − The impressive π-extended pentadecabenzo[9]helicene (Figure a, C ) exhibited a g lum value of 0.045 (Φ PL = 0.10); however, its g abs value was approximately 0.0276. For these large “superhelicene” systems, a concurrent increase of the g abs and g lum values is challenging, and simultaneously achieving a high Φ PL in a single helicene is difficult.…”