Helical Nanographenes Bearing Pentagon‐Heptagon Pairs by Stepwise Dehydrocyclization

Abstract

The incorporation of pentagon–heptagon pairs into helical nanographenes lacks a facile synthetic route, and the impact of these pairs on chiroptical properties remains unclear. In this study, a method for the stepwise construction of pentagon–heptagon pairs in helical nanographenes by the dehydrogenation of [6]helicene units was developed. Three helical nanographenes containing pentagon–heptagon pairs were synthesized and characterized using this approach. A wide variation in the molecular geometries and photophysical properties of these helical nanographenes was observed, with changes in the helical length of these structures and the introduction of the pentagon–heptagon pairs. The embedded pentagon–heptagon pairs reduced the oxidation potential of the synthesized helical nanographenes. The high isomerization energy barriers enabled the chiral resolution of the helicene enantiomers. Chiroptical investigations revealed remarkably enhanced circularly polarized luminescence and luminescence dissymmetry factors with an increasing number of the pentagon–heptagon pairs.

Synthesis and resolution of multi-chiral carbonyl-N embedded hetero[7]helicenes for efficient circularly polarized luminescence

“…4 In particular, fascinating chiroptical properties, such as circular dichroism (CD) and circularly polarized luminescence (CPL), can be detected for helicenes owing to the chiral π-conjugated geometries and helically twisted molecular orbitals. 5 Generally, helicenes with both excellent luminescence dissymmetry factors ( g lum ) and photoluminescence quantum yields ( Φ PL ) are expected for ideal CPL materials. 6 However, most carbohelicenes suffer from low Φ PL due to the small oscillator strength for the radiative electronic transition.…”