Diazananographene with Quadruple [5]Helicene Units: Synthesis, Optical Properties, and Supramolecular Assembly

Abstract

A helical diazananographene (1) was successfully synthesized by employing sterically hindered t-butyl groups to inhibit further dehydrocyclization of [5]helicene units. These t-butyl groups stabilized the conformation of [5]helicene units, thus resulting in three stable conformers of 1, comprising a pair of enantiomers (1-(P, P, P, P) and 1-(M, M, M, M)) and a mesomer (1-(P, P, M, M)). In comparison to its planar analogs, helical 1 exhibited broadened peaks in both its absorption and emission spectra, leading to an increase in the emission quantum yield from 0.3 to 0.6. The significantly enhanced radiative decay rate (k _r) accounted for the increase in the quantum yield of 1. Additionally, it was observed that the compound could be fully protonated upon the addition of an equivalent acid. Furthermore, 1 assembled into a chiral trimeric metallosupramolecular complex upon coordination with the Pd^II units. Both protonated 1 and the metallosupramolecular complex exhibited an enhanced circular dichroic response. These findings revealed that the incorporation of a helical structure and pyridinic nitrogen-doping into the nanographene can allow the synthesis of responsive chiroptical graphenic materials, which could serve as fundamental components for constructing chiral hierarchical metallosupramolecular structures.