Stepwise π-extension of double [5]helicene diimides to planar nanographene diimides.

A facile, stepwise synthetic route has been developed to access symmetric double [5]helicene diimides and their planar nanographene diimide counterparts via a C-shaped asymmetric [5]helicene. The synthetic strategy employs benzannulation and Scholl reaction methodologies to achieve progressive π-extension, yielding a new class of n-type rylene diimides with reversible redox characteristics. These helical and planar diimides exhibit variable crosswise π-conjugation and structural tunability, resulting in emission wavelengths that can be tailored alongside enhanced photoluminescence quantum yields-from 12% for the S-shaped diimide, to 57% for the C-shaped intermediate, and up to 63% for the fully planar nanographene diimide. Such properties make them promising candidates for quantum photonics, particularly as single-photon emitters. Photophysical properties, including time-resolved photoluminescence and transient absorption spectroscopy, reveal correlations between molecular structure, exciton dynamics, and emission behaviour. Notably, these helical rylene diimides demonstrate high photoluminescence efficiency in the solid state, reaching up to 32%, positioning them as strong contenders for next-generation optoelectronic devices.