Single-layer graphene has found broad applications in many emerging fields such as energy storage materials, bioengineering, photovoltaic devices. But zero band-gap of graphene possesses several limitations in the field of optoelectronics. In contrast, nanographene (NG), a precise segment of graphene, possess a tuneable finite band-gap along with other unique photophysical properties. Further, heteroatom-doped (B, N, O, S) NGs offer to modulate their physical and optoelectronic properties. Besides major application in the field of CPL and non-linear optics (NLO), helical NGs has versatile potential application in the field of the organic photovoltaics, photochromism, spin filter etc. However, due to the limitation of the traditional organic synthetic procedure, the library of cationic and N-doped nanographenes are still scarce. Here, we report a rollover π-expansion (RoPE) strategy for rapid access of a new class of nanographene via C-H activation/annulation at NHC platform. The helical nanographene molecules have a diameter of approx. 1.3 nm. This strategy involves a Rh(III)-catalyzed quadruple C–H activation followed by annulation with alkyne partner. Considering NHC as a template, a novel class of helical nanographene with a rare 6-6-5-6-6-6-5-6-6 skeleton are easily synthesized in a single-step procedure. Single crystal X-ray diffraction studies reveal twisted molecular geometry and intra-/inter-molecular π-π interactions. Isolation of bimetallic helicene intermediate, a metallo-nanographene supports the origin of helicity of the nanographene molecules. Furthermore, we succeed in the synthesis of a thiophene based photochromic nanographene using the same procedure. Further, we disclosed detailed photophysical properties for selected nanographene.

Rollover π-Expansion (RoPE): A unique approach to access N-enriched helical nanographene molecules

ACS Spring Meeting 2021

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