Piercing Molecular Graphenes: Precision Synthesis and Photophysics of NBN-Edged Porous Molecular Carbons

Abstract

Bottom-up solution-phase synthesis of atomically precise porous nanographenes is a challenging endeavor. In particular, molecular carbons with multiple pores and heteroatoms remain unknown. Herein, we report three porous molecular carbons (2PNG, 3PNG, and 7PNG) with precise NBN-doped zigzag edges, in which 7PNG possesses seven pores. X-ray crystallographic diffraction and scanning tunneling microscopy reveal their unique pore structures and self-assembly behaviors. Interestingly, the HOMO–LUMO overlap of these molecules gradually decreases as the size of the molecule increases, which induces peripheral-to-core excitations and promotes intersystem crossing. Steady-state and transient spectroscopy, along with DFT calculations, reveal the excited-state dynamics and the size-dependent energy-transfer mechanism in these NBN-doped molecular systems. Our study describes a new strategy for producing minimal wave function overlaps at almost planar geometry by segmenting the electronic structures of molecular graphene by insertion of pores, forcing the excitation to occur between the periphery and the core, with great potential for new phosphorescent and delayed fluorescence emitters.