Synthesis and Excited-State Dynamics in Molecular Nanographene: Herzberg–Teller Vibronic Coupling and Energy Transfer to Porphyrins

Nanographenes (NGs) and graphene nanoribbons (GNRs) are molecular-level bridges to bulk-carbon materials. When synthesized with atomic precision via, for example, bottom-up strategies, a direct connection between the structure and properties is demonstrable. This is of key interest, especially considering practical applications. In the current work, we report the synthesis and comprehensive photophysical characterization of a full-benzenoid nanographene (NG-Br) and its covalent conjugate featuring a porphyrin (NG-(Zn)Por). Our synthetic approach relies on a cascade of Suzuki coupling, reduction, and Sandmeyer bromination reactions, starting from halogenated nitrobenzene derivatives. Knowing at which concentration aggregation occurs is important to study either monomers of NG-Br or its aggregates. In organic solvents, the association constant of NG-Br exceeds 1 × 10⁶ M^-1. Photophysical and theoretical analyses on the monomer revealed a subtle energy proximity between (S₁)/(L_b) and (S₂)/(L_a) that is the basis for strong vibronic coupling via the Herzberg-Teller mechanism, as well as (S_1,1) and (S_2,0) vibronic mixing. In NG-(Zn)Por, an ultrafast (S₁-S₁) energy transfer from NG to the porphyrin was observed. Our findings are essential for establishing an unambiguous structure-property relationship for NGs and 9-armchair GNRs, providing a blueprint for their use in optoelectronic devices ranging from single-electron transistors to OLEDs and organic solar cells.