Regioselective π‐Extension of Helical PAHs: Tuning Energy Gaps and Excited‐State Dynamics Directed by Clar's Sextet Migration

ABSTRACT The arrangement of Clar's sextets of π‐extension critically dictates the physicochemical properties of helical systems. Herein, we report the synthesis and characterization of two novel π‐extended helical polycyclic aromatic hydrocarbons (HPAHs), 1 and 2 , derived from per‐ peri ‐perbenzo[7]helicene ( PB[7]H ) via central and terminal ring fusion, respectively. These modifications induce Clar's sextet migration within the π‐framework. Structural analysis reveals that terminal extension in 2 yields a unique dispersion‐dominated “π‐clamped” geometry. While optical energy gaps decrease monotonically with extension size, oscillator strengths exhibit a non‐monotonic trend, a “decoupling” phenomenon rationalized by transition‐moment density (TMD) analysis, which visualizes spatial cancellations of transition densities. Furthermore, transient absorption spectroscopy demonstrates that the rigidified inner‐rim of 1 delays nonradiative decay compared to PB[7]H , highlighting the role of polyene‐like electronic character in excited‐state dynamics. Importantly, these Clar's sextet migration‐directed fusions bridge the discrete energy gaps of the homologous PB[ n ]H series ( n = 5, 7, 9). This work establishes a design strategy for the continuous fine‐tuning of chiroptical properties and dynamics in HPAHs beyond the limitations of discrete homologous series.