Topology-Directed π-Extension of Helical PAHs: Tunable Energy Gap and Excited-State Dynamics

The topology of π-extension critically dictates the physicochemical properties of helical systems. Herein, we report the synthesis and characterization of two novel helically π-extended polycyclic aromatic hydrocarbons (HPAHs), 1 and 2 , derived from per - peri -perbenzo[7]helicene ( PB[7]H ) via central and terminal ring fusion, respectively. 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 retards non-radiative decay compared to PB[7]H , highlighting the role of polyene-like electronic character in excited-state dynamics. Importantly, these topology-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.