From Structure to Function: Nonlinear Optical Properties and Excited-State Dynamics of Ladder-Type π-Conjugated Benzo[<i>k</i>]tetraphene Oligomers

Ladder-type oligomers are emerging building blocks for optoelectronic and quantum applications, owing to their rigid, planar backbones that enhance charge transport, exciton delocalization, and structural stability compared to their linear counterparts. However, the fundamental structure–function relationships in unsubstituted ladder systems remain largely unexplored, as most studies focus on chemically modified derivatives. In this work, we investigate the excited-state dynamics and nonlinear optical (NLO) properties of a series of unsubstituted benzo[k]tetraphene (BTp) ladder-type oligomers with systematically extended conjugation lengths (BTp 5, BTp 7, BTp 9, and BTp 11). Our results reveal that electron delocalization, induced by the extended conjugation, facilitates intersystem crossing (ISC) that enhances efficient formation of the triplet excited states. However, both classical and entangled two-photon absorption measurements show a clear saturation of the NLO response beyond a critical conjugation threshold. This trend is consistent with excited-state lifetimes, which remain largely unchanged with further molecular extension, revealing a critical limit of the electronic tunability in the BTp series. These findings provide fundamental insights for optimizing the excited-state dynamics and NLO properties of unsubstituted BTps.