Molecular Architecture-Driven Enhancement of Multiphoton Absorption in a Branched Polyfluorene Relative to a Linear Analogue

Owing to their remarkable linear and nonlinear optical properties, conjugated polymers have attracted considerable attention as promising candidates for advanced photonic applications. However, achieving efficient multiphoton absorption through precise control of structural architecture remains a significant challenge. In this context, this work presents the synthesis and investigation of the photophysical properties, excited-state dynamics, and two- and three-photon absorption (2PA and 3PA) of a linear polyfluorene (PF) and a pyrene-centered branched polyfluorene (PFPy). The introduction of pyrene-centered branching significantly modulates the electronic structure in both solution and the solid state, leading to an average increase of ∼60% in the fluorescence quantum yield of PFPy relative to PF. A 4-fold lower nonradiative decay rate sustains this improvement in the branched system. Notably, PFPy exhibited 2PA and 3PA cross-sections of ∼4400 GM and ∼1.0 × 10–78 cm6·s2/photon2, respectively, about five- and two-folds higher than those of PF. Furthermore, normalizing the multiphoton absorption cross-sections as a function of the effective number of electrons highlights the intrinsic efficiency of the branched system. These findings indicate that the enhanced 2PA and 3PA responses of PFPy arise from the combined influence of the central pyrene chromophore and the multibranched architecture, with the 2PA analysis supporting a significant contribution from cooperative electronic coupling among the branches.