Femtosecond Single-Particle Spectroscopy of Exciton Diffusion in Individual Copper Phthalocyanine Nanofibers

Exciton diffusion in individual copper phthalocyanine (<b>CuPc</b>) nanofibers of β- and η-crystalline phases was investigated using femtosecond single-particle spectroscopy. The exciton diffusion coefficient (<i>D</i>) ranged from 1.6 × 10^-5 to 0.079 cm² s^-1 for β-<b>CuPc</b> and from 0.040 to 0.48 cm² s^-1 for η-<b>CuPc</b>, with the η phase showing an average <i>D</i> (0.16 cm² s^-1) nearly ten times larger than that of the β phase (0.020 cm² s^-1). The enhanced transport in η-<b>CuPc</b> is attributed to a stronger excitonic coupling and greater intermolecular overlap. The broad distributions of <i>D</i> within each phase strongly correlated with nanofiber size, with longer fibers exhibiting reduced diffusion, likely due to molecular packing misalignments and shape-induced electronic variations. These findings highlight the essential roles of the crystalline phase and morphology in governing exciton transport in organic nanostructures.