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

Exciton diffusion in individual copper phthalocyanine (CuPc) nanofibers of β- and η-crystalline phases was investigated using femtosecond single-particle spectroscopy. The exciton diffusion coefficient (D) ranged from 1.6 × 10^-5 to 0.079 cm² s^-1 for β-CuPc and from 0.040 to 0.48 cm² s^-1 for η-CuPc, with the η phase showing an average D (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-CuPc is attributed to a stronger excitonic coupling and greater intermolecular overlap. The broad distributions of D 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.