image: Femtosecond time-resolved single-particle spectroscopy directly visualized exciton diffusion within individual copper phthalocyanine（CuPc）nanofibers. The exciton diffusion coefficient（D）differs significantly between the η- and β-phase nanofibers, with faster diffusion in the η-phase. A “size effect” was also observed, where longer nanofibers exhibited lower D values, highlighting how molecular stacking and π–π interactions govern photoenergy transport in organic crystals.
Credit: American Chemical Society
In recent years, organic semiconductor materials have attracted considerable attention as key components for next-generation photoenergy conversion devices and organic solar cells due to their light weight and mechanical flexibility. A crucial factor determining their performance is how photoexcited excitons migrate between molecules, that is, the process of exciton diffusion. However, previous studies have provided only ensemble-averaged information, making it difficult to directly observe the diffusion behavior within individual crystals or nanostructures.
In this study, a research group led by Associate Professor Yukihide Ishibashi at the Graduate School of Science and Engineering, Ehime University, developed a femtosecond time-resolved single-particle spectroscopy technique, enabling the visualization of exciton diffusion in individual copper phthalocyanine（CuPc）nanofibers. CuPc molecular crystals are known to exhibit two crystalline phases —η（eta）and β（beta）—, which differ in molecular packing and the strength of π–π interactions.
The measurements revealed that the exciton diffusion coefficient of η-phase nanofibers is approximately three times greater than that of β-phase nanofibers, indicating longer-range energy transport. This enhancement arises from the larger molecular tilt angle and stronger π-electronic overlap in the η-phase, which lead to enhanced intermolecular excitonic coupling. Furthermore, even within the same crystalline phase, the diffusion coefficient exhibited a distribution, suggesting that microscopic defects and structural disorders influence exciton transport efficiency.
This work represents the first direct nanoscale observation of exciton diffusion in organic crystals, clarifying the relationship between molecular packing and photoenergy migration. The findings provide new design principles for achieving higher efficiency in organic photoenergy conversion and optoelectronic devices.
Journal
The Journal of Physical Chemistry Letters