The mechanism of long-range exciton diffusion in a nematically organized porphyrin layer

The exciton diffusion length in a nematically organized meso-tetra(4-n-butylphenyl)porphyrin (TnBuPP) layer was found to exceed 40 nm at a temperature of 90 K and to be equal to 22 ± 3 nm at 300 K. The exciton diffusion coefficient decreases from =3.1 × 10-6 m2/s at 90 K to (2.5 ± 0.5) × 10-7 m2/s at 300 K. This thermal deactivation is attributed to exciton motion via a band mechanism. The motion of an exciton is not limited by polaronic effects; that is, the deformation of the atomic lattice around the exciton. The absence of polaronic self-trapping implies that the exciton diffusion coefficient can be enhanced by improvement of structural order and rigidity of the material.