Para-hexaphenyl layers exhibit dramatic changes in their steady-state optical and transient photoluminescence (PL) responses when they are grown with different orientations. For a film in which molecules stand perpendicular to the substrate, radiative recombination of intramolecular Frenkel singlet excitons occurs in less than 2 ps. If the layer is grown with molecules lying parallel to the substrate, short-lived and long-lived PL decay components are found. The former is best reproduced with an annihilation rate equation, characteristic of mobile excitons relaxing in 1 ns while the latter, spectrally redshifted, confirms intermolecular exciton migration.

• Received 11 January 2002

DOI:https://doi.org/10.1103/PhysRevB.65.212202