The photophysics of luminescence in multilayered organic nanofibers

Multilayered crystalline nanofibers are exemplary model system for the study of exciton dynamics and lasing in organic materials due to their well-defined morphology, high luminescence efficiencies, and color tunability. The multi-layered nanofibers are composed of alternating layers of two different organic molecules (para-hexaphenyl (p6P) donor and α-sexithiophene (6T) acceptor materials). The excited donor (p6P) is able to sensitize the acceptor (6T) via resonance energy transfer (Förster) as a consequence of the overlap between the p6P photo emission spectrum and the 6T absorption spectrum. The highly efficient exciton diffusion in p6P and energy transfer to 6T in these nanofibers is quantified by time-resolved photoluminescence (PL) studies, where the extracted amplitudes and time constants are used to explain the PL temperature dependence obtained in steady state measurements. This enables us to extract the activation energy for exciton diffusion in crystalline p6P (19 meV). The weak PL temperature dependence of the acceptor material (6T) and hence its high quantum yield in combination with the small activation energy for exciton diffusion of the donor material (p6P) explain the high efficiency of the exciton diffusion and energy transfer in this system.