Efficient Exciton Diffusion and Resonance-Energy Transfer in Multilayered Organic Epitaxial Nanofibers

Abstract

Multilayered epitaxial nanofibers are exemplary model systems for the study of exciton dynamics and lasing in organic materials because of their well-defined morphology, high luminescence efficiencies, and color tunability. We use temperature-dependent continuous wave and picosecond photoluminescence (PL) spectroscopy to quantify exciton diffusion and resonance-energy transfer (RET) processes in multilayered nanofibers consisting of alternating layers of para-hexaphenyl (p6P) and α-sexithiophene (6T) serving as exciton donor and acceptor material, respectively. The high probability for RET processes is confirmed by quantum chemical calculations. The activation energy for exciton diffusion in p6P is determined to be as low as 19 meV, proving p6P epitaxial layers also as a very suitable donor material system. The small activation energy for exciton diffusion of the p6P donor material, the inferred high p6P-to-6T resonance-energy-transfer efficiency, and the observed weak PL temperature dependence of the 6T acceptor material together result in an exceptionally high optical emission performance of this all-organic material system, thus making it well suited, for example, for organic light-emitting devices.

Modeling temperature dependent singlet exciton dynamics in multilayered organic nanofibers

“…The model begins by reproducing the system's morphology used in the experimental study, 4 that is, 10 alternating layers of P6P and 6T. This is done by creating a matrix whose first column corresponds to the 6T monolayer followed by a number L of columns representing the number of P6P monolayers that compose the actual P6P layer (Fig.…”

“…In order to obtain the best values for the five parameters, we took advantage of the results of time-resolved photoluminescence measurements performed at several different temperatures. 4 In the genetic algorithm, the five parameters of the model (L, ρ ex , P6P d/R F , 6T d/R F , P XX ) are the genes. We evaluate the "fitness" of our simulations quantitatively by calculating the root mean squared deviation between the simulated curve and the experimental one.…”

“…3 Among the molecules often employed in this kind of study, two stand out for their capacity of self-assembling into nanofibers: parahexaphenyl (P6P) and α-sexithiophene (6T). [4][5][6][7][8] In different materials, time-resolved photoluminescence (PL) measurements allow for the probing of exciton dynamics. In P6P/6T multilayered nanofibers, experimental results 4 show that exciton diffusion increases with temperature, changing the optical properties of the system significantly.…”

“…[4][5][6][7][8] In different materials, time-resolved photoluminescence (PL) measurements allow for the probing of exciton dynamics. In P6P/6T multilayered nanofibers, experimental results 4 show that exciton diffusion increases with temperature, changing the optical properties of the system significantly. This feature results from exciton migration from one material to the other.…”

“…These are, in turn, compared to the experimental results performed in a system composed of 10 alternating monolayers of 6T (∼ 4 Å of thickness) and thicker (∼160 Å) P6P layers. 4 To find the model parameters that better reproduce the physical picture and understand the effects of temperature in exciton diffusion, we use a genetic algorithm (GA) [20][21][22][23][24] that approximates simulated time-resolved PL curves to experimental ones.…”