Charge transfer (CT) is an essential phenomenon relevant to numerous fields including biology, physics and chemistry.1-5 Here, we demonstrate that multi-layered hyperbolic metamaterial (HMM) substrates alter organic semiconductor CT dynamics.6 With triphenylene:perylene diimide dyad supramolecular self-assemblies prepared on HMM substrates, we show that both charge separation (CS) and charge recombination (CR) characteristic times are increased by factors of 2.5 and 1.6, respectively, resulting in longer-lived CT states. We successfully rationalize the experimental data by extending Marcus theory framework with dipole image interactions tuning the driving force. The number of metal-dielectric pairs alters the HMM interfacial effective dielectric constant and becomes a solid analogue to solvent polarizability. Based on the experimental results and extended Marcus theory framework, we find that CS and CR processes are located in normal and inverted regions on Marcus parabola diagram, respectively. The model and further PH3T:PCBM data show that the phenomenon is general and that molecular and substrate engineering offer a wide range of kinetic tailoring opportunities. This work opens the path toward novel artificial substrates designed to control CT dynamics with potential applications in fields including optoelectronics, organic solar cells and chemistry.
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Plasmonic nanostructures have recently been shown to alter the photonic density of states and to provide opportunities to control semiconductor photophysical properties.1-4 Experimentally and theoretically,5 we investigated the effects of a range of hyperbolic metamaterial (HMM) lamellar structures consisting of metal and dielectric multilayers on the photoluminescence (PL) lifetime of several organic chromophores which emission range from UV to visible. These molecules were immersed in a polymeric matrix spin-coated on top of the HMM substrates and streak camera measurements were completed to monitor the evolution of the chromophores spontaneous emission. The ratio of the PL lifetimes of chromophores located on top of HMM nanostructures and on top of fused silica was shown to vary in a non-monotonous way. We then showed that normalized PL lifetime of the chromophore strongly depends on the HMM phase and the number of metal-dielectric pairs. To analyze systematically this behavior and fully understand the involved mechanisms, we also developed a theoretical analysis and took advantage of both invariant imbedding method and FDTD simulation as computational tools to quantitatively explain the experimental results and predict the responses, which could be observed when varying further the HMM nanostructures.
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5. K. J. Lee, , et al. In preparation, 2016.