Molecular Excitonics

In recent years, quantum biology has emerged as an exciting new field of inquiry. Non-trivial quantum mechanical phenomena, such as spin entanglement, wave-function delocalization, and proton tunneling, have been invoked in speculative mechanisms of biological magnetic field sensing, photosynthetic energy transport, enhanced ion transport and even smell.

Quantum Biology in popular press

Exciton Transport: Lessons from Quantum Biology

Quantum effects most clearly play a role in enhancing energy transport in photosynthetic bacteria and plants. These organisms make use of a hierarchy of tuned pigment-protein antenna complexes to capture light and transport it to the reaction center with near unity quantum yield. This energy is conducted in the form of an exciton (a coulombically bound electron-hole pair), which has both wave and particle character, even at room temperature. Excitons are neutral composite bosons, and which allows them to weakly interact with their environment, leading to delocalization over multiple chromophore sites. Transient delocalization, in turn, allows them to avoid low energy trap states, leading to fast efficient exciton diffusion, even in a disordered energetic environment at room temperature. Biology thus carefully tunes energetic heterogeneity to induce efficient directed energy transport.

Molecular Aggregates

Self-assembled molecular aggregates, consist of modular chromophores, whose energetic interactions can be tuned to produce large, structurally uniform nanostructures. Their geometry, morphology and chemical properties can be changed in analogy to photosynthetic antenna. Photosynthetic organisms make use of chlorophyll J-Aggregation to increase transport rates and redshift absorption and emission spectra. J-aggregates are aligned multi-chromophore networks with nearest neighbor symmetrical coupling that leads to an increase in oscillator strength and a stabilization of the lowest energy transition. These systems can be highly ordered and show long-range exciton delocalization, which, in turn suppresses large scale energetic fluctuations. Therefore, nanostructured J-aggregates represent excellent model systems for microscopically probing long-range exciton diffusion, and creating practical exciton antennas.newfigure5

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