Design principles for long-range energy transfer at room temperature.
Typical room temperature conditions hinder ballistic long-range transfer of excitations, rendering quantum phenomena unimportant as potential tools for the design of efficient and controllable energy transfer over significant time and length scales. However, it is well-known that many properties of macroscopic systems depend on the quantum properties of minimal repeating units and, as we show here, excitonic energy transfer is no exception. With the support of an exactly solvable model, we are able to show how exciton delocalization and the ensuing formation of dark states within unit cells can be harnessed to support classical propagation over macroscopic distances. We specifically discuss the role of such factors in nano-fabricated arrays of bacterial photosynthetic complexes via extensive simulations. This allows us to resolve the to-date unexplained experimental observation of exciton diffusion lengths in such arrays in terms of an interplay between intra-unit cell thermalization and delocalization, which conspire to create and use robust dark states at room temperature.