Tunable Molecular Photonic Wires via Geometric Control of Excitonically Coupled Cyanine Dimer Relays

The assembly of molecular photonic wires (MPWs) on DNA scaffolds offers a powerful platform for controlling nanoscale energy transfer. This work demonstrates how the geometric configuration of an excitonic relay, composed of a cyanine (Cy5) dye dimer, regulates energy flow within an MPW. Exploiting linker chemistry, either H-type or J-type aggregates are selectively formed at room temperature. H-type dimers act as energy transfer inhibitors, while J-type dimers function as effective energy relays. In an optimized architecture, J-dimer MPWs outperform equivalent systems using monomeric relays. This performance is significantly amplified upon transitioning the system from solution to solid-state films, where the energy transfer efficiency of J-dimer wires is enhanced by up to 300% relative to monomeric versions. Experimental results also support approximating the dimers as single-point dipoles for Förster resonance energy transfer considerations. These results establish a robust strategy for engineering the optical properties of molecular materials, where nanoscale energy transport is precisely directed by controlling the geometry of excitonic aggregates.