Dipole-Dependent Waveguiding in an Anisotropic Metal–Organic Framework

The interaction between excitons and photons underlies a range of emergent technologies, such as directional light emission, molecular lasers, photonic circuits, and polaritonic devices. Two of the key parameters that impact exciton-photon coupling are the binding energy of excitons and the relative orientations between the exciton dipole and photon field. Tightly bound excitons are typically found in molecular crystals, where nevertheless the angular relationship of excitons with photon fields is difficult to control. Here, we demonstrate directional exciton dipoles and photon fields, anchored by metal-ligand coordination. In a pyrene-porphyrin bichromophoric metal-organic framework (MOF), we observe that the perpendicular arrangement of the pyrene- and porphyrin-based exciton dipoles engenders orthogonal polarizations of their respective emissions. The alignment of the directional exciton and photon fields gives rise to an anisotropic waveguide effect, where the pyrene- and the porphyrin-based emissions show distinct spatial distribution within microplate-shaped MOF crystals. This capability to simultaneously host heterogenous excitonic states and anisotropic photon fields points toward MOFs' yet-to-be-realized potential as a platform for advancing the frontier in the field of exciton-photonics, which centers around engineering emergent properties from the interplay between excitons and photons.