Superradiance and Directional Exciton Migration in Metal–Organic Frameworks

Rajasree, S. S.; Yu, J.; Pratik, S. M.; Li, X.; Wang, R.; Kumbhar, A. S.; Goswami, S.; Cramer, C. J.; Deria, P.
J. Am. Chem. Soc. 2022, 144, 1396 (doi:10.1021/jacs.1c11979).

Crystalline metal–organic frameworks (MOFs) are promising synthetic analogs of photosynthetic light-harvesting complexes (LHCs). The precise assembly of linkers (organic chromophores) around the topology-defined pores offers the evolution of unique photophysical behaviors that are reminiscence of LHCs. These include MOF excited states with photo-absorbed energy that is spatially dispersed over multiple linkers defining the molecular excitons. The multi-linker molecular excitons display superradiance—a hallmark of coupled oscillators seen in LHCs—with radiative rate constant (k_rad) exceeding that of a single linker. Our theoretical model and experimental results on three zirconium MOFs, namely PCN-222(Zn), NU-1000, and SIU-100 with similar topology but varying linkers suggest that the size of such molecular excitons depend on the electronic symmetry of the linker. This multi-linker exciton model effectively predicts the energy transfer rate constant; corresponding single-step exciton hopping time, ranging from a few picoseconds in SIU-100 and NU-1000 to a few hundreds of picoseconds in PCN-222(Zn), matches well with the experimental data. The model also predicts the anisotropy of exciton displacement with preferential migration along the crystallographic c-axis. Overall, these findings establish various missing links defining the exciton size and dynamics in MOF-assembled linkers. The understandings will provide design principles—especially, positioning the catalysts or electrode relative to the linker orientation for low-density solar energy conversion systems.