Symmetry-Breaking Charge Transfer in Metal–Organic Frameworks

Abstract

High quantum-yield charge carrier generation from the initially prepared excitons defines a key step in the light-harvesting and conversion scheme. Photoinduced charge transfer in molecular electron donor-acceptor assemblies is driven by a sizable Δ<i>G</i>₀, which compromises the potential of the generated carriers. Reminiscent of the special pair at the reaction center of the natural light-harvesting complex, symmetry-breaking charge transfer (SBCT) within a pair of identical struts of metal-organic framework (MOF) will facilitate the efficient generation of long-lived charge carriers with maximized potentials without incorporating any foreign redox species. We report SBCT in pyrene-based zirconium metal-organic framework (MOF) NU-1000 that leads to efficient generation of radical ions in a polar solvent and bound CT states in a low-polar solvent. The probe unveils the role of the low-lying non-Franck-Condon excitonic states as intermediates in the formation of the SBCT state from the initially prepared Franck-Condon S₁ states. Ultrafast and transient spectroscopy─probed over 200 fs-30 μs time scale─evinces a <i>k</i>_SBCT = (110 ps)^-1 in polar media (ε_s = 37.5) forming solvated radical ions with recombination rate <i>k</i>_CR = (∼45 ns)^-1. A slower rate with <i>k</i>_SBCT = (203 ps)^-1 was recorded in low-polar (ε_s = 7.0) solvent manifesting a bound [TBAPy^•+ TBAPy^•-] state with <i>k</i>_CR ≈ (17 μs)^-1. This discovery, along with other unique photophysical features relevant to light harvesting, should define a MOF-based platform for developing heterogeneous artificial photon energy conversion systems.