Switching-Off to Switching-On Triplet Formation Through Singlet-Triplet Intersystem Crossing in Porphyrin Metal-Organic Frameworks.

Solid-state artificial photosystems need precise control over their singlet or triplet excited states to enable desired photochemical transformations. While persistent triplets are often selected to drive chemical bond formation or spin-specific photoreactions, singlets can allow for ambient applications. Among different heterogeneous systems, metal-organic frameworks (MOFs) offer a solution-stable platform with many benefits, including large chemically accessible interior surfaces where excitons can be transferred from their original formation sites. Porphyrins demonstrate high intersystem crossing efficiency, with QY ≈ 80% for free-base (FB) cores, which can be increased to about 92% for palladium-metalated cores. This study shows that framework assembly prevents ISC in FB-MOFs; whereas, ISC is enhanced in Pd-MOFs compared to monomeric linkers. The MOF topology influences the excited state dynamics, resulting in short-lived triplets with τ ≲10 ps in microporous MOFs. This extensive control over QY (from 0 to approximately 100%) and triplet behavior through framework assembly offers new design principles for creating artificial photosystems that operate exclusively in their singlet or triplet states manifold.