Light-Harvesting and Ultrafast Energy Migration in Porphyrin-Based Metal–Organic Frameworks

Given that energy (exciton) migration in natural photosynthesis primarily occurs in highly ordered porphyrin-like pigments (chlorophylls), equally highly ordered porphyrin-based metal–organic frameworks (MOFs) might be expected to exhibit similar behavior, thereby facilitating antenna-like light-harvesting and positioning such materials for use in solar energy conversion schemes. Herein, we report the first example of directional, long-distance energy migration within a MOF. Two MOFs, namely <b>F-MOF</b> and <b>DA-MOF</b> that are composed of two Zn­(II) porphyrin struts [5,15-dipyridyl-10,20-bis­(pentafluorophenyl)­porphinato]­zinc­(II) and [5,15-bis­[4-(pyridyl)­ethynyl]-10,20-diphenylporphinato]­zinc­(II), respectively, were investigated. From fluorescence quenching experiments and theoretical calculations, we find that the photogenerated exciton migrates over a net distance of up to ∼45 porphyrin struts within its lifetime in <b>DA-MOF</b> (but only ∼3 in <b>F-MOF</b>), with a high anisotropy along a specific direction. The remarkably efficient exciton migration in <b>DA-MOF</b> is attributed to enhanced π-conjugation through the addition of two acetylene moieties in the porphyrin molecule, which leads to greater Q-band absorption intensity and much faster exciton-hopping (energy transfer between adjacent porphyrin struts). The long distance and directional energy migration in <b>DA-MOF</b> suggests promising applications of this compound or related compounds in solar energy conversion schemes as an efficient light-harvesting and energy-transport component