Triplet Generation Through Singlet Fission in Metal-Organic Framework: An Alternative Route to Inefficient Singlet-Triplet Intersystem Crossing

High quantum yield triplets, populated by initially prepared excited singlets, are desired for various energy conversion schemes in solid working compositions like porous MOFs. However, a large disparity in the distribution of the excitonic center of mass, singlet-triplet intersystem crossing (ISC) in such assemblies is inhibited, so much so that a carboxy-coordinated zirconium heavy metal ion cannot effectively facilitate the ISC through spin-orbit coupling. Circumventing this sluggish ISC, singlet fission (SF) is explored as a viable route to generating triplets in solution-stable MOFs. Efficient SF is achieved through a high degree of interchromophoric coupling that facilitates electron super-exchange to generate triplet pairs. Here we show that a predesigned chromophoric linker with extremely poor ISC efficiency (k(ISC)) but ES1 & GE;2ET1 ${{E}_{{S}_{1}}\ge {2E}_{{T}_{1}}}$ form triplets in MOF in contrast to the frameworks that are built from linkers with sizable k(ISC) but ES1 & LE;2ET1 ${{E}_{{S}_{1}}\le {2E}_{{T}_{1}}}$ . This work opens a new photophysical and photochemical avenue in MOF chemistry and utility in energy conversion schemes.