Förster Energy Transport in Metal–Organic Frameworks Is Beyond Step-by-Step Hopping

Metal–organic frameworks (MOFs) with light-harvesting building blocks designed to mimic photosynthetic chromophore arrays in green plants provide an excellent platform to study exciton transport in networks with well-defined structures. A step-by-step exciton random hopping model made of the elementary steps of energy transfer between only the nearest neighbors is usually used to describe the transport dynamics. Although such a nearest neighbor approximation is valid in describing the energy transfer of triplet states via the Dexter mechanism, we found it inadequate in evaluating singlet exciton migration that occurs through the Förster mechanism, which involves one-step jumping over longer distance. We measured migration rates of singlet excitons on two MOFs constructed from truxene-derived ligands and zinc nodes, by monitoring energy transfer from the MOF skeleton to a coumarin probe in the MOF cavity. The diffusivities of the excitons on the frameworks were determined to be 1.8 × 10^–2 cm²/s and 2.3 × 10^–2 cm²/s, corresponding to migration distances of 43 and 48 nm within their lifetimes, respectively. “Through space” energy-jumping beyond nearest neighbor accounts for up to 67% of the energy transfer rates. This finding presents a new perspective in the design and understanding of highly efficient energy transport networks for singlet excited states

Ag₂Se to KAg₃Se₂: Suppressing Order–Disorder Transitions via Reduced Dimensionality

We report an order–disorder phase transition in the 2D semiconductor KAg₃Se₂, which is a dimensionally reduced derivative of 3D Ag₂Se. At ∼695 K, the room temperature β-phase (CsAg₃S₂ structure type, monoclinic space group C2/<i>m</i>) transforms to the high temperature α-phase (new structure type, hexagonal space group <i>R</i>3̅<i>m</i>, <i>a</i> = 4.5638(5) Å, <i>c</i> = 25.4109(6) Å), as revealed by in situ temperature-dependent X-ray diffraction. Significant Ag⁺ ion disorder accompanies the phase transition, which resembles the low temperature (∼400 K) superionic transition in the 3D parent compound. Ultralow thermal conductivity of ∼0.4 W m^–1 K^–1 was measured in the “ordered” β-phase, suggesting anharmonic Ag motion efficiently impedes phonon transport even without extensive disordering. The optical and electronic properties of β-KAg₃Se₂ are modified as expected in the context of the dimensional reduction framework. UV–vis spectroscopy shows an optical band gap of ∼1 eV that is indirect in nature as confirmed by electronic structure calculations. Electronic transport measurements on β-KAg₃Se₂ yielded <i>n</i>-type behavior with a high electron mobility of ∼400 cm² V^–1 s^–1 at 300 K due to a highly disperse conduction band. Our results thus imply that dimensional reduction may be used as a design strategy to frustrate order–disorder phenomena while retaining desirable electronic and thermal properties

Förster Energy Transport in Metal–Organic Frameworks Is Beyond Step-by-Step Hopping

Metal–organic frameworks (MOFs) with light-harvesting building blocks designed to mimic photosynthetic chromophore arrays in green plants provide an excellent platform to study exciton transport in networks with well-defined structures. A step-by-step exciton random hopping model made of the elementary steps of energy transfer between only the nearest neighbors is usually used to describe the transport dynamics. Although such a nearest neighbor approximation is valid in describing the energy transfer of triplet states via the Dexter mechanism, we found it inadequate in evaluating singlet exciton migration that occurs through the Förster mechanism, which involves one-step jumping over longer distance. We measured migration rates of singlet excitons on two MOFs constructed from truxene-derived ligands and zinc nodes, by monitoring energy transfer from the MOF skeleton to a coumarin probe in the MOF cavity. The diffusivities of the excitons on the frameworks were determined to be 1.8 × 10^–2 cm²/s and 2.3 × 10^–2 cm²/s, corresponding to migration distances of 43 and 48 nm within their lifetimes, respectively. “Through space” energy-jumping beyond nearest neighbor accounts for up to 67% of the energy transfer rates. This finding presents a new perspective in the design and understanding of highly efficient energy transport networks for singlet excited states

Ag₂Se to KAg₃Se₂: Suppressing Order–Disorder Transitions via Reduced Dimensionality

We report an order–disorder phase transition in the 2D semiconductor KAg₃Se₂, which is a dimensionally reduced derivative of 3D Ag₂Se. At ∼695 K, the room temperature β-phase (CsAg₃S₂ structure type, monoclinic space group C2/<i>m</i>) transforms to the high temperature α-phase (new structure type, hexagonal space group <i>R</i>3̅<i>m</i>, <i>a</i> = 4.5638(5) Å, <i>c</i> = 25.4109(6) Å), as revealed by in situ temperature-dependent X-ray diffraction. Significant Ag⁺ ion disorder accompanies the phase transition, which resembles the low temperature (∼400 K) superionic transition in the 3D parent compound. Ultralow thermal conductivity of ∼0.4 W m^–1 K^–1 was measured in the “ordered” β-phase, suggesting anharmonic Ag motion efficiently impedes phonon transport even without extensive disordering. The optical and electronic properties of β-KAg₃Se₂ are modified as expected in the context of the dimensional reduction framework. UV–vis spectroscopy shows an optical band gap of ∼1 eV that is indirect in nature as confirmed by electronic structure calculations. Electronic transport measurements on β-KAg₃Se₂ yielded <i>n</i>-type behavior with a high electron mobility of ∼400 cm² V^–1 s^–1 at 300 K due to a highly disperse conduction band. Our results thus imply that dimensional reduction may be used as a design strategy to frustrate order–disorder phenomena while retaining desirable electronic and thermal properties