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.