Controlling Symmetry Breaking Charge Transfer in BODIPY Pairs

ConspectusSymmetry
breaking charge transfer (SBCT) is
a process in which
a pair of identical chromophores absorb a photon and use its energy
to transfer an electron from one chromophore to the other, breaking
the symmetry of the chromophore pair. This excited state phenomenon
is observed in photosynthetic organisms where it enables efficient
formation of separated charges that ultimately catalyze biosynthesis.
SBCT has also been proposed as a means for developing photovoltaics
and photocatalytic systems that operate with minimal energy loss.
It is known that SBCT in both biological and artificial systems is
in part made possible by the local environment in which it occurs,
which can move to stabilize the asymmetric SBCT state. However, how
environmental degrees of freedom act in concert with steric and structural
constraints placed on a chromophore pair to dictate its ability to
generate long-lived charge pairs via SBCT remain open topics of investigation.In this Account, we compare a broad series of dipyrrin dimers that
are linked by distinct bridging groups to discern how the spatial
separation and mutual orientation of linked chromophores and the structural
flexibility of their linker each impact SBCT efficiency. Across this
material set, we observe a general trend that SBCT is accelerated
as the spatial separation between dimer chromophores decreases, consistent
with the expectation that the electronic coupling between these units
varies exponentially with their separation. However, one key observation
is that the rate of charge recombination following SBCT was found
to slow with decreasing interchromophore separation, rather than speed
up. This stems from an enhancement of the dimer’s structural
rigidity due to increasing steric repulsion as the length of their
linker shrinks. This rigidity further inhibits charge recombination
in systems where symmetry has already enforced zero HOMO–LUMO
overlap. Additionally, for the forward transfer, the active torsion
is shown to increase LUMO–LUMO coupling, allowing for faster
SBCT within bridging groups.By understanding trends for how
rates of SBCT and charge recombination
depend on a dimer’s internal structure and its environment,
we identify design guidelines for creating artificial systems for
driving sustained light-induced charge separation. Such systems can
find application in solar energy technologies and photocatalytic applications
and can serve as a model for light-induced charge separation in biological
systems.