Controlling Symmetry Breaking Charge Transfer in BODIPY Pairs

Abstract

Metrics & MoreArticle Recommendations * sı Supporting Information CONSPECTUS: Symmetry 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.

Excimer evolution hampers symmetry-broken charge-separated states

“…44,45 Hence controlling the conformational exibility and the concomitant manifestation of undesired radiative and non-radiative deactivation pathways of the SB-CS state is vital for a long-lived CS state, thereby enhancing photovoltaic efficiency. 3,46 Core-annulated near-orthogonal arranged perylenediimide dimers have found extensive applications as non-fullerene acceptors in organic solar cells owing to their greater solubility, reduced aggregation and excimer formation. 47,48 Though vital for photovoltaic applications, the excited-state dynamics of heteroatom annulated PDI dimers remain elusive.…”

Ultrafast Charge Separation Driven by Torsional Motion in Orthogonal Boron Dipyrromethene Dimer

“…The SB-CS reaction dynamics and the associated solvent effect were well-resolved via femtosecond electronic and vibrational spectroscopy. − Dynamic solvent effects were observed in SB-CS reactions for which the dominant contribution to the activation energy comes from the solvent reorganization motions. − In such cases, the energy of the CS system is mainly a function of solvent configuration, and hence, the CS reaction is mainly driven by solvent dynamics. Although these studies defined the role of solvent effects on the occurrence of CS, the contribution of intramolecular nuclear motion, especially the low-frequency vibration, to the SB-CS reaction was scarcely studied …”

Solvent Tuning Excited State Structural Dynamics in a Novel Bianthryl

“…Bianthryls (BAs) played a key role in the development of photophysics . In particular, 9,9′BA is the prototype for symmetry breaking charge separation (SBCS), a topic which continues to excite wide interest. − Lippert, Mataga, and co-workers showed that this symmetrical ( D 2 d ) molecule unexpectedly supports charge transfer type emission in polar solvents. − This anomaly was widely investigated, and in an important series of papers Barbara and co-workers showed that the dynamics of CS in 9,9′BA closely follow those of solvation, suggesting a dominant role for solvent orientation dynamics in symmetry breaking and CS state stabilization. , Given the apparently dominant role of the solvent, it seems pertinent to ask whether the initial molecular symmetry plays a role in the CS. To date, there is only one photophysical study of other unsubstituted BAs; here we employ ultrafast transient absorption (TA) and femtosecond stimulated Raman spectroscopy (FSRS) to study excited state structural dynamics in the newly synthesized 1,9′BA, which has both reduced symmetry compared to 9,9′BA ( C s ) and its two anthracene moieties are linked at different carbons, giving them slightly different electronic structures …”

“…where k B is Boltzmann's constant. Using the a value calculated for the slower relaxation time and assuming ΔΘ to be 27°(from the DFT/TDDFT calculation), we recover the hydrodynamic volume of the rotating particle as 54 Å 3 . This should correspond to the hydrodynamic volume of the rotating body (here the anthracene unit).…”