Probing Through-Bond and Through-Space Interactions in Singlet Fission-Based Pentacene Dimers

Abstract

Interchromophoric interactions such as Coulombic coupling and exchange interactions are crucial to the functional properties of numerous π-conjugated systems. Here, we use magnetic circular dichroism (MCD) spectroscopy to investigate interchromophoric interactions in singlet fission relevant pentacene dimers. Using a simple analytical model, we outline a general relationship between the geometry of pentacene dimers and their calculated MCD response. We analyze experimental MCD spectra of different covalently bridged pentacene dimers to reveal how the molecular structure of the “bridge” affects the magnitude of through-space Coulombic and through-bond exchange interactions in the system. Our results show that through-bond interactions are significant in dimers with conjugated molecules as bridging units and these interactions promote the overall electronic coupling in the system. Our generalized approach paves the way for the application of MCD in investigating interchromophoric interactions across a range of π-conjugated systems.

Magnetic Circular Dichroism Elucidates Molecular Interactions in Aggregated Chiral Organic Materials

“…Unlike in ECD, under an external magnetic field of 1.4 T we observed a weak but significant MCD for Th‐CBZ in dichloromethane solution associated to the main absorption band centred at 370 nm. The magnetic field (H) normalized dissymmetry factor, $${{g}_{MCD}={{{\rm \Delta }A}\over{A}}\cdot{}{{1}\over{H}}}$$ , is approximately 5⋅10 −5 T −1 , of the same order as those observed for other simple aromatic molecules [52,61–64] . In the 320–500 nm region, the MCD spectrum consists of 3 bands of alternating sign, with a prominent negative feature at 390 nm.…”

“…[67] The Faraday B-term associated with each transition depends on the monomercentered magnetic/electric transitions (mμμ terms), the energy of each level, as well as on the angle α. By adapting Lakhwani's formulae [52] to the case of 3 interacting excited states, the B-term becomes (for the transition to the xlevel):…”

“…The Faraday B ‐term associated with each transition depends on the monomer‐centered magnetic/electric transitions ( mμμ terms), the energy of each level, as well as on the angle α. By adapting Lakhwani's formulae [52] to the case of 3 interacting excited states, the B ‐term becomes (for the transition to the x ‐ level): $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr {{\rm B}}_{{{\rm x}}_{-}}={{1}\over{4}}{{\rm m}}_{{\rm x}{\rm y}}{{\rm \mu }}_{{\rm x}}{{\rm \mu }}_{{\rm y}}{{2-2{\rm cos}{\rm \alpha }}\over{{{\rm E}}_{{{\rm x}}_{-}}-{{\rm E}}_{{{\rm y}}_{-}}}}+{{1}\over{4}}{{\rm m}}_{{\rm x}{\rm y}{\rm { {^\prime}}}}{{\rm \mu }}_{{\rm x}}{{\rm \mu }}_{{\rm y}{\rm { {^\prime}}}}{{2-2{\rm cos}{\rm \alpha }}\over{{{\rm E}}_{{{\rm x}}_{-}}-{{\rm E}}_{{{\rm y}{\rm { {^\prime}}}}_{-}}}}\hfill\cr}}$$$ …”

“…[51] A full quantitative model describing the MCD of interacting molecules is not yet available; however, a simplified exciton model for MCD was recently proposed by Lakhwani et al in the case of pentacene dimers. [52] This model considers degenerate coupling within the framework of a dipolar approximation. We can consider a Th-CBZ dimer, where each of the 3 excited state levels of the monomer (x, y, y') is split into 2 components (x � , y � , y' � ) due to dipolar/exciton interaction (Figure 1).…”

Optimizing through-space interaction for singlet fission by using macrocyclic structures

“…86 Furthermore, the through-space interactions dominate the iSF process in some covalently bridged bipentacenes with unique spatial proximity of the two pentacenes. 60,79,87–90 According to previous experimental and theoretical experience on bipentacenes, we speculate that the through-space interactions of the bipentacene macrocycles are the key factor in the occurrence of iSF, but is it the dominant factor? In addition to the role of structural confinement mentioned in the experiment, does the rigid scaffold have other functions?…”