The exciton theory formulated by Kasha and co-workers and Davydov independently gave the fundamental understanding of the photophysics of molecular aggregates.¹ Kasha demonstrated exciton splitting for various geometrical orientations of molecular transition dipoles based on the long-range Coulombic coupling employing the point dipole approximation. The molecular exciton theory highlighted the significance of the orthogonal arrangement of transition dipoles in isolating the exciton and subsequent null exciton coupling mediated monomer-like optical properties.² The emergent photophysical properties from the crafted architectures were explored using both experimental tools and the state-of the-art computational methods for understanding the aggregates beyond the Kasha’s model.^3,4 The ability of selective activation of one of the charge transfer couplings occasioned ultrafast dissociation of null exciton and evolution of charge separated state in polar solvent facilitated by the selective hole transfer coupling in the edge-to-edge arranged perylenediimide (PDI) dimer Greek cross (+), providing a way forward in fundamental understanding as well as to identify the emergent properties of null aggregates.⁵

References

• Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. The Exciton Model in Molecular Spectroscopy. Pure Appl. Chem. 1965, 11, 371–392.
• Sebastian, E.; Philip, A. M.; Benny, A.; Hariharan, M. Null Exciton Splitting in Chromophoric Greek Cross (+) Aggregate. Chem. Int. Ed. 2018, 57, 15696–15701.
• Lijina, M. P.; Benny, A.; Ramakrishnan, R.; Nair, N. G.; Hariharan, M. Exciton Isolation in Cross-Pentacene Architecture. Am. Chem. Soc. 2020, 142, 17393–17402.
• Benny, A.; Ramakrishnan, R.; Hariharan, M. Mutually Exclusive Hole and Electron Transfer Coupling in Cross Stacked Acenes. Sci. 2021, 12, 5064–5072.
• Sebastian, E.; Hariharan, M. Null Exciton Coupled Chromophoric Dimer Exhibits Symmetry Breaking Charge Separation, Am. Chem. Soc. 2021, 143, 34,13769-13781.