Understanding the photophysics and photochemistry of molecular π-stacked chromophores is important for utilizing them as functional photonic materials. However, these investigations have been mostly limited to covalent molecular dimers, which can only approximate the electronic and vibronic interactions present in higher oligomers typical of functional organic materials. In this work, we have synthesized a null-type, slip-stacked PDI dimer (2) and trimer (3) and explored the fundamental mechanism that drives ultrafast SB-CS in 3 using the electronic (TA) and vibrational (fsIR and VC) spectroscopies. Following photoexcitation, we find that a mixed FE/CT electronic excited state rapidly forms through coherent vibronic coupling primarily to a 1590 cm-1 high-frequency mode. Solvent relaxation alters the balance between the FE and CT contributions as indicated by growing VCs from the PDI radical cation. While 2 remains in the mixed state due to these vibronic and CT couplings, 3 reaches the SB-CS state by interacting with the adjacent chromophore assisted by solvent relaxation and low-frequency interchromophore vibrational modes (e.g. 390 cm-1). Vibronic coupling ultimately accelerates SB-CS between the two outer PDI units, following collapse of the FE/CT mixed state involving two of the three PDI units. This work expands our fundamental understanding of the SB-CS process and provides design principles for efficient charge separation or SB-CS beyond the contact radical ion pair characteristic of dimers.

Accelerating symmetry-breaking charge separation in a perylenediimide trimer through a vibronically coherent dimer intermediate

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ACS Fall 2022

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