The Role of Charge Transfer and Parity Symmetry of the Wave Function in N , N ′-Bis(2-phenylethyl)-3,4,9,10-perylenedicarboximide Crystals: A Frenkel-Holstein Approach

Abstract

Abstract Charge-transfer (CT) configurations can provide an efficient pathway to the triplet pair (TT) manifold in singlet fission (SF), yet diagnosing CT-mediated TT participation from optical spectra remains challenging because TT signatures are often weak, obscured, or missing. Here, we develop an extended Frenkel–Holstein Hamiltonian that explicitly includes CT and intermolecular TT diabatic states. We show that site-exchange parity controls optical activity. In the Condon limit, only the even (+) sector is bright, and TT borrows oscillator strength only through the parity-matched chain |FE+⟩ →|CT+⟩ → |TT+⟩ off the Frenkel exciton (FE) as determined by the relative phase of the electron/hole transfer coupling t+ and relative phase of the CT–TT coupling g+. CT–TT mixing also raises the energy of the active CT state, increasing the effective FE–CT detuning and suppressing CT-mediated superexchange. When applied to the α- and β-phase N,N′-bis(2-phenylethyl)-3,4,9,10-perylenedicarboximide (EP–PDI) crystals, the model reproduces polymorph-dependent vibronic ratios and CT features and provides packing-level design rules for tuning TT visibility and triplet yield via the relative phases of electron/hole transfer and CT–TT exchange couplings.