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.

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.