Synergistic Dual Steric-Electronic Effects of Bay-Substituents on Singlet Fission in Perylene Diimide Derivatives

Abstract

Singlet fission (SF), a carrier multiplication mechanism capable of surpassing the Shockley-Queisser efficiency limit, requires precise balancing of geometric and electronic effects for optimal performance. However, clear molecular design guidelines remain lacking, particularly regarding how to synergistically control substituent-induced torsional distortion and electronic modulation of SF energetics. To address this, we systematically designed twelve perylene diimide (PDI) derivatives functionalized at the bay positions with electron-donating or electron-withdrawing groups, aiming to decouple the individual contributions of structural distortion and electronic effects to the thermodynamic driving force of SF. Computational results reveal that torsional distortion of the molecular framework raises both the singlet (S 1 ) and triplet (T 1 ) excited-state energies. While increased dihedral angles enhance the SF driving force, they also elevate the T 1 energy to meet the required bandgap in device settings.Furthermore, strong electron-donating or -withdrawing substituents reduce both S 1 and T 1 energies. A key finding is that in SF systems, the substituent contribution ratio to the HOMO (HOMO%) serves as a decisive electronic parameter that directly governs excited-state energy levels. This work establishes a dual-parameter design principle for SF-active materials, emphasizing the synergistic optimization of geometric distortion and substituent electronic effects, thereby providing clear guidance for expanding the library of SF-capable molecules.