Polyene Bond Resonances Drive Intramolecular Singlet Fission in Diphenylhexatriene Dimers

Singlet fission (SF), a spin-allowed process generating multiple excitons, promises to overcome the Shockley–Queisser limit in photovoltaics. Diphenylhexatriene (DPH) dimers have recently emerged as promising intramolecular SF (iSF) materials due to their high triplet energy (∼1.5 eV), compatible with silicon photovoltaics. However, their iSF mechanism remains elusive. Employing ab initio molecular dynamics and time-dependent density functional theory, we investigate the iSF mechanisms of a representative DPH dimer in toluene. Static calculations fail to reconcile experimental observations (positive iSF driving force, Δ E SF > 0), whereas dynamical sampling reveals that high-probability transient configurations enable efficient iSF. We identify three distinct iSF channels, with the dominant pathway (>80% probability) proceeding through a genuine charge-transfer (CT) intermediate state (S*(CT)). Crucially, resonance effects within the hextriene backbone’s single–double bonds linearly modulate the HOMO–LUMO gap, excitation energies, and consequently iSF energetics. This enables over 80% of sampled configurations to satisfy both the thermodynamic (Δ E SF ≤ 0) and kinetic (Δ E CT-LE ≤ 0) criteria for iSF. Furthermore, this resonance-governed mechanism proves temperature-independent, as corroborated by simulations spanning 250–350 K. This work underscores the critical roles of solvation-influenced structural dynamics, specifically polyene bond resonances, in dictating the fluctuating characteristics of frontier molecular orbitals and their energies. Thereby, these resonances govern the excited-state properties and iSF pathways, providing a subensemble or bond-specific design strategy for high-performance SF materials.