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, ΔE_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 (ΔE_SF ≤ 0) and kinetic (ΔE_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.