General Dual-Switched Dynamic Singlet Fission Channels in Solvents Governed Jointly by Chromophore Structural Dynamics and Solvent Impact: Singlet Prefission Energetics Analyses.

The singlet fission (SF) channels in many systems are controlled by the thermodynamic driving force (Switch-1) and kinetic barrier (Switch-2), both of which could be modulated by chromophore structure dynamics and solvent properties. Using ab initio molecular dynamics (AIMD), we here simulate how the structural dynamics and solvent jointly govern singlet prefission energetics, taking a covalent BODIPY dimer (di-BODIPY) in solvents as an example. We report a general dual-switched dynamic channel for intramolecular SF in solvents and suggest an effective AIMD sampling method to characterize the joint effect of chromophore structure dynamics and the solvent impact on SFs. Results reveal that the joint effect not only provides di-BODIPY more chances for meeting the SF thermodynamic requirement (Switch-1 ON) but also tunes the charge-transfer state toward removing the kinetic barrier (Switch-2 ON). Two factors jointly govern each switch in the dual-switched SF channel, and any one does not open the channel alone. We suggest a general principle for dynamically dual-switching the SF channel in solvents by utilizing the joint effect to tune the pre-SF energetics for photoexcitation and the opening of the subsequent channel. AIMD sampling is used for the first time to discover explicit solvent-solute interaction and dynamics information and thus their effect on excitation energetics. This work also shows the statistical information for an ensemble of SF chromophores in solvents, which can undergo different photoexcitations and possible SFs. The high agreement of the findings here with the experiments justifies our AIMD sampling-based pre-SF energetic prediction as a reliable way for exploring novel SF systems and their controllability.