Intramolecular Singlet Fission in Quinoidal Dihydrothiophene

Singlet fission (SF) provides a promising mechanistic pathway to overcome the Shockley-Queisser limit of solar cell efficiencies. There are hardly handful of molecules which are known to exhibit intramolecular SF (iSF). Most of the investigated iSF systems based on the donor/acceptor architectures have low-lying triplet-state energies which cause serious limitation for efficient opto-electronic devices. Herein, we demonstrate that quinoidal bis(diarylmethylene) dihydrothiophene (QDT) acts as a promising candidate for iSF with the triplet state arising near similar to 1.0 eV. Based on ab initio quantum chemical calculations, ground and excited states of QDT are thoroughly investigated using time-dependent density functional theory and complete active space self-consistent field which corroborate the criteria for iSF. The potential energy surface scan along the normal modes indicates that the terminal C=C bond is responsible for plausible bright-to dark-state transition. Restricted active space spin flip calculations reveal the nature of multiexciton states arising in the higher singlet excited states and efficient iSF pathway in QDT. Attachment detachment density analyses reveal that charge transfer states play a pivotal role during the formation of a triplet pair from the initially singlet state. Nonadiabatic coupling between the lowest dark and bright single exciton states demonstrates favorable formation of coupled triplet pairs and a significant rate for the formation of the independent triplet states that promotes efficient iSF in QDT.