Entangled spin-polarized excitons from singlet fission in a rigid dimer

Singlet fission is recognized as an enabling process for next-generation solar cells. Here the authors design a molecular system where specific spin sub-levels can be initialized to produce a highly entangled state and demonstrate that the coherence between magnetic sub-levels of that state is preserved at higher temperatures than those encountered in conventional superconducting quantum hardware. Singlet fission, a process that splits a singlet exciton into a biexciton, has promise in quantum information. We report time-resolved electron paramagnetic resonance measurements on a conformationally well-defined acene dimer molecule, TIPS-BP1', designed to exhibit strongly state-selective relaxation to specific magnetic spin sublevels. The resulting optically pumped spin polarization is a nearly pure initial state from the ensemble. The long-lived spin coherences modulate the signal intrinsically, allowing a measurement scheme that substantially removes noise and uncertainty in the magnetic resonance spectra. A nonadiabatic transition theory with a minimal number of spectroscopic parameters allows the quantitative assignment and interpretation of the spectra. In this work, we show that the rigid dimer TIPS-BP1' supports persistent spin coherences at temperatures far higher than those used in conventional superconducting quantum hardware.