Thermodynamic Quasi-Equilibrium between Low-Lying Excited States in Molecular Nonfullerene Acceptors.

Understanding the intrinsic excited-state dynamics of nonfullerene acceptors (NFAs) is essential for linking molecular design to device performance. Here, we study the prototypical NFA Y6 in dilute solution using femtosecond transient absorption and fluorescence upconversion spectroscopy. We resolve two coupled low-lying excited-state components with LE-like and CT-like character. Distinct initial populations prepared by different excitation energies evolve into the same long-time spectral and kinetic fingerprint, providing direct evidence for a reversible quasi-equilibrium. Temperature-dependent exchange rates follow Arrhenius behavior with an apparent activation energy of ∼76 meV. This quasi-equilibrium biases the population toward the weakly emissive CT-like branch, shortening the effective exciton lifetime and constraining exciton utilization in strongly coupled donor-acceptor systems.