Excitonic Dark States in Molecular Monolayer Crystals

The tightly bound excitons and strong dipole-dipole interactions in two-dimensional molecular crystals enable rich physics. Among them, superradiance (SR), the spontaneous coherent emission from bright excitons, has sparked considerable interest in quantum-information applications. In addition, optically forbidden states (dark exciton states) have potential to both achieve Bose-Einstein condensation and modulate exciton dynamics. Here, we report a unique series of dark exciton states in highly crystalline organic monolayers (MLs) via two-photon excitation spectroscopy (TP-PLE). These dark exciton states convert to the emissive, delocalized exciton states that undergo room temperature SR. Using a vibronic exciton model, we show that these dark exciton states are mixed character states of Frenkel exciton (FE) and charge transfer exciton (CTE) with majority intralayer CTE character (>99.9%) and weak coupling to the emissive FE states. We observe significantly higher photochemical stability of MLs under two-photon excitation, which we attribute to the suppression of exciton-exciton annihilation.