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Updated: Jun 5, 2025
Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids
Published on: May 27, 2020
Excitonic Dark States in Molecular Monolayer Crystals
Md Mehedi Hasan1, Paul A Hume2,3, Linglong Zhang1,4
1School of Engineering, ANU College of Engineering, Computing and Cybernetics, The Australian National University, Canberra, ACT 2601, Australia.
Abstract:
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.
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