Exciton excitation energy. According to Frenkel exciton model, The energy difference between the exciton state and the block-localized molecular complex, i.e., the exciton-excitation energy Δ𝐸𝐸 Ex 𝐾𝐾 , corresponds to the transition from the ground state of the monomer complex (𝑋𝑋 • 𝑌𝑌) to the exciton resonance state (𝑋𝑋 • 𝑌𝑌) * :