where IPD, EAA, ES1, and 𝐸 S1 b are the electron-donor ionization potential, electron-acceptor electron affinity, lowest singlet excited state energy, and binding energy of the singlet exciton, respectively . Quantum chemical calculation results have demonstrated that the CT-to-LE transition is significant when ΔELE-CT is small (Fig. ). However, direct experimental evidence for CT-to-LE upconversion is lacking.