Fig. 2: Schematic diagrams of contrasting single-molecule UCEL mechanisms.

In the upconversion region, the first tunneling electron excites the neutral molecule from the S₀ ground state to the T₁ triplet state by either IES (a) or via a transient anionic state through two sequential carrier injection steps (b). Then, the second tunneling electron can promote the molecule from the T₁ state to the S₁ state via a transient cationic state through another two sequential carrier injection steps, as shown identically on the right panels in a and b. Here we use solid black arrows for carrier injections (CI), dashed green arrows for inelastic electron scattering (IES), red arrows for photon emission, vertical dashed lines to illustrate level shifting due to charging/discharging, and pink wavy lines to connect transitions that occur simultaneously. The same annotations are adopted for other similar figures throughout the whole manuscript. See Supplementary Notes 3 & 4 for more details.