Figure 6.

A schematic representation of the four level AND and OR single-molecule gates. The optically accessible excited state, ae, is shown as a level of finite width. It can be accessed by a two-photon transition (AND) of quite different colors or by either one-photon transition (OR) of similar color. The ultrafast radiationless transition ae → re is shown, as well as the conical intersection leading to the ground state isomer, gd*. Compare with Figs. 2 and 4. Fig. 5 shows other molecular types that can be used in this manner. Our proposal is based on the experimentally easier task of detecting the photoinduced change. If instead, or in addition, one can spectroscopically monitor the depletion of the ground state, the logical repertoire becomes richer. For example, a Not“AND” (≡“NAND”) gate can be so operated.