[structure: see text]. Efficient strategies for the communication of signals between molecules must be identified for the development of molecule-based nanoprocessors. We have demonstrated that photoinduced proton transfer can be exploited to implement intermolecular digital transmission. Light inputs operate a three-state molecular switch inducing the transfer of a proton to or from a two-state molecular switch. The signal transduction protocol of the two communicating molecular switches is equivalent to that of a sequential logic circuit incorporating three logic gates.