The design of a molecular logic circuit based on sequential forward S-2-S-2 energy transfer and back S-1-S-1 energy transfer (cyclic energy transfer) for a system comprising of an azulene and a zinc porphyrin is discussed. Using a computational dynamics simulation, the effect of varying the orientation factor via the viscosity of the medium on the cyclic energy transfer dynamics is investigated and is incorporated into the logic circuit. (C) 2003 Elsevier B.V. All rights reserved.