We propose a new class of designer molecules with functional units which are driven by entropic forces. This idea profits from the mechanically interlocked nature of topological molecules such as catenanes and rotaxanes. which allows for mobile elements whose accessible configuration space gives rise to entropic intramolecular forces. Stich entropy-driven designer molecules open the possibility for externally controllable functional molecules and nanomechanical devices. (C) 2002 Elsevier Science B.V. All rights reserved.