Coupling two molecules means to link them together, either covalently or by weak interactions. The term "coupling" is also frequently used to refer to the combination of two chemical reactions into one. Two chemical reactions can be coupled such that the more enegetically favorable reaction drives the less energetically favored reaction. Such energy coupling is analagous to how the engine of a mechanical engine couples the burning of fuel to mechanical motion. An example of nanomechanical coupling is photosynthesis, which couples the energy in sunlight with the synthesis of sugar from carbon dioxide.
