Has there ever been a diesel-electric helicopter? Occasionally I come across the diesel-electric (DE) design for submarines and freight trains. They say something like "a diesel engine powering an electric drive lets the two operate at their most efficient RPMs, and the electric drive has very little losses in efficiency."

So the question hit me: Has there even been a DE helicopter? Is there something about the DE system that makes it bad for the aviation industry? Weight maybe? But note we do not need a bunch of batteries like on a submarine. We don't need to store the power, we just convert it to electricity which goes immediately into the electric drive for the rotors.
Helicopters currently use lightweight but fairly inefficient gas turbines. Diesel-electric engines have great energy efficiency, in terms of mechanical energy delivered per kilo of fuel. However, they're quite heavy. This means that you need more lift, which in turn means more power is needed. This negates the initial advantage. 

With the advent of lightweight materials, the latter effect decreases so this situation might change.

Diesel engines can throttle up and down quite a bit, unlike gas turbines,  but this capacity is irrelevant to helicopters. Note that helicopters need a pretty constant amount of lift.