Which wavelength range in the electromagnetic spectrum has been the most fruitful in the search for complex organic molecules in space?

Light can come in many frequencies, only a small fraction of which can be seen by humans. Between the invisible low-frequency radio waves used by cell phone and the high frequencies associated with infrared light lies a fairly wide swath of the electromagnetic spectrum, and this spectrum are called "terahertz", or sometimes called submillimeter, waves. The terahertz of spectrum is full of information. Everything in the universe that is warm to about 10 degrees gives off terahertz radiation. Even at these very low temperature molecules can rotate in space, yielding unique fingerprint in the terahertz.