This SO discussion here lists some chemicals (and interestingly chlorophyll) that absorb visible radiation and emit infra red. I was wondering if there were chemicals that absorbed near infra-red and emitted much larger wavelengths (near microwave).
All molecules emit in the microwave region. They all have rotational energy levels and they absorb and emit all the time as energy flows between these levels. But this not what you are asking I suspect. If you blast thin Gallium Sulphide wafers with a femtosecond laser then it is possible to generate terahertz radiation.