There are materia

The materials absorb photons of shorter wavelength and emit photons of longer wavelength, this means they absorb photons of higher energy and emit photons of lower energy.

If a stable substance is to absorb photons of larger wavelength and emit light of shorter wavelength, it will have to absorb photons of lower energy and emit photons of higher energy. This additional energy in the emission photons must be provided by the substance which is not possible for a stable substance. Hence, such a substance can’t exist.