The phosphor coating of fluorescent tubes downconverts UV light into visible wavelengths (is, lower energy/frequency) through the process of fluorescence. The UV photons excite the electrons of the phosphor molecules to higher energy states, after which they emit photons at lower frequencies/energies (visible light) as they fluoresce back to the ground state.

In this case however, the IR to visible light conversion actually increases the energy and frequency of emitted photons.

Total energy must be conserved, so extra energy is incorporated in some way for the upconversion.

There was a plastic film material I saw a few decades ago that lights up when you shine infrared light on it, which was useful for testing IR transmitters and receivers. IIRC, that material only worked when it was bathed in visible light for some time prior to use, where the absorbed visible light acted as an energy bath to provide the required energy for the photon upconversion.