What fluorescent materials absorb visible light and emit UVC light? What fluorescent materials, if any, exhibit anti-stokes shift such that they absorb visible light and emit ultraviolet light in the range of 250-285 nm? I'm interested in particular in materials that are efficient in their conversion and not difficult to acquire.
It is not possible in a single molecule as energy conservation will forbid it. There is not enough thermal energy, $k_BT \approx 210 $ wavenumbers, visible to uv thousands of wavenumbers.  It is sort of possible using triplet-triplet annihilation ($\ce{T + T \to S^* + S}$ ) but this needs significant initial excitation and long lived triplets, which means de-oxygenated solution, and produces the same singlets as were excited initially. There are also inorganic up-conversion materials, for example some lanthanides, such as ytterbium based ones will convert near ir light to visible light.