Electrons in fluorescent substances get excited from the ground state($S_o$) to the first excited state($S_1$) or higher excited states in case of upconversion. Depending on the statistical distribution they reach a certain energy sublevel within the respective excitation manifolds.
They then go down through "vibrational relaxation" to the lowest excited state possible and eventually to the ground state again, causing a difference between absorption and emission energies. I've read that the vibrations might be caused by external sources (e.g. solvent), but that brings up the following questions:
What is the intrinsic property of fluorescent material that makes them fluoresce?
What is the origin of these vibrations that lead to vibrational relaxation and how do they affect the phenomenon of fluorescence?