Question

Why do some absorbing compounds fluoresce but others do not?

Short answer

Molecules which have smaller relaxation time will show flourescence.

Step-by-step solution

Step 1. Given information

Fluorescence is one of the ways that an excited molecule might return to its original state. The molecule is stimulated by radiation absorption and returns to its ground state in a variety of methods, both radiative and non-radiative.

Step 2. Reason for some absorbing compounds fluoresce but others do not 

Fluorescence is one of the raciative ways in which the molecule returns to its ground state. Nonradiative decay techniques such as internal conversion and vibrational relaxation occur at a faster pace than fluorescence for the other molecules that do not demonstrate this. Non-fluorescent molecules have a longer relaxation time, which means they stay in the excited state for longer and return to the ground state slowly, whereas fluorescent molecules have a shorter relaxation time.