Question

What are the structural properties of quinine that make it fluoresce compared to the structural properties of caffeine that make it absorb?

Answer 1

quinine

caffeine

for a molecule to show a fluorenscence in UV light it must satisfy many criteria.

first of all it should absorb a photon corrosponding to energ in UV region.

The absorbed photon must excite molecule to higher energy.

When a photon is absorbed, most intense fluorescence is found in compounds containing aromatic group with low-energy π→π∗ transitions

this energy gap is possible in quinine due to its structure consisting two aromatic ring.

The  π→π∗ transitions in caffeine has high energ gap.

Most of the transition shown by caffeine will be non radiative

but quinione due to low band gap will emit light when a photon

abstracted by it will relax to ground state.