PHOSPHORESCENCE OF FLUORESCEIN IN TARTARIC ACID

NEEDED

     -  Flame (Bunsen burner, ...) and test tube
     -  Tartaric acid (a food additive)
     -  Fluorescein
     -  electric torch

PROCEDURE

         A tiny little bit of fluorescein is added to
     tartaric acid (let's say 2 g) to a test tube and the
     mixture is heated till melting (yellow melt).
          The resulting phosphorescence is
     temperature dependent: Hot = nothing at all;
     room temperature = slightly; ice-cooled = great!

COMMENTS

         Neither of the starting materials is
     phosphorescent (the tartaric acid is not even
     fluorescent), but in the tartaric acid matrix, the
     Fluorescein becomes phosphorescent.
         In solution, the excited state of Fluorescein
     (S1) relaxes by fluorescence (S1>S0) or by non-
     radiative relaxation via collisions with other
     molecules. If the Fluorescein is fixed in the
     tartaric acid matrix, the chances for such
     collisions are decreased and the transition to
     the triplet state (S1>T1) is favoured and hence
     phosphorescence may occur. If the matrix is
     hot, intersystem crossing occurs again - this
     time backwards (T1>S1) - and hence
     fluorescence relaxes the excited state
      
The powder (left), the yellow melt (middle), and the way the melt should be spread over the tube before solidifying (right).
Fluorescence of the melt.
Phosphorescence of the melt
at -10°C (ice salt bath).