In this video we're going to demonstrate fluorescence thermochromism.

Simply dissolve a small amount of copper iodide into pyridine to form a clear yellow solution of trispyridine copper iodide.

Normally this does not fluoresce but if cooled to liquid nitrogen temperatures it will fluoresce a strong yellow color.

To understand why this happens one must first understand a little about fluorescence. The ultraviolet light is absorbed by the molecule and excites electrons in it to higher energy levels. Normally the molecule bumps into other molecules and looses this energy, dropping the electrons back into their ground levels. In fluorescent materials though, the electrons release their energy first as fluorescent light before they loose their energy through collisions. In trispyridine copper iodide, the molecules vibrate and collide too much at room temperature and thus destroy any chance of fluorescence, so the substance looks non-fluorescent. But if cooled low enough, the molecules vibrate less and thus the electrons have more opportunity to emit light before a collision.

Now moving on, if a few drops of the trispyrdine copper iodide solution is put on a sheet of filter paper and allowed to dry. it looses a pyridine molecule to become bispyridine copper iodide. This substance is more stable and will fluoresce at room temperature. But cooling will not improve the fluorescence noticeably since the substance is already stable enough to resist collisional deactivation.

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https://www.youtube.com/watch?v=8jDFx7yeBvw