Fluorene (Twist or shine, choose one)

Fluorene is a very simple aromatic hydrocarbon. It's often used in physical chemistry classes to teach an important lesson about fluorescence: rigidity matters:

Fluorene is essentially a rigidified biphenyl:

When a molecule absorbs a photon of light, it carries around an extra packet of energy; this is termed an "excited electronic state." Just what happens to that packet of energy depends on the molecule. Lots of molecules absorb light - your red dry-erase marker and pink highlighter both contain dyes that absorb green light. The difference between the dry-erase marker and the highlighter, however, is that the highlighter's ink has a profound propensity to re-emit light.

If you have a green laser pointer around, try an experiment: write with a red marker (dry-erase, sharpie, anything that doesn't "day-glo") and a pink highlighter on a piece of paper. Shine the laser on the red marker and you'll see some reflected green light. Shine the laser on the pink highlighter and you'll see some reflected green light, and some orange light! This has to do with what the dye does with its packet of energy (a green light, 532nm photon) once it has gotten hold of it. The red marker will convert most of it into heat (not much, but that's where it's going - I used to work with high-powered lasers that could actually burn a tiny hole in your finger if it got in the way of the light and it was focused just so!). The highlighter, however, will take that green photon, and emit some of that energy as a lower energy, orangish photon (it's probably a rhodamine type dye in there).

The probability of the dye in the red marker re-emitting a photon is very low; less than one in one thousand. The probability of the pink highlighter re-emitting a photon is high - between 10% and 100%! This is called the "fluorescence quantum yield." Certain structural characteristics of a molecule make it more or less likely to fluoresce. One is rigidity. Fluorene, thanks in part to that bridging methylene, is quite rigid; nearly every photon it absorbs will be re-emitted as lower-energy light. Biphenyl, however, will only do this about 20% of the time.