In fluorescence polarization, which observation is true regarding molecular size and polarization?

• A. Small molecular complexes show a greater amount of polarization
• B. Large complexes show a greater amount of polarization
• C. Polarization is independent of molecular size
• D. Polarization increases with temperature

Answer: (A)

Fluorescence polarization reflects how fast a molecule reorients during the excited-state lifetime. When a molecule is large, its rotational diffusion is slow, so it doesn’t change orientation much before it emits light. That means the emitted light stays highly polarized. Conversely, small molecules tumble quickly, rotating to random orientations and depolarizing the emission, which lowers the observed polarization.

So the true observation is that larger complexes show a greater amount of polarization because their slower rotation preserves the polarization of the emitted light. Temperature can affect this too: higher temperature increases molecular motion and tends to reduce polarization, not increase it. Polarization is not independent of size; it rises with increasing size due to slower rotational diffusion.