Several compounds show fluorescence characteristics due to their molecular structural features. However, the fluorescence intensity of a molecule is dependent on several external factors such as:
- Dissolved oxygen
- Solvent changes
In this article the influence of such parameters on fluorescence is discussed
Increase in temperature reduces the viscosity of the solvent medium as a result of which the frequency of collisions of the fluorescent molecule with the solvent increases. This contributes to deactivation by internal relaxation. As a result temperature rise is generally accompanied by decrease in fluorescence intensity and lower temperatures are preferable for better fluorescence activity.
Fluorescence intensities are highly sensitive to changes in pH. It has been observed that some entities show fluorescence only over a limited pH range. Aniline has fluorescence properties in neutral media but loses its fluorescence in acidic media due to formation of anilinium ion.
The common reason for changes in fluorescence behavior with pH change is due to structural changes such as extent of aromaticity, degree of conjugation or degree of ionization of ionic molecules. pH changes can also result in shift of emission to longer wavelengths.
Presence of dissolved oxygen has a tendency to decrease the fluorescence intensity. The paramagnetic property of molecular oxygen promotes singlet to triplet state transitions in other molecules. The longer lifespan of triplet state increases probability of radiation less deactivation resulting in decrease in fluorescence intensity. Likewise presence of paramagnetic transition metals in solution decreases fluorescence intensities.
Dissolved oxygen quenches phospherence to a greater degree than fluorescence and can even completely eliminate it. Hence dissolved oxygen needs to be completely excluded for measurements on phosphorescence measurements.
Changes in polarity or hydrogen bonding capacity of the solvent can influence fluorescent characteristics of molecules. Polar solvents are preferred for improvement in fluorescence signal strength. Similarly an increase in solvent viscosity can lead to increase in fluorescence intensity as deactivation due to collisions is lowered.
Due to the high sensitivity of fluorescence to external factors it becomes necessary to carry out fluorimetric analysis under strictly controlled conditions for greatest reliance on measured results.