What are the factors that cause fluorescence quenching?

There are several factors that can cause fluorescence quenching. One factor is the nature of the molecule. Saturated molecules (e.g. alkanes) with sigma bonds do not give off fluorescence. Unsaturated molecules (e.g. alkynes) with pi bonds and have good resonance exhibit fluorescence. Molecular weight is another factor, and an atom with a high molecular weight tends to exhibit low fluorescent intensity. The functional group is another factor, as an electron donor group (e.g. methoxy) increases fluorescence and an electron accepting group (e.g fluorine) decreases fluorescence intensity. Concentration of solutions is another factor, as a dilute solution exhibits high intensity due to the solution being equally distributed. In a concentrated solution, the intensity is less because the upper layer of solution is absorbed by more radiation than the lower layer. Temperature is also a factor, and an increase in temperature will cause an increase in fluorescence and vice versa. The presence of oxygen also causes a decrease in fluorescence intensity. Lastly, the pH is a factor; fluorescence decreases with increasing pH and remains stable after pH 6.