Intermolecular electronic energy:
The intermolecular electronic energy transfer from the excited molecule to a quencher molecule is one of the common ways through that the fluorescence quenching occurs. The procedure could be represented as follows.
M* + Q → Q* + M
while, the excited analyte molecule (M*) transfers its excitation energy to a quencher molecule Q, whereby it gets de-excited to M creating an excited quencher molecule, Q*. In the case Q* happens to be a fluorescent species, then its fluorescence is known as sensitised fluorescence. The sensitisation of fluorescence could be one of the ways to observe fluorescence from a molecule (Q) that might otherwise be difficult to excite directly. Therefore, the quenchers are commonly not desirable for the fluorimetric determinations. Mathematically, the quenching commonly follows the Stern-Volmer equation given below.
φ0F/ φF = 1 + K SV (Q)
where φ0F and φF are the fluorescence quantum yields for the analyte within the absence and presence of quencher respectively, and KSV is the quenching constant.
In an alternative mechanism of quenching the quencher molecule forms a complex along with the analyte molecule that then deactivates using a few internal conversion mechanisms. This mechanism could be represented as follows.
A* + Q ↔ A Q* → A Q
Within this mechanism, an effect of the concentration of the quencher molecule or the analyte could be understood in terms of easy law of mass action. A higher concentration of any of the two shifts the equilibrium to the right and the analyte fluorescence is quenched to a greater extent.
Commonly in the compounds containing heavy atoms such as, halogens are expected to cause fluorescence quenching. Further, the paramagnetic species such as oxygen, holding unpaired electrons are prominent quenchers.