This term applies broadly to variations of
quantum yields
of
photophysicalprocesses (e.g.
fluorescence or
phosphorescence ) or
photochemical reactions
(usually reaction
quantum yield) with the concentration of a given
reagent which may be a substrate or a
quencher. In the simplest case, a plot of
(or
for emission) vs.
concentration of
quencher,
, is linear obeying the equation:
In equation (1)
is referred to as the Stern–Volmer constant. Equation (1) applies when a
quencher inhibits either a
photochemical reaction or a photophysical process by a single reaction.
and
are the
quantum yield and emission intensity
radiant exitance, respectively, in the absence of the
quencher Q, while
and
are the same quantities in the presence of the different concentrations of Q. In the
case of
dynamic quenching the constant
is the product of the true
quenching constant
and the
excited statelifetime,
,
in the absence of
quencher.
is the
bimolecular reaction
rate constant for the
elementary reaction of the
excited state with the particular
quencher Q. Equation (1) can therefore be replaced by the expression (2):
When an
excited state undergoes a
bimolecular reaction with
rate constant
to form a product, a double-reciprocal relationship is observed according to the equation:
where
is the
quantum efficiency of product formation,
the efficiency of forming the reactive
excited state,
the fraction of reactions of the
excited state with substrate S which leads to product, and
is the concentration of reactive ground-state substrate. The
intercept/slope ratio gives
.
If
,
and if a photophysical process is monitored, plots of equations (2) and (3) should
provide independent determinations of the product-forming
rate constant.
When the
lifetime of an
excited state is observed as a function of the concentration of S or Q, a linear relationship should
be observed according to the equation:
where
is the
lifetime of the
excited state in the absence of the
quencher Q.
Source:
PAC, 1996, 68, 2223
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
on page 2277