Geoscience Reference
In-Depth Information
ground and excited states that govern the fluorescence properties of a molecule in a given
solvent. During fluorescence, changes in the
π
electron distribution causes changes in the
dipolar and hydrogen-bonding properties of the solute (Lakowitz,
2006
, chapter 6). If the
solute is more polar in the excited state than in the ground state then the fluorescence will
occur at longer wavelengths if the fluorophore is present in a polar solvent, as opposed to
a nonpolar solvent. This is due to the stabilization of the excited state, by the more polar
solvent. Because photoluminescence originates from an excited state, fluorescence emis-
sion occurs at longer wavelengths the more interactive the solvent is, that is, the stronger
the hydrogen bonding or polarity.
1.3.4.8 The Heavy Atom Effect
The interactions between certain ions and conjugated fluorescing ligands are known to affect
fluorescence in several ways. The fluorescence of a ligand may be somewhat enhanced or
quenched depending on the influence that the ion has on the nonradiative processes com-
peting with luminescence. In the case of the heaviest non-transition metal ions, for example
Hg (II) and Bi (II), static quenching of fluorescence and sometimes phosphorescence, often
results from a “heavy atom effect.” This term is used to describe the influence of heavy atom
substitution on spin-forbidden transitions. It is usually assumed that the dominant influence
of the heavy atom is to enhance spin-orbit coupling, and enhances the probability of radi-
ationless processes such as intersystem crossing. The fluorescence of luminescing ligands
is usually quenched by complexation of the ligand with main group transition metal ions.
This is believed to occur by way of a process known as paramagnetic quenching, in which
the unpaired electrons of the metal ion interact initially with the
π
electrons of the ligand,
thereby producing a pathway for intersystem crossing from the directly excited singlet state
of the ligand to states of higher multiplicity introduced by interactions with the metal ion.
1.3.4.9 Fluorescence Spectra
Traditionally a fluorescence spectrum is either a plot of luminescence intensity at a fixed
excitation wavelength as a function of emission wavelength (an emission spectrum) or a
plot of luminescence intensity at a single emission wavelength as a function of excitation
wavelength (an excitation spectrum).
The fluorescence of a complex mixture of fluorophores can be represented as a two-
dimensional matrix of fluorescence intensity as a function of both excitation and emission
wavelength. Cross sections of this excitation-emission matrix (EEM) at fixed excitation
wavelengths and at fixed emission wavelengths are, respectively, standard emission and
excitation spectra. The distribution of emission intensities among the emission wave-
lengths, that is, I(
λ
em
), may be expressed as
{
}
I
(
λ
)
=
K
ηλ λ
(
)
I
() exp(
1
−
−
αλ
())
cl
(1.16)
em
em
0
ex
ex
where
K
is a constant that accounts for wavelength-interdependent experimental param-
eters,
η
(
λ
em
) is the quantum yield of fluorescence at
λ
ex
and
λ
em
. The intensity of the inci-
dent radiation is defined as
I
0
(
λ
ex
), and
α
(
λ
ex
) represents the absorption cross section at
λ
ex
.
