Download Luminescence spectroscopy

Luminescence spectroscopy
Emission techniques
What happens to the molecule once it is electronically excited?
The excess energy is transferred into vibration, rotation, and
translation of surrounding molecules through collisions.
� Molecule discards the excitation energy as a photon in a radiative
decay process.
� Fluorescence
� Phosphorescence
� The molecule dissociates and carries away the energy in the form
of translation motion of the molecular fragments.
Molecular Luminescence Spectroscopy
Luminescence spectroscopy is a technique which studies the fluorescence,
phosphorescence, and chemiluminescence of chemical systems.
Fluorescence is light emission caused by irradiation with light (normally visible
or ultraviolet light) and typically occurring within nanoseconds to milliseconds
after irradiation.
Phosphorescence is a light emission which can occur over much longer times
(sometimes hours) after irradiation. It involves storage of energy in metastable
states and its release through relatively slow (often thermally activated)
processes. The phenomenon was discovered early on for phosphorus.
Chemiluminescence is light emitted during (cold) chemical reactions.
Fluorescence, phosphorescence, chemiluminescence all follow an electronic
excitation.
1
Fig. 1. Time domain of fluorescence and phophorescence
Electronic excited states are thermodynamically unstable. They persist for
lifetimes that are normally about 10 nanoseconds for many medium sized
organic molecules. A number of transition metal complexes have lifetimes of
about 1 microsecond. Triplet states, resulting from a spin interconversion
process in the excited state, have quite long lifetimes, sometimes to beyond 1
millisecond.
Singlet and Triplet States
Electrons in molecular orbitals are paired, according to Pauli exclusion
principle. When an electron absorbs enough energy it will be excited to a higher
energy state; but will keep the orientation of its spin. The molecular electronic
state in which electrons are paired is called a singlet transition. On the other
hand, the molecular electronic state in which the two electrons are unpaired is
called a triplet state. The triplet state is achieved when an electron is transferred
from a singlet energy level into a triplet energy level, by a process called
intersystem crossing; accompanied by a flip in spin.
2
Fig. 2. Electronic transitions, paired electrons in ground and singlet state.
In a singlet state, the spins of the two electrons are paired and thus exhibit no
magnetic field and called diamagnetic. Diamagnetic molecules, containing
paired electron, are neither attracted nor repelled by a magnetic field. On the
other hand, molecules in the triplet state have unpaired electrons and are thus
paramagnetic which means that they are either repelled or attracted to magnetic
fields. The terms singlet and triplet stems from the definition of multiplicity
where:
Multiplicity = 2S + 1
Where, S is the total spin. The total spin for a singlet state is zero (-1/2, +1/2)
since electrons are paired which gives a multiplicity of one (the term singlet
state).
Multiplicity = (2 * 0) + 1 =1
In a triplet state, the total spin is one (the two electrons are unpaired) and the
multiplicity is three:
Multiplicity = (2 * 1) + 1 = 3
It should also be indicated that the probability of a singlet to triplet transition is
much lower than a singlet to singlet transition. Therefore, the intensity of the
emission from a triplet state to a singlet state is much lower than emission
intensities from a singlet to a singlet state.
Energy Level Diagram for Photoluminescent Molecules
The following diagram represents the main processes taking place in a
photoluminescent molecule when it absorbs and emits energy.
3
Fig. 3. Schematic representation of different processes in luminescence
transitions
The different processes will be discussed below:
1. Absorption
The absorption of UV-Vis radiation is necessary to excite molecules from the
ground state to one of the excited states (S0 → S2). There are four different types
of electronic transitions which can take place in molecules when they absorb
UV-Vis radiation.
A σ−σ* and a n−σ* are not useful for quantitative determination while the n−π*
transition requires low energy but the molar absorptivity for this transition is low
and transition energy will increase in presence of polar solvents. The most
frequently used transition is the π−π* transition for the following reasons:
a. The molar absorptivity for the π−π* transition is high allowing sensitive
determinations.
b. The energy required is moderate, far less than dissociation energy.
c. In presence of the most convenient solvent (water), the energy required for a
π−π* transition is usually smaller.
Therefore, best molecules that may show absorption are those with π bonds or
preferably aromatic nature as discussed earlier. Absorption to higher excited
singlet states requires a very short time (in the range of 10-14 s).
2. Vibrational Relaxation (VR)
Absorption of radiation will excite molecules to different vibrational levels of
the excited state. This process is usually followed by successive vibrational
relaxation as well as internal conversion (INC) to lower excited states.
4
INC: (An example of this process is the quinine sulfate fluorescence, which can
be quenched by the use of various halide salts. The excited molecule can deexcite by increasing the thermal energy of the surrounding solvated ions.)
In cases where transitions occur to the first excited state, vibrational relaxation
to the main excited electronic level will take place and/or an intersystem
crossing (ISC) to the triplet state can occur.
3. Fluorescence (FL)
After vibrational relaxation to first excited electronic level takes place, a
molecule can return to the ground state by emission of a photon, called
fluorescence. The fluorescence lifetime is much greater than the absorption time
and occurs in the range from 10-7 to 10-9s. As the lifetime in the excited state is
increased, the probability of fluorescence will be decreased since radiationless
deactivation processes may take place. However, not all excited molecules can
show fluorescence by returning to ground state and most return to ground state
by losing excitation energy as heat or through collisions with other molecules or
solvent.
Fig. 4. The mechanism of fluorescence. The initial excitation takes place
between states of same multiplicity.
The ground singlet state molecule, S0, goes into first excited singlet state, S1,
Fluorescence may appear as an approximate mirror image of the absorption at a
lower energy, and hence lower frequency (longer wavelength) than the
absorption, the difference (in wavenumbers) between the two corresponding
bands being known as the Stokes shift.
Fluorescence emission may show vibrational structure which can provide
information about the force constants of the molecule in its ground state (the
5
electronic structure provides information about the force constants in the excited
state). Time of radiation = time of fluorescence.
4. Internal and External Conversion
Internal conversion (INC) is a radiationless deactivation process whereby
excited molecules return to the ground state without emission of a photon. This
process seems to be the most efficient deactivation process in luminescence
spectroscopy, since most molecules do not show fluorescence.
However, molecules with close electronic energy levels, to the extent that their
vibrational energy levels of ground and excited states are overlapped, are
believed to cause efficient internal conversion.
5. Intersystem Crossing
Electrons present at the first excited electronic level can follow one of three
choices including emission of a photon to give fluorescence, radiationless
deactivation to ground state, or intersystem crossing (ISC). The process of
intersystem crossing involves transfer of the electron from an excited singlet to a
triplet state. This process can actually take place since the vibrational levels in
the singlet and triplet states overlap. However, crossing of the singlet state to the
triplet state involves a flip in electron spin in order to satisfy the triplet state.
Intersystem crossing is facilitated by presence of nonbonding electrons as well
as heavy atoms. The presence of paramagnetic atoms or species also enhances
intersystem crossing. An electron in the triplet state can also cross back to the
singlet state and can result in a photon as fluorescence but at a much longer time
than regular fluorescence. This process is termed delayed fluorescence and has
the same characteristics as direct fluorescence except for the large increase in
lifetime.
6. Phosphorescence
Phosphorescence may be defined as the emission occurring due to the radiative
transition between two states of different spin multiplicity, for example between
T1 and S0.
Electrons crossing the singlet state to the triplet state with a flipped spin can also
follow one of three choices including returning to the singlet state (including a
flip in spin), relax to ground state by internal or/and external conversion, or lose
their energy as a photon (phosphorescence, Ph) and relax to ground state with a
second flip in spin to satisfy the singlet ground state. As can be rationalized
from the processes involved in collecting phosphorescence photons, this
involves an intersystem crossing and two flips in spin. This, in fact, requires a
much longer time than fluorescence (10-4 s to upto few s). Therefore, the
probability of phosphorescence, and hence the intensity of the phosphorescence
spectrum, is very low due to high possibility of radiationless deactivation.
6
Fig. 5. The mechanism of phosphorescence.
Fig. 6. Phosphorescence occurs at a lower energy (lower wavenumber, longer
wavelength) than fluorescence.
Chemiluminescence:
Chemiluminescence occurs when a chemical reaction produces an electronically
excited species which emits a photon in order to reach the ground state. These
sort of reactions can be encountered in biological systems; the effect is then
known as bioluminescence. The number of chemical reactions which produce
chemiluminescence is small. However, some of the compounds which do react
to produce this phenomenon are environmentally significant.
7
A BC* D
C* Ch

Example:
NO O3 NO2* O2
NO2* NO2 h(600 2800 nm)
Used to detect NO from 1 ppb to 10 ppt.
Intensity depends on rate of reaction of production of C*
I cl  k cl
dc 
*
dt
8
Reading
Spin Orbit Coupling
The magnetic interaction is responsible for spin orbit coupling.
An electron is a charged particle so, its angular momentum from its orbit will
result in a magnetic field. This orbital angular momentum (L) allows it to act
like a tiny bar magnet we call spin angular momentum (S). Another magnet, the
orbital angular momentumn (we refer to as quantum number L) can interact with
the spin angular momentum. The interaction of these two magnets is called spin
orbit coupling.
a. A high total angular momentum corresponds to a parallel arrangement of
magnetic moments (represented by bar magnets), and hence a high energy (J).
We can think of it as a constructive interference.
b. A low total angular momentum corresponds to a anti-parallel arrangement of
magnetic moments and hence a low energy, interaction is destructive.
Nuclei with a high number of protons will be at the centre of a strong magnetic
current and the electron will experience a strong magnetic field. This strong
field can cause the spin to flip.
9