Heavy Atom Quenching is a process inducing radiationless intersystem crossing converting molecules from a vibrationally active S1 state into an iso energetic triplet state T1. Heavy atoms or Atoms of high nuclear charge, either as substituents of fluorescent compounds or part of solvent, assumed to quench fluorescence by perturbation of fluorescencing state S1 via spin orbit coupling and hence deactivation into induced triplet state.

1. PRESENTED BY:
VIDUSHI SHARMA,
DEPARTMENT OF CHEMISTRY

2. • Molecular absorption of photons triggers the
emission of a photon with a longer wavelength
•Electron in the ground state is excited to a higher
energy state
•After loss of some energy in vibrational relaxation,
the high energy electron returns back to the ground
state by emitting fluorescent photon.
• If the spin of electron is flipped (intersystem
crossing), electron goes to the triplet state, whose
return to ground state is forbidden.
•Triplet state can result either in phophorescence or
in delayed fluorescence

3. Static Quenching
• Ground state complex formation
Dynamic quenching
• Collisional: electron transfer, spin-
orbit coupling, inter molecular H-
bonding, and intersystem crossing
to the excited triplet state without
chemical alteration

4. Heavy Atom Quenching is a process inducing radiationless intersystem crossing
converting molecules from a vibrationally active S1 state into an iso energetic
triplet state T1.
Heavy atoms or Atoms of high nuclear charge, either as substituents of
fluorescent compounds or part of solvent, assumed to quench fluorescence by
perturbation of fluorescencing state S1 via spin orbit coupling and hence
deactivation into induced triplet state.
Spin orbit interaction
Causes shift in electron’s atomic energy levels due to electromagnetic interaction
between electron spin and magnetic field generated by electron’s orbit around
nucleus

5. •Presence of Heavy atom (Mostly Halogens, eg. Cl, Br, I)
•Energy difference between S1 and accepting triplet < 10kk
Complex formation between fluorophore and quencher , in which orbitals of
heavy atoms can overlap those of excited molecules.
Distance of Quencher from fluorophore also effects exponentially
Probability of quenching W®= A exp(-2r/L)
Where A- strength of interaction
L- effective mean Bohr radius of fluorophore quencher pair

6. Internal H.A.Q
Heavy atom is
present in
fluorescent
molecule itself as
substitutent
External H.A.Q
Heavy atom is
present in solvent
as quencher.
Quenching is
Diffusion controlled

7. Quenching could be concieved as sequencial process of:
•Electron transfer by which triplet exciplex is formed
•Decay of triplet exciplex by dissociation into free radical/ heavy atom
induced electron back transfer

8. J. M. C. Martinho, “Heavy-Atom Quenching of Monomer and Excimer Pyrene
Fluorescence”, J. Phys. Chem. 1989, 93, 6687-6692

9. Herbert DREESKAMP and Joachim PABST, “THERMALLY ASSISTED
AND HEAVY-ATOM ASSISTED INTERSYSTEM CROSSING IN MESO-
SUBSTITUTED ANTHRACENES” CHEMICAL PHYSICS LETTERS
A singIe Iinear correlation between the activation energy of St-T intersystem
crossing and the logrithm of the bimolecuhr fluorescence quenching rate
constant by haloalkanes for mcso-substituted anthracenes in fluid solutions is
established- It is concluded that both the monomolecular and the hcvy-atom-
assisted deactivation of St depend in the same way on the change of Sr energy
by substitution or solvent relative to the energy of the accepting triplet T,,. The
anomalous increase of fluorescence of Br-substituted anthracenes in heavy-atom
containing solvents is consistent with a widening of the S, -Trr energy gap due to
the changing solvent

10. H. DREESKAMP, ;. KOCH, “FLUORESCENCE OF BROMOPERYLENES AND TFE
REQUIREMENTS OF HEAVY-ATOM QUENCHING”, CHEMICAL PHYSICS
LETTERS

11. Rudolf E. FOll, Horst E. A. Kramer, “Role of Charge Transfer and Spin-Orblt
Coupling in Fluorescence Quenching. A Case Study with Oxonine and Substituted
Benzene”, J. Phys. Chem. 1990, 94, 2476-2487
Quenching reasons:
•Radical formation
•Induced Triplet formation
•Induced Internal conversion
In case of heavy atom quenching:
It has been reported that Quenching due to radical formation is
minimum.
Maximum quenching is seen due to Internal conversion and
induced triplet formation.

13. Quenching due to Radical Formation:
Evans et al. 2013, “Magnetic field effects in flavoproteins and related
systems”, Interface Focus 3:
20130037.
http://dx.doi.org/10.1098/rsfs.2013.0037

14. Quenching due to Inter state crossing
Event is relatively rare, but ultimately results either in emission of a photon
through phosphorescence or a transition back to the excited singlet state that
yields delayed fluorescence. Transitions from the triplet excited state to the
singlet ground state are forbidden, which results in rate constants for triplet
emission that are several orders of magnitude lower than those for
fluorescence.
The primary importance of the triplet state is the high degree of chemical
reactivity exhibited by molecules in this state, which often results in
photobleaching and the production of damaging free radicals.
Upon transition from an excited singlet state to the excited triplet state,
fluorophores may interact with another molecule to produce irreversible
covalent modifications. The triplet state is relatively long-lived with respect
to the singlet state, thus allowing excited molecules a much longer timeframe
to undergo chemical reactions with components in the environment. The
average number of excitation and emission cycles that occur for a particular
fluorophore before photobleaching is dependent upon the molecular
structure and the local environment.

15. In solution, solvent molecules surrounding the ground state fluorophore also have
dipole moments that can interact with the dipole moment of the fluorophore to
yield an ordered distribution of solvent molecules around the fluorophore. Energy
level differences between the ground and excited states in the fluorophore produce
a change in the molecular dipole moment, which ultimately induces a
rearrangement of surrounding solvent molecules. However, the Franck-Condon
principle dictates that, upon excitation of a fluorophore, the molecule is excited to a
higher electronic energy level in a far shorter timeframe than it takes for the
fluorophore and solvent molecules to re-orient themselves within the solvent-solute
interactive environment. As a result, there is a time delay between the excitation
event and the re-ordering of solvent molecules around the solvated
fluorophore,which generally has a much larger dipole moment in the excited state
than in the ground state.

16. APPLICATIONS OF HEAVY ATOM BASED QUENCHING
•Wineforner andcoworkers: useof heavyatomssolvents improvesboth
detectionlimit and dynamicrange
•Fluorescent probe experiments
•Control certainphotochemicalreactions
•Creating ON-OFF-ON halidesensing switchingunit

17. Application in Halide sensing
Optical halide sensing using fluorescence quenching: theory and simulations:
Optically excited luminescence is generally thought of as the emission of light from
an electronically excited state .When the electron in the excited orbital has the
same spin orientation as the ground state electron, the transition to the ground
state, T1 → S0, is spin forbidden and the emission of the photon is relatively slower,
i.e. phosphorescence. Since phosphorescence is ‘long lived’ it is not normally
observed in fluid solution at room temperature, whereas fluorescence is commonly
observed from fluid solutions in the absence of a quencher. This is because many
deactivation processes compete with emission, such as non-radiative quenching
processes (figure 1). It is these fluorescence and phosphorescence quenching
processes that allow us to quantitatively sense halide.
Modified Jablonski diagram
illustrating quenching by halide
ions, where the resultant triplet
state can be depopulated by: non-
radiative processes; radiative
decay, i.e. phosphorescence hνP ;
or quenched, e.g. by dissolved O2.