Topics

• Definition• Theory’s• Factor’s• Instrumentation• Applications

Luminescence

• Emission of previously absorbed radiation

•Types : Luminescence is two types 1. Florescence

2.Phosphorescence

Fluorescence

• When a beam of light is incident on certain substances, they emmit visible light or radiations.

• ( The substance is fluorescent substance. Measurement is called as fluorimetry. )

Phosphorescence

• When a beam of light is incident on certain substances, they emit radiations continuously even after the incident light is cut off .

• (The substance is phosphorescent. Measurement of radiations is Phosphorimetry. )

Types of fluorescence

• 1. Based upon the wave length of emitted radiation when compared to absorbed radiation

- Stokes's fluorescence

- Anti- Stokes's fluorescence

- Resonance fluorescence

• 2. Based upon the phenomenon

- Sensitized fluorescence

- Direct line fluorescence

- Stepwise fluorescence

- Thermally assisted fluorescence

Theory1.Singlet and triplet states2.Excited-state state process in

molecules3.Relation between fluorescence

intensity and concentration4.Factors affecting fluorescence

Singlet and triplet states

• A molecular electronic state in which all of the electron spins are paired is called a singlet state. (spine state)

• If ‘no’ un pair electrons are present (n=0), there is only n+1 or 0+1 spin state. Such state is called a singlet state. Similarly, systems having 1,2,3,4…… unpaired electrons refer to doublet, triplet, quartet etc. state respectively

Excited –state processes in molecules

• 1.Collisional deactivation

• 2.Fluorescence

• 3.phosphorescence

S0

S1

v2

v4

v0

v3

v5

v0

v3

v5

v7v0

v3

v5

v7

AbsorptionCollisional deactivationFluorescencePhosphorescenceIntersystem crossingVibrational relaxation

processes in molecules

T

Relation between intensity of fluorescence and concentration

• We know that “less number of molecules absorb lesser radiation and so emit lesser radiation”.

• “similarly more number of molecules absorb more radiation”.

• Fluorescence intensity=Q x Ia

• Ia -intensity of absorbed light

• Q – fluorescence efficiency

• Q= fluorescence quanta emitted

EMR quanta absorbed

• Since Emission is proportional to Absorption.

• Ia = Io- It• I0 = Intensity of incident light

• It = Intensity of transmitted light

• It = I0 e –act

• Ia = I0 – I0 e –act

• Ia = I0 – (1-e –act)

• Ia = I0(1-(1-act)) because e –act =1-act

• Ia = Io (1-1+act)

• Ia = I0 x act

• F.I = Q x Ia

• F.I = QI0act

• Q = constant for a particular substance

• I0 = constant for an instrument

• a = molecular extinction coefficient• T = path length • C = concentration

• Fluorescence intensity is proportional to the concentration of the substance.

• If the concentration is high it does not fallow due to quenching effect.

concentration concentration

% in

tensi

ty

% in

tensi

ty

Low con High con

Quenching

• Quenching is the decrease in fluorescence intensity

Types :

• Self or concentration quenching

• Chemical quenching(ph,o2,halides,heavy metals)

• Static quenching

• Collisional quenching

Factors• Nature of molecules• Nature of substitutent’s• Effect of concentration• Light• Methods of illumination• Oxygen• Photodecomposition• Temperature and viscosity• Adsorption

Nature of molecules

• Compounds which absorb U.V and VIS radiations shows the fluorescence.

• Molecules having conjugated double bond are suitable.

• Aliphatic and Saturated cyclic organic compounds are not suitable.

Nature of substitutents

• Electron donating groups like –NH2, -OH enhance the fluorescence. some of the groups like -SO3H,-NH4 and Alkyl groups not effect Phosphorescence and Fluorescence.

• Electron withdrawing groups like -COOH, -NO2,-N=N- and Halides decrease or even destroy the Fluorescence

Effect of concentration

• Fluorescence α Concentration (only for small values )• Concentration should low to get good

results strongweak

• Light: Monochromatic light is essential.

• Methods of illumination :Right angle method is only used ,another method is frontal method.

• Oxygen : It shows two effects - Oxidation of fluorescent

substance in to non-fluorescence substance.

- Quenching the fluorescence.• Photodecomposition : Radiation

passing through the solutions is weak adequate for the measuring.

• Photodecomposition : Radiation passing through the solution is weak adequate for the measuring

• Temperature and viscosity : variations in temperature and viscosity will cause variations in the frequency

• Adsorption: For the fluorimetry we need 10 – 100 weaker solution than the spectrophotometry

Instruments

• Source : mercury vapor lamp, xenon arc lamp, tungsten lamp.

• Filters and monochromators

primary filter

secondary filter

• Sample cell

• Detectors

Instruments

• Source :Mercury vapor lamp, Xenon arc lamp, Tungsten lamp.

• Filters and monochromators

primary filter

secondary filter

• Sample cell

• Detector’s

Single beam fluorimeter

Tungsten source

Low passfilter High pass

filter

cuvette

Photomultiplier tube

Lamp

Sample Solution

Reference or blank

Secondary Filter

Secondary Filter

Photo Multiplier Tube

Primary Filter

Primary Filter

Double beam fluorimeter

Applications

• Inorganic chemistry• Organic chemistry• Biological• Food products• Pharmaceutical• Clinical• Natural products

Notes

• Fluorescence- relaxation through the emission of a photon

• Generally occurs on the ms-ns timescale.

• Phosphorescence is the emission of a photon from a excited triplet state

• Phosphorescence is an extremely rare property

Notes

• Absorption occurs on the 10-15 seconds• Relaxation from the ground state occurs

through the fastest available process• Most molecules do not fluoresce because

the excited vibrational states of S0 overlap with the S1 state and relaxation can take place rapidly by vibrational relaxation

• Generally on the picoseconds timescale

Thank you