Quantum yeild

Quantum yeild

1. Quantum yield and lifetime Quantum yield of fluorescence, F f , is defined as In practice, is measured by comparative measurements with reference compound for which has been determined with high degree of accuracy. Fluorescein Ideally, reference compound should have the same absorbance as the compound of interest at given excitation wavelength similar excitation-emission characteristics to compound of interest Same solvent, because intensity of emitted light is dependent on refractive index (otherwise, apply correction 1a. Quantum yield of fluorescence

2. Quantum yield and life time Another definition for F f is where k r is the radiative rate constant and S k is the sum of the rate constants for all processes that depopulate the S 1 state. In the absence of competing pathways F f =1 The observed fluorescence lifetime, is the average time the molecule spends in the excited state, and it is Fluorescence lifetime

3. Illumination source Broadband ( Xe lamp) Monochromatic (LED, laser) Light delivery to sample Lenses/mirrors Optical fibers Wavelength separation (potentially for both excitation and emission) Monochromator Spectrograph Detector PMT CCD camera Fluorescence Instrumentation

4. PMT Xenon Source Excitation Monochromator Emission Monochromator Sample compartment Fluorescence Instrumentation Spectrofluorometer schematic

5. Phosphorescence: T 1  S 1  S + h n Intersystem crossing Because intersystem crossing is a “forbidden” process which occurs with low intensity with low probability phosphorescence is a slow process where photons may be emitted over a period of fractions of seconds to hours Phosphoresence – return from a triplet excited state to a ground state; electron requires change in spin orientation

6. The sequence leading to phosphorescence: The intersystem crossing is the key step, where T 1  S 1 . The triplet state acts as a slowly leaking reservoir, in that this transition is normally spin-forbidden T 1 S 1 S T 1  S 1 Phosphorescence:

7. * Phosphorence spectra occure at longer wave length Long life time, (10-6 to 102 Sec Phosphoresence is not routinly obsereved at room temp. (quenching of triplet excited state by oxygen) Degasing is required Several methods have been used to enable the observation of phosphorescence, One of the most commontechniques is to supercool solutions to a rigid glass state usually at the temperature of liquid nitrogen (77 K). At these low temperatures, molecular collisions are greatly reduced and strong phosphorescence signals are observed. Due to long life-times, the molecule has a very high probability of losing its excess energy by radiativeless routes such as internal conversion, bimolecular collision, and photodecompositions

8. Inorganic salts and oxides of rare earths, europium, and Uranium show phosphoresence Compounds exhibit phosphorescence when adsorbed onto certain substrates such as paper, cellulose, silica, etc. *Phosphoroscope *Two types of Phosphoroscopes 1) Rotating disc 2) Rotating can Phosphoroscope

9. *Measurement of Phosphoresence *Samples are prepared in rigid media (polymer matrix) *Temperature (liquid nitrogen) • good solubility of the analyte • formation of a clear rigid glass at 77 K • low phosphorescence background (high purity) Polar compounds , ethanol is an excellent solvent and small quantities of base or acid may be added to produce a clear solid. For non-polar compounds , the most popular solvent is a mixture of diethyl ether, isopentane and ethanol in the ratioof 5 : 5 : 2 respectively, called EPA