Basic principles of Fluorescence

Dr. Nagendra Kalva

Fluorescence is a luminescence—The process of the molecule emit light from the electronically excited states created by the absorbance of light, mechanical or chemical reaction.

Photo luminescence

Fluorescence Phosphorescence

Hot body emits radiation solely because of high temperature is called incandescence. Other form of light emission from the solids is called luminescence.

At room temperature most molecules occupy lowest vibrational level of the ground electronic state

If molecule is illuminated with light at a resonance frequency

Fluorescence process governed by three important events

1) Excitation of molecule by photon to the first excited electronic level and its sub vibrational levels (femtoseconds 10 x E-15 sec)

2) The molecule loses its excess vibrational energy and falls to the lowest vibrational level of excited state. ( picoseconds 10 x E-12 )

3) Emission of longer wavelength photons and returned to the ground state of the molecule. (nanoseconds 10 x E-9 )

Excitation - following light absorption, a chromophore is excited to some higher vibrational energy level of S1 or S2

Internal conversion: relaxation to the lowest vibration energy level of The first excited state (S1 0) (loss of energy without emission of light) . ( picoseconds 10 x E-12 ) Faster than the fluorescence process. Excess vibrational energy converted into heat which is absorbed by the neighboring solvent molecules

Relaxation of a molecule from the long-lived state to the ground state accompanied with emission of light is known as fluorescence.

The well-studied probe fluorescein isothiocyanate (FITC) can undergo excitation and relaxation for approximately 30,000 cycles before the molecule no longer responds to incident illumination.

Several other relaxation processes are possible with fluorescence

1) Excited fluorophore can colloid with another molecule ( quenching –non radioactive process)

2) The excited molecule may spontaneously reverse its electron spin (spin flip) and forms the triplet state. This process is called intersystem crossing. Electrons in both orbitals now have same spin orientation

Relaxation from the triplet excited state to the ground state is known as phosphorescence

Inter system crossing: It occurs via inversion of the spin of the excited electron resulting in two unpaired electrons with the same spin orientation, resulting in a state with Spin=1 and multiplicity of 3 (triplet state).

S0

S1

T1

absorptionfluorescence

phosphorescence

Intersystemcrossing

Phosphorescence is forbidden process. Transitions between states of different multiplicity are formally forbidden.Transition excited triplet state to the ground state takes long time 10-4 to 102

The triplet state has lower energy than its associated singlet state so that emission of light with lower energy.

The phosphorescence occurs at longer wavelength.

Absorption, Fluorescence and Phosphorescence spectrum of Chrysene.

Absorption spectrum occurs results from

several favored electronic transitions from the ground state to the lowest excited electronic level.

These transitions occurs at highest photon energy (lower wavelength or higher wavenumber).

Wide range of photon energies associated with absorption transitions causes the spectra appear as broad band rather than discrete lines.

S0

S1

v=0

v=1

v=2v=3v=4v=5

v’=0v’=1v’=2v’=3v’=4v’=5

Mirror image rule

Emission of the fluorescence takes place always from the lowest vibrational level of first excited state, so the shape of emission spectrum is always same.

The lowest vibrational level in the ground electronic state to the lowest vibrational level in the first excited state 0 0 transition is common both absorption and emission process so that we observe emission spectrum to overlap the absorption spectrum corresponds to the 0 0 transitions.

Deviations from the mirror image rule

Stokes shift Energy associated with fluorescence emission is less compared with absorption so that the emitted photons with less energy and are shifted to longer wavelength this phenomenon is known as stoke shift.

heat

The shift varies with molecular structure. Stoke shift in some fluorophores. Flurescein 20 nm, Quinine 110 nm and porphyrins 200 nm.