Molecular Luminescence
• Molecular fluorescence, phosphorescence, and
chemiluminescence
• Molecules excited followed by emission
• Fluorescence and phosphorescence by absorbance of photons
• Chemiluminescence from excitation due to chemical reaction
 Oxidation product
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Theory of fluorescence and phosphorescence
Instruments
Applications
Chemiluminescence
9-1
Theory
• Electrons excited

State lifetime 10 microseconds to 1 nanosecond
 Resonance fluorescence
* No change in frequency
 Stokes shift (longer wavelengths)
• Fluorescence and phosphorescence based on

Electron spin

Singlet and triplet states
• Spin

Electrons in same orbital must have opposite spin
 Each electron has unique quantum number

Spin pair, no magnetic field
 Diamagnetic

Unpaired e- have magnetic moments
 Paramagnetic
9-2
Theory
• Quantum numbers

n=1,2,3,4
 prinicipal

l=0 to n-1
 Angular momentum

ml= +-l
 magnetic

s=+-1/2
 Spin
* Electron in same orbital (n, l, ml) must have different
spin
• 2S+1LJ
• S is spin from unpaired e
+½

L is written as S, P, D

J=L+S
• Li= 1s22s1

L=0, S =+ ½
2S
9-3

1/2
Theory
• Excited states
 Singlet
 All electron spins paired
 Double
 Unpaired ground state electron
 Triplet
 Excited state electron promoted with unpaired
spin
* Less energetic that excited signlet state
9-4
Theory
• Properties in excited
state can differ
 Paramagnetic
triplet, diamagnetic
signlet
• Change between signlet
and triplet unlikely
 Higher lifetime
9-5
Energy diagram
9-6
Excitation
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•
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Rates of absorption are high (fs)

Fluorescence emission has a longer lifetime (ns)
Relaxation

Vibration

Internal conversion
 Overlap vibration levels

External conversion
 Quenching with solvent

Intersystem crossing
 Spin flip
Quantum Yield

Ratio of molecules that luminence to total excited
molecules

Mainly n->p* or p ->p*
9-7
Fluorescence
• Increase fluorescence
 Aromatic groups
 Rigidity
• decrease fluorescence
 Temperature
increase
 Heavy atoms in
solvent
 Dissolved O2
• pH can change species
• Self absorption at high
concentration
9-8
Emission and Excitation
9-9
Fluorometer
Excitation and emission wavelengths
9-10
Spectrofluorometer
Scanning excitation and emission
9-11
Spectra
9-12
Applications
• Direct fluorescence of compound
 Can be applied to some inorganics
• Fluorometric reagent
 Evaluate change in property
• Lifetime measurements
9-13
Chemiluminescence
• Chemical reaction based
 A+B->C*+D
 C*->C+g
* Utility depends upon production of C*
9-14