1. 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
Theory of fluorescence and phosphorescence
Instruments
Applications
Chemiluminescence

2. 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

3. 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
2S1/2

4. 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

5. Theory Properties in excited state can differ
Paramagnetic triplet, diamagnetic signlet
Change between signlet and triplet unlikely
Higher lifetime

6. Energy diagram

7. Excitation 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*

8. 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. Emission and Excitation

10. Fluorometer
Excitation and emission wavelengths

11. Spectrofluorometer
Scanning excitation and emission

12. Spectra

13. Applications Direct fluorescence of compound
Can be applied to some inorganics
Fluorometric reagent
Evaluate change in property
Lifetime measurements

14. Chemiluminescence Chemical reaction based A+B->C*+D C*->C+g
Utility depends upon production of C*