9-1

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

9-2

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

9-3

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

9-4

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

9-5

Theory

• Properties in excited state can differ Paramagnetic

triplet, diamagnetic signlet

• Change between signlet and triplet unlikely Higher lifetime

9-6

Energy diagram

9-7

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->or ->

9-8

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

9-9

Emission and Excitation

9-10

Fluorometer

Excitation and emission wavelengths

9-11

Spectrofluorometer

Scanning excitation and emission

9-12

Spectra

9-13

Applications

• Direct fluorescence of compound Can be applied to some inorganics

• Fluorometric reagent Evaluate change in property

• Lifetime measurements

9-14

Chemiluminescence

• Chemical reaction based A+B->C*+D

C*->C+* Utility depends upon production of C*