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

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

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

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

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

Energy diagram

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*

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

Emission and Excitation

Fluorometer Excitation and emission wavelengths

Spectrofluorometer Scanning excitation and emission

Spectra

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

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