CH 415 Chapter 15: Molecular Luminescence Spectroscopy: Fluorescence, Phosphorescence, and Chemiluminescence

What happens to the absorbed EM energy determines whether you have...

1. Absorbance:

– molecule returns to the ground or lower energy state via a non-radiative transition such as vibration, collision with other molecules, etc. These give off the energy absorbed rather than the emission of light.

2. Fluorescence:
– Some energy is lost through various processes (e.g. non-radiative transitions) and then light is given off.

3. Phosphorescence:

– The molecule transitions from an excited triplet state to a lower energy singlet state (t*→s) and gives off light. Non-radiative transitions intervene.

Fluorescence, Phosphorescence, and Chemiluminescence

Absorption:For UV/Vis need to observe P0 and P difference, which limits detection (10^-14 to 10^-15s)

• M* → M+heat (10^-8 to 10^-9 s)

• Mass detection limit: 10^-13 to 10^-16 mole; concentration detection limit: 10^-5 to 10^-8 M, advantage: universal

1. Theory of Fluorescence (10^-5 to 10^-8 s)and Phosphorescence (10^-4 to 10 s):

-excitation of e- by absorbance of hv

-Re-emision of hv as e- goes to ground state

-Use hv2 for qualitative and quantitative analysis

• Mass detection limit: 10^-15 to 10^-17 mole; concentration detection limit: 10&-7 to 10^-9; advantage: sensitive

Deactivation processes

1. vibrational relaxation: solvent collisions

• l emission > l excitation (Stokes shift)

• vibrational relaxation is efficient and goes to lowest vibrational level of electronic state within 10^-12 s or less.

• significantly shorter life-time than electronically excited state

•  fluorescence occurs from lowest vibrational level of electronic excited state, but can go to higher vibrational state of ground level.

2. internal conversion:

• crossing of e- to lower electronic state.

• S1 to S0 would also happen .

• efficient, therefore many compounds don’t fluoresce (aliphatic)

• especially probable if vibrational levels of two electronic states overlap, can lead to predissociation or dissociation.

- dissociation: direct excitation (absorption) to vibrational state

with enough energy to break a bond

- predissociation: relaxation to

vibrational state of a lower electronic state with enough energy to break a bond

3. external conversion:

• deactivation via collision with solvent (collisional quenching)

• decrease collision→ increase fluorescence or

  phosphorescence

• decrease temperature and/or increase viscosity

• decrease concentration of quenching (Q) agent

4. intersystem crossing:

• spin of electron is reversed

- change in multiplicity in molecule occurs (singlet to triplet) - enhanced if vibrational levels overlap
- more common if molecule contains heavy atoms (I, Br)

1. Phosphorescence:

Deactivation from an ‘triplet” electronic

state to the ground state producing a photon

Variables Affecting Fluorescence

1. Quantum Yield (phi):

• ratio of # of molecules that luminesce to the total # of excited molecules → efficiency

• determined by the relative rate constants (kx) of deactivation process

• Increase quantum yield by decreasing factors that promote other deactivation processes