CHEM420'lecture'5

1
Molecular Luminescence Spectrometry Chap 15
2
Factors Affecting Fluorescence and
Phosphorescence

• Quantum Yield

• Expressed in terms of rate constants
• where knr kvib kic kec kisc kpd kd
• varies from 0 to 1

3
Factors Affecting Fluorescence and
Phosphorescence

• Transition Types in Fluorescence
• s ? s Transitions (seldom observed)
• p ? p Transitions (most common, higher )
• p? n (longer lifetime, lower )
• Fluorescence typically from lowest vib. level,
  v' 0,
• of excited electronic state

4
Factors Affecting Fluorescence and
Phosphorescence

• Fluorescence and Structure
• Most intense from aromatic p ? p transitions
• increases with
• number of fused rings
• rigidity of structure (lowers kic)
• Fluorescence and Temperature, Solvent, pH
• All must be monitored to minimize kec

5
Excitation and Emission Spectra of
Phenanthrene Fig. 15-5
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Components of a Fluorometer or Spectrofluorometer
Fig. 15-8
?
?
7
Filter FluorometerFig. 15-10
Less expensive, rugged, good for concentration
measurements
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Components of a Spectrofluorometer
Fig. 15-11
300 W at 300-1100nm
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Fig. 15-11

• Resulting emission is isotropic
• If source is at fixed ? and monochromator is
  scanned an emission spectrum results.
• If source is scanned, and monochromator is at a
  fixed ?, an excitation spectrum results.

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Fig. 15-9
Fluorescence Spectra for 1 ppm anthracence in
alcohol
Emission Spectrum
Constant ?ex Variable Info on vib levels in S1
?em
Excitation Spectrum
Constant ?em Variable ?ex Info on vib levels in
S0