• Absorption and emission spectra are usually mirrored.
• Due to Kasha's Rule, all observed transitions are between S1, S0, and T1 states.
• A fine structure is observed from the relaxation to higher vibrational states of S0.
• The most intense transitions occur at the highest orbital overlap, according to the Frank Condon principle.
• If orbitals are shifted and there is imperfect overlap, the 0,0 transition may lose intensity or disappear, resulting in a Stokes Shift.
• Transitions are numbered in the form ((\nu,\nu^{\prime})), and depending on the overlap of wavefunctions, the major peak may vary.
  • 

Kinetics

• 
• There are multiple competing processes that govern the kinetics.
• Quantum yield (\phi = \text{Quantum yield} = \frac{\text{No. } M^* \text{ that give } h\nu}{\text{No. } M^* \text{ made}} = \frac{k_r}{k_{tot}})
• The higher the quantum yield, the better the spectral imaging.
• If a molecule cannot do ISC, (\phi = \frac{k_{rad}}{k_{nr}+k_{rad}}), where (nr) denotes non-radiative processes.
• 
• Lifetime (\tau) is the duration for which (M^*) exists: (\tau = \frac{1}{k_{tot}}).
• (\frac{d[M^]}{dt} = -k_{rad}[M^]-k_{nr}[M^*])
  • (\frac{d[M^]}{[M^]} = -(k_{rad}+k_{nr})dt)
  • (M^* = [M^*]_{(t=0)}\exp{(-t/\tau)})
• Fluorescent: (\tau) = 10 ps - 100 ns
• Phosphorescent: (\tau) = 100 ns - 1 s

Time Resolved Emission Spectroscopy (TRES)

• Excitation is achieved using a sharp, short-pulsed laser.
• After each pulse, emission is measured with respect to time.
• The shorter the pulse, the better the spectrum. Poor pulses can be deconvoluted.

• Quenching

  • A Quencher can accept the excitation from a molecule and let it relax without light formation.
  • (M^* + Q \rightarrow Q^* + M \rightarrow Q + M)
  • The more Q is added, the less emission is measured.
  • (\frac{d[M^]}{dt} = -(k_{nr}+k_{rad})[M^]-k_q[Q][M^*])
  • Spectra can be compared with and without quencher analysis via a Stern-Volmer analysis specdef.
    • (\frac{I_f^o}{I_f} = \frac{\phi_f^o}{\phi_f} = \left(\frac{k_{rad}}{k_{nr}+k_{rad}}\right)\left(\frac{k_{rad}}{k_{nr}+k_{rad}+k_{Q}}\right)^{-1} = 1 + \tau_0 k_q [Q])
    • A Stern-Volmer plot quantifies fluorescence loss as a function of quencher addition specdef.
  • Quenching mechanisms
    • (^{1}M^* + Q \rightarrow \ ^{1}M + Q^*)
      • Forster Mechanism and Dexter Mechanism
    • (^{3}M^* + \ ^{1}Q \rightarrow \ ^{1} M + \ ^{3}Q^*)
    • (^{3}M^* + \ ^{3}Q \rightarrow \ ^{1} M + \ ^{1}Q^*)
      • Oxygen concentration can be determined due to triplet oxygen.
    • 
  • Use of the Stern-Volmer plot in real life:
    • A polymerization is carried out with a chromophore and quencher.
    • In the liquid state, quenching occurs easily.
    • In the solid state, quenching can no longer occur, so fluorescence increases.

• Polarization

  • The transition dipole moment results in polarized light being emitted, but tumbling molecules mean this cannot be measured.
  • To measure polarization, the sample must be frozen as a glass.
  • Anisotropy is the extent of polarization of light specdef.
    • This can be plotted over a reaction showing when solidification takes place, e.g., resin curing.

Instrumentation #UV-vis

• 
• Emission scan - Fixed incident wavelength, emission wavelength scanned specdef.
• Excitation scan - Scanned incident wavelength, fixed emission detection specdef.
  • Can pick up the emissive parts of absorption experiments.
• Inner filter effect - Highly concentrated samples may result in lower emissions due to all excitation happening at the light source point and not through the sample.
• Raman shifts may be visible in very low concentration spectra.

• How is quantum yield calculated? spcards
  • (\phi = \frac{No. \ M^* \ that \ give \ h\nu}{No. \ M^* \ made} = \frac{k_f}{k_{tot}})
  • (\phi = \frac{k_{rad}}{k_{rad}+ k_{non \ rad}})
• How is the lifetime (\tau) of an excited molecule calculated? spcards
  • (\tau = \frac{1}{k_{tot}})
• What is the typical lifetime of a fluorescent compound? spcards
  • 10 ps - 100 ns
• What is the lifetime of a phosphorescent compound? spcards
  • 100 ns - 1 s
• In Time Resolved Emission Spectroscopy (TRES), what kind of light source is used? spcards
  • Sharp, short-pulsed laser.
• How can badly pulsed spectra be improved in emission spectroscopy? spcards
  • Deconvolution
• What is a Stern-Volmer plot? spcards
  • (\frac{I_f^0}{I_f} = \frac{\phi_f^0}{\phi_f} = 1 + \tau_0 k_q [Q])
  • Fluorescence as a function of quencher addition.
• Give the Forster Mechanism for quenching. spcards
  • (^{3}M^* + \ ^{1}Q \rightarrow \ ^{1} M + \ ^{3}Q^*)
• Give the Dexter Mechanism for quenching. spcards
  • (^{3}M^* + \ ^{3}Q \rightarrow \ ^{1} M + \ ^{1}Q^*)
• Why can we not measure polarization in liquid state emission spectroscopy? spcards
  • The molecules are tumbling.
• What is Anisotropy? spcards
  • The extent of light polarization.
• What is an Emission scan? spcards
  • Fixed incident wavelength, emission wavelength scanned.
• What is an Excitation scan? spcards
  • Incident wavelength scanned with fixed emission wavelength detection.
• What is the Inner filter effect? spcards
  • Highly concentrated samples show low emission due to emission occurring close to the light source and not through the whole sample.