LASER FLASH PHOTOLYSIS

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LASER FLASH PHOTOLYSIS

• SWATHI GANNAVARAM
• CHE 466-LASER MOLECULAR SPECTROSCOPY

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PHOTOCHEMICAL REACTION

• Photochemical reaction - induced by light.
• Molecules produce electronic excited states.
• Very fast
• Hence FAST REACTION techniques are needed to
  study such reactions.

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FLASH PHOTOLYSIS

• Important FAST REACTION technique.
• Flash lamps like xenon lamp
• moderate reactions
• slow decay time of the light emitted from it.
• Not useful for fast reactions.

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FLASH PHOTOLYSIS

• The pulse width of the light source must be less
  than the half time of the chemical reaction.
• xenon flash lamps - pulse width µs time scale.
• Specially designed lasers with pulse widths in
  the ns or ps scale are required for fast and very
  fast reactions.

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LASER FLASH PHOTOLYSIS

• Allows the study of a very wide range of
  photochemical processes.
• The set up usually uses a Nd-YAG
  (neodymium-yttrium aluminum garnet) laser or an
  eximer (excited dimer) laser.

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LASER FLASH PHOTOLYSIS

• Nd-YAG (synthetic mineral) - excited by flash
  lamps to produce light in the IR region at 1064nm
• Photon frequency then
• doubled(532nm)
• tripled(355nm)
• quadrupled(266nm)
• Power - greatly reduced at each successive step.
• For many conjugated aromatic compounds, output at
  355nm works well

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• Eximer laser
• dimer stable in its excited state
• dissociates readily in its ground state.
• XeF(351nm),XeCl(308nm) etc are usually used
• spectral width - 0.3nm
• pulse width 10ns

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Monitoring fast reactions

• Using different techniques
• Conductivity
• IR
• Raman
• Mass spectrometry
• Most commonly used - UV/Visible
  spectrophotometry.
• A very fast digital oscilloscope digitizes the
  signal from the photo detector.

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APPLICATION
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Laser flash photolysis study of the
aminophtalimide derivativesElucidation of the
nonradiative deactivation route
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What are Aminophthalimides (AP)?

• Highly fluorescent compounds.
• Excitation leads to enhanced separation of charge
  between the amino nitrogen and carbonyl moieties.
• Intramolecular charge transfer causes them to
  fluoresce.
• Highly sensitive to the polarity and hydrogen
  bond donating nature of the solvent.

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What are they studying?

• AP - deactivated due to specific hydrogen bonding
  interaction with protic solvent.
• No attempts made to know if the decrease in
  fluorescence efficiency is due to
• internal conversion or
• inter system crossing or
• both.
• Can be studied by monitoring the triplet state of
  the system.
• Transient absorption studies- LASER FLASH
  PHOTOLYSIS.

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OBJECTIVE

• Study the fluorescence behavior of three
  different aminophthalimide derivatives with
  different ring sizes in
• less polar (1,4-dioxane),
• polar aprotic (acetonitrile) and
• protic (methanol) solvent.
• Report the results of triplet state of the system
  using laser flash photolysis.

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EXPERIMENTAL

• Cary100 UV-VIS spectrophotometer steady state
  absorption
• Fluorolog-3 Spectrofluorimeter Fluorescence
  spectra
• Fluorescence quantum yields (ff) obtained using
  AP as the reference compound (ff0.63 in
  acetonitrile)
• Transient absorption measurements Laser flash
  photolysis setup
• Applied photophysics LKS60 Kinetic spectrometer
  work station software - record and analyse the
  data

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SET UP

• Q switched ND-YAG laser (pulse width 8ns)
• Spectrometer consisted of
• 150W pulsed xenon lamp,
• programmable f/3.4 grating monochromator,
• digitized oscilloscope,
• photomultiplier tube
• The solutions were excited by third
  harmonic(355nm) of laser

Monitoring light
Detector
Sample
Amplifier
Triplet-triplet absorption
ISC
Laser
VR
Oscilloscope
Flash excitation
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RESULTS
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Steady State absorption

• Absorption maxima observed between 380 and 400nm
• Shift in absorption maxima with increasing
  polarity

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Fluorescence behavior

• Influence of solvent on the fluorescence spectra.
• Solvent effect

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Time resolved absorption studies

• Transient absorption 450-550nm
• Maximum absorption at 470nm in dioxane
• Red shift due to increase in polarity
• Triplet-triplet trasition
• A.4P in 1,4-dioxane(270ns,2µs,5µs)
• B.4P in acetonitrile(400ns, 2µs,5µs)

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• Additional support involves triplet sensitization
  study using thioxanthone
• Thioxanthone sensitized triplet-triplet
  absorption spectra of 4P in acetonitrile

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Estimation of Extinction coefficient values

• The extinction coefficient(eT) of triplet-triplet
  absorption is calculated using
• eT eTR(?OD)8kobs/(?OD)0R(kobs-k0)
• ( eTR 3 x 10-4M-1cm-1 at 630nm)

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Estimation of Triplet Quantum Yield(FT )

• Comparitive Actinometry method using
  4,4-dimethoxybenzophenone
• FT FTR(?OD)0 eTR/(?OD)0R eT
• (eTR 5200 M-1cm-1 at 545nm FTR 1.0)

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Trends from the table

• 1,4-dioxane(least polar)
• singlet triplet yields(85-93)
• IC(7-15)
• Acetonitrile(polar,aprotic)
• singlet triplet yields(44-78)
• IC(22-56)
• Methanol(protic)
• IC(87-96)

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CONCLUSIONS

• Aminophthalimides characterized by low triplet
  yields and moderate values of molar extinction
  coefficients of triplet-triplet absorption
• In protic solvent(methanol) where the
  fluorescence was found to be weak,the triplet
  yield also was found to be low
• Hence from the results it can be concluded that
  an enhanced internal conversion process is
  responsible for low fluorescence efficiency in
  polar protic and aprotic solvents.

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References

• 1) Bhattacharya, B. Samanta, A. Chemical Physics
  Letters 2007, 442, 316-321.
• 2) Hollas, J.M.,Modern Spectroscopy, Wiley, 4,
  2004, 356-357.
• 3) Gilbert, A, Baggot, J, Essentials of
  Molecular Photochemistry, CRS Press,1991, 115.

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Special thanks

• Dr.Cedeno
• Audience

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QUESTION

• What are the pulse widths for laser flash
  photolysis and what kind of reactions (fast slow
  or medium) can be studied by them and also list
  an application?
• Nanosecond or Picosecond
• Fast and very fast
• Transient triplet-triplet absorption