Turro Group Meeting Phosphorescence and the Triplet State

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Turro Group MeetingPhosphorescence and the
Triplet State Radical Pair Recombination and
MFEs

• Jeffrey Lancaster
• November 11, 2008

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Summary

• How do we know triplet (T) states exist?
• How do we know that phosphorescence comes from
  the triplet state?
• What effect does an external magnetic field have
  on
• triplet lifetimes?
• rates of intersystem crossing?
• product yields?
• enantiomeric excess (e.e.)?
• in confined systems?
• What effect does nuclear spin (via isotopes)
  have on the above?

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Identification of the Phosphorescent State with
the Triplet or Biradical State

• A molecule excited to T (phosphorescent state)
  can relax to S via b or with thermal excitation
  to S (fluorescent state)
• S relaxes to S via a
• At low temperatures, there is no a pathway from T

• Observation molecules in T can be excited to
  T, T - then will relax back to T, NOT to S,
  S, S
• Hypothesis Phosphorescent state is unique
• will not change (change likely from degradation,
  1 e- oxidation)

Lewis, G.N. Kasha, M. J. Am. Chem. Soc. 1944,
66, 2100-2116.
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Identification of the Phosphorescent State with
the Triplet or Biradical State

• Hypothesis Long lifetime of phosphorescent
  state is due to prohibition of fall from
  phosphorescent (T) state to normal state (S)
• Explain as either
• different multiplicities
• tautomers (distortion of molecule)
• With vibrational energy released in T, T,
  return to T would be unlikely under tautomer
  explanation

• b is not the only deactivation mechanism, also
  non-radiative deactivation (dissipation)
• Rate of deactivation is sum of b and dissipation
• rate is only b when no dissipative deactivation

Lewis, G.N. Kasha, M. J. Am. Chem. Soc. 1944,
66, 2100-2116.
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Identification of the Phosphorescent State with
the Triplet or Biradical State

• Observation increasing the temperature and/or
  fluidity of a sample decreases the
  phosphorescence
• Explanation rigidity of the medium protects
  from dissipative deactivation
• Predication triplet state should have a
  measurable paramagnetic susceptibility
• (A TESTABLE PARADIGM!)

• Definitions of fluorescent state and
  phosphorescent state adjusted to include
  species like O2 where T is lower in energy than S
• Define fluorescence (a) and phosphorescence (b)

Lewis, G.N. Kasha, M. J. Am. Chem. Soc. 1944,
66, 2100-2116.
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Radical Pair Recombination as a Probe of Magnetic
Isotope and Magnetic Field Effects
Scheme 1_at_NaY Faujasite Zeolite?
Lem, G. Turro, N.J. Chem. Commun. 2000, 293-294.
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Radical Pair Recombination as a Probe of Magnetic
Isotope and Magnetic Field Effects
Scheme 1_at_NaY Faujasite Zeolite?

• External Magnetic Field - no effect
• Magnetic Isotope Effect - 2-fold increase in
  e.e. of dl-1
• Can interpret based on T-S ISC for triplet
  biradicals
• Separation of Angstroms leads to decreased T-S
  energy gap, increased hyperfine couplings,
  increased nuclear spin interactions
• 13CO may have faster ISC - decrease in
  decarbonylated products
• 12CO may have slower ISC - increase in
  decarbonylated products
• Lack kinetic model to explain experimental
  results
• Can probe radical pair dynamics in zeolites!

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Lem, G. Turro, N.J. Chem. Commun. 2000, 293-294.
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