Turro Group Meeting Phosphorescence and the Triplet State & Radical Pair Recombination and MFEs Jeffrey Lancaster November 11, 2008

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? 2

Lewis, G.N.; Kasha, M. J. Am. Chem. Soc. 1944, 66, Identification of the Phosphorescent State with the Triplet or Biradical State 3 A molecule excited to T (phosphorescent state) can relax to S via  or with thermal excitation to S’ (fluorescent state) S’ relaxes to S via  At low temperatures, there is no  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, Identification of the Phosphorescent State with the Triplet or Biradical State 4 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  is not the only deactivation mechanism, also non-radiative deactivation (‘dissipation’) Rate of deactivation is sum of  and dissipation – rate is only  when no dissipative deactivation

Lewis, G.N.; Kasha, M. J. Am. Chem. Soc. 1944, 66, Identification of the Phosphorescent State with the Triplet or Biradical State 5 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 O 2 where T is lower in energy than S Define fluorescence (  ) and phosphorescence (  )

Radical Pair Recombination as a Probe of Magnetic Isotope and Magnetic Field Effects Lem, G.; Turro, N.J. Chem. Commun. 2000, Scheme Faujasite Zeolite?

Radical Pair Recombination as a Probe of Magnetic Isotope and Magnetic Field Effects 7 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 13 CO may have faster ISC - decrease in decarbonylated products 12 CO may have slower ISC - increase in decarbonylated products Lack kinetic model to explain experimental results – Can probe radical pair dynamics in zeolites! Scheme Faujasite Zeolite? Lem, G.; Turro, N.J. Chem. Commun. 2000,

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