Turro Group Meeting Phosphorescence and the Triplet State & Radical Pair Recombination and MFEs

1. Turro Group MeetingPhosphorescence and the Triplet State &Radical Pair Recombination and MFEs Jeffrey Lancaster November 11, 2008

2. 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

3. 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. 3

4. 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. 4

5. 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. 5

6. Radical Pair Recombination as a Probe of Magnetic Isotope and Magnetic Field Effects Scheme 1@NaY Faujasite Zeolite? Lem, G.; Turro, N.J. Chem. Commun. 2000, 293-294. 6

7. Radical Pair Recombination as a Probe of Magnetic Isotope and Magnetic Field Effects Scheme 1@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! Lem, G.; Turro, N.J. Chem. Commun. 2000, 293-294. 7

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