excepaons molecular oxygen o2

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Unformatted text preview: Aon to Photochemistry 14 SelecAon Rules During an electronic transiAon, mulAplicity must be ______________ •  ‘allowed’ transiAons : singlet singlet and triplet triplet •  ‘forbidden*’ transiAons : singlet triplet CHM3122 IntroducAon to Photochemistry 15 Singlet excited states (SES) •  usually, the energy of an excitaAon photon is larger than the S0 → S1 gap hν S0 CHM3122 Sn IntroducAon to Photochemistry 16 The fate of Sn •  physical decay of Sn → S1 where the excited molecule loses the excess of energy via collision with other molecules •  this process: VIBRATIONAL RELAXATION, VR VR hν S0 Sn S1 Kasha’s Rule: the Sn → S1 decay is SO FAST that we will never observe photochemistry from upper singlet excited states CHM3122 IntroducAon to Photochemistry 17 The 3 fates of S1 •  3 possible decay pathways –  non ­radiaAve decay to S0 –  radiaAve decay to S0 –  photochemical process (ISC, energy transfer, bond homolysis etc.) CHM3122 IntroducAon to Photochemistry 18 Triplet excited states (TES) •  in some molecules: SES TES •  non ­radiaAve process where the spin of the promoted electron is flipped •  according to the SelecAon Rules, this is formally ‘forbidden’ CHM3122 IntroducAon to Photochemistry 19 The 3 fates of T1 •  3 possible decay pathways –  non ­radiaAve decay to S0 –  radiaAve decay to S0 –  photochemical process (energy transfer, etc.) CHM3122 IntroducAon to Photochemistry 20 Jablonski Diagram A graphical representaAon S1 T1 S0 CHM3122 IntroducAon to Photochemistry 21 Jablonski Diagram A graphical representaAon RadiaAve transiAon Non ­radiaAve transiAon Sn VR, 10 fs – 10 ps S1 CHM3122 IntroducAon to Photochemistry phosphorescence, µs - min ISC fluorescence, 1 – 100 ns S0 IC, 10 fs – 10 ps absorption, ~1 fs ISC, ns - µs T1 22 More detail… CHM3122 IntroducAon to Photochemistry 23 Some important notes… 1.  Energy: absorpAon > fluorescence > phosphorescence abs fluor phos λ E 2.  Fluorescence: no change in spin allowed Phosphorescence: involves 2 spin flips forbidden CHM3122 IntroducAon to Photochemistry 24 Some important notes… 3.  We forgot the sub ­levels! –  each electronic states has associated vibraAonal and rotaAonal sub ­levels –  in general, we’ll only include the vibraAonal states and ignore the rotaAonal sub ­ levels S1 T1 S0 CHM3122 IntroducAon to Photochemistry 25...
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This note was uploaded on 11/10/2012 for the course SCIENCE 1, 2, 3 taught by Professor All during the Winter '12 term at University of Ottawa.

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