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Chemistry 2

Chemistry 2. Lecture 13 Everything. Learning outcomes from lecture 12. Be able to explain Kasha’s law by describing internal conversion Be able to define fluorescence quantum yield Be able to describe intersystem crossing and how it leads to phosphoresence

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Chemistry 2

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  1. Chemistry 2 Lecture 13 Everything

  2. Learning outcomes from lecture 12 • Be able to explain Kasha’s law by describing internal conversion • Be able to define fluorescence quantum yield • Be able to describe intersystem crossing and how it leads to phosphoresence • Be able to explain why the phosphorescence occurs at lower energy (“red-shifted”) and is slower than fluorescence Assumed knowledge The sequence of events that can occur after absorption, including emission, fluorescence, phosphorescence, non-radiative decay, internal conversion and intersystem crossing. The use of Jablonski diagrams to describe these processes.

  3. Energy is the most important thing

  4. Extraterrestrial solar spectrum 6000 K thermal spectrum

  5. Atmospheric absorption Absorption by water, Carbon dioxide and ozone. Scattering too! O3 H2O H2O & CO2 overtones!

  6. Blue ice is due to overtone absorption overtones!

  7. Absorption of light by the earth ISC ISC ISC IC IC Absorption Fluorescence Chlorophyll fluorescence in satellite image S2 T2 While the earth fluoresces a little, the majority of incoming energy is internally converted into heat, and re-radiated in vibrational infrared transitions of water, rocks, asphalt… S1 T1 Phosphorescence S0

  8. Greenhouse (average)

  9. Absorption and re-emission of infrared radiation by atmospheric molecules E E/2 E/4 E ATMOSPHERE E/2 E E/4 E E/2 2E EARTH

  10. The greenhouse effect is due to IR absorption With no atmosphere, average temperature on earth is T0. If earth was blackbody with albedo of 0.40, then E E=168 But, with single layer blackbody atmosphere absorbing outgoing radiation, Earth heats to irradiate twice the incoming energy. E 2E EARTH But the atmosphere does not absorb all outgoing radiation…. And is best represented as a multlayer. Clouds and weather complicate matters.

  11. Greenhouse gases If atmosphere was purely N2 and O2, all surface-emitted radiation would escape into space. Gases which have oscillators that overlap the emission spectrum of earth’s 300K blackbody convey blackbody behaviour to the atmosphere (statistical emission/absorption). Peak emission of 300K object is 590cm-1.

  12. Green plants ISC ISC S2 T2 Absorption Fluorescence ISC IC IC S1 T1 Phosphorescence S0 Green plants absorb CO2 and synthesize sugars using light energy from the sun. S2-S0 S1-S0 Can be modelled as a particle on a ring system.

  13. Light harvesting The energy in photosynthesis is transferred from chlorophyll to chlorophyll and funnelled into the reaction centre. chlorophylls S2-S0 S1-S0

  14. Energy transfer is internal conversion in bichromophoric molecule IC IC Absorption Fluorescence * S2-S0 S2 T2 S1 hn T1 S0 hn

  15. Photodynamic cancer therapy Cytotoxic singlet oxygen is produced from energy transfer after intersystem crossing. Specific tissues can be targeted by laser irradiation of triplet sensitizer. ISC S1 T1 hn S1 energy transfer T0 S0 O2 triplet sensitizer Skin transmits red and near infrared light effectively (800nm is great), but absorbs most visible and UV. Much effort goes into finding near IR absorbing triplet sensitizers for cancer therapy.

  16. what is wrong with this picture?

  17. triplet-triplet annihilation (TTA) TTA sensitizer sensitizer S1 S1 S1 S1 ISC ISC T1 T1 T1 T1 S0 S0 S0 S0 emitter emitter

  18. spin states of two triplets 1/9 collisions statistically gives singlet which can decay into excited S1 of one chromophore, and S0 opf the other.

  19. requirements for TTA upconversion TTA sensitizer S1 Step down by >>kBT S1 ISC More than half S1 T1 T1 S0 emitters

  20. single threshold solar cells unoccupied energy levels IC! ~32% max V electrons

  21. Up-conversion Limiting efficiency of an Upconversion cell is about 50% cell Up-conversion unit

  22. Nothing wrong with this picture!

  23. Good Luck! Week 13 homework • Electronic spectroscopy worksheet in the tutorials • Complete the practice problems at the end of the lectures • Note: ALL of the relevant past exam problems have been used as practice problems (either on the worksheets or as ‘end of lecture problems’. Other questions on past papers include parts which are no longer part of the course.

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